GIFT OF 
 _ Fhoebe Si - Means fi. 
 
PETROGRAPHIC METHODS 
 
 PART I 
 THE POLARIZING MICROSCOPE 
 
 PART II 
 ROCK MINERALS 
 
Published by the 
 
 McGraw-Hill Boole Company 
 
 _N<e>v 
 
 \Succe5sons to theBookDepartments of tKe 
 
 McGraw Publishing Company Hill Publishing Company 
 
 Publishers of Books for 
 
 Electrical World The Engineering and Mining Journal 
 
 Engineering Record American Machinist 
 
 Electric Railway Journal Coal Age 
 
 Metallurgical and Chemical Engineering Power 
 
PETROGRAPHIC METHODS 
 
 THE AUTHORIZED ENGLISH TRANSLATION OF 
 
 PART I 
 
 Anleitung zum Gebrauch des Polarisationsmikroskops 
 (Third revised edition) 
 
 AND 
 PART II 
 
 Die Gesteinsbildenden Mineralien 
 (Second revised edition) 
 
 BY 
 
 DR. ERNST WEINSCHENK 
 
 PROFESSOR EXTRAORDINARIUS OF PETROGRAPHY 
 
 IN THE 
 UNIVERSITY OF MUNICH, GERMANY 
 
 RENDERED INTO ENGLISH BY 
 
 ROBERT WATSON CLARK, A. B. 
 
 INSTRUCTOR IN PETROGRAPHY IN THE UNIVERSITY OF MICHIGAN 
 
 McGRAW-HILL BOOK COMPANY 
 
 239 WEST 39TH STREET, NEW YORK 
 
 6 BOUVERIE STREET, LONDON, E. C. 
 
 1912 
 
COPYRIGHT, 1912 
 
 BY THE 
 
 McGRAw-HiLL BOOK COMPANY 
 
 Printed and Electrotyped 
 
 by The Maple Press 
 
 York, Pa. 
 
PREFACE 
 
 Although there are several excellent treatises on rock minerals 
 in thin section there is, nevertheless, a demand for a text which 
 sets forth all the methods used in a detailed study of rocks, in a 
 clear and concise manner. The fact that Part I of the original 
 German text, published by the Herder Publishing Company, 
 of Freiburg, in Breisgau, appeared in the third edition in 1910 
 and Part II in the second edition in 1907, seems recommendation 
 enough for presenting it to the English reading student. As it 
 was found inexpedient to adhere strictly to a mere translation 
 of the German an attempt has been made rather to render it 
 freely into English. 
 
 I am greatly indebted to Professor E. H. Kraus, of the Univer- 
 sity of Michigan, for encouragement in this effort, for very help- 
 ful suggestions in its execution, and for material aid in reading 
 the manuscript and correcting the proof. I wish to express here 
 my sincere appreciation for his interest in this work. Also to 
 Mr. W. F. Hunt, of the University of Michigan, I express my 
 hearty thanks for aid in reading the manuscript and correcting 
 proof. . 
 
 R. W. CLARK. 
 MlNERALOGICAL LABORATORY, 
 
 UNIVERSITY OF MICHIGAN, 
 January, 1912. 
 
CONTENTS 
 
 PAGE 
 PREFACE . v 
 
 INTRODUCTION xiii 
 
 PART I 
 
 CHAPTER I 
 
 THE MICROSCOPE 1 
 
 The Simple Microscope or Lens 1 
 
 The Compound Microscope 4 
 
 Polarizing Apparatus 7 
 
 The Polarizing Microscope 12 
 
 Material for Observation . 21 
 
 ERRATA. 
 
 Page 18 Caption to Fig. 32a should read Lomb. 
 
 Page 22 Line 14 " " 1/2 mm. 
 
 Page 64 " t 
 
 Page 132 Caption to Fig. 161 Heating Apparatus. 
 
 Page 184 " " " 183 " " Trachyte. 
 
 Page 188 " " " 195 " " Pyroxene. 
 
 Clark Petrographic Methods. 
 
 . 30 
 
 ' . 30 
 
 .uinerences in inaices ol Ketraction under the Microscope '. . 33 
 
 Determination of the Index of Refraction, Immersion Method . 36 
 
 Determination of Form and Cleavage 41 
 
 Measurement of Size and Thickness 45 
 
 Inclusions 47 
 
 Color 48 
 
 Observations in Reflected Light 49 
 
 CHAPTER IV 
 
 OBSERVATIONS IN PARALLEL POLARIZED LIGHT 51 
 
 1. Optical Properties of Crystals 51 
 
 vii 
 
viii CONTENTS 
 
 PAGE 
 
 Single and Double Refraction 51 
 
 Double Refraction of Light in Calcite 52 
 
 Uniaxial Crystals 55 
 
 Biaxial Crystals 58 
 
 2. Investigations with One Nicol the Polarizer 60 
 
 Surface Color and Pleochroism 60 
 
 The Phenomenon of Pleochroism . . 62 
 
 3. Investigations with Crossed Nicols 65 
 
 Recognition of Double Refraction 66 
 
 Determination of the Position of the Vibration Directions . . 67 
 
 Strauroscopes 71 
 
 Strength of the Double Refraction. Interference Colors ... 73 
 
 Modification of the Interference Colors 77 
 
 Measurement of the Double Refraction by Means of Interference 
 
 Colors 79 
 
 Interference Colors in Variously Orientated Cross Sections . . 82 
 
 Chromoscope for Interference Colors 83 
 
 Character of the Double Refraction 87 
 
 Compensators 88 
 
 Use of Compensators 93 
 
 CHAPTER V 
 
 OBSEKVATIONS IN CONVERGENT POLARIZED LIGHT 95 
 
 Direction of the Rays in Convergent Light 95 
 
 Methods of Observation in Convergent Polarized Light 97 
 
 Optically Uniaxial Crystals 99 
 
 Crystals with Circular Polarization 104 
 
 Character of the Double Refraction of Uniaxial Crystals .... 105 
 
 Biaxial Crystals 107 
 
 Dispersion of the Optic Axes 109 
 
 Measurement of the Optic Angle 112 
 
 Character of the Double Refraction of Biaxial Minerals 115 
 
 CHAPTER VI 
 
 TWINS AND OPTICAL ANOMALIES 120 
 
 Twins 120 
 
 Optical Anomalies 123 
 
 APPENDIX. (ACCESSORY APPARATUS) 125 
 
 1. Rotation Apparatus 125 
 
 Rotation Apparatus for Observations between two Plano- 
 convex Lenses 126 
 
 Rotation Apparatus for Investigation in Liquids 128 
 
 2. Heating Apparatus 129 
 
 3. Microphotographic and Projection Apparatus 132 
 
 Drawing Apparatus 134 
 
 SUMMARY OF METHODS . . 135 
 
CONTENTS ix 
 
 PART II 
 
 CHAPTER VII 
 
 PAGE 
 
 PREPAKATION OF MATERIAL 143 
 
 Investigation of Rock Pow.der 143 
 
 Preparation of Thin Sections 144 
 
 CHAPTER VIII 
 
 METHODS OF SEPARATION 148 
 
 1. Chemical Methods of Separation 148 
 
 2. Physical Methods of Separation 152 
 
 Analyses by Washing 152 
 
 Separation According to Specific Gravity 153 
 
 Heavy Organic Liquids 156 
 
 Heavy Solutions 157 
 
 Heavy Molten Liquids 159 
 
 Magnetic Separation 160 
 
 Other Methods of Separation 161 
 
 CHAPTER IX 
 
 METHODS OF INVESTIGATION 162 
 
 1. Chemical Methods of Investigation 162 
 
 (a) General Reactions 162 
 
 Reactions with Ftuosilicic Acid 163 
 
 Reactions with Certain Elements 164 
 
 (b) Special Reactions 172 
 
 Staining Methods 173 
 
 Precipitates on Thin Section 175 
 
 Alteration by Calcination 176 
 
 2. Physical Methods of Investigation 177 
 
 Determination of Specific Gravity 177 
 
 Determination of Hardness, Cleavage, etc 179 
 
 CHAPTER X 
 
 DEVELOPMENT OF ROCK CONSTITUENTS * . 181 
 
 External Form 181 
 
 Twinning 184 
 
 Aggregates 185 
 
 Growth and Solution 188 
 
 Cleavage, Parting and Mechanical Deformation 190 
 
 Intergrowth and Inclusions 192 
 
 Directions for the Use of the Descriptive Section 199 
 
 CHAPTER XI 
 
 DESCRIPTIVE SECTION 204 
 
 1. Opaque Minerals 204 
 
 Pyrite 204 
 
CONTENTS 
 
 PAGE 
 
 Pyrrhotite 205 
 
 Chalcopyrite 205 
 
 Galena 205 
 
 Metallic Iron 206 
 
 Magnetite 206 
 
 Chromite 207 
 
 Hematite 207 
 
 Ilmenite 208 
 
 Graphite : . 209 
 
 Carbonaceous Matter 210 
 
 2. Isotropic Minerals 212 
 
 Perovskite 213 
 
 Sphalerite 213 
 
 Garnet Group 214 
 
 Spinel Group 218 
 
 Periclase 219 
 
 Boracite 219 
 
 Rock Salt 219 
 
 Leucite 219 
 
 Glass 220 
 
 Analcite 222 
 
 Sodalite Group 223 
 
 Opal. .' 224 
 
 Fluorite 225 
 
 3. Uniaxial Minerals 225 
 
 Rutile 227 
 
 Anatase 229 
 
 Cassiterite 230 
 
 Wurtzite 230 
 
 Zircon 230 
 
 Xenotime 231 
 
 Corundum 232 
 
 Vesuvianite 233 
 
 Gehlenite Group 233 
 
 Tourmaline 234 
 
 Apatite 236 
 
 Rhombohedral Carbonates 238 
 
 Eudialyte 242 
 
 Scapolite Group 242 
 
 Alunite 243 
 
 Beryl 243 
 
 Brucite 244 
 
 Quartz, Chalcedony, Tridymite 244 
 
 Nepheline 249 
 
 Apophyllite 250 
 
 Chabazite 250 
 
 Cancrinite 250 
 
 Hydronephelite 251 
 
CONTENTS xi 
 
 PAGE 
 
 4. Biaxial Minerals 251 
 
 Brookite 254 
 
 Goethite 254 
 
 Pseudobrookite 255 
 
 Sulphur 255 
 
 Baddeleyite 255 
 
 Titanite ._. . . 255 
 
 Lievrite 256 
 
 Monazite 257 
 
 Lavenite 257 
 
 Chrysoberyl 257 
 
 Epidote Group 258 
 
 Staurolite : 266 
 
 Diaspore 267 
 
 Cyanite 267 
 
 Sapphirine 268 
 
 Serendibite 269 
 
 Prismatine 269 
 
 Astrophyllite 269 
 
 Brittle Micas 269 
 
 Margarite 272 
 
 Olivine Group 272 
 
 Pyroxene Group 277 
 
 Lawsonite 286 
 
 Amphibole Group 287 
 
 Dumortierite 295 
 
 Axinite . 295 
 
 Rinkite 295 
 
 Sillimanite 295 
 
 Datolite 296 
 
 Mosandrite 297 
 
 Barite 297 
 
 Andalusite 297 
 
 Lazulite 298 
 
 Carpholite 299 
 
 Prehnite . . 299 
 
 Celestite ' 299 
 
 Aragonite 299 
 
 Wollastonite Group 299 
 
 Humite Group 300 
 
 Topaz 301 
 
 Anhydrite . . 301 
 
 Mica Group 302 
 
 Chlorite Group and Serpentine 310 
 
 Hydrargillite 316 
 
 Talc 316 
 
 Pyrophyllite 317 
 
 Bertrandite. . 317 
 
xii CONTENTS 
 
 PAGE 
 
 Wagnerite 318 
 
 Kaolin 318 
 
 Nontronite 319 
 
 Hydromagnesite 319 
 
 Cordierite 319 
 
 Wavellite 320 
 
 Gypsum 321 
 
 Feldspar Group .321 
 
 | Biaxial Zeolites 341 
 
 TABLES 343 
 
 INDEX . . 385 
 
Introduction 
 
 The investigation of inorganic substances does not entail a 
 study of an almost inexhaustible number of forms as is the case 
 in the organic world where the structure is often so detailed that 
 even the strongest objectives of a modern microscope cannot 
 resolve them. Inorganic bodies occur in a comparatively large 
 number of different forms and strong magnification is rarely 
 needed in the investigation of them, because their structure is 
 seldom very minute. 
 
 The methods of microscopic investigation employed in the 
 study of organic nature are not applicable in inorganic research 
 because of the infinite number of chemical compounds already 
 known and the large additions that are being made daily. If 
 the microscope were merely an instrument for magnification, as 
 it is with zoologists, botanists, and medical investigators, neither 
 the chemist nor the mineralogist would be warranted in under- 
 taking microscopic studies. 
 
 Nevertheless, the modern microscope is very useful in the 
 investigation of inorganic substances. It is a sort of optical 
 universal apparatus which reveals not only the outer form, but 
 also the inner structure of a substance. It leads to results 
 rapidly and easily, which if obtained by other methods would 
 involve much time and labor. The introduction of the micro- 
 scope into chemical and mineralogical laboratories has been 
 made possible by numerous improvements devised in the last 
 thirty years and by its transformation into a polarizing instru- 
 ment. It is not yet used as much as it might be or as extensively 
 as its adaptability warrants. The splendid aid such a micro- 
 scope affords, is too little appreciated, especially in synthetic 
 and analytical chemistry. In many instances there is no other 
 way of determining the characteristics of a substance so rapidly 
 and positively, making lengthy tests often unnecessary. 
 
 Microscopic methods are especially serviceable in the investi- 
 gation of rocks. Petrography, the science of rocks, owes the 
 great progress it has made in the last third of the nineteenth 
 century to microscopic investigations. These methods, so 
 
 xiii 
 
xiv INTRODUCTION 
 
 fruitful to the petrographer, have only recently been applied to 
 chemical investigations, but their general introduction can be 
 accomplished only with considerable effort. However, micro- 
 chemical investigations based upon modern microscopic technic 
 have yielded results that are extremely promising. 
 
 Anyone, who has had opportunity to study the methods of 
 microscopic analysis thoroughly, must confess that, with a little 
 practice, they are very useful in recognizing rapidly, easily, and 
 positively the composition of a substance even in very small 
 quantities. Nevertheless, the optical methods found at first only 
 a very limited use in this promising field. Synthetic chemistry 
 has made use of microscopic methods still less, although they 
 would be very helpful to organic chemists in particular on 
 account of the large number of compounds. The unnecessary 
 loss of time which would be saved if organic chemists had a 
 means of identifying their compounds both rapidly and accu- 
 rately, no one can estimate. If it were only known how 
 quickly all the optical properties of a crystalline precipitate can 
 be determined after a little practice, it would be found justifiable 
 to recommend most urgently that chemists become skilled in 
 microscopic technic. The usual statements made in the litera- 
 ture concerning such precipitates generally take into considera- 
 tion only the simplest morphological properties and these are 
 frequently the least constant characteristics of a substance. 
 
 Chemists have adopted petrographic methods in spite of the 
 fact that their materials are quite different. In a way, a petro- 
 grapher is at a disadvantage because, in the investigation of 
 slides, he has to deal with sections orientated at random while in 
 the crystalline powders, which a chemist studies, the various 
 crystals assume a common characteristic position on the object 
 glass on account of their development. It is much easier to 
 determine the crystal system in the latter case, which is not easily 
 done in many instances in rock slides. On the other hand, 
 variously orientated sections of one and the same mineral can 
 be studied by a petrographer and he is able to determine the 
 optical properties of a substance in all directions, while with 
 isolated crystals this is often impossible. Some knowledge of 
 physical crystallography is an absolute prerequisite for investi- 
 gations in both these fields and its successful application to 
 microscopic studies requires much practice. 
 
 The first demand made of a polarizing microscope is that its 
 
INTRODUCTION xv 
 
 combinations of lenses shall be as perfect as possible. Mag- 
 nification is of secondary importance, the other optical adjust- 
 ments being the most important part of the instrument. A 
 large, plane, achromatic field combined with objectives and ocu- 
 lars transmitting as much light as possible are the first things 
 demanded of a microscope by a mineralogist as well as by a 
 botanist, zoologist, or medical investigator. The lens systems are 
 used for the various optical methods, which have been developed 
 in microscopic technic, and the optical properties, as well as the 
 observations of the image of the object investigated, are depen- 
 dent upon the sensitiveness of the lenses. 
 
 The mineral constituents of rocks can be recognized by the 
 naked eye only in a few cases. Macroscopic examination is 
 never sufficient to reveal all the details of the composition of a 
 rock. Texture also, which is of great importance in the classi- 
 fication of many rocks, can be observed by the unaided eye only 
 when it is comparatively coarse. Before a thorough study of 
 the composition and texture of rocks is undertaken, the student 
 should become well acquainted with the means by which such 
 knowledge may be obtained. 
 
 Microscopic examination of thin sections is very important. 
 It is quite necessary for the student to become as well acquainted 
 with microscopic optical methods as possible even though only a 
 superficial knowledge of petrography is desired, because an 
 approximate determination of rocks is frequently impossible 
 without them. On the other hand, to think that the macroscopic 
 characteristics of a rock should be ignored and the same deter- 
 mined only in thin section under the microscope, is an error that 
 must be avoided, although it was prevalent during the earlier 
 stages of the development of petrography. The macroscopic 
 appearance of a rock with reference to texture and constituents 
 furnishes a clue, which in many instances simplifies or supple- 
 ments the microscopic investigation. For this reason the 
 general exterior appearance of a rock must be carefully observed 
 before the microscopic investigation is begun. Mineralogy is 
 the science that teaches one to recognize the minerals by their 
 macroscopic properties. Petrographic investigation should not 
 be undertaken without a thorough knowledge of mineralogy, 
 although the list of rock-forming minerals of frequent occurrence 
 is very small. Mineralogy is the indispensable foundation upon 
 which petrographical investigation is based. 
 
xvi INTRODUCTION 
 
 Good results cannot be obtained in the microscopic study of 
 rocks unless the student has a comprehensive knowledge of the 
 optical properties of crystals and has had much practice in the 
 application of microscopic methods. Familiarity with a polariz- 
 ing microscope is a prerequisite for all petrographic work. 
 Serious errors in the determination of rock constituents may be 
 avoided by such knowledge. Special emphasis must be laid 
 upon the fact that simply knowing the appearance of certain 
 minerals in some rocks, or in a large number of rocks, is not 
 sufficient to give that confidence which is necessary in petro- 
 graphic investigation. As the appearance of one and the same 
 mineral varies greatly in different rocks, a continual source of 
 error is thus introduced and only general confusion results if the 
 knowledge of minerals is purely superficial and not sufficiently 
 based on microscopic optical methods. 
 
 The microscopic optical methods necessary for petrographical 
 investigation are discussed thoroughly in the first part of this 
 book. A knowledge , of these fundamental principles must 
 precede all petrographic work. 
 
 A positive statement concerning the detailed composition of a 
 rock cannot always be made upon the results of microscopic 
 investigations alone. A thin section confines the examination to 
 an extremely limited portion of the rock. In general, therefore, 
 one cannot expect to ascertain the complete composition of a 
 rock even after a very exhaustive study of one or more thin 
 sections. This is especially true of certain constituents that are 
 only sporadically present, but still are of considerable importance 
 for the rock as a whole. It often happens, after careful investi- 
 gation by optical methods, of certain mineral sections in a slide, 
 that the student does not feel sure that he has determined the 
 mineral correctly. An attempt is made to supplement these 
 methods as much as possible in various ways and this may be 
 done either by a series of chemical tests on the slide itself or by 
 isolating the different constituents by chemical or physical means 
 so as to facilitate studies on them. We distinguish, therefore, 
 between methods of separation and methods of investigation, and 
 each of these is divided into a chemical and a physical group. 
 A clear conception of the mineral composition of a rock, upon 
 which a safe classification is possible, can be obtained only 
 through a combination of all these methods. 
 
 Finally it may be added that the most accurate determination 
 
INTRODUCTION xvii 
 
 of the composition of a rock, obtainable by a combination of all 
 these methods, is one of the goals of petrography, but that with 
 it, the final object of petrographical research has by no means 
 been reached. It strives to explain not only the present con- 
 dition of the rocks, but also the origin and alteration of them. 
 All descriptions of rocks should be full enough to permit of the 
 determination of their geological relations. Petrographical 
 Investigation becomes important only in connection with geo- 
 logical studies and in this respect it is one of the most important 
 chapters in the whole science of geology. Without its aid geology 
 comes to some conclusions which have been proved in many 
 instances to be erroneous. The purpose of this book is to make 
 the determination of the rock-forming minerals possible and 
 space cannot be devoted to the consideration of these other 
 points. Description of the minerals and the methods of studying 
 them is the sole object of this text. 
 
PART I 
 
 THE POLARIZING MICROSCOPE 
 
CHAPTER I 
 
 FIG. 1. Real Image. 
 
 The Microscope 
 
 The Simple Microscope or Lens. The simplest form of the 
 microscope, the lens, serves to shorten the focal distance of the 
 eye and to increase the focal angle so that objects which are too 
 near to be seen distinctly by the naked eye can be seen by the 
 use of the lens. The shortest distance that a normal naked eye 
 can see distinctly is about 25 
 cm. If the distance from the 
 eye be less, the image becomes 
 indistinct and it requires a 
 system of lenses to make it 
 sharp. Magnification is, there- 
 fore, always reduced to the 
 focal length of 25 cm. A convex lens casts a real image SR, 
 Fig. 1, of the object rs, which is farther from the lens than its 
 focal length. The image is inverted, on the opposite side of the 
 lens from the object, and is beyond the focal length of the lens. 
 It usually cannot be observed by the eye directly, but becomes 
 
 visible when cast upon a 
 screen and can be repro- 
 duced directly on a photo- 
 graphic plate. If the ob- 
 ject is less than the focal 
 distance from the lens, the 
 rays do not unite in a 
 point on the other side of 
 the lens, Fig. 2, but the image RS appears in a normal position 
 on the same side as the object rs, and is designated as a virtual 
 image. It cannot be projected. 
 
 The faces of a cor. vex lens are portions of spherical surfaces and, in conse- 
 quence of this, the rays passing through various parts of the lens converge 
 approximately at a common point only when the curvature of the lens is 
 
 1 
 
 FIG. 2. Virtual Image. 
 
FETROGRAPHIC METHODS 
 
 FIG. 3. Spherical Aberration. 
 
 very small. If the faces have a greater curvature the rays through the edge 
 
 unite at another point than that at which the rays through the center con- 
 verge, and we see the phenomenon 
 of spherical aberration, Fig. 3. This 
 causes convexity of the image and in- 
 distinctness in various portions of the 
 field of vision, which is very annoy- 
 ing and fatiguing to the eye. The 
 image appears distorted because all 
 parts of the field are not magnified 
 equally. The border is magnified 
 
 more than the center so that a cross-sectioned object like Fig. 4 appears as 
 
 indicated in Fig. 5. 
 
 Another deficiency of a simple lens 
 
 is its dispersion, i.e., the property of 
 
 bodies to refract light of various wave 
 
 lengths differently. In the normal 
 
 case, colors having the shortest wave 
 
 lengths are refracted most, i.e., violet 
 
 rays are deviated more than red. 
 
 The formula for such dispersion is 
 
 v> p. It thus happens that the ob- 
 ject appears to have a colored border 
 
 because the various colors converge at 
 
 different places, Fig. 6. This phenomenon is called chromatic aberration. 
 
 FIG. 4. FIG. 5. 
 
 FIGS. 4 and 5. Cross-sectioned Object 
 
 Magnified 
 Uniformly. Ununiformly. 
 
 FIG. 6. Chromatic Aberration. 
 
 These two difficulties can be reduced if only those rays which 
 L 'pass through the center 
 
 of the lens are used. 
 This was accomplished 
 in the older types of 
 lenses in the manner 
 indicated by Figs. 7 and 
 8, the former being the 
 Brewster, the latter the Coddington model. The outer part of 
 the lens is cut off in each case by grinding out a portion. The 
 elongated cylindrical lens devised 
 by Stanhope, Fig. 9, also reduces 
 this deficiency. On it the face 
 turned toward the object has less 
 curvature than the other face. All 
 of these models have the disad- 
 vantage that their focal lengths 
 are very short. Fraunhofer im- 
 proved upon them by combining two plano-convex lenses set 
 
 FIG. 7. 
 Brewster's Lens. 
 
 FIG. 8. 
 Coddington's Lens. 
 
THE MICROSCOPE 
 
 with their convex faces toward each other, Fig. 10. This 
 
 model has been varied in many ways. 
 Lenses which are completely or 
 
 almost entirely corrected for spherical 
 
 and chromatic aberration are said to 
 
 be aplanatic and achromatic. Such a 
 
 condition is obtained, however, only 
 
 by a combination of different kinds 
 
 of glass. Thus, flint glass has nearly twice as great a dispersion 
 
 as crown glass. A system of lenses 
 can be arranged as in Fig. 11, con- 
 sisting of a converging lens L of 
 crown glass and a double concave 
 lens of flint glass L'. The colors are 
 FIG. 11. Correction of chromatic dispersed by the convex lens, but are 
 Aberration. reunited in the point p by the con- 
 
 cave lens, the so-called Bruecke lens. The best modern lens is 
 
 one composed of a double convex lens of 
 
 crown glass L, Fig. 12, between two diverging 
 
 meniscuses of flint glass F. It is almost 
 
 perfectly aplanatic and achromatic and com- 
 bines with these advantages a large field, a 
 
 long focal length, and distinctness. The 
 
 entire field, which is very large, is only used 
 
 when the lens is placed very close to the eye. 
 
 FIG. 12. Steinheil 
 Triplet. 
 
 It is used princi- 
 pally in the investigation of 
 rocks when a magnification 
 of six to twelve is desired. 
 
 The Verant lens, consisting of a 
 combination of two lenses, is 
 quite serviceable -for a smaller 
 magnification up to about four. 
 It has a horn eyepiece which 
 must be fitted to the eye and this 
 places it in the proper position to 
 I obtain an image entirely free 
 ] from distortion. A Zeiss anastig- 
 j matic lens can be used for stronger 
 ' magnification up to about 
 twenty-seven. It consists of a 
 combination of four lenses and in 
 spite of its comparatively large 
 magnification has a wide field and the object can be placed quite a con- 
 
 FIG. 13. Lenstand. 
 (After Voigt & Hochgesang.) 
 
PETROGRAPHIC METHODS 
 
 siderable distance from it. The lens ought to be held firmly in a stand for 
 that purpose while the eye is moved slowly over it. 
 
 It is best to fasten the lens upon a stand, especially when stronger magni- 
 fication is used, to insure the necessary stability and to leave the hands free 
 for manipulation. Fig. 13, page 3, shows such a stand equipped to use 
 polarized light. It may also be provided with a rotating stage. 
 
 The Compound Microscope. The compound microscope, or 
 simply the microscope, is distinguished from the simple micro- 
 scope or lens by a combination of two independent systems of 
 
 lenses. The simplest form 
 of such a microscope consists 
 of two double convex lenses 
 as shown in Fig. 14, in which 
 the path of the rays through 
 the microscope is shown 
 diagrammatically. One of 
 the lenses, ab Fig. 14, has a 
 shorter focal length than the 
 other, and since it is always 
 next to the object it is called 
 the objective. It casts a 
 real magnified image US of 
 the object rs. This image is 
 a little further from it than 
 the focal length of the lens 
 and is inverted. It is pro- 
 jected within the focal length 
 of the eyepiece or ocular and 
 is, therefore, again magnified 
 and appears as a virtual- 
 image at S'R'. This has 
 the same relative position as 
 the real image, i.e., it is in- 
 verted with respect to the 
 object. It thus appears that 
 the objective casts a real 
 image of a close object and this image is in turn observed by 
 means of the ocular, as a magnified virtual image. Since the 
 real image is inverted with respect to the object and the virtual 
 image not, objects observed through the compound microscope 
 always appear reversed. For purposes of projection such as 
 
 Fia. 14. Passage of Rays through a Compound 
 Microscope. 
 
THE MICROSCOPE 5 
 
 is common in photography, the ocular is generally removed and 
 the real image of the object is projected upon a photographic 
 plate or upon a screen. 
 
 The ocular and objective are placed in a metallic tube to exclude all dis- 
 turbing outside light between them and, as shown in Fig. 14, they are placed 
 much farther apart than the sum of their focal distances. They are cor- 
 rected for a fixed distance, but nevertheless the ocular must be so arranged 
 that this distance can be altered somewhat, as desired, because even in a 
 correct apochromatic system, object glasses of various thickness may require 
 lengthening or shortening of the tube. 
 
 The ocular is generally set in the upper part of the tube and (in the simpler 
 instruments) the objective is screwed into the lower part of it. However, 
 if the objective is changed frequently, considerable time is lost in mani- 
 pulating, which is very annoying. To avoid this the whole series of 
 
 FIG. 15. Revolver. FIG. 16. Tongs. Objective Holders. FIG. 17. Groove. 
 
 objectives can be fastened on a revolver so that the change can be effected by 
 simply rotating it, Fig. 15, or the tube can be fitted with an objective clamp, 
 Fig. 16, into which the objective can be slipped by means of a ring on the 
 upper part of it, so arranged that it is always approximately centered. 
 Finally, a guide bar can be constructed on the objective, which fits accurately 
 into a groove in the tube, Fig. 17. 
 
 A more thorough discussion of the construction of modern objectives 
 and oculars would lead us too far and they are generally very perfectly con- 
 structed by the more reliable firms. It is evident that a system of lenses must 
 be even more achromatic and aplanatic than a simple lens. At best a system 
 of lenses is only approximately perfect in both these respects and the image 
 appears quite perceptibly distorted when a periscopic ocular is employed in 
 order to use the whole field of the objective. This is especially true with low 
 magnification. Generally, the correction for chromatic aberration is 
 parfect only for a part of the spectrum, so that a faint coloring appears 
 particularly with strong objectives. This is entirely avoided in apochromatic 
 lenses, which are constructed of special glasses. In them there is an equal 
 difference of magnification for the colors in all zones of the field of vision and 
 this is completely counter-balanced by a compensating ocular, constructed 
 in the reverse order. 
 
6 
 
 PETROGRAPHIC METHODS 
 
 To obtain a high magnification it is generally advisable to use a strong 
 objective with a medium to weak ocular, because the amount of light in 
 the image is entirely dependent upon the numerical aperture of the objec- 
 tive. Abbe defines the numerical aperture, or simply aperture, as the pro- 
 duct of one-half of the angle of aperture of the lens and the index of refrac- 
 tion of the medium between the objective and the object. Figs. 18 and 19 
 show that the angle of aperture of the system of lenses, i. e., the angle of the 
 cone of rays that is taken in by the objective is not the sole criterion for the 
 amount of light from a given point. In each case the angle of the cone of 
 light, and therefore the amount of light transmitted from the object, is the 
 same. Fig. 18 represents a type of a dry system of lenses in which the outer 
 rays of the cone of light, when extended, fall outside of the objective be- 
 cause, when they pass from the slide into the air, they are refracted away 
 from the normal and in this case are diverging at a larger angle than the angle 
 of aperture of the lens. Thus the object is illuminated by a smaller cone 
 of light. 
 
 If a liquid is placed between the object and the objective the latter be- 
 comes an immersion system, Fig. 19. The liquid should have an index of 
 refraction approximately the same as that of the cover-glass on the object 
 
 FIG. 18. Passage of Rays in a 
 Dry System. 
 
 FIG. 19. Passage of Rays in 
 an Immersion System. 
 
 and the lower lens of the objective, e. g., oil. The light upon passing into 
 the object is refracted, as in the other case, but when it passes through the 
 oil into the objective it suffers but little deviation. Every point of the 
 object is illuminated by the full strength of the cone of light used. 
 
 The aperture of a dry objective cannot be more than 1 theoretically, 
 because n = 1 and the maximum of sin u is 1 . With a water immersion the 
 theoretical value may be as high as 1.33, when half of the angle of aperture 
 is 90, but this is impossible in practice. With oil it may reach 1.5 and with 
 bromnaphthalene it may be over 1.6. It must always be kept in mind that 
 the lenses of the objective, the cover-glass, and the liquid between them must 
 have the same indices of refraction. The increased illumination of an im- 
 mersion objective is shown by the fact that even a water immersion takes 
 in 1.77 times as much light as a dry system with the same angle of aperture. 
 
 The resolving power of objectives will now be briefly discussed. The 
 shortest distance that can be distinguished by an objective is represented by 
 the quotient of A divided by the aperture of the objective, where A. is the 
 wave length of the light employed. Structures of 0.00015 mm. can be dis- 
 tinguished by a monobromnaphthalene immersion making use of the violet 
 rays and taking their impression on a photographic plate. This is more 
 
THE MICROSCOPE 7 
 
 easily accomplished by oblique illumination with a ray of light or with a 
 central cone of light. On account of the greater wave length of the light 
 in the illuminous part of the spectrum, the distances must be about twice 
 as great to make the object distinct. Finer detail can be obtained by 
 photographic methods than by direct observation, especially when only 
 ultraviolet rays are used. The objectives must, however, be especially 
 corrected for these rays. Ultramicroscopy, devoted to the study of ex- 
 tremely small particles of approximately molecular dimensions, is only 
 mentioned here for the sake of completeness. 
 
 It may be suggested that the best source of light for microscopic investi- 
 gation is daylight and indeed a northern sky covered with thin white clouds. 
 Direct sunlight as a source of illumination is extremely disadvantageous for 
 the eye and must be changed into diffused light by the use of a screen. 
 Only two of the artificial sources of light are of importance, namely, the 
 electric arc and the Lassar lamp. The latter gives a blue light produced by 
 passing it through a color filter. 
 
 To produce monochromatic light, either a flame colored by sodium, 
 lithium and so forth or Geissler tubes or a light filter may be used. If very 
 intense illumination is employed, the Abbe spectro-polarizer may be advan- 
 tageously used. It is inserted in place of the regular polarizer and gives 
 illumination from a small part of the spectrum. 
 
 The tube is fastened to the stand by means of a large screw for 
 coarse adjustment and a micrometer screw for fine adjustment. 
 It is movable vertically to and from the stage, which is firmly 
 fixed on the stand. The illuminating apparatus is seen through 
 a hole in the stage. It serves as a collecting system for the light 
 reflected from a concave mirror below. It is necessary that the 
 aperture of the illuminating system be at least as great as that 
 of the objective if the entire aperture of the latter is to be used. 
 The former, however, need not be so carefully corrected for 
 spherical and chromatic aberration. The Abbe illuminating 
 apparatus is of the most perfect construction for, when it is 
 employed as an immersion system, its aperture is the same as the 
 strongest objective and at the same time it affords .a sufficiently 
 large illuminated field, when used with the weakest objectives. 
 A condenser is used in the simpler microscopes, but it must be 
 removed when using low power objectives because of the small 
 illuminated field. 
 
 Polarizing Apparatus. The polarizing microscope differs 
 from an ordinary microscope chiefly in having an attachment for 
 producing plane polarized light. Ordinary light vibrates in all 
 planes at right angles to the direction of propagation of the ray 
 as shown in Fig. 20. Plane polarized light, on the other hand, 
 vibrates only in one plane, Fig. 21. The plane, of polarization 
 
8 
 
 PETROGRAPHIC METHODS 
 
 PP, Fig. 22, of a ray of light AB, i.e., the plane in which the poles 
 lie, is always perpendicular to the plane of vibration SS. 
 
 In using transmitted polarized light, the direction of propaga- 
 tion AB, Fig. 22, is perpendicular to the plane of the object. 
 The light vibrates in a plane at right angles to this direction, this 
 plane being called the plane of vibration SS', Fig. 22, while its 
 trace on the plane of the object SS', Fig. 21, is the vibration 
 
 FIG. 20. Ray of Ordinary 
 Light. 
 
 FIG. 21. Plane of Vibration of a 
 Ray of Polarized Light. 
 
 direction of the light. The velocity of the light is dependent 
 upon the rate of vibration, or in other words, the elasticity of the 
 medium under investigation in a direction at right angles to the 
 direction of propagation. In examining a slide we observe the 
 differences in the optical elasticity in different directions, which 
 lie in the plane of the object itself, and are thus perpendicular to 
 the direction of propagation of the light. The plane of polariza- 
 tion PP', Fig. 22, is perpendicular to the plane of vibration SS'. 
 
 FIG. 22. Ray of Polarized Light with Plane of Vibration, Plane of Polarization and Direc- 
 tion of Propagation. 
 
 Ordinary light can be changed into partially polarized light by 
 reflection or refraction. Polarization by reflection is most com- 
 plete when the light strikes the reflecting surface at a certain 
 angle, which is dependent upon the reflecting substance. This 
 angle of .incidence i is determined by the formula tan i = n. 
 This angle possesses another notable property, viz., that the 
 reflected portion of the ray incident at that angle travels at 
 right angles to the refracted portion. It is called the angle of 
 
THE MICROSCOPE 9 
 
 polarization, and for ordinary glass is about 57. Naturally 
 it varies with different colors, although only to a small extent. 
 By repeated reflection at this angle the amount of light may be 
 increased and for this reason a series of thin glass plates is often 
 employed, which produce almost perfectly polarized light. 
 
 A ray of ordinary light L, Fig. 23, which falls upon a series of 
 glass plates at the polarizing angle is partially reflected as a ray 
 of plane polarized light P. Its plane of vibration is perpendicular 
 to the plane of the incident light LOB. The refracted portion 
 of the ray P', which passes through the plates is likewise polarized, 
 
 P" 
 
 FIG. 23. Polarization by Reflection and by Refraction. 
 
 but not so perfectly. Polarization becomes more complete the 
 oftener the ray is refracted, i.e., the greater the number of plates. 
 The plane of vibration of the refracted polarized ray is perpen- 
 dicular to that of the reflected ray and is therefore parallel to the 
 plane of incidence. Polarization of the refracted ray is the more 
 complete the smaller the angle of incidence. Polarization by 
 refraction often becomes quite annoying, especially in the case 
 of very strong objectives, for example, those with a very large 
 angle of aperture, because in them peripheral rays -of light often 
 pass through at oblique angles of considerable size. 
 
 The simplest and oldest polarizing instruments are based upon 
 the use of a reflecting glass plate. Such is the Noerremberg 
 polariscope or orthoscope, Fig. 24. Light from a strikes the 
 reflecting plate AB, the polarizer, at the polarizing angle and is 
 reflected as plane polarized light to the mirror c, and from it 
 into the axis of the instrument. The polarized light passes 
 through the plate AB and the rotating stage above it, and strikes 
 the blackened mirror S, the analyzer, which can be rotated about 
 a vertical axis. When S is parallel or antiparallel to AB, it 
 
10 PETROGRAPHIC METHODS 
 
 reflects the light polarized by the latter, but when in a crossed 
 position it extinguishes the light and appears dark. 
 
 A simple polarizing apparatus can be prepared from double 
 refracting crystal plates in which the absorption of light in one 
 direction is much greater than in the other, for example, in deeply 
 colored tourmaline. Fig. 25 shows tourmaline tongs of this sort. 
 Each arm of the tongs contains a plate of tourmaline cut parallel 
 to the optic axis and so fastened that it can be rotated. The 
 plates are so thick that no light passes through in the direction 
 of greatest absorption. If like directions in the two plates are 
 parallel light is transmitted, but in crossed positions the field of 
 vision appears dark. 
 
 FIG. 24. Orthoscope. FIG. 25. Tourmaline Tongs. 
 
 Polarization of light by reflection or refraction is by no means 
 complete enough for finer investigations, while that produced by 
 absorption gives polarized light which is deeply colored. For 
 this reason such apparatus is used only in the simplest and 
 cheapest instruments, while in a good polarizing microscope, nicol 
 prisms or similar devices are employed. These completely 
 polarize the light and produce colorless illumination with but 
 little loss. 
 
 The construction of nicol prisms, or simply nicols, depends upon 
 the ability of double refracting crystals to decompose a ray of 
 ordinary light into two rays of polarized light vibrating at right 
 angles to each other. These rays are refracted differently. 
 The original form of a nicol, Fig. 26, was an elongated cleavage 
 piece of calcite, the ends of which were cut so as to make an angle 
 of 68 instead of 71 with the long edges. Then the calcite is 
 
THE MICROSCOPE 
 
 11 
 
 cut through at right angles to the artificial faces and the two 
 halves are cemented together in the same position with Canada 
 balsam. The two polarized rays vibrating at right angles to each 
 other pass through the lower half of the prism as far as the 
 layer of Canada balsam, but the ordinary ray (o) is refracted 
 more than the extraordinary ray (e). In the direction in which 
 the ordinary ray vibrates, calcite is a much denser medium for 
 light than Canada balsam, and since this ray e 
 
 impinges upon the layer of balsam with a suffi- 
 ciently high angle of incidence it is totally reflected 
 and eliminated. The other ray (e) is refracted 
 about equally in calcite and Canada balsam and 
 since it impinges upon the balsam with a smaller 
 angle of incidence, it passes through the combina- 
 tion with only slight refraction. Thus a nicol prism 
 produces light polarized in one plane, the plane of 
 vibration being that of the extraordinary ray and a 
 principal section of the calcite. The vibrations of 
 the ray are parallel to the short diagonal of a 
 cleavage piece of calcite. 
 
 This original form was afterward altered and modified 
 in many ways. One modification which has been used 
 quite extensively was suggested by Hartnack and Praz- 
 mowsky. Aside from having the end faces perpendicular 
 to the others they used linseed oil instead of Canada balsam 
 as the cement. This allows the calcite to be used more 
 advantageously. Ahrens produced a double prism of 
 similar construction but much shorter, and it is character- 
 ized by an especially large angle of aperture. 
 
 Prisms of sodium saltpeter have recently been recommended extensively. 
 On account of its optical properties, it is more adaptable for such apparatus 
 than calcite. However, its application has been limited on account of its 
 hygroscopic nature. Combinations of glass with calcite or saltpeter have 
 been used with great success. The upper portion of the prism may be con- 
 structed of glass having an index of refraction and dispersion as nearly as 
 possible identical with that of the extraordinary ray in calcite, or only a thin 
 cleavage piece of the double refracting material may be cemented between 
 two glass wedges. 
 
 More completely polarized light can be obtained with such a combination 
 than with a simple calcite prism in which the light reflected from the edges 
 is always a disturbing element. Since the indices of refraction and particu- 
 larly the dispersion of the glass and the extraordinary ray in calcite are not 
 exactly the same, such combinations can only be used as polarizers and not 
 as analyzers because they are not entirely achromatic. The name "nicol" 
 has been retained for all these constructions. 
 
 FIG. 26. 
 
 Construction 
 
 of a Nicol Prism. 
 
12 
 
 PETROGRAPHIC METHODS 
 
 The Polarizing Microscope. Fig. 27 shows a cut of one of the 
 simpler polarizing microscopes made by W. and H. Seibert. A 
 cross section of this instrument is shown in Fig. 28. The illumi- 
 nation by a mirror, as well as the ocular, objective, and the 
 
 FIG. 27. Polarizing Microscope by W. & H. Seibert in Wetzlar. 
 
 movement of the tube by a coarse and a micrometer screw are 
 the same as in an ordinary microscope. This instrument differs, 
 however, from the ordinary model in many ways. Cross hairs, 
 F, Fig. 28, consisting of two fine threads of spider's silk stretched 
 exactly at right angles to each other, are placed in the inner focus 
 
THE MICROSCOPE 
 
 13 
 
 of the Huygen's ocular, which is arranged to fit into the upper 
 end of the tube. 
 
 The part of the tube carrying the ocular can be drawn out and 
 contains a slit in which a Bertrand-Amici lens can be placed, S, 
 Fig. 28. The drum m on the micrometer screw for fine focusing 
 of the tube is graduated, es- 
 pecially on the larger instru- 
 ments, so that the vertical 
 movement of the objective, 
 can be measured accurately. 
 
 The screw k for coarse ad- 
 justment of the tube should 
 be placed quite high so that 
 the objective can be moved 
 vertically within wide limits, 
 thereby allowing the use of 
 high apparatus on the stage, 
 for example, the universal 
 rotating stage. The arm A, 
 of the stand which holds the 
 tube must be curved as much 
 as possible so that it will not 
 prevent the rotation of the 
 stage with such accessory ap- 
 paratus on it. 
 
 A nicol prism, the analyzer, 
 is generally placed within the 
 tube so that it can be with- 
 drawn horizontally. Under 
 certain conditions it is ad- 
 vantageous to rotate the 
 nicol through 90 by means 
 of /, a circular scale indicat- 
 ing the angle of rotation. In 
 some cases the nicol is fixed and then another nicol W, placed 
 over the ocular, is used when rotation is necessary. The ocular 
 is then supplied with a graduated ring R so that the amount of 
 rotation can be read. The latter form of analyzer is found only 
 in the older models. 
 
 The clamp Z, Fig. 27, on the lower end of the tube holds the 
 objectives firmly. A slit c into which the compensators, placed 
 
 FIG. 28. Cross Section through the Micro- 
 scope in Fig. 27. 
 
14 
 
 PETROGRAPHTC METHODS 
 
 between two small glass plates, can be inserted, is made in the 
 tube above the clamp. The objective holder is fitted with two 
 micrometer screws, v and v' , at right angles to each other with 
 springs opposite each, so that the objective can be moved slightly 
 in a horizontal direction. After each change of objectives the 
 
 axis of the latter can be 
 centered exactly with the 
 axis of rotation of the stage 
 by means of these screws. 
 They are therefore called 
 the centering screws. 
 
 On other models the 
 centering is effected on the 
 stage, i.e., instead of ad- 
 justing the axis of the ob- 
 jective with the axis of 
 rotation of the stage, the 
 center of the stage itself 
 is shifted by a similar pair 
 of centering screws. Cen- 
 tering on the stage is not 
 so advantageous, for it 
 lessens the stability of the 
 stage, which is of consider- 
 able importance, especially 
 with high magnifications. 
 
 Frequent centering 
 Hi causes much time to be 
 ^aSSBiiPyflP wasted, especially with the 
 
 older instruments on which 
 the objectives are always 
 screwed in. This is very 
 annoying, however, so 
 some instruments have been constructed which make centering 
 unnecessary. Thus in the French model, Fig. 29, devised by 
 Nachet (Paris) , the objective and the stage are connected by a 
 strong arm and can be rotated simultaneously, thus avoiding 
 all eccentric rotation. This can also be accomplished on the 
 microscope shown in Fig. 30, which was first constructed in 
 Geneva. The objective holder TM with the micrometer screw 
 M is fastened onto the stage and rotates with it. Especially 
 
 FIG. 29. Polarizing Microscope 
 by A. Nachet in Paris. 
 
THE MICROSCOPE 
 
 15 
 
 beginners are at a disadvantage in using these models for the 
 head of the micrometer s.crew changes its position with rotation 
 of the stage, and when it is necessary to use the screw there is 
 a tendency to remove the eye from the ocular. 
 
 Ok 
 
 FIG. 30. Polarizing Microscope by C. Reichert in Vienna. 
 
 This defect was avoided by a model first made in England, 
 Fig. 31 being a similar instrument devised by Voigt and Hoch- 
 gesang. Here the two nicols can be rotated simultaneously, the 
 rotation of one being transmitted to the other by means of a 
 vertical rack and pinion. The object and stage do not move. 
 
16 
 
 PETROGRAPHIC METHODS 
 
 Models of this sort are strongly recommended for work with the 
 rotation apparatus, to be described later, and are made in various 
 designs. 
 
 By improving this model, Voigt and Hochgesang have recently 
 produced a new instrument called a polarizing microscope-poly- 
 meter, Fig. 32. S is the coarse focusing screw of the microscope 
 which is constructed particularly wide, and T is a lateral micro- 
 
 FIG. 31. Polarizing Microscope with Rotating Nicols. 
 in Gottingen.) 
 
 (Voigt & Hochgesang 
 
 meter screw. This is a new feature and is entirely independent 
 of the rest of the instrument. The analyzer can be rotated with 
 the circle R alone and likewise the tube with the ocular and cross 
 hairs can be rotated on the circle K 2 , while the objective remains 
 stationary. The two nicols can 1 be rotated simultaneously by 
 
THE MICROSCOPE 
 
 17 
 
 shifting F. Thus this new model is characterized by great 
 stability and convenience, together with a wide range of use- 
 fulness. 
 
 FIG. 32. Polarizing Microscope Polymeter. (Voigt & Hochgesang in Gottingen.) 
 
 The stage T, Figs. 27 and 28, can be rotated about a vertical axis and is 
 graduated so that the amount of rotation can be noted. A vernier is often 
 placed on the scale, but it is generally of little value because the accuracy of 
 2 
 
18 
 
 PETROGRAPHIC METHODS 
 
 microscopic measurements is much too small to permit closer reading than 
 i, an amount that is easily estimated. The stage is also equipped with a 
 series of holes for attaching accessory apparatus. Of these the object clamp 
 is very important and is used to hold the object in place particularly when 
 the stage is inclined. The form shown in Fig. 33 is the most satisfactory 
 
 FIG. 32a. Polarizing Microscope. Bausch & Lamb. 
 
 because it allows a definite point on the object to be orientated rapidly in 
 the center of the field of vision. It is also useful when the stage can be fixed 
 in any position by means of a set screw. 
 
 A mechanical stage is frequently used on the larger instruments for 
 investigations particularly with strong objectives, Fig 34. By means of 
 
THE MICROSCOPE 
 
 19 
 
 FIG. 33. Object Clamp. 
 
 two screws at right angles to each other the object can be moved laterally. 
 With such an apparatus the whole slide can be investigated much more 
 evenly because one is independent of the more or less arbitrary movement of 
 the hand. If the grooves in which the stage slides are equipped with scale 
 divisions, the stage itself may be used to locate the portions of the slide 
 investigated by simply noting the position of the scale with reference to a 
 fixed position of the object on the stage. 
 
 It is better though to indicate the position of a mineral in the slide either 
 by a free hand colored ring or by means of an object marker. The latter is 
 adjusted in place of the objective after the 
 mineral has been accurately centered. This 
 little apparatus may have an eccentric diamond 
 point that can be regulated and set in a holder 
 with a spring so that it rests upon the cover- 
 glass when the tube is lowered. Then the stage 
 
 with the slide held firmly in place is rotated and a small circle is scratched 
 on the cover-glass marking the proper spot. The marker may also have the 
 form of an objective similar to the one above, but on the lower end there is 
 a round opening which can be smeared with some oil color, ink, or a solution 
 of shellac. This leaves a small circle on the cover-glass of the slide. 
 
 Beneath the stage on the models shown in Figs. 27 and 28 
 there is a plate with a hole in the center through which a tube 
 can be moved in a vertical direction by means of a lever. The 
 illuminating apparatus and the second nicol, the polarizer P, 
 are placed in the tube. It is very desirable to be able to remove 
 the nicol and the illuminator separately from the tube, i.e., 
 
 each piece should be 
 fastened in a separate 
 holder. The illuminat- 
 ing apparatus consists 
 of a lens of small aper- 
 ture upon which rests 
 the condenser with large 
 aperture. In case of 
 observations with very 
 weak objectives the con- 
 denser can be swung out 
 of the path of the rays 
 
 by means of tongs placed within the rotating stage. When the 
 condenser rests upon the polarizer and the tongs are freed from 
 it, it moves in a vertical direction with the polarizer and other 
 illuminating apparatus. An iris diaphragm which is never lack- 
 ing on zoological and medical instruments, is placed only on the 
 larger models of the polarizing microscope, because it requires 
 
 FIG. 34. Mechanical Stage by R. Fuesz in Steglitz. 
 
20 
 
 PETROGRAPHIC METHODS 
 
 rather complicated apparatus and the same effect is obtained by 
 the vertical movement of the illuminating apparatus. 
 
 The polarizer can be rotated and there is either a series of 
 notches on the holder into which a catch on the polarizer slips, 
 
 or there is a graduated scale so 
 that it can be placed parallel to 
 the cross hairs in the ocular at any 
 time. 
 
 FIG. 35. Device for removing the 
 Condenser by A. Nachet. 
 
 Many other devices have been sug- 
 gested to overcome the annoyance of 
 removing the condenser by hand. The 
 condensing lens may be placed in a 
 simple slide in the stage of the micro- 
 scope, but as vertical movement is 
 then impossible, there can be no grada- 
 tion of the illumination or adjustment 
 
 for variation of thickness of the object glasses. It may also be fastened in 
 a fixed clamp, but here again the limit of vertical movement is small, which 
 makes an iris diaphragm necessary. The lens may also be fastened in a 
 slide at a fixed distance above the illuminator in such a manner that it 
 retains the vertical movement, but can be drawn out laterally. Finally 
 Nachet perfected a very simple device which does not interfere with the 
 movement of the illuminating 
 
 la 
 
 system. It consists of a lateral 
 screw below the stage by means 
 of which the condenser can be 
 rotated into position over the 
 illuminating apparatus like a 
 visor, Fig. 35. 
 
 In some cases, especially with 
 very strong objectives or with 
 objects that transmit but little 
 light, it is advantageous to re- 
 move the polarizer entirely, be- 
 cause it absorbs more than half 
 the light at one's disposal. For 
 this reason it had better be set 
 in a holder separate from that of 
 the rest of the illuminating appa- 
 ratus, for then it can be with- 
 drawn from the tube at any 
 time. Orientation of the nicol by hand wastes a good deal of time. 
 
 FIG. 36. Device for throwing out the Polarizer 
 by W. & H. Seibert. 
 
 By 
 
 means of a device made by W. and H. Seibert, Fig. 36, the polarizer P can be 
 moved in a groove by the knobs K and K' and its -place taken by a hollow 
 cylinder with an iris diaphragm J. Thus the polarizer can be thrown in or 
 out without changing its orientation and this has the advantage that the 
 instrument can be used for the most exacting investigations in organic 
 
THE MICROSCOPE 
 
 21 
 
 microscopy at any time. The illuminating apparatus in this device can be 
 moved together with the other in a vertical direction by the screw S'. The 
 condenser C can be slipped out by the tongs Z. This complicated device 
 can also be replaced by the simple visor arrangement and it is also apparent 
 that an Abbe illuminating apparatus might be used. 
 
 Fig. 32A represents one of the latest models of a petrographic microscope 
 made by the Bausch and Lomb Optical Company in Rochester, N. Y. 
 
 Material for Observation. The largest number and the most important 
 microscopic observations are made in transmitted light. It is therefore 
 
 FIG. 37. Cutting and Grinding Machine by Voigt & Hochgesang. 
 
 necessary to prepare the objects for investigation so that they will be suffi- 
 ciently transparent to allow the inner structure to be studied and so that 
 there will be no overlapping of different individuals, and at any one 
 place on the object the material be homogeneous. In organic microscopic 
 investigations this is accomplished to a large extent by means of an instru- 
 ment called a microtome. In mineralogy and petrography such thin prepa- 
 rations can rarely be used on account of the difficulty of preparing them. 
 However since the structure of the formations considered here are by no 
 means so fine, a slice of a rock or mineral ground to a thickness of from 
 
22 
 
 PETROGRAPHIC METHODS 
 
 0.03 to 0.04 mm. is sufficient. Slices of 0.02 mm. thickness or less are neces- 
 sary only in rare cases for detailed study of objects with fine structure. 
 
 A cutting or grinding apparatus is used for preparing thin sections, the 
 larger ones being driven by motor or foot power as in Fig. 37. The smaller 
 machines are supplied with a crank to be turned by hand, but this hinders 
 the operation considerably. The specimen is fastened on to the holder a with 
 shellac cement and, by means of the weight c, is pressed against the cut- 
 ting plate d, which rotates above it. The best cutting plates are covered 
 around the edge with diamond dust and must always be kept moist with oil. 
 Emery or carborundum powder and water can also be used. When the 
 specimen has been cut once the holder a is moved to the left a short distance, 
 depending upon the desired thickness of the section and a second cut is 
 made. The necessary thickness naturally varies with the coherence of the 
 rock, but must be in the neighborhood of 12 mm. Next, one surface is 
 finely ground with emery or carborundum dust on the slightly convex disk 
 m which rotates horizontally and then it is cemented with warmed Canada 
 
 FIG. 38. Cross Section through a Slide. 
 
 balsam on to a thick glass. Care must be taken that the cement is neither 
 too brittle nor too soft because in either case the preparation may be easily 
 torn loose in the grinding. It is best to evaporate Canada balsam on a 
 water bath to the proper consistency. Its viscosity is then reduced by 
 heating on a hot plate. Then the section is carefully ground down to the 
 desired thickness by using successively finer grained carborundum or 
 emery. 
 
 The slides are usually cemented on to an object glass by means of Canada 
 balsam and are then covered with a cover-glass, using the same cement. 
 The thickness of the cover-glass may be 0.10-0.15 mm., but not more on 
 account of the short focal lengths of the stronger objectives. The thickness 
 of the cover-glass is of little importance with weak objectives, but the 
 stronger ones are corrected for a fixed thickness, and if other thick- 
 nesses are used the length of the tube must be changed. For certain 
 special investigations, principally the determination of the index of re- 
 fraction, the preparation is left uncovered. When the grinding has 
 been carefully done the thickness of the slide may be quite uniform except 
 that it may decrease toward the edges as shown in Fig. 38, which represents 
 a cross section through the edge of a thin section magnified about 50 times, 
 a is the cover-glass, d the object glass, and 6 the rock slide on which the rough 
 surfaces can be distinctly seen. The slide itself is enveloped on both sides 
 with Canada balsam. 
 
THE MICROSCOPE 23 
 
 The form and size of the object glass plays an important role, while the 
 thickness may vary between limits, which depend upon the focal length of 
 the illuminating apparatus. Square object glasses about 32x32 mm. are 
 most satisfactory for investigations with the polarizing microscope, and at 
 the present time these are almost always used in petrography. If a longer 
 form is preferred, it must not be so long that, when placed on the stage of a 
 polarizing microscope and rotated, it will strike some part of the instru- 
 ment and be displaced. 
 
 The study of rock powders often leads to good results for rapid orientation. 
 This was the earliest method of microscopic rock analysis. Thin cleavage 
 plates of minerals having a good cleavage, often give very characteristic 
 optical reactions and when artificial crystals are to be investigated the best 
 results are obtained in many cases when the crystals are allowed to form on 
 the object glass by the evaporation of a drop of the solution; or a fine 
 crystalline powder may be imbedded in Canada balsam or some other 
 liquid. 
 
 Reflected light is used comparatively rarely for observations with a 
 polarizing microscope except for opaque objects, minute crystals, and etch 
 figures. Preparations of this character are best left uncovered. 
 
 It may be mentioned finally that the methods of staining, which are so 
 important in organic microscopy, are used only, in exceptional cases in the 
 investigation of inorganic bodies and, then particularly in the investigation of 
 loose fibrous or scaly structures, but even then they are of less value than in 
 organic preparations. 
 
CHAPTER II 
 The Adjustment of a Polarizing Microscope 
 
 Before a microscope can be used it must be tested to see that 
 the various parts of the instrument perform their functions 
 properly. The tests consist of: 
 
 1. Testing the system of lenses; 
 
 2. Centering the stage; 
 
 3. Adjusting the cross hairs and the nicol prisms. 
 
 These operations are called "the adjustment of the instrument." 
 
 i. Testing the Lenses 
 
 Aplanatic and Achromatic Properties. Although accurate correction for 
 spherical and chromatic aberration in the illuminating lenses plays a com- 
 paratively small role and need only be considered for microphotography, yet 
 the sharpness of the image and the clearness of the observations depend upon 
 the most perfect aplanatic and achromatic properties of the lenses. Con- 
 vexity of the image is the first thing that appears when using a defective 
 objective. It may be recognized by inability to make the center and the 
 edge of the image of equal, sharpness. The haziness of the image in differ- 
 ent parts of the field makes work with such objectives extremely fatiguing to 
 the eye. It is encountered in fairly well constructed weak objectives only 
 with a periscopic ocular, which uses the entire field of the objective, but in 
 stronger lens systems it may affect and fatigue the eye with ordinary oculars. 
 Along with the haziness, there is a distortion of the image due to the fact 
 that different zones in the field are magnified differently. A cross-sectioned 
 micrometer is used to recognize this imperfection, Figs. 4 and 5, page 2. 
 Chromatic aberration is likewise troublesome. It occurs with stronger 
 magnification and is not entirely corrected in the best achromatic lenses. 
 Minute opaque bodies appear colored, especially if the reflecting mirror is 
 placed obliquely or if the light from one side is shut off by a screen. If the 
 illumination is principally from the left, an object will appear reddish on 
 the left and violet on the right if the chromatic correction has been insuffi- 
 cient, and if too great, the colors are reversed. The only lenses that are 
 perfectly corrected for chromatic aberration are the apochromatic lenses, 
 but even with these a compensating ocular must be used. 
 
 The system of lenses may be tested for perfect achromatic correction by 
 the Abbe test plate. This is a silvered glass plate on which series of parallel 
 lines are etched at different microscopic distances from each other. An 
 apochromatic system makes such a series appear perfectly clear and with- 
 out colored borders. 
 
 24 
 
ADJUSTMENT OF A POLARIZING MICROSCOPE 25 
 
 Light Intensity, Magnification, and Resolving Power. The 
 
 amount of light which passes through an objective is dependent 
 upon the aperture, if properly constructed. It is, however, 
 influenced quite appreciably by small errors in the construction 
 of the lens system so that there may be an appreciable variation 
 in the amount of light in different systems with the same aper- 
 ture. Since the amount of light is also dependent upon the 
 magnification demanded, only the most carefully designed and 
 constructed objectives may be used for the highest magnification, 
 less perfect ones being practically useless. There is also serious 
 objection to the unlimited increase in the magnification of lens 
 combinations, which in many cases, seems to be the chief object 
 of the manufacturers. On the one hand the amount of light is 
 not sufficient for clear observations, while on the other hand the 
 maximum sensitiveness of the objective is limited by the aper- 
 ture. Abbe compiled a table showing the total magnification 
 that may be expected from a microscope with objectives with 
 the following apertures: 
 
 Aperture 0.1 0.2 0.3 0.6 0.95 1 1.20 1.30 2 
 
 Magnification 53.0 106.0 159.0 317.0 501.0 635.0 688.0 
 
 Some idea of the amount of light passing through an objective 
 can be obtained by testing it with a weak ocular in diffused light 
 produced by a white thinly clouded sky. One testing of this 
 character is generally quite sufficient for ordinary practice. 
 
 The magnification, however, must be measured directly, because it is often 
 necessary in work with the microscope to give numerical values of the sizes 
 of the objects observed. A table showing the possible magnifications with 
 the various objectives and oculars generally accompanies each instrument, 
 but it is important to check it. The simplest method of accomplishing 
 this is to focus on an object micrometer with the lens system to be tested. 
 One millimeter is divided into a hundred parts and the image is projected 
 upon a sheet of paper by means of an Abbe sketching device, to be described 
 later. A few of the lines are then sketched on the t paper and the distance 
 between them is measured. It must be remembered that the amount of 
 magnification varies with the length of the tube and the measurement 
 should be made with the normal length to which the objectives are 
 corrected. 
 
 Granting that its construction is perfect, the resolving power of an ob- 
 jective is a function of the aperture. This can be determined theoretically 
 with an Abbe apertometer. For practical ^purposes a test object, which 
 generally accompanies a microscope, is to be preferred, especially one made 
 
 1 Greatest aperture of a dry system. 
 
 2 Greatest aperture of an ordinary oil immersion. 
 
26 PETROGRAPHIC METHODS 
 
 of diatoms, the fine surface structures of which may serve as a standard of 
 the resolving power of the objective. 
 
 The pleurosigma angulalum with its characteristic fluting in three direc- 
 tions is the best object for testing the strongest dry systems. The fine 
 details of sketch of the surirella gemma, especially the striations running 
 perpendicular to the fine cross fluting, serve to estimate the sensitiveness 
 of an oil immersion up to an aperture of 1.30. An Abbe test plate with 
 its various systems of lines can be used for the same purpose. The observa- 
 tions are more perfect when the object is illuminated by oblique rays of light. 
 
 Finally false light, i.e., light produced by reflection of any sort, must be 
 avoided in microscopic observations, although it is much less annoying 
 here than in microphotography. For microphotographic purposes all 
 metallic parts, especially within the tube of the microscope, should be 
 blackened to prevent reflection. 
 
 Under certain conditions such reflection may be caused by the lenses of 
 the ocular itself, so that it seems advisable to place a device on the ocular in 
 some cases, which corresponds to a lengthening of the holder of the ocular 
 lenses and cuts off the light falling obliquely on the upper lens of the ocular. 
 In other instances, especially with weak objectives having a long focal 
 length, the light under the objective, which falls on the slide and is reflected 
 by it into the tube, causes a great deal of annoyance. Confusion can be 
 avoided by shutting out the light with the hand or by placing a black screen 
 around the instrument. Such devices must always be arranged so that 
 they do not interfere with the movement of the instrument. 
 
 A thin cleavage plate of mica, observed in convergent polarized light is 
 the best device to determine whether the angle of aperture of the illuminat- 
 ing system is sufficient for the objectives with the largest numerical aperture 
 that may be employed. If an immersion objective is used, the illuminating 
 apparatus must also be used as an immersion. The interference figure 
 will be equally il uminated throughout the field, if the objective and the 
 illuminating system are properly adjusted. If the angle of aperture of the 
 illuminating apparatus is not sufficient, the edge of the image will appear 
 dark. If the adjustment is not proper the field of vision is unevenly illu- 
 minated or a sharply outlined portion of the field, corresponding to the 
 cross section of the polarizer, appears light. 
 
 Microscopes made by the best firms rarely require the adjustments 
 mentioned above. Centering of the axis of rotation of the stage and ad- 
 justment of the vibration directions of the nicols, on the other hand, are 
 dependent upon numerous contingencies and are not infrequently altered 
 during work, so that a frequent revision of them is strongly advised. The 
 steps necessary will therefore be described more thoroughly. 
 
 2. Centering the Stage 
 
 The center of rotation of the stage must fall as nearly as pos- 
 sible in the axis of the objective, i.e., the microscope must be 
 centered, or the displacement, which an object undergoes upon 
 rotating the stage, prevents exact observation to a large extent. 
 
ADJUSTMENT OF A POLARIZING MICROSCOPE 27 
 
 Centering is naturally unnecessary in those microscopes in which 
 the object rotates simultaneously with the objective, Figs. 29 
 and 30, page 14, or the nicol prisms rotate, Figs. 31 and 32, 
 page 16. In centering other models a small speck on the 
 object is brought exactly to the intersection of the cross hairs 
 and the stage is rotated through 360, while the displacement of 
 the speck is constantly observed. The speck will generally 
 describe a circle whose center x, Figs. 39 and 40, does not coincide 
 with the intersection of the cross hairs o. The centering screw 
 
 FIG. 39. 
 
 FIG. 40. 
 
 Centering the Stage. 
 
 A is turned until the point x has passed through the distance xr, 
 i.e., until it falls upon the cross hair lying transverse to this screw. 
 Then with the screw B it is moved through the distance ro, i.e., 
 the center of the circle x appears to lie at the intersection of the 
 cross hairs. The speck first selected is again brought to the center 
 of the cross hairs and is tested to see whether it changes its 
 position upon a complete rotation of the stage. Generally there 
 is a slight movement and this is corrected in the same manner 
 as before until centering is perfectly accomplished. 
 
 In general each change of objectives introduces a small eccentricity, 
 although in perfectly constructed instruments having objective tongs, Fig. 
 16, page 5, this error is not large enough to cause a noticeable interfer- 
 ence with the work. The centering screws operate at an angle of 45 to 
 the cross hairs instead of parallel to them in certain instruments, especially 
 where they are placed on the stage. The instructions given above must be 
 changed then according to the diagram, Fig. 40. 
 
 The line of sight of the microscope must coincide exactly with the axis 
 of rotation of the stage and this cannot be obtained by simply centering. 
 
28 PETROGRAPHIC METHODS 
 
 The adjustment of the instrument for this is checked by placing a signal 
 such as a dark cross on the lower lens of the objective and illuminating 
 it by a vertical illuminator from above. This is then reflected by a plane 
 parallel mirror laid upon the stage. If the instrument is properly adjusted 
 the image from the mirror must coincide with the cross and must not change 
 its position upon rotating the stage. 
 
 3. Adjustment of the Cross Hairs and the Nicol Prisms 
 
 It is important to know whether the vibration direction of the 
 polarizer is from front to rear or from right to left when it has 
 been set in the proper position indicated on it. In the earlier 
 nicols with rhombic cross section, as already explained, the vibra- 
 tion direction is parallel to the short diagonal, but in the various 
 later makes it must be determined in each case. A thin deeply 
 colored tourmaline crystal, which shows darker color and stronger 
 absorption of the light 'when its principal crystallographic axis 
 is at right angles to the vibration direction of the polarizer, is 
 used. A pile of glass plates or, in the simplest case, a reflecting 
 surface can also be employed. The light polarized by reflection 
 vibrates perpendicular to the plane of incidence. If such a 
 reflecting surface inclined at the polarizing angle is observed 
 through a nicol prism, it will appear dark as soon as the vibration 
 direction of the nicol is perpendicular to it, i.e., parallel to the 
 plane of incidence. 
 
 The adjustment of both nicols must be tested. If the micro- 
 scope is to be used for any kind of measurements in polarized 
 light the vibration directions of one of the nicols must be as nearly 
 as possible parallel to one of the cross hairs, which are exactly at 
 right angles to each other. This is best accomplished by the aid 
 of a colorless needle-like crystal imbedded in Canada balsam. The 
 crystal must have parallel extinction and an index of refraction 
 as near that of the Canada balsam as possible. The reaction is 
 especially distinct when the crystal shows an intense interference 
 color of a low order between crossed nicols. 
 
 Long needle-like crystals of quartz about 0.1 to 0.15 mm. thick 
 are particularly useful for this purpose. Cleavage pieces of 
 anhydrite and so forth are a little less adaptable, but can be more 
 readily obtained. If a long edge of the crystal imbedded in 
 Canada balsam is placed parallel to one of the cross hairs and the 
 vibration direction of the polarizer is exactly the same, light will 
 pass through the crystal unaltered. Quartz, which has an index 
 
ADJUSTMENT OF A POLARIZING MICROSCOPE 29 
 
 of refraction nearly the same as Canada balsam, is entirely invis- 
 ible between crossed nicols and remains invisible upon rotation of 
 the analyzer through 90. If, however, upon rotating the ana- 
 lyzer a slight illumination or coloration of the quartz takes place, 
 it is an indication that the vibration direction of the polarizer 
 does not correspond exactly with the direction of the cross hair. 
 This is corrected by rotating the polarizer until the phenomenon 
 can no longer be noticed. The analyzer is checked in the same 
 manner, the polarizer being marked in its proper position and 
 then used as the movable nicol. The quartz crystal is then placed 
 parallel to the second hair and the test is again made to, see that 
 no coloration takes place. When such is the case, we have proof 
 that the vibration direction of one of the nicols is parallel to one 
 of the cross hairs and further that the two hairs are exactly at 
 90 to each other. 
 
 In many instances it is impossible to find a position of the nicol prism 
 in which the crystal entirely disappears because very frequently an optical 
 disturbance is produced by the lenses. Partial polarization by refraction, 
 especially in the stronger systems, may also occur. Further, the lenses may 
 acquire considerable double refraction, due to tension in the metallic holder 
 caused by rapid changes of temperature. If the tension in the latter case 
 is not too great, a normal condition of equilibrium will be attained after a 
 short time. However, if the effect on polarized light is quite distinct, such 
 a lens system will give rise to much inconvenience and inaccuracy. It may 
 be noted that if the optical disturbance becomes apparent upon inserting 
 the polarizer, the defect belongs to it, but in other cases to the objective. 
 Since nicol prisms are generally set in cork, the volume of which changes 
 as it gradually dries out or is affected by changes in temperature and in the 
 humidity of the air, the tests for the orientation of the nicol prisms must 
 be frequently repeated. This is especially advisable before accurate 
 measurement of the vibration directions of a crystal is attempted. 
 
CHAPTER III 
 Observations in Ordinary Light 
 
 Ordinary light is unpolarized. The observations to be con- 
 sidered next can be carried out without the use of the nicol prisms. 
 The following properties can be studied in ordinary light: (1) 
 index of refraction, (2) form and cleavage, (3) size and thickness 
 of the object, (4) inclusions, (5) color, and (6) appearances in 
 reflected light. 
 
 Methods of Determining the Index of Refraction. The first 
 and most important observation that we are able to make with 
 a simple microscope is that of refraction. A ray of light which 
 passes obliquely from one medium to another is deflected from 
 its original direction, i.e., it is refracted because the rate of trans- 
 mission of light in various mediums is generally different. 
 
 Kin i v 
 Refraction may be expressed by the law: = Fig. 41, in 
 
 sin r v 
 
 which i is the angle between the incident ray and the normal, r, 
 that of the refracted ray, and v and v' are the respective velocities 
 of light in the two mediums. It follows 
 from the above formula that when a ray of 
 " light passes from a medium with greater 
 
 velocity v to one with smaller velocity v', it 
 is refracted toward the normal, because then 
 r<i. If in the above equation the first 
 FIG 41 medium is air in which the velocity of light 
 
 Refraction of Light. may be assumed to be equal to one, v =1, 
 the index of refraction of the second me- 
 dium with respect to air may then be expressed by the formula, 
 
 , or the velocity is the reciprocal of the index. 
 v' sin r 
 
 If light passes from a medium with smaller velocity v' into one 
 with greater velocity v, the conditions are reversed, and the 
 angle between the normal and the refracted ray is larger than for 
 the incident ray. The ray is refracted away from the normal. 
 For a certain angle of incidence i, the value of r will equal 90, 
 i.e., light incident at this or a greater angle does not pass into the 
 
 30 
 
OBSERVATIONS IN ORDINARY LIGHT 31 
 
 second medium, but is totally reflected. The angle of incidence 
 for which r = 90 is called the critical angle. The greater the 
 difference of velocity in the two mediums, the smaller the critical 
 angle, e.g., for air and glass (n = 1.5) it is 40 45', for the diamond 
 (n -2.4) it is 23 45'. 
 
 If the ray falls perpendicularly upon the contact of the two 
 different mediums sin i = 0, and hence, sin r = as well, i.e., 
 normal incident light suffers a change in velocity, but it is not 
 deflected. 
 
 If isolated crystals are observed under the microscope only 
 those parts will appear clear and transparent which are bounded 
 by smooth parallel surfaces that lie at least approximately per- 
 pendicular to the axis of the microscope. It is only in such cases 
 that the transmitted rays suffer very little or no deviation. If 
 the inclination of the surfaces is considerable, total reflection 
 takes place at the contact between the crystal and air. Such 
 portions then appear dark. If the faces of a crystal are uneven 
 or if unpolished plates, such as thin sections, are being studied, 
 they appear more or less clouded because the unevenness of the 
 surface allows only a portion of the light to pass through unre- 
 fracted, while the flank surfaces of the depressions and elevations 
 cause total reflection of the light due to their inclined positions, 
 Fig. 38, page 22. 
 
 If a slide with rough surfaces is immersed in water (n =1.33) 
 and covered with a cover-glass, the rough appearance will entirely 
 disappear, but the slide becomes far more transparent because 
 now the difference in refraction between the crystal and the 
 medium surrounding it is less and the critical angle becomes 
 greater. If a liquid with higher index is used the rough appear- 
 ance will disappear more the more nearly the index of the liquid 
 approximates that of the substance in question. When the 
 indices of the two mediums are exactly alike the section will 
 appear as though polished and the uneven surfaces perfectly 
 even, but if a liquid of higher index is employed, the unevenness 
 of the surface will reappear. The outline of the crystal, which 
 when uncovered shows very distinctly, will gradually disappear 
 as the index of refraction of the surrounding liquid approaches 
 that of the crystal and is entirely invisible when the two indices 
 are alike, provided the crystal is colorless. With more strongly 
 refracting liquids it reappears. 
 
 In many instances the best method for observing this difference 
 
32 
 
 PETROGRAPHIC METHODS 
 
 of index of refraction is in as nearly parallel light as possible. 
 If the rays which pass through a preparation are quite divergent, 
 then at each point on the uneven surface light impinges from 
 many different directions. Therefore a portion of the light will 
 pass through all parts of the preparation, and the unequal 
 illumination and, consequently, the rough appearance of the 
 surface, show much less. Since the index of refraction is one of 
 the most important properties in microscopic determinations, it 
 is customary not to polish the slide, and observations are begun 
 in approximately parallel light so that this difference will appear 
 more distinctly. 
 
 The illuminating apparatus of the microscope produces rays of 
 light which are more or less convergent so that, for the investiga- 
 tion of the index of refraction, rays as nearly parallel as possible 
 
 FIG. 42. FIG. 43. 
 
 Cone of Light narrowed by Iris Diaphragm. Sinking Condenser. 
 
 must be isolated from the cone of light. Narrowing of the cone 
 of light is generally accomplished by means of an iris diaphragm 
 J, Fig. 42, placed a short distance below the illuminating appa- 
 ratus. This cuts out those rays which pass through the outer 
 edge of the lens. The light passing through the middle part of 
 the lens is only slightly convergent, as shown in Fig. 42. The 
 effects of this device, which is generally considered to be indis- 
 pensable, can be produced even better by sinking the whole 
 illuminating apparatus as shown in Fig. 43. When it is lowered 
 in its holder the outer part of the cone of light is eliminated and 
 that which passes through the slide consists of rays which diverge 
 but slightly. In each method it seems to be expedient to use as 
 strong an illuminating system as possible. In using a microscope 
 
OBSERVATIONS IN ORDINARY LIGHT 
 
 33 
 
 without an Abbe illuminating apparatus it is always advisable 
 to leave the condenser in place as long as it does not diminish the 
 field of vision, although this is contrary to the usual instructions. 
 With the weakest objectives, however, it must, in all cases, be 
 removed. It is therefore expedient to carry out all investigations 
 on the microscope with the condenser in place and it is only 
 removed on passing over to the lowest magnifications. 
 
 Differences in the Indices of Refraction Under the Microscope. 
 Since complete disappearance of a crystal in a liquid is only to be 
 observed when the indices of refraction of the two are about 
 equal, and since a crystal stands out in relief from the surrounding 
 medium in the same manner whether the index is higher or lower, 
 one is in doubt whether to ascribe the relief to one or the other 
 
 FIG. 44. Higher 
 
 Index of Crystal in Canada 
 
 FIG. 45. Lower 
 with Raised Tube. 
 
 cause. The determination as to whether the index of the crystal 
 is higher or lower than that of the liquid, however, is very simple. 
 The cone of light is cut down until the contact between the 
 crystal and the liquid, or between two crystals, appears as a 
 sharp line, and then the objective is raised. A distinct band of 
 light can be recognized parallel to the contact between the two 
 mediums and this moves toward the substance with the higher index 
 upon raising the tube, while the more weakly refracting object 
 appears to have a dark border. Upon lowering the tube, the 
 opposite phenomenon is observed. 1 
 
 Fig. 44 shows crystals of barium nitrate (n =1.57) in Canada balsam with 
 the tube raised. The band of light can be distinctly recognized within the 
 edges of the crystal. The opposite is the case in Fig. 45, which shows tetra- 
 
 1 This is the Becke method. 
 3 
 
34 
 
 PETROGRAPHIC METHODS 
 
 hedrons of sodium uranyl acetate having a lower index of refraction than 
 the surrounding Canada balsam, and therefore when the tube is raised, a 
 sharp bright band can be seen outside, and a dark one inside the crystal. 
 
 This phenomenon can be explained in the following manner. 
 All the rays from the illuminating apparatus striking obliquely 
 on the contact of two differently refracting substances, Fig. 46, 
 pass from the medium with lower index on the left into the 
 one with higher index on the right. On 
 the other hand, only a portion of the 
 light will pass from the higher refracting 
 medium into the lower, because within 
 a certain angle /? total reflection of the 
 light takes place. This angle is de- 
 pendent upon the ratio of the indices of 
 refraction in the two bodies and is larger 
 the greater the difference between the 
 indices. 
 
 FIG. 46. Behavior of Rays on the Con- 
 tact between two Different Media. 
 
 FIG. 47. Observation 
 through the Microscope. 
 
 If the cone of light is narrowed down so that only those rays 
 are present which fall within the angle f) it is evident that a 
 strongly illuminated zone will appear on the side of the contact 
 line on which the light passing through the border is added to 
 that which is totally reflected, i.e., on the side of the more 
 strongly refracting substance, and on the other side where no 
 light passes through, there will be an equally pronounced dark 
 band. Fig. 47 shows diagrammatically this phenomenon as 
 observed in the microscope. If the ocular is placed in the 
 
OBSERVATIONS IN ORDINARY LIGHT 
 
 35 
 
 plane of the image B, the contact between the two substances 
 appears as a sharp line. When the tube is raised to the position 
 B" the band of light will be observed on the side of the substance 
 with the higher index of refraction, due to the inversion of the 
 image, and when the tube is lowered so that the ocular comes 
 to the position B', the light band appears on the side of the 
 
 FIG. 48. High FIG. 49. Medium 
 
 Index of Mineral in Canada Balsam. 
 
 FIG. 50. Mineral with Low Index in Canada Balsam. 
 
 lower refracting medium. The following general rule may be 
 applied: Upon raising the tube the light zone moves toward the 
 substance with the higher index of refraction. 
 
 Figs. 48-50 show this phenomenon as ordinarily observed on 
 a thin section embedded in Canada balsam. If a mineral like 
 quartz (n = 1 . 54, Fig. 49) has approximately the same index 
 
36 PETROGRAPHIC METHODS 
 
 as Canada balsam, the unevenness of the surface, represented by 
 Fig. 38, p. 22, can scarcely be observed at all, if very nearly 
 parallel light passes through the slide. Thin sections of such 
 colorless minerals are scarcely visible when the illuminating 
 apparatus is completely lowered. It is different in Figs. 48 
 and 50 representing slides of olivine, ft = 1.68, and haiiyne, 
 ft = 1.48, respectively. The index of refraction of the former is 
 considerably higher, that of the latter lower than Canada 
 balsam, ft = 1.54. The irregularities of the surface are filled 
 up with a medium that is optically different and therefore, 
 when the illuminating apparatus is lowered, they appear very 
 distinctly because various points on the surface are inclined 
 at different angles to the rays of light which are deflected 
 differently and some are totally reflected. The result is the 
 rough chagrined appearance of the thin section under investi- 
 gation. Since the roughness appears more readily the greater 
 the difference of index of refraction between the mineral and 
 Canada balsam, the amount of lowering of the illuminating 
 apparatus or of closing of the iris diaphragm necessary to 
 show this difference is an excellent means for the approximate 
 determination of the index of refraction of minerals in thin 
 sections. Care must be taken that the slides are uniformly 
 ground, otherwise there will be great differences in the properties 
 of the surfaces. 
 
 If isolated crystals are studied instead of thin sections, the roughness of 
 the surface is rarely of importance. Observation of the crystal form in 
 ordinary light can only be accomplished when there is a great difference 
 between the indices of refraction of the crystal and that of the surrounding 
 medium, and this can be recognized by narrowing up the cone of light 
 from below. 
 
 If a grain is embedded in a liquid with approximately the same index of 
 refraction, its form will not be observed at all, or at best will be very indis- 
 tinct, even when the cone of light is narrowed down as much as possible. 
 This is well illustrated by quartz embedded in Canada balsam (n = 1 . 54) . 
 If, however, the quartz crystal is embedded in linseed oil (n = 1.48) or 
 monobrom-naphthalene (n = 1 . 66) its form will be distinct even when the 
 illuminating cone is much wider. 
 
 Determination of the Index of Refraction. Immersion Method. 
 By this method the difference in the indices of refraction of two 
 mineral grains in contact with one another in a thin section can 
 be determined accurately as well as the individual indices of 
 refraction of isolated crystals or fragments embedded in a 
 
OBSERVATIONS IN ORDINARY LIGHT 37 
 
 liquid of known index. A series of liquids POTASSIUM MERCURIC 
 
 IODIDE SOLUTIONS. 
 called indicators is arranged so that the gi n j) 
 
 index of each exceeds that of the pre- 3.2 1.733 
 
 ceding one by 0.01-0.02. A series of 3.1 1.715 
 potassium mercuric iodide solutions with 
 
 different concentrations furnishes a scale, 2 ' g i 658 
 
 which is quite easily controlled, and the 2.7 1.640 
 
 index of refraction is readily obtained 2.6 1.621 
 
 from the accompanying table by determin- 2 -5 1.602 
 ing the specific gravity. 
 
 These indicators have the disadvantage 2 ' 2 i 546 
 
 that the index of refraction changes some- 2.1 1 . 527 
 
 what during the operation, due to the 2.0 1.509 
 
 evaporation of water. Hence, homogen- 1.491 
 
 eous liquids of definite chemical composi- 1.473 
 
 tion are better. The following table is 1 ' 6 
 
 given by Schroeder van der Kolk: 1,5 1.419 
 
 Water 1.33 
 
 Ethyl alcohol 1 .36 
 
 Amyl alcohol 1 . 40 
 
 Chloroform 1 . 45 
 
 Castor oil 1 .48 
 
 Benzol 1 . 50 
 
 Monochlorbenzol 1 . 52 
 
 Clove oil.... 1.54 
 
 Monobrombenzol 1 .56 
 
 Aniline 1 . 58 
 
 Bromoform 1 . 59 
 
 Cinnamon oil 1 . 60 
 
 Carbon disulphide 1 . 63 
 
 a-Monochlor naphthalene 1 . 64 
 
 a-Monobrom naphthalene 1 . 66 
 
 Klein's solution 1 . 70 
 
 Thoulet solution 1 . 73 . 
 
 Methylene iodide 1 . 75 
 
 Solutions with intermediate values can be prepared by mixing 
 these indicators and this process is continued until a mixture 
 has been found in which the crystal disappears as completely as 
 possible. Complete disappearance can usually not be obtained 
 in white light, on account of the difference of dispersion be- 
 tween the crystal and liquid. For this reason it is better to 
 use monochromatic light. When the proper mixture has been 
 
 X S = specific gravity.nD index of refraction for the Na line. 
 
38 PETROGRAPHIC METHODS 
 
 found, its index of refraction should be determined by means of 
 a Bertrand total reflectometer, which allows a very accurate 
 determination of the index to be made rapidly, using only a 
 single drop of the liquid for this purpose. This immersion 
 method gives sufficiently good results for all practical purposes, 
 provided the comparison of indices of the liquid and mineral is 
 carried out according to the methods described, pages 33-36. 
 
 The method of oblique illumination proposed by Schroeder 
 van der Kolk gives more accurate results. Oblique illumination 
 is produced either by placing the mirror on the microscope 
 obliquely or by inserting a piece of cardboard laterally be- 
 tween the mirror and the illuminating apparatus. In the larger 
 microscopes the iris diaphragm can be moved horizontally or 
 there is a device which when pushed in stops the light eccentric- 
 
 \ 
 
 FIG. 51. FIG. 52. 
 
 Oblique Illumination. 
 
 Crystal higher Crystal lower 
 
 Index than the Liquid. 
 
 ally. Usually, however, a piece of cardboard is quite efficient. 
 Oblique illumination can also be obtained by screening one side 
 above the ocular. An Exner refractometer can be used for this 
 purpose. It consists of a hood to be placed over the ocular 
 with a diaphragm at the focal distance from the ocular. This 
 diaphragm can be closed laterally by means of a slide. If a grain 
 is immersed in a liquid it affects transmitted parallel light in 
 much the same manner as a converging lens, i.e., it converges 
 the rays if its index is higher than that of the liquid and in the 
 opposite case it causes them to be divergent. If a bundle of 
 parallel oblique rays passes through the preparation, only that 
 light is taken up by the objective, in the first case, which is on 
 the same side as the screen, Fig. 51, while in the second case, only 
 that on the opposite side is received by the objective, Fig. 52. 
 
OBSERVATIONS IN ORDINARY LIGHT 39 
 
 However, since the objective produces inversion, the reverse 
 phenomenon is seen in the microscope. // the grain has a 
 higher index of refraction than the liquid, the dark band is seen 
 on the side next to the screen, but if the grain has a lower index 
 it appears on the opposite side. 
 
 This method is extremely delicate when liquid and crystal 
 have very nearly the same indices, and especially when their 
 dispersions are at the same time quite different. It may happen 
 that the mineral, which generally has the weaker dispersion, will 
 possess a lower index for blue and a higher index for red than the 
 liquid. A brilliant colored border, blue on the side of the screen 
 and red on the opposite side, is an indication that liquid and min- 
 eral have very nearly the same indices of refraction. In rare 
 cases, when the mineral has a stronger dispersion than the liquid, 
 the position of the colors is reversed. To determine the indices 
 of refraction of double refracting crystals by this method, it' 
 must be possible to make a determination of each index succes- 
 sively. The ordinary ray of uniaxial crystals can be recognized 
 in polarized light, after its vibration direction has been placed 
 parallel to that of the polarizer, in that the optical reaction is 
 very sharp even though the grains be very irregular, because the 
 refraction of the ordinary ray is the same in all directions. The 
 extraordinary ray, however, cannot be determined as accurately 
 especially in substances which have no good cleavage because it 
 appears at every point on the surface with different values due 
 to the different inclinations of the surface toward the optic axis. 
 To determine the principal indices of refraction of double refracting 
 substances accurately, a section must be taken whose orientation 
 has been previously determined in convergent polarized light. 
 
 A center screen is very serviceable for observing the differences between 
 the indices. It causes the object to be illuminated by the strongly diverging 
 outer rays from the illuminating apparatus and can be placed in a slide 
 in the polarizer, but must be adapted for the angle of aperture of the ocular. 
 This method is especially useful in the investigation of minute objects, 
 and in the study of inclusions and so forth. 
 
 An accurate method for determining the index of refraction of micro- 
 scopic objects, consisting of a combination of an Abbe total reflectometer 
 with a microscope, was suggested by C. Klein. By this method all indices 
 of refraction in any microscopic cross section of a mineral can be determined, 
 but the part of the slide surrounding this section must be covered with 
 black varnish, and good results can be obtained even with comparatively 
 small individuals. The method of observation is the same as with an 
 Abbe total reflectometer itself and need not be discussed further here. It 
 
40 PETROGRAPHIC METHODS 
 
 need only be stated that the stand upon which the hemisphere of the Abbe 
 apparatus rests is hollow, the hemisphere itself is placed over the opening 
 on a ground plane surface, so that the object to be investigated can be 
 observed on the microscope in transmitted ordinary or polarized light. 
 However, this apparatus, which is quite complicated and comparatively 
 expensive, has not been used as extensively as it deserves to be. 
 
 The total reflectometer constructed by Wallerant is somewhat simpler 
 and can be attached to any microscope. It consists of a plate to be set 
 on the stage and carries a prism of strongly refracting glass, which can be 
 rotated about a horizontal axis parallel to its refracting edge, either alone 
 or in combination with an alidade reading to 2'. The edge itself is ground 
 off parallel to the base of the prism, so that when the apparatus is used 
 on the microscope, light is transmitted through it. An uncovered thin 
 section, polished as perfectly as possible, is firmly pressed on the base of 
 the prism by two clamps, the contact being made with methylene iodide. 
 The extinction directions of the section are then determined in transmitted 
 light. The prism is now rotated until convergent rays from a powerful 
 source of light are incident upon one of the lateral faces. The light is 
 reflected by the section and emerges from the other surface of the prism in 
 the axis of the microscope. The object is centered as accurately as possible 
 and the vibration directions as previously determined by the extinction 
 are brought into position, and the prism rotated until the distribution of 
 light and shadow indicate the critical angle for total reflection. The iris 
 diaphragm in the tube is now closed until only the section to be studied 
 is visible. Now the ocular is replaced by a telescope containing a spec- 
 troscope, which isolates the yellow rays from the white light. The edge of 
 the section is then placed parallel to a cross hair in the telescope and a 
 reading taken. Then the cross hairs are illuminated by light sent in from 
 the side by means of a total reflecting prism. The prism bearing the 
 section is rotated with the alidade until the image of the cross hairs reflected 
 from its base coincides with the object A second reading is then made 
 and the angle of total reflection determined. 
 
 The liquids employed so frequently in organic microscopy to increase the 
 illumination of an object depend upon the principle of compensation of the 
 refraction of light. Preparations of crystals and rocks are generally 
 embedded in Canada balsam, the index of which may vary between 1 . 53 
 and 1 . 545, depending upon the amount of evaporation. Canada balsam 
 is especially adapted for this purpose because it is quite liquid when fresh 
 and upon heating slightly is transformed into a clear transparent almost 
 co orless cement that sticks firmly and becomes very slightly double 
 refracting if at all, upon cooling as other resins generally do. Solutions of 
 hardened Canada balsam in benzol, xylol, etc., are sometimes used especially 
 with substances that cannot be warmed. The index of refraction is then 
 somewhat less on account of the dilution. For chemists Canada balsam has 
 a disadvantage in that it is a very good solvent not only for organic sub- 
 stances, but also for numerous inorganic salts, and its solvent action is 
 sometimes increased by the addition of benzol. A more feebly refracting 
 cement is needed in rare instances and then linseed oil is employed, n = l . 48. 
 When placed in the direct rays of the sun for a few weeks it bleaches and 
 polymerizes. 
 
OBSERVATIONS IN ORDINARY LIGHT 
 
 41 
 
 Determination of Form and Cleavage. A stereoscopic micro- 
 scope, with which an object can be observed as a body, is well 
 adapted for determining the form- of crystals and especially for 
 the investigation of etch figures that are not very deep. The 
 distinctness of form that can be obtained, especially with a Zeiss 
 binocular microscope simplifies greatly the deciphering of micro- 
 scopic crystals. The model shown in Fig. 53, constructed by the 
 
 FIG. 53. Grenough Binocular Microscope by C. Zeiss in Jena. 
 
 above firm according to the principle of Grenough, has the advan- 
 tage that the image of the crystal appears in its true position 
 due to prisms in the tubes. Since there are two objectives, each 
 in the axis of an observation tube, strong magnification cannot 
 be used. Other devices for stereoscopic investigation with a 
 microscope, such as a double ocular or stereoscopic ocular which 
 can be placed on a simple microscope, give good results only with 
 low magnification, but they do not at the same time produce the 
 distinctness of form obtained by a binocular microscope. The 
 
42 PETROGRAPHIC METHODS 
 
 importance of stereoscopic observation is confined to a small 
 number of cases and the binocular microscope cannot replace a 
 polarizing microscope. The next step in the microscopic 
 determination of crystals is to observe and sketch the form as 
 accurately as possible. (See methods for sketching crystals at 
 the end of Part I.) This must not be confined to the determi- 
 nation of the outline alone ; but care must be taken to decipher 
 as well as possible the properties of the crystal as a whole. 
 Without this, entirely erroneous conclusions may be reached 
 because the angles between the edges often appear greatly dis- 
 torted in consequence of the irregular position of a crystal in the 
 slide. 
 
 The measurement of characteristic angles goes hand in hand 
 with the determination of crystal form, but only such angles are 
 to be considered as are bounded by edges lying exactly in the 
 plane of the stage, or by planes perpendicular to this direction. 
 
 One leg of the angle to be measured is placed parallel to one of the cross 
 hairs and a reading of the position of the stage taken. Then the stage is 
 rotated until the other leg is parallel -to the same cross hair and another 
 reading taken, the difference between the two being the angle sought. 
 The edge can be placed in parallel position much more perfectly, the longer 
 and straighter it is and the more aplanatic the ocular lenses are. It seems 
 better too in making such measurements, not to make the edge coincide 
 
 FIG. 54a. FIG. 54b. FIG. 54c. 
 
 Measurement of Angles with a Lesson's Prism. 
 
 exactly with the cross hair, but to leave a small space between the two so 
 that the parallel position can be found much more accurately. In favor- 
 able cases, if the position is not perfect, the reading to whole degrees and the 
 estimating to quarter degrees are quite sufficient. 
 
 A Leeson's prism can be advantageously used for the accurate measure- 
 ment of plane angles, especially on very small crystals. It consists of an 
 achromatic quartz prism that is placed over the ocular so that it can be 
 rotated in a graduated holder. With it, two images of the object partially 
 overlying each other are seen, Fig. 54a. The prism is rotated until one 
 edge of the angle to be measured coincides in both images, Fig. 546. A 
 reading of the scale is then made and the other edge treated in the same 
 manner, Fig. 54c. More accurate results can also be obtained when a 
 glass plate with a system of fine parallel lines etched upon it is set at the 
 focus of the ocular in place of the cross hairs. A long line crosses the 
 
OBSERVATIONS IN ORDINARY LIGHT 
 
 43 
 
 middle of the system perpendicularly. This and the middle line of the 
 system are somewhat heavier so that the object can be easily centered. 
 The edges of the angle to be measured are then brought into parallel 
 position with the fine lines, one after the other. 
 
 An ocular goniometer also deserves some mention at this time. This is 
 an ocular in which one of the cross hairs can be rotated with respect to the 
 other, and the amount of rotation read on a circular scale. The movable 
 hair is placed parallel successively to the legs of the angle to be measured. 
 This method has the advantage that the slide need not be moved. Micro- 
 scopes in which the nicols may be rotated simultaneously possess the 
 advantages of an ocular goniometer inasmuch as the cross hairs also rotate 
 with respect to the object. 
 
 Actual reflection goniometric measurements can be made with a micro- 
 scope if a small reflection goniometer or the rotating apparatus, to be 
 described in the appendix, is combined with a microscope. However, 
 such microscopic-goniometric measurements will only give good results 
 after considerable practice and after all, are of very little use. If in such 
 investigations the microscope itself is to be used to observe the reflection 
 of the surfaces, a Gauss mirror appliance is valuable. It consists of a 
 
 FIG. 55. Distorted Crystal. 
 
 FIG. 56. Normal Development. 
 
 mirror placed obliquely on the ocular and a black cross hair under the 
 objective. After the surface has been placed approximately perpendicular 
 to the axis of the microscope, the tube is lowered half the focal distance of 
 the objective and the mirror image of the cross hairs is seen in the ocular. 
 When the surface is accurately adjusted the image coincides with the 
 cross hairs. This device can only be used with the lowest power objectives. 
 
 The form of crystals observed in the microscope is frequently distorted 
 in many ways, especially in cases of rapid crystallization, so that the 
 image of a crystal can be deciphered only with difficulty on account of the 
 irregular development of similar faces. ' 
 
 In such cases, however, the angles between the edges are always constant 
 and a measurement of them affords a means of finding the faces belonging 
 to a form and of recognizing the symmetry of the crystal. Fig. 55 represents 
 a cross section of a distorted crystal, which at first glance appears entirely 
 unsymmetrical. Measurement of the plane angles shows that the angles 
 be and b'c' are both approximately 90, while the other angles have equiva- 
 lents among themselves. It is presumably an orthorhombic crystal in 
 which the prism faces be and b'c' have been developed unequally, Fig. 56. 
 
44 
 
 PETROGRAPHIC METHODS 
 
 Rapid crystallization produces in many cases imperfect growth and de- 
 pressions bounded by more or less regular forms appear on the surface of 
 crystals. The cause is more rapid growth on the edges than in the middle of 
 the faces. These irregularities pass over into skeletal crystals, Fig. 57, 
 which may be developed in a large variety of delicate forms. Star-shaped 
 
 FIG. 57. Skeletal Crystals of Olivine. 
 
 skeletal crystals of snow, ice flowers, etc., are the best known forms of this 
 type. These skeletal forms in turn pass over into minute growths resem- 
 bling arrow heads, rods, etc., which cannot be definitely determined as 
 crystals and are called crystallites, Fig. 58. 
 
 f Mi, i 
 * <$$ i 
 
 FIG. 58. Crystallites. 
 a, Globulete; b, Margarite; c, Cumulite; d, Baculite; e, and/, Trichite. 
 
 Minerals observed in thin sections very frequently show more 
 or less regular cracks which indicate cleavage. In many cases 
 a few straight sharp cracks cut across the whole section, for ex- 
 ample, mica, Fig. 59. The cleavage is perfect even though the 
 number of cracks is small. The cracks may be discontinuous, 
 
 FIG. 59-61. Perfect. 
 
 Distinct. 
 
 Imperfect. 
 
 Cleavage. 
 
 as in hornblende, Fig. 60. Here the cleavage is distinct, or good. 
 If the cracks are more or less curved, but following on the whole 
 certain directions, the cleavage is said to be imperfect, garnet, 
 Fig. 61. Different systems of cleavage, of equal or unequal 
 value, may be present enclosing a characteristic angle, which can 
 
OBSERVATIONS IN ORDINARY LIGHT 45 
 
 be determined by measurement. Much more importance is 
 generally ascribed to the measurement of cleavage angles in thin 
 sections than it really deserves, because the size of the angles 
 is largely dependent upon the orientation of the various cross 
 sections of a crystal and this of course is variable. In any case, 
 it must be shown by raising and lowering the tube that the cleav- 
 age cracks traverse the section as nearly vertically as possible. 
 
 Investigation of cleavage fragments of crystals or of rock- 
 forming minerals often gives very characteristic results, and it is 
 often serviceable for rapid orientation or for supplementing the 
 observations on thin sections. . 
 
 In isolated crystals, cleavage cracks are not often observed 
 and then only if the cleavage is very perfect. Sometimes 
 crystals can be crushed and cleavage fragments formed, the form 
 of which is a more characteristic feature of the substance than 
 can be found in a thin section. Cracks parallel to gliding planes 
 may appear similar to cleavage cracks in microscopic objects 
 and there is no way of distinguishing them. Occasionally more 
 or less regular parting, which may be due to various causes, 
 appears quite similar to cleavage cracks. 
 
 Measurement of Size and Thickness. While it is frequently 
 interesting to know the size of an object, its thickness is more 
 often determined. A micrometer, which is sometimes placed on 
 the stage and sometimes in the ocular, is used for the determina- 
 tion of size. When placed on the stage, it gives the size directly, 
 but when in the ocular the results must be recalculated for the 
 objective used and for the length of the tube. On the other hand, 
 errors made in the object micrometer divisions are increased by 
 the total magnification of the instrument, while those of the 
 ocular micrometer are only increased by the magnification of the 
 ocular itself. 
 
 In certain cases a scale on the mechanical stage, Fig. 34, 
 page 19, in which divisions on the drum of the screw permit 
 1/1000 mm. to be estimated, serves as an object micrometer. 
 The small errors, which are apt to occur in cutting the thread 
 of the screw, make such operations unreliable. 
 
 Divisions of only 1/10 mm. are demanded of a micrometer 
 placed in the ocular so that the requirements of the execution are 
 much less and errors likewise smaller. Such an ocular microm- 
 eter can simply be placed in the ocular at the focal distance, 
 or a micrometer ocular can be constructed in such a manner that 
 
46 PETROGRAPHIC METHODS 
 
 a cross hair at the focus of the ocular can be moved by means of a 
 screw with a measuring drum working at right angles to the axis 
 of the instrument, while an immovable cross hair indicates the 
 center of the field. In other cases the scale itself is fixed and a 
 Ramsden ocular with a central division is moved over the scale 
 by means of a measuring drum. The division to which this 
 mark must be brought as nearly as possible, is made upon the 
 upper plane surface of a lens, which is in turn adjusted for the 
 objective so that the eccentricity produced by lateral displace- 
 ment of the ocular is compensated. 
 
 A micrometer possessing divisions as represented in Fig. 4, 
 page 2, and placed in the ocular, may serve to determine the rela 
 tive amounts of minerals in thin sections. Such an adjustment 
 is called a planimeter ocular. 
 
 The measurement of the thickness of microscopic objects is 
 generally possible only when the optical properties are quite 
 accurately known. Good results can seldom be obtained with 
 isolated crystals, but frequently in thin sections quite accurate 
 determinations of the thickness are possible because certain 
 
 known minerals are nearly always 
 present and their thickness can be 
 estimated by means of the inter- 
 ference colors. The measurement of 
 the thickness is not very reliable with 
 other methods even in the most 
 
 s \/ r favorable cases. 
 
 io 
 
 A method, which was frequently 
 
 used formerly, is that of Duke de 
 
 FIG. 62,-D^deChauines Chaulnes for the determination of 
 
 the index of refraction. It can be 
 
 also used in reversed manner for measuring the thickness. 
 A point o, Fig. 62, is sharply focused and a plane parallel plate 
 of a transparent substance is inserted between it and the ob- 
 jective. In consequence of the refraction of the rays or and os 
 one receives the impression that the light emanates from o' ' . The 
 objective must then be raised by means of the micrometer screw 
 until it is focused on o'. With a known thickness of the plate, the 
 distance oo f is dependent upon its index of refraction. If the 
 latter is known, we have a basis for estimating the thickness of 
 the plate. The change of focus of the objective can be read 
 directly from the divisions on the drum of the micrometer screw. 
 
OBSERVATIONS IN ORDINARY LIGHT 47 
 
 This method is often used but care must be exercised not to overestimate 
 the accuracy of it because a great deal of practice is necessary to obtain 
 reliable results. A small foreign body, e.g., a dust particle, which occurs 
 between the object and the cover-glass is focused and then the apparatus 
 lowered until the same signal is seen reflected from the under side of the 
 slide. The amount of lowering of the objective constitutes the value 6 and 
 the thickness of the slide is given by the equation d = bn, where n is the 
 index of refraction of the section measured. 
 
 The measurements are most accurate when stronger magnifications are 
 used because the amount of displacement of the real image from the ob- 
 jective increases with an increase of magnification. The image of the 
 point chosen must be brought exactly into the plane of the cross hairs and 
 the proper adjustment can be recognized when, upon moving the eye above 
 the ocular, the image and cross hairs do not show relative displacement. 
 Since, in general, the index of refraction n is not positively known and con- 
 stitutes the greatest error in this method, an immersion system can be 
 employed with good results for this determination. Then it is not neces- 
 sary to fix the value of the index of refraction itself, but only to determine 
 the difference between the index of the mineral and the immersion oil. 
 
 Methods that depend upon observations of the interference colors are 
 much more exact. There may be well orientated sections of minerals 
 present in the slide with known indices of refraction from which their 
 thickness, which indicates the thickness of the slide, can be estimated. 
 Such cross sections can be introduced into the slide artifically by cementing 
 cleavage plates of barite on different sides of the rock section and grinding 
 them down with the section. Measurements thus made are at any rate a 
 close approximation. 
 
 Inclusions. The presence of inclusions is of importance in 
 certain cases because a large number of crystallized bodies, es- 
 pecialy minerals, possess the ability to enclose large amounts 
 of foreign substances upon crystallization. This is often so 
 extensive that the greater part of the crystal consists of such 
 foreign bodies. These inclusions can be recognized in ordinary 
 light by the differences in the indices of refraction and color. 
 
 Some of the inclusions occurring in this way are crystallized' substances 
 and must be determined according to the ordinary methods. Inclusions of 
 gas, liquid, and glass, which frequently furnish important clues to the 
 genetic relations, are also found. Gas inclusions in crystallized bodies 
 always stand out in strong relief, because the index of refraction of the gas 
 is far below that of the crystallized substance. In parallel light it is sur- 
 rounded by a single broad, dark band produced by refraction and total 
 reflection, Fig. 63. Separation into different zones is never observed in 
 gas bubbles because all gases are miscible. In observing gas inclusions 
 care must be exercised to note that the gas bubble is really in the object 
 and not in the Canada balsam. Air bubbles in the balsam naturally appear 
 very similar to gas inclusions, but by sharp focusing with the micrometer 
 screw they can be seen to lie either above or under the object itself. 
 
48 
 
 PETROGRAPHIC METHODS 
 
 Liquid and glass inclusions on the other hand appear to be separated into 
 different zones. The liquids include salt solutions, water, and, in some 
 minerals, liquid carbon dioxide. In all cases a small bubble of gas or air 
 is present in the liquid, which under certain conditions, especially in very 
 small inclusions and under variations of temperature, moves about quite 
 perceptibly. The index of refraction of all liquids existing in nature lies 
 between that of the mineral and the gas bubble, so that the edges of both 
 liquid and bubble appear as heavy lines in parallel light, Fig. 64. Some- 
 
 FIG. 63. 
 Gas Inclusions. 
 
 FIG. 64. 
 Liquid Inclusions. 
 
 FIG. 65. 
 Glass Inclusions. 
 
 times liquid inclusions seem to consist of several immiscible liquids with 
 different indices of refraction beside each other, or they may contain 
 small crystals. If the liquid in such an inclusion is carbon dioxide it can 
 be recognized by heating the preparation for some time a trifle above the 
 critical temperature of carbon dioxide. The bubble will disappear and the 
 border of the inclusion will become darker. 
 
 Glass inclusions likewise contain one or more bubbles, but they are always 
 immovable and do not show such heavy contours compared with the in- 
 cluding crystal if the latter has a low index of refraction. They appear 
 more distinct the higher the index of refraction of the crystal. The bubble 
 itself is always surrounded by a broad dark 
 band, Fig. 65. A regular arrangement of inclu- 
 sions is very characteristic in many cases, for 
 example, Fig. 66, leucite. 
 
 FIG. 66. Leucite with 
 Slag Inclusions. 
 
 Color. Another observation that can be 
 made in ordinary light is that of the color. 
 Particulars concerning ih& naming of the 
 colors are scarcely necessary. It may be 
 pointed out that many bodies, which are 
 very dark or entirely opaque in large 
 crystals are colorless or nearly so in thin section, for example, 
 augite, while others which are very light colored in macroscopic 
 crystals show their color quite distinctly in the thinnest sections, 
 thus, andalusite and cyanite. It need not be further emphasized 
 that a layer of any substance appears lighter colored the thinner 
 it is. The thinnest sections are generally not chosen for deter- 
 mining the color. 
 
 When light passes through a body it suffers diminution of 
 intensity and is either partially or entirely absorbed. If the 
 
OBSERVATIONS IN ORDINARY LIGHT 49 
 
 absorption is only slight and is about the same for the different 
 colors of the spectrum, the body appears colorless and trans- 
 parent, but if the absorption is complete even in the thinnest 
 sections, the body is said to be opaque. If certain colors of the 
 spectrum are absorbed more than others the substance appears 
 colored. Distinction is made between characteristic colors, or 
 those belonging to the substance itself, and dilute colors produced 
 by intermixture of foreign material present in such a fine state 
 of division that it cannot be detected with the strongest magni- 
 fication. Coloring produced by inclusions, on the other hand, 
 can always be detected in this manner. 
 
 Since the naming of the colors is in general very subjective, attempts 
 have been made to express them accurately by means of comparison with 
 an objective color scale. A Radde color scale is especially serviceable for 
 this purpose. It shows the various colors in a horizontal series while the 
 various shades of each are arranged in the vertical columns. Spectroscopic 
 dispersion of the colors obtained with a spectroscope-ocular is of very little 
 value. See page 49 concerning false colors, pseudochroism, which is often 
 observed on small opaque bodies with strong objectives. The color of 
 thin plates, which are in themselves colorless, belongs in the same category. 
 It is most beautifully observed in preparations of diatoms. 
 
 Observations in Reflected Light. These observations are con- 
 fined, in general, to surface properties, which cannot be deter- 
 mined in transmitted light, such as the luster of opaque sub- 
 stances, metallic luster, etch figures, etc. Light coming in from 
 below is cut out for this purpose and only light falling on the 
 object from above is used, but it can be increased by reflectors 
 or converging lenses. 
 
 If light incident on the preparation from above is used, it is desirable to 
 use an objective with long focal length because otherwise the objective holder 
 cuts out a large portion of the light. Low power 
 objectives are therefore more serviceable for such 
 observations and if stronger magnification is needed, 
 in spite of the loss of light, stronger oculars, which 
 are supplied almost entirely for this purpose with 
 'the larger instruments, are used. 
 
 If objectives of shorter focal length are employed, 
 a vertical illuminator inserted above the objective is 
 used. Fig. 67 gives a cross section of such an illumi- JT IG 57 
 
 nator. It consists of a total reflecting prism K wliich Vertical Illuminator, 
 sends the light L, coming in through the opening D, 
 
 down through the objective O upon the preparation. The prism covers 
 one half of the objective and leaves the other half free for observation 
 4 
 
50 PETROGRAPHIC METHODS 
 
 and through it the light, reflected from the object, reaches the eye. It 
 must be noted that, in using this method, the object must not be covered 
 with a cover-glass because it would reflect the greatest part of the light. 
 Uncovered preparations are best for investigations in reflected light. A 
 vertical illuminator cannot be used with the stronger objectives on ac- 
 count of the great loss of light produced by the double transit of the light 
 through the objective. 
 
CHAPTER IV 
 Observations in Parallel Polarized Light 
 
 Observations in polarized light are of especial importance in 
 the investigation of crystallized bodies because by this means 
 the inner structure of a crystal can be determined. For this 
 purpose a nicol prism, the polarizer, is placed between the source 
 of light and the object to be studied. By rotating the stage 
 through 360 the phenomena can be observed which a crystal 
 shows in various planes in which the plane polarized light 
 from the polarizer is vibrating. A second nicol prism, the 
 analyzer, with its vibration direction exactly at right angles to 
 that of the polarizer may be placed above the objective and 
 the phenomena of interference of light between crossed nicols 
 may then be observed. Before the phenomena thus observed 
 can be discussed in detail, the optical properties of crystals 
 must be reviewed briefly. 
 
 i. Optical Properties of Crystals 
 
 Single and Double Refraction. If a small illuminated open- 
 ing in a dark screen is viewed perpendicularly through a cleavage 
 piece or a plane parallel plate cut in any direction from a crystal 
 of fluorite or halite, it appears in the same place and in the 
 same manner as it would if the plate were not there. If instead 
 of the plane parallel plate a prism of the -same substance is 
 used and the opening is viewed through two surfaces oblique to 
 each other, the image will appear removed from its original 
 position and will be surrounded by a colored border, but only one 
 image of the opening will be seen. All amorphous bodies and 
 crystals of the cubic system behave in the same manner. They 
 are called singly refracting or isotropic substances. 
 
 If the index of refraction is determined by the deviation of 
 light in a series of such prisms of a singly refracting substance, 
 it will be found to be the same regardless of the orientation 
 of the prism in the crystal. Singly refracting bodies deviate 
 
 51 
 
52 
 
 PETROGRAPHIC METHODS 
 
 light in all directions in the same manner. They are there- 
 fore called isotropic. Since the refraction of light in a prism 
 is dependent upon the index of refraction of the substance in 
 question and, since the index is inversely proportional to the 
 velocity of the light, it follows that in an isotropic body light 
 is propagated in all directions with the same velocity. 
 
 Crystals of the other systems do not behave in this manner. 
 If a spot of light is observed in the same manner as before, 
 through a transparent cleavage piece of calcite it appears double. 
 One of the images is seen in the true position and the other is 
 removed from it. Depending upon the position of the calcite 
 the second image is above or below, to the right or to the left. 
 If the calcite be rotated, it will be observed that the latter 
 
 FIG. 68. Double Refraction in Calcite. 
 
 FIG. 69. A Principal Section in Calcite. 
 
 image moves in a circle around the former. It may also be 
 noted that the line joining the two images is always the bisector 
 of the obtuse angle of the cleavage fragment. All crystals that 
 are not cubic, act in a similar manner and are therefore called 
 doubly refracting or anisotropic. 
 
 Double Refraction of Light in Calcite. Of the two rays 
 produced by the double refraction of calcite, or any other 
 hexagonal or tetragonal crystal, the one, w, Fig.. 68, gives an 
 image of the object in its true position, i.e. with perpendicular 
 incidence of the light it suffers no deviation. It follows the 
 ordinary laws of refraction and is therefore designated as the 
 ordinary ray. The other ray, e, is deviated from its path. 
 Different laws of refraction govern it and it is, therefore, called 
 the extraordinary ray. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 53 
 
 Any plane of a crystal having a principal crystallographic 
 axis, i.e., those belonging to the hexagonal or tetragonal crystal 
 systems, is called a principal section, provided it contains the 
 optic axis. Thus, the dotted plane, Fig. 69, is one of an infinite 
 number of principal sections containing the c axis of calcite and 
 it is, indeed, the one passing through two edges of the rhom- 
 bohedron. A line joining the two images produced by double 
 refraction in every section of calcite corresponds to the projec- 
 tion of a principal section on the face in question. From this, 
 it follows that the deviation of the extraordinary ray in calcite, 
 or in any hexagonal or tetragonal crystal, is in a principal 
 section. In a cleavage piece of calcite the line joining the two 
 images produced by double refraction is the bisector of the 
 obtuse angle of the calcite rhombohedron, Fig. 70. 
 
 FIG. 70. FIG. 71. 
 
 Double Refraction in a Double Refraction in two parallel 
 
 Rhomb of Calcite. Rhombs of Calcite. 
 
 If the images seen through a piece of calcite are observed 
 through a second cleavage piece of the same thickness, it will be 
 found that they do not behave as two illuminated openings. 
 When the two cleavage pieces are exactly parallel, Fig. 71, it will 
 be noted that the distance between the two images is doubled, 
 i.e., the double refraction of one piece is simply added to that of 
 the other. If one calcite is rotated, four images are generally 
 seen, which are equally bright when the principal sections of 
 the calcite form an angle of exactly 45, Fig. 72. 
 
 The significance of the phenomenon that both images from the 
 first calcite did not, upon rotation of the second, behave as two 
 openings illuminated by ordinary light, was formerly very 
 difficult to understand. An explanation was only possible 
 after the difference between ordinary and polarized light was 
 discovered. The former consists of light vibrating in all 
 azimuths at right angles to the. direction of propagation, while 
 in the latter the vibrations are always executed in but one plane, 
 
54 PETROGRAPHIC METHODS 
 
 which is likewise perpendicular to the direction of propagation, 
 Fig. 22, page 8. It is a noteworthy fact that our eye is not 
 adjusted to discern this difference between ordinary and polarized 
 light and, that appliances which themselves produce polarization, 
 are necessary to recognize it. 
 
 The phenomena represented in Figs. 71-74 can only be 
 explained on the supposition that the vibrations of light in the 
 two images produced by the first calcite are polarized in two 
 directions at right angles to each other; and further, that the 
 vibration directions of the two rays are perpendicular to each 
 other in calcite, the one taking place in the principal section and 
 the other at right angles to it. According to Fresnel's hypoth- 
 esis, which is here taken as a basis, the vibration direction of 
 the extraordinary ray is always in the principal section while that 
 of the ordinary ray is perpendicular to it. 
 
 FiG.JT2. FIG. 73. FIG. 74. 
 
 < Double Refraction in two Rhombs of Calcite Rotated. 
 45 90 180 
 
 The phenomena of Figs. 71 74 can be explained in a very 
 simple manner. In Fig. 71 the light which emerges from the 
 first calcite as the extraordinary ray e passes through the second 
 parallel to it, likewise as the extraordinary ray e and suffers the 
 same amount of deviation in the second as in the first and in fact 
 in the same direction. The other ray passes through both calcites 
 as the ordinary rays co and o and is not deviated. If the second 
 calcite is rotated through an angle of 45, Fig. 72, the ordinary ray 
 CD of the first is resolved by the second into two components, which 
 correspond to the two vibration directions of the ordinary ray 
 o and the extraordinary ray e in the second calcite. Thus, two 
 images are formed, of which one coo produced by a light passing 
 through both calcites as an ordinary ray, suffers no refraction. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 55 
 
 The other cue owes its origin to the component corresponding to 
 the extraordinary ray of the second calcite and is therefore re- 
 fracted in its principal section. In the same manner the extra- 
 ordinary ray of the first calcite e gives rise to two images eo 
 and ee, which are shown in corresponding positions in Fig. 72. 
 In a position in which the two principal sections enclose an angle 
 less than 45, the two images coo and se are brighter than the 
 other two. Upon rotating more than 45 the two latter become 
 brighter at the expense of the former until, after a rotation of 
 90, the images appear as shown in Fig. 73, only we and so being 
 visible. When at last the calcite has been rotated through 180 
 the conditions are as in Fig. 74. The ordinary ray of the first 
 passes as an ordinary ray through the second, and the extraordi- 
 nary ray as an extraordinary. The former is not deviated at all, 
 while the latter is refracted just as far in one direction in the first 
 calcite as it is in the opposite direction in the second calcite so 
 that all four images coincide. 
 
 If several differently orientated plates are cut from a crystal 
 of calcite, two images of an illuminated pinhole can generally be 
 seen through them, but the distances between the images will 
 vary according to the orientation of the plates. If the indices 
 of refraction are determined in these plates it will be observed 
 that the index of the more strongly refracted ray is the same in all 
 the plates, while that of the other varies. The minimum 
 value of the latter is obtained in a plate cut parallel to the prin- 
 cipal crystallographic axis, i.e., in a plate in which the light pass- 
 ing through the calcite vibrates parallel and perpendicular to that 
 axis. The difference between the velocities of the two rays is 
 greatest when the light is propagated perpendicular to the prin- 
 cipal crystallographic axis. Plates parallel to the base give but 
 one value for the index of refraction. In this case 'the light is 
 propagated parallel to the principal crystallographic axis. Hence 
 it follows that for light traveling in that direction calcite behaves 
 as a singly refracting substance and further that the elasticity 
 is the same in all vibration directions perpendicular to the prin- 
 cipal axis. 
 
 Uniaxial Crystals. That direction of propagation in a doubly 
 refracting crystal in which light suffers no double refraction, 
 i.e., perpendicular to which all vibration directions are equal, is 
 called the optic axis. Calcite and all other hexagonal and tet- 
 ragonal crystals, i.e., all crystals with a principal crystallographic 
 
56 
 
 PETROGRAPHIC METHODS 
 
 axis, possess but one such direction and are therefore called 
 uniaxial. The ray which is propagated in all directions with 
 equal velocity is the ordinary ray. The other ray, the index of 
 refraction and the velocity of propagation of which are depend- 
 ent upon the direction in which the vibrations take place, is 
 the extraordinary ray. It follows from the above that the index 
 of refraction or its reciprocal, the velocity of the extraordinary 
 ray, more nearly approaches that of the ordinary ray the smaller 
 the angle between the optic axis and the direction of propaga- 
 tion of the ray through the crystal. In the direction of the optic 
 axis the double refraction or the difference between the indices 
 disappears because all vibrations perpendicular to the optic axis 
 have equal values. 
 
 The difference between the indices of the two rays passing 
 through a uniaxial crystal, i.e., its double refraction, is greatest 
 when the light travels through the crystal perpendicular to its 
 principal crystallographic axis, which is also the optic axis. It 
 becomes zero when the direction of propagation coincides with 
 
 FIG. 75. FIG. 76. 
 
 Principal Section of a Wave Surface of a Uniaxial Crystal. 
 
 Positive. Negative 
 
 the optic axis and polarization of the light no longer takes place. 
 In the directions lying between these two, the double refraction 
 is less the smaller the inclination of the ray of light to the optic 
 axis. 
 
 This phenomenon can be best understood by considering a 
 wave surface, which includes all points to which light would 
 travel from the center of a crystal in a given unit of time. Since 
 the portion of the light passing through as an ordinary ray vi- 
 brates in all directions with equal velocity it reaches the surface 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 57 
 
 of a sphere in the time interval. On the other hand the extra- 
 ordinary ray possesses different velocities in different directions 
 and its velocity is equal to that of the ordinary ray only in a 
 direction parallel to the optic axis. Its ray surface must be in 
 contact with that of the ordinary ray at the ends of the axis. 
 These are the only points which the two wave surfaces have in 
 common. The change of velocity of the extraordinary ray is 
 the same on all sides for equal inclinations of the direction of 
 propagation to the optic axis and increases regularly up to the 
 maximum perpendicular to the optic axis. Therefore the wave 
 surface of the extraordinary ray must be a rotational form differ- 
 ent from a sphere. It is an ellipsoid of rotation, Figs. 75 and 76. 
 The index of refraction of the ordinary ray is indicated by 
 cu, and that of the ray differing from it the most, the extra- 
 ordinary ray, by e. The double refraction is expressed by the 
 
 FIG. 77. Negative FIG. 78. Positive 
 
 Double Refraction. 
 
 difference e-co. There are two possible cases; one, as in calcite, 
 where oj > e that is, the ordinary ray is more strongly refracted 
 than the extraordinary. Then e-co is negative and such crystals are 
 said to be negative or because the extraordinary ray is farther from 
 the normal than the ordinary ray, they are also called repellant 
 crystals, Fig. 77. In the other case >a>, the crystal -is said to be 
 positive or attractive, Fig. 78. Since the velocity is the reciprocal 
 of the index of refraction, the extraordinary ray of a negative 
 crystal and the ordinary ray of a positive crystal have the greatest 
 velocities. The direction of greatest elasticity and least index 
 of refraction is called tt 1 and the corresponding index a; the 
 direction of least elasticity c and its index ?-, while c is the prin- 
 cipal crystallographic axis. c = a is the symbol of a negative 
 
 x Usage differs somewhat. In America X, Y, Z is proposed by Iddings for 0, B, t re- 
 spectively, while in France Up (p = petit, small) ng (g = grand, large) and nm (m = 
 mo yen, medium) are used for CLfft respectively, b and /3 refer of course to biaxial 
 crystals. 
 
58 PETROGRAPHIG METHODS 
 
 crystal, c = t that of a positive, while f-a. shows the amount of 
 double refraction in all cases. 
 
 Biaxial Crystals. The conditions are more complex in crystals 
 of the orthorhombic, monoclinic, and triclinic systems. If an il- 
 luminated opening is observed through several plates of aragonite 
 variously orientated, it will be noted that, in general, two images 
 are seen, but both appear to be displaced from their original 
 positions. In this case the light is resolved into two extraordinary 
 rays which are always polarized at right angles to each other. 
 If the indices of refraction are determined with a series of 
 prisms of aragonite it will be noted that none of them is con- 
 stant. The maximum and minimum, i.e., the greatest double 
 refraction, is observed in a prism in which the direction of pro- 
 pagation of the ray is parallel to the crystallographic a axis. 
 The two rays vibrate at right angles to each other and parallel 
 to the other two crystallographic axes, which are at the same time 
 the directions of greatest and least velocity of light or the direc- 
 tions of smallest and greatest index of refraction in the crystal. 
 In all other directions the indices of refract r on are intermediate 
 between these two extremes, including also light vibrating in the 
 direction of the a axis. Three optical directions perpendicular 
 to each other must be distinguished in aragonite and these coin- 
 cide with the a, b, and c axes. On the other hand, there is no 
 relationship existing between the value of a vibration direction 
 whether a, b, or c and the crystallographic axis because in 
 the orthorhombic system the crystallographic axes can be placed 
 in any position desired. The three principal vibration directions 
 of light, the axes of optical elasticity in a biaxial crystal, can be 
 designated in various ways depending upon whether the index 
 of refraction or its reciprocal value, the velocity of light, is to be 
 emphasized. 
 
 f smallest a 
 
 Index of refraction < medium ft 
 [ largest 7- 
 
 a = 1 : a largest. ] Velocity of 
 
 B = 1 : /? medium [ light. 
 t = \if smallest j Elasticity. 
 
 Let us now consider more in detail the bundle of planes, the 
 axis of which is the direction of medium index, /?. Light passing 
 through the mineral perpendicular to one of these planes is 
 resolved into two rays, one of which always vibrates in the 
 direction of medium velocity and the other is perpendicular to 
 it. Its vibration direction coincides either with that of the 
 largest (/) or of the smallest (a) index or it corresponds to a 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHTS 
 
 medium value between these two. Among these values there 
 is one /?', Fig. 79, equal to jt. For the direction of propagation 
 AA, which lies in the plane a f and perpendicular to the plane 
 $ /?', there are two perpendicular vibration directions of equal 
 value. A A is an optic axis. Since the plane a ft in aragonite 
 is a plane of symmetry, another direction equal to AA and 
 symmetrical to it, must lie on the other side of a in the plane 
 a f j and a and y will bisect the angle between these two optic 
 axes. Another such direction is crystallographically impossible 
 and therefore aragonite, and with it all crystals of the ortho- 
 rhombic, monoclinic, and triclinic systems, are biaxial. 
 
 The plane in which the two optic axes lie is called the plane 
 of the optic axes. The directions of largest (f) and smallest (a) 
 
 cc A 
 
 A a 
 FIG. 79. Construction of an Optic Axis in Aragonite. 
 
 index bisect the angles of the optic axes and are called middle 
 lines or bisectrices. The direction of medium index '(/?), perpen- 
 dicular to the plane of the optic axis, is called the optic normal. 
 The optic axes form an acute angle on one side and an obtuse 
 on the other. The bisector of the acute angle is called the first 
 middle line or the acute bisectrix Bx a and the bisector of the 
 obtuse angle, the second middle line or obtuse bisectrix Bx . 
 If the acute bisectrix is the direction of the smallest index (a) 
 the crystal is said to be negative and in the other case positive. 
 The value of the intermediate index can be the arithmetic mean 
 of the largest and smallest indices for a certain color only when 
 the optic angle for that color is 90, otherwise it is the nearer 
 
60 PETROGRAPHIC METHODS 
 
 that of the obtuse bisectrix the smaller the acute optic angle, 
 and when it becomes the two directions are equivalent and 
 the crystal is optically uniaxial. 
 
 Since the index of refraction is different for different colors 
 in one and the same crystal, the size of the optic angle must 
 vary with the color. Sometimes the acute angle for red (p) is 
 larger than that for violet (u), which is indicated by the symbol 
 of dispersion p > u. The opposite o > p is also encountered. 
 
 As seen in the case of aragonite, the plane of the optic axes 
 in orthorhombic crystals is always parallel to one of the three 
 pinacoids because the directions of greatest and least elasticity 
 coincide with two of the crystallographic axes. In the mono- 
 clinic system only one of the three principal vibration directions 
 coincides with the b axis and this is the case for all colors. The 
 plane of the optic axes lies parallel or perpendicular to the 
 clinopinacoid and the other two vibration directions fall in the 
 plane of symmetry, wherever they may, but are always per- 
 pendicular to each other. They may have quite different 
 positions in that plane for different colors, thus causing dis- 
 persion of the bisectrices or the planes of the optic axes. In the 
 triclinic system the optical elements are orientated entirely 
 independently of the crystallographic axes and likewise the 
 relationships for the different colors are independent of each 
 other. Even here, however, the three principal vibration 
 directions for any one color are perpendicular to each other. 
 
 The velocity of light in any direction in a biaxial crystal 
 can be easily shown by the Fresnel principle. An ellipsoid is 
 described about the three axes of elasticity of a biaxial crystal 
 and a plane is placed in the center perpendicular to the direction 
 of propagation of the ray. The form of such a section is always 
 an ellipse, except in the two positions perpendicular to the 
 optic axes, where it is a circle. The lengths of the large and 
 small axes of the elliptical section correspond to the velocities 
 of the two rays propagated perpendicular to the section. 
 
 2. Investigations with One Nicol the Polarizer 
 
 Surface Color and Pleochroism. When light enters an aniso- 
 tropic crystal it is generally resolved into two rays propagated 
 with different velocities. These rays are absorbed differently 
 and, when this difference is great enough, variation in color may 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 61 
 
 be noted. If a doubly refracting crystal is illuminated with 
 ordinary light, the mean of the two vibrations is observed at 
 any moment because, as a result of the rapid change of the 
 planes of vibration in a ray of ordinary light, equal amounts 
 of light pass through the crystal at a given instant parallel to 
 each of the vibration directions. Upon looking through 
 a surface of a crystal in ordinary light a mixed color is observed, 
 which is called the surface color. If, on the other hand, plane 
 polarized light enters a doubly refracting crystal, the two com- 
 ponents corresponding to the vibration directions in the crystal 
 will be alike only when they form an angle of 45 with the 
 vibration directions of the nicol. One direction will become 
 zero, i.e., only those vibrations parallel to the other direction 
 will be observed, when the vibration direction of the nicol 
 coincides with it. 
 
 The properties of each ray passing through a crystal can be 
 studied in succession by placing the vibration directions parallel 
 to that of the polarizer between crossed nicols and then removing 
 the analyzer to observe the color. Thus the color for certain 
 directions and the differences for different directions of vibration 
 can be observed. These colors are called the colors of the axes. 
 Variation in color for different directions in a crystal is called 
 pleochroism. 
 
 For these investigations the polarizer beneath the preparation is used, 
 while the analyzer is pushed out of the tube. If the analyzer were used 
 the interference phenomena, caused by the partially polarized light reflected 
 
 from the mirror, although not very dis- 
 tinct, would greatly interfere with the 
 
 [L y= ^i|j!!!!| results. If the colors parallel to the 
 
 rri > ^\ K \ Ili axes are to be observed side by side, in- 
 
 stead of one after the other, a Haidinger 
 lens, Fig. 80, is used. This consists of 
 a tube with a rectangular opening in 
 FIG. 80. Haidinger Lens. O ne end and contains an elongated 
 
 piece of calcite K, which is of the 
 
 proper length to give two images of the opening side by side. The vibra- 
 tion directions of the polarized light are perpendicular to each other in 
 these two images produced by double refraction. If the crystal in front 
 of the opening is rotated until the two images show the greatest possible 
 difference in color, the two principal colors of the face are observed. A 
 similar device is the dichroscopic ocular, which may be used for observing 
 the two principal colors under the microscope. Here, however, the results 
 are disturbed by the partial polarization of light reflected from the mirror, 
 as indicated above. 
 
62 PETROGRAPHIC METHODS 
 
 Cross sections of cubic crystals or of amorphous substances 
 always show the same color upon a complete rotation of the stage 
 above the polarizer because in them the absorption of light is 
 the same in all directions. In uniaxial minerals all directions 
 perpendicular to the optic axis are of equal value in an optical 
 sense. Only the color of the ordinary ray is observed on the base 
 of a uniaxial crystal in ordinary light and this is the same as 
 the surface color. Such a section does not change its color upon 
 rotation of the stage. The absorption of light is the same for 
 the ordinary ray in any section and its color can be observed 
 in any direction, while the absorption of the extraordinary ray 
 is variable and differs the more from that of the ordinary ray 
 the nearer its vibration direction approaches the direction of 
 the optic axis. The color of the ordinary ray can be observed 
 in any section, while that of the extraordinary ray appears only 
 in a section parallel to the optic axis of the crystal. If the 
 difference in absorption for the two principal directions of an 
 uniaxial crystal is sufficiently large to be observed it will be seen 
 most perfectly in a section parallel to the optic axis. Since in 
 the case of uniaxial crystals there are only two principal colors, 
 it is common to speak of dichroism. 
 
 In biaxial crystals there are three different vibration directions 
 for color as well as for indices of refraction. These crystals possess 
 three principal colors and are termed trichroic. The pure color 
 belonging to one of these vibration directions can be observed 
 only in a section parallel to it. In all others mixed colors are 
 seen. In the orthorhombic system the vibration directions of 
 the greatest, least and intermediate indices of refraction are at. 
 the same time the color axis. In the monoclinic system only the 
 b axis is simultaneously a color axes. The other two lie anywhere 
 in the plane of symmetry, but are not very far removed from the 
 vibration directions of light. In the triclinic system all these 
 directions are independent of each other but even in this case, 
 if a vibration direction is placed parallel to the polarizer instead 
 of a color axis, the error is scarcely noticeable. 
 
 The Phenomenon of Pleochroism. The difference of absorption of light 
 in various directions, which is known as pleochroism, is so slight in numerous 
 cases, especially in colorless crystals, that it cannot be observed even with 
 thick plates of the mineral. Since the color of a section, and likewise the 
 difference of color in different directions, is less distinct the thinner the 
 layer, it follows that in thin section, pleochroism can only be observed in 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 63 
 
 substances having a great difference of absorption in different directions. 
 Although in the observation of weak pleochroism the subjective ideas of the 
 observer and a certain amount of practice are important, still the investi- 
 gation of pleochroism is a valuable aid in microscopic research. 
 
 It is very useful in the study of pleochroism to keep three possible cases 
 in mind. This can be done best by the aid of a color scale such as a Radde 
 color scale, which shows the different colors in a horizontal row and the 
 different intensities of each from the darkest to the lightest tints in vertical 
 columns. The following cases are possible : 
 
 1. Different parts of the spectrum are absorbed in different parts of the 
 crystal in such a manner that the intensity of the light which travels in 
 these directions remains approximately the same. Various colors appear 
 in different directions, but one is about as bright as the other. Compare 
 horizontal series of the color scale. 
 
 2. In each direction approximately the same portions of the spectrum are 
 absorbed but with unequal intensity. The same color is observed in all 
 directions, but with different intensities. Compare vertical series of the 
 color scale. 
 
 3. In different directions different colors of the spectrum are absorbed 
 with different intensities. The different colors are seen in different shades. 
 Compare the diagonal of the color scale. 
 
 These three cases often give rise to very characteristic phenomena, e. g., 
 pleochroic pyroxene almost always shows the same depth of color, in one 
 direction light green, in the other light yellow. Common hornblende of 
 the same color shows stronger absorption of the green ray and its pleo- 
 chroism is from dark green to light yellow, while in biotite only simple 
 differences of absorption of the same color are effective and it changes from 
 light brown to dark brown. 
 
 The strength of pleochroism in one and the same crystal is dependent 
 upon its thickness and very frequently the phenomenon can be observed in 
 thicker layers when it is not noticed at all in thin sections. The absorp- 
 tion is rarely so great that no light is observed passing through the crystal 
 in that direction. The color of the more strongly absorbed ray can almost 
 always be recognized, even in the most strongly absorbing minerals if the 
 section is thin enough. 
 
 The orientation of the differently absorbed rays in a crystal is of import- 
 ance because it tends to be strikingly constant in one and the same sub- 
 stance, as also in the larger isomorphic series. Two points of view are to 
 be considered in practice: 1. The relationship of the more strongly 
 absorbed ray to the crystallographic development of the substance in 
 question, i. e., to the long direction of its cross section, the principal zone. 
 Almost all important rock-forming minerals with great differences of 
 absorption show stronger absorption of the ray vibrating parallel or nearly 
 parallel to the principal zone. They have a deeper color when the longer 
 edge of the cross section is parallel to the vibration direction of the polarizer. 
 Fig. 81. Tourmaline behaves in the opposite manner and shows a lighter 
 color, Fig. 82. Stronger absorption of light in the direction perpendicular 
 to the principal zone is one of the most important means for determining 
 this mineral, which often occurs in minute individuals that are difficult to 
 
64 
 
 PETROGRAPHIC METHODS 
 
 determine by other optical properties. 2. The relation between the 
 refraction and absorption of the two rays vibrating in a section. There is 
 no regular connection between these two properties and although in most 
 cases in rock-forming minerals the most strongly refracted ray is the most 
 strongly absorbed, this is by no means always the case. In the above 
 mentioned example, tourmaline, the extraordinary ray is the least refracted 
 and the least absorbed. Its absorption symbol is >o or since tourmaline 
 is optically negative w>. In apatite, which is likewise negative, the 
 more strongly refracted ray is orientated in reverse manner. Here the 
 symbol is o> or >&). Similar results are encountered in optically 
 biaxial minerals. Common hornblende, for example, universally shows 
 c>6 > a, while with riebeckite and arfvedsonite, on the other hand, o >B >. 
 
 Pleochroism can be produced in an artificial manner under certain 
 conditions and may become a very good characteristic of a substance. 
 When members of the olivine or amphibole groups containing iron are 
 roasted, they assume strong colors and decided differences of absorption. 
 This serves to distinguish them from the pyroxenes which generally do not 
 
 FIG. 81. Stronger absorption Parallel 
 to Principal Zone. 
 
 FIG. 82. Stronger Absorption Perpen- 
 dicular to Principal Zone. 
 
 show any pleochroism as the result of such operations. Fibrous and 
 scaly mineral aggregates can be colored directly with aniline dyes, where- 
 upon many of them such as talc and serpentine become distinctly pleochroic. 
 Many artificial crystals, colorless in themselves, acquire not only color but 
 often distinct pleochroism upon crystallization from a dye solution. This 
 phenomenon has been known a long time in the case of strontium nitrate. 
 For the sake of completeness pleochroic halos, Figs. 83 and 84, will be 
 considered briefly. In numerous minerals, particularly in cordierite, 
 andalusite, mica, hornblende, tourmaline, etc., it will be noticed that certain 
 rounded spots show stronger pleochroism than the rest of the mineral 
 generally does. If these portions are studied more carefully small inclusions 
 of foreign minerals will be discovered in the center of such pleochroic halos 
 and the outline is surrounded on all sides by a more pleochroic zone. 
 Since it is possible to produce phenomena analogous to the pleochroism in 
 these minerals by illuminating with radium preparations and, further since 
 most of the inclusions forming the centers of such halos zircon, rutile, 
 orthite, etc. are radio-active, there can be no doubt that the pleochroism 
 of the halo depends, for the most part at least, upon the action of active 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 65 
 
 rays. It is noteworthy that refraction, double refraction, and frequently 
 also dispersion are greatly changed in the pleochroic halos. They are 
 sometimes decidedly lower, sometimes much higher. Besides the radio- 
 active inclusions surrounded by pleochroic halos there are frequently other 
 crystalline inclusions in the same crystal, which are not classed with radio- 
 active minerals (and do not show this phenomenon), for example, the long 
 apatite needles in Fig. 84. 
 
 Two other phenomena which are observed now and then must also be 
 mentioned. One of these is pseudodichroism, a phenomenon which occurs 
 in perfectly colorless crystals. If there are a large number 
 of parallel orientated inclusions in a crystal and the refrac- 
 tion of these differs considerably from that of the surround- 
 ing crystal, light passing through is disturbed by the inclu- 
 sions if they are steeply inclined to the direction of propa- 
 gation of the light. The less refracted red rays are bent 
 toward the middle and the more strongly refracted violet 
 rays toward the edge of the field of vision. Only those 
 rays vibrating par- 
 allel to the direction 
 of the inclusions suffer 
 this sort of refraction, 
 while those vibrating 
 at right angles to it 
 are not changed. If 
 the crystal is so orien- 
 tated in the center of 
 
 the field that the direction of the inclusions is parallel to the principal 
 direction of the polarizer, it appears a brilliant brown. If it is on the edge, 
 it is colored gray, but in each case it becomes colorless upon rotating 
 through 90. 
 
 The occurrence of interference colors without the use of the analyzer is 
 the other of the phenomena referred to above. If a layer of a doubly 
 refracting mineral, orientated at random occurs in a slide under a thin crystal 
 of a very strongly absorbing mineral biotite or tourmaline the absorb- 
 ing crystal, which scarcely allows one of the rays to pass through, acts as an 
 analyzer itself and the mineral beneath it shows interference colors. Com- 
 plementary colors appear upon a rotation through 90, because the absorb- 
 ing direction of the preparation is first perpendicular and then parallel to 
 the principal direction of the polarizer. The occurrence of interference 
 colors in minerals with a high double refraction calcite and rutile which 
 are crossed by twininng lamellae greatly inclined to the face of the slide, is a 
 similar phenomenon. 
 
 3. Investigations with Crossed Nicols 
 
 Observations between two nicols are carried out almost 
 entirely with the planes of vibration of the two crossed, and for 
 this reason they are called observations with crossed nicols 
 
 FIG. 83. FIG. 84. 
 
 Pleochroic Halos around Inclusions of Zircon in 
 Tourmaline. Biotite. 
 
66 
 
 PETROGRAPHIC METHODS 
 
 The following determinations are to be made in parallel polarized 
 light with the microscope, as commonly constructed, which 
 affords an enlarged image of the object investigated: 
 
 (a) Recognition of double refraction. 
 
 (b) Determination of the position of the vibration directions. 
 
 (c) Measurement of the strength of double refraction. 
 
 (d) Determination of the relative velocities of the two rays 
 vibrating in a cross section. 
 
 Recognition of Double Refraction. If a singly ^refracting 
 substance, or a section through such a substance, is placed 
 between crossed nicols, the plane polarized light from the polar- 
 izer passes through the crystal unaltered to the analyzer, and 
 since its vibration direction is at 90 to that 
 of the approaching ray no light will pass 
 through. No change occurs upon rotating 
 " Q/ the stage' through 360. A singly refracting 
 substance appears dark in all positions be- 
 tween crossed nicols. 
 
 If a doubly refracting crystal is observed 
 between crossed nicols, the light coming from 
 the polarizer suffers no change in the crystal 
 only when the vibration directions RR' and 
 SS' in the crystal are exactly parallel to the 
 vibration directions PP' and QQ' in the two 
 nicols, Fig. 85. If the vibration directions 
 RR' and SS' are oblique to PP 7 and QQ', as in 
 Figs. 86 to 88, the plane polarized light from 
 the polarizer is resolved into two components 
 Or and Os' corresponding to the vibration 
 FIG. 85. vibration directions of the crystal. After another 
 Directions in Crystal reso lution in the analyzer the components 
 
 Parallel to Those of the . v ,.".' v\ J /^* 
 
 Nicols. passing through it, Op and Oo are com- 
 
 bined. The plate appears illuminated in all 
 other' positions because a portion of the light always passes 
 through the analyzer. The magnitude of the components Op 
 and OS indicates the brightness and it follows from a com- 
 parison of Figs. 86 to 88 that the maximum is reached when 
 the vibration directions of the crystal are at 45 to those of the 
 nicols, Fig. 87. 
 
 Since the vibration directions RR' and SS' are parallel to PP' 
 and QQ' four times in a horizontal rotation of the stage through 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 67 
 
 360, doubly refracting crystals are alternately light and dark 
 four times in a rotation of 360 between crossed nicols. Maxi- 
 mum brightness is obtained when the vibration directions of the 
 crystal are at 45 to those of the nicols. It diminishes upon 
 further rotation and passes gradually over into complete darkness 
 when these directions are respectively parallel. This latter 
 position is also called the position of extinction, and the vibration 
 directions in the crystal, the extinction directions. The determi- 
 nation of the latter is an important aid in many cases for ascer- 
 taining the crystal system. 
 
 -Q' Q 
 
 ' Q- 
 
 p' 
 
 FIG. 87. 
 Vibration Directions in Crystal Oblique to Those of the Nicols. 
 
 Gray interference colors of the lowest order (see Fig. 96, page 75), corre- 
 sponding to the lowest degrees of double refraction, are extremely difficult 
 to distinguish from total darkness. Therefore, when thin sections of very 
 weak double refracting crystals are examined, it is sometimes difficult to 
 distinguish whether or not, there is any effect on polarized light at all. To 
 recognize very weak double refraction, use is made of certain accessory 
 devices sensitive tint plate, Bravais double plate, etc. which will be 
 described more in detail under the head of stauroscopes. The change of 
 interference color in such devices, caused by even extremely low double 
 refracting plates, is much more easily determined than the difference be- 
 tween absolute darkness and very weak illumination, so that the presence 
 of double refraction can be definitely determined, even in the most doubtful 
 cases. 
 
 Determination of the Position of the Vibration Directions. 
 
 The position of the vibration directions can be determined 
 from the above by placing the crystal between crossed nicols in 
 the position of perfect darkness. Then the vibration directions 
 of the crystal lie parallel to those of the two nicols, and the latter 
 are exactly parallel to the cross hairs in an adjusted microscope. 
 
68 
 
 PETROGRAPHIC METHODS 
 
 A characteristic crystal edge or cleavage direction is brought 
 parallel to one of these hairs by rotating the stage, and a reading 
 is made on the scale on the stage. Then the analyzer is inserted 
 and the stage rotated until the crystal appears perfectly dark, 
 and another reading of the scale is made, the difference being 
 the angle which the vibration direction makes with the edge or 
 cleavage in question. 
 
 The position of absolute darkness may not be found accurately 
 on account of the gradual transition from light to dark. The 
 beginner will experience considerable difficulty in finding the 
 true position of darkness. The method of procedure is as 
 represented in Fig. 89. PP' and QQ' represent the vibration 
 
 FIG. 89. Determination of Extinction Direction. 
 
 directions of the two nicols. The stage is rotated in the direction 
 of the arrow I until the crystal appears entirely dark and its 
 long edge, which may formerly have been parallel to PP', has 
 reached the position A'. Then it is rotated further toward QQ', 
 and the crystal becomes light again. It is then rotated back in 
 the direction of the arrow II until darkness is obtained, say in 
 the position A". The direction a, which bisects the angle between 
 the two directions a' and a", i.e., the mean of the two positions, is 
 the true vibration direction. If this operation is repeated 
 several times, the mean of a large number of readings will come 
 very near to the true position of darkness. Even though the 
 greatest care is exercised, an error of J, as is generally the 
 case with microscopic measurements, can scarcely be avoided. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHTS 
 
 In general parallel or symmetrical extinction, Figs. 90 and 91, is observed 
 in hexagonal, tetragonal and orthorhombic crystals, because the vibration 
 directions coincide with the crystal axes. The determinations are made 
 in white light because the vibration directions are the same for all colors. 
 Monoclinic crystals show parallel or symmetrical extinction only upon 
 faces in the zone of the 6 axis, and for all colors, while on all other faces the 
 extinction is oblique, Fig. 92, i.e., the extinction directions are unsymmet- 
 rical with respect to the crystal edges. Since this variation is different 
 for different colors, the position of absolute darkness cannot be obtained 
 with white light. The extinction is best determined in monochromatic 
 light if an accurate measurement, and not merely an approximate orienta- 
 tion is to be made. 
 
 
 FIG. 90. Parallel 
 
 FIG. 91. Symmetrical. 
 Extinction. 
 
 FIG. 92. Oblique 
 
 When the extinction angle is determined it must be noted whether the 
 measurement from the crystal edge is to the front or to the rear. The 
 difference caused by confusion of these two directions is shown in Fig. 92. 
 The dotted arrow shows the false vibration direction, the proper direction 
 of which is given by the solid arrow. A beginner will do well to sketch 
 the crystal and place the sketch parallel to the extinguished crystal, and 
 then draw lines parallel to the vibration directions, i.e., parallel to the cross 
 hairs. This shows the vibration directions sought in their true positions. 
 
 As mentioned above, hexagonal, tetragonal and orthorhombic 
 crystals generally show parallel, extinction because their develop- 
 ment is usually parallel or symmetrical to the principal vibration 
 directions. Monoclinic crystals are not uncommonly developed 
 so as to be long prismatic parallel to the b axis, e.g., epidote and 
 wollastonite, and then the principal zone is the position of 
 parallel extinction. It is obvious that crystals thus developed 
 may be confounded in thin section with those showing parallel 
 extinction in all sections, because cross sections transverse to the 
 direction of elongation are not very numerous. The true charac- 
 ter of monoclinic crystals can, however, be easily determined if 
 they are developed tabular parallel to the base or prismatic 
 
70 
 
 PETROGRAPHIC METHODS 
 
 parallel to one of the directions lying in the plane of symmetry. 
 Triclinic crystals may be developed in various ways and can 
 rarely be orientated properly. 
 
 The value of the extinction angle of monocline crystals is 
 determined on the clinopinacoid and where there is no statement 
 to the contrary, this face is always understood. Other faces can 
 be used for the determination of a crystal only when their ex- 
 tinction directions are accurately known. The directions can- 
 not generally be determined from the inclination of the face to 
 the b axis, because the extinction angle does not diminish in a 
 regular manner in passing from the clinopinacoid to the ortho- 
 pinacoid, and may even have a higher value on an intermediate 
 face than on the clinopinacoid itself. Since the extinction angle 
 on crystal faces that are easily determined has a characteristic 
 value for any given substance, it should be given for the prism 
 faces as well as for the clinopinacoid, especially in the case of 
 monoclinic substances having a good cleavage parallel to the 
 prism, for if the prism angle is known, the true optic angle can be 
 calculated from these values. 
 
 Diopside with an extinction angle of 38 on the clinopinacoid is the best 
 example for illustrating this relationship. The crystal is mounted on a 
 otation apparatus, as described in the appendix, and the extinction meas- 
 ured on the faces inclined at least 
 10 to each other in the zone of the 
 c axis. These values are plotted 
 as ordinates, while the abscissas 
 represent the inclination of the 
 various faces to the clinopinacoid, 
 Fig. 93. The extinction for any 
 face lying in that zone can be read 
 from the curve plotted from this 
 data. Thus, for example, a prism 
 is inclined about 45 to the clino- 
 pinacoid, the prism angle being 
 not far from 90 in pyroxene, and 
 
 the extinction on that face is about 30. Great importance must be at- 
 tached to the character of such curves in the investigation of thin sections. 
 The extinction on the orthopinacoid is symmetrical, but the angle increases 
 very rapidly on faces to either side so that parallel extinction is obtained 
 only on faces very accurately orientated, as is rarely the case in thin sec- 
 tions. It is therefore easy to reach the false conclusion that the mineral 
 is triclinic because it shows oblique extinction in all sections. 
 
 In triclinic minerals the determination of the extinction angles 
 is of value only when the orientation of the face upon which 
 
 38" 
 30 
 
 2<T 
 10 
 
 
 ( 
 
 
 
 
 
 
 
 
 
 
 
 
 
 . 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 X. 
 
 
 
 
 
 
 
 
 
 ^\ 
 
 N 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 ) 10 20 30 40 50 60 70 80 9( 
 LO 110 10 
 
 FIG. 93. Extinction Curve for Diopside. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 71 
 
 they are observed is accurately known. The orientation of the 
 clinopinacoid of monoclinic crystals can be easily determined 
 by convergent light, as will be described later, because this face 
 is either parallel or perpendicular to the plane of the optic axes, 
 but such determinations are generally impossible in triclinic 
 minerals. However, now and then the direction of cleavage 
 cracks, twinning, etc.. offer a clue, and the measurement of the 
 extinction angle in triclinic minerals is important only in such 
 sections. Extinction is of especial importance in the investiga- 
 tion of cleavage pieces. 
 
 Stauroscopes. The simple method of ascertaining the position of ab- 
 solute darkness, described above, is not sufficient for refined measurements 
 and a number of methods have been devised which yield more accurate 
 results, especially with colorless minerals. After the position of extinction 
 has been found as accurately as possible with crossed nicols, one observes 
 upon rotating the analyzer, whether in all positions of the nicols it shows 
 the same tint as the surrounding field. The position is correct only when 
 that is the case. 
 
 There are a number of accessory devices, which can be used for special 
 purposes. These are all grouped together under the name of stauroscopes. 
 The most important are : 
 
 1. The Kobell stauroscope. 
 
 2. The Brezina double plate. 
 
 3. The Caldron double plate. 
 
 4. The Bertrand plate. 
 
 5. The sensitive tint plate, gypsum test plate. 
 
 6. The Bravais double plate. 
 
 7. The Biot quartz plate. 
 
 8. The half shadow polarizer. 
 
 9. The twin polarizer. 
 
 1. The Kobell stauroscope consists of a plate of calcite cut perpendicular 
 to the optic axis, and can be inserted between the upper lens of the ocular, 
 and the ocular analyzer placed over it. An interference figure of an opti- 
 cally uniaxial crystal is seen, but it is destroyed as soon as even the slightest 
 double refracting substance is placed between the two nicols. The vibration 
 directions of the crystal studied are properly placed only when the inter- 
 ference figure is perfectly restored. 
 
 2. The Brezina double plate consists of two plates of calcite placed side 
 by side. They are cut equally oblique to the base, but in opposite directions 
 and are symmetrical with respect to the plane of contact between them. 
 The interference figure in this case shows a single band surrounded by 
 colored curves in the middle of the field of vision. If the vibration directions 
 of the crystal are not exactly the same as those of the nicols that part of the 
 band over the crystal appears to be broken off from the rest of the dark bar. 
 
 3. The Caldron double plate is placed within an ocular of its own, because 
 the inserted plate demands a greater distance between the ocular lenses than 
 
72 
 
 PETROGRAPHIC METHODS 
 
 is usual. It consists of two wedge-shaped pieces of calcite so placed that 
 the plane of contact between them, when viewed from above, appears as a 
 sharp line and the vibration directions in the two halves are symmetrical to 
 it. If this border line is exactly parallel to the vibration directions of the 
 two nicols, each half shows the same degree of extinction. If a double re- 
 fracting plate is placed in the path of the rays, and its vibration directions 
 do not coincide with those of the nicols, the luminosity of the two halves 
 of the double plate does not change uniformly because their vibration direc- 
 tions are oblique to each other. Equal luminosity can only be regained by 
 orientating the extinction directions of the crystal plate accurately. 
 
 4. The Bertrand plate, like the Caldron, is usually placed in an ocular 
 of its own. It consists of four sectors of alternating right and left rotating 
 quartz cut perpendicular to the optic axis with a thickness of about 2 . 5 mm. 
 The interference colors of the four sections are bluish-white, but they are 
 changed uniformly when a double refracting 
 crystal is placed on the stage with its vibration 
 directions not exactly parallel to those of the 
 nicols, Fig. 94. If they are parallel, the four 
 sectors must all have and retain the same color 
 when the nicols are rotated horizontally. 
 
 5. The sensitive tint, violet I, Fig. 96, page 
 75, at the beginning of the second order of 
 colors is produced by a thin plate of a colorless 
 feebly refracting mineral, quartz or gypsum, of 
 the proper thickness to give this particular 
 interference color between crossed nicols. It is 
 
 fastened between two small strips of glass and inserted into a slot above the 
 objective in such a manner that its vibration directions are at 45 to those of 
 the nicols. As shown in Fig. 96 a retardation of . 00001 mm. is sufficient 
 to change this color to purple on the one side or to indigo on the other. The 
 same figure also shows that violet I is more susceptible to a distinct change 
 of color than all the interference colors occurring between crossed nicols. 
 Deep violet of the first order, which is produced with parallel nicols by a plate 
 of half the thickness of the one above can also be used as a sensitive tint. 
 Violet II, at the beginning of the third order of colors, although it is often 
 used as a stauroscope, is however, less valuable for this purpose because 
 with it marked changes of color take place only with great differences of 
 thickness. The sensitive tint is not only used to determine the vibration 
 directions accurately, but frequently also as a compensator or as a means 
 of recognizing very low double refraction, because the insertion of a crystal 
 with a very feeble double refraction is sufficient to influence the color 
 decidedly. 
 
 6. A Bravais double plate extends the applicability of the sensitive tint. 
 It is set in the inner focal plane of the ocular and on account of its thinness, 
 the ocular scarcely requires correction. One of the small plates described 
 under 5 is cut diagonal to its vibration directions and the two halves are 
 rotated through 180 to each other. Thus the vibration directions of the 
 halves are at 90 to each other, Fig. 95. The plate is so placed in an ocular 
 that the junction of the two parts coincides with one of the cross hairs. 
 
 FIG. 94. Bertrand Plate. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 73 
 
 The change which one half suffers when a double refracting crystal is intro- 
 duced is reversed in the other half and these two lying side by side allow the 
 slightest change of color imaginable to be recognized. This method is so 
 accurate that the double refraction of a glass cube can be observed when 
 pressed between the fingers. This is the most accurate method of all for 
 recognizing the double refraction as well as for determining the extinction 
 direction. The determination of the latter is, however, less accurate 
 because it depends upon the change of the interference colors and cannot 
 be made in monochromatic light, i. e., this 
 method cannot be used for determining accur- 
 ately the extinction directions of monoclinic 
 and triclinic crystals. 
 
 7. The Biot quartz plate, cut perpendicular 
 to the c axis, is 3 . 75 mm. thick and upon 
 rotating the analyzer gives various character- 
 istic interference colors between two nicols. 
 In a definite position of the analyzer the, deli- 
 cate violet of the sensitive tint is obtained. 
 This serves ostensibly for the determination 
 
 of the extinction directions of colorless crystals, Fl0 ' 95.-Bravais Double Plate, 
 while other tints give better results with colored 
 
 substances. When the proper position of extinction has been found, it 
 can be checked by rotating the analyzer and noting the interference colors. 
 The Biot quartz plate is generally so mounted that it can be inserted in 
 the slot above the objective. 
 
 8. The half shadow polarizer is a nicol which has been cut through the 
 middle in a direction slightly oblique to its principal section and the pieces 
 recemented together in reversed positions. The vibration directions form 
 a small angle with each other. It is used in place of the polarizer and the 
 two halves appear in equal half shadows when the vibration direction of the 
 analyzer is parallel to the line of division. 
 
 9. The twin polarizer differs from the above in that the vibration directions 
 in the two halves are at 90 to each other. The application of both devices 
 is very limited because the line, of junction and the crystal must be seen 
 sharply at the same time which means that only objectives with large focal 
 lengths can be used. 
 
 Those stauroscopes which depend upon a change of interference colors 
 can only be employed for determinations in white light. They cannot 
 be used when accurate determinations of the extinction directions of strongly 
 dispersing monoclinic and triclinic crystals are to be made. This deter- 
 mination must be carried out in monochromatic light. A Brezina double 
 plate or Kobell stauroscope is to be preferred in such cases. 
 
 Strength of the Double Refraction, Interference Colors. The 
 
 two rays into which light is resolved by a double refracting 
 crystal are propagated differently in the crystal. Upon emerging 
 from the crystal one ray is retarded a certain amount behind the 
 other and this retardation depends upon the difference of elas- 
 
74 PETROGRAPHIC METHODS 
 
 ticity of the two vibration directions perpendicular to the direc- 
 tion of propagation, and upon the thickness of the crystal. 
 When the two rays are brought back to one plane of vibration 
 by the analyzer they show a difference of phase. The rule is that 
 two plane polarized rays produced by the double refraction of a 
 single plane polarized ray of light, interfere with each other when 
 they are reverted again to the same plane of polarization. 
 
 Since ordinary light is composed of various colors with different 
 wave lengths, X, the phasal difference will be different for the 
 various colors for a definite thickness of the crystal, which corre- 
 sponds to a fixed retardation of one ray over the other. For a 
 definite color the two rays are compounded by the interference 
 and that color appears with especial brilliancy, while for other 
 colors the two vibrations destroy each other and no light of that 
 color passes through the analyzer. Thus, instead of white light, 
 colors appear and they are called interference colors. 
 
 There are two cases to be distinguished in the interference 
 of light depending upon whether the plane of vibration of the 
 analyzer is parallel or perpendicular to that of the polarizer. In 
 the first case the rays emerging from the crystal interfere with 
 the same phase, and in the latter case with reversed phase. With 
 parallel nicols those colors appear with greatest intensity which 
 are extinguished with crossed nicols, and vice versa, so that the 
 interference colors observed with parallel nicols are complemen- 
 tary to those seen between crossed nicols. 
 
 Let us consider more in detail the latter case, which is prac- 
 tically the only one used in microscopic technic, and assume 
 for the sake of simplicity, that with a double refracting crystal 
 of definite thickness the same absolute retardation of the two 
 rays is produced for all colors. The wave length A of extreme 
 violet amounts to about 380 ///*, that of extreme red about 775 
 fjLjji, millionths millimeters or microns. With crossed nicols those 
 colors are emphasized which interfere with a phasal difference of 
 1/2 ^ or an odd multiple of 1/2 X, while those colors are ex- 
 tinguished which interfere with a retardation of one or several 
 whole wave lengths. The interference color of a crystal can be 
 determined when the thickness and double refraction are known, 
 because the retardation is dependent upon both of these factors. 
 If the thickness of the preparation is known, the double refrac- 
 tion can be determined from the interference color, or inversely 
 if the double refraction is known, the thickness can be estimated. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 75 
 
 + Nicols 
 
 II Nicols 
 
 white 
 
 steel-gray 50 
 
 
 white 
 
 lavender-gray __.100 
 grayish-blue 150 
 greenish-white 200 
 
 
 
 ^ yellowish-white 
 O2-__brownish-white 
 
 brownish-yellow 
 
 
 
 -,,^_ 
 
 white 250 
 
 FVA- 
 
 light-red 
 
 light-yellow. .300 
 bright-yellow .350 
 
 orange-yellow. 430 
 
 EK\ 
 
 .carmine 
 
 reddish-brown 
 deep violet 
 
 7"P / \ 
 
 cy 2 \- 
 
 S^, indigo 
 
 blue 
 
 -H\ 
 r t \ 
 
 AHX- 
 
 grayish-blue 
 
 orange 450 
 
 
 
 bluish-green 
 pale-green 
 
 reddish-orange ..500 
 red 530 
 
 -F\ 
 
 7?\ 
 
 
 deep-red- -550 
 
 
 
 
 purple- 565 
 
 
 light-green 
 83 greenish-yellow 
 - golden-yellow 
 
 violet I 575 
 
 
 indigo. .589 
 
 -D\ 
 -CX 
 
 H%\- 
 
 sky-blue 664 
 
 _orange 
 
 greenish-blue 728 
 
 -B\ 
 
 
 brownish-orange 
 
 green 747 
 
 F%\- 
 
 light-carmine 
 
 green 800 
 
 -AX 
 
 -H2\ 
 
 E%\- 
 
 <-. 
 _purple 
 
 yellowish-green 850 
 yellow 910 
 
 
 violet 
 
 -G2X 
 
 
 
 indigo 
 
 orange. __. 948 
 
 
 "t_ _ dark-blue 
 
 orange-red 1000 
 
 -F2\ 
 
 C 3 / 2 X_ 
 
 greenish-blue 
 
 red 1060 
 
 -E2\ 
 
 9 
 green 
 
 violet-red 1100 
 
 A 3 / 2 X_ 
 G%\ 
 
 
 violet II 1128 
 
 
 y ello w i sh-green 
 
 indigo . 1151 
 
 F%.\- 
 
 .yellow 
 
 
 iR 2X 
 
 
 Fio. 96. The Interference Colors. 
 
76 PETROGRAPHIC METHODS 
 
 If it is assumed that the retardation of the two rays in a crystal 
 illuminated by polarized light amounts to only 50 ///*, Fig. 96, 
 a small phasal difference will be experienced by all colors upon 
 emerging from the crystal, and small but equal amounts of light 
 for each color will pass through the analyzer, and the impression 
 of a gray color is made between crossed nicols, +nicols. If the 
 retardation is a little greater, violet will be more prominent, giving 
 a lavender gray tint. The other colors follow in order, grayish- 
 blue, grayish-white, white and yellow, until when the retardation 
 is about 300 /*/*, or about half the wave length of very intense 
 yellow, that color is especially bright, while at the same time 
 extreme violet, /} = 380, is almost entirely extinguished. The 
 general impression is that of a brilliant yellow color. If these 
 interference colors are observed with a spectroscope, the yellow 
 part of the spectrum will appear brilliantly illuminated, but 
 diminishes toward both sides until where violet is reached no 
 light is visible. 
 
 If the retardation is about twice as great, or about 575 /*/*, 
 it equals the wave length of yellow, lying near the Na line. This 
 color will be destroyed, while a brilliant violet interferes with 
 nearly 1J ^ phasal difference, and appears with double inten- 
 sity, while at the other side of the spectrum some is visible. 
 The resulting interference color is a reddish-violet, violet I, Fig. 
 96. When studied spectroscopically the violet part of the 
 spectrum is seen to be strongly illuminated and a dark line ap- 
 pears in the yellow. Violet is the least luminous color in the 
 spectrum and is therefore greatly influenced by extremely small 
 changes in the retardation as shown in Fig. 96. It is called the 
 sensitive red or violet. 
 
 In the same manner the changes which white light undergoes 
 with different retardations can be calculated, and it is found 
 that blue follows violet, green follows blue, etc., until finally 
 violet is reached again, and the same order of colors repeated. 
 A series of colors from one violet to another is called an 
 order and distinction is made between first, second, third 
 orders, etc. 
 
 Let us consider more carefully a color of the third order, one 
 which results by a retardation of 1500 /*//. This is twice the 
 wave length of brilliant red, three times that of a blue-green, 
 and four times that of extreme violet. These colors will all be 
 extinguished, while yellow and blue will be intensified. The 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 77 
 
 total result is a yellowish tint, because of the greater intensity 
 of the yellow part of the spectrum. This color, however, appears 
 somewhat mixed when compared with the one above, and does 
 not give the impression of a pure yellow. If the retardation 
 amounts to about 5000 /*/<, it corresponds to 7 A for intense red, 
 8 X for orange, 9 X for yellowish-green, 10 X for green, 11 X 
 for blue, 12 X for bluish-violet, and 13 X for violet. These colors 
 will all be extinguished, while the intermediate red, yellow, green, 
 blue, and violet colors are intensified; when these colors are united 
 they form white. This interference color is called white of the 
 higher order in contradistinction to the white of the first order 
 produced by very slight retardations. It may be very difficult 
 for beginners to distinguish between these two white interfer- 
 ence colors. The white of the higher order produced by mani- 
 fold interference frequently has a delicate appearance similar to 
 mother of pearl, while white of the first order is generally mixed 
 with gray, green, or yellow. 
 
 The differentiation of the various orders of interference colors 
 is quite difficult for a beginner. In a normal case the colors of 
 the first order with their deep hard tones can be separated from 
 those of the second order, which are unusually brilliant, luminous 
 colors. In the third order the colors become duller and more 
 mixed, and farther up the brilliancy is rapidly lost, until in 
 the fifth and sixth orders white of the higher order is reached. 
 It may be held as a rule for the estimation of interference colors 
 under all conditions that the colors appear harder and purer the 
 lower they are. This difference will appear quite distinctly 
 even to an inexperienced eye by comparing the red of the first 
 and second orders. 
 
 The two nicols are generally crossed for observing the inter- 
 ference colors and only in comparatively rare cases is observation 
 between parallel nicols advisable. Such a case would be that of a 
 very low double refracting crystal, which is not very bright be- 
 tween crossed nicols, and the presence of double refraction can- 
 not be positively determined. Fairly distinct colors may be 
 observed then between parallel nicols, see Fig. 96. 
 
 Modification of the Interference Colors. In deriving the interference 
 colors above, it was assumed that the retardation of one ray over another 
 was the same for all colors, but this is by no means the case. Just as the 
 index of refraction is different for different colors, so the difference between 
 the indices, which produces the interference color in polarized light, varies. 
 
78 
 
 PETROGRAPHIC METHODS 
 
 This can be deduced from the table of the indices of calcite for various lines 
 of the spectrum. 
 
 A 1.650 
 
 B 1.653 
 
 D 1.658 
 
 F.. . 1.668 
 
 H.. 
 
 1.683 
 
 1.483 
 1.484 
 1.486 
 1.491 
 1.498 
 
 r -OL 
 0.167 
 0.169 
 0.172 
 0.177 
 0.185 
 
 The interference colors for various substances are not entirely the same, 
 although the mean retardation may be identical, because of the differences 
 in indices of refraction for the various colors. These differences are generally 
 not very large but in spite of that, the difficulty of observing the inter- 
 ference colors is considerably increased by the dispersion of the double 
 refraction even in ordinary cases. If that is the case, as in the example 
 already cited, calcite, and likewise in quartz and most of the other rock- 
 forming minerals, the double refraction for red is less than that for violet 
 and there appears according to Becke a brilliant supernormal interference 
 color of the first order. With greater dispersion it may attain the luminous 
 properties of the second order. If 7- a is greater for red than for violet, 
 which is more rarely the case, the interference colors are duller, subnormal. 
 Naturally this behavior often causes great inconvenience in the methods of 
 compensation to be described later. The various colors cannot be entirely 
 compensated because they have varying double refraction in various parts 
 of the spectrum. 
 
 It is possible that in the dispersion, the double refraction for some color 
 may be zero, so that, for one end of the spectrum the double refraction 
 will be positive and for tne other end negative, while for some intermediate 
 color it will be zero. The latter color will be extinguished and instead of a 
 
 ABC 
 
 7-CT + 0.001 0.0 -0.002 
 
 FIG. 97. Double Refraction of Fuggerite for Different Parts of the Spectrum. 
 
 normal gray or grayish-blue color of the lowest order, a deep color appears 
 that is complementary to the extinguished color. Such interference colors 
 are called abnormal in the narrower sense. 
 
 This is the case in the mineral fuggerite with low double refraction, Fig. 
 97. It is weakly positive for one end of the spectrum and weakly negative 
 for the other, but between the two, near the Na line, the double refraction 
 is 0. That color is always extinguished in polarized light by a plate of any 
 thickness, while the light emerging from the preparation combines to form 
 not a gray of the first order characteristic of low double refraction, but a 
 color complementary to the yellow, that is, a deep blue which becomes more 
 luminous the thicker the slide. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 79 
 
 The blue color can be seen only after prolonged observation in very thin 
 slides, but in thicker slides it may appear as a very brilliant Prussian blue. 
 An experienced eye can recognize at once the difference in shade between it 
 and a normal interference color. A beginner can easily recognize the ab- 
 normal property of the color by making use of the \ \ mica plate as a com- 
 pensator. Anomalous interference colors are never entirely compensated 
 by the plate, but are changed to a dirty brownish-gray when the vibration 
 directions of the mineral are crossed with respect to those of the com- 
 pensator. Under similar conditions indigo or blue of the second order, 
 which may appear similar to the abnormal color, are reduced to a brilliant 
 orange by a reduction of 150 /*/*, Fig. 91, so that the anomalous property 
 of the other color can be seen most distinctly. The same is true if the 
 equivalent directions are parallel in the two objects. The anomalous blue 
 changes to a dirty white, while the normal passes over into a brilliant 
 greenish-blue. Various complementary colors may occur as abnormal 
 interference colors according to the location in the spectrum of the zero 
 point of double refraction. 
 
 Another series of abnormal interference colors depends upon the dis- 
 persion of the vibration directions in monoclinic and triclintc crystals. In 
 cross sections of these minerals the vibration directions for different colors 
 differ from each other. In consequence of this all colors are not extinguished 
 at the same time upon rotating between crossed nicols. Instead of complete 
 extinction there is a change of colors from yellow to violet. The properties 
 of the interference colors are materially influenced by this dispersion when 
 the vibration directions are at 45 to those of the nicols. In sections nearly 
 perpendicular to an optic axis extremely abnormal colors may occur. 
 Such interference colors are called dispersion colors. It must also be 
 remembered that the interference colors are greatly modified by the color 
 of the mineral itself and by considerable differences of absorption in different 
 directions. 
 
 The larger the angle of aperture of the illuminating and observing systems 
 of the microscope the less pure will the interference colors appear. This is 
 partly because light passing through the lenses in a very oblique direction 
 is partially repolarized, and partly because light travels through the crystal 
 in very diverging directions and therefore suffers different retardations. 
 The polarization colors are mixed with each other and this naturally makes 
 exact determinations very difficult. When wide-angled systems are used, 
 the illuminating cone of light should be narrowed down for observations in 
 parallel polarized light as far as the source of light will permit. 
 
 Measurement of Double Refraction by Means of Interference 
 Colors. An interference color depends upon the difference of 
 retardation of one ray over another. In one and the same 
 substance it bears direct relation to the thickness of the plate. 
 Therefore, the color can be used to determine the double refrac- 
 tion or the thickness of a slide. The appended table shows the 
 thickness in millimeters which plates of various crystals, cut 
 parallel to the optic axis or to the axial plane, must have in 
 
80 PETROGRAPHIC METHODS 
 
 order to give a brilliant reddish-violet of the first order, the 
 retardation being 0.000575 mm. 
 
 y-a. Thickness in mm. 
 
 Mercurous chloride . 640 . 00086 
 
 Triphenyl benzol . 348 . 0016 
 
 Rutile and sulphur . 287 . 0019 
 
 Calcite 0.172 0.0032 
 
 Olivine 0.036 0.015 
 
 Quartz 0.009 0.06 
 
 Apatite . 004 0.14 
 
 Apophyllite . 001 . 55 
 
 A few of the more important rock-forming minerals show 
 the following maximum interference colors in a normal section 
 about 0.03 mm. thick: 
 
 a. Gray to grayish- white; pennine, vesuvianite, apatite, 
 zoisite, nepheline. 
 
 b. White to yellowish- white ; feldspar, quartz, serpentine, 
 topaz, cordierite, enstatite, andalusite, clinochlore, gypsum. 
 
 c. Yellow to orange; hypersthene, wollastonite, cyanite. 
 
 d. Red I to indigo; tourmaline, sillimanite, glaucophane. 
 
 e. Blue to green; augite, hornblende. 
 
 /. Yellow to orange; chondrodite, olivine. 
 
 g. Red II to blue; mica, segirine, etc. 
 
 If the section is . 04 mm. thick most of the minerals under b 
 above will be yellow to orange and the interference colors of the 
 others will be raised accordingly, mica, for example, assuming 
 a dull color of the fourth order. If on the other hand the thick- 
 ness of the slide is only 0.02 mm. all the minerals under b will 
 have a dull gray interference color of the lowest order, and the 
 color for mica will scarcely be as high as the middle of the 
 second order. In either case the interference color is of no use 
 in determining the mineral. 
 
 Various methods can be used to determine the double refrac- 
 tion of a crystal by means of the interference colors observed 
 between crossed nicols. These methods may depend in part 
 upon a comparison of the interference color of an unknown, 
 with that of a known crystal, and in part upon the compensation 
 of the double refraction by the use of the compensators to be 
 described later. Each of the latter shows a series of interfer- 
 ence colors so placed that they can be compared directly. It is 
 necessary to know accurately the thickness of the slide and the 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT SI 
 
 orientation of the cross section for all methods of measuring 
 the double refraction by means of the interference colors. 
 
 Approximate determinations are the simplest and give results 
 most rapidly. For this purpose the interference color of the 
 object is compared with a table of interference colors such as the 
 one devised by Michel-Levy and Lacroix. After a little practice 
 very good results may be obtained with it. 
 
 Somewhat more accurate determinations can be made with the quartz 
 wedge comparator, likewise constructed by Michel-LeVy. A cross section 
 of it is shown in Fig. 98. It is attached to the side of an ocular O. The 
 light L is reflected into it by the mirror S, and after total reflection by the 
 prism C, passes through the quartz wedge G between two nicols A and P. 
 The quartz wedge can be moved by means of a screw. The light finally 
 passes into a prism R, the reflecting face of which is silvered, and is totally 
 reflected into the field of vision of the ocular. There is a small, round, 
 
 
 \ 
 L 
 
 FIG. 98. Quartz Wedge Comparator devised by Michel-Levy. 
 
 unsilvered spot in the center of the latter prism and upon this, the small 
 prism R' is cemented. Through this opening the interference color of the 
 crystal M lying under the objective is observed while the rest of the field, 
 which receives its light from the totally reflecting prisms, shows the inter- 
 ference color of that portion of the quartz wedge which is in the path ,of the 
 rays. A circular area is observed in the field of the ocular, which shows 
 the interference color of the mineral lying upon the stage and this is sur- 
 rounded by a ring of the interference color from the quartz wedge. The 
 latter can be made to agree with the color of the inner area by moving the 
 wedge. The thickness of the quartz wedge at this particular point can be 
 observed on a scale and the order of the interference color determined 
 directly. 
 
 This apparatus is very interesting, but is only applicable in exceptional 
 cases. The modifications which the interference colors undergo, (1) by the 
 varying double refraction for different colors, (2) by the different indices of 
 refraction of the crystal, and (3) by its transparency are sufficient to make 
 
82 , PETROGRAPHIC METHODS 
 
 an absolute comparison of the interference colors very difficult. In addition 
 there are a number of obstacles in the determination of the interference 
 colors of which the difference in the illumination of the objects to be 
 compared may be mentioned, so that this method can be used advantage- 
 ously with colorless minerals only. The thickness of the slide can be 
 measured as it is being ground, by placing small basal cleavage pieces of 
 barite on each of the four corners of the slide and grinding them down 
 with it. The interference color of the barite can be used to indicate the 
 thickness, provided the surface is not clouded as is very often the case. 
 
 This determination of thickness is very accurate when the interference 
 colors are gray or grayish-blue of the lowest order. It has been proposed in 
 such cases to place a horizontal mirror directly under the preparation. 
 The light would then be returned through the analyzer by means of a mirror 
 in the tube above it and through the slide to the mirror below, from which 
 it would be reflected a second time through the slide and analyzer, before 
 it reaches the eye, and the impression would be that a plate of double the 
 thickness was being observed between parallel nicols. Even in such cases 
 more reliable results are obtained by means of compensators. It is to 
 be noted that most of the minerals show low gray colors in sections with an 
 approximate thickness of 0.02 mm. and even more so with a thickness of 
 0.01 mm. It is advisable for ordinary investigations not to have the 
 slide ground too thin. 
 
 The determination of the double refraction in thin sections by interference 
 colors is not very accurate, because of the fact that the section is accurately 
 orientated only in exceptional cases. To determine the difference in 
 velocity of the two rays in an optically uniaxial mineral, it is necessary to 
 use a section cut absolutely parallel to the optic axis because the true value 
 for the extraordinary ray can be obtained only in such sections, and in like 
 manner the true value of ? a in biaxial substances is seen only in a section 
 parallel to the plane of the optic axes. In every case in which an accurate 
 determination is to be made, the orientation of the cross section must be 
 accurately established. 
 
 Interference Colors in Variously Orientated Cross Sections. 
 
 Sections parallel to the optic axis show the highest interference 
 colors produced by a uniaxial mineral. The colors become 
 lower the greater the inclination of the section to the optic axis, 
 until with an inclination of 90 there is no effect on parallel 
 polarized light whatever. Sections of optically uniaxial minerals 
 cut perpendicular to the optic axis behave in perfectly, parallel 
 polarized light as isotropic substances. However, more or less 
 convergent rays pass through microscopic preparations, and 
 if the double refraction is high, or the thickness of the slide con- 
 siderable, these rays passing obliquely through the sections 
 produce considerable illumination of the image, because they 
 do not traverse the crystal in the direction of its isotropic axis. 
 Hence, sections of uniaxial minerals perpendicular to the optic 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT S3 
 
 axis are weakly illuminated and remain so upon the rotation 
 through 360. This corresponds entirely to the phenomenon, 
 which biaxial crystals cut perpendicular to an optic axis show, 
 except, as will be pointed out directly, the cause of the illumi- 
 nation in the latter case is for the most part different. 
 
 Sections of biaxial crystals parallel to the plane of the optic 
 axes show the highest interference colors of all, because here the 
 maximum and minimum velocities enter into consideration. 
 In a section perpendicular to the acute bisectrix, the inter- 
 ference colors are lower the smaller the angle between the optic 
 axes. In any case the interference color of such a section is 
 lower than that of a section perpendicular to the obtuse bisec- 
 trix, the interference colors of which more nearly approach those 
 of a section parallel to the axial plane the larger the obtuse 
 optic angle. In a section perpendicular to an optic axis, a ray 
 of light is resolved into an infinite number of rays lying upon the 
 surface of a cone each polarized in a different direction. This 
 phenomenon is called interior conical refraction and the result is 
 no interference color, but equal illumination. Such sections of 
 biaxial crystals appear light between crossed nicols and remain 
 uniformly light upon a complete rotation of the slide through 
 360. This phenomenon is less distinct the thinner the slide 
 and the lower the double refraction of the crystal. It can 
 scarcely be recognized at all in thin sections of very feebly 
 double refracting crystals, which appear uniformly dark in 
 parallel polarized light. 
 
 Chromoscope for Interference Colors. The accurate determi- 
 nation of the interference colors is one of the most important pro- 
 cesses in the technic of a polarizing microscope. Even the 
 beginner must be able to do it in as positive a manner as possible. 
 The differentiation of the various orders of color is quite difficult, 
 particularly in the first three orders, which occur most frequently 
 and this difficulty can be overcome only by careful practice and 
 the repeated observation of interference colors, which can be 
 easily determined. Various methods have been used to present 
 the subject of interference colors in as objective a manner as 
 possible. At first a color table was prepared, which corresponded 
 more or less with the natural colors and showed an arrangement 
 of the important series of colors from the first to the fourth 
 orders. Even in the most favorable cases a poor idea of the 
 actual conditions could be obtained from it, because subjective 
 
84 PETROGRAPHIC METHODS 
 
 and technical factors played a great r61e in the preparation of 
 the colors, which made an approximation to the true properties 
 of the original very difficult. The ink used generally constitutes 
 another bad factor, for aniline dyes are always used for this 
 purpose, but they fade in a comparatively short time, and thus 
 make the general impression even more inaccurate. The excel- 
 lent reproductions of interference colors in the table of bire- 
 fringences by Michel-Levy and Lacroix lose their true value for 
 instructional purposes in a comparatively short time. 
 
 The interference colors are a simple function of the thickness 
 of a crystal, the double refraction being constant. Therefore, 
 a thin wedge of a homogeneous crystalline substance, observed in 
 white parallel polarized light, shows the colors in parallel bands 
 beside each other analogous to a table of colors. Such a wedge 
 usually made of quartz generally accompanies a polarizing micro- 
 scope as a compensator. With a length of 4-5 cm., it shows the 
 first four orders of colors which are the principal ones to be 
 distinguished. Above these the interference colors so nearly 
 approach white of the higher order that they can scarcely be dis- 
 tinguished even by a skilled observer. If such a wedge is 
 pushed slowly across the field in such a position that its vibra- 
 tion directions are at 45 to those of the nicols, the various 
 orders of color can be observed in succession. Only a small 
 portion of the series can be seen at one time because of the 
 narrowness of the field of vision of the microscope, and hence, 
 analogous colors of the various orders cannot be observed 
 simultaneously so as to be compared with each other, the 
 differentiation of which is of great importance. Smaller wedges 
 of this character could be made, which, when observed with a 
 low power objective would show the first four orders of color in 
 the field at once. Instead of such a wedge, a wedge-shaped 
 crystal of ammonium metavanadinate, which is quit easily 
 prepared, could be used to bring the interference colors beside 
 each other in distinct bands in the field of vision. Careful study 
 of a single color is interfered with by the narrowness of the bands 
 in the field, and also by the fact that analogous colors of the 
 various orders are separated by bands of different colors. 
 
 An apparatus devised by Weinschenk, called a chromoscope for 
 interference colors, has been found to be very useful in these 
 fundamental studies. It consists of a glass plate a, Fig. 99 ; as 
 a polarizer and an arm b attached at the angle of polarization of 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 85 
 
 the glass and carrying a nicol prism d that can be rotated and used 
 as an analyzer. The arm also bears a lens e with a two fold mag- 
 nification and so arranged that it can be thrown out of the line 
 of vision. The holder c is so placed that its front surface corre- 
 sponds to the normal to the direction of polarization. A quartz 
 wedge 8 cm. long and 3 cm. wide is placed in the holder with its 
 
 FIG. 99 Chromoscope for Interference Colors. 
 
 vibration directions at 45 to the edges of the holder. Then a 
 mica plate of the same size is placed under it with its equivalent 
 directions crossed so that with it, demonstrations can be made 
 with the lowest interference colors. With this arrangement, the 
 first four orders of colors from low gray to red of the fourth order 
 are obtained in regular, broad, parallel bands. By inserting the 
 lens the magnification is doubled, but two orders of colors can 
 still be seen simultaneously. Two plane parallel gypsum plates, 
 
 FIG. 100. Quartz Wedge in Chromoscope with Parallel Orientated Gypsum Plates Red I 
 
 and Red II. 
 
 each 8 cm. long but only 1 cm. wide, with vibration directions at 
 45 to the edges, accompany the apparatus. These plates alone 
 yield red of the first and second orders. The two gypsum 
 plates are then placed beside each other on the quartz wedge so 
 that their equivalent directions are parallel, the lower plate show- 
 
86 
 
 PETROGRAPHIC METHODS 
 
 ing red of the second order, the upper red of the first order, as 
 shown in Fig. 100. From the top downward, bands of red of the 
 first, second and third orders, each 1 cm. wide, are in contact 
 with each other and can be compared directly. Thus the phe- 
 nomenon of the addition of double refraction can be studied. 
 Fig. 101 shows the gypsum plates rotated through 180 from 
 which the compensation of the double refraction can be derived. 
 The numerals in Figs. 100 and 101 indicate the order of the colors 
 in their respective positions. The longest arrow shows the di- 
 rection of the fastest ray in the quartz wedge, the smaller ones 
 being used for the gypsum plates. 
 
 FIG. 101. Quartz Wedge in Chromoscope with Oppositely Orientated Gypsum Plates 
 
 Red I and Red II. 
 
 Interference colors that occur in wedge-shaped layers within a slide not 
 infrequently serve to determine the order of the color in microscopic study 
 of thin sections and isolated crystals. They often aid materially in denning 
 the crystal form. Wedge-shaped individuals may be formed in thin section 
 (1) by the flaking off of a mineral with good cleavage, (2) by the edge of the 
 individual being quite oblique to the surface of the slide, and (3) by the edge 
 of the slide itself being somewhat thinner, which is generally the case. The 
 interference color in the principal part of the section can be determined by 
 beginning in the thinnest place where the color is lowest and counting the 
 bands in order, i.e., noting how often red is repeated. In the case of oblique 
 twinning lamellae of very strong double refracting minerals, such color bands 
 produced by wedge-shaped sections often lead to the erroneous conclusion 
 that there is a whole series of such lamellae, when in fact there is but a single 
 one cut across very obliquely. An example of this phenomenon is shown 
 in Fig. 102, which represents an augite crystal cut very obliquely through a 
 simple twin. 
 
 Interference colors on the wedge-shaped outer portions of isolated crystals 
 are frequently of importance in determining the crystal form, as for ex- 
 ample, the bands around the edge of an artificial crystal of anglesite, Fig. 
 103, from which the form can be easily observed. They also aid in the 
 approximate determination of the double refraction. If these bands are 
 very close to the edges of a crystal that are quite flat, it is evident that the 
 substance has high double refraction, etc. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 87 
 
 Crystals are found now and then which do not extinguish at all when 
 rotated between crossed nicols, but show a change in interference colors. 
 In a very few cases this may depend upon a strong dispersion of the vibra- 
 tion directions or upon the fact that the crystal is twinned and the twinning 
 plane is approximately parallel to the plane of the stage of the microscope 
 in which case the vibration directions of the two individuals will be oblique 
 to each other. When the vibration directions of one individual are parallel 
 to those of the nicols, the interference color of the other is seen and vice 
 versa, while in intermediate positions transition colors are present. These 
 two cases may be distinguished by observation in monochromatic light 
 when there will be complete extinction if the cause of the change of color 
 is based upon dispersion of the vibration directions, but there will be no 
 extinction in the case of overlapping twins. 
 
 FIG. 102. Simple Twinning Lamellae FIG. 103. Artificial Crystal of Lead Sulphate 
 in Augite. between Crossed Nicols. 
 
 Character of the Double Refraction. The interference color 
 is dependent upon the retardation of one ray over the other in its 
 passage through a double refracting crystal. In a plate cut par- 
 allel to the optic axis of a negative uniaxial mineral the extra- 
 ordinary ray, vibrating parallel to the optic axis, exceeds by a 
 definite amount the ordinary, vibrating perpendicular to it and 
 is refracted less. If a second crystal plate is laid upon the first 
 in parallel position, that ray, which in the first traveled with 
 the greater velocity and less refraction, will likewise be the faster 
 and less refracted ray in the second, while the other is propagated 
 in each case with a smaller velocity and greater refraction, Fig. 
 104. The phasal difference between the rays upon emerging 
 from the second crystal will be equal, to the sum of the retarda- 
 tions in each. The double refraction and with it the interference 
 
88 
 
 PETROGRAPHIC METHODS 
 
 color of two crystals are added together when equivalent vibra- 
 tion directions lie parallel. If one of the crystals is rotated 90 
 or if, as represented in Fig. 105 ; an equivalent plate of a pos- 
 itive crystal is used instead of a second negative one, that ray 
 which travels in the first with the greater velocity and smaller 
 refraction will be propagated in the second with a smaller velocity 
 and greater refraction, while the reverse holds for the other ray. 
 The phasal difference, which the two rays show upon emergence 
 from this combination, will be equal to the difference of the re- 
 
 FIG. 104. Addition FIG. 105. Compensation 
 
 of the Double Refraction. 
 
 tardations in each. The double refraction and with it the inter- 
 ference color of two crystals is compensated when equivalent 
 vibration directions are crossed. 
 
 If the retardation in one plate is equal to the retardation in 
 the other a combination of them in crossed position will act 
 as an isotropic body in parallel polarized light, i.e., the double 
 refraction of one entirely nullifies the double refraction of the 
 other, as respresented in Fig. 105. 
 
 Compensators. The use of compensators depends upon the 
 phenomena described above. They serve to determine the char- 
 acter of the double refraction by raising or lowering the inter- 
 ference color and sometimes an accurate determination of the 
 interference color can be made by perfect compensation. The 
 most important compensators are: 
 
 1. Quarter undulation mica plate. 
 
 2. Plates with reddish-violet I, II, etc. 
 
 3. Fedorow mica wedge. 
 
 4. Quartz wedge. 
 
 5. Birefractometer. 
 
 6. Babinet compensator. 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 89 
 
 7. Twin compensator. 
 
 8. Biot rotating quartz. 
 
 9. Wright wedge. 
 
 Of these accessories 1 and 2 accompany the simplest polarizing micro- 
 scopes, while the others are used more for special investigations. Numbers 
 5-7 are generally placed in a special ocular from which they can be removed 
 by a screw. When they are used, the nicol in the tube must be thrown out, 
 which is then replaced by an analyzer placed over the ocular. The others 
 are inserted in a slot in the tube just above the objective or in a slot above 
 the ocular in the older models where the analyzer was placed on top. The 
 slots are sometimes arranged so that the long edge of the plate lies at 45 
 to the vibration directions of the nicols and sometimes so that they are 
 parallel. In all cases, however, the vibration directions of the compensators 
 must form an angle of 45 with those of the nicols, hence they are con- 
 structed differently according to the different modifications of the instru- 
 ments. In the first case the vibration directions are parallel to the edges 
 
 Q 
 
 FIG. 106. Parallel. FIG. 107. Diagonal. 
 
 Compensator with Vibration Direction. 
 
 of the rectangle and the direction of greatest or of least elasticity may be 
 parallel to the long edge. Formerly the quarter undulation plates were 
 nearly always made with the direction of the smallest velocity corresponding 
 to the long edge, and the other plates with reversed orientation, Fig. 106. 
 At the present time the quarter undulation plate is also orientated in this 
 jvay, in order to obviate the great confusion which arises from the use of 
 the other plates. It is still better to construct them all as shown in Fig. 107, 
 where the vibration directions form an angle of 45 with the edges of the 
 plate. The direction of greatest velocity a is marked on it with a diamond 
 point. It has the added advantage in that it is only necessary to invert the 
 plate to bring about an exchange of the principle vibration directions while 
 the crystal remains stationary. 
 
 Very frequently complete compensation cannot be obtained on account 
 of the dispersion of the double refraction in different substances, which 
 may be very troublesome for accurate determinations of the double re- 
 
90 
 
 PETROGRAPHIC METHODS 
 
 fraction. In crystals with distinct super- or sub-normal interference colors, 
 measurements cannot be made very exactly. Although compensators 
 can be made from any colorless, double refracting material, gypsum, mica 
 and quartz are used in general for this purpose. 
 
 1. A quarter undulation mica plate is a thin cleavage piece 
 of colorless mica which causes a retardation of both rays of 
 about 1/4 wave length for sodium light, i.e., about 150 /*/*. 
 Observed between crossed nicols it appears grayish-blue of the 
 first order, and between parallel nicols brownish- white. In many 
 instances plates which cause a retardation of only 1/8 A are 
 of good service. The quarter undulation mica plate is used to 
 orientate the other compensators. It shows a biaxial inter- 
 ference figure in convergent light. Since it is optically negative 
 the direction of the axial plane corresponds to the direction of 
 least elasticity. 
 
 2. The compensators with reddish-violet I, II, etc., have al- 
 ready been described under the head of stauroscopes. Their 
 additional use as compensators justifies repetition at this time. 
 
 FIG. 108. Mica Wedge. 
 
 Violet I is generally used to determine the character of the double 
 refraction, while violet II and plates with higher interference 
 colors are used with very strong double refracting substances. 
 They are generally prepared from gypsum or quartz. 
 
 3. The Fedorow mica wedge, Fig. 108, consists of a pile of 16 
 plates of mica of equal thickness. Each plate produces a retarda- 
 tion of 1/4 ^ and is shorter by a definite amount than the one 
 next under it. With this apparatus the first four orders of 
 color appear in 16 divisions and each division can be made s<5 
 wide that the field of vision of the microscope shows a single 
 homogeneous interference color at any given time. 
 
 4. The quartz wedge is a small plate of quartz cut in the 
 shape of a wedge and the retardation at various places is shown 
 by a scale. It is inserted in the slot above the objective until 
 complete darkness or, still better, the sensitive violet I is obtained 
 by means of compensation. The thickness of the quartz is read 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 91 
 
 off at this point, from which the double refraction of the crystal 
 can be calculated. 
 
 5. The birefractometer consists of an ocular within which 
 a quartz wedge can be moved by means of a rack and pinion. A 
 micrometer scale is etched upon the glass strip that covers the 
 wedge. From it the thickness of that portion of the wedge in use 
 can be read directly. It is still better to have the quartz wedge 
 so made that only one half of the field of vision is colored, while 
 the scale is placed upon the other half. With this arrangement 
 the apparatus can be used not only as a compensator but also 
 as a comparator in which case the crystal is placed in the vacant 
 half of the field and its interference color is compared with the 
 color of the movable quartz wedge. 
 
 6. The Babinet compensator consists of two thin quartz 
 wedges with equal refracting angles, lying one on the other, Fig. 
 109. In one wedge the optic axis is parallel, in the other perpen- 
 dicular, to the sharp edge. The 
 
 upper wedge is not movable but 
 the lower one can be moved by 
 a rack and pinion, and the 
 
 amount Of the movement Can FIG. 109.-Babinet Compensator. 
 
 be read on a drum. In the 
 
 zero position there is a dark band in the center of the field 
 marked by an oblique cross hair, Andreas cross hair, because 
 both wedges are of equal thickness at that place and since their 
 equivalent directions are crossed their double refraction is entirely 
 compensated. On either side there is a symmetrical series of 
 bands with an increasing order of interference colors. When a 
 double refracting plate is inserted the dark band is removed from 
 the middle of the field. The movable wedge is then moved in by 
 means of the screw until the band lies again exactly in the middle 
 and the amount of displacement is read on the drum. If the crystal 
 is rotated 90 the black band is removed toward the opposite side. 
 It is again placed on the cross hair and the mean of the two 
 readings is indicative of the double refraction of the crystal. 
 An approximate adjustment of the central band should be made 
 in white light, because in monochromatic light dark bands occur 
 in place of the colored ones, but the fine adjustment should be 
 consummated in sodium light. The double refraction of the plate 
 
 2 T tg tynn 
 in question can be calculated form the formula y a = 
 
92 PETROGRAPHIC METHODS 
 
 in which r is the amount of displacement for one rotation of the 
 screw, $ is the angle of the wedge of the comparator, m is the 
 number of rotations of the screw, and d is the thickness of the 
 plate under investigation. 
 
 7. The twinning compensator can be described most easily by 
 calling it a Babinet compensator which is cut through the center 
 parallel to the direction of the long arrow in Fig. 109. One half 
 is rotated on the fresh cut through 180 with respect to the 
 other, and each portion is left movable in opposite directions. 
 In the normal position there is a dark band across the entire 
 field of vision corresponding to the complete compensation of 
 the double refraction in the two oppositely orientated portions 
 of the wedge. If a double refracting crystal is placed in the 
 
 path of the rays the band recedes just 
 as far toward the right in one half of the 
 field as it does toward the left in the 
 other half, and the amount of displace- 
 
 FIG. 110. Bravais Twin Com- 
 pensator, ment necessary to unite both parts of 
 
 the band again indicates very accurately the double refraction. 
 Bravais constructed another type of twinning compensator, 
 Fig. 110. The two parts, rotated through 180 to each other, 
 were not placed side by side but one over the other. Then the 
 whole field of vision is dark in the normal position because the 
 double refraction of the oppositely orientated portions of the 
 wedge is compensated completely. When a double refracting 
 substance is placed in the path of the light, darkness is restored 
 by moving one of the wedges. The displacement of the wedge 
 is indicative of the double refraction but it is by no means as 
 accurate as in the previous case. 
 
 8. The Biot rotating quartz is also a very useful device for 
 determining the character of the double refraction. A plate of 
 quartz perpendicular to the axis is ground so thin that circular 
 polarization is no longer apparent. This is set in a holder so 
 that it can be inserted in a slot above the objective and rotated 
 on a horizontal axis. - In the normal position it produces no 
 change but as soon as it is rotated, the vibration direction of 
 the ordinary ray being the axis of rotation, it becomes double 
 refracting. Any amount of compensation can be obtained by 
 more or less rotation. 
 
 9. The simple wedges cannot always be used advantageously 
 because the lowest interference colors cannot be perceived on 
 
OBSERVATIONS IN PARALLEL POLARIZED LIGHT 93 
 
 account of the thinness of the wedge. Wright therefore 
 fastens a gypsum or quartz wedge on an oppositely orientated 
 plane parallel plate of gypsum which gives violet I. By the 
 subtraction of one order of colors the lowest colors of the first 
 order, beginning at the zero point, are obtained instead of those 
 bordering violet I. 
 
 Use of Compensators. To use any of these compensators the crystal 
 under investigation is centered and its vibration directions placed at 45 
 to those of the nicols. Except when using a Babinet or a twinning com- 
 pensator, the uses of which were described above, the interference color of 
 the object is first observed and then the compensator inserted and the 
 direction of change in the interference color noted, i.e., whether the inter- 
 ference color of the combination corresponds to the sum or the difference 
 of the retardations in the crystal and in the compensator. If, for example, 
 an optically uniaxial negative crystal with a prismatic development is 
 orientated on the stage of the microscope so that its long direction coincides 
 with the direction of greatest velocity in the compensator, the interference 
 color increases. It appears as if the crystal had become thicker. If the 
 conditions are reversed, the interference color is lowered as if the layer 
 were now thinner. Those crystals, which show very low interference colors, 
 are apparently an exception to this rule. For example, a crystal that 
 shows gray-blue of the first order, when compensated by violet I in crossed 
 position, shows a brilliant orange-yellow interference color, which never- 
 theless indicates subtraction because that is lower than violet I. This 
 can be best shown by rotating the crystal 90. Then a higher blue of the 
 second order appears. In general it is not known from simple observations 
 in parallel polarized light that the crystal observed is uniaxial and pris- 
 matically developed. That the direction of greatest velocity is parallel to 
 its long edge, its principal zone, is all that can be determined. The de- 
 termination of the relative velocities of the two rays vibrating in a cross 
 section is designated as determination of the optical character of the principal 
 zone. 'That direction in which a crystal or crystal section is elongated is 
 taken as the principal zone. The principal zone is negative when the direc- 
 tion of greatest elasticity, and positive when the direction of least elasticity 
 is parallel to the long edge. 
 
 The determination of the optical character of the principal zone is very 
 useful in the determination of crystals. Its value can be best appreciated 
 for the study of thin sections by comparing the optical character of the 
 principal zone with the true optical character of the crystal. This 
 indicates the crystallographic habit, the position of the optic plane, and so 
 forth. 
 
 The example cited above shows that, in an optically uniaxial crystal in 
 which the principal axis corresponds to the principal zone, the optical 
 character of the latter is the same as the optical character of the crystal. 
 If a uniaxial negative crystal is tabular parallel to the base, a transverse 
 section will show a decided elongation but the principal axis is perpendic- 
 ular to it, i.e., the direction of smallest velocity is parallel to the principal 
 
94 
 
 PETROGRAPHIC METHODS 
 
 zone and its character is positive, 
 possibilities. 
 
 The following table shows the various 
 
 Character of crystal 
 + or - 
 + or - 
 
 + or - 
 + or - 
 + or - 
 
 Uniaxial 
 
 Prismatic habit 
 
 Tabular habit 
 
 Biaxial 
 
 Prismatic; Bx |j principal zone 
 Tabular; Bx J_ tabular face 
 Prismatic; Bx || principal zone 
 Tabular; Bx^j J_ tabular face 
 Prismatic; optic normal J_ principal zone 
 Tabular; optic normal I tabular face 
 
 Character of principal zone 
 + or -. 
 - or +. 
 
 - or +. 
 
 . 
 
 The character of the double refraction of the principal zone varies in 
 different sections of a biaxial substance when the vibration direction in 
 that section of the zone is the optic normal. 
 
 The determination of the character of the principal zone is frequently 
 impossible, either because an actual characteristic zone cannot be recognized 
 or because the interference color is white of the higher order. In the former 
 case it is impossible to speak of a principal zone, while in the latter, other 
 methods must be used. Either the wedge-shaped borders of the crystal 
 or slide are used and the ordinary methods of compensation applied to the 
 brilliant bands of color, or compensators corresponding to a higher order of 
 color can be employed, and by subtraction brilliant interference colors of 
 lower orders produced. 
 
CHAPTER V 
 Observations in Convergent Polarized Light 
 
 Direction of the Rays in Convergent Light. It is important 
 in many cases to study the optical properties of a crystal or 
 crystal section not only in one direction but in as many different 
 directions as possible. Either convergent polarized light is used 
 for this purpose, by means of which the optical properties of a 
 crystal can be observed in various directions at one time, or a 
 rotating apparatus, described in the appendix, is used and the 
 different directions studied in sequence. 
 
 Fig. Ill shows diagrammatically the passage of the rays in 
 convergent light. The lenses L and I/ are so arranged that the 
 upper focus of L coincides with the lower focus 
 of L' at the point /. The crystal K to be investi- 
 gated is placed between the two lenses. Each 
 of the illuminating cones of light with its apex 
 in the lower focal plane F of the lens L is con- 
 verted by that lens into a bundle of parallel rays, 
 and these bundle's emerge from the lens with all 
 possible inclinations to the axis of the lens within 
 its angle of aperture. These parallel rays pass 
 through the object between the lenses and are 
 reunited in points in the upper focal plane F' of 
 the lens L'. 
 
 Every point in the image, thus produced, 
 corresponds to a direction in the crystal. In 
 convergent light the optical phenomena, which an object shows 
 in all the directions in which light is propagated through it, are 
 observed side by side. The different retardations which the 
 light has experienced in the various directions in a crystal 
 cannot be observed without the aid of other devices, because 
 the eye is not properly adjusted and, hence, nothing at all is 
 seen in convergent light until such apparatus is used. 
 
 If instead of ordinary light polarized light produced by the 
 polarizer P and the analyzer A is used, interference phenomena 
 become visible at each point of the image when double refracting 
 
 95 
 
 F 1~ -i 
 
 P 
 Fia. 111. 
 
 Convergent Light. 
 
96 
 
 PETROGRAPHIC METHODS 
 
 FIG. 112. Field of Vision in Convergent 
 Light. 
 
 crystals are studied. This corresponds to a retardation of one 
 ray over the other upon passing through the crystal in that 
 particular direction. The interference figure thus obtained is 
 one of the most important means of determining the optical 
 properties of a crystal. 
 
 It must always be kept clearly in mind that in observations 
 in convergent light any given portion of the interference figure 
 
 does not correspond to a definite 
 point on the crystal but rather to 
 some direction in it. The image 
 is not changed, if a homogeneous 
 crystal is moved parallel to itself 
 on the stage. It is, in general, 
 altered immediately if the direc- 
 tion of observation is changed by 
 rotating the crystal on an axis. 
 
 The simple lenses L and I/ are 
 now replaced by combinations of 
 plano-convex lenses. The special 
 case here considered is one in which a converging lens is placed 
 over the polarizer and L' is replaced by any objective. 
 
 The circle AB, Fig. 112, represents the field of vision in convergent 
 polarized light. For the sake of simplicity, the angle of aperture may be 
 taken equal to 90. Rays, which have left the crystal plate with an in- 
 clination of 45 to the vertical, emerge around the outer edge of the field. 
 Assuming the index of refraction /? for the crystal to be 1.60, it follows 
 
 from the law of sines, sin V = that the rays at the edge of the image 
 
 P> 
 
 have passed through the crystal at an angle of 26 1 /2 to the vertical, or the 
 entire field includes a cone of light in the crystal with an angle of 53. If a 
 smaller circle is taken in the field of vision, for example, one with 2/3 the 
 diameter, CD, it embraces all rays which emerge from the crystal under an in- 
 clination of about 28 I/ 2 to the vertical or pass through the crystal at 172/3, 
 Fig. 113. Then, if a circle with only 1/3 the diameter of the field is taken, EF, 
 it includes those rays which leave the crystal at an angle of 13, and are 
 propagated through it under an angle of about 9. Finally, in the center of 
 the image O, Fig. 112, the optical phenomena correspond to those rays 
 which pass through the crystal plate exactly vertically. From the above, 
 it appears that with an objective with a definite angle of aperture the cone of 
 rays passing through a crystal will be greater the smaller the value of ft, the 
 denominator of the equation for the law of sines. Feebly refracting minerals 
 give a much more extensive perspective of their optical properties than 
 those with higher indices of refraction. The value of /? can be greatly 
 reduced by increasing the aperture by means of an immersion system. 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 97 
 
 Methods of Observation in Convergent Polarized Light. The 
 
 simplest method to pass from the observation of the object to 
 that of the interference figure consists in removing the ocular 
 and observing the interference phenomena, RS, Fig. 113, 
 produced by the objective alone. It consists of a small image 
 visible in the upper focal plane of the objective. This is the 
 Lasaulx method. Inversion of the image does not occur in this 
 
 FIG. 113. A Biaxial Interference Figu-e. 
 
 method of observation. The observed interference figure has 
 the same position as the object, but is inverted with respect to 
 the microscopic image. The rays which have passed through 
 the crystal from left below to right above appear in the image on 
 the right. Images thus produced are generally very small but 
 sharply defined. They can be magnified but a part of the dis- 
 tinctness is lost by the process. A low power microscope can 
 
 7 
 
98 
 
 PETROGRAPHIC METHODS 
 
 be obtained by using the ocular A, Fig. 113, and the Bertrand 
 lens B, inserted in the tube as an objective. This can be focused 
 on the small interference figure by elongating the tube and it 
 gives a magnified image R"S", which is observed in the ocular. 
 The image thus observed is inverted with respect to the object 
 and has the same position as the microscopic image of the object. 
 The proper centering of the Bertrand lens must always be checked first 
 by placing a uniaxial crystal, tabular parallel to the base, in the micro- 
 scope. If the position of the lens is correct, the dark cross, described below, 
 must coincide with the cross hairs of the ocular. The interference figure 
 can also be magnified by placing a Klein lens above the ocular. This lens 
 can be moved in a vertical direction in its holder. The magnified image 
 in this case is likewise inverted with respect to the object, but its position 
 coincides with the microscopic image of the object. A real image R'S', 
 adapted for projection and photography, is obtained by using the Bertrand 
 lens without the ocular, or by using a projection ocular. This image is 
 likewise inverted with respect to the object. 
 
 The interference figure observed appears smaller the higher the 
 power of the objective used. More convergent bundles of rays 
 are obtained from stronger objectives, because in general the 
 aperture of the system increases with the magnification. The 
 interference figure obtained with a high power objective, allows 
 directions at greater inclinations to each other to be studied, 
 
 II 
 
 (Homogeneous 
 
 Immersion) 
 
 M. 
 
 OOa 
 
 n-\ 
 
 n=l 
 
 w = l 
 
 1.515 
 
 2w = 8 2w=30 2w = 74 2w = 128 2tt = 118 
 
 FIG. 114. Angle of Aperture of the most Important Objectives of W. and H. Seibert. 
 
 provided the aperture of the illuminating apparatus is great enough for that 
 of the objective. For observations in convergent light, the condenser 
 must always be in position and high power objectives 1 used. 
 
 Cross sections of the various objectives made by W. and H. Seibert are 
 shown in Fig. 114 in which 2u is the angle of aperture. Fig. 115 shows 
 the corresponding interference figure of each in an optically biaxial mineral 
 with an apparent optic angle of about 85. 
 
 If the aperture of the illuminating apparatus is not sufficiently large for 
 
 1 When such objectives are not available convergent light can be produced by a small 
 air bubble in the Canada balsam or by a soap bubble, which takes the place of the lens O. 
 The interference figure so produced appears magnified in the microscope. 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 99 
 
 the objective, only the inner part of the image will be illuminated and the 
 border will appear dark. If one of a large number of minute individuals 
 is to be studied in convergent polarized light, or if a crystal consists of many 
 small twinned individuals, the optical nature of which is to be determined, 
 an ocular diaphragm should be used. With it only the light passing through 
 that portion of the object in the center of the field of vision reaches the eye. 
 The simplest method for such observations is to center the desired portion 
 
 OOa, 
 
 FIG. 115. Interference Figures Corresponding to the Objectives in Fig. 114. 
 
 of the object carefully and upon removing the ocular, place a diaphragm 
 with a small perforation in its center over the end of the tube. The object 
 can be centered by using a Ramsden ocular with an iris diaphragm in the 
 lower focal plane. The diaphragm is narrowed until only the portion of the 
 object to be studied is visible in parallel light. Then the ocular is removed 
 while the iris diaphragm remains in the tube. In investigations by the 
 Bertrand method the iris diaphragm is placed in the upper focal plane of the 
 lens which is inserted into the tube. 
 
 Optically Uniaxial Crystals. Let us consider more closely the 
 behavior in convergent polarized light of a plate of a uniaxial 
 crystal cut perpendicular to the 
 optic axis. One of the several 
 bundles of parallel rays AA', Fig. 
 116, passes through the plate per- 
 pendicularly and is parallel to the 
 optic axis. It is at the same time 
 the axis of the cone of light. There 
 is no double refraction in the direc- 
 tion of the optic axis so the center 
 of the interference figure will ap- 
 pear dark. If a second bundle of 
 rays A'w, which passes through 
 the crystal slightly inclined to its 
 
 axis, is considered, the light will be resolved into two rays, one 
 vibrating in the principal section of the crystal and the other 
 perpendicular to it. These rays will emerge from the crystal 
 with a small phasal difference corresponding to the small inclina- 
 
 FIG. 116. Construction of an Uniaxial 
 Interference Figure. 
 
100 PETROGRAPHIC METHODS 
 
 tion to the axis, and weak illumination will appear at that point. 
 Other bundles of rays A'Z pass through the crystal more inclined 
 to the axis and have a decidedly greater retardation correspond- 
 ing to this greater inclination. Thus, there is a continual increase 
 of interference color from the center toward the edge of the 
 image. All rays with equal inclination to the axis have the same 
 retardation so the interference colors appear in concentric rings 
 around the dark middle point in the order shown in Fig. 96. 
 They are most brilliantly illuminated in those sections forming an 
 
 angle of 45 with the vibration 
 directions of the two nicols. As 
 light is not resolved in those 
 sections coinciding with the 
 vibration directions of the nicols, 
 they remain dark under all con- 
 ditions. Therefore, the inter- 
 ference figure consists of a dark 
 cross, the arms of which are 
 parallel to the vibration direc- 
 tions of the nicols and cut across 
 the colored concentric rings, 
 
 FIG. 117. Interference Figure of a high ^ *S' ' 
 
 Double Refracting Uniaxial Crystal. All directions perpendicular 
 
 to the optic axis are equal in 
 
 uniaxial crystals so there is no change in the image when the 
 stage is rotated through 360, and all these directions coincide 
 in turn with the vibration directions of the nicols. Interference 
 figures of optically uniaxial crystals cut perpendicular to the 
 optic axis are not changed by a complete horizontal rotation of 
 the section. 
 
 The behavior of uniaxial crystals in sections perpendicular to the optic 
 axis between parallel nicols can be understood from the above discussion. 
 Light is not resolved in that principal section of the crystal which corre- 
 sponds to the vibration direction of one of the nicols and in the direction 
 perpendicular to it. A light cross appears instead of the dark one, de- 
 scribed above, cutting across the concentric colored rings, whose colors are 
 complementary to those obtained between crossed nicols. 
 
 If the face through which the observation of the interference figure is 
 made is not exactly perpendicular to the optic axis, the rays observed in 
 the center of the field are not those which pass through the crystal parallel 
 to the optic axis. The intersection of the arms of the dark cross is dis- 
 placed from the center of the field and the colored rings, the center of which 
 
OBSERVATIONS, CONVERGENT POL ARPZED* LIGHT it)! 
 
 is this intersection point, are eccentric with respect to the field of vision. 
 When the slide is rotated, the intersection of the cross arms describes a 
 circle about the center of the field. See the upper series in Fig. 118. The 
 directions of the arms of the cross are dependent only upon the vibrations 
 of the nicols and, since these are not changed, the arms retain their directions 
 but are displaced across the field parallel to the original positions. 
 
 If the section is cut so obliquely that w sin /JL > u, in which w is the index 
 of refraction of the ordinary ray in the crystal, /* is the angle of inclination 
 of the section to the optic axis, and u is the aperture of the ocular, and the 
 apparent direction of the optic axis falls outside of the angle of aperture of 
 the ocular, a comparatively reliable clue can still be obtained that the 
 crystal is uniaxial. As shown in the lower series of Fig. 118, the extreme 
 
 FIG. 118. Interference Figures of Uniaxial Crystals Cut Cblique to the Axis. 
 
 portions of the black cross pass through the field of vision in a regular man- 
 ner when the crystal plate is rotated. The difference between interference 
 figures in oblique sections of uniaxial crystals and those of biaxial crystals 
 is emphasized in Fig. 118, in that the black bars which pass across the field 
 are represented exactly parallel to the vibration directions of the two nicols. 
 This is not the case when the axis emerges very obliquely and the aperture 
 of the objective is quite large. Then, as they are shifted, their directions 
 are changed very decidedly just a.s in a biaxial interference figure. But 
 sections of biaxial crystals with small cptic angles, cut not too obliquely, 
 show perfectly parallel shifting of the bars, particularly when objectives 
 with small apertures are used. It may also be noted that the emergence of 
 the optic axis from a cleavage fragment of calcite, where w sin fj. = 1.165, can 
 only be se.en by means of an immersion system. 
 
 The behavior of a plate of a uniaxial crystal cut parallel to the optic axis 
 in a rock section cannot be distinguished in ordinary light from that of a 
 biaxial crystal cut perpendicular to its obtuse bisectrix. This will be 
 treated more fully farther on. It is impossible to distinguish, by simple 
 means, a section of a uniaxial crystal parallel to the optic axis from a bi- 
 axial. The greater the inclination of tne plate to the optic axis the more 
 positive the determination. However, the direction of the optic axis or that 
 of the acute bisectrix can be detei mined in the interference figure in a 
 biaxial crystal with not too large an optic angle. Let us assume that the 
 
102 'P&TiiOGRAPHIC METHODS 
 
 cross section given in Fig. 119 is parallel to the optic axis cc of a crystal, 
 which is observed in convergent light at 45 to the vibration directions of 
 the nicols and the characteristic distribution of the interference colors is 
 seen. The observed phenomena can be best shown in a plate cut from the 
 section parallel to its long direction, Fig. 120, and one perpendicular to it, 
 Fig. 121. The simplest explanation can be given with the assumption that 
 the angle of aperture of the objective is 180, which cannot actually be 
 obtained in practice but can be approached approximately. Ifthebehav- 
 
 FIG. 119. Section of an FIG. 120. FIG. 121. 
 
 Uniaxial Crystal Parallel Section through Fig. 119. 
 
 to the Optic Axis. Parallel to cc. Parallel to aa. 
 
 ior of the rays in the section cc perpendicular to Fig. 119, as shown in Fig. 
 120, is studied, it will be seen that the rays whose image appears in the 
 center of the field of vision have passed through the crystal perpendicular to 
 its optic axis. They interfere with the full value of the double refraction 
 of the crystal f-a. The rays, however, which appear on the extreme 
 edge of the field, have passed through the crystal in the direction cc, i.e., the 
 direction of the optic axis, and have suffered no double refraction whatever. 
 In those quadrants through which the optic axis passes the interference 
 
 \ 
 
 FIG. 122. Uniaxial Crystal Parallel to the Optic 
 Axis in Convergent Polarized Light. 
 
 color is lowered toward the edge of the field but it is not reduced to zero 
 as it would be in the theoretical case under consideration. Red I, in the 
 middle of the field, changes into a distinct yellow as shown in Fig. 122. In 
 the section given in Fig. 121 -the rays in the middle of the image are likewise 
 propagated perpendicular to the optic axis and interfere with the greatest 
 double refraction. The rays in the outer portion of the image have the same 
 direction of propagation and hence the same double refraction. The dis- 
 tance traversed by the rays increases with their obliquity, causing an increase 
 in the retardation and consequently a raising of the interference colors, red 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 103 
 
 I passing over into blue. The section of quartz cut parallel to the optic 
 axis may serve to illustrate this as shown in the following table: 
 
 I. 
 
 II. 
 
 u 
 
 P 
 
 d 
 
 w 
 
 d 
 
 w 
 
 
 
 1.0000 
 
 0.0091 
 
 0.0091 
 
 0.0091 
 
 0.0091 
 
 20 
 
 1.0652 
 
 0.0069 
 
 0.0073 
 
 0.0091 
 
 0.0095 
 
 40 
 
 1.3054 
 
 0.0038 
 
 0.0050 
 
 0.0091 
 
 0.0119 
 
 60 
 
 2.0000 
 
 0.0010 
 
 0.0020 
 
 0.0091 
 
 0.0182 
 
 u is the angle of inclination of the rays toward the normal to the section, p 
 is the distance traversed in the direction indicated, d the amount of double 
 refraction in this direction, and w the retardation of the two rays resulting 
 from the last two factors. The columns under I give the value for those 
 directions which lie in the section corresponding to Fig. 120, while those 
 for the section corresponding to Fig. 121 are to be found in the columns 
 under II. 
 
 The interference figures of uniaxial minerals are often affected by inclu- 
 sions, twinning lamellae, etc. When disturbing inclusions are p.resent the 
 interference figure is undistorted only 
 when the vibration directions of the 
 inclusions correspond to those of the 
 nicols. In other cases the dark cross 
 in the interference figure does not 
 remain unaltered upon rotating the 
 object, but it opens giving rise to 
 two rather irregular hyperbolas that 
 reunite on further rotation. Such 
 minerals behave like biaxial crystals 
 with a very small optic angle. The 
 latter phenomenon is very common 
 so that positive determinations can- 
 not always be made. It may occur 
 that the black cross appears open 
 normally, and does not clo~se upon 
 rotating the object. This is caused 
 
 by double refraction in the lenses and a normal image can frequently be 
 produced by rotating the objective. 
 
 In very thin plates of low double refracting substances the bundles of 
 rays inclined most to the optic axis do not suffer sufficient double refraction 
 to produce brilliant interference colors. Illumination occurs only at the 
 extreme border of the field in the four sectors lying between the principal 
 sections of the nicols. The greater part of the image is a broad, poorly de- 
 fined cross, Fig. 123. For recognizing very weak double refraction in con- 
 vergent light, a sensitive tint plate may be used. When it is inserted the 
 interference figure, which was formerly scarcely visible, becomes distinct by 
 a blue color in two opposite quadrants and orange in the other two. If on 
 the other hand, the plate is thick or the substance has a very high double 
 
 FIG. 123. Interference Figure of a low 
 Double Refracting Uniaxial Crystal. 
 
104 PETROGRAPHIC METHODS 
 
 refraction, the rings are very close and the color is of a lower order in the 
 center, but gives way rapidly to white of a higher order toward the edge. A 
 distinct interference figure can be obtained even with a low power objective. 
 Crystals with Circular Polarization. It is not necessary to go very deeply 
 into the phenomenon of circular polarization in convergent light because it 
 can only be observed distinctly in much thicker slides than are used in ordi- 
 nary microscopic studies. Circular polarization is the property possessed 
 by certain bodies, whereby light is resolved into its components in that 
 direction in which double refraction does not usually take place. Since all 
 directions perpendicular to this one are equivalent, resolution of the light 
 can only give rise to circular vibrations which move in opposite directions 
 and possess slightly different velocities. Circular polarization can take 
 place in isotropic crystals in all directions but in double refracting crystals 
 only in the direction of an optic axis. Upon emerging from the crystal the 
 two circular vibrations unite in a plane polarized vibration, the plane of 
 which is rotated somewhat corresponding to the retardation of one ray over 
 the other. Circular polarizing crystals rotate the plane of polarization of 
 light 
 
 When a circular polarizing crystal section is observed in parallel, mono- 
 chromatic light, it appears light between crossed nicols, if it is sufficiently 
 thick, and there is no change in its luminosity when the plate is rotated 
 
 horizontally. When one of the nicols 
 is rotated through a definite distance, 
 the section becomes dark. Depend- 
 ing upon whether this rotation is to 
 the right or to the left, crystals are 
 classified into right- or left-handed 
 crystals, and the amount of rotation 
 of the nicol necessary to produce 
 complete darkness indicates the 
 strength of the circular polarization, 
 provided the thickness of the plate is 
 known. The rotation of the plane of 
 polarization is generally quite differ- 
 ent for different colors, e.g., in quartz 
 
 FXG. 124-Interferenc7Figure of a Uniaxial extreme violet is rotated 21/2 times 
 
 Crystal with Circular Polarization. as much as extreme red. If a plate 
 
 cut perpendicular to the optic axis is 
 
 thick enough, it will show interference colors in parallel polarized light, 
 and these colors remain unchanged during a complete rotation of the 
 plate. When one nicol is rotated, the colors change in such a manner 
 that the various colors of the spectrum are extinguished in order, the 
 others uniting to form a mixed color. Upon rotating the nicol in the 
 direction of the hands of the clock, these complementary colors follow in the 
 order of the spectrum for a right-handed crystal, and in the reversed order 
 for one which is left-handed. 
 
 The phenomena of a uniaxial, circular polarizing crystal can be very 
 easily deduced form the above. An interference color pioduced by circular 
 polarization appears in the center of the image and the dark cross disappears, 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 105 
 
 Fig. 124. The color is constant throughout the whole of the center of the 
 field and behaves exactly as the interference color of the plate did in parallel 
 polarized light. This phenomenon is generally not seen in microscopic 
 preparations because circular polarization is nearly always too weak to 
 produce an illumination which is observable in a thin layer. 
 
 Character of the Double Refraction of Uniaxial Crystals. 
 
 In observing the interference figure of a uniaxial crystal it 
 was seen that the phenomenon was produced by double refraction 
 of the rays passing obliquely through the plate. The light is 
 resloved into two vibrations 'corresponding to the extraordinary 
 
 FIG. 125. Vibration Directions in an Uniaxial Interference Figure. 
 
 and ordinary rays. The extraordinary ray vibrates in a prin- 
 cipal section and, since all planes through the optic axis are prin- 
 cipal sections, all radii of the interference figure are vibration 
 directions of the extraordinary ray. The tangents' which are 
 perpendicular to the radii are the vibration directions of the 
 ordinary ray, Fig. 125. 
 
 If one of the compensators already described, e.g., violet I, is 
 inserted in the microscope so that its vibration directions are at 
 45 to those of the nicols, the directions of greatest and least 
 elasticity in the plate will be parallel to a principal section of the 
 crystal. A reddish-violet color appears instead of the dark cross, 
 as indicated in Fig. 126, while the corners of the four sectors are 
 colored yellow and blue. A negative crystal is represented in 
 Fig. 126. Its extraordinary ray is the faster, or the velocity 
 
106 
 
 PETROGRAPHIC METHODS 
 
 of the ray vibrating in the principal section or in the radius of the 
 image is the greater, and is equal to a. Addition of the double 
 refraction takes place in those sectors through which the vibration 
 direction of the faster ray in the compensator passes radially, and 
 a higher interference color appears. It is the brilliant blue follow- 
 ing violet I. In the other sectors the 
 vibration direction of the slower ray in 
 the compensator is parallel to the radius 
 of the interference figure and compensa- 
 tion takes place. The interference color 
 is lowered in these two sectors, i.e., it 
 changes to yellow. 
 
 The simple mnemonical rule for this 
 phenomenon is: The line joining the 
 blue sectors and the direction of greatest 
 velocity in the test plate form a minus sign 
 
 for optically negative crystals. In positive crystals the other sectors 
 show the blue interference color. The line joining the blue sectors 
 and the direction of greatest velocity in the test plate form a plus 
 sign for optically positive crystals. 
 
 A quarter undulation plate gives characteristic reactions in a 
 similar manner. The dark cross breaks up into two black spots 
 
 FIG. 126. Interference Figure 
 of a Negative Uniaxial Crystal 
 with Gypsum Test Plate. 
 
 Q' 
 
 p' 
 
 P' 
 
 FIG. 127. Positive FIG. 128. Negative 
 
 Uniaxial Interference Figure with a Mica Test Plate. 
 
 and the line joining them corresponds to a vibration direction in 
 the test plate. It is parallel to the direction of greatest velocity 
 in the test plate for positive crystals, Fig. 127, and perpendicular 
 to it for negative crystals, Fig. 128. The colored rings may be 
 observed to move out in those two quadrants and move in in the 
 other two. 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 107 
 
 Figs. 127 and 128 show diagrammatically the conditions described above. 
 The phenomenon is based on the direction of greatest velocity of light in 
 the mica plate, but if, as is frequently the case, the direction of least elasticity 
 is parallel to the long direction of the test plate, the two images must be 
 interchanged. 
 
 The eighth undulation plate likewise gives a characteristic reaction in 
 which the dark cross breaks up into two curves similar to the middle portion 
 of a figure 8. The figure is closed on both ends by close colored rings and 
 lies parallel to the direction of greatest velocity in the mica when the crystal 
 is negative, and in the reverse position when it is positive. 
 
 The investigation of the character of the double refraction of a thin, feebly 
 double refracting mineral with a quarter undulation mica test plate between 
 crossed nicols does not give very positive results, because the dark spots as 
 well as the rings fall outside of the field of vision. The violet I plate is 
 better in such cases. Its brilliant color reaction is distinct even with the 
 feeblest double refracting substances. Great precautions are necessary in 
 the investigation of the weakest double refracting crystals with violet I 
 because the object glass itself often gives a very weak interference figure 
 which becomes quite distinct with the sensitive tint. A quarter undulation 
 plate is sufficient in almost all cases if the observations are made between 
 parallel instead of crossed nicols. The four angles of the brownish cross, 
 which now appears, are alternately white and deep brown passing over into 
 blue. The two latter quadrants, which in negative crystals are parallel, 
 are in positive crystals crossed to the 
 direction of greatest velocity in the 
 mica plate, and can be easily recog- 
 nized by their color. A mica test 
 plate gives the best results with very 
 strongly double refracting substances 
 in which the innermost colored ring 
 is very small, and also with highly 
 colored substances. 
 
 Biaxial Crystals. If a plate of 
 a biaxial crystal with not too 
 small an optic angle is cut per- 
 pendicular to an optic axis and 
 
 Observed in Convergent polar- Fla - ISO. Interference Figure of a Biaxial 
 , , . , . .. Crystal Perpendicular to an Optic Axis. 
 
 ized light, an image similar to 
 
 that of a uniaxial crystal will be seen. There is only one bar in 
 place of the dark cross and nearly circular ovals replace the truly 
 circular rings of the uniaxial figure, Fig. 129. The black bar is 
 parallel to the vibration direction of one of the nicols, when the 
 plane of the optic axes lies parallel to it, i.e., when the only 
 principal section that can be placed through the plate is parallel 
 to the vibration direction of a nicol. When the plate is rotated 
 the bar with the rings rotates also, but in the opposite direction, 
 
108 PETROGRAPHIC METHODS 
 
 about a point corresponding to the place of emergence of the 
 optic axis. After a rotation of 90 the bar is parallel to the 
 vibration direction of the other nicol. The vibration directions 
 of the two nicols always form the bisectors of the angles between 
 the dark bar and the direction of the plane of the optic axes. 
 The bundle of rays passing through the crystal parallel to the 
 optic axis suffers no double refraction in this case and the 
 corresponding portion of the image is therefore dark. The point 
 at which the axis emerges is the pivot for the black bar and is the 
 only portion of the interference figure that always remains dark. 
 
 FiG.30. FIG. 131. 
 
 Interference Figures of Biaxial Crystals Perpendicular to Acute Bisectrix. 
 
 A plate of an orthorhombic crystal cut perpendicular to the 
 acute bisectrix shows the image represented in Fig. 130, when the 
 plane of the optic axes is parallel to the prinicpal section of one 
 of the nicols. The figure shows a black cross, corresponding to 
 the two principal sections that can be placed through the plate. 
 That bar of the cross, which is parallel to the axial plane, is 
 sharply defined. The other bar, at right angles to it, is broad 
 and indistinct. This cross cuts the system of lemniscates sym- 
 metrically. The inner curves are closed ovals while the outer 
 ones unite to form a figure 8. The axes themselves emerge in 
 the vertices of these curves. When the plate is rotated the cross 
 opens, the arms uniting to form a curve which is very nearly an 
 hyperbola when the plate has been rotated through 45, Fig. 131. 
 The colored rings rotate also with the plate -but their form is not 
 changed. The only points which always remain dark when the 
 plate is rotated are the points where the axes emerge, and they 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 109 
 
 form the vertices of the hyperbola. The distance between the 
 vertices when the axial plane is at 45 to the nicols indicates the 
 size of the optic angle. 
 
 The interference figure of biaxial crystals with very small optic angles 
 approximates that of uniaxial crystals. The black cross appears to be sur- 
 rounded by a single system of colored circular curves, Fig. 132, and it opens 
 only slightly upon rotating the slide. As already remarked, this is some- 
 times seen in uniaxial crystals so that a definite distinction between uni- 
 axial and biaxial substances may become quite difficult. 
 
 If the optic angle is very large, Fig. 133, it may happen that the axes do 
 not emerge within the field of vision even when the strongest objectives are 
 
 FIG. 132. FIG. 133. 
 
 Interference Figure of a Biaxial Crystal with a very 
 Small Optic Angle. Large Optic Angle. 
 
 used. That is the case when ft sin V>A, aperture of the objective used. 
 If ft sin V> 1, the optic axes do not emerge from the crystal into the air at 
 all, because then total reflection of the light takes place, and the apparent 
 optic angle is greater than 180. If an immersion system is used with such 
 crystals, the optic axes can be observed in all cases in a section perpendicular 
 to the acute bisectrix, because then the difference between the indices of 
 the crystal and the medium surrounding it is decreased, whereby the refrac- 
 tion of the rays emerging from the crystal into the immersion liquid is 
 reduced, and the angle of 'total reflection increased. 
 
 With an immersion system the two axes can often be seen in the field of 
 vision in a section cut perpendicular to the obtuse bisectrix if the index of 
 refraction of the crystal is not too great, and the optic angle is very large. 
 
 Dispersion of the Optic Axes. The optic angle is different for 
 different colors and is sometimes larger for red than for blue, 
 Fig. 134. It is indicated by the dispersion formula p>v, or the 
 reverse may be the case, v> p. Dispersion can scarcely be recog- 
 nized in the microscope if it is very slight, especially when the 
 
no 
 
 PETROGRAPHIC METHODS 
 
 FIG. 134. Axial Plane in an 
 Orthorhombic Crystal. 
 
 substance has very low double 
 refraction. If the dispersion is 
 greater, there will be brilliantly 
 colored bands around the vertices 
 of the hyperbola in the interfer- 
 ence figure and this band is 
 broader and more brilliant the 
 stronger the dispersion. Very 
 strong dispersion of the optic 
 axes can be noted even in parallel 
 polarized light in the so-called 
 dispersion colors, see page 79. 
 If the hyperbolas are yellow on 
 the convex side, and blue on the 
 concave, the optic angle for blue 
 
 is smaller than that for red, 
 Fig. 135. Those rays which pass 
 through a crystal parallel to an 
 optic axis for a definite color suffer 
 no double refraction for that color, 
 and the complementary color ap- 
 pears in its place in the interfer- 
 ence figure, i.e., in the vertex of FlG ' 135 -- rthoAombi ' 
 
 the hyperbola. 
 
 In the orthorhombic system the 
 bisectrices and the optic normal for 
 all colors coincide with the crystal- 
 lographic axes, Fig. 134. One of 
 the axes can be the optic normal for 
 one color and another axis for 
 another color, i.e., the planes of the 
 optic axes for different colors can be 
 crossed, as for example, in brookite. 
 In any case the interference figure 
 is symmetrical to the plane of the 
 optic axes and the plane perpendicu- 
 lar to it. In the monoclinic system 
 in which only the b axis coincides 
 with a principal vibration direction, 
 FIG. i36.-Axiai Plane in a Monoclinic symmetry with respect to one or 
 Crystal with inclined Dispersion, both of these planes may be lacking. 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 111 
 
 There are three different kinds of dispersion in the monoclinic 
 system. 
 
 1. Inclined dispersion, the b axis is the optic normal, Fig. 136. 
 The optic axes lie anywhere in a plane parallel to the clinopina- 
 coid and the two axes suffer different amounts of dispersion. 
 
 FIG. 137. 
 Inclined Dispersion. 
 
 FIG. 138. Optic Plane in a Monoclinic Crystal 
 with Crossed (Horizontal) Dispersion. 
 
 The interference figure is symmetrical with respect to the plane 
 of the optic axes but not to the plane perpendicular to it, because 
 the bands of color are different on each arm of the hyperbola, 
 Fig. 137. 
 
 2. Horizontal dispersion, the b axis is the obtuse bisectrix, 
 
 FIG. 139. Horizontal Dispersion. FIG. 140. Crossed Dispersion. 
 
 Fig. 138. (The bisectrices I and II must be interchanged.) 
 The planes of the optic axes for different colors are any planes 
 perpendicular to the clinopinacoid. The interference figure is 
 not symmetrical with respect to the plane of the optic axes but 
 to the plane perpendicular to it, Fig. 139. 
 
112 
 
 PETROGRAPHIC METHODS 
 
 3. Crossed dispersion, the b axis is the acute bisectrix, Fig. 138. 
 The position of the plane of the optic axes is the same as in the 
 previous case. In the interference figure the distribution of 
 
 color to the right above is the same 
 as to the left below, Fig. 140, that is, 
 the colors are symmetrical to a point. 
 As there is no relation, in the 
 triclinic system, between the optic 
 directions for the different colors and 
 the crystallographic axes, the distri- 
 bution of the axes as well as of the 
 planes of the optic axes is entirely 
 unsymmetrical, Fig. 141. 
 
 In general these differences of dis- 
 persion are quite small and are 
 observed only in very favorable 
 cases. The presence of distinct in- 
 clined or crossed dispersion must be 
 considered as characteristic of a 
 monoclinic crystal, but the absence 
 of this phenomenon is not sufficient 
 to determine the crystal as ortho- 
 rhombic. 
 
 FIG. 141. Optic Plane in a 
 Triclinic Crystal. 
 
 The presence of strong dispersion of the optic axes is a good earmark for 
 a substance. In monoclinic crystals the character and strength of the dis- 
 persion are not definitely related to each other. Titanite, for example, 
 shows very strong dispersion of the optic 
 axes but the inclined character is scarcely 
 noticeable. On the other hand in a certain 
 group of pyroxenes, showing inclined dis- 
 persion, one axis is very weakly dispersed, 
 the other very strongly. 
 
 Measurement of the Optic Angle. 
 The determination of the size of the 
 optic angle is of some importance 
 and can be carried out by various 
 methods. The distance between the 
 
 w- 
 
 A 
 
 A 
 
 /}' -1.015 
 
 FIG. 142. Apparent Optic Angle. 
 
 hyperbolas gives only the size of the 
 
 apparent optic angle, which differs most from the true angle, 
 the larger the latter, and the higher the intermediate index of 
 refraction of the crystal. Those rays which pass through the 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 113 
 
 plate parallel to the optic axes impinge upon the upper 
 surface of the crystal obliquely and, emerging from a stronger 
 to a lower refracting medium, are refracted away from the 
 normal, Fig. 142. Thus, the apparent optic angle in air is 
 always larger than the true optic angle. If 2E is the apparent, 
 
 sin E 
 and 2V the real optic angle, sin V = - . Thus the real optic 
 
 angle of a crystal can be calculated when /? is known. Diopside 
 and orthoclase, for example, have very nearly the same apparent, 
 optic angle in air, about 118. However, in diopside, the value 
 /?, which is 1.678, is decidedly higher than in orthoclase, where 
 /? = 1.524. Thus, in the former the apparent optic angle corre- 
 sponds to a much smaller real angle, 61 1/2, while the real angle 
 of orthoclase is 71. 
 
 Only the apparent optic angle can be measured in a microscope. 
 The distance between the vertices of the hyperbolas is used for 
 this determination, which is made, if possible, in monochromatic 
 light. With a micrometer scale in the ocular the law of Mallard 
 can be used; sin E = DK, where D is the number of divisions on the 
 scale and K is the constant determined, once for all time, for one 
 of the objectives. It can be determined best with a plate cut 
 perpendicular to the acute bisectrix with a known apparent optic 
 angle. 
 
 In using the simplest methods of observation in convergent light there is 
 generally no need of a micrometer scale in the ocular. Such a scale can, 
 however, be placed in the objective, so that it can be seen sharply at the same 
 time as the interference figure, or it can be etched on the front lens of the 
 objective. This has a disadvantage, however, that such an objective can- 
 not be used so much for ordinary observations and, further, that the image 
 produced by this method is extremely small, and the error in the measure- 
 ment is therefore very large. 
 
 The measurement of the apparent optic angle can be best made with the 
 microscope by using a Bertrand or Klein lens, each of which produces an 
 enlarged interference figure. With a Bertrand Jens, which transforms the 
 ocular into a microscope, measurement can be made by means of an ocular 
 micrometer, i.e., by means of a scale placed in the focus of the ocular itself. 
 With a Klein lens, on the other hand, the scale can be shifted in the holder 
 of the lens which is placed over the ocular. When the proper adjustment 
 of the image and scale has been attained, there should be no relative dis- 
 placement when the eye is moved back and forth over the lens. Otherwise 
 there is considerable error resulting from the parallax. 
 
 If, instead of a fixed micrometer, the cross hairs in the ocular are so 
 arranged as to be moved by means of a micrometer screw, their intersection 
 8 
 
114 
 
 PETROGRAPHIC METHODS 
 
 can be made to coincide in turn with the vertices of 
 the hyperbolas. The amount of the displacement can 
 be read off on the drum of the screw. This method 
 gives results that are a little more accurate than 
 with a fixed micrometer. Finally the interference 
 figure can be transferred to coordinate paper by 
 means of a sketching device. Now and then this 
 method has many advantages. 
 
 The sine of the optic angle is obtained from the 
 distance between the hyperbolas, and the simplest 
 way to make this determination is to use a sine rule, 
 for example, Schwarzmann's optic angle scale, Fig. 143. 
 The two parts a and b slide on each other. The upper 
 part a gives the number of divisions in the ocular mi- 
 crometer and the lower part b the size of the corres- 
 ponding optic angle when it has been adjusted for the 
 ocular used. In Fig. 143 it is adjusted by measuring 
 the optic angle of aragonite on a fixed micrometer in 
 sodium light. There were 5.9 divisions on the scale 
 corresponding to an apparent optic angle of 30 15'. 
 30 15' on the lower scale is placed exactly under 5.9 
 of the upper, which constitutes adjustment of the 
 scale for that objective. If a section of topaz is 
 studied, 17.6 scale divisions will be noted. The divi- 
 sion on the lower scale corresponding to that is 99, 
 which is the apparent optic angle for topaz. 
 
 The scale has another value, viz., that the real optic 
 angle can be deduced from the apparent without any 
 calculation if the value of /? is known. The value of 
 is marked in the upper scale, for example 1.61 for 
 topaz, and x the distance from division 1 to the new 
 division is measured with a pair of dividers. This 
 distance, when measured backward from the apparent 
 optic angle, gives the value of the real angle, which in 
 this case is about 5.6. 
 
 The optic angle can also be measured in those 
 sections which are not exactly orientated. This is 
 very important for petrographic investigation because 
 perfectly orientated cross sections are rare in slides. 
 The trace of the optic axes is best transferred to a 
 projection from which the calculation of the optic 
 angle can be made, if the intermediate index of re- 
 fraction is known. These measurements are not of 
 very great importance in the microscopic study of 
 rocks, because in most of the isomorphous groups, 
 which are the most widespread constituents of rocks, 
 the optic angle varies within wide limits and no rela- 
 tionship appears to exist between the optic angle and 
 the chemical composition. 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 115 
 
 Character of the Double Refraction of Biaxial Minerals. The 
 
 determination of the character of double refraction of biaxial 
 minerals is important only when it it positively known whether 
 the determination was made on the acute or obtuse bisectrix. 
 It frequently happens in rock-forming minerals that the optic 
 angle is not far from 90, as in plagioclase, olivine, epidote, and 
 then the distinction is very difficult. The optic axes do not then 
 emerge from a section perpendicular to the acute bisectrix any 
 more than they do in a section perpendicular to the obtuse bisec- 
 trix; thus this characteristic of the mineral must be abandoned 
 because a positive acute bisectrix corresponds to a negative 
 obtuse bisectrix, and vice versa. 
 
 FIGS. 144-145. Determination of the Character of the Double Refraction in a Section 
 Oblique to an Optic Axis. 
 
 Sections that show the emergence of an optic axis in the field 
 of vision of the microscope are very important. If the optic 
 angle is not too near 90 the dark bar produced by a slightly 
 oblique axis is distinctly curved with the convex side toward the 
 acute and the concave side toward the obtuse bisectrix. When 
 the plane of the optic axes is placed at 45 to the nicols and violet 
 I is inserted, the bars appear violet but the convex and concave 
 sides of the curves are colored differently as indicated in Figs. 
 144 and 145. The axial plane in each case is parallel to the 
 direction of the greatest elasticity in the test plate, i.e., the bars 
 are perpendicular to it. Hence in a negative crystal the convex 
 side is yellow and the concave blue, Fig. 144, and this is reversed 
 in Fig. 145. In that portion of the image toward the bisectrix a, 
 the ray vibrating in the axial plane is propagated with the velocity 
 c. In this case the axial plane lies parallel to a in the compensator 
 and hence subtraction of the double refraction takes place on this 
 side of the axis. The interference color sinks from violet in the 
 bar to yellow. 
 
116 PETROGRAPHIC METHODS 
 
 With a section about perpendicular to a bisectrix, the change 
 produced by a quarter undulation plate inserted at 45 to the 
 nicols is observed, when the axial plane is parallel to the vibration 
 direction of one of the nicols. The black cross disappears and 
 the curves in each pair of opposite quadrants are either moved 
 closer together or farther apart. Thinning of the rings takes 
 place in those quadrants cut by the direction of greatest elas- 
 ticity in the test plate when the crystal is positive, and vice 
 versa if it is negative. This test can only be made with safety 
 in well orientated sections and is much less used than the method 
 which depends upon compensation of the double refraction. 
 
 When the interference figure is placed in the 45 position with 
 respect to the nicols, the middle of the field shows an interference 
 color which corresponds to the retardation of the rays propagated 
 in the direction of the bisectrix in question. One of these rays 
 vibrates parallel to the axial plane and is propagated with the 
 velocity belonging to the ray whose vibration direction is the 
 other bisectrix. The second ray vibrates parallel to the optic 
 normal b and has the corresponding intermediate velocity. If, 
 for example, the section is perpendicular to the acute bisec- 
 trix of a negative crystal, that ray which vibrates in the axial 
 plane is parallel to the obtuse bisectrix, which in this case is the 
 direction of least elasticity for the crystal and is therefore smaller 
 than the value of the optic normal perpendicular to it. A 
 compensator is then inserted so that its direction of greatest 
 elasticity is parallel to the axial plane in the crystal. Thus, 
 equivalent directions are crossed and the impression is obtained 
 that the thickness of the crystal has been diminished. The 
 interference colors are lowered and the rings spread out. A 
 biaxial crystal is negative if the double^ refraction is lowered, 
 when the direction of greatest elasticity in the compensator is 
 parallel to the axial plane in a section perpendicular to the acute 
 bisectrix, and positive when it is increased. The reverse of this 
 is true for a section perpendicular to the obtuse bisectrix. 
 
 Beginners often experience difficulty in making this determination with 
 feebly double refracting substances, which show no color in the field, or 
 only gray of the first order. This is especially true for those groups of rock- 
 forming minerals on which observations in convergent polarized light are 
 most frequently made, as the piagioclases. With sections perpendicular to 
 the negative bisectrix of such crystals, the original gray changes to yellow 
 when violet I, which is most frequently used for these determinations, is 
 inserted. Apparently the interference color is raised instead of lowered but 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 117 
 
 the gray in the slide has actually been subtracted from the red of the com- 
 pensator, and yellow remains. This fact can be proved by rotating the 
 compensator, for the field becomes blue, i.e., it acquires the next higher 
 color after violet I, while the yellow is the next one lower and signifies 
 subtraction of the interference colors. 
 
 It is often interesting to determine the position of the axial plane with 
 respect to the edges of a crystal, this being sometimes a valuable characteris- 
 tic of the mineral. The interference figure is placed in the 45 position. 
 The line joining the vertices of the hyperbolas in the interference figure, or 
 
 FIG. 146. Biaxial Interference Figures in Sections Oblique to the Acute 
 Bisectrix. 
 
 the line perpendicular to one 'arm of the curve is the direction of the plane 
 of the optic axes. It is unimportant which method of observation in con- 
 vergent light is used, because the rotation of the image is of no consequence 
 in determining the direction of the plane. 
 
 This rotation of the image is considered when the bisectrix is inclined 
 toward tho vertical, and it must be determined in which direction the devia- 
 tion takes place. It must always be remembered that in observing without 
 the ocular, the interference figure is reversed with respect to the image of 
 the object, while in observing with the ocular the two images are placed 
 parallel. 
 
 The phenomena produced by a biaxial crystal cut obliquely to the prin- 
 cipal vibration directions are shown in Fig. 146. It represents the behavior 
 of three differently orientated sections of topaz upon a rotation through 90. 
 Unlike the behavior of uniaxial crystals, Fig. 118, p. 101, it can be distinctly 
 seen that the dark bar rotates about the point of emergence of the axis 
 as a center, because in this case its direction is not dependent upon the 
 position of the principal sections of the nicols alone, but upon the relation of 
 them to the principal vibration directions of the plate. 
 
118 PETROGRAPHIC METHODS 
 
 Besides sections perpendicular to the acute bisectrix and those perpen- 
 dicular to the optic axes, which have been amply described above, sections 
 perpendicular to the obtuse bisectrix and those parallel to the plane of the 
 optic axes are of special interest. If the real optic angle of a certain sub- 
 stance is nearly 90, it is difficult to distinguish between acute and obtuse 
 bisectrices especially in minerals with low indices of refraction. If one does 
 not wish to make accurate measurements, or if they cannot be made, because 
 there is no objective with a sufficient aperture, very useful results can be ob- 
 tained in parallel polarized light. With two well orientated sections of equal 
 thickness, perpendicular respectively to each of the bisectrices, the one 
 perpendicular to the obtuse bisectrix gives the higher interference color. As 
 the size of the acute optic angle decreases, and the obtuse becomes larger, the 
 difference of the interference colors of the two sections naturally increases. 
 In convergent light the lemniscates in the interference figure, perpendicular 
 to the obtuse bisectrix, are wider, and the dark broadened bar cuts them 
 only at the extreme edge 01 Jie field. The change is the same as if the obser- 
 vation were made with objectives with constantly decreasing magnification. 
 The reverse order of the images in Fig. 114, p. 98, gives the best idea of this 
 gradual change. When the apparent obtuse optic angle becomes very 
 large, there is only an indication of the symmetrical distribution of color in 
 the different quadrants, when observed in white light. The dark bars corre- 
 sponding to the hyperbolas first appear in the field, when the plane of the 
 optic axes is almost perfectly parallel to the vibration directions of the nicols. 
 They are then very broad and not sharply defined so that it gives the im- 
 pression that the plate is alternately light and dark throughout its whole 
 extent. 
 
 The phenomena described above can be observed in plates parallel to the 
 plane of the optic axes. They cannot be distinguished in white light from 
 a plate perpendicular to the bisector of a large obtuse optic angle or those 
 parallel to the optic axis of a uniaxial mineral. They can be distinguished 
 sometimes in monochromatic light because in a section perpendicular to the 
 obtuse bisectrix black curves occur corresponding to the colored ones in 
 white light. These curves are portions of lemniscates which curve toward the 
 point of emergence of the optic axes near the edge of the field, Fig. 132, p. 109. 
 Analogous curves in a section parallel to the axial plane of a biaxial crystal 
 or to the optic axis of a uniaxial crystal are hyperbolas, which do not show 
 such bending. If this reaction does not possess the necessary sharpness for 
 positive differentiation, it is still helpful, under certain conditions, to observe 
 the interference figure in monochromatic light because with it the orienta- 
 tion of the section can be determined much better. The determination 
 of the direction of extinction in a section of a monoclinic crystal should never 
 be undertaken before it is proved in convergent polarized light that the sec- 
 tion is really parallel to the clinopinacoid of the crystal, which is always 
 perpendicular to one of the three principal vibration directions. It always 
 gives an interference figure in monochromatic light, which is symmetrical 
 with respect to two directions perpendicular to each other. 
 
 The direction of the acute bisectrix can be recognized distinctly by the 
 distribution of colors in the interference figure in the 45 position of a section 
 parallel to the axial plane in the same manner as was described on page 102 for 
 
OBSERVATIONS, CONVERGENT POLARIZED LIGHT 119 
 
 sections parallel to the optic axis of a uniaxial mineral. It is necessary, how- 
 ever, that the real optic angle does not approximate 90 too closely. In the 
 quadrants through which the acute bisectrix passes there is a lowering of the 
 color or when the optic angle is quite large a slight raising of the color. In 
 the other two quadrants the color is raised and in case the optic angle is 
 large, the increase is much greater than in the other two quadrants. The 
 following figures give the values for gypsum, which is optically positive and 
 has an optic angle of 59 corresponding to the table on page 103. The values 
 under I are for the planes inclined to the acute bisectrix and those under II 
 for those inclined to the obtuse bisectrix. 
 
 I II 
 
 u 
 
 P 
 
 d 
 
 w 
 
 d 
 
 w 
 
 
 
 1.0000 
 
 0.0098 
 
 0.0098 
 
 0.0098 
 
 0.0098 
 
 20 
 
 1.0652 
 
 0.0089 
 
 0.0095 
 
 0.0096 
 
 0.0100 
 
 40 
 
 1.3054 
 
 0.0066 
 
 0.0086 
 
 0.0089 
 
 0.0116 
 
 60 
 
 2 . 0000 
 
 0.0040 
 
 0.0080 
 
 0.0082 
 
 0..0164 
 
 The rays in plane I, inclined about 60 to the normal to the plate, show 
 not quite half the retardation of the corresponding directions in plane II. 
 
CHAPTER VI 
 
 Twins and Optical Anomalies 
 
 Twins. Many crystals do not consist of a single homogeneous individual 
 but of two or more individuals intergrown in a regular manner. They are 
 called twins, fourlings, etc. The crystallographic laws according to which 
 such intergrowth takes place can be determined in simple cases by careful 
 microscopic observation, but with more complex inter- 
 growths the results of such observations must be used 
 with great care, if they have not been verified by 
 accurate goniometric measurements. 
 
 Twinned intergrowths of cubic crystals can only be 
 determined under the microscope by their outline. 
 Reentrant angles are a clue to the presence of twins, 
 Fig. 147. Optical recognition of twins is not possible 
 
 FIG. 147. Cubic m uniaxial minerals, if the various sections are parallel 
 
 Twin with Reentrant to the principal axis. The vibration directions and 
 Angle. Magnetite. flte velocities of the extraordinary rays are the same 
 for both individuals in every section through such a 
 crystal. In convergent polarized light the behavior of the various indi- 
 viduals is the same in this case. Quartz is a characteristic example. It 
 forms intergrowths and twins having parallel optic axes, which cannot be 
 determined microscopically. 
 
 When the principal axes of the twinned 
 individuals are inclined to each other it will 
 be observed in every section oblique or per- 
 pendicular to the twinning plane, that the 
 parts do not extinguish simultaneously. 
 Thus in Fig. 148, when one part of the twin 
 is dark, the other part, sharply separated 
 from it by the twinning plane, is light, and 
 vice versa. The angle, which the extinction 
 directions in the two individuals form with 
 each other must not always be taken as a 
 basis for the calculation of the twinning law, 
 because the true inclination of the two 
 principal axes can only be determined when 
 the slide is perpendicular to the twinning 
 plane. 
 
 Twins in the orthorhombic system are generally more easily determined. 
 A difference in color in various parts of strongly pleochroic substances is 
 indicative of the presence of twins, even though the principal axes are 
 parallel. Such twins are difficult to recognize, however, in case the pleo- 
 
 120 
 
 FIG. 148. Uniaxial Twin with 
 Inclined Axes. Rutile. 
 
TWINS AND OPTICAL ANOMALIES 
 
 121 
 
 chroism is not distinct. This is also the case when the axial plane is per- 
 pendicular to the twinning plane and the optic angle is very large, or the 
 bisectrix of a very small optic angle lies in the twinning plane. In both 
 these cases the extinction directions coincide and the interference colors are 
 quite similar in the sections in which the twinning can usually be recognized 
 most easily. In such cases the orientation of the different parts can be 
 established with convergent polarized light. If, on the other hand, the 
 crystallographic axes of the twinned individuals are oblique to each other, 
 the intergrowth can be recognized by different extinction directions. The 
 two individuals appear equally light when the inclination of each to the 
 nicols is the same, Fig. 149. 
 
 Most of the twins in the monoclinic system are those in which the 6 axes 
 of the two individuals are parallel. If the crystals are developed prismatic 
 to this axis, the zone of the 6 axis will be the one most frequently studied. 
 What was said for orthorhombic crystals with parallel principal axes applies 
 also for sections of monoclinic inclined at a large angle to the 6 axis. Such 
 twins cannot be positively determined in all cases by optical means. This- 
 is seen in the extremely common twins of epidote, which are but rarely 
 recognizable. But the different positions of the vibration directions in the 
 twins can be easily determined in sections perpendicular or oblique to the 
 b axis, as shown in the example of diopside, Fig. 150. 
 
 Twins of triclinic crystals can be recognized in all sections because the 
 vibration directions do not generally lie parallel. 
 
 FIG. 149. Penetration 
 Twin with Inclined Axes. 
 Staurolite. 
 
 FIG. 150. Monoclinic 
 Twin. Diopside. 
 
 FIG. 151. Penetration 
 Trilling. Cordierite. 
 
 It is sometimes noted that not only two, but three or even more indi- 
 viduals are intergrown in a regular manner. Intergrowths of three indi- 
 viduals are called trillings. Characteristic penetration trillings of certain 
 orthorhombic and monoclinic crystals are noteworthy. They possess a 
 prism angle of approximately 120, and unite to form 'an apparently hex- 
 agonal crystal, as shown in Fig. 151. 
 
 Fourlings generally assume the form of lamellar intergrowths. Twinning 
 lamination is particularly common in monoclinic and triclinic substances 
 but is not lacking in orthorhombic (olivine), hexagonal (calcite), tetragonal 
 (rutile), and in cubic crystals (fluorite). In the latter it cannot be observed 
 optically. Crystals, twinned in this manner, apparently consist of two or 
 
122 
 
 PETROGRAPHIC METHODS 
 
 more individuals which penetrate each other in a lamellar manner, e.g., pla- 
 gioclase, Fig. 152, so that one system of lamellae belongs to one individual 
 and the other to a second, etc. It sometimes happens that one system of 
 twinning lamellae, produced by twinning according to a certain law, is 
 crossed by another system belonging to a second law. The phenomenon 
 may assume very fine development and is called lattice lamination or cross 
 hatching. Fig. 153 shows cross hatching in albite produced by twinning 
 according to the albite and pericline laws. 
 
 This complicated structure of crystals consisting of numerous alternating 
 twinning lamellae is very widespread especially in the larger polymorphous 
 groups, e.g., the epidote and feldspar groups. Modifications of lower sym- 
 metry approach those of higher symmetry with respect to external form 
 by such repeated twinning. The approximation is the greater the finer the 
 individuals, until, finally, a minuteness of the single components may be 
 
 FIG. 152. Twinning Lamination. 
 
 FIG. 153. Lattice Lamination. 
 
 attained, such that they cannot be differentiated under the microscope. 
 Then the optical properties agree with those of the modifications with higher 
 symmetry. This gradual transition from lower to higher symmetry gave 
 rise to the theory of Mallard, that numerous crystals belonging to groups 
 with higher symmetry are the result of such twinning of individuals with 
 lower symmetry. 
 
 It has often been observed that substances which crystallize in one crystal 
 system at a certain temperature, are resolved into a pile of complicated 
 twins of individuals with lower symmetry when the temperature is changed. 
 This was shown long ago for leucite and boracite, which, at the moment of 
 their formation at higher temperatures, crystallize cubic, but upon cooling 
 this modification is not stable and passes over into an apparently confused 
 complex of double refracting lamellae, Fig. 154. The cubic condition of 
 equilibrium can be restored by heating the crystal up to a certain tempera- 
 ture, while cooling is followed again by the breaking up into the double 
 refracting modification. 
 
 Many methods have been used to determine the monoclinic and triclinic 
 crystals composed of twinning lamellae, which occur so frequently in rocks. 
 The sections of such crystals are orientated in various ways and the methods 
 employed for their determination are based in part upon the mutual 
 relations of the twinned individuals. The simplest method is the determi- 
 nation of the extinction angle in a series of sections in which the two indi- 
 
TWINS AND OPTICAL ANOMALIES 123 
 
 viduals extinguish symmetrically with respect to the twinning plane, i.e., 
 they are perpendicular to the twinning plane. Another method is the 
 determination of the position of equal luminosity, which very thin plates 
 of feebly, double refracting crystals show. Thicker plates, or those with 
 higher double refraction, can only be used in monochromatic light. A 
 twin, in which the vibration directions are oblique to each other in the two 
 parts, cannot be recognized as such in all positions between crossed nicols. 
 On rotating the crystal horizontally through 360, there are eight positions 
 in which the two halves appear equally light and the twinning plane entirely 
 disappears. Four of these are crossed with respect to the other four. 
 These two groups of positions of four each can be distinguished from each 
 other in that when the lamella partially overlap like wedges, these over- 
 lapping parts in one group are just as light as the rest of the crystal, but in 
 the other group they are darker. This method of determining the position 
 of equal luminosity was formerly successfully employed in the determina- 
 tion of the plagioclases, but it did not find any general application and has 
 been replaced by modern methods of determination. 
 
 FIG. 154. Twinning Lamination FIG. 155. Segments in 
 
 in Leucite. Garnet. 
 
 Optical Anomalies. Optical anomalies are quite often observed in micro- 
 scopic studies. Variations in the optic angle of biaxial crystals, the slight 
 opening of the dark cross in uniaxial crystals upon rotating in convergent 
 polarized light, and the uneven illumination of cubic crystals between crossed 
 nicols are very common occurrences. A very faintly illuminated halo cut 
 by a dark cross, Brewster's cross, often appears around inclusions in glass 
 and cubic crystals in parallel polarized light. 
 
 Crystals with an external cubic form may be sometimes observed, which 
 do not show normal optical behavior either in their entirety or in regularly 
 bounded portions. They appear double refracting. The phenomenon in 
 leucite and its decomposition into a complex of twins has been mentioned 
 above.. Strictly speaking, this phenomenon cannot be classed with anom- 
 alies as generally considered, but it is a genuine and characteristic para- 
 morph. A characteristic type of optical anomalies for cubic crystals may be 
 observed when a second substance is present in such minute quantities that 
 it cannot be detected chemically. The two substances may or may not 
 be isomorphous. Thus a crystal of potassium alum may contain slight 
 amounts of ammonium alum and retain its outward cubical symmetry but 
 internally it is composed of a number of double refracting pyramids. 
 
 It can generally be observed that the structure of such a crystal is most 
 
124 PETROGRAPHIC METHODS 
 
 intimately related to its external form. It is composed of as many pyramids 
 as there are faces, and each face of the crystal is the base of a pyramid, the 
 apexes of which unite in the center of the crystal, Fig. 155. If the substance 
 is without definite crystal form, its internal structure cannot, naturally, be 
 influenced by it. The optical properties, which are based upon the internal 
 structure of the crystal, are disturbed before the development of the form is 
 completed. Such a division of the field into segments is observed quite 
 frequently in cubic crystals and not infrequently also in uniaxial crystals. 
 They are, however, rarer the lower the symmetry of the crystal, because 
 anomalous crystals always correspond in their optical behavior to groups 
 with lower symmetry. It is noteworthy that certain compounds are very 
 susceptible to such optical influences so that they are rarely found in a 
 normal condition, -while in others optical anomalies are not known at all. 
 
 The phenomena grouped together as optical anomalies of isotropic bodies 
 are of three kinds. 1. An uneven illumination between crossed nicols with 
 a Brewster's cross in the neighborhood of inclusions. This is undoubtedly 
 produced by tension and is found in amorphous as well as in cubic sub- 
 stances. 2. A division of the field corresponding to the external form. This 
 is generally produced by foreign substances between the molecules and is 
 likewise a tension phenomenon. 3. The occurrence of twin laminations or 
 cross hatching. These can generally be referred to dimorphism of the sub- 
 stance in question, and paramorphism resulting from it. 
 
Appendix 
 Accessory Apparatus 
 
 A number of more or less complicated devices are quite desir- 
 able in some cases as accessories to the microscope. They are 
 of great importance in certain special investigations, and are 
 mentioned here for the sake of completeness. It is not intended 
 to give a long list of them or a detailed description of each but 
 to give some idea only of their general applicability. There are 
 three principal groups of such accessory apparatuses which are 
 of great importance. 
 
 1. Rotation apparatus. 
 
 2. Heating apparatus. 
 
 3. Projection and reproduction apparatus. 
 
 The more important types of these will be briefly described. 
 
 i. Rotation Apparatus 
 
 Under this head are included all those simple or complex 
 attributes to the microscope by which the object under investiga- 
 tion can be rotated about one or more axes other than the axis of 
 the stage. The oldest of these were used exclusively for gonio- 
 metric investigation. They were called microscope goniometers 
 but they are scarcely of any importance at the present time. 
 There is a whole series of adjustments from the simplest types to 
 the universal stage, which are used principally for investigating 
 the optical properties of an object in various directions. 
 
 These devices depend chiefly upon two different principles. 
 One tends to eliminate refraction and total reflection of the rays 
 coming from the oblique surfaces of the preparation. This is 
 accomplished by placing the crystal or the microscopic prepara- 
 tion between two plano-convex lenses so that at all points both 
 surfaces of the apparatus are perpendicular to the rays of light, 
 which are sent through the preparation, and no deviation of the 
 central rays, at least, can take place. The other makes it 
 possible to observe an object immersed in a liquid of very similar 
 index of refraction. The liquid is in the form of a horizontal or 
 
 125 
 
126 PETROGRAPHIC METHODS 
 
 a vertical layer bounded by plane surfaces, and the crystal can 
 be rotated in it in any manner. The first method is not very 
 valuable in the study of thin sections. The other is used more 
 frequently with isolated crystals but can also be employed very 
 profitably with thin sections. 
 
 Rotation Apparatus for Observations between Two Plano-convex 
 Lenses. The simplest of these accessories was suggested by 
 Schroeder van der Kolk and consits only of a single pair of lenses. 
 One lens with a diameter of 25 mm. is placed in the opening of 
 the stage and can be rotated in all directions. The other lens 
 with a diameter of about 8 mm. is placed on the center of the slide. 
 Both lenses must be so made that the preparation lies as nearly as 
 possible in the focus of the system, i.e., the lower lens must differ 
 from the form of a hemisphere by the thickness of the object 
 glass and the upper one by the thickness of the cover-glass. The 
 object glass of the preparation, which should be circular and 
 have a smaller diameter than the lens itself, is laid upon the larger 
 hemisphere, contact being made by a drop of glycerine. The 
 particle to be investigated is then centered accurately when the 
 preparation is as nearly horizontal as possible. The second 
 lens is fastened on the cover-glass of the preparation by a small 
 drop of glycerine also. This lens is moved about until the object 
 to be observed, which appears twice as large as before, is exactly 
 in the middle of the field of vision. The two principal vibration 
 directions of the object are determined and then the lower lens 
 is rotated about one of the directions as an axis, permitting 
 observations to be made in parallel polarized light continuously. 
 An object holder, Fig. 33, page 19, can be used to eliminate many 
 of the accidents which may result by the rotation by hand. It 
 rests on the preparation like a spring and can be rotated about a 
 horizontal axis. The amount of rotation can be read off a small 
 circular scale. 
 
 The principal vibration direction of the object under investigation is 
 placed parallel to the axis of rotation of the apparatus and then the rotation 
 can be carried out accurately and measured. With a prismatic crystal, 
 which shows parallel extinction and gives no characteristic interference 
 figure in convergent light, the rotation is made with the long axis of the 
 crystal as the axis of rotation until it lies parallel to one of the vibration 
 directions of the nicols. Two cases are possible. Either the crystal remains 
 constantly extinguished during the rotation, or more and more illumination 
 appears and, after a definite rotation of the accessory apparatus, the 
 crystal must be rotated on a vertical axis through a definite arc to produce 
 
APPENDIX 127 
 
 darkness again. In the latter case the crystal is monoclinic. If it remains 
 constantly dark it is rotated on a second axis perpendicular to the first 
 horizontal axis and the observations repeated in the same order. Now, 
 if it remains dark during the whole rotation the crystal is tetragonal, hex- 
 agonal or orthorhombic, but if it gets gradually lighter during the second 
 rotation it is monoclinic with a prismatic development parallel to the 6 axis. 
 
 If there is parallel extinction in all cases the long direction of the crystal 
 is used as the axis of rotation and it is placed at 45 to the nicols so that the 
 crystal shows its most brilliant interference color. If, when the crystal is 
 rotated, there is no change in the interference color, it is uniaxial. If there 
 is a change, it is orthorhombic. It is thus obvious that this apparatus is an 
 extremely important aid in the investigation of crystals. All the changes 
 which take place in convergent polarized light can be followed by lengthen- 
 ing the tube slightly without otherwise changing the adjustments. 
 
 The apparatus would be much more useful if it were constructed of a 
 series of large lenses with a hemispherical cup cut in the center of the plane 
 surface. Large crystals, or precious stones, could be immersed in this cup 
 
 FIG. 156. Fedorow Universal Stage. 
 
 in a liquid with the same index of refraction and the various directions 
 studied in parallel and convergent light. This gives excellent results in the 
 study of precious stones. 
 
 There are a large number of other apparatuses of this kind based upon 
 the same principle but only the large model of the Fedorow universal stage 
 will be described in detail. Fig. 156 shows this stage with three axes of 
 rotation. It consists of a stand l l l l which can be placed on the stage of a 
 microscope. The stage in the stand can be rotated about a horizontal axis 
 by means of the screw k. The circle T with its vernier n shows the amount 
 of rotation. The axis can be fixed by means of the screw /. The vernier 
 n l is firmly fixed on this axis and the stage K is rotated on an axis perpen- 
 dicular to the first. The stage K bears a third axis TLd which can be rotated 
 about some axis lying in the plane of this stage. Finally the glass stage S 
 carrying the preparation can be rotated in its own plane. 
 
 The preparation is placed on the stage, contact being made by glycerine. 
 Then a plano-convex lens a is placed under the stage S and another over the 
 preparation concentrically. Here also glycerine serves to insure a good 
 contact. The efficiency of the apparatus can be increased for the investiga- 
 
128 
 
 PETROGRAPHIC METHODS 
 
 tion of crystals with higher indices of refraction, by using a more strongly 
 refracting glass for the stage, lenses and holders of the object. The liquid 
 used for the contact must also have a higher index of refraction than in the 
 former case. Although this apparatus has proved its usefulness for certain 
 investigations, for example, in the study of the feldspars, the results obtained 
 are not commensurate with its complicated construction. The necessity 
 of having to use special kinds of glass with this apparatus naturally prevents 
 its being used extensively. 
 
 Rotation Apparatus for Investigation in Liquids. This method 
 was first proposed by C. Klein and used for studying isolated 
 
 crystals or fragments, but was later 
 also employed in the investigation 
 of thin sections. The simplest ap- 
 paratus consists of a low glass dish 
 the bottom of which is a plane 
 parallel glass plate. On one side 
 there is a tapering neck in which a 
 glass stopper can be rotated. The 
 inner end of this stopper is pro- 
 vided with a crystal holder and the 
 outer end with a scale, Fig. 157. 
 The dish is filled with a liquid hav- 
 ing as nearly as possible the same 
 index of refraction as the crystal to 
 be studied. The crystal is placed 
 
 in the holder exactly parallel to the zone under consideration. 
 The extinction direction on the various faces in the zone can be 
 determined with this apparatus. The real optic angle can also 
 be measured directly if the index of refraction of the liquid 
 corresponds exactly with that of the crystal. 
 
 Special objectives with a wide focal angle and not too small 
 a field as well as the appropriate condensers generally accom- 
 pany this rotation apparatus, so that observations in convergent 
 polarized light can be made although object and objective are 
 separated considerably. 
 
 Klein's larger universal rotation apparatus for crystals can be 
 used more extensively. The crystal can be quite accurately ad- 
 justed and centered on its holder and the measurements made 
 with considerable precision. For using this apparatus the micro- 
 scope is placed in a horizontal position and a small rectangular 
 glass dish placed in the path of the rays of light. This dish con- 
 tains the liquid in which the crystal is immersed, and can be 
 
 FIG. 157. Simple Rotation Appa- 
 ratus by R. Fuesz. 
 
APPENDIX 129 
 
 rotated without striking the sides. The plane parallel walls are 
 placed as nearly perpendicular as possible to the axis of the 
 microscope. 
 
 A small goniometer can also be combined with the stage of a microscope 
 in a similar manner. It however has the disadvantage that either the focal 
 length of the objective must be much larger on account of the elevation of 
 the adjusting and centering apparatus on the goniometer, or the 'goniometer 
 must be placed entirely outside of the axis of the microscope and the crystal 
 mounted on an especially long holder. In the latter case, the lever arm is so 
 long that a very slight movement of the adjusting screws produces so large 
 a displacement of the crystal that such a device has not proved practical. 
 
 The apparatus shown in Fig. 158 is constructed. for investiga- 
 tion of thin sections according to this method. The dish B, 
 which holds the liquid, is closed at a by a plane parallel glass 
 
 FIG. 158. Universal Rotation Apparatus for Slides by C. Klein. 
 
 plate. S is the stage, the middle part of which consists of glass. 
 The object is fixed on it by means of the clamps e and e 1 . The 
 screw k is used to rotate the stage S in its own plane, the motion 
 being transmitted by a cog wheel. With T it can be moved on a 
 second axis perpendicular to the first. The instrument is made 
 for various sizes of object glasses. Ordinary slides can be studied 
 with it but the cover-glass and any adhering Canada balsam must 
 first be removed. 
 
 2. Heating Apparatus 
 
 Since the optical properties of crystals are dependent upon the 
 temperature and in some substances change considerably with 
 comparatively slight variations in temperature, a number of 
 devices have been constructed for making microscopic observa- 
 tions at high and constant temperatures. Such an apparatus is 
 9 
 
130 
 
 PETROGRAPHIC METHODS 
 
 also used to observe crystallization in its incipient stages at 
 higher temperatures. For this reason a microscope equipped 
 with such an apparatus, Fig. 159, is also called a crystallizing 
 microscope, and can be used especially for physical-chemical in- 
 vestigations. Very high temperatures can be obtained with it 
 without injuring the lenses because the lenses of the objective 
 are continually water-cooled. 
 
 FIG. 159. Chemical Microscope by Voigt & Hochgesang. 
 
 In a special, heating microscope recently constructed by 
 Doelter an electric current is used to produce the heat. With 
 regulating devices, high and very constant temperatures can be 
 obtained. With the microscope shown in Fig. 160, which is also 
 
APPENDIX 
 
 131 
 
 arranged for photography at high temperatures, temperatures of 
 over 1000 with a magnification of 132 can be obtained by using a 
 small furnace. These temperatures can be controlled accurately 
 in intervals of 5 and can be held constant for a long time. With 
 larger furnaces higher temperatures up to 1600 can be obtained 
 but with a smaller magnification, up to 88. Observations in po- 
 larized light are possible up to about 1200, when the object itself 
 
 KS 
 
 FIG. 160. Electric Heating Microscope with Photographic Camera by Doelter. 
 
 Reichert in Vienna). 
 
 (C. 
 
 is too strongly luminous. The furnaces used are well insulated 
 and are closed top and bottom with a quartz-glass plate. The 
 lenses of the objective are not cemented and are water-cooled. 
 In Fig. 160 this heating microscope is combined with a photo- 
 graphic camera. 
 
 A very useful heating apparatus for smaller tests is shown in Fig. 161 at 
 about 1/3 natural size . It is covered with asbestos and placed upon f our[glass 
 legs to insulate it from the stage and the optical apparatus in the stage. 
 The heating box A' is closed at 6 and the opposite under-side by ajplane 
 
132 PETROGRAPHIC METHODS 
 
 parallel glass plate, while the heated air passes through the flue A. The 
 preparation is placed on an object holder located in the hole b and heated by 
 means of the gas burner gg' '. To cool it off rapidly, cold air can be intro- 
 duced through the tube r. The thermometer on the flue A can be read to 
 450. It rests on the object like a fork and thus the temperature can be 
 determined with great accuracy. 
 
 P 
 FIG. 161. Projection Apparatus by W. & H. Seibert. 
 
 3. Projection and Reproduction Apparatus 
 
 There are a large number of devices made to obtain and repro- 
 duce microscopic images without any subjective influence. In 
 so far as such a reproduction can be made by microphotography, 
 the method deserves preference on account of its absolute objec- 
 tivity. There are, however, a whole series of phenomena which 
 a photographic plate does not reproduce with sufficient distinct- 
 ness, especially when the image possesses some comparatively 
 unimportant feature the importance of which should, however, 
 be emphasized for certain purposes. 
 
 Microphotographic and Projection Apparatus. Any micro- 
 scope can be adapted for projection and demonstration purposes 
 with microscopic preparations, by using a source of light with a 
 system of lenses, as shown in Fig. 162. An electric arc is the 
 best source of light but if electricity is not available, a lime or 
 zircon light or a Welsbach burner can be used. The intensity of 
 light diminishes in the order named. The real image produced 
 by the objective is best for projection, at least with the com- 
 paratively low magnifications generally used in a polarizing 
 microscope. An image, a yard in diameter, can be projected by 
 using an arc light and a medium magnification. It may be 
 thrown either on an opaque white screen or on a white transparent 
 
APPENDIX 
 
 133 
 
 screen and viewed from the other side, but it must be exactly 
 focused by means of the micrometer screw of the microscope. 
 The large amount of heating produced by the system of lenses 
 LjL/2 must be avoided by inserting a cooling bath with circulating 
 
 FIG. 162. Projection Apparatus by W. & H. Seibert. 
 
 water K in the path of the rays. Even then great care is neces- 
 sary on account of the sensitiveness of the Canada balsam in the 
 nicols. As the nicols must be left so long in the path of the rays, 
 
 even though cooled, it is advisable to 
 use a microscope equipped with an ad- 
 justment for throwing out the polarizer. 
 The larger and more complicated 
 microphotographic apparatuses can be 
 left entirely out of consideration for 
 reproduction of the phenomena seen in 
 a polarizing microscope, because their 
 magnification is comparatively unim- 
 portant to the petrographer. Perfectly 
 good results can be obtained with any 
 camera after a little practice, provided 
 it is properly set up with the micro- 
 scope, and there is a strong, well-cen- 
 tered source of light. The real image 
 of the object produced by a microphotographic objective, 
 corrected for the chemically active light rays, is reproduced 
 on the plate. A projection ocular is used only for stronger 
 magnifications. The simple apparatus represented in Fig. 
 
 FIG. 163. Photographic Camera 
 
 by R. Fuesz. 
 (1/5 Natural Size). 
 
134 
 
 PETROGRAPHIC METHODS 
 
 163, which can be set on the microscope, is very good for low 
 magnifications. 
 
 Photomicrographs taken in diffused daylight often lack in 
 sharpness, which is the result of long exposure and not very 
 careful manipulation. Various sources of artificial light are 
 much to be preferred on account of the greater constancy of the 
 luminosity of these sources. An arc or lime light is the best. 
 Recourse to color filters, which play so great a r6le in organic 
 microphotography, is had here, only in special cases particularly 
 for photography at very high temperatures. 
 
 Photographing with convergent polarized light is compara- 
 tively simple. The real image from a Bertrand lens without the 
 
 FIG. 164. Drawing Apparatus by Abbe. 
 
 ocular is focused directly on a photographic plate. The Bertrand 
 lenses for ordinary investigations generally have too small an 
 aperture to project the image beyond the tube of the microscope, 
 and hence special lenses with larger focal lengths are needed for 
 photography and projection purposes. It may also be remarked 
 that colored photographs of interference figures have been suc- 
 cessfully made by the use of Lumiere plates. 
 
 Drawing Apparatus. There is a large number of drawing 
 devices depending upon the principle of the camera lucida. 
 The image observed in the microscope is thrown upon a sheet of 
 drawing paper, where it appears uniformly distinct over the 
 whole field and can be traced with a sharp pointed pencil. Two 
 of these constructions have been found to be especially useful. 
 The first of these is the Abbe drawing apparatus, the second the 
 
APPENDIX 
 
 135 
 
 
 FIG. 165. Nachet Drawing 
 Apparatus. 
 
 Nachet. The former is to be preferred because there is no loss 
 of light even when the strongest objectives are used. 
 
 The Abbe apparatus, Fig. 164, consists of two prisms RR' cemented to- 
 gether, forming a cube. They are silvered on the plane of contact with the 
 exception of a small spot in the center. The eye receives the image o from 
 the ocular through this opening, while a mirror placed at a distance of 70 mm. 
 reflects an image of the drawing paper on the silvered surface from which it 
 is reflected to the eye. The difference in 
 intensity of luminosity of the object and 
 the drawing surface can be regulated by 
 inserting plates of smoked glass with differ- 
 ent depths of color in the path of the rays 
 from the latter. 
 
 The Nachet drawing apparatus, Fig. 165, 
 consists of a prism with a rhombic cross 
 section abed on the front of which a small 
 prism efg is cemented over the ocular. The 
 rays o' coming from the object strike the 
 face ef perpendicularly and pass to the eye 
 
 unrefracted. The rays from the drawing pencil p suffer reflection at cb 
 and ad, and then pass out of the apparatus the same as the others. 
 
 It is obvious that an interference figure can also be drawn with this 
 apparatus. It is very useful for measurements under certain conditions, 
 especially when the interference figure has been traced on cross-section 
 paper. 
 
 Summary of Methods 
 
 When all the methods that may be employed for determining 
 crystals with a polarizing microscope are assembled, the following 
 are noted: 
 
 1. Observation of the index of refraction. 
 
 2. Determination of crystal form and cleavage. 
 
 3. Observation of inclusions. 
 
 4. Determination of color and pleochroism, including obser- 
 vations in reflected light. 
 
 5. Recognition of double refraction. 
 
 6. Determination of the position of vibration directions. 
 
 7. Measurement of the double refraction. 
 
 8. Determination of the optical character of the principal 
 zone. 
 
 9. Distinction between uniaxial and biaxial crystals. 
 
 10. Determination of the optical character. 
 
 11. Determination of the position of the optic plane. 
 
136 PETROGRAPHIC METHODS 
 
 12. Measurement of the optic angle and determination of 
 the dispersion. 
 
 Except for the sake of practice for beginners it is not necessary 
 to adhere strictly to the order given. 
 
 In work with the polarizing microscope with a definite object 
 in view, the crystal system is first determined and then the vari- 
 ous properties necessary for the determination of the substance 
 are observed. 
 
 The determination can only be made between crossed nicols 
 and in many instances, especially with biaxial crystals, observa- 
 tions in parallel light must be combined with those in convergent 
 light. 
 
 The following scheme is given for this determination: ' 
 
 1. All individuals of a substance remain dark between crossed 
 nicols through a rotation of 360: Cubic crystals. 
 
 2. Most individuals become alternately light and dark in a 
 rotation through 360 between crossed nicols: Not cubic 
 crystals. 
 
 2a. The vibration directions always lie parallel or sym- 
 metrical to the outlines of the crystal: Hexagonal, 
 tetragonal, or orthorhombic crystals. 
 
 26. The vibration directions lie partly parallel or sym- 
 metrical and partly oblique to the outlines: Mono- 
 clinic crystals. 
 
 2c. The vibration directions always lie oblique to the 
 outlines: Triclinic crystals. 
 
 2a 1$ A uniaxial interference figure is obtained in con- 
 vergent polarized light: Hexagonal and tetragonal 
 crystals. 
 
 2a 2 . A biaxial interference figure is obtained in convergent 
 polarized light: Orthorhombic crystals. 
 
 Crystals of the hexagonal and tetragonal systems can be dis- 
 tinguished from each other only by crystal form and cleavage. 
 
 This schematic arrangement can be modified in its detail, and there is a' 
 considerable difference between the investigation of isolated crystalline 
 powders or cleavage fragments and determinations in thin sections. In the 
 former case the individuals are frequently all, or nearly all, orientated in 
 the same manner on account of crystallographic development or perfectness 
 of cleavage and lie on one certain crystal face. In thin sections, on the other 
 hand, accurate crystallographic orientation occurs only in exceptional cases. 
 The cross sections are orientated in various ways rendering the determina- 
 
APPENDIX 137 
 
 tion of the crystal system easier. These two cases will be considered 
 separately. 
 
 When the observations are made on a powder and none of the individuals 
 affect polarized light, the conclusion cannot be drawn that the substance is 
 cubic. Whether or not it is uniaxial and all the individuals accidentally lie 
 on the basal pinacoid can be determined by observations in convergent light. 
 In a thin section determinations in convergent light are rarely necessary. 
 
 If there are individuals of a substance in the powder or in a thin section, 
 which give different interference colors when the thickness is the same, 
 recourse is taken at once to observations in convergent light. Whether the 
 substance is uniaxial or biaxial can be best determined on those individuals 
 which show the lowest interference color, because these are most inclined to 
 an optic axis which will appear within the field of vision. It will be very 
 difficult for a beginner to find all those individuals of one substance, especially 
 when the crystals are colorless. The interference colors may be different 
 and this may be accompanied by a difference in the whole habit in various 
 directions. The appearance of colorless mica in thin section may be taken 
 as an example. All sections transverse to the basal pinacoid show lath- 
 shaped crystals, sharp cleavage cracks, and high interference colors. Sec- 
 tions of the same mineral parallel to the base have very weak or no double 
 refraction on account of the small optic angle. There is also no trace of 
 cleavage or elongation. The mineral appears this way also when powdered. 
 The brilliant colors observed in the interference figure are proof, however, 
 that the mineral has strong double refraction. 
 
 If it has been found that the substance in a powder is biaxial, an attempt 
 should be made to obtain an interference figure as nearly symmetrical as 
 possible. If the interference figure, obtained from an individual with 
 symmetrical extinction, is symmetrical to two planes, the crystal is ortho- 
 rhombic. If it is symmetrical to one plane only it may be monoclinic. 
 The latter is certainly true if the fragment shows oblique extinction in 
 parallel polarized light. If the crystals have a prismatic development or a 
 prismatic cleavage they can be determined as orthorhombic by the parallel 
 extinction of the individuals oblique to the plane of the optic axes. Dis- 
 tinction between monoclinic and triclinic individuals is very difficult 
 because individuals with oblique extinction are also oblique to the axes, as 
 is generally the case in triclinic crystals. 
 
 The distinction of uniaxial from biaxial crystals in powders -is often very 
 difficult when the crystals are elongated parallel to the principal axis or to 
 the acute bisectrix or when they have good cleavages parallel to these faces. 
 All individuals then lie approximately parallel to the corresponding princi- 
 pal optical directions. An interference figure parallel to the optic axis of a 
 uniaxial crystal and that in a section parallel to the plane of the optic axes 
 or perpendicular to the obtuse bisectrix are of such a character that even an 
 experienced observer may often draw erroneous conclusions from them. 
 When many crystals of approximately the same size are observed, it is 
 noted that all needles of uniaxial crystals show the same interference colors 
 for the same thickness and, when embedded in a mobile liquid and rolled by 
 the movement of it, the interference color does not change. In biaxial 
 crystals, however, there will be a difference when the thickness is the same, 
 
138 
 
 PETROGRAPHIC METHODS 
 
 depending upon Whether the observation is made parallel to the optic nor- 
 mal or the obtuse bisectrix, and this change can be verified when the 
 crystal is rolled by the movement of the liquid. 
 
 The beginner usually experiences difficulty in determining the crystal 
 system, partly on account of the optical anomalies and partly on account of 
 the great similarity which the systems of lower symmetry show to those of 
 higher symmetry in their optical behavior. Optical anomalies in cubic 
 crystals are often of such magnitude that they show quite brilliant inter- 
 ference colors and in basal sections of uniaxial minerals the double refrac- 
 tion may be so low that no definite reaction can be obtained either in con- 
 vergent or parallel polarized light and the mineral may be considered cubic. 
 Such is the case in a cross section of apatite. 
 
 Uniaxial minerals frequently show a transition to biaxial by the opening 
 of the dark cross of the interference figure when rotated, for example, quartz, 
 and vesuvianite, while biaxial crystals approach uniaxial by a small optic 
 
 
 = Bluish -green 
 A ^#= Green 
 
 ^Yellowish- 
 green 
 
 FIQ. 166. 
 
 Fio. 167. 
 
 Optical Sketch of a Crystal. 
 
 angle, thus, phlogopite. Monoclinic and triclinic crystals assume the 
 properties of orthorhombic when their oblique extinction cannot be meas- 
 ured, for example, muscovite and epidote, and oblique sections of ortho- 
 rhombic minerals with a decided deviation from symmetrical extinction can 
 sometimes be observed. The determination of the crystal system in such 
 cases can be made accurately only by a skilled observer. 
 
 Further investigation of minerals, after the crystal system has been de- 
 termined, is best carried out systemmatically according to the scheme given 
 on page 135, using the material in the corresponding chapters as a basis. All 
 the optical properties of the crystal investigated can be shown in a sketch 
 as represented in Figs. 166 and 167. 
 
 These methods of crystal drawing have proved very practical and give 
 a clear idea of the results of the microscopic study. After the form, cleavage, 
 inclusions, etc., have been drawn as true to nature as possible with the aid 
 of a drawing apparatus, the vibration directions are determined and indi- 
 cated on the drawing by arrows. The determination of the relative values 
 
APPENDIX 139 
 
 of two rays vibrating in a cross section, combined with the investigations 
 in convergent light, gives the directions of the axes of greatest, medium, and 
 least elasticity which are indicated in the figure by c, B and c. The index 
 of refraction in any direction is indicated in the following way: if it corre- 
 sponds with that of Canada balsam the corresponding arrow is drawn very 
 lightly; if it is still lower the arrow is dotted; if it is higher the arrow is drawn 
 heavier and is the heavier the greater the difference between the index of 
 refraction for the corresponding vibration direction and that of Canada 
 balsam. If pleochroism is present, it is determined and the arrow for the 
 corresponding direction is labeled accordingly. The amount of the double 
 refraction estimated from the interference color is indicated by arcs connect- 
 ing the arrows for the vibration directions. Here, again, very low double 
 refraction is indicated by dotted lines, while the heavier lines indicate 
 stronger double refraction. When the double refraction is very strong, the 
 lines are doubled. Investigation in convergent light reveals whether the 
 substance is uniaxial or biaxial. If the former is the case and the axis 
 emerges perpendicularly, a small circle with a black cross is drawn in the 
 center of the sketch. If the plane of the drawing is parallel to the axis the 
 same signal is made outside of the sketch. If the section is oblique the 
 interference figure is sketched on the edge of the crystal about as it appears 
 in the microscope. If the crystal is biaxial, the true position of the optic 
 plane is indicated and the symmetry of the emergence of the axes as well as 
 the approximate size of the optic angle is shown in the sketch. There are, 
 naturally, cases in which a sketch of this character does not give the entire 
 perspective of the optical properties, but in a large number of cases detailed 
 descriptions are unnecessary, and a conception of all the optical properties of 
 a crystal can be had at a glance, as far as they can be determined by the 
 qualitative methods of microscopic technic. Fig. 166 is based upon crys- 
 tals of hydrated basic copper sulphate, and Fig. 167 upon an artificially 
 prepared manganese salt corresponding to vivianite, 
 
PART II 
 
 ROCK-FORMING MINERALS 
 
CHAPTER VII 
 Preparation of Material 
 
 Investigation of Rock Powder. Rock powders were used for the earliest 
 studies of rocks under the microscope. The powder on the slide was covered 
 with water (n = 1.333) to reduce the total reflection as much as possible. 
 Later, cedar oil (n = 1.52) or Canada balsam (n = 1.545) was used. This 
 very simple method can still be used to get very good results in determining 
 rapidly the approximate properties of a rock or a mineral. There may be 
 too little of some of the constituents to determine them in the usual way and 
 then recourse is taken to this method, in which case the residues of chemical 
 or mechanical separations are used. There is a large number of minerals 
 that are difficult to determine in a slide, but which show characteristic 
 features in powdered form. This is particularly true for minerals that 
 cleave easily. Cleavage plates may be studied in convergent light, and 
 minerals with similar characters can be easily distinguished. Scapolite, the 
 different members of the mica group and similar .minerals, topaz, cyanite, and 
 prehnite are some which in many instances can be positively determined 
 in cleavage plates only. 
 
 Table 19, I will aid in these determinations. The best method of pre- 
 paring the material is to crush a small grain of the mineral between two 
 object glasses by pressing between the fingers. The larger pieces resulting 
 from this treatment are placed on another object glass and covered with 
 eugenol, which has an index of refraction the same as Canada balsam 
 (n = 1.545). Finally a cover-glass is placed on the preparation. 
 
 This method is especially applicable in the study of porous materials such 
 as sand, and in the investigation of soil of any sort. The index of refraction 
 of the grains may be determined accurately according to the Schroeder van 
 der Kolk method (see Part I, page 38). This is of great importance in 
 the determination of the feldspars which are otherwise difficult to recognize 
 in a powder. (See feldspar group, method No. 7.) 
 
 Various grains of a mineral in a powder may have different thickness. 
 This may make them appear to have different optical properties, which 
 gives rise to much confusion in all investigations on powders, especially for 
 a beginner. Slides, however, can be prepared from such material in the 
 following manner: the powder is mixed with a thick paste of zinc oxide 
 and potassium water glass and the mass is left to harden in a short glass 
 tube of large diameter. A slide is made of the mixture, thus prepared, by 
 the method of making rock sections, to be described below. 
 
 It will thus be seen that the microscopic investigation of rock powders is 
 often quite important and, since these methods frequently lead to conclusions 
 quite different from those obtained in studying a thin section, they should 
 be strongly emphasized in petrographical studies. 
 
 143 
 
144 PETROGRAPHIC METHODS 
 
 In some cases, which are, however, quite infrequent, certain clues are ob- 
 tained by observing the crystallographic form of the rock constituents. 
 Great care must be taken in the preparation of such material that the grains 
 are not broken. The isolated material is examined either without any 
 liquid or immersed in one such as water, which has an index of refraction 
 widely different from that of most of the rock-forming minerals. 
 
 The rock is coarsely crushed in a mortar for mechanical and chemical 
 separations. Then by pounding and hammering, but not by rubbing, a 
 powder is prepared in which the grains are as nearly of equal size as possible. 
 Most of these grains consist of but a single mineral. Grains of various sizes 
 are isolated by means of a series of sieves of from 25 to 250 meshes per square 
 inch. The fine dust, which is formed in large amounts in all these opera- 
 tions, is thus removed. This is quite necessary, at least for the mechanical 
 methods of separation, because the dust would remain suspended for a long 
 time in the rather viscous liquids used in these operations. The dust is 
 only used where there are microlitic constituents in the rock to be studied. 
 On the other hand, a fine dust-like condition is especially favorable for 
 chemical methods of separation and, in this case, it is the coarser grains that 
 must be removed. 
 
 It is evident that the material to be studied and prepared by these 
 various methods should be as fresh as possible and must not be taken from 
 a weathered surface or otherwise altered portion of the rock. The material 
 should be carefully studied first under the microscope before any further 
 investigations or attempts at separation are made. 
 
 Preparation of Thin Sections. Thin sections are the most im- 
 portant preparations for the study of rocks ; not only because a 
 large number of variously orientated mineral sections are shown 
 side by side, but also because the texture of a rock, which is so 
 important in petrography, can be studied. 
 
 The preparation of thin sections, especially of the harder rocks, 
 was formerly an operation requiring unlimited time. They were 
 ground by hand on an iron plate covered with wet emery powder. 
 This method is still used for polishing small rock chips. The 
 operation has been gre'atly simplified by modern cutting and 
 polishing machines but, in spite of this, it is nevertheless a 
 tedious task requiring much practice and skill. It is advisable 
 to have such work done by a specialist, as for instance Voigt 
 und Hochgesang in Goettingen, Germany, or W. H. Tomlinson, 
 in Swarthmore, Pa. Nevertheless, every petrographer should be 
 able to prepare usable sections. This may be necessary in some 
 cases where a knowledge of a rock is desired at once. For this 
 reason, the principal hand machines will be briefly described 
 here. 
 
 A small cutting and polishing machine driven by hand or, if 
 
PREPARATION OF MATERIAL 
 
 145 
 
 the necessary power plant is at hand, one equipped to run by 
 power is an indispensable part of the equipment of a modern 
 petrographical laboratory. Fig. 168 illustrates such a machine 
 designed by Voigt and Hochgesang. Thin plates of quite hard 
 rocks can be cut in a comparatively short time by means of a 
 rapidly rotating metallic disk set with bort, emery, or carborun- 
 dum powder. The plates can then be ground down on the 
 polishing disk. 
 
 FIG. 168. Cutting and Polishing Machine. 
 
 The polishing plate consists of a slightly convex iron disk, a lead plate 
 being used for the coarser manipulations. It can be made to rotate rapidly. 
 At first coarse powder, emery or carborundum, is placed on it with water, 
 while fine powder is used for the finer polishing operations. The plate to be 
 polished is pressed by hand, or by a mechanical device, as evenly as possible 
 on the rotating disk until a plane surface is obtained. Then it is rubbed 
 with fine wet powder and, finally, with dust entirely free from grains until 
 the surface is polished. Thin sections are not given a high polish except 
 10 
 
146 PETROGRAPHIC METHODS 
 
 in special cases, e.g., when they are to be used with a Wallerant total 
 reflectometer (see Part I, p. 40). 
 
 When one surface has been ground as carefully as possible, it is firmly 
 cemented to a small glass plate. Canada balsam, which is used as a cement, 
 is placed on the glass and evaporated on a water bath until it is hard but 
 not brittle. Then it is heated carefully until it becomes fluid and the 
 ground fragment is warmed and carefully pressed into it. The fragment 
 should be moved back and forth slightly to remove the air bubbles from the 
 balsam. When no more bubbles can be seen through the glass the fragment 
 is held firmly for a moment until the balsam has completely cooled. Then 
 the same grinding process is carried out on the other side of the fragment, 
 but naturally great care must be taken that the abrasive agent does not 
 tear a portion of the fragment away. The final polishing is accomplished 
 under special precautions using only a very small quantity of the finest 
 grinding material. It is tested from time to time under the microscope to 
 see whether the proper thickness has been obtained and also to see that the 
 entire section has a uniform thickness. A perfect thin section should be 
 a plane parallel plate 0.03 to 0.04 mm. thick and the surface should cover 
 three or four square centimeters. 
 
 When the section is finished, it is carefully cleaned and removed from its 
 bed by heating. A sufficient quantity of Canada balsam is warmed on an 
 object glass and the section is pushed on to this and worked around until 
 the balsam is free from air bubbles. Then a cover-glass about 0.10 to 0.15 
 mm. thick is cemented over it with Canada balsam. Great care must be 
 used to remove all the air bubbles from the balsam and also not to break the 
 section in transferring it or in pressing the cover-glass on it. The whole 
 process of making sections requires much care and attention, and is ex- 
 tremely tedious, especially for one who has had very little practice. Even 
 with as much care as is possible, it is difficult to prepare a section of the 
 proper thickness large enough to be of much use, until the operator has 
 had considerable experience. Since this involves the loss of a great amount 
 of time, the average petrographer rarely acquires sufficient skill to make a 
 good section. 
 
 The preparation of slides from schistose or clastic rocks, or from any loose 
 fragmental rock is especially difficult. It often is a very tedious operation, 
 and the results may be ruined by an intimate mixing of the abrasive agent 
 with the rock. It is absolutely impossible to clean such a section. To be 
 sure, it is possible to fill the capillary spaces, which exist in such rocks, by 
 treating the fragment from which the section is to be cut, with a dilute 
 solution of mastic or saponlac for a long time under pressure. However, 
 the organic substance, thus introduced, may be dissolved out again by the 
 Canada balsam so that extreme precaution is necessary in the subsequent 
 operations. 
 
 Thin sections are used not only for microscopical investigations, but very 
 frequently also chemical tests may be made upon them. If the section is 
 covered, a portion of the cover-glass is cut with a diamond glass cutter, which 
 is a stylus with a small cleavage piece of diamond fastened in the end, and 
 removed by warming it slightly. The balsam still adhering to the uncovered 
 portion must be carefully washed away with benzol or alcohol so that the 
 
 
PREPARATION OF MATERIAL 147 
 
 chemical reagents may have easy access to the rock. For other investiga- 
 tions the rock section may be cut with a diamond stylus and a portion 
 removed from the object glass by warming slightly so that it is isolated from 
 everything else. After it has been thoroughly cleaned in benzol or alcohol, 
 it may be digested in acid or ignited on a platinum foil, as the case requires. 
 Sections of individual minerals may be removed from such a rock section 
 and they can be investigated micro-chemically. 
 
CHAPTER VIII 
 Methods of Separation 
 
 All methods employed to resolve a rock into its components or 
 to isolate a single rock constituent are designated as methods of 
 separation. Such processes are for various purposes. In some 
 cases a rock constituent, whose accurate determination is not 
 possible under the microscope, is isolated from the. rock, and can 
 then be investigated further, and a chemical analysis made. In 
 other cases it is interesting, as well as profitable, to determine the 
 proportional amounts of the several constituents of a rock. 
 Finally, those minerals, which are only sporadically present, may 
 be concentrated and important clues to the genetic relationships 
 of the rocks concerned may be obtained by studying them. 
 
 The methods of separation generally give good qualitative 
 results, but it often requires a combination of several of these 
 methods to isolate a constituent for quantitative analysis, be- 
 cause the material must then possess a high degree of purity. 
 
 As mentioned above these methods are divided into: 
 
 1. Chemical methods of separation. 
 
 2. Physical methods of separation. 
 
 i. Chemical Methods of Separation 
 
 Chemical methods of separation are for the most part quite 
 simple. They are based upon the different solubilities of various 
 minerals in chemical reagents, but such differences are not so 
 great that one rock constituent may be completely dissolved 
 before the others show signs of solution. These methods, there- 
 fore, require great care and skill in order to obtain satisfactory 
 results. Besides these difficulties, a part of the rock mass is 
 always rendered unavailable for further investigation and a large 
 amount of by-products, which must be removed, is precipitated 
 in the course of chemical separations. Such methods become 
 so involved in their application that they should be employed 
 only under especially favorable conditions, where the physical 
 methods, to be described later, are not effective. 
 
 148 
 
METHODS OF SEPARATION 149 
 
 In some cases acids, in others alkalies, are used for these separa- 
 tions, which maybe carried out at ordinary or at elevated tempera- 
 tures according to the conditions. High pressure may be 
 necessary in some instances, and occasionally a molten solvent 
 may be found quite valuable. The most important reagents are : 
 
 1. Hydrofluoric Acid. In general, hydrofluoric acid attacks 
 quartz and silicates high in silicic acid much more readily than 
 it does those low in silicic acid. Among the more important 
 rock-forming minerals, anatase, andalusite, cyanite, hematite, 
 corundum, magnetite, perovskite, rutile, sillimanite, spinel, brit- 
 tle micas, staurolite, topaz, tourmaline, cassiterite, and zircon 
 are not attacked by hydrofluoric acid or a mixture of it with 
 hydrochloric acid. Minerals of the garnet group, micas, horn- 
 blende, pyroxenes, titanite, etc., are attacked with difficulty, 
 while the feldspars (plagioclase more readily than orthoclase), 
 leucite, quartz and minerals of the sodalite group are easily 
 attacked. Rock glass is the most susceptible of all. 
 
 Isolation of completely insoluble rock constituents is best 
 carried out in an evaporating dish of hammered lead in which 
 powdered fluorite may be treated with sulphuric acid. Thin 
 layers of the rock powder to be treated are spre'ad over one 
 another separated by layers of fluorite and the whole mass is 
 moistened with dilute sulphuric acid or hydrochloric acid. A 
 tight cover is placed on the dish and all joints are sealed with 
 wax or some other cement. The preparation is left standing 
 until the hydrofluoric acid evolved has acted upon the rock 
 powder. A crust of salts forms very rapidly on the mineral mass 
 during the process and it must be destroyed from time to time. 
 The reaction usually proceeds quite slowly, but it can be accel- 
 erated considerably by warming the lead dish a little. The 
 advantage of this process is that large quantities of material can 
 be used, which is of special importance in the isolation of constit- 
 uents occurring in very small quantities. 
 
 In other cases the rock powder may be poured slowly, under 
 constant stirring, into a concentrated solution of hydrofluoric 
 acid, but naturally great precaution must be taken for protection 
 from the vapors evolved. It is advisable to place the platinum 
 dish, in which such a process is carried on, in a cold water bath 
 or a freezing mixture until the first violent reaction has moder- 
 ated, because the action of hydrofluoric acid on such powders fre- 
 quently causes a decided increase in temperature. The reaction 
 
150 PETROGRAPHIC METHODS 
 
 is interrupted at the proper moment by the addition of a large 
 excess of water. The residue must be washed thoroughly 
 and the gelatinous silica, formed during the process, separated 
 from the unaltered mineral grains by pressing first with a plati- 
 num spatula and finally with the fingers. It is poured off with 
 
 the wash water. 
 
 * 
 A quantitative separation of the gelatinous silica cannot be 
 
 made in this manner and the residue must either be heated to 
 change the balance of the silica into a powder that can be washed 
 away, or, after thorough washing, it may be treated with an 
 alkali, which dissolves the separated silica. Hydroxide of iron 
 is generally precipitated in this process and must be extracted 
 with hydrochloric acid. If a small quantity of silica still remains, 
 the treatment is repeated until the material appears sufficiently 
 pure when examined under the microscope. 
 
 2. Hydrofluosilicic Acid. Pure hydrofluosilicic acid does not 
 attack quartz. It can, therefore, be used as a means for the 
 quantitative separation of quartz from a rock. However, 
 secondary reactions take place producing a solvent for quartz 
 so that a considerable portion of it is dissolved. 
 
 3. Hydrochloric, Chlorous and Organic Acids. Dilute hydro- 
 chloric acid is used to isolate silicates from carbonate rocks. How- 
 ever, if the silicates occurring in a granular limestone are easily 
 attacked by hydrochloric acid, as e.g., forsterite and gehlenite, 
 strong organic acids such as acetic and citric are recommended. 
 Even these do not always produce favorable results because they 
 also attack silicates considerably. An aqueous solution of chlorous 
 acid is the best reagent to use in such cases. It is best to prepare 
 this solution fresh by treating a solution of barium chlorate with 
 the calculated amount of dilute sulphuric acid, care being taken 
 to avoid an increase in temperature. After removing the pre- 
 cipitate of barium sulphate, the clear filtrate is used. It attacks 
 calcite quite energetically and has no effect upon most of the 
 silicates. Being an oxidizing agent it may also be used for 
 removing organic substances from rocks. 
 
 Pure, warm hydrochloric acid can be employed and by a 
 skillful modification of the process, it is frequently possible to 
 remove the calcite before any effect on the silicates is noticeable. 
 Usually, better results are obtained than with organic acids. The 
 carbonates are soluble in hydrochloric acid with evolution of car- 
 bon dioxide; the sulphides with the development of hydrogen 
 
METHODS OF SEPARATION 151 
 
 sulphide, and usually with the separation of sulphur. Leucite 
 dissolves with the formation of powdery silica, while scapolite 
 and basic plagioclase are more difficultly soluble under the same 
 conditions. Minerals of the sodalite group, nepheline, melilite, 
 olivine, wollastonite, and most of the zeolites dissolve with the 
 formation of gelatinous silica, while serpentine and chlorite are 
 very difficultly soluble. Apatite, powdered hematite and magne- 
 tite dissolve without any formation of by-products. Magnetite 
 dissolves most readily after addition of potassium iodide. 
 
 Concentrated hydrochloric acid is used to separate various 
 difficultly soluble silicates from each other. However, a perfect 
 quantitative separation cannot often be affected by such means, 
 because the mass must be digested at higher temperatures for 
 some time until one of the constituents is entirely decomposed. 
 During this process the other minerals have either been partially 
 dissolved or considerably altered. For this reason the earlier 
 custom of dividing a rock into a portion soluble in hydrochloric 
 acid and one insoluble, for analytical purposes, has been entirely 
 abandoned. 
 
 4. Sulphuric Acid. Sulphuric acid alone is rarely used to 
 separate rock constituents. Quartz can be separated quantita- 
 tively from the main mass of the silicates by digesting the rock 
 powder with dilute sulphuric acid in a sealed tube at a tempera- 
 ture of 150 to 200. The other constituents are destroyed by 
 this reagent. Application of sulphuric acid in other cases is 
 scarcely to be recommended. 
 
 5. Alum. The methods for separating silicates from carbonate 
 rocks, mentioned above, usually do not produce accurate results, 
 which, however, can be obtained by the use of an alum solution. 
 This dissolves the calcite rapidly and leaves most of the other 
 constituents, even dolomite itself, unattacked. It may be used in 
 cases where a quantitative determination of the proportions of 
 calcite and dolomite in a carbonate rock is desired. 
 
 6. Sodium Hydroxide. Opal can be dissolved out of a rock 
 with sodium hydroxide. This process proceeds quite rapidly 
 even at ordinary temperatures. If a rock powder is digested for 
 a long time with sodium hydroxide on a water bath a large 
 number of the silicates are materially altered; for example, the 
 feldspars are changed into compounds easily soluble in hydro- 
 chloric acid. Other minerals, as pyroxene, quartz, etc., are not 
 altered. 
 
152 PETROGRAPHIC METHODS 
 
 Finally some methods, that are used only in special cases, should be 
 mentioned. Thus spinel and corundum remain unchanged when a rock 
 powder is decomposed with sodium potassium carbonate. Carbonaceous 
 substances can be removed from a rock by heating in an oxidizing flame. 
 A solution of cuprammonium chloride will dissolve metallic iron. Finally 
 bituminous substances can be extracted with ether, benzol, etc. 
 
 2. Physical Methods of Separation 
 
 Physical or mechanical methods of separation depend upon 
 differences in physical properties of the minerals. Specific 
 gravity is by far the most important of these so that the methods 
 dependent upon it must be described in detail. Methods based 
 upon magnetic differences and other properties of the minerals 
 are of much less importance. 
 
 Analyses by Washing. The methods so universally employed in com- 
 mercial processes for separating substances according to their specific 
 gravities can only be used where great differences in specific gravity are 
 involved, as in the separation of ores from rock material. Specific gravity 
 is not the only important factor in this process of separation either- in quiet 
 or circulating water. The size and development of the individual mineral 
 particles are of special significance for, on the one hand, fine dust will remain 
 suspended for a much longer time than coarse sand, and on the other hand, 
 thin plates will settle to the bottom more slowly than rounded grains. 
 Therefore, the washing process for separating rock constituents cannot be 
 used except where the minerals have very different habits, as mica and 
 quartz, or where the difference in specific gravity is quite considerable. 
 Even then complete separation cannot be effected by washing in most cases. 
 
 The apparatus used in soil analysis for separating the various constituents 
 according to their abilities to remain suspended in a liquid is not generally 
 applicable in rock investigation, but another device may be employed for 
 the washing tests. It consists of a series of funnel-shaped glass vessels one 
 above another, each terminating at top and bottom in a glass tube. The 
 powder to be separated is placed in the smallest funnel at the bottom. 
 The lower tube of this funnel is connected with a water tap. A hose from 
 the upper tube leads to the lower tube of the funnel next in size above it. 
 All the connections are made in this manner and the upper glass tube of the 
 largest funnel at the top of the series is made to discharge into a sink. A 
 slow stream of water is passed through the apparatus and it carries the 
 lighter particles along with it and deposits them in the funnel above, 
 where the eddies are weaker on account of the larger size of the vessel. An 
 Eichstaedt apparatus, based on the reduction of eddies with an increase of 
 cross section, gives similar results. 
 
 The methods used in washing precious stones from a matrix give better 
 results, especially after some practice. A shallow porcelain dish may be 
 used for this purpose, or better still a flat shield-shaped safety trough in 
 which the powder is spread out with a little water. The trough is held in 
 
METHODS OF SEPARATION 153 
 
 one hand and is jarred by sharp quick taps with the other. The individual 
 minerals become separated from each other in sharply denned zones. A 
 percussion table, such as is used in ore dressing, can also be constructed, 
 and operated most easily by hydraulic pressure. Serviceable results may 
 be obtained occasionally by blowing or sucking with a regulated stream of 
 air. Mica and graphite are but slightly affected by crushing, while other 
 constituents are reduced to a fine dust that can be blown away by this 
 method. 
 
 Separation According to Specific Gravity. The so-called 
 heavy liquids are used to isolate the rock constituents according 
 to their specific gravities. This process is frequently employed, 
 as the liquids have specific gravities higher than those of the 
 individual rock constituents (Table 21, No. 11). After the rock 
 powder to be separated is immersed in one of them it is well 
 shaken and allowed to stand for some time. The heavier min- 
 erals sink to the bottom while the lighter ones float. 
 
 If the pure powder of two different minerals is well mixed, a 
 quantitative separation of each can be made by using a liquid 
 whose specific gravity is about the mean between those of the 
 powders. The specific gravities of the two powders may, how- 
 ever, be so nearly equal as to differ by one unit in the second 
 decimal place. 
 
 The relations are not so simple in the rocks. Here we rarely 
 have to deal with particles that consist of but one mineral, as 
 was the case in the example given, but the particles generally 
 consist of several minerals intergrown in different proportions. 
 It is, therefore, impossible to separate the individual minerals of 
 a rock by this method. The separation must be carried out in 
 single stages so as to separate the combinations resulting from 
 such intergrowth from the pure minerals. All this reduces the 
 efficacy of the method so that great differences in specific gravity 
 are necessary to make a complete separation. Powders that 
 have passed through a 25 to 50 mesh sieve are best for all these 
 separations. If the grains are much finer, separation takes place 
 very slowly until finally, if the division is as fine as dust, separa- 
 tion does not occur at all, because the fine particles remain sus- 
 pended in the quite immobile liquid. The finest dust must be 
 sifted out or washed away in all cases before the powder is placed 
 in a heavy liquid. 
 
 The mobility of the liquid employed has a great deal to do 
 with the rapidity and accuracy of the separation. Heavy 
 organic liquids, which may be used, are generally quite mobile. 
 
154 PETROGRAPHIC METHODS 
 
 On the other hand, the heavy solutions of inorganic salts in 
 water are but slightly mobile when concentrated, and some are 
 so viscid that small particles may rise or sink in them very 
 slowly. 
 
 Organic liquids have an unpleasant odor and evaporate quite 
 rapidly, but do not act upon the minerals to be investigated. 
 Inorganic solutions are odorless, attack certain minerals quite 
 strongly, also the skin, and are, for the most part, quite poison- 
 ous. Thus each has its advantages and disadvantages. 
 
 To be of value for separations according to specific gravity, 
 liquids must be capable of being easily evaporated and concen- 
 trated so that every gradation of density can be readily produced. 
 In general the process is as follows : grains of the heavier and those 
 of the lighter rock minerals are picked out of the powder. They 
 are placed in the concentrated heavy liquid. The concentration 
 is reduced, by dilution with water in the case of inorganic solu- 
 tions and with ether or benzol in the case of organic liquids, until 
 some of the grains sink while the others float. The whole mass 
 of rock powder is now put into the liquid thus prepared. Another 
 method of obtaining the proper density of the heavy liquid 
 consists in the use of a scale of indicators of known density. 
 The density of the minerals to be separated must also be known. 
 Two of the minerals from the scale, corresponding in specific 
 gravity to the minerals to be investigated, are put in the liquid, 
 and then it is diluted until one sinks while the other floats. The 
 following list of test minerals has been found to be quite practical: 
 
 Sp. gr. Sp. gr. 
 
 1. Sulphur, Girgenti 2 . 070 11. Quartz, St. Gotthard 2 . 650 
 
 2. Hyalite, Waltsch 2 . 160 12. Labradorite, Labrador .... 2 . 689 
 
 3. Opal, Scheiba 2.212 13. Calcite, Rabenstein 2.715 
 
 4. Natrolite, Brevik 2.246 14. Dolomite, Muhrwinkel ... 2.733 
 
 5. Pitchstone, Meissen 2 . 284 15. Dolomite, Kauris 2 . 868 
 
 6. Obsidian, Lipari 2 . 362 16. Prehnite, Kilpatrick 2 . 916 
 
 7. Pearlite, Hungary 2 . 397 17. Aragonite, Bilin 2 . 933 
 
 8. Leucite, Vesuvius 2.465 18. Tremolite, Zillertal 3.020 
 
 9. Adularia, St. Gotthard... 2.570 19. Andalusite, Bodemais.... 3.125 
 10. Nepheline, Brevik 2 . 617 20. Apatite, Ehrenfriedersdorf 3 . 180 
 
 Better results will be obtained in all separations with heavy 
 liquids if the dimensions of the grains are about equal. This 
 method fails completely with thin, scaly minerals like mica, for 
 a portion of the mica will be found with each of the separated 
 parts. Mica scales can be removed by allowing the powder to 
 
METHODS OF SEPARATION 
 
 155 
 
 glide over an inclined plane of rough paper, or better by breathing 
 into quite a large funnel and causing the rock powder to move 
 over the moistened walls; then mica scales will stick to the walls 
 and can be removed later by inverting the funnel and tapping 
 it. This manipulation must be repeated several times. 
 
 Separations with heavy liquids can be carried out in any kind of a vessel, 
 but a beaker is the best and simplest. Comparatively large quantities of 
 the liquid must be used and this is always 
 very expensive. It is also difficult to re- 
 move entirely the light mineral grains 
 floating on the surface of the liquid, 
 before the heavy residue is poured out. 
 
 Under certain conditions, especially 
 when large quantities of heavy constit- 
 uents are to be treated, the separation 
 can be made more rapidly in a beaker than 
 in a separating funnel, because the large 
 mass of powder sinking to the bottom of 
 the funnel easily clogs up the delivery tube. 
 The beaker is filled over two- thirds full of 
 the liquid, whose specific gravity has been 
 accurately fixed for the minerals in ques- 
 tion, and then the powder is poured into 
 it under constant stirring. The powder 
 separates into two distinct parts. The 
 beaker is then set in a large porcelain 
 evaporating dish and left to stand until a 
 layer of clear liquid appears between the 
 floating material and that on the bottom. 
 A hollow conical vessel, a, Fig. 169, is set 
 into the liquid and held in a vertical posi- 
 tion by a clamp around the neck b. A 
 small funnel, c, rests in the upper end of 
 the neck. A large dropping funnel, d, is 
 so arranged above this that the mercury, 
 which it contains, can drop into the small 
 funnel. As the mercury flows slowly into 
 the conical vessel, the latter gradually sinks 
 
 until the beaker overflows. Then the influx of mercury is increased so that 
 the vessel sinks very rapidly. This causes the floating material to be 
 washed over quantitatively into the porcelain dish below. 
 
 It is generally better to carry out the separations in a separat- 
 ing funnel which can be arranged in various ways. The funnel 
 sketched in Fig. 170 has a very simple but practical form. It 
 can be used in nearly all cases so that it will not be necessary to 
 describe more complicated devices constructed for this purpose. 
 
 FIG. 169. Apparatus for Separating 
 Large Amounts of Rock Powder. 
 
156 PETROGRAPHIC METHODS 
 
 The stopcock B is closed and the funnel filled up to E with the 
 liquid. Then the stopcock A is closed and the powder poured 
 into the upper funnel. After shaking vigorously the stopcock 
 A is opened again, whereupon the heavier constituents collect in 
 D. Then A is closed again and the process repeated 
 until the separation is complete. 
 
 Organic liquids, aqueous solutions of inorganic 
 salts, and molten inorganic salts may be used for 
 separations depending upon specific gravity. 
 
 Heavy Organic Liquids. Many organic liquids are 
 characterized by a high density. Only those liquids 
 are applicable for these processes that do not have 
 an injurious effect upon the operator, do not evap- 
 orate too easily, and are quite permanent and not 
 too expensive. 
 
 Tetrabromacetylene and methylene iodide have been 
 FIG. 170. found to be extremely useful. When pure, they are 
 
 Separating 
 
 Funnel. light yellowish, exceptionally mobile liquids, that 
 can be diluted easily to any proportions by the addi- 
 tion of ether or benzol. Since the diluent is very volatile it can 
 be readily removed from the liquid, thus concentrating it. Still 
 there is always a considerable loss, because some of the heavy 
 liquid is evaporated along with the diluent. This makes such 
 work quite expensive, especially when methylene iodide is used. 
 The loss is of less importance with tetrabromacetylene because 
 the operator can prepare it himself. 
 
 Tetrabromacetylene is an almost colorless liquid that can be 
 very easily prepared at any time. Pure acetylene is passed into 
 bromine until the whole solution becomes light yellow. It is 
 very stable and has no destructive effect upon the rock constit- 
 uents. When concentrated its specific gravity is only 3.0, so 
 that its application is quite limited. 
 
 Methylene iodide is distinguished from tetrabromacetylene 
 by a much higher specific gravity, viz., 3.32 at 16 C. It is con- 
 siderably above that of most of the rock-forming minerals. The 
 density increases quite rapidly with a decrease in temperature 
 until at its freezing-point, 5 C., the specific gravity is 3.35. 
 Methylene iodide is decomposed quite readily even by the action 
 of light, and this is a great disadvantage. Iodine separates out 
 and colors the liquid brown. It can be removed by shaking the 
 solution alternately with potassium hydroxide and water after 
 
METHODS OF SEPARATION 157 
 
 which it can be dried with calcium chloride and filtered. An- 
 other method of purifying it is to reduce the temperature a little 
 lower than 5 C. and allow the liquid to freeze. Light colored 
 crystals of methylene iodide are formed and these are separated 
 from the dark brown liquid by squeezing. Still this process is 
 very expensive because methylene iodide is so high priced and 
 some is always lost in the process. A powder that has been 
 treated with methylene iodide should be washed with ether and 
 the washing must be continued a long time before every trace 
 of iodine disappears. 
 
 The specific gravity can be materially increased by dissolving 
 iodine in the methylene iodide. It retains its mobility, but loses 
 its transparency by this process. If iodoform instead of iodine 
 is dissolved in the liquid, a strong smelling, transparent, yellow 
 liquid is obtained with a specific gravity 3.45. However, it can 
 only be diluted with pure methylene iodide. Finally iodine 
 may be added to this solution and a liquid is obtained which is 
 quite mobile, but nontransparent and has a specific gravity as 
 high as 3.6. 
 
 The high index of refraction of methylene iodide (n = 1.75) 
 makes it very valuable for optical determinations. This value 
 is increased by the fact that, upon addition of iodoform or 
 sulphur, the index can be increased to 1.78. It cannot be used 
 with a Bertrand or an Abbe .total reflectometer because, like all 
 liquids containing iodine, it attacks the strongly refracting glass 
 used on these instruments. 
 
 Heavy Solutions. One of the first liquids used to separate 
 rock minerals was a solution of acid mercuric nitrate in water 
 with a specific gravity of 3.3 to 3.4. It was recommended by 
 Count Schaffgotsch. It has a destructive effect on a number of 
 the minerals because of its acid properties and has therefore a 
 limited application. The loss of expensive material may be 
 reduced to a minimum by exercising some care in working with 
 heavy solutions. Since distilled water is used for dilution and 
 washing, little or no expense is involved. Work with heavy 
 solutions is less expensive under all conditions than work with 
 organic liquids. On the other hand, the greater viscosity of the 
 concentrated heavy solutions must be considered. The solutions 
 generally used are : 
 
 1. Potassium Mercuric Iodide, Thoulei Solution. A solution 
 of potassium mercuric iodide, HgI 2 .2KI, is the most convenient of 
 
158 PETROGRAPHIC METHODS 
 
 the various heavy solutions because it can be easily prepared and 
 can be diluted or concentrated to any desired density. It is 
 made by dissolving potassium iodide and red mercuric iodide 
 in the ratio of 4:5 in water, and after the addition of a little 
 metallic mercury it is evaporated on a water bath (not over a 
 direct flame!) until a fragment of fluorite (sp. gr. =3.15) floats on 
 it. Several needle-like crystals form upon cooling and the clear 
 yellow transparent liquid has a specific gravity of nearly 3.2 in 
 summer only 3.17. It is not very mobile when concentrated, 
 but it can be diluted with water to any extent and concentrated 
 again on a water bath. A small excess of potassium iodide 
 does no harm because, upon evaporation, it separates out as a 
 crystalline crust over the liquid, but an excess of mercuric 
 iodide is injurious because it is precipitated upon the addition 
 of water. 
 
 If a solution of a definite specific gravity is to be prepared by 
 dilution, it cannot be done accurately by adding a measured 
 amount of water to the concentrated solution, because there is a 
 strong contraction of the solution when it is diluted. The density 
 of the prepared liquid must be checked by a Westphal balance 
 or by an indicator. The final steps in fixing the specific gravity 
 of a solution should not be attempted by adding water, but by 
 adding a less concentrated solution of the same kind. 
 
 Thoulet solution has some disadvantages among which are its 
 poisonous character and its cauterizing effect on the skin. It 
 attacks metals vigorously with the separation of mercury. It is 
 also very difficult to remove from the mineral powder. Some- 
 times this can only be accomplished by repeated digestion with 
 potassium iodide solution and water. Attention was called to 
 its high index of refraction in Part I, page 37. 
 
 2. A solution of cadmium borotungstate, Klein's solution, 
 9W0 3 .B 2 3 .2(CdOH) +16H 2 O is much more difficult to prepare 
 and is usually not made by the operator himself. When a dilute 
 solution of it is evaporated on a water bath until a film of crystals 
 forms over it, a yellow liquid with a specific gravity of 3.36 is 
 obtained upon cooling. It is quite viscid, but it can be mixed 
 with water in any proportions without decomposition, and its 
 mobility is increased very rapidly upon dilution. The liquid is 
 harmless, but is decomposed by carbonates as well as metals, and 
 these must be removed before using it. The crystals of the 
 tungstate, which form upon concentration, can be purified by 
 
METHODS OF SEPARATION 159 
 
 draining off the mother liquor. They melt at 75 C. to a liquid 
 whose specific gravity is 3. 6, but it is so viscid that it cannot be 
 used to separate fine powders. 
 
 3. Barium mercuric iodide, Rohrbach solution, is quite mobile 
 with a specific gravity of 3.59. It decomposes very easily so 
 that even its preparation (100 parts BaI 2 + 130 parts HgI 2 ) must 
 be carried out with great care. The concentrated solution can- 
 not be diluted with water. A less concentrated solution must 
 be added very slowly by pouring it over the concentrated solu- 
 tion in a thin layer and allowing the two to diffuse. The diffi- 
 culties encountered in diluting this solution are so great that its 
 application is limited and of late it is rarely employed. 
 
 Heavy Molten Liquids. It is necessary to work at high temperatures with 
 molten inorganic salts. Cadmium borotungstate has already been referred 
 to. Other salts recommended for this purpose are: 
 
 Lead chloride, sp. gr. = 5.0, and zinc chloride, sp. gr. = 2.4. When melted 
 they can be mixed with each other in all proportions. The melting-point 
 is about 400 C. The operations may be carried on best in a small test-tube 
 on a sand bath, and the powder to be separated should be poured into the 
 melted mass in small portions. The liquid is very viscid and splutters a 
 great deal so that it is quite dangerous to use it. When the separation is 
 completed, the mass is cooled and the cake, thus forced, is cut through the 
 middle horizontally. The mineral particles are freed from the mass and 
 purified by digestion with water to which a little nitric acid has been added. 
 Mercurous nitrate melts at 70 C. to a very mobile liquid with a specific 
 gravity of 4.3. It can be liquefied very easily on a water bath and diluted 
 in any proportions with warm water. It possesses, however, the very 
 disagreeable property of decomposing with a rapid separation of a basic salt 
 which has a high melting-point. For this reason the molten salt often begins 
 to solidify before the separation of the rock powder is complete. If the melt 
 is diluted with water it is somewhat less decomposable. Molten silver 
 nitrate is also very useful. By heating to 200 C. a mobile liquid is obtained 
 with a specific gravity of 4. 1 . This can be diluted in all proportions with potas- 
 sium nitrate and a rock powder can be completely separated in this liquid. 
 However, the high temperature at which one must work makes the use of 
 this salt quite unpleasant. Thallium silver nitrate is prepared by dissolving 
 equivalent amounts of thallium and silver in nitric acid. At 75 C. the 
 double salt forms a liquid, as mobile as water, and has a specific gravity of 
 nearly 5.0. It can be diluted in all proportions with water and concentrated 
 again by evaporation on a water bath. It is necessary to work very care- 
 fully with this melt because the specific gravity increases upon cooling as 
 well as upon the evaporation of the water. The separation should be 
 carried out as rapidly as possible on a water bath at constant temperature. 
 Then the mass should be solidified rapidly by directing a stream of cold 
 water upon the outside of the containing vessel. Its properties are very 
 similar to those of thallium mercurous nitrate with a specific gravity of 5.3, 
 
160 
 
 PETROGRAPHIC METHODS 
 
 except that it is decomposed by sulphides. Both of these salts are, however, 
 quite expensive on account of the high price of thallium. 
 
 Magnetic Separation. The methods of magnetic separation give good 
 results in many cases. Sometimes a simple magnet and, sometimes, an 
 electro-magnet is used for this purpose. A simple magnet is very useful to 
 draw out steel fragments which have been introduced into the powder dur- 
 ing pulverization and would decompose Thoulet's solution. Besides metal- 
 lic iron, only magnetite and pyrrhotite are attracted by a simple magnet. 
 They are best removed by spreading the dry rock powder out on a paper 
 stretched on a frame supported by four legs. Then a magnetized steel comb 
 is drawn along underneath the paper. The magnetic particles are thus 
 drawn to the edge of the paper. 
 
 FIG. 171 a. Electro-magnet. 
 
 An electro-magnet has a much more extensive application. The best 
 form of this is a horseshoe. A rectangular piece of soft iron is screwed on 
 each pole and so arranged that by turning the pieces the distance between 
 the poles can be regulated, Fig. 17 la. Thus, the strength of the electro- 
 magnet can be controlled. Most of the minerals containing iron are 
 attracted by it, while those free from iron quartz, feldspar, etc. remain 
 behind unless they contain magnetic inclusions. The degree of magnetic 
 attraction is independent of the amount of iron present. As yet, minerals 
 have not been systematically arranged according to the degree of attrac- 
 tion, but a short classification of the most important minerals can be found 
 in Table 19, No. VI. 
 
 To separate a rock powder electro-magnetically, it is spread out on a stiff 
 piece of paper and passed back and forth under the poles of an electro-mag- 
 
METHODS OF SEPARATION 
 
 161 
 
 net, the strength of the magnet being regulated as described above. This 
 paper is removed from time to time and a clean sheet is placed under the 
 magnet. Then by breaking the circuit the attracted particles fall off. 
 
 The separation can be made in water or alcohol when very fine powder is to 
 be treated. The rock powder is washed in the liquid and then the latter is 
 caused to pass over the magnet. The process can be accomplished in much 
 the same manner as it is on a large scale in the commercial separation of 
 ores. Numerous minerals that are only slightly magnetic become strongly 
 magnetic when heated to redness in 
 an oxidizing flame. This phenom- 
 enon finds extensive application in 
 science as well as in commercial 
 processes. 
 
 Other Methods of Separation. 
 Difference in fusibility has been 
 suggested as a means of separating 
 minerals, but this method will not 
 give good results except when the 
 difference is very pronounced and 
 the mineral particles are of even 
 size. The powder is spread out oh 
 a platinum foil and heated to 
 glowing in a blast flame until the 
 more easily fusible part is melted 
 together, then the unmelted portion 
 is simply shaken off. 
 
 Finally, there are cases in which certain rock constituents must be sorted 
 by hand. A lens is used in this work, but it must not have too strong a 
 magnification because, on the one hand, the short focal length of a high 
 power objective lowers it so as to be in the way of the operator and, on the 
 other hand, the unsteadiness of the hand is more evident. The instrument 
 shown in Fig. 171 is quite useful for such operations. A strip of glass whose 
 width equals the diameter of the lens, can be pushed back and forth under 
 the lens. The powder is spread out evenly on this glass by means of a 
 distributer and is observed through the lens. The individual grains are 
 then removed with a pair of small forceps or a sharp moistened splinter of 
 soft wood. The powder can also be immersed in a liquid on the glass strip 
 to make the grains more transparent. It is often advantageous to use a 
 pointed glass tube attached to a suction pump to remove the desired grains. 
 These are drawn through the tube over into a glass flask arranged for this 
 purpose. With a little practice very fine rock particles can be sorted out 
 with this apparatus. It can also be used to examine the particles in polar- 
 ized light by placing a glass plate at the proper angle below the stage and a 
 nicol prism above the lens. In this way the apparatus has an extensive 
 applicability. A binocular microscope, Part I, page 41, also gives good 
 results in such cases. 
 
 FIG. 171. Lens Stand. (W. and H. Seibert.) 
 
 11 
 
CHAPTER IX 
 Methods of Investigation 
 
 Investigation with a polarizing microscope is the most impor- 
 tant and most frequently used of all the methods of petrograph- 
 ical research. Next in importance to the microscope are the 
 quantitative and qualitative chemical analyses which enable 
 one to make as complete a determination of a rock as does an 
 optical examination of a thin section. A determination of the 
 specific gravity and other physical properties is also made to 
 complete the information concerning a mineral. 
 
 The methods of investigation, like those of separation, fall 
 naturally into two groups. These are the chemical and physical 
 methods. The treatment here includes only certain subordinate 
 sections of each because the general chemical methods are not 
 within the province of this book, while the optical methods have 
 been thoroughly discussed in Part I. 
 
 Under the heading of chemical methods, only such chemical 
 reactions are discussed as are adapted to a rapid recognition of 
 minerals in the simplest way possible, especially with very small 
 amounts of the material. Special reactions, which are very 
 characteristic for certain minerals, will also be given. The 
 physical part includes all physical methods except the optical 
 investigations. 
 
 i. Chemical Methods of Investigation 
 (a) General Reactions 
 
 The methods of micro-chemistry may be mentioned first, as of 
 importance in the general reactions of a number of the elements. 
 These can only be applied successfully when there is nothing to 
 interfere with them. In many cases the tests are so simple and 
 easy that good results may be obtained with them, but in just as 
 many other cases they are very complicated and the obtaining of 
 unquestionable results is not dependent upon a large amount of 
 practice alone but also upon numerous other contingencies, the 
 
 162 
 
METHODS OF INVESTIGATION 
 
 163 
 
 efficaciousness of which cannot be positively determined in every 
 case. Only such of the numerous micro-chemical methods will 
 be discussed as are comparatively simple and give definite 
 results. 
 
 Reactions with Fluosilicic Acid. The reactions with fluosilicic acid, which 
 is replaced by hydrofluoric acid in the investigation of silicates, are especially 
 important for petrographical studies because the fluosilicates formed are 
 very soluble and crystallize easily. They do not possess the importance 
 generally ascribed to them because, in addition to the characteristic salt of 
 any base, frequently double salts also separate out, and the composition of 
 these as well as the conditions under which they form have not as yet been 
 thoroughly studied. They do not seem to give good results in the investi- 
 gation of minerals like the feldspars containing aluminium and alkalies 
 
 FIG. 172. Sodium Fluosilicate. 
 
 FIG. 173. Calcium Fluosilicate. 
 
 because a double salt is always formed in varying amounts and its exact 
 composition is unknown. These investigations are carried out on an object 
 glass covered with a uniform thin layer of Canada balsam free from bubbles 
 and not completely hardened. It can be prepared by placing some balsam 
 on a glass and warming it slightly. A celluloid plate can also be used, but 
 celluloid always shows some double refraction phenomena, and this inter- 
 feres with the optical investigation. A drop of pure fluosilicic acid (with 
 silicates pure hydrofluoric acid suffices) is placed on the mineral splinter 
 and the solution is left in a closed box of dry wood to evaporate completely. 
 The crystals are formed first toward the end of the process. Every trace of 
 acid must be positively absent before investigating under a microscope, or 
 there is danger of etching the objective. The fluosilicates of sodium, 
 potassium, and calcium have higher indices of refraction than Canada ' 
 balsam, while the indices of magnesium, iron, and manganese fluosilicates 
 are lower. 
 
 Sodium fluo silicate, hexagonal; low double refraction, negative; mostly in 
 short, thick prisms and twins as shown in Fig. 172, or in long needle-like or 
 pyramidal crystals rarely tabular, parallel to the base. Potassium fluosilicate, 
 
164 PETROGRAPHIC METHODS 
 
 cubes and octahedrons, usually quite large ; often distorted, tabular octahed- 
 rons, quite similar to the plates of sodium fluosilicate mentioned above. Lith- 
 ium fluosilicate, not very characteristic ; acicular and monoclinic, apparently 
 hexagonal. Magnesium fluosilicate, as well as the similar compounds of iron, 
 manganese, nickel and cobalt : rhombohedral, colorless or very light colored ; 
 short to long prismatic, often with many faces; high double refraction, 
 positive; characterized by brilliant interference colors. Calcium fluosili- 
 cate, short, monoclinic prisms with inclined terminations, Fig. 173, low 
 double refraction, negative; 6 = 0, c : B = 40 /. Irregular growths with 
 rounded outline are generally observed; crystals, rare. The crystals of 
 strontium and barium fluo silicates are quite similar to these. The latter are 
 usually very small, since they are quite insoluble. Aluminium fluosilicate 
 is a gelatinous substance. It forms rhombohedral double salts with the 
 alkalies and these salts have strong double refraction. 
 
 Reactions with Certain Elements. For these reactions the 
 minerals to be tested are dissolved in sulphuric or hydrochloric 
 acid, or they can be fused in a platinum crucible with sodium 
 potassium carbonate and a small amount of saltpeter or potas- 
 sium acid sulphate. Sometimes potassium fluoride or fluorite 
 
 is added. The salts obtained 
 from these operations may 
 then be further studied. 
 Flame or blowpipe reactions 
 give more definite results in 
 many cases than micro-chem- 
 ical tests, the latter being 
 used in special cases. The 
 most important and reliable 
 reactions that will be consid- 
 ered are the following: 
 
 Sodium. The yellow color 
 of the sodium flame serves as 
 FIG. 174. Sodium Uranyi Acetate. an indication of this element. 
 
 The slightest trace of sodium 
 
 impurities will produce the yellow flame, hence it is better to make 
 a micro-chemical test for the sake of accuracy. A drop of an 
 acetic acid solution (N. B. free from sodium!) of uranyl acetate 
 is added to the solution to be investigated. Sharply defined, 
 light yellow tetrahedrons of sodium uranyl acetate are formed 
 with an index of refraction less than that of Canada balsam. 
 Fig. 174 shows these crystals under the microscope with the 
 tube slightly raised. If magnesium, or a similar element, is 
 present in the solution, a triple acetate containing all these ele- 
 
METHODS OF INVESTIGATION 165 
 
 merits is formed. It is easily obtained in large rhombohedral 
 crystals of very light yellow color, low index of refraction, and 
 very high negative double refraction. It generally forms com- 
 binations similar to a cubic icositetrahedron. If there is but a 
 small amount of sodium present, some magnesium salt can be 
 added, if not already present, and a very sensitive reaction for 
 sodium can be obtained with the triple acetate, since it contains 
 only 1.5 per cent, sodium. 
 
 Potassium. The violet colored flame of potassium is obscured 
 by sodium if it is present at the same time. The potassium color 
 can be recognized when the colored flame is observed through a 
 blue cobalt glass, but numerous potassium silicates do not give 
 the characteristic coloration. These must be heated with 
 gypsum, free from alkali, or with a mixture of gypsum and fluor- 
 ite. Potassium is best precipitated as potassium platinochloride 
 which begins to crystallize only after a long time has elapsed. 
 Crystallization can be accelerated by adding a drop of alcohol. 
 Sharp, yellow octahedrons and combinations of it with the cube 
 are formed, which have a very high index of refraction. Am- 
 monium, rubidium and caesium give rise to the same type of 
 crystals. 
 
 Lithium. The carmine red colored flame of lithium is obscured by sodium. 
 Nevertheless a very small quantity of lithium can be recognized in the 
 presence of a predominating amount of sodium in an ordinary flame. The 
 powdered mineral is stirred by a heated platinum wire, which with the 
 adhering particles is dipped into tallow. The lithium color appears when 
 this preparation is burned in an ordinary candle flame. The determination 
 by means of a spectroscope is absolutely positive, while micro-chemical re- 
 actions, e.g., precipitation as a carbonate, are less reliable. If a little potas- 
 sium carbonate is added and the solution evaporated, lithium carbonate 
 crystallizes in slender needles having a strong double refraction, parallel 
 extinction, and a negative character of the principal zone. These needles 
 almost invariably unite to form sheaf-like or plume-shaped aggregates, and 
 are characterized by a lower index of refraction than Canada balsam. 
 
 Calcium. The yellowish-red flame of calcium salts is not 
 characteristic enough. A micro-chemical test for calcium is 
 better. A drop of very dilute sulphuric acid is added to the 
 solution of a calcium salt and upon evaporation slender needles 
 of gypsum crystallize out, which tend to unite into star- or 
 sea urchin-shaped aggregates, especially in acid solutions. When 
 the calcium content is small, it is better to evaporate the sul- 
 phuric acid off of the object glass and to take up the residue with 
 
166 PETROGRAPHIC METHODS 
 
 a drop of water. Then, the individuals appear as needles with 
 oblique terminations making an angle of 65 1/2, Fig. 175. They 
 unite very readily to form the characteristic swallow-tail twins 
 with a reentrant angle of 131. See the description of the mineral 
 gypsum on a subsequent page for its optical properties. Calcium 
 oxalate forms small crystals very difficult of determination. 
 
 Strontium and Barium. The purplish-red flame of strontium and the 
 yellowish-green of barium are not very reliable tests without the aid of a 
 spectroscope, but good tests may be obtained micro-chemically. Some 
 potassium oxalate is added to a neutral solution of the salts, which is left 
 
 to stand for some time on an object 
 glass, covered so as to retard evapora- 
 tion. Barium oxalate crystallizes out 
 in fan-shaped crystal skeletons and 
 needles with oblique end faces, 115. 
 They have an extinction c : fi =24. 
 The lowest index of refraction exceeds 
 that of Canada balsam only slightly. 
 The double refraction is very high. 
 Strontium oxalate usually shows two 
 types of crystallization, but four- 
 pointed stars are especially charac- 
 teristic. When they lie horizontally, 
 they do not affect parallel polarized 
 light and in convergent light they 
 FIG. 175. Gypsum. give an expanded cross that shows 
 
 positive double refraction. It is quite 
 
 significant that when they are embedded in liquid Canada balsam they 
 disappear entirely and cannot be found again in polarized light. The other 
 type of crystallization shows small rectangles with horn-like projections 
 on the corners. The crystals have strong double refraction and an 
 index of refraction somewhat higher than the other type of crystals. 
 Barium oxalate crystallizes from hot solutions in elongated six-sided, 
 tabular crystals that can be recognized as penetration twins of two or 
 more individuals. They are perhaps triclinic with cross sections like 
 crystals of stilbite. The double refraction is rather low. Strontium 
 salts, under these conditions, give small lancet-shaped bars with a positive 
 principal zone, parallel extinction, and medium double refraction. The 
 index of refraction is less than that of Canada balsam. They form star- 
 shaped twins at an angle of about 90 and give no interference figure in 
 convergent light. These tests, as well as numerous other micro-chemical 
 tests, are of little value to prove the presence of these two alkali earths. 
 
 Beryllium. There is no practical method to prove absolutely the presence 
 of beryllium in small amounts. 
 
 Magnesium. The micro-chemical test for magnesium is car- 
 ried out after the iron and manganese salts have been precipi- 
 
METHODS OF INVESTIGATION 
 
 167 
 
 tated by a slight excess of ammonia in the presence of sal ammo- 
 niac. The solution is greatly diluted and sodium phosphate 
 added. It is then allowed to evaporate, and as the solution is 
 concentrated, hemimorphic orthorhombic crystals of magnesium 
 ammonium phosphate, Fig. 176, form. They are much more 
 perfect than when they are precipitated rapidly by an excess of 
 ammonia. They have a rather low double refraction, and lower 
 indices of refraction than Canada balsam. The positive bisectrix 
 has a somewhat oblique posi- 
 tion in the trapezoidal crys- 
 tals. The axial angle is very 
 small and the optic plane coin- 
 cides in direction with the 
 parallel edges of the crystals. 
 
 Iron. The formation of 
 Prussian blue upon the addi- 
 tion of potassium ferrocya- 
 nide to an oxidized solution 
 of iron salts is a far better 
 test for the presence of iron 
 than any micro-chemical reac- 
 tion. Even if there is an ex- 
 ceptionally small content of 
 
 iron, the solution is colored a distinct green. If there are but 
 traces of iron in the solution, the best test is to note the red color 
 upon the addition of potassium sulphocyanide. 
 
 Nickel and Cobalt. These two elements are rare constituents of rock- 
 forming minerals. Small amounts of nickel can be detected by adding 
 yellow ammonium sulphide to an ammoniacal solution of nickel. The 
 liquid becomes brownish if only a trace is present. Still smaller traces can 
 be detected by the rose-red to brown color resulting upon the addition of 
 sodium trithiocarbonate to a nickel solution. Sodium trithiocarbonate is 
 prepared by shaking a sodium sulphide solution with carbon disulphide. 
 Cobalt must be removed as potassium cobalt nitrate before this test can be 
 carried out, because cobalt blackens the solution. The best test for nickel 
 is the following: a strong ammoniacal solution is either violently shaken in 
 air or is boiled with powdered a-dimethyl glyoxime when a scarlet precipi- 
 tate is formed. 
 
 The simplest test for cobalt is the blue bead of microcosmic salt. If 
 potassium nitrite is added to a dilute ammoniacal solution of cobalt, which 
 is then acidified with acetic acid, small cubes of deep yellow potassium 
 cobalt nitrite are precipitated. They often appear entirely opaque under 
 the microscope. The following reaction is more sensitive: potassium 
 
 FIG. 176. Magnesium Ammonium 
 Phosphate. 
 
168 PETROGRAPHIC METHODS 
 
 sulphocyanide is added to a solution containing cobalt and ethyl alcohol 
 carefully poured over it. The alcohol assumes an intense blue color. 
 
 Manganese. The smallest traces of manganese can be detected by fusing 
 the mineral with sodium carbonate and saltpeter on a platinum foil. If 
 there is considerable manganese, the melted mass appears green, but if 
 the content of manganese is quite small and silica is present, it is sky 
 blue. This color may be clearly observed with extremely small traces of 
 manganese. 
 
 Chromium. The presence of chromium can be proved by fusion in a 
 bead of microcosmic salt in an oxidizing flame. The bead is red when hot, 
 and emerald green when cold. The mineral may also be fused with sodium 
 carbonate and saltpeter on a platinum foil and the melt dissolved in hot 
 water. When the solution is acidified with nitric acid and a drop of silver 
 nitrate added, it becomes brilliant red and deep red rhombohedrons of silver 
 chromate, with an angle of about 70 and a high index of refraction, are 
 formed. With only traces of chromium the solution has a fluorescent yel- 
 low color like that characteristic of a dilute solution of eosine. 
 
 Tungsten. Minerals containing small traces of tungsten form a blue glass 
 when fused with phosphorous pentoxide in a platinum crucible. 
 
 Molybdenum. A solution of 1 gr. phenyl hydrazine in 70 gr. of 50 per 
 cent, acetic acid gives a good reaction for molybdenum. The mineral is 
 dissolved and digested two minutes in the reagent, and if molybdenum is 
 present, the solution becomes red and can be decolorized again by shaking 
 with chloroform. 
 
 Cerium, Lanthanum, Didymimium, Yttrium, Erbium, Thorium, etc. 
 No reactions are known for positively detecting the so-called rare earths 
 in the small quantities in which they occur in rocks. The micro-chemical 
 methods that might be used do not give rise to well developed crystals and, 
 furthermore, the difference between the various earths has not been thor- 
 oughly studied. Spectrum analysis appears to be the only method that 
 affords any degree of certainty in the determination of the rare earths. 
 Even by this method, only the recognition of neodymium and praseodymium 
 is positively certain. The mineral grain to be studied is focused under the 
 microscope with such a magnification that the grain nearly covers the whole 
 field of vision. Then a good spectroscope is placed over the ocular when 
 the presence of neodymium will be noticed by a broad absorption band in 
 the yellow and a somewhat narrower one in the green. If praseodymium is 
 present, the strongest absorption occurs between blue and violet and a 
 weaker one in the blue. This test is used on rock-forming minerals only 
 when monazite is a constituent of the rock. Monazite generally contains 
 both elements. 
 
 Aluminium. A fragment of a mineral is moistened with cobalt 
 nitrate solution and heated to redness in a blowpipe flame in 
 order to test for aluminium. It becomes blue, if aluminium is 
 present, but this color is easily obscured by oxide of iron. The 
 presence of aluminium can be proved micro-chemically by adding 
 slightly diluted sulphuric acid and a small grain of some caesium 
 
METHODS OF INVESTIGATION 169 
 
 salt to a solution of the mineral, when well developed octahedrons 
 of caesium alum will be formed. These are characterized by an 
 especially high index of refraction. If dendrites form instead of 
 the crystals they can be recrystallized in water and octahedrons 
 obtained. Ferric salts give the same reaction, but the two can be 
 differentiated by treatment with ammonium sulphide vapor. If 
 larger amounts of ferric oxide are present, the reaction is indis- 
 tinct. Iron must be reduced first with sulphurous acid before 
 the test for aluminium is made. 
 
 Boron. The green flame is a sufficient test for the presence of boric acid 
 in minerals. It is obtained when the powdered mineral is mixed with 
 fluorite and potassium bisulphate in a platinum crucible and placed in a 
 flame Turner's test. The reaction proceeds better if the mineral is decom- 
 posed by fusion and the melt dissolved in hydrochloric acid. Methyl 
 alcohol is added to this solution and when ignited it burns with a green 
 flame. For a micro-chemical test, the mineral, for example tourmaline, is 
 fused in a platinum crucible with sodium carbonate, if it is too difficultly 
 soluble in hydrofluoric and sulphuric acids. The fused mass is then dis- 
 solved in concentrated sulphuric and hydrofluoric acids. A platinum cover 
 with a drop of water adhering to the under side of it is placed on the crucible 
 and the latter is warmed until the acid begins to vaporize. The drop of 
 water takes up the silicon tetrafluoride as well as the boron fluoride which 
 develops. This solution is placed on an object glass covered with Canada 
 balsam and potassium chloride is added. Upon evaporation rhomboidal 
 or elongated six-sided tabular crystals of potassium fluoborate separate out. 
 When these are recrystallized from hot water they develop prisms with 
 domes and pyramids. The indi ces of refraction are lower than that of Canada 
 balsam and the double refraction is so low that it can only be recognized 
 in thick crystals with a gypsum test plate. The principal zone is negative. 
 
 Carbon. If carbon is present in a rock as such or as a constit- 
 uent of an organic compound, it is converted into a carbonate by 
 fusing the substance with potassium antimonate. The carbonate 
 can then be tested by its effervescence with acid. If only a very 
 small quantity of carbonate is present, the rock powder is spread 
 out with water on an object glass and a cover-glass is placed over 
 it. Then a drop of dilute hydrochloric acid is placed in contact 
 with the edge of the cover-glass and a piece of filter paper is 
 inserted under the other side of the glass. The acid is slowly 
 drawn under the cover and the formation of small gas bubbles in 
 the solution can be observed under the microscope. 
 
 Silicon. The formation of skeletons of silica in a bead of 
 microcosmic salt cannot always be definitely observed. The 
 best method for the detection of silica is a micro-chemical test 
 
170 PETROGRAPHIC METHODS 
 
 carried out in the same manner as was described for boron, except 
 that sodium chloride is used instead of potassium chloride. The 
 powder may also be decomposed by fusion with sodium carbonate 
 and the melted mass treated with pure hydrofluoric acid. In 
 both cases the crystallization of sodium fluosilicate is to be 
 observed. 
 
 Titanium. Minerals in which, titanium is suspected are fused 
 with potassium bisulphate, dissolved in water and treated with 
 hydrogen peroxide. With small traces of titanium the solution 
 is colored light yellow, and with larger quantities deep brown. 
 The color is most easily recognized if the operation is carried out 
 on a porcelain plate. 
 
 Zirconium. It is difficult to prove the presence of zirconium 
 oxide in rocks with absolute certainty, because it is usually 
 present only in very small quantities. The substance is fused 
 with about double its amount of sodium carbonate for a long 
 time and the fusion product dissolved in hot acidified water. 
 Zirconium is found in the insoluble residue as small six-sided 
 crystals similar to tridymite. The index of refraction is lower 
 than that of Canada Balsam. They often gather into packets 
 similar to a pile of coins. Another method of testing for zircon- 
 ium consists in fusing the powder with potassium bisulphate and 
 precipitating the zirconium oxide with ammonia. After filtering, 
 the precipitate is dissolved in hydrochloric acid and a little tin 
 foil is dropped into the solution. If zirconium oxide is present 
 the solution colors turmeric paper brown, while solutions contain- 
 ing titanium do not color it after reduction. 
 
 Tin. If a mineral is thought to contain tin, it is fused on a charcoal 
 tablet with sodium carbonate and potassium cyanide. The tin is reduced to 
 a metallic bead that can be seen with a lens. If the bead is not observed, 
 the melt is placed on a platinum foil and dilute hydrochloric acid with a 
 very little platinum chloride added. An intense red-brown color results if 
 tin is present. A borax bead, containing a little copper oxide and tin, 
 heated alternately in an oxidizing and reducing flame, becomes ruby 
 colored. This is a very sensitive reaction. 
 
 . Vanadium. Vanadium is often present in rocks in very small quantities. 
 The rock powder is fused with sodium carbonate and saltpeter and the melt 
 dissolved in slightly acidified water. The solution becomes red in color 
 when hydrogen peroxide is added, provided vanadium is present. When a 
 grain of sal ammoniac is added to a solution of the melt, microscopic crystals 
 of ammonium metavanadinate are formed. They have a very character- 
 istic shape like that of a whetstone or a hatchet. 
 
 Niobium and Tantalum. A mineral is fused with a large amount of 
 
METHODS OF INVESTIGATION 171 
 
 caustic potash to test for niobates and tantalates. The melt is dissolved in 
 water and hydrochloric acid added. Both salts are thrown down as a 
 white precipitate, and after filtering, the precipitate is dissolved in sulphuric 
 acid. A piece of zinc is dropped into the solution which is then heated to 
 boiling. Niobates color the solution smalt blue, the color being retained 
 for a long time upon dilution until gradually the white powder forms again. 
 Tantalates produce a grayish-blue color which rapidly fades when the 
 solution is diluted. 
 
 Phosphorous. Phosphoric acid is present in rocks only in 
 small amounts. When a rock powder is heated to redness with 
 magnesium powder in a glass tube, the phosphoric acid is reduced, 
 causing a brilliant pyrotechnic display. When the product 
 formed is thrown into water the odor of phosphine can be readily 
 recognized even though only a small trace of phosphorous is 
 present. By another method the rock powder is digested with 
 nitric acid and the solution treated with ammonium molybdate. 
 After a long time a sulphur yellow precipitate of ammonium 
 phosphomolybdate is formed. Under the microscope it seems to 
 consist of small, yellow, rounded dodecahedrons or of delicate 
 skeletal crystals. However, if soluble silicates are present at the 
 same time, there is danger of confusing the ammonium phospho- 
 molybdate with ammonium silicon molybdate which may be 
 formed. It is well to render the silica insoluble by evaporating 
 and heating on the object glass, and then to redissolve the powder 
 before the addition of ammonium molybdate. The formation 
 of magnesium ammonium phosphate, mentioned above un- 
 der magnesium, is also a useful reaction for the detection of 
 phosphorous. 
 
 Sulphur. Sulphur in any form can be tested by fusing a 
 mineral containing it with sodium carbonate on charcoal. The 
 melt is crushed on a silver coin and moistened. A brownish- 
 black spot is produced Hepar test. The melt also colors an 
 alkaline solution of sodium nitrocyanide violet. 
 
 Chlorine. Chlorine can be detected when a mineral powder 
 is fused in a bead of microcosmic salt saturated with copper oxide. 
 The flame becomes distinctly blue. The micro-chemical test is 
 also good. Silver nitrate is added to an ammoniacal solution of 
 a chloride and is left in the dark to evaporate. The resulting 
 octahedrons of silver chloride have a high index of refraction and 
 become clouded and violet when left in the light. 
 
 Fluorine. To test for fluorine, the mineral is fused with salt 
 of phosphorous in a small glass tube. The tube is etched near 
 
172 PETROGRAPHIC METHODS 
 
 the melted material. The mineral can also be heated with con- 
 centrated sulphuric acid in a platinum crucible on the cover of 
 which a drop of dialyzed silica adheres. This drop is later re- 
 moved and placed on an object glass covered with Canada balsam 
 and some sodium salt is added. The characteristic crystals of 
 sodium fluosilicate are formed upon evaporation if fluorine were' 
 present. 
 
 Finally, the test for water must be mentioned. Attention 
 must be called to the fact that with many minerals, very careful 
 and extended drying is necessary to remove all the hygroscopic 
 water. The best test for water is carried out in the following 
 manner. A narrow glass tube about 10 cm. long is drawn out at 
 one end into a capillary and dry air is drawn through it while 
 heating. Then the capillary is closed by fusion and the rock 
 powder introduced into the other end of the tube which is like- 
 wise closed. During this process care must be taken that no 
 moisture is introduced into the tube from the burner gas. Upon 
 heating the powder in the sealed tube, whatever water is present 
 can be driven into the capillary containing a small quantity of 
 some dry dye. The dye is dissolved by the drop of water as it 
 condenses and imparts an intense color to the drop. 
 
 b. Special Reactions 
 
 Special reactions can be applied either to isolated grains or directly to 
 thin sections. If the slide is to be subjected to chemical investigation it 
 is left uncovered, or if covered, the cover-glass can be removed by warming 
 it slightly. The section itself must then be thoroughly cleaned from 
 Canada balsam by washing with benzol. For certain reactions, such as 
 heating tests, etc., the rock section must also be removed from the object 
 glass. This can be accomplished by heating gently to melt the Canada 
 balsam, after which the section is washed for some time in benzol. In 
 most cases the whole section is not subjected to chemical tests at one time, 
 but only a portion of it is used, reserving the rest of it for other reactions. 
 The cover-glass is cut between the two portions with a diamond cutter and, 
 if the section also is to be removed from the object glass, it is cut through 
 at the same place. In some cases the cover-glass is removed and the 
 section covered with liquid Canada balsam. Then the particle to be iso- 
 lated is focused under the microscope and a perforated cover-glass is placed 
 over it so that the hole in the glass falls directly over the grain to be studied. 
 If a reaction with hydrofluoric acid is to be made the cover-glass can be 
 replaced by a platinum foil. The section must be left for a long time to 
 dry and then the balsam must be thoroughly washed out of the small 
 opening over the mineral grain. It is then ready for the reaction. 
 
METHODS OF INVESTIGATION 173 
 
 Certain chemical reactions serve to clarify rock sections because 
 they dissolve out those constituents which make it clouded or 
 nontransparent. Such substances are chlorite, zeolites and 
 other scaly decomposition products, or iron oxide which is 
 present as a finely divided pigment. They c'an be removed by 
 separating the section from the object glass and digesting it for 
 a long period with dilute hydrochloric acid. If the section is 
 impregnated with graphite or carbonaceous material, it is clari- 
 fied by roasting on a platinum foil. A wet method for removing 
 carbonaceous material is most favorable under certain conditions, 
 e.g., in the study of the organic structure of coals. The section is 
 treated for a long period with chlorous acid, page 150, which 
 dissolves the carbonaceous matter. Graphite cannot be removed 
 in this way. 
 
 The observation of etch figures has often been suggested as a means of 
 investigating thin sections, but no reliable results can be obtained in this 
 way. Etch figures may, however, be useful to distinguish between amor- 
 phous substances and those which crystallize in the cubic system because the 
 latter would give regularly bounded figures. 
 
 Staining Methods. The staining methods, which play so 
 important a role in organic microscopy, are only used occasionally 
 in the investigation of thin sections because only a few minerals 
 in their natural condition would absorb the stains and those that 
 do are principally thick, scaly to fibrous aggregates of good 
 cleavable minerals, but even they can scarcely be differentiated 
 or recognized in this way. Usually, before a mineral in a thin 
 section is stained, it is treated on the object glass with dilute 
 acid until it has been so altered that it is covered with a gelatinous 
 precipitate. This gelatinous material absorbs the stain evenly and 
 retains it so that it cannot be washed out. After the etching 
 action, which is always only superficial, the acid must be thor- 
 oughly washed away before the stain is applied or it may be 
 destroyed. The section is placed in an evaporating dish filled 
 with distilled water to which a few drops of a solution of Congo 
 red or malachite green or any other permanent dye have been 
 added. If it is left to stand for several hours, the dye is absorbed 
 quite evenly by the gelatinous precipitate. It is then thoroughly 
 washed in running water, dried, and covered again with Canada 
 balsam. 
 
 Certain silicates dissolve in hydrochloric acid and the silica 
 
174 PETROGRAPHIC METHODS 
 
 separates in a gelatinous condition, covering the thin section. 
 A thin film of dilute hydrochloric acid is put on the mineral in 
 the thin section so that the etching will take place only on the 
 surface and the whole mineral will not be dissolved. This reac- 
 tion is characteristic for nepheline, which is otherwise often 
 very difficult to recognize in the ground mass of a basaltic rock. 
 But plagioclase and scapolite rich in lime, sodalite, melilite, 
 wollastonite, chlorite, serpentine, zeolites, and certain rock 
 glasses give the same reaction, so that the conclusion that 
 nepheline is present is only warranted if, after it has been 
 colored, the mineral shows the characteristic cross section of 
 nepheline. 
 
 Aluminium fluosilicate, resulting by etching aluminium sili- 
 cates with hydrofluoric acid, is also a gelatinous substance well 
 adapted to such color tests. It is especially serviceable for 
 differentiating between feldspar and quartz in very fine grained 
 aggregates. Hydrofluoric acid is allowed to act for a short time 
 on the section and then it is entirely removed by heating on a 
 water bath. The stain is absorbed by the gelatin formed on the 
 feldspar while the quartz is not colored at all. In this case, it is 
 best to use dilute alcohol to wash the section. Other silicates of 
 aluminium that are attacked by hydrofluoric acid, e.g., cordierite, 
 can be distinguished from quartz or from silicates containing no 
 aluminium in this manner. By careful operation the feldspars 
 can be differentiated by these tests because orthoclase is more 
 difficultly attacked than the plagioclases and, among the latter, 
 those richest in lime are the most easily attacked. 
 
 Haiiyne, containing calcium sulphate, can be determined by 
 the gypsum crystals formed on its surface when it is treated with 
 dilute hydrochloric acid and dried. Characteristic hopper- 
 shaped growths of salt also appear with the gypsum. The salt 
 crystals are seen alone under the same conditions on sodalite, 
 noselite and nepheline. A characteristic reaction has been 
 found for certain minerals in a modification of this test. If, 
 e.g., an aluminium silicate containing chlorine (sodalite and 
 scapolite) or sulphur (lazurite) is treated with a solution of silver 
 nitrate in dilute hydrofluoric acid, the gelatin is impregnated 
 in the first case with silver chloride, which becomes brown when 
 treated with a photographic developer, and in the second case 
 with black silver sulphide. In this manner these elements that 
 are not common in silicates, can be detected. 
 
METHODS OF INVESTIGATION 175 
 
 Precipitates on Thin Sections. A dilute nitric acid solution of 
 silver nitrate can be used to test for chlorine in such minerals as 
 apatite. Apatite can also be determined by a localized lemon- 
 yellow precipitate which tends to form in a wreath around the 
 apatite grain when it is treated with a concentrated nitric acid 
 solution of ammonium molybdate. Heating should be avoided in 
 this reaction because then the silica is easily dissolved out of the 
 surrounding minerals and gives rise to a very similar precipitate 
 of ammonium silicon molybdate. , 
 
 There are numerous precipitation reactions by means of which 
 the various carbonates, which cannot be distinguished by their 
 optical properties, can be differentiated. Linck's solution is a solu- 
 tion of ammonium phosphate in dilute acetic acid. If a thin sec- 
 tion containing carbonates is treated with it, calcite will be 
 entirely dissolved out, while dolomite is slowly covered with 
 crystals of magnesium ammonium phosphate arranged in charac- 
 teristic aggregates. A dilute solution of copper sulphate be- 
 comes brilliant blue upon the addition of calcite, but with dolo- 
 mite it remains unchanged. Calcite rapidly precipitates 
 colored gelatinous alumina from a solution of alum or neutral 
 aluminium sulphate, containing also brilliant green or fuchsine. 
 A coating of ferric hydroxide forms on calcite in solutions of 
 ferric salts and this coating can eventually be colored black by 
 ammonium sulphide. Dolomite remains completely unaltered 
 for^a long time in both of the above cases, but it gives the same 
 reactions upon heating. Calcite cannot be distinguished from 
 aragonite by any of the above reactions, but the tests by which 
 they can be differentiated are very interesting. If the minerals 
 are finely powdered and boiled with a dilute solution of cobalt 
 nitrate, calcite remains perfectly colorless while aragonite 
 becomes bright violet. If boiled long, ten minutes or so, calcite 
 is colored slightly, but in the meantime aragonite has turned dark 
 violet. A solution o ferrous ammonium sulphate can be used 
 to distinguish between calcite and aragonite, and this reaction 
 can be carried out on a thin section. If a drop of the cold solu- 
 tion is placed on the slide, calcite becomes rust colored, while 
 aragonite turns dark blackish-green. 
 
 Metallic iron occurs now and then as a constituent of rocks. 
 It can be detected by a copper sulphate solution in which it 
 becomes coated with metallic copper, or by Klein's solution which 
 turns deep blue. 
 
176 PETROGRAPHIC METHODS 
 
 It is quite evident that all these precipitation reactions can be 
 carried out on single isolated grains. This is of great importance 
 in the case of such minerals as scapolite, which is present only in 
 very small quantities and is very difficult to recognize. 
 
 Alteration by Calcination. Attempts are frequently made to 
 produce characteristic changes in minerals by calcining the 
 section. Thus zeolites, brucite, hydromagnesite and cancrinite 
 become cloudy when heated a little and their optical properties 
 are somewhat changed. If the slide is heated to redness and 
 afterward treated with silver nitrate, brucite and hydromagne- 
 site will be coated with a dark brown film of silver hydroxide. 
 Other hydrous minerals such as chlorite, serpentine, etc., must 
 be calcined at very high temperatures for some time before they 
 become cloudy. They turn brown first and at higher tempera- 
 tures black, due to the oxidation of iron. 
 
 Numerous minerals containing iron change their color when 
 moderately heated in an oxidizing flame. Thus, olivine becomes 
 brownish-red and quite pleochroic; light-colored ' hornblende 
 assumes the color and absorption of basaltic hornblende. Chlorite 
 changes in a similar manner. Epidote and orthorhombic 
 pyroxenes become darker when heated very slightly, while 
 the monoclinic pyroxenes free from sodium darken only at very 
 high temperatures, and those rich in sodium and iron melt easily 
 to a black glass. Some varieties of colorless cordierite can be 
 made blue and pleochroic by slow and careful heating, but they 
 lose their color again if heated higher. Many members of the 
 sodalite group are colored an intense blue by heating in air 
 while, with other members, this change takes place only when 
 heated in sulphur vapor. Finally, the calcination test can be 
 used to detect aluminium in a thin section. It is heated with 
 cobalt solution, then washed with dilute acid, and the blue color 
 is observed in reflected light. This serves to distinguish talc 
 from sericite. 
 
 The calcination process can be used to distinguish between 
 carbonaceous substances or graphite and ores in a rock section. 
 It is always well to treat the section with hydrochloric or nitric 
 acid before and after heating, because the organic substance is 
 very frequently intergrown with iron ore. Graphite and carbo- 
 naceous material both burn off more easily if they are in a finely 
 divided state. 
 
METHODS OF INVESTIGATION 177 
 
 2. Physical Methods of Investigation 
 
 Determination of the Specific Gravity. Specific gravity is the 
 most important physical property, aside from the optical proper- 
 ties, that can be used to determine a mineral. The different 
 methods, which the physicists employ to determine it, require 
 for the most part, too much time. The accuracy of measurement 
 with a hydrostatic balance or a pycnometer is not at all commen- 
 surate with the small degree of purity generally possessed by 
 the rock minerals. Further, it is necessary, for such accurate 
 determinations of specific gravity, to have a considerable amount 
 of pure material available, which is often impossible. We must 
 be contented with less accurate methods, but this has the 
 advantage that the results can be obtained even with a very 
 small grain of a mineral and the measurement possesses a sufficient 
 degree of accuracy for our purpose. 
 
 The method generally employed in petrography to determine 
 the specific gravity consists in putting a small splinter of the 
 mineral in. question into one of the heavy liquids described on 
 pages 156 to 159. The liquid is gradually diluted until that 
 point is reached at which liquid and grain have the same specific 
 gravity, so that the grain remains suspended in any part of the 
 liquid when it is agitated and is not driven up or down. If a 
 crystal, which is apparently homogeneous, is broken into frag- 
 ments, it will be found impossible to bring all these pieces into 
 a state of suspension at the same time, but a medium grain will 
 be suspended, while some will sink slowly and others will rise. 
 These various fragments of one and the same crystal appear to 
 have different densities. The phenomenon depends upon the 
 fact that there are minute inclusions, cleavage cracks, etc., in the 
 mineral and these are not constant throughout the crystal, but 
 are different in the different fragments. 
 
 Cleavage cracks, gas bubbles, and liquid inclusions lower the 
 specific gravity of a mineral, while inclusions of heavy minerals 
 raise it. In general then, the specific gravity is most approxi- 
 mately obtained in a solution in which part of the grains float, 
 while the others sink. The density of the liquid is then obtained 
 by means of an aerometer, the best form of which is the Westphal 
 balance, Fig. 177. 
 
 The two arms of the balance beam are in equilibrium. The 
 pointer on the arm J plays exactly on the point opposite it when 
 12 
 
178 
 
 PETROGRAPHIC METHODS 
 
 / \ 
 
 FIG. 177. Westphal Balance. 
 
 the hydrometer, usually prepared in the form of a thermometer, 
 is suspended from E by a fine platinum wire. If the thermome- 
 ter is immersed in water at 4 C., it is evident that it will be buoyed 
 up and the balance will be thrown out of equilibrium. The 
 buoyant force is equal to the weight of the volume of water at 
 4 C., displaced by the thermometer when it is immersed. If a 
 weight m, equal to the weight of this volume of water, is suspended 
 
 with the thermometer at E, the 
 balance will again return to a state 
 of equilibrium. 
 
 The balance is supplied with four 
 different weights in the form of 
 riders and these are calibrated ac- 
 cording to the volume of water at 
 4 C. displaced by the hydrometer. 
 They correspond to 0.01, 0.1, one 
 and three times the weight of it. 
 In addition the balance beam is 
 divided into ten equal parts so that 
 the specific gravity of a liquid can be measured accurately to the 
 third decimal place and can be estimated' to the fourth. Thus, 
 it can be measured far more accurately than is necessary when 
 the impurity of the mineral is considered. 
 
 The specific gravity of the heavy liquids that are employed changes quite 
 rapidly with a change in temperature, so the whole determination must be 
 carried out at a temperature as nearly constant as possible. If this is not 
 possible, the temperature, at which the mineral fragments were brought into 
 suspension and that at which the specific gravity of the liquid was deter- 
 mined, must be taken. The error introduced by the change of temperature 
 can be eliminated by considering the coefficient of expansion of the liquid. 
 It is quite useless to make an extra effort to work at constant temperatures 
 because the temperature variations ordinarily experienced do not affect the 
 specific gravity of a mineral and only the changes in the liquid need to be 
 considered. The expansion and contraction of a heavy liquid can be 
 demonstrated best by bringing a crystal into suspension in a heated room 
 and then placing the vessel in a cool room. After a short time the crystal 
 rises again and floats. If the vessel is placed near a stove the crystal will 
 be seen to sink rapidly. It is also to be noted that a crystal fragment turns 
 upon its sharp edge when the specific gravity of it and of the liquid are about 
 the same. This is an indication that further dilution must be continued 
 extremely slowly and carefully. If the proper point is exceeded and the 
 crystal begins to sink, the density must be increased again by the addition 
 of the concentrated liquid. 
 
 It is not necessary at this time to consider any apparatus for determining 
 
METHODS OF INVESTIGATION 179 
 
 the specific gravity of soluble substances because petrographers rarely have 
 to deal with such minerals. Furthermore, the solvent effect of the organic 
 liquids, which are used, and of the solutions of inorganic salts is extraordinar- 
 ily variable. If one group of liquids cannot be used because of its solvent 
 action on a mineral, another can be employed in almost all cases. On the 
 other hand, minerals with a higher density than the heaviest liquid obtain- 
 able are not rare. The method of procedure then is to knead a mass of 
 wax and minium thoroughly in such proportions that the specific gravity 
 shall be about 2.5. The mineral is pressed into a weighed amount of this 
 mass called the float. The amount of the float is so chosen that the specific 
 gravity of the combination amounts to about 3.0. If g equals the weight 
 of the float and p its specific gravity, g f the weight of the mineral and P the 
 specific gravity of the combination, the specific gravity of the mineral p / 
 
 g'P 
 
 can be calculated from the formula p' = . Fine feathery 
 
 g+g' -Pg/p 
 
 tongs of thin glass threads can be made and used as floats. The mineral 
 grain is fastened in the tongs. In either case it is absolutely necessary not 
 to use too small a fragment of the mineral because in that case the error 
 introduced becomes too large. 
 
 Another apparatus for determining the specific gravity of liquids is 
 based upon the law that the heights of columns of liquids retaining equilib- 
 rium in communicating tubes, are inversely proportional to their specific 
 gravities. If the liquid to be determined is in one arm of the tube and 
 water in the other, the difference in height of the columns gives the specific 
 gravity of the liquid directly. This apparatus can also be arranged to 
 make separations according to specific gravity. 
 
 Determination of Hardness, Cleavage, etc. Now and then it is advan- 
 tageous to determine the hardness of a mineral. However, the small grains 
 that are obtainable are too small to make a hardness test by hand. A 
 thin bar of lead is planed on one end. An indicator of known hardness is 
 chosen from the scale of hardness and one surface is polished. The mineral 
 grains, whose hardness is to be determined, are placed on this surface. 
 Then the lead bar is pressed against it firmly and moved back and forth. 
 The grains sink into the soft lead and are held firmly as if in a setting and 
 are moved about over the polished surface. If the grains are harder than 
 the indicator, fine scratches will be observed on the surface by. the aid of 
 a lens. The more accurate methods used in physics for determining the 
 hardness of substances are of no consequence in the case of rock-forming 
 minerals. It may be mentioned that the hardness of a mineral or a rock, 
 i.e., its resistance to being scratched, is often confused with its resistance to 
 abrasion. There are soft minerals that can be but slightly abraded. They 
 are tough like serpentine. Then there are others that are very hard, but 
 because of their brittleness, are more easily worn away than would be 
 expected from their hardness. . Quartz is one of this kind. 
 
 The determination of the hardness and the tendency to abrasion of a 
 mineral is frequently greatly influenced by cleavage, because, parallel to 
 the cleavage, scratching as well as abrasion encounters less resistance. 
 Cleavage is nearly always quite evident in a thin section because the 
 minerals are shattered more or less along their cleavage planes during the 
 
180 PETROGRAPHIC METHODS 
 
 process of grinding. The degree of cleavage can be easily estimated in a 
 mineral cross section by the number of the cleavage cracks, and whether 
 they are long and straight or not. Compare Part I, p. 44. The cleavage 
 form and the cleavage planes can be determined from the angle which the 
 cracks \iake with each other in a thin section, but care must be taken to 
 determine the cleavage only in sections perpendicular to the cleavage 
 planes. Those are sections in which the cracks do not shift sideways when 
 the focus of the microscope is changed from the upper to the lower surface 
 of the mineral. An example of this is the distinction of pyroxene, with a 
 prismatic cleavage of approximately 90, from amphibole with a prismatic 
 cleavage of about 124. A section of pyroxene cut obliquely to the cleavage 
 can show exactly the cleavage angle of hornblende and vice versa. 
 
 The production of a percussion figure is of special interest in the investi- 
 gation of micaceous minerals. A minute scale of the mineral is placed on 
 a smooth cork and a vertical needle is arranged in a support over it so 
 that it can move up and down freely. The needle is struck a sharp blow 
 with a watch spring and driven into the plate. A series of hexagons can 
 be observed around the hole thus produced. See the mica group, p. 302. 
 The angles of the hexagons are bisected by quite straight cracks of which 
 two, lying diametrically opposite each other, are more pronounced than the 
 others and are called the principal rays. The direction of these principal 
 rays corresponds to the plane of symmetry of mica. 
 
 The study of magnetism is confined, for the most part, to the effect of a 
 mineral on a magnetic needle so mounted as to be delicately sensitive. 
 Only a very few minerals affect it naturally, particularly magnetite and pyr- 
 rhotite. All minerals rich in iron will affect a magnetic needle after they have 
 been fused. Polar magnetism also occurs in rocks, but the distribution of the 
 two poles is usually very complicated and apparently without any regularity. 
 Conductivity of electricity is not often used in studying rock minerals, but 
 is valuable to distinguish between graphite and coal. Graphite, fastened 
 in zinc, becomes coated with metallic copper in a copper sulphate solution. 
 
CHAPTER X . 
 Development of Rock Constituents 
 
 External Form. The outlines of the rock-forming minerals are 
 extremely simple. Well developed crystals, the outlines of which 
 are generally not very sharp, are quite rare and, when they do 
 occur, they are simple and do not show combinations of many 
 faces. Rock constituents crystallographically developed in this 
 manner, as the phenocrysts of the porphyries, Fig. 178, or the 
 garnets of a mica schist, are termed automorphic (Greek autos, 
 self; morphe, form) or idiomorphic (Greek idios, own). A partial 
 crystallographic development is observed far more frequently. 
 Feldspar in granite shows plainly its own crystal development 
 next to the quartz, but not to the mica. This is called hypidio- 
 morphic development (Greek hypo, almost) . The most frequent 
 
 FIG. 178. Quartz Porphyry, Rochlitz, Saxony. 
 
 case is that in which the mineral grains lie beside each other 
 without any indication of crystallographic form. Such a gran- 
 ular aggregate is said to be xenimorphic (Greek xenos, foreign) or 
 allotriomorphic (Greek allotrios, foreign). This type of develop- 
 ment is seen most plainly in granular limestones and may be 
 observed macroscopically. 
 
 Under the microscope also, these three types of development can be 
 sharply differentiated from each other, as Figs. 179 to 181 show. In Fig. 
 
 181 
 
182 
 
 PETROGRAPHIC METHODS 
 
 180 the development of the quartz, the lightest material in the cut, is very 
 distinctive. It fills up the spaces between the other constituents and ob- 
 tains its form entirely from them. It is designated as interstitial material 
 or mesostasis (Greek mesos, middle; stasis, mass). 
 
 FIG. 179. Porphyric Structure. Granite 
 Porphyry, Ernsthofen, Odenwald. 
 
 FIG. 180. Hypidiomorphic Structure. 
 Quartz Diorite, Schwarzenberg, Vogesen. 
 
 After F. Berwerth. 
 
 Rocks do not in all cases consist entirely of crystallized constituents. 
 Sometimes amorphous substances make up part of the rock. This is some- 
 times rock glass formed from the magma, occurring either as a subordinate 
 base or as a prominent constituent. Sometimes it is deposited from an 
 
 FIG. 181. Allotriomorphic Structure. 
 Quartzite, Perosa, Cottic Alps. 
 
 FIG. 182. Sharp crystals, Uralite and Plagio- 
 clase in Uralite Porphyry, Predazzo, Tyrol. 
 
 aqueous solution and is then usually opal. Naturally these substances 
 never possess characteristic forms. 
 
 It may be said that the smaller the amount of a constituent in a rock and 
 the smaller the individuals, the more perfect is the crystallographic develop- 
 
DEVELOPMENT OF ROCK CONSTITUENTS 183 
 
 ment. Those minute crystals occurring as accessory components of the 
 eruptive rocks, like zircon, xenotime, apatite, monazite, and chrysoberyl, 
 which are only present in very small quantites and belong to the oldest 
 period of solidification of the rock, are often as well developed as models 
 and sometimes show an abundance of faces. Likewise small crystals of 
 feldspar, pyroxene, etc., which have separated from the glassy ground mass 
 of an eruptive rock, sometimes have quite sharp forms. Larger individuals 
 are not so apt to show this, either because they did not have a good develop- 
 ment originally or because they have later been partly redissolved. The 
 smaller crystals are not so liable to resorption. 
 
 The size of the rock-forming constituents is quite variable. 
 Especially large dimensions are observed only in pegmatites, 
 which are scarcely to be considered as rocks in the narrower sense. 
 Feldspar crystals are found in them with edges several meters 
 long and mica plates as big as a table. In rocks, mineral individ- 
 uals as large as a head or fist are quite scarce; those whose 
 largest dimension is one or more centimeters are more common. 
 Even these are more apt to be found among the phenocrysts of 
 porphyric rocks. The dimensions are usually smaller in granular 
 rocks from a diameter of about 1 cm. down to individuals which 
 cannot be seen with the naked eye. The granular rocks may be 
 medium granular, or fine granular, or may appear to the naked eye 
 to be aphanitic. The ground mass of porphyric rocks, is generally 
 macroscopically aphanitic and often under the microscope is 
 composed of such minute individuals that a very thin section 
 and strong magnification are necessary to resolve it. The 
 minutest rock constituents, often well bounded crystallograph- 
 ically, which are so small that they appear in the slide as specks, 
 are called microlites (Greek micros, small) . This name was orig- 
 inally given to a rod-like formation which was afterward called 
 belonite (Greek belone, arrow heads) . 
 
 The crystallographic habit is in general quite characteristic 
 for any one species. Apatite develops in needles; mica is 
 tabular; epidote is elongated parallel to the b axis; hornblende 
 parallel to the c axis; and many feldspars parallel to the a axis, Fig. 
 183, p. 184. In thin sections the cross sections have a character- 
 istic form and a distinguishing principal zone, the optical char- 
 acter of which aids in determining the mineral. This is found in 
 column Chz in the table at the end of the book. Acicular or thin 
 tabular minerals give mostly lath-shaped cross sections. Other 
 minerals, e.g., quartz or calcite are developed more nearly iso- 
 metrically. Their cross sections show no characteristic elonga- 
 
184 PETROGRAPHIC METHODS 
 
 tion and they have no principal zones, Fig. 184. In other cases, 
 e.g., corundum and topaz, the individuals are sometimes elong- 
 ated parallel to one crystallographic axis and sometimes to 
 another, and the optical character of the principal zone varies 
 accordingly. 
 
 The mineral individuals of a rock are frequently simple crystals. If 
 they are well bounded crystallographically, the form of the crystal can 
 generally be inferred from the outlines of several sections differently ori- 
 entated. However, too much importance should not be given to the ap- 
 pearance of one or more cross sections. The predominant section of a 
 mineral developed prismatic or tabular is in both cases lath-shaped. 
 The laths from prismatic crystals have a width about equal to the thick- 
 ness of the crystal, while those from tabular crystals are much broader. 
 
 FIG. 183. Crystals with Principal Zone FIG. 184. Crystals without Principal 
 Grachyte, Nolles. (After Berwerth.) Zone. Quartzite, Steiermark. 
 
 Such minerals, as apatite, often give six-sided sections which have no effect 
 on polarized light and even in convergent polarized light cannot be readily 
 distinguished from isotropic minerals. Again in a great number of rocks 
 including some igneous types, tabular or acicular crystals of uniaxial min- 
 erals are all arranged parallel so that they extinguish four times when 
 rotated between crossed nicols, but as a uniaxial figure cannot be obtained, 
 these are often mistaken for biaxial minerals. 
 
 Twinning. Twinning is exceptionally widespread in rock-forming 
 minerals. It can be recognized partly in the form of the section. In 
 cubic twins and in uniaxial twins, in which the optic axes are parallel, 
 this is the only way to recognize them. If reentrant angles cannot be seen, 
 the twinning will not be observed in rocks, even though it is very widespread, 
 as in the case of quartz. Cruciform twins, Fig. 185, and juxtaposition twins, 
 Fig. 186, show readily in the external form. In penetration twins this in- 
 dication is lacking and is never observed where the rock constituents do 
 not have an external form. 
 
DEVELOPMENT OF ROCK CONSTITUENTS 185 
 
 Twins of uniaxial crystals with their optic axes oblique to each other 
 and twins of biaxial crystals can generally be recognized in polarized light. 
 In the first case, the two individuals separated by a sharp plane extinguish 
 differently, and in the second case even though the elasticity axes are 
 parallel the two parts will show different interference colors. This is par- 
 ticularly notable in cyanite. In such cases as epidote where the twinning 
 plane is very nearly parallel to the bisectrix of an optic angle, that is, almost 
 90, the twinning may not be apparent. 
 
 Besides simple twinning, repeated twinning or lamination is as common 
 in rock-forming minerals as it is in other minerals. It is rarely observed 
 in cubic minerals, but is very frequent in hexagonal (calcite), tetragonal 
 (rutile), orthorhombic (cordierite), monoclinic (diallage), and triclinic 
 minerals (plagioclase) . The two series of differently orientated individuals 
 are intergrown with each other in a more or less regular manner in the form 
 
 FIG. 185. Penetra- FIG. 186. Mono- FIG. 187. Twinning FIG. 188. Lattice 
 tion Twin with Inclined clinic Twin. Lamination. Lamination. 
 
 Axis. Staurolite. Diopside. 
 
 Plagioclase. 
 
 of lamellae, Fig. 187. In the crystal systems of higher symmetry several 
 series of lamellae may cross, producing a lattice effect. These lamellae are 
 of the same shape and the twinning is parallel to the different faces of one 
 form. Such is the case with calcite twinned parallel to 1/2 R. In the crystal 
 systems of lower symmetry, this lattice structure may appear, but is due to 
 the presence of two series of lamellae twinned according to different laws. 
 Albite, Fig. 188, is a good example of this, where albite and peri-cline twin- 
 ning occur together. Twinning lamination may be caused by the effect of 
 pressure on a mineral. This can be demonstrated with calcite which pos- 
 sesses the property of gliding. In other cases, particularly in the plagio- 
 clases, which are so important to petrographers, this is positively not the 
 case as is shown in the schist belt of the Central Alps. Here some of the 
 rocks have been greatly deformed locally, but the plagioclases show almost 
 no lamination. 
 
 Aggregates. The occurrence of several individuals of a min- 
 eral together is called an aggregate. Minerals, which have no 
 characteristic principal zone generally form granular aggregates, 
 while those with prismatic or acicular development tend to form 
 
186 
 
 PETROGRAPHIC METHODS 
 
 columnar to fibrous aggregates, such as are macroscopically 
 apparent in sillimanite intergrown with quartz. Minerals 
 possessing a tabular ' habit develop scaly or flaky aggregates. 
 Finely divided aggregates of colorless, low double refracting 
 minerals appear as homogeneous individuals in a thin section in 
 ordinary light, but their true nature becomes apparent in 
 polarized light, if they are not isotropic substances, because the 
 various parts extinguish differently aggregate-polarization. If 
 the aggregates are very fine an even illumination will be observed 
 between crossed nicols with low magnification and this will not 
 
 FIG. 189. Liparite. Glashuttental, 
 Hungary. 
 
 FIG. 190. Oolite, Vilstal, Algan. 
 
 be affected by rotating the stage. Only with high power ob- 
 jectives is it possbile to resolve such aggregates into their 
 constituent parts. This phenomenon is quite apparent in dense 
 aggregates of sericite or talc. 
 
 The most important of the fibrous or scaly aggregates are those 
 in which the various individuals are arranged in a radiating man- 
 ner around a point or a line spherulites or axiolites. Such 
 structures are characteristically developed in eruptive rocks 
 that are rich in silica and have solidified rapidly. The spheru- 
 lites, Fig. 189, correspond in general to a eutectic mixture of 
 quartz and feldspar. Analogous structures are by no means 
 rare in secondary depositions in cracks of the rocks, e.g., in 
 chalcedony or aragonite. They constitute the formation known 
 as oolite, Fig. 190. In organic nature the spherulitic develop- 
 ment is .sometimes quite marked, e.g., in the skeletal portions 
 
DEVELOPMENT OF ROCK CONSTITUENTS 187 
 
 of foraminifera. In these, axiolites predominate, e.g., in the 
 cross sections of nummulites. Frequently the radial structure 
 is lost and a granular aggregate is developed in its place by 
 
 FIG. 191. Granospherite. Recrys- 
 talized Oolite. 
 
 FIG. 192. Marble (Ordinario), 
 Carrara, Italy. 
 
 recrystallization. The outer form and sometimes also the 
 concentric structure may be preserved during this process. It 
 gives rise to a granospherite, Fig. 191. Distinctions can 
 be made in the character of the granular aggregates. Both 
 
 FIG. 193. Marble (Statuarid), 
 Carrara, Italy. 
 
 FIG. 194. Zonal Plagioclase in Pitchstone. 
 Cunardo near Lugano. (After E. Cohen.) 
 
 end members of an even grained, granular rock show the min- 
 eral grains simply lying beside each other mosaic structure, 
 Fig. 192, or very intimately dovetailed with each other, 
 Fig. 193. 
 
188 
 
 PETROGRAPHIC METHODS 
 
 Growth and Solution. The composition of a magma gradually 
 changes during the course of crystallization. This gives rise to 
 zonal development in isomorphous minerals, particularly in the 
 plagioclases and pyroxenes, Fig. 194, in which twinning, if it is 
 present, continues unchanged through all 
 zones. In pyroxenes rich in sodium and 
 titanium various pyramids are so grown 
 together as to produce an hourglass structure, 
 Fig. 195. It is noteworthy that this appear- 
 ance, which has been shown to be dependent 
 
 FIG. 195. Hourglass 
 Structure in Ryroxene. 
 
 FIG. 196. Skeletal Crystals of Olivine. 
 
 upon a gradual change of the chemical composition of the crys- 
 tallizing mass, occurs but exceptionally in contact rocks and, if 
 it does occur, the law of zonal construction may be just reversed. 
 In eruptive rocks this law seems to be constant, e.g., in the 
 plagioclases where the outer zones are always richer in silica. 
 
 Various phases in the process 
 of the formation of rocks may 
 be recognized from the develop- 
 ment of the rock constituents 
 as has been thoroughly estab- 
 lished in the Allgemeine Ges- 
 teinskunde by E. Weinschenk. 
 Only the external forms will 
 be described here. The crys- 
 tallization of a mineral is more 
 complete if it takes place 
 slowly from a very dilute solu- 
 tion. In the glassy eruptive 
 
 rOCks, Which have COOled SUd- Fl - ^.-Skeletal Crystals .in Pitchstone. 
 . . . (From Arran.) 
 
 demy, incomplete crystalliza- 
 tions are frequently observed. The growth on the edges and 
 corners is most noticeable and the faces remain as more or less 
 deep indentations ruin-like development. Crystal skeletons 
 may result, which are hard to define crystallographically, but 
 
DEVELOPMENT OF ROCK CONSTITUENTS 189 
 
 are sometimes quite regular, Fig. 196, and at other times grow 
 in variously shaped structures not unlike organic forms. The 
 exact nature of them cannot be determined in most cases, Fig. 
 197. The last series of growth forms is very simple. In 
 obsidian, crystallites, Fig. 198, which cannot be determined, are 
 frequently found. They may appear as dark points globulites 
 
 a h 
 
 ':*!>* 
 
 > v -::.. 
 
 ~ ; w 
 
 FIG. 198. Crystallites, 
 a Globulite, b Margarite, c Cumulite, d Baculite and e and f Trichite. 
 
 which may be grouped together like a string of pearls marga- 
 rites or in irregular bunches cumulites. Sometimes they are 
 dark rods baculites or curved fine hair-like forms trichites. 
 These often form star-shaped aggregates, Fig. 198, /. 
 
 Other irregularities in the form of the constituents of eruptive 
 rocks are, unlike those already described, of a secondary nature, 
 and are caused by the sol- 
 vent action of the magma 
 on the crystals which have 
 formed in it. This is natur- 
 ally observed most com- 
 monly in porphyric rocks in 
 which the physical condi- 
 tions have been intensely 
 changed during the process 
 of crystallization. The phe- 
 nomenon, which is called 
 magmatic corrosion or re- 
 sorption, is rarer in rocks 
 rich in glass than in rocks 
 
 fe . FIG. 199. Corroded Quartz. 
 
 poor in glass, because in the 
 
 latter the process of solidification has been slower and more 
 time given to the solution process. The external form is often 
 considerably changed. The corners and edges are rounded, 
 and deep indentations made in the faces until finally only 
 an irregular fragment of the crystal is left, Fig. 199. These 
 
190 
 
 PETROGRAPHIC METHODS 
 
 phenomena are especially pronounced in quartz, olivine and 
 the minerals of the sodalite group. 
 
 Chemical changes also result under these conditions. Min- 
 erals of the amphibole and mica groups containing hydroxyl 
 suffer molecular rearrangement in which opaque iron oxide 
 separates out and a black magmatic border is produced. This 
 process may continue until finally a more or less sharply defined 
 speck, showing aggregate-polarization, is left in place of the 
 original crystal. They are very difficult to determine, but in the 
 tuffs of such eruptive rocks, which preserve the material in the 
 first stages of its occurrence, the unaltered mineral is often seen 
 in large quantities. 
 
 It is noteworthy that the accessory minerals in granular lime 
 stones, especially the silicates, often show surface characteristics 
 analogous to magmatic resorption so that they appear to have 
 been partly fused. 
 
 Cleavage, Parting, and Mechanical Deformation. The signifi- 
 cance of cleavage in the determination of rock-forming minerals 
 has already been pointed out. The more complete the cleavage 
 the straighter and finer do the cleavage cracks appear in micro- 
 scopical preparations and, if the cleavage is perfect in colorless 
 low double refracting minerals as in feldspar, it is often quite 
 difficult, even when the condensing system of lenses is lowered as 
 
 FIG. 200. Perfect 
 
 FIG. 201. Distinct 
 
 FIG. 202. Imperfect. 
 
 Cleavage. 
 
 far as possible, to recognize the cleavage cracks distinctly. It 
 may, however, be noted that they often become visible in polar- 
 ized light especially with the gypsum test plate. If the cleavage 
 is less perfect, the cleavage cracks are not so straight, do not 
 extend all the way across the section, and are readily recognizable. 
 The number of cleavage cracks appears to have no connection 
 with the character of the cleavage. Brittle minerals show a 
 larger number in general than those that are less brittle even 
 though the latter have a more perfect cleavage. In rocks which 
 
DEVELOPMENT OF ROCK CONSTITUENTS 191 
 
 have been shattered by erogenic processes, cleavage is particularly 
 noticeable. Figs. 200 to 202 show the difference in general 
 appearance of perfect, distinct, and imperfect cleavages. 
 
 Fibrous cleavage is characteristic in cyanite and gypsum, and may be 
 recognized by an aggregation of extremely fine lines in one place on the 
 section. Parting faces appear different from the ordinary kind of cleavage 
 because they are regular and straight, as in diallage. 
 
 Besides these appearances, based on the inner structure of the rock- 
 forming minerals, cracks, which are more or less straight and of an inciden- 
 tal nature, are to be observed in the thin sections of many minerals. In gar- 
 net, for example, a checked or cracked appearance is extremely widespread 
 and the short cracks cut through the whole section in the same manner with- 
 out any relation whatever to the orientation of the garnet individuals. 
 Long needles, e.g., of apatite or sillimanite, tend to show a distinct cross 
 fracture which could easily be confused with cleavage. Such crystals are 
 very easily divided by breaking and even the movement of a liquid stream 
 of lava is sufficient to break a prismatic or tabular feldspar crystal and the 
 parts may be found in a thin section in close proximity to one another 
 
 It is more difficult to explain the fact that some minerals, e.g., pyroxene 
 in unaltered andesite, do not show complete extinction but appear to be 
 compressed into columns that 
 extinguish differently, i.e., they 
 possess undulatory extinction. This 
 can only be caused by fracturing 
 which has resulted during the 
 crystallization of the magma and 
 is called protoclase. Analogous 
 disturbance of the optical behavior 
 is far more common in plutonic 
 and in all possible metamorphic 
 rocks. Certain brittle constitu- 
 ents often show a wavy or undu- 
 latory extinction in which uniaxial 
 crystals, e.g., quartz, frequently 
 become biaxial. This very readily 
 changes into fracture and the 
 grains of quartz or of olivine ap- 
 pear as an aggregate of columnar 
 
 crystals, which show the extinction directions slightly different, but sharply 
 separated, so that the mineral under certain conditions appears quite fibrous. 
 
 The columns, further, may be cross fractured and along the 
 contact surfaces of the grains a fine friction material is formed. 
 Homogeneous quartz grains are changed to a very fine aggregate 
 of particles so intimately dovetailed into each other that they 
 still form quite a compact sand. This is seen around the larger 
 grains and is called mortar structure, Mortelstruktur, Fig. 203. 
 
 FIG. 203. Mortar Structure in Quartz. 
 
192 PETROGRAPHIC METHODS 
 
 It is not certain how much of this mechanical deformation is due 
 to stresses dominant during crystallization and how much is due 
 to orogenic pressure acting at a later period. However, the whole 
 phenomenon is generally termed cataclase. 
 
 This deformation often produces a real characteristic appear- 
 ance in minerals with a good cleavage. Individuals of horn- 
 blende, for example, may be divided up into cleavage prisms so 
 that they behave in cross section like a packet composed of 
 rhombs. Minerals with gliding planes like mica split parallel 
 
 FIG. 204. Bent Mica Plates, Mica Schist. FIG. 205. Marble with Bent Lamellae. 
 
 to these under the influence of orogenic pressure, and calcite 
 under the same conditions assumes a fibrous character due to 
 very fine twinning lamination. These minerals are, however, 
 far more flexible than the ones first mentioned, especially mica 
 and calcite, and to a less degree feldspar also. They can be 
 bent and crumpled considerably without fracturing, Figs. 204 and 
 205, but finally when the action becomes too great they also are 
 deformed in the same manner as quartz, i.e., assume the Mortel- 
 structur. 
 
 Intergrowth and Inclusions. Regular and irregular inter- 
 growths of various minerals are widespread. It is necessary to 
 distinguish between poikilitic intergrowth and peripheral develop- 
 ment. In the first case two homogeneous minerals mutually 
 penetrate each other. Pegmatitic intergrowth of quartz and 
 feldspar, graphic granite, Fig. 206, and perthitic intergrowth of 
 various feldspars (see feldspar group) are typical examples. 
 Peripheral growth is most frequent in related minerals as ortho- 
 
DEVELOPMENT OF ROCK CONSTITUENTS 193 
 
 rhombic and monoclinic pyroxenes, monoclinic and triclinic 
 feldspars, or hornblende and pyroxene. It is also known in 
 
 FIG. 206. Graphic Granite from Jekaterinburg, Ural. 
 
 minerals that are not at all related to each other, in gabbros 
 where ilmenite is frequently surrounded by a halo of biotite and 
 olivine by a halo of hypersthene or garnet, etc. 
 
 FIG. 207. Ocellary Structure, Leucitophyre FIG. 208. Quartz Eye with a Halo of Glass Rich 
 Olbriick, Eifel. in Microlites. Basalt, Neuhaus, Oberpfalz. 
 
 The borders around certain minerals often show characteristic phenomena. 
 For example, in volcanic glasses, a dark colored fringe may be seen around 
 feldspar crystals and a lighter colored one around pyroxene. This is 
 especially noticeable in leucite rocks where acicular crystals of aegirine 
 grow around the larger leucite individuals, ocellary structure, Fig. 207, or 
 13 
 
194 PETROGRAPHIC METHODS 
 
 in eclogites in which the garnets protrude into their surroundings like 
 pseudopods. Among such appearances, which are classed together as 
 concentric structures, the quartz lenses of basic eruptive rocks, Fig. 208, are 
 especially noteworthy but they have an entirely different origin. 
 
 Crystal inclusions are distinguished from intergrowths, when 
 the foreign mineral that is included in another crystal is less in 
 quantity or possesses its own crystal form against the including 
 substance, as quartz in hornblende in Fig. 209. The inclusions 
 may be variously orientated or they may be arranged parallel 
 to some crystallographic direction of the including mineral. 
 
 Single grains of various miner- 
 als may also be included by 
 another. In all these cases the 
 relations between the inclusion 
 and the including mineral are 
 undoubtedly not so intimate as 
 in the case of an intergrowth. 
 Gas, liquid, and glass inclu- 
 sions are also observed but 
 their characteristics have been 
 briefly described in part I, 
 page 48. 
 
 The ability of various rock 
 
 FIG. 209.-Perforated Hornblende Granulite, Constituents to take U P inclll- 
 
 Ampe, Ceylon. sions is quite variable. It is 
 
 quite subordinate in quartz 
 
 but plagioclase occurring right beside it, often shows abundant 
 inclusions. This is* also particularly noticeable in garnet, cya- 
 nite, staurolite, calcite, etc., where the quantities of inclusions 
 may be so large as to exceed the mass of the including mineral. 
 
 The significance of the inclusions as to the conditions of the 
 formation of rocks was early recognized, especially in the case of 
 liquid inclusions in the quartz of granites, which point to an 
 hydatopyrogenic origin. 
 
 With a low magnification, cloudy bands may be observed penetrating the 
 quartz grains of granite without any reference to the orientation of the 
 grains. With stronger magnification they are resolved into swarms of 
 thousands of the most minute liquid inclusions with gas bubbles often in 
 extremely violent motion. The liquid is sometimes water, or an aqueous 
 solution, and sometimes liquid carbon dioxide. The latter is best tested 
 by heating the preparation above the critical temperature of carbon dioxide, 
 i.e., above 31 C. At this temperature the gas bubble disappears suddenly 
 
DEVELOPMENT OF ROCK CONSTITUENTS 195 
 
 accompanied by a violent disturbance of the liquid, and the liquid inclusion 
 becomes a gas inclusion. It can also be proved analytically that it is 
 really carbon dioxide. 
 
 Liquid inclusions do not always have such small dimensions. Large 
 irregular cavities filled with liquid are sometimes seen in olivine crystals in 
 peridotites. In contact rocks the liquid inclusions frequently fill negative 
 crystals, i.e., cavities with the same shape as the crystal, and of considerable 
 size. They are sometimes apparent macroscopically in quartz crystals, 
 and particularly in halite. 
 
 Liquid inclusions are found especially in minerals in plutonic 
 rocks and in contact rocks, and these minerals do not often 
 contain glass inclusions. Glass inclusions are characteristic for 
 extrusive rocks, and it may also be observed that when they are 
 included in light minerals, they are darker and in dark minerals 
 lighter than the glassy basis of the rock itself. The form of 
 glass inclusions is sometimes irregular, sometimes oval, or in the 
 form of negative crystals with rounded edges. Their dimensions 
 are never as small as those of liquid inclusions, but they rarely 
 exceed microscopic size. 
 
 Inclusions of pure glass are found mostly in acid and inter- 
 mediate extrusive rocks and even also in such rocks in which the 
 glassy basis has been completely devitrified by secondary altera- 
 tion. Sometimes, however, the alteration 
 has effected the glass of the inclusions and 
 their character can only be recognized by 
 the outer form. Corresponding to the 
 greater ease with which basic magmas 
 crystallize, real glass is less often observed 
 as an inclusion in basic rocks, the glass 
 tending to be more or less filled with micro- 
 
 . . , , . , , , ., ~ , FIG. 210. Leucite with 
 
 lites which cloud it. Such products are siag inclusions, 
 
 better termed slag inclusions. In lava, 
 which has solidified rapidly, they entirely fill the plagioclase 
 individuals and in leucite crystals they cause a characteristic 
 appearance by their radial or zonal arrangement, Fig. 210. 
 
 The arrangement of the inclusions is often very characteristic. 
 Aggregations in the center or in border zones are especially 
 prevalent, as is also zonal arrangement. The plagioclases a"re 
 frequently built up of zones alternately rich and poor in inclu- 
 sions. In garnet the inclusions are sometimes orientated 
 parallel to crystallographic directions and sometimes tangential 
 to them. In certain pyroxenes, in olivine, and in the labradorite 
 
196 . PETROGRAPHIC METHODS 
 
 of gabbros, brown flakes and rods, which are perhaps ilmenite, 
 are regularly arranged and often cause a metallic chatoyancy in 
 the mineral visible to the naked eye. The arrangement of 
 graphitic dust in certain minerals of contact rocks is especially 
 typical, particularly in andalusite. The variety known as chias- 
 tolite presents the appearance shown in Fig. 211. In other 
 minerals, which are likewise often rich in inclusions, as cyanite 
 or staurolite, no regularity in the arrangement of the inclusions 
 is to be noted. 
 
 One type of arrangement of inclusions, which is classed with 
 the characteristic structures of contact rocks, is especially inter- 
 esting. If a contact schistose rock is cut across the schistocity 
 
 FIG. 211. Chiastolite in Chiastolite FIG. 212. Helicoidal Structure. 
 
 Schist Gunildrud on Ekernsee, Norway. Cordierite Gneiss, Bodenmais. 
 
 (After E. Cohen.) 
 
 and observed in thin section an irregular crystalline aggregate 
 appears. However, the bands of inclusions are often inter- 
 woven in the principal constituents and reveal the original 
 schistocity with all its contortions very plainly. This is called 
 helicoidal structure. In cordierite hornfels, Fig. 212, these bands 
 consist of sillimanite, biotite, and ilmenite, in other cases of 
 graphite dust, and in still others of small quartz grains, but in 
 spite of the varying composition the general appearance remains 
 the same and they are therefore extremely characteristic. 
 
 A large amount of inclusions in certain constituents, especially 
 the larger ones, is characteristic for contact rocks. Such is 
 particularly the case in garnet, in which sometimes the center of 
 the crystal is a compact mass of inclusions surrounded by a ring 
 
DEVELOPMENT OF ROCK CONSTITUENTS 197 
 
 of garnet, perimorph, Fig. 213, and sometimes the irregularly 
 arranged inclusions constitute the greater part of the crystal and 
 are interwoven with a network of fine veins of the including 
 
 FIG. 213. Garnet Perimorph. Eclogite, 
 Kleinitz in Grosvenedig. 
 
 FIG. 214. Sieve Structure. Mica Schist, 
 Zopetspitze, Grosvenedig. 
 
 mineral, sieve structure, Fig. 214. The external form of such 
 crystals so rich in inclusions may be perfectly normal. 
 
 The occurrence of pleochroic halos must be mentioned. They occur 
 around inclusions of certain minerals containing zirconium, tin, titanium, 
 and some of the rare earths, when included in certain other 
 minerals, see Table 19, No. IV. 
 
 They may be best observed when the direction of strongest 
 absorption of the including crystal is parallel to the vibra- 
 tion direction of the polarizer and then they appear as 
 quite small zones not sharply separated from the surround- 
 ing mineral, Figs. 215 and 216. The pleochroic halos show 
 a much stronger ab- 
 sorption of light than 
 does the rest of the 
 crystal, yellow to deep 
 brown colors being 
 especially common. 
 If, on the other hand, 
 the direction of least 
 absorption is parallel 
 to the polarizer the 
 phenomenon almost 
 
 completely disappears. The form of the halo varies greatly and is dependent 
 upon the form of the inclusion. It is very significant that other minerals 
 than those included in Table 19, e.g., the needles of apatite in biotite, Fig. 
 216, do not show such halos. Pleochroic halos may be of a secondary origin, 
 e.g., when augite, which does not show them, alters to hornblende. In 
 
 II 
 
 i 
 
 I 
 
 m 
 m 
 
 i': 
 
 
 1 
 
 & 
 
 
 
 If 
 
 ii 
 
 FIG. 215. FIG. 216. 
 
 Pleochroic Halos around Inclusions of Zircon in 
 Tourmaline. Biotite. 
 
198 PETROGRAPHIC METHODS 
 
 general the optical properties of a crystal are quite strongly modified in 
 the pleochroic halos. The indices of refraction are raised, the double refrac- 
 tion may become much lower or may be raised considerably, and in biaxial 
 crystals a stronger dispersion of the optic axes is generally to be observed 
 in the halos. All these phenomena do not indicate that the coloring is 
 due simply to an organic pigment but much more that it is due to an iso- 
 morphous mixture in which the elements, that are predominant in the in- 
 clusions, play a considerable role. The ease with which these halos can be 
 destroyed by heating does not in any way oppose this view for that is a 
 common property of silicates colored dark by a content of titanium, e.g., 
 melanite, and also of numerous occurrences of dark rutile, cassiterite, 
 zircon, etc. 
 
 Pseudomorphs. Alteration of rock-forming minerals is exceedingly com- 
 mon, but very frequently the form and also a considerable portion of the 
 original mineral remains unchanged indicating clearly the origin of the 
 pseudomorphs. Most of the pseudomorphs are not the result of the action 
 of ground waters. Pseudomorphs due to weathering are comparatively 
 rare. The most of these occurrences are to be ascribed to the post- volcanic 
 action of magmatic waters. They are, therefore, most important in eruptive 
 rocks and in regions where they have effected contact metamorphism. 
 These regions are the characteristic habitats of pseudomorphs. 
 
 Pseudomorphs, which exhibit a new composition in an old form, some- 
 times consist of a newly developed, homogeneous crystal, e.g., uralite. In 
 most cases, however, they are composed of a dense aggregate which may 
 still show the cleavage and structure of the original mineral, thus bastite, 
 or may form an irregular cluster, which presents the phenomenon of aggre- 
 gate polarization in polarized light, e.g., liebenerite. 
 
 An important group of alterations belongs in this class, in which simple 
 molecular rearrangement takes place without change of chemical com- 
 position paramorphs. One of the most important chemical geological 
 processes belongs here. It is the very widespread transformation of 
 aragonite into calcite in the fossilization of organic lime skeletons. Some- 
 times the original structure is retained, while at other times it is com- 
 pletely obliterated during the process. The transformation of minerals 
 free from water into those containing water is especially frequent. There 
 are two processes of this kind which take place very extensively, e.g., the 
 formation of gypsum and of serpentine. An increase of volume takes 
 place which produces great geological disturbances and its effects are even 
 noticeable on a small scale in the fracturing of the surrounding minerals, 
 which are traversed by radial cracks, Fig. 217. 
 
 Alteration begins for the most part on the border and progresses into the 
 interior along the cleavages and gliding planes, expanding these more and 
 more until a peculiar network results, Fig. 218, within which a residue of 
 the original mineral, still unaltered but greatly clouded, is often preserved. 
 In the normal case the newly developed substance is confined strictly to the 
 border of the original crystal, but the texture of the altered rock may be 
 recognized excellently, palimsest structure. The former shape is lost in the 
 newly developed minerals only when the alteration is especially extensive 
 and general. Secondary formations develop abundantly beyond the 
 
DEVELOPMENT OF ROCK CONSTITUENTS 199 
 
 former borders of the minerals and the texture of the rock is more and more 
 confused. In exceptional cases the alteration begins in the middle of an 
 individual, but only when the crystals are zonal and the nucleus is more 
 susceptible to alteration, as in the plagioclases of eruptive rocks. 
 
 One must guard himself against confusing alteration products with in- 
 clusions. Corresponding to the manner of formation, alteration products 
 have a form dependent upon cleavage cracks and other discontinuities in the 
 crystal. They form a more or less regular network in a crystal which has 
 
 FIG. 217. Radial Cracks in Plagioclase FIG. 218. Mesh Structure in Serpentinized 
 produced by Serpentinization of Olivine. Olivine. Trogen near Hof , Fichtelgebirge. 
 
 suffered alteration, but single constituents of the secondary substance lack 
 characteristic form. Inclusions, however, can have definite forms and may 
 be in definite orientation bearing no relation to the cleavage cracks. They 
 do not form continuous veins but are in small sharply defined crystals, 
 which are completely imbedded in an unaltered clear transparent mineral. 
 
 Directions for the Use of the Descriptive Section. The optical 
 properties of rock-forming minerals are taken as a basis for the 
 presentation in the descriptive portion and in the accompanying 
 tables. A thorough knowledge of the methods of determining 
 these properties, as is set forth in Part I, must be presupposed 
 and the following paragraphs will only recapitulate the points 
 particularly important in the use of this portion. 
 
 It may be remarked that, in so far as the transparent minerals 
 are concerned, the presentation of the properties in the descriptive 
 section will only supplement the statements in the tables so that 
 both must always be used jointly. The arrangement in the text 
 is the same as in the tables and the number after the name of 
 each mineral in the descriptive section indicates the page of 
 the tables where the mineral is to be found and, vice versa, the 
 
200 PETROGRAPHIC METHODS 
 
 number in the first column of the tables refers to the page in the 
 text giving the description of the mineral. 
 
 The systematic classification of rock-forming minerals must be 
 different from that used in mineralogy because of the methods 
 used to determine them. A chemical system, which miner- 
 alogists find so useful, has no significance in this case. The 
 classification, which is adhered to in this text, has proved to be 
 especially valuable and divides minerals into four groups: 
 
 1. Opaque minerals. 
 
 2. Isotropic minerals. 
 
 3. Uniaxial minerals. 
 
 4. Biaxial minerals. 
 
 This classification is based upon those properties which can generally be 
 determined quickly and positively by simple methods. In certain cases 
 some difficulties are encountered, especially by a beginner, because of the 
 overlapping of the groups, but an experienced observer will soon overcome 
 this. For example, hematite and ilmenite are found in the column of opaque 
 minerals, but in certain instances they occur in fine transparent double 
 refracting scales. Chromite is also quite frequently transparent with a 
 brown color. On the other hand, some minerals from the other three 
 groups are often so deeply colored that they do not transmit light except 
 in the thinnest sections, as certain varieties of rutile or lievrite, and cossyrite. 
 They may be distinguished from opaque minerals, however, by an entire 
 lack of metallic luster in reflected light. This is especially so with the last 
 two minerals. 
 
 Certain of the, isotropic minerals are not isotropic in all cases. Such are 
 perovskite, garnet, leucite, and analcite. However, these generally show 
 a well characterized regular structure of several double refracting parts so 
 that one, with experience at least, will scarcely ever be in doubt as to how 
 to classify it. Some double refracting minerals may also be present, 
 whose birefringence is so small that they could be overlooked in a cursory 
 examination of a section, e.g., apatite, eudialyte, zoisite, etc. Optical 
 anomalies are very common in uniaxial substances and they generally 
 appear biaxial with a small optic angle in convergent light. There are 
 also biaxial minerals in which the optic angle is nearly zero and they give 
 the impression of being uniaxial, thus, mica. Any scheme of classification 
 based upon a series of external features will show such discrepancies, but, 
 as already remarked, they can be easily overcome and with a little practice 
 any one can readily apply the division into these four groups. 
 
 That property which is first apparent, viz., refraction, is used 
 as a basis for further subdivision of the transparent minerals, so 
 that in each of the three groups the minerals are generally arranged 
 according to the decreasing index of refraction. However, 
 this principle is carried out only on general lines because there 
 
DEVELOPMENT OF ROCK CONSTITUENTS 201 
 
 are no means of measuring the index of refraction of a mineral 
 embedded in Canada balsam accurately. The necessity of 
 treating various isomorphous groups as units gives rise to 
 numerous deviations because there are sometimes great differ- 
 ences in the indices of refraction of various members of such a 
 group, but this will scarcely ever give rise to confusion. 
 
 The observation of the relief of a mineral in a thin section gives 
 a clue to the position in which that mineral is to be found in the 
 table. Table No. 3 can be used for an approximate classification 
 of the mineral. 
 
 The index of refraction is generally given to the third decimal place, 
 which is unreliable in many cases. The determinations of various in- 
 vestigators of ten show variations of several units in the third decimal place, 
 even in minerals which have a comparatively simple composition. In 
 minerals belonging to an isomorphous series of many members, the variations 
 in the determinations are much greater, e.g., almandine from Silberbach 
 in the Fichtelgebirge, n = 1.761; from Falls in the Fichtelgebirge, n = 1.770; 
 from India, n = 1.791 1.800. Three values are given for blue spinel 
 from Ceylon, n = 1.719; n = 1.720; and n = 1.726. It seems useless, there- 
 fore, to give values to the fourth decimal place. 
 
 Table 18 gives a tabulation of the minerals treated in this 
 text according to their double refraction and the interference 
 colors dependent upon it, as arranged by Michel-LeVy and 
 Lacroix. The values given there represent the difference be- 
 tween the maximum and the minimum indices of refraction, i.e., 
 f a, and generally are the mean where several values are to be 
 found. The apparent extreme values are given only where the 
 variations in the double refraction are especially large, as in 
 chloritoid and titanite. The table may be used in the following 
 manner. The thickness of the section is measured (see Part I, 
 page 45) and then a cross section is sought as nearly parallel to the 
 optic axis or to the optical plane as possible. This can be tested 
 in convergent polarized light. The interference color of the 
 section gives a very accurate means of determining the mineral. 
 If, for example, by means of the low yellowish-white interference 
 color of a section of quartz properly orientated, the thickness of 
 the slide is found to be 0.03 mm. ; while another mineral, which is 
 also colorless, shows yellow of the second order for its highest 
 interference color, the vertical line corresponding to this color 
 is followed to its point of intersection with the horizontal line 
 corresponding to the thickness 0.03 mm. The diagonal through 
 this point gives the double refraction of the mineral in question, 
 
202 PETROGRAPHIC METHODS 
 
 which in this particular case is approximately 0.030. The dis- 
 tinction between the few minerals to which the investigation has 
 thus been limited, can be made from a more careful study of the 
 other optical properties. 
 
 The color of a mineral is highly characteristic and useful in its 
 determination. Table 18 is arranged according to this prin- 
 ciple. Many minerals, which are macroscopically colored, are 
 entirely colorless in thin section and in all cases the thinner the 
 section the lighter the color and more difficult is it to determine. 
 The cause of the color varies greatly. Sometimes a character- 
 istic relation to the chemical composition can be recognized, as in 
 the pyroxenes and amphiboles. Sometimes the color changes 
 without any apparent variation in the chemical composition. In 
 this case, the color is produced by an admixture of an extremely 
 small amount of highly colored material as seen in garnet. In 
 a third case, microscopic investigation reveals the fact that in- 
 clusions are the cause of color, e.g., haiiyne and bronzite. The 
 color in this case is quite variable and many such minerals are 
 only colored occasionally. The color is not always uniform in 
 one section of a crystal. Zonal alternations or a very irregular 
 speckled distribution of the color are very common. 
 
 The presence or absence of pleochroism is likewise a highly 
 characteristic feature. The comparative depths of color, i.e., 
 absorption, which appears along certain axes, is quite constant 
 for a mineral group, and for this reason the absorption symbols 
 are usually given in the tables. The colors of different axes, 
 on the other hand, vary within wide limits and are only given 
 where they are more or less constant. 
 
 A number of minerals are included in this part which occur 
 only rarely as rock constituents. They are distinguished from 
 the more important by smaller type. The most of these are not 
 encountered by a beginner and are only observed in the course 
 of advanced study. 
 
 Only those relations are considered which are of value to petro- 
 graphers. All purely chemico-mineralogical and otherwise 
 theoretical considerations are omitted. Likewise, the crystal- 
 lographic development, which is of subordinate significance in the 
 determination of rock-forming minerals, is only discussed in a 
 cursory manner. Complicated crystal drawings are avoided and 
 in their place are numerous optical sketches for various minerals. 
 
 In explanation of the figures it may be said that a, b, c are the 
 
DEVELOPMENT OF ROCK CONSTITUENTS 203 
 
 crystallographic axes; o, B, c are the principal vibration direc- 
 tions; a, /?, f are the corresponding indices of refraction; A and B 
 are the optic axes. 
 
 A circle with a cross indicates the point of emergence of the 
 optic axis of a uniaxial mineral. The point where an optic axis 
 emerges from a biaxial mineral is indicated by a curve, printed 
 heavily where the dispersion is great. These indicate by their 
 position and distance apart the location of the axes in the section 
 in question. The apparent optic angle is often indicated on an 
 important crystal face or cleavage face, if it is parallel to the 
 optic plane and this may be a characteristic property of the 
 mineral. 
 
 The possibility of confusing various minerals is only con- 
 sidered in their descriptions when the slight differences in the 
 optical properties might be easily overlooked. For example, the 
 minerals perovskite and melanite are sometimes difficult to distin- 
 guish in spite of the great difference in the indices of refraction, 
 because they are very much higher than that of Canada balsam. 
 Nepheline and apatite, on the other hand, are not considered as 
 similar minerals, although their indices of refraction are nearly 
 the same, but the difference can readily be observed because they 
 both lie in the neighborhood of that of the Canada balsam. 
 
CHAPTER XI 
 Descriptive Section 
 
 As already mentioned, the best classification of rock-forming 
 minerals embraces the following four groups: 1. Opaque Minerals. 
 2. Isotropic Minerals. 3. Uniaxial Minerals. 4. Biaxial Minerals. 
 Those properties which are first apparent in microscopical inves- 
 tigation are made the basis for this classification. A classifica- 
 tion according to crystal systems seems to be less commendable 
 because it is not always possible to determine the system in a 
 thin section, especially in the case of hexagonal and tetragonal 
 minerals. Such a classification would also necessitate a division 
 of the most important isodimorphous groups, which can be 
 treated as a whole in the classification proposed. 
 
 i. Opaque Minerals 
 
 Included with the opaque minerals are those which are ordinarily not 
 transparent in thin section. They have more or less of a metallic luster 
 macroscopically and in reflected light under the microscope. Since from 
 their nature further optical determinations are impossible, microscopic 
 distinctions cannot be made safely if they have the same surface color, and 
 are without crystal form. 
 
 Some opaque minerals are cubic and some hexagonal. If they possess 
 crystal forms, those belonging to the first system show three-, four- or six- 
 sided cross sections which are isometric, while the typical sections of those 
 of the other systems are hexagonal or lath-shaped, corresponding to a thin 
 tabular development. 
 
 Pyrite 
 
 Pyrite (FeS 2 ) occurs in perfectly bounded crystals of various 
 dimensions consisting usually of a cube a striated by a pyrito- 
 hedron e, or in combination with it, Fig. 219, page 205. It also 
 occurs in large segregations and concretions, sometimes as a 
 cementing material, and in irregular grains and fine veins as an 
 impregnation in rocks. 
 
 It dissolves in nitric acid but hydrochloric acid scarcely 
 attacks it. Hardness, 6.5; sp. gr. ? 5. Its brass yellow color 
 
 204 
 
DESCRIPTIVE SECTION 205 
 
 is always apparent in a thin section. The ease with which it 
 weathers to red or brownish-red ferric hydroxide varies in 
 different localities and causes the rock to decompose very 
 rapidly, for example Alaunschiefer. Pyrite occurs everywhere. 
 When it is present as a constituent of eruptive rocks it is usually 
 of secondary origin and its occurrence is accompanied by many 
 other decomposition phenomena, such as 
 kaolinization, propyllitization, and the forma- 
 tion of greenstone. It is frequently found 
 also in all- kinds of contact rocks, sometimes 
 in large crystals which are greatly deformed 
 if the rock was under great pressure, and 
 sometimes in massive deposits as a constit- 
 uent Of the Contact ZOne. It OCCUrS in Cube andPyritohedron. 
 
 clastic rocks, principally in the form of con- 
 cretions in clay slates, and in coal. It is often confused' with the 
 more bronze colored pyrrhotite and the more yellowish-green 
 chalcopyrite, but it can be distinguished from both of these by 
 its superior hardness. 
 
 Pyrrhotite 
 
 Pyrrhotite (FeS), as a rock constituent, is found principally in small 
 grains or in larger dense masses and, in unaltered gabbros, occasionally in 
 tabular crystals. It is easily dissolved by acids. H. =4; sp. gr. =4.6 
 Usually strongly magnetic. Its color is bronze brown, but it is generally 
 tarnished or entirely altered to rust. It is found as a primary constituent 
 often containing nickel in basic eruptive rocks, and in green sclrsts and 
 amphibolites derived from them; also in contact limestones. It often 
 forms comparatively large, dense irregular masses. 
 
 Chalcopyrite 
 
 Chalcopyrite (CuFeS 2 ) is quite rare as a true rock constituent. It is 
 found associated with other sulphides as a constituent of basic eruptive 
 rocks and green schists, derived from them, and in granular limestones. 
 It never shows crystal form. It is usually in very irregular grains in 
 which well developed crystals of pyrite have grown. The distinction by 
 means of the greenish-yellow color of chalcopyrite is then very easy. H. =4; 
 sp. gr. =4.2. It gives a copper reaction. 
 
 Galena 
 
 Galena (PbS), associated with sphalerite, is not a rare constituent of 
 contact-metamorphic limestones. Occasionally it occurs in well developed 
 
206 PETROGRAPHIC METHODS 
 
 cubes, Fig. 220, but more often it is in grains which have 
 a light lead gray color and an extremely brilliant metallic 
 luster. It generally shows very perfect cubical cleavage 
 in reflected light. It is soft and often has a smeared 
 border, like graphite. This with the color gives it the 
 
 FIG. 220. Cube, appearance of molybdenite but it differs from the tabular 
 individuals of molybdenite in being more compact. 
 
 H. =2.5; sp. gr. = 7.5. Soluble in nitric acid with separation of sulphur. 
 
 Metallic Iron 
 
 Metallic iron is rare in terrestrial rocks and occurs only in angular grains. 
 It is always alloyed with nickel which may predominate, as in awaruite. 
 It is easily soluble in acids and decomposes Thoulet's and Klein's solutions. 
 In the latter case intense blue tungstic acid is formed. It precipitates 
 metallic copper from copper solutions. H.=4; sp. gr. = 7.8. Strongly 
 magnetic, but shows no polarity. It rusts easily in moist air. While it 
 has only been observed on the earth as a rare constituent of the most basic 
 eruptive rocks, it is known to occur extensively in meteorites. 
 
 Magnetite 
 
 Magnetite (Fe 3 O 4 ), which often contains titanium titanium 
 magnetite is a universal constituent of rocks. It occurs some- 
 times in octahedral crystals, Fig. 22 1, or twins, Fig. 222, some- 
 times in delicate crystal skeletons, and sometimes in irregular 
 grains or compact masses. It dissolves easily in hydrochloric 
 acid especially upon addition of a little potassium iodide (dis- 
 tinction from hematite, titanite, graphite, etc.). H. = 6; sp. 
 
 FIG. 221. Octahedron. FIG. 222. Twin. Spinel Law. 
 
 gr. =5.2. Strong magnetism, but not polar. Quite resistive 
 to weathering, and is ; therefore, only rarely surrounded by a 
 border of rust, and is found commonly in secondary deposits 
 magnetite sand. It appears black with a variable strong 
 metallic luster. The simplest means of isolating it is with a 
 magnet. It may be distinguished in this way from other 
 similar black opaque ores and from graphite. It is widely dis- 
 
DESCRIPTIVE SECTION 207 
 
 seminated in the eruptive rocks, particularly in the basic ones. 
 It is usually one of the first minerals to crystallize and so is 
 frequently included in the other constituents, especially in the 
 colored minerals. Black ores are very frequent in the form of 
 fine dust in the ground mass of such rocks. Although this is 
 probably mostly magnetite, it has been called opazite because of 
 the difficulty of determining it positively. It is quite as frequent 
 in all other groups of rocks. 
 
 In gabbros black compact ores, which are to be considered partly as 
 titanium magnetite and partly as ilmenite, are often surrounded with a 
 homogeneous fringe of biotite. Aggregates of magnetite grains are widely 
 disseminated in extrusive rocks, usually mixed with augite, which some- 
 times shows the form and cleavage of hornblende or biotite quite plainly, 
 and is probably produced by magmatic resorption of them. Intergrowth 
 with ilmenite is not at all rare. In such cases the magnetite can be dissolved 
 with hydrochloric acid. 
 
 Chromite 
 
 Chromite [Fe(Al,Fe,Cr) 2 OJ is found exclusively in olivine rocks in sharp 
 octahedral crystals, Fig. 221, page 206, or in compact masses often of con- 
 siderable size. It is one of the oldest products of crystallization and is 
 very frequent in sharp crystals as inclusions in olivine with which it is 
 almost universally associated. Acids attack it with great difficulty. 
 H.=5.5; sp. gr. =4.5. Macroscopically, it is brownish-black with only a 
 weak metallic luster, which appears more greasy under the microscope. 
 In thin section it is frequently transparent with a brown color particularly 
 on the edges. Its index of refraction is about 2.1. It is unaffected in 
 the alteration of olivine to serpentine because it is so resistive. A bril- 
 liant green pleochroic halo of chromeocher is often to be observed around 
 the crystals of chromite in serpentine. It is easy to isolate because of its 
 high specific gravity and its resistance to reagents, and it can be tested by 
 the bead reactions. 
 
 Hematite 
 
 Hematite (Fe 2 O 3 ) sometimes occurs in compact masses or in 
 crystals with an uncommonly brilliant metallic luster, Fig. 223. 
 They are always opaque in thin section, and 
 appear steel gray to iron black in reflected 
 light. Sometimes it is in thin hexagonal 
 plates which are transparent and red. In 
 the form of fine dust, hematite is the most 
 
 7 . Hematite Crystal. 
 
 frequent red pigment of rocks. It is difficultly 
 
 attacked by hydrochloric acid, but the more finely divided it 
 
 is, the more readily is it dissolved. It is not magnetic when not 
 
208 PETROGRAPHIC METHODS 
 
 intergrown with magnetite. H. =6.5; sp. gr. =5.25. Streak 
 red to reddish-brown. It is very resistive to weathering but is 
 now and then transformed to rust. 
 
 Compact nontransparent hematite is found in all kinds of 
 rocks. In eruptive rocks, it is more frequent in the acid varieties 
 into which it is often introduced by fumaroles. Thin yellowish- 
 red plates with properties like those of mica; which without fur- 
 ther investigation have been identified as hematite, are found as 
 orientated inclusions in feldspar. They give rise to the red 
 metallic chatoyancy, as in sunstone (Sonnensteiri) . It is not 
 positively determined whether these are hematite, but the well 
 developed platy crystals with brilliant metallic luster in reflected 
 light, which are so widespread in contact rocks, positively do 
 belong to hematite. The thinner individuals are transparent 
 with deep red color and have strong negative double refraction 
 and weak pleochroism, e yellowish; co brownish-red. Hematite 
 is known as a red pigment in all rocks from granite to clay slate 
 and is especially frequent in a group of acid porphyries and por- 
 phyrites that have been devitrified and appear felsitic. No 
 characteristic properties can be recognized in it even with the 
 strongest magnification and so this pigment sometimes, with a 
 yellowish or brownish color, is better known as ferrite. 
 
 Ilmenite 
 
 Ilmenite [(Fe,Ti) 2 O 3 ] is more frequently found in crystals than 
 compact. Two varieties can also be distinguished in this 
 mineral. One has a weak metallic luster, is iron black, and 
 occurs in tabular crystals or in dense compact masses; the other 
 is micaceous, brown transparent, and has a submetallic luster. 
 Both types have a great tendency to grow in skeletal forms. 
 Perhaps the ocherous brown pigment, e.g., in the plagioclases of 
 certain gabbros, may be referred to ilmenite. 
 
 It behaves like hematite toward acids and the magnet. H. = 
 5.5; sp. gr. =4.8 to 5.2. Unlike hematite it is frequently decom- 
 posed and is then real characteristic. It is coated with a 
 clouded film of leucoxene, white in reflected light, consisting of a 
 mixture of various titanium minerals and at the same time the 
 cleavage, which is not very apparent in the fresh mineral, becomes 
 plainly visible. Ilmenite may finally be entirely replaced by 
 leucoxene. Alteration into homogeneous individuals of titanite 
 
DESCRIPTIVE SECTION 
 
 209 
 
 or rutile, or zonal growths with these minerals are not rare. 
 It is found principally in eruptive rocks and of these it favors the 
 basic and those rich in sodium. The micaceous type with not 
 very strong double refraction, optically negative, brown, cu 
 yellow, is confined to the porphyric eruptive rocks. Perhaps the 
 orientated brown tabular inclusions, which produce the metallic 
 chatoyancy of diallage, hypersthene, etc., belong to this mineral. 
 
 Graphite 
 
 Graphite (C) is found in granular limestones rarely crystallized 
 in hexagonal plates with a brilliant metallic luster and steel gray 
 color. It generally forms shredded or flaky aggregates, which 
 have a great tendency to parallel intergrowths with altered mica, 
 Fig. 224. If it is treated with nitric acid and heated on a plati- 
 num foil, it swells up into voluminous worm-like particles. In 
 other rocks the individuals are more compact and of even size 
 and have an egg-shaped cross 
 section in the slide. Such 
 varieties are denser and do 
 not show the flaky character 
 of graphite to the naked eye. 
 They do not swell when heated 
 with nitric acid. For this rea- 
 son it has been considered as 
 an individual mineral called 
 graphitite. Graphite is a 
 widely disseminated black 
 pigment of crystalline schists. 
 As such it is as fine as dust 
 and a crystallographic devel- ,-, 00/l n ... 
 
 FIG. 224. Graphite m Graphite Gneiss from 
 Opment Cannot be Observed Pfaffenreuth near Passau. 
 
 even with the strongest mag- 
 nification. This is designated as graphitoid, and is considered as 
 a transition product to amorphous coal. The properties of 
 these three types of graphite are too similar to permit a separa- 
 tion to be made, especially since they are related by all possible 
 transition stages. 
 
 Graphite is completely insoluble in acids and molten alkalies* 
 If it is melted with metallic potassium, and ferrous and ferric 
 salts are added with some hydrochloric acid, Berlin blue is 
 
 14 
 
210 PETROGRAPHIC METHODS 
 
 frequently obtained. This is a proof of a small content of nitro- 
 gen in the graphite. In a mixture of fuming nitric acid and 
 potassium chlorate, it is oxidized to graphitic acid, which 
 crystallizes in golden yellow, metallic, hexagonal plates. They 
 are optically negative and have a strong double refraction. The 
 coarser flaky varieties of graphite burn very difficultly but when 
 finely divided it burns quite easily. It is a good conductor of 
 heat and electricity. Rocks rich in graphite feel cold. When 
 held in a zinc holder, it precipitates metallic copper from a 
 solution of copper. H. = l; sp. gr. = 2.3. The borders of its 
 cross sections never appear sharp on account of its softness. 
 It is smeared over the entire section in fine powder during the 
 process of grinding so that its distribution is often recognized 
 with difficulty. This property is one of the best characteristics 
 of graphite compared with hematite and ilmenite, which are 
 similar minerals. It is also distinguished from them by its 
 insolubility and its ability to burn. The brilliant metallic luster 
 of flaky aggregates produced by a very perfect cleavage parallel 
 to the base, becomes more and more obscured in the more com- 
 pact varieties. However, the grayish-black streak always remains 
 brilliant except in the finest varieties. 
 
 Graphite is rare as a primary constituent of eruptive rocks, 
 but if present, is mostly in the form of thick lumps. Altered 
 limestones and schists are its true habitat. It results in these 
 partly by contact-metamorphic alteration of organic matter and 
 partly by some sort of fumarolic action, which greatly alters the 
 rock at the same time. In such rocks its characteristic associate 
 is rutile. The dust-like variety of graphitoid is distinguished 
 from coal to which it is very similar by its electrical conductivity 
 and its reaction for graphitic acid. Elementary analysis seems 
 to be quite useless to distinguish between graphite and coals with 
 high fixed carbon. This is partly on account of the organic 
 impurities consisting chiefly of hydrous silicates, w r hich graphite 
 nearly always contains, and partly on account of an almost 
 constant content of nitrogen in graphite. It is not even lacking 
 in the best crystallized varieties, e.g., the coarse crystalline 
 graphite from Ceylon. 
 
 Carbonaceous Matter 
 
 Carbonaceous matter is the black pigment of numerous sedi- 
 mentary rocks. Shapeless grains are found in the thin sections of 
 
DESCRIPTIVE SECTION 211 
 
 gray to black clay slates and coal slates, and in gray to black 
 limestone, but a fine opaque dust is more frequent. This impreg- 
 nates the whole rock evenly and makes microscopic analysis of it 
 very difficult. Nothing is known about the chemical constitu- 
 tion of this material which may cause the black color of coal. 
 It contains oxygen, hydrogen, and a small quantity of nitrogen, 
 besides carbon evidently in variable proportions. It must not 
 be considered as established that a series of gradual transitions 
 from a sedimentary occurrence, rich in oxygen, hydrogen, and 
 nitrogen to a compound poor in them exists, as it was attempted 
 to show in certain phyllites and in schungite interbedded in layers 
 of compact coal. The analyses, which were designed to show 
 that such was the case, were not made on material properly 
 selected but on very impure formations and a certain amount of 
 the hydrogen and oxygen reported could easily be explained in 
 this way. The analyses which show a very small content of 
 nitrogen are those of the purest varieties of graphite. Moreover, 
 the methods of determining nitrogen in an elementary analysis 
 are not reliable. All conjectures concerning the chemical prop- 
 erties of such transition members are entirely without foundation. 
 
 Many of the opaque carbonaceous substances are at least 
 partially soluble in potassium hydroxide and become colored 
 brown. All are soluble in a mixture of potassium chlorate and 
 fuming nitric acid and give rise to a brown liquid. This appears 
 to be the only characteristic reaction to distinguish them from 
 graphite. At present it is by no means a settled question 
 whether pure amorphous carbon or a compound closely related 
 to it occurs in the black constituents of rocks, which also appear 
 as sooty films on bedding planes and cleavage faces of the sedi- 
 ments in various formations, or whether the carbonaceous 
 material is itself a definite compound or a series of compounds, 
 or whether or not they are definite compounds at all. 
 
 Finely divided black ores appear very similar to carbonaceous 
 material. They can be distinguished by treating the slide with 
 hydrochloric or nitric acid, which dissolve the ores and then by 
 heating to redness which destroys the coal. These processes are 
 often necessary before microscopical investigation can be carried 
 out on the slide. 
 
 The dark color of sediments does not depend entirely upon carbonaceous 
 matter. Frequently, other organic compounds with a brown to yellow 
 color and resinous luster occur in the sediments along with carbonaceous 
 
212 PETROGRAPHIC METHODS 
 
 matter, which is always opaque. These compounds are called bituminous 
 matter and represent a series of oxidation products of hydrocarbons. Their 
 constitution is likewise unknown but they give to a rock in which they 
 occur in great quantities a brown shiny streak. They are at least partially 
 soluble in benzol or xylol and are destroyed by a mixture of potassium 
 chlorate and nitric acid. 
 
 Finally, there are organic ingredients in sedimentary rocks, which can- 
 not be observed microscopically but can be recognized by the unpleasant 
 odor, which they produce when the rock is struck for example, stink 
 stone. They are doubtless the most resistive of all organic compounds 
 and remain unchanged even under the most intense metamorphic alteration, 
 which transforms the other kinds into graphite. They are only changed in 
 the zone lying next to the eruptive contact. Then a delicate rose-red color- 
 ing matter results from the vile smelling substance, which, together with the 
 character of the odor, indicates that they are indol derivatives, especially 
 skatol. 
 
 2 . Isotropic Minerals 
 
 Amorphous substances and cubic crystals are optically isotro- 
 pic. The former are not minerals in the narrower sense because 
 they do not have a definite stoichiometric constitution. As 
 they are sometimes predominant and sometimes subordinate as 
 rock constituents, their characteristic properties must be included 
 in this discussion. 
 
 Cubic crystals are distinguished from them by a regular struc- 
 ture which is indicated in the straight edges, cleavage, or the 
 arrangement of inclusions. The structure is sometimes so con- 
 cealed that it is not seen in direct observation but it can still be 
 determined by the formation of etch figures. 
 
 FIG. 225. Principal Cross Sections of Cubic Minerals. 
 
 Isotropic minerals are dark between crossed nicols and pro- 
 duce no interference figure in convergent polarized light. Opti- 
 cal anomalies of various sorts are very common and are recognized 
 by spotted illumination, by lamination, or by a regular division 
 of the crystal into segments that extinguish differently. 
 
 Cubic minerals generally develop in the form of cubes, octa 
 hedrons, and dodecahedrons, and therefore their cross sections 
 are commonly isometric and three-, four-, or six-sided. The 
 
DESCRIPTIVE SECTION 213 
 
 tetragonal trisoctahedron is rarer and its cross section is usually 
 eight-sided and more or less rounded. Distortions of any of the 
 forms give rise to variations in the shape of the cross sections. 
 Twins according to the spinel law are not at all uncommon. 
 Only the simplest forms are known as cleavage fragments. 
 Cleavage parallel to the cube and octahedron is frequent and 
 perfect, while that parallel to the dodecahedron is rare and 
 usually imperfect. Fig. 225 shows the forms of the cross sections 
 of minerals in the cubic system as well as the principal kinds of 
 cleavage. 
 
 Perovskite (1) 
 
 Perovskite occurs as a rock constituent either in minute, light brown, 
 octahedral crystals and skeletons of crystals, or in larger compact grains 
 with a dark color. It is yellowish in reflected light and has an adamantine 
 luster. The smaller crystals are usually optically normal but the larger 
 ones consist of a system of interpenetrated double refracting lamellae. 
 Inclusions and decomposition are not known. It is widely disseminated in 
 basic eruptive rocks associated especially with melilite. It is also 
 known in certain contact rocks. It is infusible before the blowpipe. It 
 is distinguished from various minerals that resemble it, picotite, melanite, 
 etc., by its higher index of refraction, which causes the high metallic luster 
 in reflected light, and by the reaction for titanium. 
 
 A large number of rare cubic minerals appear very similar to perovskite 
 under the microscope with respect to the index of refraction, color, and 
 geological occurrence. Such are pyrochlore, beckelite and pyrrhite, which 
 crystallize predominantly cubical and are yellowish in color, also brown 
 zirkelite, reddish koppite, knopite, which usually shows optical anomalies, 
 and dysanalyte with its metallic luster, being almost opaque in thin section. 
 All these minerals are local constituents of eruptive rocks and pegmatites 
 rich in soda, but are found also in grains in the contact rocks lying next to 
 these. Oldhamite, CaS, also occurs in meteorites but it is not observed 
 except in specially prepared sections because it decomposes in -water. 
 
 Sphalerite (1) 
 
 Sphalerite is a widespread constituent of rocks, occurring in irregular 
 grains associated with opaque sulphides. It is light yellow to brownish, 
 and appears very much like rutile and was almost always determined as 
 such until recently. It is distinguished from rutile by a more perfect cleav- 
 age and its isotropic behavior toward light. 
 
 Sphalerite may also be confused with the rare isotropic minerals mentioned 
 under perovskite with which it is associated in contact rocks, especially in 
 limestone. It can only be determined positively by a blowpipe. It gives a 
 film of zinc oxide when fused with sodium carbonate on charcoal. 
 
214 
 
 PETROGRAPHIC METHODS 
 
 Garnet Group (1) 
 
 The minerals of the garnet group are widespread rock constit- 
 uents. They are sometimes well developed crystals, and some- 
 times rounded or irregular grains. The most frequent crystal 
 form is the rhombic dodecahedron, Fig. 226, and the tetragonal 
 trisoctahedron, Fig. 227, is less common. Combinations of these 
 two forms, Fig. 228, also occur. Their dimensions vary greatly 
 and every gradation from microlites to crystals as large as a fist 
 are found. Individuals of intermediate sizes are most common 
 and they are macroscopically visible. Almandite and the lime 
 garnets often form perimorphs. Alterations that are compara- 
 tively rare, in which chlorite, amphibole, or biotite are formed at 
 the expense of the garnet, appear similar to these. 
 
 Fia. 226. FIG. 227. FIG. 228. 
 
 Dodecahedron. Tetragonal Tris- Combination. 
 
 octahedron. 
 
 Cleavage parallel to the dodecahedron is at most only suggested, 
 Fig. 225, e and /. But on account of its brittleness the garnet 
 crystals are usually quite full of short jagged cracks that do not 
 indicate any crystallographic orientation. 
 
 Optical anomalies are common but are, confined mostly to the 
 group of lime garnets free from titanium and to the manganese 
 garnets. This consists of a division of the crystal into segments, 
 that are optically different, and often also of a zonal structure 
 due to the growth of layers with very strong but variable double 
 refraction. The division into segments always corresponds to 
 one of the crystal forms present. The crystal then appears to be 
 composed of as many pyramids as there are faces to that form and 
 each face is the base of a pyramid whose apex lies in the center of 
 the crystal. The double refraction of these segments is quite 
 variable and may exceed that of quartz. They are biaxial, 
 optically positive often with strong dispersion, p< v } and 2v = 56 
 to 90. Anomalous interference figures are frequent. Each one 
 of these segments is usually not very homogeneous but is built up 
 
DESCRIPTIVE SECTION 
 
 215 
 
 of differently orientated parts that are more or less irregularly 
 bounded, so that the character of the optical anomalies stands 
 out prominently and can readily be distinguished from normal 
 double refracting substances. 
 
 The dodecahedron is the most frequent form of garnet crystals 
 and, therefore, the optical structure is usually the so-called 
 dodecahedral structure. Fig. 229 shows a section parallel to the 
 dodecahedron face and Fig. 230 shows one out of the middle of a 
 crystal parallel to an octahedron face. A similar development 
 occurs now and then in almandite unaccompanied by optical 
 anomalies, and is due to the arrangement of inclusions, or to 
 breaking parallel to the pyramids of growth. 
 
 Almandite is the most widespread member of the garnet group. 
 The larger crystals are macroscopically transparent and wine red, 
 and are termed precious garnet, while the more frequent red to 
 
 FIG. 229. Segments in Garnet. 
 
 FIG. 230. Garnet with Optical Anomalies. 
 Section Parallel to Octahedron. 
 
 yellowish-red varieties clouded by inclusions or cracks, are called 
 common garnet. Both varieties are pale reddish in thin section. 
 Almandite is always isotropic. It is an accessory constituent of 
 all eruptive rocks with the exception of the most basic ones. It 
 is characteristic for granulite. It is more common and frequent 
 as an essential constituent of various groups of contact rocks. It 
 is usually macroscopically visible. Concentric structures often 
 develop around almandite and these tend to extend outward 
 from a compact grain into the surrounding minerals like pseudo- 
 pods. The larger, well defined crystals are easily removed from 
 rocks having abundant mica, while they are apt to be broken in 
 rocks with little mica. 
 
 The mineral is sometimes altered to chlorite or hornblende, 
 and in aplites and pegmatites, in which almandite is the most 
 characteristic accessory constituent, round flakes of biotite are 
 seen which contain a residue of garnet from which the biotite 
 
216 
 
 PETROGRAPHIC METHODS 
 
 was formed. Almandite is quite resistive to weathering and is, 
 therefore, often found in secondary deposits. It is used as a 
 substitute for emery. A very fine skeletal development has been 
 observed in clastic rocks but nothing more is known about this 
 formation. Almandite has the greatest tendency to crystalliza- 
 tion of all rock-forming minerals and is, therefore, frequently 
 filled with inclusions. Almandite crystals, perforated like a 
 sieve, are the most characteristic features of contact rocks. In 
 rocks rich in. quartz the garnet itself takes a small part in the 
 composition of such a structure, Fig. 214, page 197. It also oc- 
 curs in the most beautiful perimorphs in such rocks, Fig. 213, 
 page 197. 
 
 The manganese garnet, spessartite, is very similar to almandite 
 in its occurrence as a rock constituent but is much rarer. It is 
 common, especially in pegmatite dikes. It forms also a principal 
 
 constituent of the whetstone 
 schists of the Ardennes moun- 
 tains but occurs in minute 
 colorless individuals. 
 
 Pyrope, usually containing 
 some chromium, is confined to 
 the olivine ,rocks and their 
 derivatives, the serpentines. 
 It generally shows rather in- 
 distinct crystal form, which 
 appears to be hexahedral. Its 
 individuals can be recognized 
 distinctly macroscopically by 
 the blood red color. It is dis- 
 tinguished from almandite by 
 being poor in inclusions and 
 cracks and, therefore, often transparent cinnamon-stone. The 
 occurrence of a fine fibrous, radial border of kelyphite is especially 
 distinctive, Fig. 231. It sometimes consists of hornblende 
 minerals but is more often a mixture. It often completely 
 replaces the pyrope. This border is considered a product of 
 magmatic resorption of pyrope, that has already been formed, by 
 a magma rich in magnesium. Pyrope is also found in secondary 
 deposits in comparatively large grains, which are cut as semipre- 
 cious stones Bohemian garnet. 
 
 Hessonite, rich in iron, and topazolite of the lime garnet series 
 
 FIG. 231. Pyrope with Kelyphite Border 
 in Serpentine from Karlstetten, Lower 
 Austria. (After Cohen.) 
 
DESCRIPTIVE SECTION 217 
 
 are found almost exclusively in crevices and other forms of 
 secondary development within rocks, especially in serpentine. 
 Light green topazolite, demantoid, is sometimes found in dis- 
 seminated crystals. These minerals are also the most important 
 constituents of skarn, which accompanies magnetite ore deposits. 
 Emerald green uwarowite, an optically anomalous lime garnet 
 containing chromium, is likewise confined to crevices in serpen- 
 tine. Grossularite and hessonite poor in iron are widely dis- 
 seminated constituents of contact-metamorphic limestones and 
 lime-silicate hornfels. The former is entirely colorless in thin 
 section, and the latter is light reddish, brownish, or greenish, the 
 color being entirely independent of the chemical composition. 
 Light reddish, excellently bounded microlites of lime garnet 
 often fill the plagioclase crystals of 'the granite of the Central Alps 
 in such a manner that the rock has a reddish tinge. When these 
 garnets occur in limestones, they have a crystallographic form 
 but are often rounded in a peculiar manner as though they had 
 been slightly fused on the surface. In other rocks they form 
 granular aggregates without distinct crystal form. Lime gar- 
 nets poor in iron are easily fusible, and those rich in iron fuse 
 with difficulty. 
 
 The lime garnets have fewer inclusions than almandite, but 
 perimorphs are encountered in them. Evidences of alteration 
 are almost entirely lacking. On the other hand, a structure con- 
 sisting of different zones is very common. 
 
 The members of the lime garnets containing titanium play a 
 special r61e. They are macroscopically black with a pitchy 
 luster but in thin section are brown and built up of zones in which 
 the intensity of the color varies depending upon the variations in 
 the content of titanium. These garnets occur especially in 
 eruptive rocks rich in sodium and their contact formations, and 
 are usually well developed and always poor in inclusions. They 
 are named according to the increasing content of titanium 
 melanite, schorlomite and ivaarite, the latter containing about 
 15 per cent. TiO 2 . The cause of the black color of pyrenaite, 
 which occurs in the contact rocks of the Pyrenees, is to be sought 
 not in these garnets but in finely divided graphite. 
 
 The silicates almandite, pyrope, and hessonite form isomor- 
 phous mixtures only in exceptional instances. The garnet of 
 eclogites, which corresponds apparently to normal almandite, is 
 such a mixture in which almandite predominates. Dense splin- 
 
218 PETROGRAPHIC METHODS 
 
 tery aggregates of grossularite are noteworthy in olivine rocks and 
 serpentine. They occur in lumps or form a part of the so-called 
 saussurite. Similar formations of hessonite occur on the con- 
 tact of dikes containing hessonite, with olivine or serpentine 
 rocks. The latter have been shown to be alteration products of 
 the olivine rock itself. 
 
 Almandite and pyrope are not easily confused with other 
 minerals because of the reddish tint which is always plainly seen 
 even in the thinnest sections. Colorless or greenish lime garnets 
 resemble the spinels, especially when they are in small individ- 
 uals, and when the crystal form is lacking they can only be 
 distinguished by a chemical test. They can be differentiated 
 from periclase, which they likewise simulate, by the imperfect- 
 ness of the cleavage. Titanium garnets may be confused with 
 picotite or perovskite, but the garnet usually has the zonal 
 structure. Chemical reactions will give the necessary confirma- 
 tion. Double refracting lime garnets are so similar to vesuvianite 
 and to the low double refracting members of the epidote group, 
 especially when the garnet is in fine grained aggregates, that a 
 differentiation is not possible. 
 
 Spinel Group (1) 
 
 The minerals of the spinel group (compare magnetite and chromite, page 
 206) occur in sharp octahedral crystals and twins according to the spinel 
 law, Figs. 221 and 222, page 206, without cleavage, and less often in irregular 
 grains. They are always optically normal. They are the hardest and 
 most resistive rock-forming constituents and are, therefore, scarcely ever 
 altered and are frequently found in secondary deposits. 
 
 Chrome spinel, picotite, forms minute crystals nearly everywhere in 
 olivine with which it, like chromite, is always associated. It often shows 
 no crystal form when it occurs as an independent constituent of peridotite. 
 It is macroscopically black with a submetallic luster, but under the micro- 
 scope the metallic luster is not seen. It is brown in transmitted light but 
 usually lighter than chromite to which it is exactly similar except for the 
 hardness. The iron spinels, pleonaste and hercynite, are black as is also the 
 zinc spinel, kreittonite, which occurs in certain ore deposits and is very 
 similar to the others, being distinguishable from them only chemically. 
 They have dull luster, green streak, and are green and transparent in thin 
 section. Common spinel is red, green, blue or violet to the naked eye, but 
 is always colorless in thin section. It is a characteristic product of contact 
 metamorphism particularly in granular limestones and dolomites. It can 
 be recognized macroscopically, and on account of its dearth of inclusions and 
 lack of cleavage, it is clear and transparent precious spinel. Iron spinel 
 is also found in other rocks, such as granulites and Iherzolites. It is mostly 
 
DESCRIPTIVE SECTION 219 
 
 in the outer border zone of the eruptive rock in schlieren and is often mixed 
 with other contact minerals. It is undoubtedly a residue of resorbed in- 
 clusions of the neighboring rock. The minutest microlites of colorless 
 spinel are found in the so-called fritted rocks and only the high index of 
 refraction serves to determine it. Spinel can be easily isolated in all cases 
 on account of its resistance to reagents. The distinguishing of the spinels 
 from perovskite and garnet is discussed under those minerals. The lack of 
 cleavage and its chemical behavior distinguish it from periclase. 
 
 Periclase (1) 
 
 Periclase is not a rare constituent of contact-metamorphic limestones. 
 It is usually in small octahedrons with rounded surfaces. They are macro- 
 scopically colorless to greenish-brown. When fresh it is easily recognized 
 by the cleavage, Fig. 225, c, page 212, and the index of refraction. It is usu- 
 ally strongly altered to scaly aggregates of brucite, e.g., in predazz.ite, 
 or of serpentine. In these the original cleavage is sometimes retained. 
 
 Boracite (1) 
 
 Boracite is a rock-forming mineral now and then, but is confined to the 
 rock-salt formations. It occurs in gypsum and abraum salts in well 
 developed tetrahedral crystals that are macroscopically recognizable. 
 They are colorless to light green, often possessing an abundance of faces. 
 Under the microscope it appears very much like leucite from which it is 
 distinguished by its higher index of refraction and its mode of occurrence. 
 
 Rock Salt 
 
 Rock salt as a rock constituent is not observed in thin sections prepared 
 with water and is, therefore, not included in the tables. H. = 2; sp. gr. =2.2. 
 Perfect cubical cleavage, Fig. 220, page 206. It is without crystal 
 form and is soluble in water giving a salty taste, n = 1.544. It often occurs 
 in pure, very coarse granular aggregates that are colorless, deep blue, red 
 or yellow, and rich in liquid inclusions, or in fibrous masses in crevices, or 
 finally in fine impregnations in salt clays. It also occurs as incrustations 
 on lava rocks and as small cubes in liquid inclusions in quartz. ~- 
 
 Leucite (1) 
 
 Leucite is found almost exclusively in white, perfectly 
 developed tetragonal trisoctahedrons, Fig. 227, page 214, in 
 eruptive rocks rich in alkali, especially the leucitophyres, leucite 
 tephrites, etc. In these rocks it frequently forms large, brittle, 
 and checked crystals, but in leucite basalt it is only in the form 
 of microlites. Aggregates of orthoclase and nepheline in the 
 form of large leucite crystals are found in leucite syenites and the 
 rocks produced by differentiation of such a magma. 
 
220 PETROGRAPHIC METHODS 
 
 The outlines are usually rounded. Skeletal development and 
 magmatic resorption are not rare. Small crystals are always 
 optically normal but larger crystals consist of complicated 
 lamellar twinning intergrowths and penetration of such a charac- 
 ter that a highly characteristic structure is observed in polarized 
 light, as is shown in Fig. 232. When heated to 600 this anoma- 
 lous structure disappears but returns again upon cooling. 
 
 The frequency of inclusions is an especially characteristic 
 property of leucite. They consist of crystallized minerals, slag, 
 or colorless to brownish glass, arranged in 
 a regular manner either zonal or radial, Fig. 
 210, page 195. Needles of various minerals, 
 arranged tangentially around the crystals of 
 leucite, are often observed, ocellary struc- 
 ture, Fig. 207, page 193. It is often altered 
 because it is so readily attacked. Besides 
 the pseudomorphs referred to above those 
 FiG.232 : -TwinningLa m - of analcite and other zeo lites are wide- 
 
 ination in Leucite. 
 
 spread. 
 
 It would be very easily overlooked where it occurs in the form 
 of microlites, but even they are distinguished from all other 
 minerals by the arrangement of the inclusions. It could be 
 confused with microcline when it forms compact aggregates that 
 are optically anomalous, but its mode of occurrence is entirely 
 different. If the optical anomalies, the crystal form, and the 
 inclusions are lacking, distinction from analcite, sodalite, or from 
 rock glass is scarcely possible. 
 
 Glass (1) 
 
 Glassy material is found as a principal or subordinate rock constituent 
 only in rocks which have solidified rapidly from the magma. These are 
 the comparatively small eruptive dikes, which have branched out far from 
 the vulcanic center, or the rocks poured out upon the surface and their tuffs. 
 or those rocks or fragments which have suffered partial fusion, i.e., fritting. 
 The glass occurring in rocks may have quite a variable composition, but the 
 acid mixtures tend in general more to glassy development than do the more 
 basic ones. Although in andesite, diabase and basalt, glasses are not 
 lacking, still the series of quartz porphyry, rhyolite, and trachyte is much 
 richer in rock glass. 
 
 Some of the glasses are free from water while others show an original 
 content of water up to 8 or 10 per cent. The former are called obsidian and 
 the latter pitchstone, while perlite with 2.5 to 5 per cent, of water is inter- 
 
DESCRIPTIVE SECTION 
 
 221 
 
 mediate. The specific gravity is always low, rising to 2.4 in obsidian and 
 2.7 in basic glasses, and falling to 2.25 in those containing water. 
 
 The normal rock glasses have a chemical composition corresponding to 
 that of crystalline rocks. They are principally products of rapid solidi- 
 fication of granitic rock magmas, but glasses with the composition of ande- 
 site and even trap and basalt are not altogether lacking although they are 
 in general anomalous forms of development. It has been noted that 
 partially or entirely resorbed inclusions have given rise to the formation of 
 glass because the ability of the dissolving magma to crystallize has been 
 lost through the chemical composition of the inclusions. The glassy 
 development of basic rocks may also be referred to such phenomena. These 
 glassy forms occur sometimes in rounded masses within normal rocks or on 
 the contact with certain rocks, while they are entirely lacking with other 
 rocks. Chemical analysis almost always shows variations from the 
 normal composition in such cases. 
 
 The chemical behavior of rock glasses is quite variable according to the 
 varying composition. In general, the glasses .are more rapidly attacked by 
 the atmosphere and are more easily dissolved by hydrofluoric acid than 
 are most of the crystallized rock constituents. Most of the rock glasses 
 are very little affected by hydrochloric 
 acid, but now and then they gelatinize 
 with hydrochloric acid, especially 
 basaltic glasses rich in sodium. Rock 
 glasses often appear to have double 
 refraction in the neighborhood of 
 inclusions. This is produced by strain 
 and gives rise to the Brewster's cross, 
 Part I, page 123. Obsidian some- 
 times forms the predominant constit- 
 uent of rocks and in them only a 
 few devitrification products occur. 
 It is brittle and has a perfect con- 
 choidal fracture, Fig. 233, vitreous 
 luster, and is almost invariably dark 
 grayish-black to pitch black in color. 
 In thin section the color can often be 
 plainly recognized, but it is sometimes 
 entirely colorless and frequently filled 
 full of trichites. Upon heating many 
 
 varieties swell out into foamy masses, which are light gray to white and 
 are called pumice. Pumice in the form of ejectamenta and bombs is 
 often found in nature accompanying obsidian. The obsidians belonging to 
 basic rocks, such as the bluish-black tachylite and the black hyalomelane, 
 are heavy and have comparatively high indices of refraction. They are 
 quite highly colored in thin section. 
 
 Perlite is a rock glass, which is full of curved cracks throughout its mass, 
 so that it breaks up easily into rounded fragments each of which has an 
 onion-like structure. It usually has a lower specific gravity than obsidian 
 because of these cracks, and is lighter colored macroscopically. Under 
 
 FIG. 233. Conchoidal Fracture (1/2 
 Natural Size). Obsidian, Iceland. 
 
222 
 
 PETROGRAPHIC METHODS 
 
 the microscope the onion-like arrangement of the cracks presents an un- 
 commonly characteristic appearance, Fig. 234. Pitchstone has a pitchy 
 luster and is variously colored macroscopically. It may be green, yellow, 
 red, or black. The pitchy luster may be due to an abundant development 
 of microlitic crystallization. Crystal skeletons assume manifold forms in 
 pitchstone. 
 
 Rock glasses often possess the appearance of decided flow structure. In 
 these glasses variously colored bands mixed with each other in multifarious 
 ways form the principal constituent. In other cases they flow around the 
 large crystals that have separated out, Fig. 235, eutaxite, so that it appears 
 as if the different parts were not miscible with each other even in the liquid 
 condition. Alteration of beds greatly devitrified with those less so, and 
 of porous with denser layers, is not rare in rock glasses. The phenomenon of 
 devitrification is to be especially emphasized. The original glass, which is 
 
 FIG. 234. Perlite, Glashiittental near 
 Schemnitz, Hungary. 
 
 FIG. 235. Eutaxite. Microscopic Fluidal 
 Structure in Pitchstone. Kastelruth, South 
 Tyrole. 
 
 completely amorphous, is transformed into a crystalline aggregate called 
 microfelsite, felsite, etc. The newly formed mineral aggregates structure- 
 less combinations of quartz and orthoclase in the ordinary acid glasses, 
 are considered to be the result of a long period of molecular rearrangement 
 somewhat analogous to the transformation of the metals. The cause has 
 also been sought in circulating waters, either of a meteoric or of magmatic 
 character. 
 
 When the glass occurs in the form of fine films as a subordinate base 
 between the crystalline constituents and is color'ess, it is very hard to 
 determine. It can scarcely be distinguished from nepheline or from zeolites, 
 especially in those cases where a feeble double refraction is shown on account 
 of strain. If it forms the residue from which the other constituents have 
 crystallized, it is always richer in silica than the normal rock. 
 
 Analcite (1) 
 
 Analcite is only known as a secondary rock constituent in druses and in 
 pseudomorphs. It occurs in various types of soda rocks and is usually 
 
DESCRIPTIVE SECTION 223 
 
 formed from sodalite, nepheline, and leucite. As a rock constituent it 
 does not have its own crystal form. Crystals are only found in druses and 
 cavities. Optical anomalies are present now and then, but they are mostly 
 much weaker than in garnet. Analcite usually appears clear and trans- 
 parent. Sometimes the cleavage cracks are quite plain, but frequently it 
 does not possess these and then the determination is made in a chemical 
 manner by testing for sodium. The double refraction also increases and 
 it becomes clouded when heated. The mineral itself has, however, no char- 
 acteristic microstructure and no striking properties. It is most easily con- 
 fused with leucite, sodalite, and nepheline. 
 
 Sodalite Group (2) 
 
 The usual crystal form of the minerals of the sodalite group is 
 the rhombic dodecahedron, Fig. 226, page 214. The cross sec- 
 tions are therefore six-sided or quadratic, and often somewhat 
 distorted. The edges are usually 'rounded and the faces are 
 eaten out and corroded. Sodalite is found now and then in 
 irregular grains, which is not the case with the other members of 
 the group. The dimensions are usually not very small and 
 microlites are entirely lacking. These minerals are easily 
 recognized by the naked eye when they are colored. Colorless 
 fresh varieties may be easily overlooked, but they show plainly 
 on the fracture surface of the rock if they are clouded by incipient 
 alteration. They then appear dull white or yellowish and are 
 easily confused with altered feldspar from which they can be 
 distinguished by the lack of cleavage and the typical isometric 
 form of the cross section. 
 
 The sodalite minerals are found in rather basic soda rocks free 
 from primary quartz and constitute the part most susceptible 
 to alteration. They are frequently altered to natrolite, analcite, 
 and other zeolites, forming aggregates like snowflakes Spreu- 
 stein. Alteration to dense aggregates of mica or to amorphous 
 masses is also known. 
 
 Sodalite is disseminated throughout certain granular eruptive 
 rocks and is macroscopically colorless, or at most very light blue 
 or green, but in thin section it is always colorless. Haiiyne is 
 more often found in extrusive rocks and is sometimes colorless, 
 while at other times it is gray, yellow, green, red, or deep blue. 
 It is frequently speckled. Certain colorless occurrences have 
 the property of taking on an intense blue color when heated. 
 If such varieties are treated with hydrofluoric acid and silver 
 nitrate they percipitate black silver sulphide, while sodalite 
 
224 
 
 PETROGRAPHIC METHODS 
 
 FIG. 236. Haiiyne with 
 Regular Inclusions. 
 
 becomes coated with silver chloride. Regularly arranged black 
 bars with unknown properties are frequently observed in soda- 
 lite minerals. They have a tendency to congregate on the edge 
 of the crystals and make them less transparent, Fig. 236. Such 
 borders, which are quite small but entirely opaque, are observed 
 especially around crystals, which have been 
 greatly rounded and corroded by the molten 
 magma. Ferric hydrate forms from these 
 dark inclusions when the rock is weathered, 
 and it colors the mineral yellowish to red. 
 Other inclusions, ores, pyroxene, etc., are 
 frequent, but glass is comparatively rare. 
 If the crystal form is lacking, the regularly 
 arranged inclusions together with the low 
 index of refraction is often the only 
 
 characteristic feature because they show very imperfect cleavage, 
 which is at best only irregular cracks. 
 
 Differentiation of the various members is only possible 
 chemically. The test for chlorine in sodalite has already been 
 mentioned. If haiiyne is treated with hydrochloric acid, gypsum 
 crystallizes out. The same crystals are obtained from noselite if 
 a trace of calcium carbonate is added to the solution. The index 
 of refraction being lower than that of Canada balsam distin- 
 guishes them from other similarly appearing minerals, but that 
 may give rise to confusion with zeolites, tridymite, or rock 
 glass. They can, however, be positively differentiated from 
 analcite and glass only by special chemical reactions. 
 
 Lazurite may be referred to briefly. It occurs in irregular grains forming 
 the deep blue coloring constituent of lapis lazuli. It is a member of the 
 sodalite group containing sodium sulphide and causes the blue color of the 
 other members. Lapis lazuli is a contact rock containing also calcium- 
 magnesium silicates and carbonates. 
 
 Opal (2) 
 
 Opal, as a rock constituent, is not often visible macroscopically. It 
 may be developed as precious opal with the characteristic p ] ay of colors 
 or as dull colored common opal clouded by numerous inclusions. It is 
 always secondary, filling crevices, or occurring in pseudomorphs after 
 feldspar and other minerals. It is found especially in greatly altered erup- 
 tive rocks and their tuffs. It is not easy to ascertain to what extent opal 
 occurs as a constituent of sedimentary rocks. The amorphous opal-like 
 silica of organisms is very easily transformed into crystalline aggregates 
 and, on the other hand, opal sandstones occur in such relationships that 
 
DESCRIPTIVE SECTION 
 
 225 
 
 the formation of the mineral from thermal processes is highly probable. In 
 rocks in which opal is not seen macroscopically it is observed in thin sec- 
 tions in finely divided, shapeless, and colorless impregnations which may be 
 recognized only with great difficulty. This type of occurrence also shows 
 all the earmarks of post volcanic alteration. It is always colorless under the 
 microscope and may be recognized by its very low index of refraction. It 
 often shows anomalous double refraction, Brewster's cross, and inclusions 
 of tabular crystals of tridymite are extremely common. A microscopic 
 differentiation from glass is often very difficult, although the low index of 
 refraction is very distinctive. It can be determined positively by its solu- 
 bility in potassium hydroxide. Even this reaction may give rise to error 
 because numerous decomposition products containing aluminium also yield 
 silica in potassium hydroxide. 
 
 Fluorite (2) 
 
 Fluorite is rare as a real constituent of rocks. It is found in granite, 
 especially in the facies altered by pneumatolytic processes greisen 
 together with tourmaline, topaz, etc. It is also found as a constant asso- 
 ciate of the zirconium silicates, that occur accessory in soda rocks. It is 
 always in grains and is easily recognized when it is violet, but very difficult 
 to find when it is colorless. It is characterized by the fact that it has the 
 lowest index of the isotropic rock-forming minerals and also has a perfect 
 cleavage, which usually causes a scaly appearance to the surface of the 
 section. 
 
 3. Uniaxial Minerals 
 
 The rock-forming minerals of the tetragonal and hexagonal 
 systems are sometimes long, sometimes short prismatic, with 
 
 FIG. 237. Principal Cross Sections of Uniaxial Minerals. 
 
 or without pyramidal terminations. Sometimes, they are devel- 
 oped tabular parallel to the base or are predominantly pyramidal 
 or rhombohedral. Fig. 237 shows the most important types of 
 
 15 
 
226 PETROGRAPHIC METHODS 
 
 cross sections. In the prism zone they are either elongated or 
 short, almost square rectangles with or without domatic forms 
 on the long or the short side. Rhombic forms, with or without 
 truncated corners, are also observed. In the first case the 
 extinction is always parallel and perpendicular to the sides of 
 the rectangle, while in the latter case the extinction direction 
 bisects the angles of the rhombus. 
 
 A section orientated as nearly parallel to the optic axis as 
 possible is found by choosing the one which, in parallel polar- 
 ized light, shows the highest interference color of all the sections 
 of the same mineral. This is also an indication of the strength 
 of the double refraction. In convergent polarized light such a 
 section shows a distribution of colors into curves simulating 
 hyperbolae. They are symmetrical with respect to two planes, 
 and the color is lowered in those quadrants through which the 
 optic axis passes and is raised in the other two quadrants with 
 respect to the center of the field. The optical character of the 
 principal zone, Ch 2 , is the same as that of the mineral itself, 
 Chm, because in uniaxial minerals with a prismatic development 
 the optic axis and the axis of the principal zone coincide. If 
 uniaxial minerals are tabular parallel to the base, the optic axis 
 is perpendicular to the principal zone, and the character of the 
 principal zone is opposite to that of the mineral. 
 
 Sections cut perpendicular to the optic axis remain dark when 
 rotated between crossed nicols in parallel light and appear like 
 isotropic minerals, but if the double refraction is not too small, as 
 it is in apatite, such sections give an interference figure in con- 
 vergent light. Their outlines are quadratic, or rarely eight- 
 sided in tetragonal minerals, and six- or three-sided in hexagonal 
 minerals. Occasionally where trigonal forms occur the sections 
 are nine-sided. Such sections give the interference figure of uni- 
 axial crystals in convergent light. In certain rock-forming 
 minerals of this group optical anomalies are frequent. They 
 may be shown only in convergent light by a spreading of the 
 black cross upon rotation, or they may be noted in parallel light 
 by the occurrence of double refracting segments in a basal sec- 
 tion. Still these features are not as common as are the anom- 
 alies in isotropic minerals. The circular polarization in quartz 
 is not observed in thin section. 
 
 In all cases the shapes of the sections show only the simplest 
 forms and this is true to a still higher degree for the cleavage. 
 
DESCRIPTIVE SECTION 
 
 227 
 
 In the holohedral members, the cleavage is only parallel to 
 prisms of the first and second order and to the basal pinacoid. 
 In hemihedral forms rhombohedral cleavage in addition to the 
 others is the most important. The character of the various 
 kinds of cleavage is shown schematically in Fig. 237. 
 
 Rutile (4) 
 
 Rutile is very widespread, but is only present in subordinate 
 quantities. It is found in larger quantities only in association 
 with certain intensely altered minerals. It some- 
 times shows good crystal form which is almost 
 always long prismatic. Such development is 
 apparent even in the minutest microlites in clay 
 slates. It is sometimes found in rounded grains 
 or in large compact masses that are visible to 
 the naked eye and may be recognized by the 
 blackish-red color and the submetallic to 
 adamantine luster. Twins parallel to (101), FlG - 238. Rutile 
 Poo with knee-shaped cross sections, Figs. 238 doi)poo." 
 and 239, and those parallel to (301), 3Poo with 
 heart-shaped cross sections, Fig. 240, are especially common 
 in the smaller individuals. In the compact masses twinning 
 
 Rutile Twins. 
 Parallel (101) P oo . Parallel (301) 3 P oo . 
 
 lamination according to the first law is observed. Here also 
 prismatic cleavage, which is often lacking in crystals, appears 
 as sharp cracks. 
 
228 PETROGRAPHIC METHODS 
 
 Sagenite is a name given to a characteristic lattice structure 
 of rutile needles crossing each other at an angle of about 60. 
 It is found as an inclusion in numerous micas, the asterism of 
 which is often caused by it. It appears in this same form 
 produced by the chloritization of biotite rich in titanium. 
 Rutile accompanied by other titanium minerals is very frequently 
 produced by the decomposition of bisilicates. Concerning pleo- 
 chroic halos around inclusions of rutile in biotite, hornblende, 
 cordierite, etc., see page 197. 
 
 Rutile has the highest indices of refraction and double refraction 
 of all rock-forming minerals. Only the smallest microlites show 
 brilliant interference colors, if they do not appear opaque on 
 account of total reflection. In some varieties the extraordinary 
 ray is strongly absorbed giving rise to a change in color. Some- 
 times pleochroism is entirely lacking, especially in the light 
 yellow grains so widespread in amphibolite and eclogite. The 
 larger compact particles are dark and usually strongly pleo- 
 chroic. They show a submetallic luster in reflected light. 
 
 Primary rutile is rare in the eruptive rocks, but is widely dis- 
 seminated in all groups of contact rocks. In them two types are 
 frequently associated with each other. The one is in sharply 
 developed crystals with a grayish-violet color, and the other is 
 in rounded grains having a yellow or brown color. It often has 
 a border with a crumbly appearance, which is white in reflected 
 light and consists predominantly of titanite. This is called 
 leucoxene. Rutile is sometimes intergrown with homogeneous 
 titanite or ilmenite in various zonal arrangements. It is very 
 widespread in sedimentary rocks because of its resistance to 
 weathering. It occurs in the coarse mechanical sediments in 
 the form of compact rounded grains, and in the fine sediments as 
 needles that are doubtless authigenetic. These needles are very 
 minute, often twinned and scarcely transparent. The most 
 typical development of this formation appears to be present only 
 in the extreme outer portion of a contact zone. 
 
 It is riot attacked by the atmosphere nor by acid, except by 
 hot concentrated sulphuric acid. It can be easily isolated by a 
 mixture of hydrochloric and hydrofluoric acids. The test for 
 titanium is made by fusing the residue in a bead of potassium 
 bisulphate and dipping it in hydrogen peroxide when it becomes 
 brownish. 
 
 Very dark colored rutile might be confused with opaque ores. 
 
DESCRIPTIVE SECTION 229 
 
 It can be distinguished by its adamantine luster in reflected 
 light and its transparency, that is always to be noted upon care- 
 ful observation. The light colored varieties were formerly often 
 determined as zircon. It is distinguished from zircon by being 
 always colored and by its higher double refraction, which also 
 differentiates it from anatase. Distinction from the rarer min- 
 erals brookite, cassiterite, and goethite is very difficult, as these 
 minerals, like rutile, give white of the higher order in a 'normal 
 section and have an adamantine luster like it in reflected light. 
 The safest method of differentiation is a chemical test, but the 
 effect on convergent light is also distinctive in the case of brookite 
 and goethite. 
 
 Rutile may be confused with perovskite and similar minerals 
 on account of its high index of refraction, but perovskite, when 
 it is anomalous, shows at best a very low interference color as 
 does wurtzite, which is also included here. Confusion with 
 sphalerite may occur, but that mineral never shows double re- 
 fraction. Titanite finally does not have an adamantine luster 
 in reflected light. 
 
 Anatase (4) 
 
 Anatase as a rock constituent is frequently developed tabular, while 
 sharp pyramidal forms are less common. Some typical cross sections are 
 given, Fig. 237 /, page 225. It may also form granular aggregates. The 
 individuals are mostly small, poorly developed, clouded, and have a speckled 
 color, blue, yellow or colorless. The stronger absorption of the ordinary 
 ray is not often clearly discernable, and the same is true of the cleavage. It 
 generally results from the alteration of other minerals containing titanium 
 and is, therefore, found particularly in greatly altered rocks, in contact- 
 metamorphic formations, as well as in those which have suffered decomposi- 
 tion by the action of mineralizers. It is very common in such rocks as a 
 by-product in the alteration of bisilicates and it forms a part of the dense 
 aggregate known as leucoxene. It is confused with zircon, from which it 
 can be distinguished, however, by the speckled color and cloudy appearance, 
 much higher indices of refraction, and the optical character. It is also 
 similar to rutile, and titanite from which it is distinguished by its much 
 lower double refraction, imparting to it brilliant interference colors in the 
 section. 
 
 It may be remarked, parenthetically, that grains of corundum similar 
 to anatase may be present in the section. These have been derived from the 
 polishing or grinding material. The optical properties of these grains are 
 similar to those of anatase. They are uniaxial, blue or yellowish, and 
 pleochroic, but differ in the optical character of the principal zone. 
 
230 PETROGRAPHIC METHODS 
 
 Cassiterite (4) 
 
 Cassiterite is a rare constituent of lithionite granite and the rocks in the 
 vicinity of a contact with it. Crystals are not frequent in these rocks, but 
 they have pyramidal development with characteristic twinning parallel to 
 (101), P 00 or they may be simple prisms. Grains are the most common. It 
 is rarely colorless, but is yellow, brown, or more often deep red. In the 
 last case it is strongly pleochroic in thin section. A zonal or speckled dis- 
 tribution of the color is common. If its determination is doubtful, especially 
 if it is confused with rutile and anatase, the grains should be isolated with 
 hydrofluoric and sulphuric acids and fused in a borax bead colored blue by 
 copper oxide. A ruby-red color then indicates cassiterite. (See page 170.) 
 
 Wurtzite (4) 
 
 Wurtzite is found together with sphalerite and is intergrown with it in 
 clustered incrustations. It is similar to sphalerite in color. Its distri- 
 bution as a rock-forming mineral has not been definitely determined. It is 
 distinguished from the minerals, which appear similar to it, rutile, goethite, 
 and sphalerite, by its weak double refraction. Its solubility in acids is also 
 distinctive. 
 
 Zircon(4) 
 
 Zircon is one of the most widespread rock constituents, but 
 is always present only in small quantities. It is macroscopically 
 apparent only in pegmatites. The so-called zircon syenites, 
 belonging to the soda series of rocks, are especially rich in it. 
 The mineral is abundantly present in brown, red, or greenish 
 cloudy crystals with an adamantine luster and forms an import- 
 ant constituent of the rock. As a rock constituent, it is always 
 microscopic and the individuals are in most cases very small. It 
 is entirely colorless in thin section. It is almost never wanting 
 in the acid and intermediate members of the normal series of 
 eruptive rocks and is one of the first minerals to crystallize in 
 them. It is, therefore, in minute crystals that are as perfect 
 as models and often have an abundance of faces. It is less com- 
 mon in basic eruptive rocks and is only exceptionally observed 
 in peridotites and in the members of the soda rock series rich in 
 alkali. It is found in all types of contact rocks, but it occurs more 
 frequently in grains, which are well characterized by their high 
 index of refraction and brilliant interference colors. It is nearly 
 always present in sedimentary rocks, but mostly in clouded grains. 
 
 Zircon is not attacked by acids. When decomposed in rocks 
 it often shows characteristic zonal structure in which clear and 
 more or less clouded zones alternate, Fig. 241. In spite of this 
 
DESCRIPTIVE SECTION 
 
 231 
 
 there is much doubt concerning the miner alogical and petro- 
 graphical definition of the mineral. It is often very difficult to 
 distinguish it from xenotime, which has a slightly higher double 
 refraction and according to modern investigations is found to be 
 as widespread in acid rocks as zircon. Because of their similar 
 appearance and occurrence they can be differentiated positively 
 only by the Hepar reaction. Besides that, very different varie- 
 ties can be distinguished in macroscopic crystals of zircon. One 
 variety with a normal specific gravity and quite a high double 
 refraction seems to be related by gradual transi- 
 tions with one whose specific gravity sinks to 4, 
 and which is scarcely double refracting "when 
 entirely clear and homogeneous. It cannot be 
 determined just how much this transition is 
 based upon the taking up of water, as has been 
 shown for the cloudy variety known as malacon. 
 Little is known of the distribution of the varie- 
 ties with low double refraction because the 
 mineral occurs in small amounts and is usually 
 overlooked, or is classed as a member of the 
 epidote series. 
 
 Xenotime is frequently found by chemical 
 investigations among the high double refracting 
 individuals and is generally determined as 
 zircon. Sometimes the zonal clouded crystals 
 belong principally to xenotime. Under certain conditions it 
 can be confused with colorless cassiterite, but the chemical 
 reaction and higher double refraction of cassiterite, giving 
 rise to white of the higher order in a normal section, serve to 
 distinguish them. It is distinguished from titanite by a lower 
 double refraction. It can also be confused with monazite or 
 with colorless epidote, but convergent light shows the difference 
 at once. Confusion with anatase and rutile was mentioned 
 under these minerals. The occurrence of pleochroic halos 
 around zircon individuals is noteworthy, see page 197. Very 
 perfect cleavage is only clearly observed in the larger individuals 
 and is almost entirely lacking in the ordinary microlitic forms. 
 
 Xenotime (4) 
 
 The distribution of xenotime in rocks cannot be ascertained with our 
 present knowledge of it. It appears to be present first of all in acid eruptive 
 
 FIG. 241. Zircon 
 (Xenotime). Sec- 
 tion Parallel to the 
 Principal Axis. 
 
232 
 
 PETROGRAPHIC METHODS 
 
 rocks and in sedimentary rocks derived from them, occurring with or in- 
 stead of zircon. It is easily recognized when it occurs in short pyramidal 
 crystals, which are sometimes intergrown parallel with prismatic zircon 
 crystals. It may form prismatic crystals itself like Fig. 241 and these can 
 scarcely be distinguished from zircon. The higher interference colors, 
 white of the higher order if the crystals are not too thin, and also the pleo- 
 chroism (o> light rose-red, e pale yellowish), which closer observation reveals, 
 make the recognition of isolated crystals possible, but in thin section both 
 of these properties are not distinctly evident. The pyramidal crystals are 
 usually greatly clouded by decomposition and in the prismatic crystals, 
 which were simply called zircon, this turbidity is very common, particularly 
 in the zonal development. Clear prismatic crystals are rarer. The Hepar 
 test on carefully isolated material gives positive proof of xenotime. How- 
 ever, only fresh varieties give this reaction because the sulphuric acid disap- 
 pears when the crystals become cloudy. It is difficultly fusible before the 
 blowpipe, and if moistened with sulphuric acid it colors the flame bluish- 
 green. It is slowly decomposed in a salt of phosphorous bead. 
 
 Corundum (4) 
 
 Corundum shows varying habits. Short pyramidal crystals sometimes 
 with a barrel shape, Fig. 242, and with distinct zonal structure, or thin 
 tabular crystals parallel to the base are the common forms. Cross sections 
 with a poor outline and pale speckled color are often 
 difficult to determine, because they have no character- 
 istic appearances except the high index of refraction. 
 The pleochroism is characteristic and is observed only 
 when the mineral has a deep blue color. The various 
 other macroscopic colors are not seen in a thin section. 
 A few twin lamellae parallel to the rhomb ohedron are 
 seen now and then. As a granular aggregate it forms 
 the principal constituent of emery, which presumably 
 represents a dike-like development in contact-metamor- 
 phic limestones. It is otherwise almost entirely a con- 
 tact mineral and as such is most frequently developed 
 tabular and accompanied by sillimanite, cordierite, 
 spinel, etc.; or it occurs as the result of resorbed inclu- 
 sions of aluminous rocks in eruptive rocks. These are 
 the large blue crystals of sapphire occurring especially 
 in pegmatites. It is also found in contact rocks as 
 dark irregular masses of microscopic size produced by 
 segregation. Corundum can be recognized by the naked eye in granular 
 limestones and dolomites in which it appears brilliant red, ruby, but in 
 thin section it is colorless. It is often necessary to isolate corundum 
 from a rock to determine it positively. Its great resistance even 
 toward molten carbonates of the alkalies makes its isolation easy. The 
 presence of small grains of corundum in sections prepared with emery is 
 mentioned because they often give rise to confusion. Corundum is dis- 
 tinguished from vesuvianite and apatite by its more brilliant interference 
 
 FIG. 242. Barrel- 
 shaped Crystal of 
 Corundum. 
 
DESCRIPTIVE SECTION 233 
 
 colors. Light colored varieties of tourmaline can be very similar to it, but 
 the latter shows a greater difference in absorption. Among the biaxial 
 minerals it may be confused with those which are often blue like corundum, 
 such as cyanite, sapphirine, lawsonite, zoisite, sillimanite, etc. Investiga- 
 tions in convergent light show the differences. 
 
 Vesuvianite (lodocrase) (4) 
 
 Vesuvianite is a typical contact mineral that occurs in well developed 
 short prismatic crystals only in contact-metamorphosed limestones. In 
 other contact rocks it forms irregular, elongated grains. It is macroscopic- 
 ally light green or yellow to brown in color, but in thin section it is colorless 
 and transparent. Only those varieties containing manganese are colored 
 distinctly reddish. It is decomposed by hydrochloric acid with difficulty 
 and fuses before the blowpipe with intumescence to a greenish glass. After 
 fusion it gelatinizes with hydrochloric acid. The vesuvianites form a series 
 of isomorphous mixtures of which one end member, which usually predomi- 
 nates, is optically negative, and the other has a weak, positive, double re- 
 fraction. Anomalous interference colors are therefore frequently observed 
 in the intermediate members and the colors may be arranged in a zonal 
 manner. The division of basal sections into regular biaxial fields is very 
 common. Dense aggregates like nephrite, which occur in serpentine and 
 produce a formation similar to saussurite, are noteworthy. In such an 
 occurrence it is extremely difficult to recognize Vesuvianite and distinguish 
 it from grossularite, gehlenite or zoisite, and clinozoisite, since all these 
 minerals have high indices of refraction and show anomalous interference 
 colors. It is difficult to distinguish from apatite and this can often only be 
 done by a chemical test. It appears similar to andalusite even when it has 
 normal interference colors, but the biaxial behavior of the latter serves to 
 differentiate the two. 
 
 Gehlenite Group (4) 
 
 Gehlenite and melilite are apparently an isomorphous mixture of a cal- 
 cium-aluminium silicate, which sometimes occurs pure in gehlenite, with 
 a calcium silicate free from aluminium, that is not known in nature. Gehle- 
 nite forms rounded, short, prismatic crystals in contact-metamorphic lime- 
 stones, and the length is usually quite the same as the thickness. Melilite 
 occurs more frequently in poorly bounded, tabular crystals parallel to the 
 base, and appears to be confined to basalts and tephrites. The former is 
 macroscopically grayish, and in thin section always colorless. The latter is 
 now and then colored yellow by its content of iron and the extraordinary 
 ray is absorbed more than the other humboldite. The latter also occurs in 
 large irregular particles which are entirely perforated by leucite crystals. 
 The negative double refraction diminishes with a decrease in the content 
 of aluminium and those poorest in aluminium are optically positive. Be- 
 tween these extremes there is a series of mixtures, which show anomalous 
 interference colors in the most typical manner. 
 
 Both minerals are easily altered and the melilite particularly is often 
 
234 
 
 PETROGRAPHIC METHODS 
 
 coated with a dull hazy film. The mineral itself is also clouded considerably. 
 It may pass over into a fibrous aggregate with a strong double refraction, 
 the fibers standing perpendicular to the basal pinacoid. Melilite often 
 shows the characteristic appearance of having its sections crossed by 
 numerous thin glass pegs arranged parallel to the principal axis. These 
 penetrate into the crystals from both sides and often spread out like funnels 
 in the interior. Such an appearance is called peg structure, Fig. 243. Cleav- 
 age is often entirely lacking in such 
 sections, but in other cases it is 
 seen in a few sharp cracks running 
 perpendicular to the principal axis. 
 Perovskite is a characteristic asso- 
 ciate of melilite. Gehlenite is often 
 very similar to vesuvianite even in 
 varieties with anomalous interfer- 
 ence colors fuggerite and to 
 zoisite and clinozoisite, and is often 
 only to be distinguished from them 
 positively by treatment with hydro- 
 chloric acid. Melilite was formerly 
 determined as feldspar or as neph- 
 eline, but is distinguished from 
 them by the higher index of refrac- 
 tion, the dull appearance in re- 
 flected light, and the chemical 
 properties, particularly the ease 
 with which it is dissolved in acids, and the high content of calcium. 
 
 In certain contact rocks in the Fassatal, gehlenite is replaced by fuggerite, 
 a mineral with a similar composition. It is likewise tetragonal and occurs 
 in tabular crystals. It is isotropic for sodium light and therefore has deep 
 blue anomalous interference colors. It has a more perfect basal cleavage. 
 Sp. gr. = 3.18. It is soluble in acids with the separation of powdered silica. 
 
 Tourmaline (4) 
 
 A group of complex silicates containing boric acid is included 
 under the name of tourmaline. Lithia tourmaline, macro- 
 scopically very pale, in thin section colorless, scarcely ever 
 occurs as a real rock constituent and is distinguished from the 
 strongly colored green, blue or brown magnesia tourmalines, 
 which are mostly light colored in thin section. The iron tourma- 
 line containing titanium, schorl, is, like most silicates containing 
 titanium, macroscopically black with a pitchy luster. It is 
 highly colored in thin section and is characterized by a very 
 strong absorption of the ordinary ray. It is the most important 
 member of the series as a rock constituent. 
 
 Schorl is often in zonal crystals built up of layers with different 
 
 FIG. 243. Peg Structure in Melilite. 
 Melilitebasalt, Oahu, Sandwich Islands. 
 (After E. Cohen.) 
 
DESCRIPTIVE SECTION 
 
 235 
 
 intensities of color. The hemimorphic development of the 
 crystals, the ends of which are shown diagrammatically in Figs. 
 244 and 245, is rarely apparent, but the three- to nine-sided 
 cross sections, Fig. 246, are very characteristic. The tourma- 
 lines that are light colored in thin section are especially wide- 
 spread in granular limestones and are often found in large, 
 well developed crystals with yellowish, greenish, bluish or 
 brownish color, but these are often difficult to recognize because 
 of the comparatively small difference in absorption. In other 
 rocks the blue varieties form ragged particles without any indi- 
 
 FIG. 244. 
 
 FIG. 245. 
 
 Opposite Ends of a Tourmaline Crystal. 
 
 FIG. 246. Tourmaline 
 Cross Section. 
 
 cation of crystal form. Radial aggregates tourmaline suns 
 are very widespread. It is often noted also that a crystal of dark 
 tourmaline is well bounded on one side and on the other side it 
 is grown into a fibrous aggregate of light colored tourmaline. 
 
 The optical properties vary within wide limits. The indices of 
 refraction and the double refraction appear to be highest in the 
 members rich in titanium. The indices and the double refraction 
 increase in the pleochroic halos, see page 197. 
 
 The dark colored varieties are sufficiently distinguished. in all 
 cases from the numerous and widespread members of the mica 
 and amphibole groups and from apatite, with which they have 
 been confused, by the strong absorption perpendicular to the 
 principal zone and by the trigonal cross sections. The minerals 
 with light color and low absorption, which are difficultly recog- 
 nizable, are very similar to andalusite, staurolite, lawsonite, 
 forsterite, and corundum, but by more careful observation 
 the stronger absorption of the ordinary ray can be observed 
 even though it is only seen in traces. Epidote often shows a 
 similar orientation of the absorption. In this case the usual 
 speckled appearance of the interference colors of epidote and its 
 cleavage serve to differentiate them. Investigation in conver- 
 gent polarized light is helpful here as in most of the other cases. 
 
236 PETROGRAPHIC METHODS 
 
 In any case the resistance of tourmaline to reagents allows it to 
 be easily isolated and it can be determined by its reaction for 
 boron, see page 169. 
 
 Only schorl is known as a primary constituent of eruptive rocks 
 and it sometimes occurs as an accessory constituent of granitic 
 rocks, especially the aplites. Otherwise tourmaline is the most 
 distinctive mineral of pneumatolytic processes, and its formation 
 can nearly everywhere be shown to be connected with such 
 processes. It is therefore always found where they have been 
 most intensely active. Besides in the pegmatites, it is found 
 especially in the vicinity of tin ore veins and certain copper ore 
 dikes, where the whole rock has been tourmalinized. It occurs 
 also in the kaolin deposits. 
 
 The distribution of tourmaline in contact rocks of all sorts 
 must be especially emphasized. It is never lacking in all the 
 various kinds of formations which owe their origin to contact- 
 metamorphic alteration by granitic rocks, even though the 
 granite itself contains no trace of tourmaline. Tourmaline 
 sometimes occurs in very large individuals, e.g., in the schist 
 zone of the Central Alps where the crystals are macroscopically 
 apparent, but even in these rocks it is more common in separate, 
 minute microlites, which could be easily overlooked unless one 
 accidentally observes a typical cross section or finds a pris- 
 matic crystal with the characteristic absorption perpendicular to 
 the principal zone. Tourmaline is everywhere present in this 
 form in gneisses, mica schists, amphibolites, green schists, 
 hornfels, and Knotenschiefer and it is found in distinctly 
 developed individuals in the remotest parts of the contact zone. 
 Here no alteration can be recognized in the external appearance 
 of the rock itself, and under the microscope the action of contact 
 metamorphism can only be discerned by the formation of tour- 
 maline and the development of small, clay slate needles rutile. 
 
 Apatite (5) 
 
 Apatite is everywhere present as a rock constituent, but in 
 small quantities and nearly always in minute individuals. It 
 occurs in acid eruptive rocks in well developed crystals with a 
 long, prismatic to needle-like habit, Fig. 247. In basic rocks 
 and, especially in those rich in sodium, it forms large, rounded 
 crystals that are short and thick, and in contact rocks and sedi- 
 
DESCRIPTIVE SECTION 
 
 237 
 
 FIG. 247. 
 Apatite. 
 
 ments it forms rounded grains. Apatite as a rock constituent 
 can only be seen macroscopically in exceptional cases. It appears 
 as crystals with a brilliant luster in certain camptonites and may 
 likewise be observed as light blue grains in marble. Radial 
 aggregates phosphorite frequently colored dark by 
 organic substances, are found in numerous sediments 
 as large concretions. 
 
 Apatite is always one of the first products to crys- 
 tallize in eruptive rocks and is, therefore, largely in- 
 cluded in the other constituents, especially the dark 
 ones. It is strikingly contrasted with the other 
 minerals by its colorless lath-shaped or six-sided cross 
 sections, Fig. 216, page 197. Gas, liquid, and glass 
 are observed as inclusions in apatite. It is not often 
 colored and the colored varieties are confined to 
 eruptive rocks. It may be grayish-blue, brown, or 
 brilliant orange, with a strong absorption parallel to the principal 
 zone. Six-sided sections do not give a distinct interference figure 
 because of the low double refraction. Weathering and disin- 
 tegration of the rock usually leave the apatite unaffected. It is 
 
 therefore found very wide- 
 spread in soils, which owe their 
 content of phosphate to the 
 great dissemination of apatite 
 microlites. 
 
 In many instances apatite 
 can only be distinguished posi- 
 tively from vesuvianite, zois- 
 ite, or from other colorless 
 double refracting grains be- 
 longing to orthite (allanite) by 
 chemical tests for phosphoric 
 acid. Low double refraction 
 and the negative character of 
 the principal .zone distinguish it 
 from tremolite, sillimanite, etc. 
 
 FIG. 248. Slide of a Bone with Perfectly 
 Retained Structure from the Permian Wichita 
 Beds of Texas. 
 
 Bone substance, consisting predominantly of calcium phosphate, is to be 
 appended to apatite. It likewise has a low double refraction and occurs in 
 fine fibrous aggregates that are imperfectly radial. Within them the or- 
 ganic structure is often perfectly retained and shows itself in the nutrition 
 canals, etc., Fig. 248. 
 
238 
 
 PETROGRAPHIC METHODS 
 
 Rhombohedral Carbonates (5) 
 
 The rhombohedral modifications are the only carbonates that 
 occur as actual rock constituents. The others are found prin- 
 cipally as secondary depositions in crevices and as organic remains. 
 The diagenetic transformation of these into calcite is explained 
 in the Allgemeine Gesteinskunde by E. Weinschenk, page 118. 
 Calcite is one of the most widespread rock-forming minerals. It 
 forms the principal constituent of extensive formations and con- 
 stitutes whole mountain chains. Dolomite is likewise quite 
 widespread, while the other carbonates treated here, magnesite, 
 siderite, and smithsonite, are more of local importance. 
 
 It is noteworthy that calcite seldom forms crystals in rocks. 
 They do occur in minute, isolated rhombohedrons in the central 
 granite and are here undoubtedly grown into the quartz as a 
 primary constituent. Other than this it is always a secondary 
 constituent of eruptive rocks produced by the alteration of sili- 
 cates of lime. Pseudomorphs of calcite after plagioclase or 
 augite, and even after olivine, are found in greatly altered occur- 
 rences. It is especially frequent under such conditions as an 
 
 impregnation in the rocks, 
 which it may penetrate in 
 large veins in which the calcite 
 is granular, or fibrous or the 
 ground mass of a porphyric 
 rock may be entirely impreg- 
 nated with fine aggregates of 
 calcite, which frequently pos- 
 sess a radial, fibrous structure. 
 Dense sedimentary lime- 
 stones consist predominantly 
 of irregular, granular aggre- 
 gates of calcite. The indi- 
 viduals are usually clouded 
 with inclusions and vary 
 greatly in size. They are often transformed into larger grains 
 with greater clearness by recrystallization. The organic struc- 
 ture is often distinctly retained in such a formation, Fig. 249. 
 Only rarely do the limestones fail to show crystalline aggregates 
 under the microscope, as for example in the Solenhofener schists, 
 which are considered as a brackish water formation. Radial, 
 
 FIG. 249. Crinoid Limestone with Lattice 
 Structure. Vilstal near Pfronten, Algan. 
 
DESCRIPTIVE SECTION 239 
 
 fibrous oolites, sometimes with onion structure, are found in 
 limestones. They often lose their structure by secondary 
 processes and then they consist of small spheres of granular 
 calcite, Figs. 190 and 191, pages 186-187.' 
 
 Calcite shows its best development in the various groups of 
 contact rocks, as in lime-mica schist and in calcium-silicate fels. 
 In some of these rocks it forms the principal constituent as in 
 marble, while in others it occurs in subordinate particles as in the 
 two rocks mentioned above. It never shows crystal form, but is 
 always in granular aggregates and all the other rock constit- 
 uents are well crystallized in it, while in those occurrences rich 
 in silicates the calcite takes on the character of interstitial ma- 
 terial. Pure marble lying next to the contact is often quite 
 coarse grained, the individuals measuring as much as an inch in 
 one dimension and these tend to be colored sky blue. All gra- 
 dations are found from this down to formations that are macro- 
 scopically entirely non-crystalline. Under the microscope a very 
 regular texture of crystalline grains of calcite is observed. It 
 sometimes shows mosaic structure and sometimes the grains 
 are intimately dovetailed into each other, Figs. 192 and 193, page 
 187. The individual grains are sometimes clouded with fine 
 graphite dust and then are somewhat pleochroic with the darker 
 color in the vibration direction of the 
 ordinary ray. The perfect cleavage 
 parallel to the unit rhombohedron, Fig. 
 250, which is difficult to observe in the 
 denser aggregates, shows plainly in this 
 type in a series of sharp cracks. Very 
 frequently, but not always, numerous 
 twinning lamellae parallel to R are FlG . 250. Rhombohedron. 
 seen, and these are often parallel to but 
 
 one face of that form. They are to be explained by the gliding 
 of the calcite under the influence of pressure, and have proba- 
 bly resulted during the mechanical operations of the grinding. 
 The small number of lamellae in Figs. 192 and 193 is remarka- 
 ble, and is very distinctive because the marble from Carrara is 
 considered as a type of dynamo-metamorphic limestone. 
 
 Undoubtedly the twinning lamellae are increased everywhere 
 where the marble has been subjected to oro genie stresses after re- 
 crystallization had taken place, and such occurrences consist .en- 
 tirely of fibrous grains in which the manifold bending indicates 
 
240 PETROGRAPHIC METHODS 
 
 the high degree of plasticity of calcite, Fig. 205, page 192. This 
 mineral also breaks under too high a pressure and dense shattered 
 aggregates with great solidity are developed from granular marble. 
 This is well illustrated in the ivory marble with its excellent 
 cataclastic appearance. 
 
 The great difference in the indices of refraction for the two 
 principal vibration directions can be distinctly shown under 
 the microscope with one nicol. It is also the cause of the brilliant 
 interference colors, which the twinning lamella crossing the sec- 
 tion obliquely often show; compare Part I, page 87. In a 
 normal section calcite gives a pale white of the higher order 
 between crossed nicols. Cross sections in which the cleavage 
 cracks form an equilateral triangle show an interference figure 
 distinctly even with a low-power objective. With a higher 
 objective a black cross is obtained surrounded by numerous 
 colored rings but these are often disturbed by interbedded 
 twinning lamellae. 
 
 Dolomite is distinguished from calcite by its greater tendency 
 to develop its own crystal form. If it is intergrown in calcite 
 it usually shows rhombohedral cross sections. Aggregates con- 
 sisting principally of dolomite show the mosaic structure much 
 more distinctly than calcite, and this may pass over into a fine 
 drusy granular structure like sugar. The dovetailed structure 
 is also found in pure dolomites but it is rare. It forms worm- 
 like intergrowths in calcite in a few occurrences of eozoon cana- 
 densis. The higher indices of refraction and double refraction 
 compared with calcite cannot generally be determined in an 
 ordinary section. The lack of twinning lamellae parallel to 1/2R 
 is typical, for that is not a gliding plane in dolomite. In place 
 of that, however, twinning parallel to 2R occurs in numerous 
 lamellae so that is not a good means of differentiation although 
 generally there are fewer twinning lamellae in dolomite than in 
 calcite. Furthermore, they are seldom so greatly deformed. 
 The greater absorption of the ordinary ray is more easily noted 
 than in calcite. 
 
 Dolomite is also found .as a secondary formation in altered 
 eruptive rocks and may be in pseudbmorphs or as fine impreg- 
 nations in the ground mass. If the content of iron which is 
 always present has not given rise to the formation of rust, 
 and also in the rare cases where it occurs in radial fibrous form 
 filling out crevices, dolomite can only be determined by a chem- 
 
DESCRIPTIVE SECTION 241 
 
 ical test. Dolomite rocks of the sedimentary formations gener- 
 ally show a more distinct crystalline texture than limestones, 
 and are often penetrated through and through by veins of calcite. 
 The numerous cavaties, which appear macroscopically and are 
 covered with small dolomite crystals, are especially character- 
 istic. Under the influence of contact metamorphism the dolo- 
 mites form granular, and often pure, white dolomite marble, 
 which is usually finer grained than the equivalent limestones, 
 and like the limestones show transitions into silicate fels. Gran- 
 ular dolomite with excellent mosaic structure weathers very 
 frequently to a sandy mass consisting of small rhombohedrons, 
 dolomite ash. 
 
 Magnesite is much rarer and only of local importance. If it results as a 
 by-product of serpentinization it is found in well developed unit rhombo- 
 hedrons containing considerable iron. They are intergrown in serpentine, 
 chlorite fels, pot stone, etc. It forms granular pseudomorphs after olivine 
 in certain melaphyres and in sagvandite. Larger homogeneous masses, 
 which occur within the metamorphic limestones in the border zones of the 
 Central Alps have a very coarse-grained texture and consist of flat rhombo- 
 hedrons, 1/2R, with dimensions of about an inch. This form occurs very 
 distinctly in pinolites streaked with layers of clay slate. These varieties 
 also contain iron in considerable amounts and they, therefore, assume a 
 rusty appearance upon weathering. Snow-white aggregates of magnesite, 
 free from iron, are just the opposite of those above. They are dense like 
 porcelain, have a conchoidal fracture, and form veins in serpentine and, if 
 it can be detected microscopically at all, they show a spherulitic texture. 
 
 Siderite is sometimes yellowish and weakly pleochroic in thin sections. It 
 occurs locally in extensive deposits, and in geological association with 
 magnesite it forms coarse to medium grained masses within the granular 
 limestones of the Central Alps. It is also found as a constituent of carbona- 
 ceous iron rock occurring in bedded deposits. It is fine granular and filled 
 with carbonaceous particles. It occurs also in the form of fibrous and spher- 
 ulitic concretions spherosiderite. This name is also used for radial fibrous 
 incrustations of iron carbonate in crevices of trap rock. 
 
 For the other modifications of calcium carbonate, see aragonite under the 
 biaxial minerals. Ktypeite has been found in rocks, especially as a constit- 
 uent of a few pisolites. It has a specific gravity of about 2. 65 and n about 
 1,55. f a = 0.020, optically uniaxial, positive. Conchite, which is quite 
 similar to aragonite in many respects but is undoubtedly not the same, is of 
 interest because it forms the principal constituent of shells of living mollusks 
 and many other lime organisms. Its geological significance depends upon 
 its slight stability, especially upon the ease with which it is transformed on 
 the one hand into calcite and on the other into dolomite under the influence 
 of solutions containing magnesium. Sp. gr. =2.85, a = 1.523, ^ = 1.662, 
 f a = 0.139. Optically uniaxial, negative, but frequently shows a distinct 
 opening of the cross in convergent polarized light. 
 16 
 
242 PETROGRAPHIC METHODS 
 
 Concerning the distinction of the carbonates among themselves 
 and from other similar minerals, it may be mentioned that in 
 the normal fresh occurrences a differentiation of the rhombo- 
 hedral carbonates from each other is not possible in a thin section 
 by simple optical investigations. The special reactions thor- 
 oughly discussed on page 175 must be used here. The same is 
 true for conchite and the orthorhombic series of carbonates of 
 which only aragonite occurs as a constituent of rocks and is 
 found but rarely. It almost always forms fine fibrous aggre- 
 gates, which cause great difficulty in the determination of the 
 optical properties even in convergent polarized light. The car- 
 bonates often appear very similar to titanite, but observation of 
 the index of refraction always gives a positive distinction. The 
 carbonates can always be positively determined in thin sections 
 by the effervescence with cold or warm acid. 
 
 Eudialyte (Eucolite) (5) 
 
 Eudialyte forms rounded crystals or irregular grains and can always be 
 recognized macroscopically by its reddish color. It is usually colorless in 
 thin section. The optical character is variable in one and the same cross 
 section. Parts that are weakly doubly refracting and optically positive, 
 alternate with those that are isotropic or have a negative double refraction, 
 eucolite. These sections do not show anomalous interference colors. The 
 members that are optically negative are often distinctly colored in thin 
 section and show weak absorption, &>> e. The mineral has only been found 
 in nepheline syenite. It can be distinguished from apatite by the fact that 
 the eudialyte grains are always considerably larger. 
 
 Scapolite Group (5) 
 
 The scapolites form a-n isomorphous series with quite variable 
 optical properties. The calcium-aluminium silicate, meionite, 
 forms one end member and a sodium-aluminium silicate, 
 marialite, the other. The former has higher indices and 
 double refraction. Dipyre, couseranite, etc., are intermediate 
 members but they are all referred to in general as scapolite 
 because the distribution of the various members has been very 
 little studied. 
 
 The scapolites show crystal form only in contact-metamor- 
 phosed limestones. In them, prismatic crystals are macro- 
 scopically visible, but in spite of that, they are difficult to 
 recognize; see cross sections b and d, Fig. 237, page 225. Other- 
 wise only granular or columnar aggregates are observed in which 
 
DESCRIPTIVE SECTION 243 
 
 the cleavage sometimes appears distinctly. Alteration to 
 micaceous minerals with characteristic mesh structure is frequent 
 as is also alteration to homogeneous individuals of colorless 
 chlorite leuchtenbergite. Special reactions for scapolite, page 
 174, can be made only on fresh material, because the content 
 of chlorine is lost in the process of weathering. The usual 
 occurrence of these minerals is in contact rocks and they are 
 often so filled with carbonaceous inclusions that they are non- 
 transparent. By metamorphism black crystals may be devel- 
 oped with an appearance similar to that of chiastolite, couser- 
 anite, or white knots with excellent sieve structure may be 
 formed. If alteration phenomena also appear, it is very difficult 
 to recognize the mineral. It occurs, furthermore, as granular 
 aggregates in metamorphosed diabases and gabbros, especially 
 in the vicinity of the Norwegian apatite dikes. It is likewise 
 found in various rocks called scapolite gneiss but they do not 
 have the composition nor the geological significance of a gneiss. 
 They are mostly normal contact rocks. Scapolite often plays a 
 r61e analogous to that of tourmaline in the contact formation 
 of granites and particularly in that of basic eruptive rocks. 
 Its lack of characteristic optical properties obscures an accu- 
 rate conception of its distribution which is undoubtedly not 
 unimportant. 
 
 The scapolites are most frequently confused with quartz and 
 feldspar. The positive optical character of quartz and the 
 biaxial property of feldspar distinguish them. Cordierite 
 appears quite similar to the members with low double refraction 
 but it has no cleavage in addition to being biaxial. The special 
 reactions referred to above are the safest since the optical 
 properties are so variable. 
 
 Alunite (5) 
 
 Alunite forms cube-like rhombohedrons and occasionally basal tabular 
 crystals and flaky aggregates. The cleavage appears distinctly both macro- 
 scopically and microscopically. It is only found as an alteration product of 
 acid extrusive rocks which have been subjected to the action of solfataras. 
 It is distinguished from diaspore, which accompanies it, by its lower indices 
 of refraction, and from quartz by the cleavage and high double refraction, 
 and this latter property also distinguishes it from the feldspars. 
 
 Beryl (5) 
 
 Beryl is a rare constituent of certain granites and their contact formations, 
 and occurs in poorly bounded prismatic crystals. It is more frequent in 
 
244 
 
 PETROGRAPHIC METHODS 
 
 large, clear or clouded crystals in pegmatites. It is usually visible macro- 
 scopically and is generally sky blue in the eruptive rocks, and sap green 
 emerald in contact formations. It is sometimes light blue under the 
 microscope. However, it is usually quite difficult to determine the poorly 
 denned individuals with any degree of accuracy, and principally, because of 
 its similarity to quartz, it is generally overlooked, if it only occurs in micro- 
 scopic individuals. It is distinguished from quartz by the negative charac- 
 ter of the double refraction. 
 
 Brucite (5) 
 
 Brucite is rare but is found in micaceous flaky aggregates in altered rocks 
 rich in magnesium. It is characterized by its tombac brown, anomalous 
 interference colors, which indicate a very low double refraction that approx- 
 imates that of chlorite. It can often only be distinguished from chlorite 
 by treatment with silver nitrate when the brucite becomes colored deep 
 brown. Its radial, scaly pseudomorphs after periclase in contact lime- 
 stones predazzite and its coarse flaky occurrence in crevices in serpentine 
 are especially noteworthy. It is distinguished from the micas by the nega- 
 tive character of the principal zone and its easy solubility. 
 
 Quartz, Chalcedony and Tridymite (5) 
 
 Quartz is one of the most important rock-forming mineralsr 
 It plays an important r61e in all groups of rocks. If it has 
 crystal form it is the hexagonal bipyramid alone, Fig. 251, os 
 in combination with a very subordinate prism. The crystal. 
 
 FIG. 251. Quartz. 
 
 FIG. 252. Quartz Section Parallel 
 to the Principal Axis. 
 
 are originally sharply developed but the edges and corners are 
 often considerably rounded and the faces greatly corroded 
 forming irregular, tubular indentations now filled with the rock 
 mass, Fig. 252 and Fig. 199, page 189. Sections parallel to the 
 principal axis have rounded, rhombic outlines with an angle 
 of about 100. Perpendicular to the principal axis the sections 
 are six-sided, Fig. 237e, page 225. Quartz is found principally 
 
DESCRIPTIVE SECTION 
 
 245 
 
 with this development in quartz porphyry and rhyolite, and it 
 contains a few glass inclusions filling negative crystals. The 
 same development is also seen, but less distinctly and more 
 rounded, in aplites and dike granites, in certain binary granites 
 and granulites, Fig. 253, and likewise in lime-mica schists. 
 
 Rounded crystals of quartz are found in basic eruptive rocks 
 and they may be present as numerous minute individuals pene- 
 trating the basic constituents, Fig. 209, page 194, especially in 
 certain gabbros, or they may be in large shattered individuals 
 surrounded by a border of hornblende or augite needles arranged 
 radially to the quartz quartz-augen, Fig. 208, page 193. These 
 are seen in lamprophyres and diabases. In the latter case the 
 origin of the quartz can nearly always be distinctly traced to 
 
 FIG. 253. Granulitic (Aplitic) Structure. 
 Granulite, Curunegala, Ceylon. 
 
 FIG. 254. Dentated Quartz. Perosa, 
 Cottic Alps. 
 
 the shattered neighboring rock. Most frequently all indications 
 of crystal form are lacking in the quartz. In most granites, 
 syenites, diorites, etc., it fills the interstices between the other 
 constituents, Fig. 180, page 182. Minute liquid inclusions, 
 arranged in rows, are especially common in this type. They 
 penetrate through the sections as cloudy bands and pass from 
 one grain over into the neighboring one unchanged. They are 
 the cause of the clouded, milk-white color of certain occurrences 
 of quartz. Others contain abundant microscopic needles, 
 apparently rutile, and still others are colored red by small 
 flakes of iron oxide. 
 
 In contact rocks quartz generally forms an uniform mosaic in 
 
246 PETROGRAPHIC METHODS 
 
 which the grains may simply lie in contact with each other, Fig. 
 184, page 184, or they may be intensely dovetailed into one an- 
 other, Fig. 254. Again they may form suture joints, but not 
 grow together as in itacolumite, Fig. 255. Helicoidal structure 
 is found in the quartz aggregates in contact rocks. Lines of 
 inclusions of carbonaceous substance or sillimanite, etc., corre- 
 sponding to the original schistosity of the rock, wind about 
 through the aggregates. 
 
 Quartz does not suffer any alteration by weathering and, 
 therefore, occurs always entirely fresh in secondary deposits. 
 In clastic rocks it is sometimes angular, Fig. 256, and sometimes 
 rounded. Frequently it is observed, that the fragments have 
 
 FIG. 255. Itacolumite. Ouro Preto, FIG. 256. Clastic Structure. Sandstone, 
 
 Brazil. Schramberg, Black Forest. 
 
 grown into crystals by the addition of secondary silica crystal 
 sandstone, Fig. 257. In rocks containing but little quartz, the 
 mineral is often found in well developed short prismatic crystals. 
 These may be homogeneous or they may be brownish, due to 
 organic substance stink quartz. They may be red to yellow, 
 colored by iron hydroxide, or they may be colorless, clouded, or 
 limpid. These occurrences are undoubtedly of secondary origin, 
 formed in place, and, for the greater part at least, have been 
 derived from the siliceous portion of organic skeletons which are 
 known to be very soluble. Quartz also occurs as a secondary 
 formation in granular or radial, columnar aggregates. It may 
 be in pseudomorphs after other minerals in certain eruptive 
 rocks and their tuffs, or it may be a siliceous cement as in crystal 
 sandstone, where it occurs as an enlargement of the original 
 
DESCRIPTIVE SECTION 
 
 247 
 
 clastic grains, or again it may occur in dense aggregates in sili- 
 cified tuffs cementing the rock constituents. In the last case, 
 however, quartz is frequently replaced by other modifications 
 of silica. In fritted sandstone it often has properties resembling 
 perlite. Inclusions in it are fused to glass. Quartz is extremely 
 brittle and is, therefore, unlike any other mineral, an indicator 
 of the degree of dynamic action that has modified the rock. 
 Under the influence of pressure, the appearance of cataclastic 
 or mortar structure, Fig. 203, page 191, is most distinctly devel- 
 oped. It frequently becomes biuxial under these conditions. 
 
 Quartz is characterized macroscopically by its concoidal 
 fracture with somewhat of a greasy luster and its great hardness. 
 
 FIG. 257. Clastic Structure Crystal Sand- 
 stone. Erbach, Odenwald. 
 
 FIG. 258. Micropegmatite. 
 
 In rocks quartz is generally light smoky gray, but occasionally 
 due to inclusions of (1) hematite flakes it is red, or of (2) mag- 
 netite dust grayish-blue, or of (3) chlorite or hornblende green. 
 A light blue color, which sometimes occurs, cannot be explained 
 by microscopic investigation. Under the microscope quartz 
 shows an irregular outline and is always colorless. Twinning, 
 which is also common in rock-forming quartz, cannot be recog- 
 nized in thin section. Graphic intergrowths of microscopic 
 dimensions of quartz and orthoclase are very widespread, 
 particularly in the ground mass of porphyric rocks, Fig. 258. 
 They are called micropegmatites. Sharp, angular sections of 
 clear quartz are sharply delineated against the clouded feldspar, 
 presenting a hieroglyphic appearance. Frequently the inter- 
 growth becomes more and more indistinct and passes over into 
 
248 PETROGRAPHIC METHODS 
 
 an irregular fibrous formation, granophyre, inclining to genuine 
 spherulites, Fig. 189, page 186. It may become still more irreg- 
 ular and so fine that it has no effect upon polarized light what- 
 ever, microfelsite. An intergrowth similar to the micropegmatite 
 occurs in which the parallel individuals of quartz penetrating the 
 feldspar have worm-like sections quartz vermicide myrmecitic 
 intergrowth, Fig. 259. The feldspar is plagioclase. Both kinds 
 of intergrowths are typical structures for eruptive rocks. 
 
 Quartz is easily confused with several other minerals, which 
 like it, are colorless and have a low double -refraction. It is 
 
 most similar to nepheline, 
 which, however, has a lower 
 double refraction and is soluble 
 in hydrochloric acid. It re- 
 sembles optically negative beryl 
 and the various zeolites, which 
 can be distinguished from it 
 by cleavage, solubility, and in 
 many instances the biaxial be- 
 havior. Scapolite is distin- 
 guished by its negative double 
 refraction and cleavage, and 
 cordierite by its biaxial proper- 
 
 FIG. 259. Quartz Vermicule. . r r . 
 
 ties and frequent pleochroic 
 
 halos. The feldspars can be distinguished in general by cleavage, 
 indices of refraction, and biaxial behavior. This last property 
 becomes the most important earmark for plagioclase in contact 
 rocks where cleavage, twinning, etc., are lacking. In order to 
 establish its distribution compared with these minerals, the 
 special reactions cited on page 174 can be employed to best 
 advantage. 
 
 A large number of fibrous, siliceous minerals, occurring in radial aggre- 
 gates or having an onion-like structure or both of these together and 
 penetrated by opal, have been shown to be rock-forming minerals. They 
 are found as concretionary masses in crevices and air holes in eruptive 
 rocks, as a cement in their tuffs, and as a cementing material in sediments. 
 Chalcedony is the best characterized member of this group. It has a some- 
 what stronger double refraction and negative principal zone, and the acute 
 bisectrix of a very small optic angle lies perpendicular to this zone. Quartz- 
 ine and lussatite are similar to it. They are biaxial with a small optic 
 angle, but are positive and have positive principal zones. A variety known 
 as lutezite is distinguished from the others by an oblique extinction of the 
 
DESCRIPTIVE SECTION 249 
 
 fibers, while pseudochalcedony is distinguished by the negative character of 
 the principal zone and of the mineral. Distinction from zeolites similar 
 to it is only possible chemically. 
 
 Tridymite is more characteristic. Very small tabular crystals with an 
 hexagonal outline are clustered together and the little plates overlap each 
 other like shingles on a roof. It shows division into segments in polarized 
 light, and in convergent polarized light it gives a badly distorted, biaxial 
 interference figure. The very low index of refraction is its most distinctive 
 feature, causing it to appear in thin section with decided relief. It occurs 
 particularly in acid eruptive rocks, rhyolite and trachyte, but not as a 
 primary constituent. It is of secondary origin produced by fumaroles and 
 is, therefore, not uniformly distributed, but is segregated in individual 
 clusters. It is also found in a few fritted sandstones. 
 
 Nepheline (6) 
 
 Nepheline is never found in association with quartz and it 
 occurs entirely in the basic soda rocks of the series frorn nepheline 
 syenite to theralite, and from phonolite to basalt. Two types 
 can be distinguished. The one is fresh, has a vitreous luster and 
 is limpid in thin section. It is called nepheline. The other is 
 macroscopically reddish or greenish with a greasy luster and it 
 is called elaeolite. The latter is distinguished under the micro- 
 scope by being less fresh. It is filled with inclusions and de- 
 composition products of various sorts but it is by no means to be 
 considered as an independent mineral species. In 
 coarse granular rocks the mineral can be distinctly 
 seen macroscopically in the last mentioned form and 
 the same variety is also observed by the naked eye 
 in nepheline porphyry. On the other hand, the fresh 
 form is not so readily observed partly because the 
 individuals are smaller and partly because they are 
 clear, colorless, and transparent. Rocks, which con- 
 tain nepheline in fine particles, frequently show a characteristic 
 greasy luster. 
 
 When nepheline shows distinct crystal form its habit is short, 
 thick prismatic, Fig. 260. Its cross sections are short rectangu- 
 lar to six-sided, Fig. 237 e, page 225. Frequently inclusions of 
 pyroxene needles are arranged in a zonal manner. It is usually 
 without distinct cleavage. Even the variety known as elaeolite 
 forms such individuals in rocks rich in nepheline. Characteristic 
 for the shapeless particles of elaeolite are the irregular, clouded 
 bands of decomposition products, and the inclusions of scales 
 of brownish-red ferric hydroxide or of needles of aegirine. Like- 
 
250 PETROGRAPHIC METHODS 
 
 wise, the large grains of the clear, transparent type are easily 
 recognized by the index of refraction which agrees almost ex- 
 actly with that of Canada balsam. However, when the dimen- 
 sions are very small, as in numerous nepheline basalts, or finally, 
 when it occurs only as a fine cement of colorless substance be- 
 tween the other constituents, it can only be identified with an 
 approximate degree of accuracy by gelatinization with hydro- 
 chloric acid and the formation of cubes of salt. It can only 
 be positively distinguished from zeolites and rock glass, if in 
 addition to this the etched grains show cross sections of a typical 
 form after they have been stained. 
 
 The fact that nepheline is confined to the soda rocks helps 
 to make it more easily recognized because it is almost always 
 accompanied by bisilicates rich in sodium and, when these are 
 present, it .is always well to look for nepheline. Its similarity to 
 the zeolites must, however, not be lost sight of. Numerous 
 colorless minerals have been confused with nepheline. Apatite 
 is distinguished by the higher indices of refraction, quartz and 
 cordierite by the higher double refraction, scapolite by both of 
 these properties, and sanidine by a lower index of refraction. 
 
 Nepheline is one of -the most easily attacked of the rock con- 
 stituents. Alteration into zeolites such as analcite, hydro- 
 nephelite, and natrolite is especially common Spreustein. In 
 certain rocks pseudomorphs of matted mica after nepheline are 
 found and these have received the name liebenerite or gieseckite. 
 
 Apophyllite (6) 
 
 Apophyllite is rare and like all the zeolites is only known as a secondary 
 product mostly in basic eruptive rocks. The mineral is characterized in 
 thin section by an extremely low double refraction, the occurrence of anomal- 
 ous interference colors, and a very perfect cleavage. It is a very rare 
 mineral as a rock constituent. 
 
 Chabazite (6) 
 
 Chabazite is also found among the zeolitic decomposition products of 
 silicates containing lime. It occurs in cavities and crevices in basic eruptive 
 rocks and is found often in splendidly developed limpid crystals. It occurs 
 as a rock constituent in the same kind of rocks, but is usually poorly developed 
 and difficult to distinguish from the other zeolites. 
 
 Cancrinite (6) 
 
 Cancrinite is confined to the soda rocks. It is sometimes associated with 
 nepheline after which it also forms pseudomorphs. It sometimes replaces 
 
DESCRIPTIVE SECTION 251 
 
 nepheline. It appears macroscopically in poorly bounded grains and scaly 
 aggregates with a yellow to reddish color, and is distinguished by its good 
 prismatic cleavage. Recently, rhombohedral crystals have been observed 
 in tephrites. Under the microscope its individuals are colorless and have 
 inclusions of iron hydroxides, etc. 
 
 The mineral is easily distinguished from all other rock constituents by 
 its low indices of refraction and brilliant interference colors. The identity of 
 the mineral can also be proved by treating the section under the microscope 
 with warm hydrochloric acid, when the development of small bubbles of 
 carbon dioxide can be observed. If the preparation is heated, the mineral 
 becomes cloudy. Alteration is the same as in nepheline. Cancrinite fuses 
 before the blowpipe with intumescence to a vesicular glass. 
 
 Hydronephelite (Ranite) (6) 
 
 Hydronephelite and ranite, which is distinguished from it by a small 
 content of lime, are the most frequent alteration products of nepheline. 
 They are observed in confused flaky aggregates in pseudomorphs after 
 nepheline Spreustein. They can only be recognized with any degree of 
 accuracy under the microscope in sections perpendicular to the c axis in 
 convergent polarized light, and in all cases they are difficult to distinguish 
 from the other zeolites, which are associated with them in the composition 
 of spreustein. 
 
 4. Biaxial Minerals 
 
 When the rock-forming minerals of the orthorhombic, monoclinic and 
 triclinic crystal systems show distinct crystal form they possess prismatic or 
 tabular development. Pyramidal forms alone rarely occur, but they are 
 frequent as terminations on prismatic crystals. Pyramid faces also have 
 no significance as cleavage directions. Fig. 261 gives the principal types 
 of cross sections observed. 
 
 In the orthorhombic system, sections parallel to the a and c axes are more 
 or less lath-shaped with a square or a domatic end. Sections parallel to a 
 macropinacoid are quadratic when there are but two end faces, and rhombic 
 when the development is that of a prismatic form. When both of these 
 types of forms occur in combination, the sections are six- to eight-sided. It 
 is significant that the prism angle of numerous orthorhombic minerals, and 
 the same is true for monoclinic and triclinic crystals, approaches 90 or 
 120, and their cross sections are similar to those of tetragonal and hexago- 
 nal minerals. Quite frequently crystals with a lower symmetry are observed 
 simulating the symmetry of a higher class. In the lath-shaped cross 
 sections of the orthorhombic crystals the extinction is usually parallel and 
 perpendicular to the edges, and the same is true for the sections across the 
 front of the crystal, if its form is determined by the basal pinacoidal faces. 
 If a prismatic form predominates in such a section, the extinction is sym- 
 metrical. 
 
 Monoclinic minerals are sometimes prismatic parallel to the axis of 
 symmetry or in the direction perpendicular to it. Sometimes they are 
 
252 
 
 PETROGRAPHIC METHODS 
 
 tabular parallel to the basal pinacoid or the plane of symmetry, and occa- 
 sionally to the macropinacoid. In the prismatic type all the sections with 
 parallel extinction are lath-shaped, while those with oblique extinction are 
 much shorter and have four-, six- or eight-sided outlines. If the prism axis 
 is perpendicular to the direction of the axis of symmetry, the sections with 
 parallel extinction may show all of the forms referred to in the orthorhombic 
 crystals, while the sections with oblique extinction appear elongated and 
 have unsymmetrical end faces. Minerals in the monoclinic system, devel- 
 oped tabular parallel to the base, have lath-shaped cross sections, which 
 may show parallel or oblique extinction, while sections parallel to the base 
 
 FIG. 261. Principal Cross Sections of Biaxial Minerals, 
 
 are nearly always regular six-sided. Among the minerals on which the 
 plane of symmetry predominates, all sections with parallel extinction are 
 lath-shaped, while those with oblique extinction are rhombic or irregular 
 six-sided. 
 
 The triclinic minerals, which are not abundant as rock constituents, are 
 very similar to the monoclinic .in form. Generally it is quite difficult to 
 establish the membership of the variously orientated cross sections in the 
 triclinic system. 
 
 Sections of a biaxial mineral parallel to the plane of the optic axes give 
 the highest interference colors of all sections. Such a cross section must be 
 sought to determine the value of the double refraction by means of the 
 interference colors, and one can be assured of the proper orientation by the 
 behavior in convergent polarized light (see Part I, page 118) . It is also useful 
 to find a cross section parallel to the plane of the optic axes to determine the 
 directions of extinction of monoclinic minerals, because in many monoclinic 
 substances the axial plane lies in the plane of symmetry, and it is in this 
 latter plane that the extinction angle must be determined. Sections of 
 biaxial minerals perpendicular to one of the bisectrices give interference 
 colors which under all circumstances are lower than those of a section paral- 
 lel to the plane of the optic axes. The colors in the one case are about half 
 as high as in the other, and they are about the same in both sections perpen- 
 
DESCRIPTIVE SECTION 253 
 
 dicular to the acute or obtuse bisectrix if the optic angle approaches 90. 
 The interference color in a section perpendicular to the obtuse bisectrix is 
 always the higher, and it is the more like that of a section parallel to the 
 optic plane, the smaller the acute optic angle. The interference color is 
 very low in a section perpendicular to the acute bisectrix if the optic angle 
 is small, even though the double refraction of the mineral itself is high. 
 This phenomenon is especially interesting in the investigation of micaceous 
 minerals in the form of powder. In consequence of the perfect cleavage only 
 small flakes are found and these are perpendicular to the acute bisectrix 
 of a small optic angle. The best example of this is muscovite. It is optic- 
 ally negative, and in certain occurrences, f a. = 0.042. Therefore the inter- 
 ference colors in a section parallel to the axial plane are quite brilliant in the 
 thinnest slides. /? a = 0.039 shows that in a section perpendicular to the 
 obtuse bisectrix the interference colors are quite the same as in the first 
 section. On the other hand the double refraction in a cleavage plate is 
 quite different, 7- /? = 0.003. Distinct interference colors appear only in 
 very thick plates. 
 
 Biaxial minerals with a small optic angle cannot always be positively 
 recognized even in sections perpendicular to the acute bisectrix, because of 
 the frequency of optical anomalies in uniaxial minerals. It is not possible 
 to make a distinction in sections quite oblique or parallel to the acute bisec- 
 trix. It was noted in Part I, page 118, that in minerals with a large optic 
 angle and high indices of refraction the optic axes cannot be seen in conver- 
 gent polarized light even in sections perpendicular to the acute bisectrix, 
 with a dry system of lenses, because the axes are totally reflected. The 
 determination of the optical character of such a mineral is only possible in 
 oblique sections according to the method described in Part I, page 115. 
 Even this method is not always reliable, especially when the optic angle is 
 nearly 90, as in olivine and plagioclase. It is quite evident that the deter- 
 mination of the optical properties is more positive and can be made in more 
 sections where the optic angle is large than where it is small, because most of 
 the sections in the former case will show a good characteristic interference 
 figure. 
 
 The position of the plane of the optic axes relative to the principal zone is 
 a valuable characteristic in determining a mineral. The plane is sometimes 
 parallel and sometimes perpendicular to the principal zone. 
 
 The size of the optic angle is different for various colors. For this reason, 
 if an interference figure is rotated into the 45 position with respect to the 
 planes of vibration of the nicols, a fringe of color will be seen around the 
 vertices of the hyperbolae. It is sometimes yellow on the convex side and blue 
 on the concave, and sometimes the reverse. In the first case the optic angle 
 is greater for red than for blue, and the dispersion formula is ^>y. In 
 monoclinic and triclinic minerals, the bisectrices for various colors do not 
 generally coincide. Inclined dispersion is the most frequent in the mono- 
 clinic rock-forming minerals (see Part I, page 111) . Strong dispersion of the 
 optic axes or of the bisectrices becomes apparent in many sections even in 
 parallel polarized light by the occurrence of anomalous interference colors. 
 
 When biaxial minerals show pleochroism three color axes can be distin- 
 guished corresponding to the three principal vibration directions. In the 
 
254 PETROGRAPHIC METHODS 
 
 orthorhombic system these two sets of axes coincide. In the monoclinic 
 system the two color axes lying in the plane of symmetry may be oblique 
 to the principal" vibration directions, while in the triclinic system there is no 
 regularity in the position of either set of axes. In both cases the differences 
 are quite small. An accurate orientation of the mineral section in conver- 
 gent polarized light should always precede an accurate determination of the 
 pleochroism. 
 
 Brookite (7) 
 
 Brookite occurs as a rock constituent in small rather elongated plates with 
 domatic terminations, but it is a rare constituent and is always in small 
 amounts. It is of secondary origin formed by the alteration of silicates con- 
 taining titanium, especially of biotite in granite and quartz porphyry, and 
 it is also often found in clastic rocks. It is always accompanied by rutile and 
 anatase. It is brown and transparent under the microscope. It shows no 
 pleochroism on the broad face and has an adamantine luster in reflected 
 light. The crossed position of the optic planes for green and red is charac- 
 teristic. It can be noted by observing the tabular crystals in convergent 
 polarized light. The mineral can be determined by its adamantine luster 
 in reflected light and high indices of refraction as well as high double refrac- 
 tion. Observations in convergent light distinguish it from rutile, cassi- 
 terite and pseudobrookite. It is distinguished from goethite principally by 
 chemical tests. 
 
 Goethite (Needle Iron Ore) (7) 
 
 A few remarks concerning goethite must be made here. Its occurrence 
 in thin needle-like inclusions in various other minerals has been proved, but 
 it is scarcely ever observed as a rock constituent on account of its similarity 
 to rutile, brookite, etc. Fine fibrous aggregates of velvet blend are easily 
 recognized in ore deposits, but otherwise it can only be positively determined 
 by chemical tests. 
 
 The mineral, which was originally called goethite, is quite different from 
 needle iron ore and it is now called ruby mica. Its composition is the same 
 as that of goethite, but it occurs in short, rhombic, tabular crystals with 
 very perfect cleavage parallel to the broad face and perfect cleavage per- 
 pendicular to it, and in addition it has a fibrous fracture. It is red to yel- 
 lowish-red pleochroic, has high indices of refraction and comparatively low 
 double refraction. 2V is not far from 90 for all colors in the same plane. 
 Many of the red inclusions producing the red color or aventurine chatoyancy 
 of minerals belong to this type. 
 
 Fine grained to scaly aggregates of brown iron hydroxides with brilliant 
 interference colors are widely observed as pseudomorphs after pyrite. It 
 cannot be positively determined whether they belong to goethite or not. 
 The same is true for brown iron ore limonite 2Fe 2 O 3 .3H 2 O, which under 
 certain conditions forms similar aggregates. It is fibrous with perfect 
 cleavage in the direction of the fibers and has a positive principal zone and 
 negative double refraction. It has high indices. f~ a: = 0.05. 2V is very 
 large. It is brown to yellow pleochroic fc> c> 0. 
 
DESCRIPTIVE SECTION 
 
 255 
 
 Pseudobrookite (7) 
 
 Small rectangular tabular crystals of pseudobrookite are but rarely found 
 in the recent eruptive rocks and their tuffs. It is quite similar to brookite, 
 but is usually deeper colored and is probably always secondary, formed by 
 fumaroles. It is red, transparent under the microscope only when it is 
 extremely thin and is weakly pleochroic. It is distinguished from brookite, 
 goethite and wurtzite, which appear quite similar to it in ordinary light, 
 by the depth of color and observations in convergent light. 
 
 Sulphur (7) 
 
 Sulphur as a rock constituent is confined on the one hand to volcanic 
 rocks and is then either a by-product in alumstone or a binding material 
 in tuffs of volcanic ash. It is difficult to recognize in the latter case, but can 
 always be determined by its ability to ignite and by the odor given off upon 
 combustion. On the other hand, it is found in sediments especially in 
 gypsum and in organic deposits in which it is apparent even macroscop- 
 ically. Its microscopic distribution has been very little investigated. 
 
 Baddeleyite (7) 
 
 Up to the present time baddeleyite is only known in basic granular soda 
 rocks. The cross sections are usually elongated and show twinning lamina- 
 tion. They are pleochroic, being green parallel, and brownish perpendicular 
 to the principal zone. It has perfect cleavage and is characterized beyond 
 question by its high indices of refraction and double refraction. 
 
 Titanite (7) 
 
 Titanite is always only an accessory constituent of rocks, but 
 it may be so concentrated locally that it assumes considerable 
 importance. It is distributed throughout all rocks with the 
 
 FIG. 262. Envelope Form 
 of Titanite. 
 
 FIG. 263. Titanite. Horizontal Cross Section 
 through Fig. 262. 
 
 possible exception of pure magnesium silicate rocks. It has a 
 great tendency to be associated with hornblende. Its crystallo- 
 graphic habit is quite variable. In granite and related rocks 
 the envelope form, Fig. 262, predominates. It gives a sharp 
 
256 PETROGRAPHIC METHODS 
 
 rhombic cross section, Fig. 263, caused by the predominance of 
 {1123}, 2/3 PS. In soda rocks it has sharp angular prismatic 
 development with {01 1 } , P 06 predominating. Individuals of the 
 first type generally belong to grothite. It is macroscopically 
 dark brown and is often quite distinctly colored, generally 
 reddish, in thin section and is pleochroic. Those of the second 
 type resemble more the yellow sphene, which is colorless in thin 
 section. Twinning parallel to the basal pinacoid is not rare. 
 It often divides the rhombic cross sections in halves and it also 
 occurs in the form of lamellae. 
 
 The mineral is further found in grains, which are frequently 
 elongated especially in schistose rocks. Minute rounded particles 
 may be assembled together, insect eggs, and the finer grained 
 these are the more clouded they appear. Titanite is also wide- 
 spread as a border around other titanium minerals and as pseudo- 
 morphs after them. It occurs especially with anatase as the 
 principal constituent of leucoxene. 
 
 The cleavage is usually developed only in a very few cracks 
 and it is very characteristic that these are generally not parallel 
 to the edges of the crystals, Fig. 263. The optical properties 
 vary within wide limits, but the high indices of refraction and 
 double refraction together with the strong dispersion of the 
 optic axes afford good marks of recognition. Deep colored 
 varieties can be confused with rutile under some conditions and 
 the colorless or light colored ones with cassiterite or xenotime 
 from which a distinction is very difficult, if it cannot be made 
 by the interference figure. This cannot always be done on 
 account of the small optic angle of titanite. In this case a 
 chemical test for calcium, made upon the isolated material, is 
 necessary to make a positive distinction. It is easily distin- 
 guished from anatase, zircon, epidote and monazite by the much 
 lower double refraction of these. See page 242 concerning 
 confusion with calcite. 
 
 Lievrite (Ilvaite) (7) 
 
 Lievrite is found in large masses in some ore deposits and in more isolated 
 individuals in contact rocks. It forms pseudomorphs after the bisilicates 
 in soda rocks. It is also encountered with fine hair-like development in the 
 pores of trachytes and is known as breislakite. Its determination is made 
 very difficult by the fact that it is almost entirely nontransparent but, on 
 the other hand, it is favored &y its solubility in hydrochloric acid and 
 the micro-chemical test for calcium as well as by its fusibility. Anig- 
 matite is similar to it, see page 294. 
 
DESCRIPTIVE SECTION 
 
 257 
 
 (100) 
 
 FIG. 264. Monazite. 
 
 Section Parallel Plane of 
 
 Symmetry. 
 
 Monazite (7) 
 
 Monazite is a widespread constituent of granitic and syenitic rocks, 
 especially aplites and pegmatites, and the schists injected by them, but it 
 is always only sporadically present. It occurs in tabular to prismatic 
 crystals, Fig. 264, which have separated early from the magma. It is very 
 easily confused with zircon and epidote in thin 
 section, but observation with the spectroscope 
 readily distinguishes them. The characteristic 
 absorption bands of neodymium and praseo- 
 dymium, page 168, characterize monazite. Cleav- 
 age plates show an almost symmetrical position 
 of the positive bisectrix of a small optic angle. 
 Since it is quite resistive to weathering, it is 
 found also in clastic rocks, especially in sand 
 stones, but it is frequently clouded and generally 
 forms rounded grains. 
 
 Lavenite (7) 
 
 When lavenite occurs in irregular grains it is 
 often difficult to distinguish from epidote. In 
 most of the prismatic crystals the position of the 
 optical plane parallel to the principal zone can be 
 determined. It is reddish-brown or yellow macro- 
 scopically and in thin section it is quite distinctly 
 
 colored. Twinning lamellae parallel to {100} are frequent. One of the 
 axes emerges from a cleavage plate obliquely. It is always fresh. 
 
 Woehlerite is very much like lavenite. It is also monoclinic, but is some- 
 what lighter colored and has lower double refraction. It forms larger 
 individuals, which are usually tabular parallel to {100} and have twinning 
 lamination parallel to the same form. The extinction angle is about 43 
 and that, together with the position of the optical plane perpendicular to 
 the plane of symmetry, is the distinctive characteristic. The obtuse bisec- 
 trix, which is negative, is seen in laminated cross sections. It is more easily 
 attacked by hydrochloric acid and is more difficultly fusible than lavenite. 
 Both minerals are confined to the soda rocks, but are widely distributed in 
 them in isolated grains or crystals, accompanied by other zirconium silicates 
 and violet fluorite. 
 
 Another silicate containing zirconium and titanium, not unlike lavenite, 
 has been observed in jagged needles that are frequently greatly honeycombed 
 and often twinned, in certain phonolites and is called hainite. Monoclinic? 
 It cleaves parallel to a direction perpendicular to the positive bisectrix of 
 a large optic angle with strong dispersion p > v. Negative character of the 
 principal zone. Extinction angle up to 16. Index of refraction about 1.7. 
 f a about 0.012. Pleochroism light yellow to colorless. 
 
 Chrysoberyl (7) 
 
 Chrysoberyl undoubtedly plays no subordinate role among the constit- 
 uents that are constant, but are only present hi extremely small amounts in 
 17 
 
258 
 
 PETROGRAPHIC METHODS 
 
 acid eruptive rocks, especially in aplites, pegmatites, and in schists injected 
 by them. It is very difficult to determine it, when it occurs in small isolated 
 crystals because of its optical properties which are unusually variable. It 
 sometimes has a large optic angle with a weak dispersion, and often the optic 
 angle is for red and 90 or over for violet. It often shows anomalous 
 interference colors in consequence of the great dispersion. 
 
 Epidote Group (8) 
 
 The epidote group is one of the most difficult to recognize of all 
 the rock-forming minerals, partly because of the great similarity 
 in the optical properties of the various members, which are quite 
 different chemically and cry stallographic ally, and partly because 
 of the enormous differences displayed by 
 compounds, chemically very closely related 
 to each other. Minerals of this group with 
 an extremely low double refraction are 
 distinguished from those which show 
 medium to very strong double refraction. 
 There are types in the first class, which 
 show the highest degree of anomalous interference colors, and 
 others that are scarcely observable in thin section and give a 
 pure gray of the first order. 
 
 All the minerals considered here are developed prismatic with 
 a tendency to a long, tabular growth. In the monoclinic 
 
 FIG. 265. Epidote. 
 
 FIG. 266. Section through 
 a Twin of Epidote. 
 
 FIG. 267. Section through 
 a Twin of Orthite. 
 
 members the principal zone is that of the axis of symmetry, 
 Fig. 265, and the orthorhombic members, contrary to custom, 
 are set up accordingly, so that the principal zone is that of the 
 transverse axis. Sharply bounded crystals are rare. The pre- 
 dominating cross sections of the principal zone are mostly 
 rounded on the ends. Corresponding to their development, 
 the monoclinic members show chiefly sections with parallel 
 
DESCRIPTIVE SECTION 
 
 259 
 
 extinction. In transverse sections it is not easy to distinguish 
 monoclinic from orthorhombic members. The extinction direc- 
 tion is almost parallel to the trace of the front pinacoid in many 
 epidote minerals, which is usually also the twinning plane. 
 The two halves or the lamellae of which such crystals are con- 
 structed extinguish at about the same time, Fig. 266. 
 
 Orthite (allanite) shows a characteristic difference in this 
 respect. Its extinction angle referred to this direction is very 
 considerable, Fig. 267, and its twins therefore appear most dis- 
 tinctly in polarized light. It is noteworthy, however, that when 
 crystal form or twinning are lacking, and the extinction is meas- 
 ured from the cleavage cracks, which are few in number but very 
 sharp, both varieties show about the same extinction, 30. 
 
 FIG. 268. Zoisite a, 
 
 FIG. 269. Zoisite 
 
 Section Parallel (010). 
 
 There is a difference here in the axes of elasticity. In epidote 
 and clinozoisite the axis of elasticity which forms this angle with 
 the cleavage direction is that of the least velocity c, while in 
 orthite it is the direction of the fast ray, a. 
 
 Figs. 268 to 273 show the usual development of the most im- 
 portant members of the epidote group together with their most 
 important optical properties. In the normal members of the 
 series the plane of the optic axes lies perpendicular to the direc- 
 tion of elongation. This is one of the most important means of 
 distinguishing epidote from the pyroxenes and other minerals, 
 which otherwise possess similar properties. In each of the sub- 
 divisions of the members with low double refraction, there are 
 those in which the position of the axial plane coincides with the 
 trace of the cleavage. In the first group transverse sections of 
 the prismatic individuals are parallel to the axial plane and, 
 
260 
 
 PETROGRAPHIC METHODS 
 
 therefore, show the highest interference colors. In these sections 
 also, twinning appears most distinctly. In the other group such 
 transverse sections show the obtuse bisectrix and have medium 
 interference colors. 
 
 The minerals of the epidote group are very frequently inter- 
 grown with each other partly in a very regular, zonal manner, so 
 
 (001) 
 
 V 
 
 *s* 
 
 \p 
 
 y 
 
 
 
 v v- 
 
 
 p^ 
 
 (010) 
 
 (100) 
 
 FIG. 270. Zoisite a: Axial Plane Parallel 
 Cleavage. Zoisite 0: Axial Plane Perpen- 
 dicular Cleavage. Section Parallel Base. 
 
 (010) 
 
 4 
 
 Jt 
 
 (101) 
 
 FIG. 271. Clinozoisite and Epidote. 
 Section Parallel (100). 
 
 that the content of iron, and with it the double refraction, de- 
 creases from the center toward the edge, or vice versa. Alternat- 
 ing zones also occur. In other cases the center may be one of 
 the zoisites which grows into a monoclinic member toward the 
 edge. One of the most characteristic features of orthite is an 
 
 ^(001) 
 
 FIG. 272. Clinozoisite, 
 
 or >4 
 
 FIG. 273. Epidote, 
 
 Section Parallel Plane of Symmetry. 
 
 enveloping growth of Clinozoisite. In such cases the following 
 parallelism of elasticity axes is to be noted: 
 
 zoisitea zoisite/9 Clinozoisite, epidote, orthite. 
 
 r r a 
 
 P * r 
 
 a 
 
DESCRIPTIVE SECTION 261 
 
 The crystallographic orientation of the members of the group 
 referred to above is taken from these observations. In other 
 cases the intergrowth is entirely irregular and the various 
 members penetrate each other. The great differences in double 
 refraction of the members thus intergrown often give rise to 
 the brilliant speckled appearance of the sections in polarized 
 light, and this does not occur in any other series of minerals. 
 
 Zoisite and clinozoisite are scarcely ever distinctly seen macro- 
 scopically, but rocks in which they participate in the composition 
 to a great degree have a light yellowish- or grayish-green tint. 
 Epidote itself with its pistachio green color pistazite is often 
 seen macroscopically. It is the most frequent yellow pigment of 
 crystalline rocks and as such is in a fine state of division. Nor- 
 mal zoisite and clinozoisite are colorless in thin section. Mono- 
 clinic epidote rich in iron is quite light, usually yellowish, with 
 strong absorption parallel to the principal zone. The intensity 
 of the color and the pleochroism are increased by heating to 
 redness in air. A deep brownish-yellow color appears in an 
 unroasted section of the members with the highest double refrac- 
 tion and the highest content of iron, but this seems to be a rare 
 occurrence. Similarity to lavenite appears in the most highly 
 colored members. 
 
 There is a series of members containing chromium which are macroscopic- 
 ally almost emerald green. In this series chrome zoisite and chrome epidote can 
 be distinguished, and there are also varieties corresponding to clinozoisite. 
 These minerals are deeply colored in thin section and the pleochroism (o= c 
 light green, B deep orange) can be noted in occurrences that have such a low 
 double refraction that they scarcely affect polarized light. The color and 
 strength of the double refraction are in no way related to each other. 
 
 The whole series can be distinguished also in the members containing 
 manganese. Thulite corresponds to the zoisites. It is macroscopically 
 rose-red and has a distinct color in thin section; a yellowish, B red, c rose. 
 It usually has anomalous interference colors. A series with varying inten- 
 sity of color and strength of double refraction leads to manganese epidote 
 piemontite. It appears blackish-red macroscopically and it is characterized 
 in thin 'section by deep color and strong pleochroism. It is often the cause 
 of the deep red color of rocks, especially in altered porphyrites, e.g., in 
 porfido rosso antico, and in phyllites. Next to hematite it is the most impor- 
 tant red pigment of crystalline rocks. It often appears not unlike dumor- 
 tierite, but is distinguished from it by a different optical orientation. 
 
 Finally orthite or ceriwn epidote, allanite, is to be mentioned. It usually 
 shows good crystal form in the granular eruptive rocks, and these as well as 
 the isolated grains are characteristically enveloped by clinozoisite. It has 
 a brown, sometimes also blood red, color and is strongly pleochroic with a 
 
262 
 
 PETROGRAPHIC METHODS 
 
 high double refraction, 7- a = up to about 0.3. Isolated black grains with 
 a pitchy luster, surrounded by a red border, are frequently observed macro- 
 scopically. It is noteworthy that these deep colored orthites are not 
 infrequently transformed into an amorphous yellowish to red gum-like 
 mass, a phenomenon which numerous minerals containing cerium show. The 
 color of orthite is very much lighter in the soda rocks, contact rocks, and 
 injected schists. In thin section it is light brownish, greenish-yellow, or 
 light violet, probably due to titanium. Some individuals are indeed colorless. 
 The double refraction is decreased in such occurrences to about 0.002, and 
 the index of refraction of these lighter colored members also seems to be 
 much lower, about 1.70 or less. The determination of small prisms is thus 
 often extremely difficult. They often show normal interference colors and 
 are then nearly always determined as apatite. Sometimes they are charac- 
 terized by brilliant anomalous colors. 
 
 There are many reasons for classing these small crystals as orthite. Fre- 
 quent zonal growths with clinozoisite point to their membership in the epi- 
 dote group. The appearance, of pleochroic halos is often observed near 
 them, and this is distinctive for all occurrences of orthite, and, finally, the 
 transverse sections nearly always show twinning with the orientation of 
 Fig. 267, page 258. Hence there can be no doubt of their relation to orthite. 
 It may be mentioned that in spite of the high content of rare earths, spectro- 
 scopic investigation of orthite usually gives no results. 
 
 The following table gives a classification of the phenomena described and 
 can be used to differentiate these minerals. 
 
 
 Double 
 refraction 
 
 Inter- 
 ference 
 color 
 
 Position of 
 optical 
 plane 
 
 Optic 
 angle 
 
 Extinction 
 
 Zoisite a. 
 
 
 Anomal- 
 
 || cleavage 
 
 Medium. 
 
 i 
 
 
 
 
 ous. 
 
 and princi- 
 
 
 
 
 
 
 
 pal zone. 
 
 
 
 
 Zoisite /?. 
 
 Low. 
 
 Normal. 
 
 1 
 
 Small. 
 
 
 > Approxi- 
 mately or 
 
 Clinozoi- 
 site. 
 
 
 Strongly 
 anomal- 
 
 J_ cleavage 
 
 1 Very 
 
 
 entirely 
 parallel. 
 
 
 
 ous. 
 
 i and prin- 
 cipal zone. 
 
 large. 
 
 > 
 
 
 Epidote. 
 
 High. 
 
 Normal 
 
 
 
 
 
 
 speckled . 
 
 | ' 
 
 
 
 Orthite. 
 
 Medium 
 
 Anomal- 
 
 Variable. 
 
 Variable. 
 
 Oblique 36. 
 
 
 to low. 
 
 ous or 
 
 
 
 
 
 
 normal. 
 
 
 
 
DESCRIPTIVE SECTION 263 
 
 The great difficulties encountered in making an accurate 
 determination of the epidote minerals can be seen from this 
 classification. A distinction is quite important where the mem- 
 bers are of considerable size and to some extent well developed. 
 Much practice is required to distinguish the various members, 
 where the minerals occur in dense aggregates as in the saussurite, 
 which is macroscopically greenish-gray and has a splintery 
 fracture, or where they are in sharply defined microlites grown 
 in plagioclase as is generally the case in the central granite. 
 If cleavage plates of these minerals can be investigated, it is 
 noted that in epidote rich in iron an axis emerges slightly oblique 
 to the cleavage face. In clinozoisite it is more oblique and 
 cleavage plates of the zoisites show no characteristic interference 
 whatever. 
 
 Distinction of the two zoisites, which are not positively ortho- 
 rhombic but possibly crystallize triclinic, from clinozoisite, which 
 is certainly monoclinic, is made difficult by the fact that the 
 latter has an extinction almost parallel to the vertical axis. The 
 degree of abnormality of the interference colors is the safest 
 means of differentiation. It is pure gray in zoisite /?, distinct 
 bluish-gray in zoisite a, and brilliant Prussian blue in clino- 
 zoisite. Investigation in convergent polarized light also aids 
 to distinguish them. Thorough study shows that clinozoisite 
 is by far the most widespread of the low double refracting mem- 
 bers, which were formerly simply classed together as zoisite. 
 Zoisite a is rather frequent, but zoisite /? is quite rare as an in- 
 dependent rock constituent. 
 
 Concerning the geological distribution of these minerals, it 
 may be said they belong to the most typical formations of con- 
 tact rocks rich in aluminium and calcium, and as such are found 
 especially abundant in amphibolites, chlorite schists, green 
 schists, and eclogites, in which they are regularly mixed with 
 the other minerals or are separated in lighter colored layers. 
 They are observed less frequently in mica schists, but in them 
 they have a tendency to occur in knots rich in graphite, which 
 are macroscopically black. As already mentioned they form 
 a constituent of saussurite, which occurs in pseudomorphs after 
 plagioclase rich in calcium. In it these pseudomorphs are usually 
 very poorly developed and the structure of the saussurite is so 
 matty that the individual grains cannot be separated from 
 each other even under the microscope. Since numerous other 
 
264 
 
 PETROGRAPHIC METHODS 
 
 minerals with high indices and low double refraction, frequently 
 also with anomalous interference colors, such as vesuvianite, 
 garnet, gehlenite, prehnite, etc., take part in the formation of 
 these aggregates, the question whether saussurite really belongs 
 to the epidote minerals must remain unsettled. 
 
 The low double refracting members occur alone in some rocks, 
 especially in amphibolites and saussurites. In other rocks, 
 however; they are in zonal intergrowth with epidote, which is 
 richer in iron and has a higher double refraction. In numerous 
 
 chlorite schists, eclogites, etc., 
 epidote entirely replaces the 
 other members. In lime- 
 silicate fels rich in aluminium, 
 lime" mica schist, etc., epidote 
 is in general more frequent 
 than the other members. 
 These rocks pass over into 
 yellowish epidote fels or epi- 
 dosite. In the latter rock, 
 epidote is observed here and 
 there in crystals well bounded 
 on all sides, while in other 
 occurrences it, like the others, 
 shows good faces at best only 
 in the zone of the axis of sym- 
 metry. Sections parallel to the a axis generally appear as 
 rounded tapering laths. 
 
 The widespread occurrence of abundant sharply defined 
 microlites of clinozoisite in the plagioclase of the central granite 
 is very noteworthy. Fresh glassy feldspar consisting mostly of 
 oligoclase is often so abundantly filled with these microlites 
 without any orientation that the rock shows a yellowish-green 
 tint macroscopically, and extremely thin sections are necessary 
 to make the formation transparent at all. Fig. 274 gives a 
 representation of this phenomenon, which differs greatly from 
 all forms of secondary development and can only be designated 
 as an original formation. It represents a typical feature of 
 piezocrystallization. Besides this, clinozoisite and epidote are 
 encountered in much larger grains as primary constituents of 
 acid eruptive rocks. They are generally confined to the border 
 zone of the intrusive and represent constituents taken up from 
 
 FIG. 274. 
 
 Plagioclase with Microlites of Clinozoisite. 
 Central Granite, Mosele, Zillertal. 
 
DESCRIPTIVE SECTION 265 
 
 the neighboring rock. The zoisites are also found in intruding 
 aplites and pegmatites. They occur in large crystals in peg- 
 matites, which intrude the eclogites of the Fichtelgebirge, and 
 in microscopic individuals in the aplites, which intrude the 
 amphibolites of Mount Grossevenedig. They are much more 
 frequent in the eruptive rocks, where they are secondary. The 
 alteration of bisilicates into chlorite is almost always accompanied 
 by the formation of greater or smaller amounts of epidote in 
 irregular grains. If this form is but weakly colored it appears 
 similar to anatase, which occurs under like conditions but is 
 distinguished from the latter by much lower indices of refraction. 
 The manner of occurrence of manganese epidote has already 
 been referred to and likewise that of orthite. Aside from the 
 extinction and the frequent occurrence of pleochroic halos in 
 the neighborhood of orthite, it is characterized by the fact that 
 it is always only a subordinate and purely accessory rock 
 constituent. 
 
 Minerals of the epidote group are not infusible and the richer 
 they are in iron the more easily they fuse. Orthite is distinctly 
 attacked when fresh by hydrochloric acid, but the other members 
 resist the acid unless they have previously been roasted to drive 
 off water and then they gelatinize readily. 
 
 Confusion of epidote minerals with numerous others has 
 already been referred to many times, thus with vesuvianite, 
 etc., in saussurite where a separation is not possible. Light 
 colored epidote is often not unlike the pyroxenes. They have 
 numerous cleavage cracks parallel to which lies the optical plane, 
 while in epidote it is perpendicular. In lavenite also the optical 
 plane lies in the principal zone. Olivine often appears similar 
 to epidote, but it never has the speckled interference, colors and 
 generally has no distinct cleavage cracks. It also turns deep 
 brown upon roasting and gelatinizes readily with hydrochloric 
 acid. The isolated occurrence and spectroscopic investigation 
 distinguish monazite. Confusion of low double refracting 
 members with apatite has been mentioned above. When no 
 morphological distinction can be found, apatite can be deter- 
 mined by its solubility in dilute nitric acid and its reaction 
 for phosphorus. Brown orthite often appears not unlike basaltic 
 hornblende. It shows a smaller difference in absorption than 
 does the hornblende and the position of the optical plane trans- 
 verse to the principal zone. Further than this the border of 
 
266 
 
 PETROGRAPHIC METHODS 
 
 clinozoisite and epidote around the individuals of orthite, which 
 are always isolated, is exceedingly characteristic. 
 
 Staurolite (8) 
 
 Staurolite is a typical mineral of the contact rocks, particu- 
 larly those formed by piezocontact metamorphism. It is rarely 
 found in the granites, but is found in the clastic rocks in con- 
 sequence of its great resistance to weathering. 
 It sometimes occurs in quite large, well developed, 
 flat prisms and twins, in which the individuals 
 interpenetrate each other at 90 or 60, Fig. 275. 
 They are dark brown in color. Sometimes the 
 borders are very irregular. Smaller microscopic 
 
 FIG. 275. ...... 11 i 
 
 Staurolite Twins, individuals usually have a much poorer form, 
 but in general the prismatic development is 
 distinct. The larger crystals are usually filled with inclu- 
 sions like a sieve. They may be penetrated by quartz in an 
 irregular manner or in a helicoidal manner as shown in Fig. 
 276. Graphite dust is observed in them much the same as in 
 chiastolite. Microscopic individuals tend to be purer, but are 
 difficult to determine on account of their poor development. 
 
 FIG. 276. Staurolite Twin Filled 
 with Quartz Inclusions. 
 
 FIG 277. 
 Staurolite Cross Section. 
 
 Sections corresponding to Fig. 277 are the most characteristic 
 for the mineral. It is distinguished from the yellow varieties 
 of tourmaline or from colored epidote by the lower double 
 refraction and by the orientation of the absorption, which is 
 strongest in the direction of the principal zone. Now and then 
 
DESCRIPTIVE SECTION 267 
 
 pleochroic halos are present in the mineral. It can be easily 
 isolated because of its insolubility even in cold hydrofluoric 
 acid. It is strongly attracted by an electro-magnet and is 
 infusible before the blowpipe. 
 
 Diaspore (8) 
 
 Diaspore has been positively recognized only as a decomposition product 
 in completely altered rhyolites, in the spreustein of nepheline syenites, in 
 kaolin, in emery, and in other rocks bearing corundum. Its distribution 
 other than this cannot be determined. Small crystals of it are rounded 
 and rather elongated plates. Sharp cracks occur parallel to the direction 
 of the a axis. It is occasionally colored and is then pleochroic. Its in- 
 solubility allows it to be isolated. The development and negative char- 
 acter of the principal zone distinguish it from sillimanite. Higher double 
 refraction and a good cleavage distinguish it from andalusite; parallel ex- 
 tinction and the lack of fibrous fracture from cyanite, and the double re- 
 fraction and cleavage from sapphirine. It is most similar to lawsonite. 
 Aside from its indices and double refraction, it may be distinguished from 
 lawsonite by its inf usibility. 
 
 Cyanite (Disthene) (8) 
 
 Cyanite forms broad, tabular individuals usually poorly 
 bounded with oblique four- or six-sided cross sections. It 
 often occurs with a light blue macroscopic color and perfect 
 cleavage. It is also found in certain piezocontact rocks in 
 radial aggregates or sheaves, which are macroscopic ally black 
 and entirely filled with graphite dust rhaetizite. Twins are 
 very widespread, the macropinacoid being the twinning plane. 
 Closely assembled cracks representing fibrous fracture parallel 
 to the base are frequently observed, especially in bent sections. 
 These are perpendicular to the sharp cracks representing cleav- 
 age parallel to the macropinacoid and to the less regular cracks 
 of the cleavage parallel to the brachypinacoid. A light bluish 
 tint can frequently be discerned in thin sections. Pleochroism 
 can also be recognized if some care is exerted. Deeper colors 
 are rare and a mottled appearance is characteristic of their 
 presence. 
 
 Cleavage plates parallel to the macropinacoid are nearly 
 perpendicular to the negative bisectrix of a large optic angle, 
 Fig. 278. The axial plane forms an angle of about 30 with the 
 cleavage cracks parallel to the brachypinacoid. This makes 
 
268 
 
 PETROGRAPHIC METHODS 
 
 (001) 
 
 the mineral easily distinguished from all other rock constituents. 
 Cleavage plates from twins show a brilliant change of color in 
 parallel polarized light, but no difference in extinction. The 
 fact that macropinacoidal sections of twins show different colors 
 in the two halves, but extinguish nearly at the same time, is the 
 most characteristic feature of the mineral and prevents it from 
 being confused with any other. 
 
 The principal field of distribution of cyanite is in mica schists 
 where it is frequently extremely rich in inclusions of other 
 minerals. It is penetrated by microlites of rutile, tourmaline, 
 and quartz grains in a sieve-like manner. 
 It is often so filled with graphite dust 
 that it appears almost nontransparent. 
 Then its determination in thin section 
 is extremely difficult. In eruptive rocks, 
 especially in cyanite granulites and in 
 pegmatites, it is macroscopically visible 
 and in thin section it is poor in inclu- 
 sions and often distinctly blue. The 
 most beautiful occurrences of the mineral 
 paragonite schists belong here. It is 
 frequently associated with staurolite in 
 such rocks and is often in parallel growth 
 with it. 
 
 In thin section it is quite similar to a 
 whole series of minerals, such as sillimanite, andalusite, topaz, 
 colorless epidote, diaspore, lawsonite, sapphirine, serendibite, and 
 finally light blue hornblende. It is distinguished from all of 
 them by its fibrous fracture, the oblique extinction in sections 
 perpendicular to the negative bisectrix or in the cleavage plates 
 and by the nearly parallel extinction of macropinacoidal sections 
 of twins. 
 
 FIG. 278. 
 Cyanite, Cleavage Plate. 
 
 Sapphirine (8) 
 
 Sapphirine has been positively determined only in rocks from Greenland 
 forming masses in gneiss and having the character of contact rocks. It 
 may be found widely disseminated in the eclogites. The properties of 
 sapphirine in thin section are not very distinctive. It may be discovered 
 if it is blue or bluish-green, but its determination is very difficult if it is 
 colorless or occurs as poorly bounded tabular crystals in parallel, scaly 
 aggregates or those which are very imperfectly radial. It must be isolated 
 with hydrofluoric acid in such cases, but even then it can be identified only 
 
DESCRIPTIVE SECTION 269 
 
 with difficulty. It may be confused with all the minerals mentioned under 
 cyanite. Lack of cleavage distinguishes it from most of them. It is 
 distinguished from serendibite by lack of twin lamination, from corundum 
 by its biaxial properties, and from cordierite by its much higher indices of 
 refraction. 
 
 Serendibite (8) 
 
 This mineral has only been found in the skarn-like border between a 
 pegmatite and limestone in Ceylon. The lack of cleavage, typical twinning 
 lamination, large extinction, and the pleochroism from nearly colorless 
 to sky blue or indigo are sufficient indication of its presence. 
 
 Prismatine (Kornerupine) (8) 
 
 Prismatine and kornerupine, which is closely related to it, are rare 
 minerals in pegmatites and are associated with sapphirine. In thin section 
 they are very similar to sillimanite in form and habit, but may be distinctly 
 separated from it by the negative character of the principal zone. They 
 are distinguished from apatite by higher double refraction, from tremolite 
 and the brittle micas by parallel extinction in all sections, and from anda- 
 lusite by the small optic angle. 
 
 Astrophyllite (9) 
 
 Astrophyllite is not rare in soda rocks as an accessory constituent. It is 
 macroscopically visible in micaceous individuals that are brittle and bronze 
 colored or in star-shaped groups. In thin section it appears as laths with a 
 perfect cleavage and strong pleochroism. It is distinguished from micas 
 by its high indices of refraction, from the brittle micas by its high double 
 refraction, and from yellowish-brown hornblende by the perfect cleavage, 
 which shows only in one direction in sections parallel to the front pinacoid. 
 
 Brittle Mica Group (9) 
 
 Many micaceous minerals are classed together under the term 
 of the brittle micas. They are distinguished from mica and other 
 flaky, micaceous minerals by greater hardness, decidedly higher 
 indices of refraction and lower double refraction. 
 
 The following table is arranged to show a comparison of the 
 properties of these minerals. 
 
270 
 
 PETROGRAPHIC METHODS 
 
 
 
 Index of 
 refraction 
 
 Double 
 refraction 
 
 Extinction 
 
 Optic angle 
 
 Astrophyllite. . . 
 
 
 ITTio-Vi 
 
 High. 
 
 Parallel. 
 
 Medium large. 
 
 Brittle micas. . . 
 
 
 xiign. 
 
 Medium to 
 
 
 Quite large. 
 
 
 j 
 
 
 
 Oblique. 
 
 
 Margarite 
 
 < 
 
 ^uite high. 
 
 Low. 
 
 
 Large. 
 
 Mica 
 
 
 
 
 High. 
 
 } 
 
 Very small to 
 
 
 
 
 
 I Parallel. 
 
 medium large. 
 
 Chlorite. 
 
 
 
 Low. 
 
 
 Very small. 
 
 
 
 
 
 
 
 Clinochlore 
 
 
 
 
 }oh1iniip 
 
 Medium large. 
 
 Hydrargillite. . . 
 
 
 > Low. 
 
 
 
 1 Small 
 
 Tale 
 
 
 
 1 
 
 ] 
 
 J 
 
 
 
 
 I TTio-V 
 
 I Parallpl 
 
 
 Pyrophyllite 
 
 
 
 f fli ^ u - 
 
 
 Medium large. 
 
 Kaolin 
 
 
 
 Low 
 
 Oblique. 
 
 Large. 
 
 
 
 
 
 
 
 They possess a distinct cleavage parallel to the basal pinacoid 
 and one or more imperfect cleavages transverse to that. The 
 optical properties of the brittle micas are not very constant. 
 Differences in index of refraction are uncommonly distinctive 
 and the differences in double refraction are not small. The 
 optical character is sometimes positive and sometimes negative. 
 The color is likewise quite variable. Some are colorless, while 
 others are more or less deeply colored, and then pleochroic. They 
 show stronger absorption of the ray vibrating perpendicular to 
 the cleavage, which is uncommon for micaceous minerals. 
 
 Certain varieties of chloritoid are undoubtedly triclinic. In 
 cleavage plates of such varieties the optical plane lies unsym- 
 metrical to the percussion figure. Other varieties approach the 
 hexagonal system in their behavior. The specific gravity 
 varies within just as wide limits. Most of the brittle micas are 
 not attacked by acids, but some are comparatively easily dis- 
 solved ottrelite. Chemical analysis does not point to a common 
 formula for the brittle micas so that the question of the relation- 
 
DESCRIPTIVE SECTION 
 
 271 
 
 ship of the members must remain open. The two members 
 given in the tables represent the end members of the series with 
 ottrelite, brandisite, clintonite, sismondine, masonite, seybertite, 
 etc., arranged intermediately. Nothing is known about the 
 relative delimitations of these in rocks. 
 
 The brittle micas are widely disseminated constituents^ in 
 contact rocks, especially in cases of piezocontact metamorphism 
 and are found but rarely in the inner contact zone, e.g., in the 
 coarse crystalline silicate fels in the Fassatal and in certain 
 eclogites, etc. On the other hand, they are the most character- 
 istic constituents of the outer contact 
 zone and their principal field of dis- 
 tribution is in such rocks as phyllites. 
 They appear macroscopically in the 
 
 FIG. 279. Chloritoid, Cleavage Plate. 
 
 FIG. 280. Chloritoid, 
 Section Parallel Plane of Symmetry. 
 
 phyllites in elongated or disc-shaped knots ottrelite and occa- 
 sionally also in sheaf-like and radial aggregates. They may be 
 recognized by the greenish-black color and brilliant luster. 
 
 Abundant inclusions are usually observed under the micro- 
 scope, especially in the knots. These are often penetrated with 
 quartz grains, giving them the sieve structure or filled with graph- 
 ite dust until they are nontransparent. Zonal structure, hour- 
 glass development, and abundant twinning lamination are often 
 observed. Index of refraction, double refraction, and particu- 
 larly the pleochroism of the larger individuals, are very important 
 properties but, nevertheless, they are frequently confused with 
 cyanite. The large tabular cross sections of the latter show more 
 distinct cleavage than the basal sections of the brittle micas. 
 Cyanite is also characterized by a larger optic angle. The brittle 
 micas are often confused with bluish-green hornblende from 
 which they can only be distinguished by observations in con- 
 vergent light, particularly in cleavage plates, Fig. 279. 
 
272 
 
 PETROGRAPHIC METHODS 
 
 The brittle micas, as microscopic constituents, are much more 
 widespread than the larger individuals. The development is 
 such that they can hardly be determined positively. The nor- 
 mal occurrence is in small irregular particles, grouped in radial 
 aggregates, in which the graphite content of the rock is so con- 
 centrated that they are nontransparent. These individuals, 
 which often cannot be seen at all until after they have been 
 roasted in an oxidizing flame, are quite light colored and some- 
 times entirely colorless. Twinning and cleavage can scarcely 
 be recognized in them. They are constituents which can be 
 determined more by the general character of the rock than by 
 the properties of the mineral itself. They can be easily isolated 
 from the rock because they are quite heavy and are not attacked 
 very much by hydrofluoric acid. 
 
 Margarite (9) 
 
 Margarite is undoubtedly much more widespread than it has been shown 
 to be up to the present time. It is a constant associate of emery and is 
 also found in contact rocks rich in alumina. It is distinguished from mica 
 by the index of refraction and double refraction, and from colorless chlorite 
 by the former property. It can scarcely be distinguished from the brittle 
 micas in thin section except by the very large optic angle. A lower hard- 
 ness can be observed on the larger isolated individuals. 
 
 Olivine Group (9) 
 
 Olivine, or peridote, is by far the most frequent mineral of the 
 olivine group. Forsterite and monticellite are rare and confined 
 to contact-metamorphosed rocks. Fayalite, ferrous silicate,, in 
 
 crystals often with tridymite and 
 alone in compact, dark brown 
 masses, is characteristic in certain 
 rocks and is probably of pneumato- 
 lytic origin. Chemically, the mem- 
 bers are quite similar, but their 
 optical properties vary greatly. The 
 indices of refraction and the double 
 refraction, likewise the positive optic 
 angle, increase very appreciably with 
 
 the content of iron, while the double refraction is very much 
 lowered by the content of calcium, as noted in monticellite. 
 Members rich in iron fuse more easily than those poor in iron. 
 
 FIG. 281. 
 Olivine Tabular 
 Parallel (100). 
 
 \ 
 
 FIG. 282. 
 Olivine Tabular 
 Parallel (010). 
 
DESCRIPTIVE SECTION 
 
 273 
 
 Olivine with an average content of ferrous iron of from 11-13 
 per cent, is the most important member of the group. Hyalo- 
 siderite is much richer in iron. Olivine is characteristic of the 
 most basic eruptive rocks. It seldom possesses crystal form in 
 the granular rocks of the peridotite series except when numerous 
 individuals are intergrown poikilitically in pyroxene. The 
 same is true in gabbro, norite, and trap. On the other hand, it 
 is one of the first products to form in porphyric rocks, especially 
 in melaphyre and basalt, and less often in andesite, and then it is 
 well bounded crystallographically. It is also found in large 
 amounts in certain soda rocks. A mineral, which appears 
 quite similar and is widespread in contact rocks, has a much 
 
 FIG. 283. Olivine, Section 
 Parallel (100). 
 
 FIG. 284. High 
 Index of Mineral in Canada 
 
 lower content of iron and is classified as forsterite. In general 
 the crystals are short prismatic parallel to the vertical axis or 
 tabular parallel to the macropinacoid, Fig. 281. They are less 
 often tabular parallel to the brachypinacoid, Fig. 282, and that 
 particularly in mixtures rich in lime. They show poorly defined 
 cleavage cracks parallel to the direction of elongation and the 
 plane of the optic axes is perpendicular to them, Fig. 283. Faces 
 and edges are generally rounded and corroded, and indentations 
 filled with the solidified magma are not rare, Fig. 284. Basal 
 sections are generally eight-sided and show two unequal cleav- 
 ages perpendicular to each other, Fig. 285. The mineral is not 
 often prismatic, except when it is rich in lime. Then the prin- 
 cipal zone is parallel to the a axis and is positive. Olivine is 
 
 18 
 
274 
 
 PETROGRAPHIC METHODS 
 
 occasionally found with a skeletal development as a constituent 
 of the ground mass in rocks poor in olivine and containing 
 abundant glass. Then it shows domatic or swallow-tail forms 
 and various thread-like growths, Fig. 196, page 188. 
 
 It may be remarked that olivine is present in numerous rocks 
 of the basalt series in large sharp angular fragments and grains. 
 These can be referred to the inclusions of olivine fels which occur 
 in these rocks. The entire olivine content of the rock may be 
 traced back to crushing of these inclusions. In this form it is not 
 equivalent to the other constituents of a basalt. 
 
 Olivine is macroscopically bottle green (hyalosiderite 
 reddish-brown to golden yellow) and has a distinctive con- 
 
 choidal fracture. Under the micro- 
 scope it is almost colorless, and 
 transparent, but when heated in 
 air it always becomes reddish- 
 brown. The ray vibrating parallel 
 to the vertical axis is then less 
 strongly absorbed. Hyalosiderite 
 shows this property when fresh. 
 Sometimes twinning is noted proba- 
 bly parallel to {011} and this can 
 also be developed in the form of laminations. It is very brittle 
 and frequently shows the cataclastic structure under the influ- 
 ence of mountain-forming processes. 
 
 Its characteristic associate is chromite, which nearly always 
 occurs as small, sharp crystals included in the olivine. Inclusions 
 of other opaque ores and apatite are also found in olivine. Like- 
 wise, the little brown tabular crystals, which are regularly 
 arranged and are so characteristic in hypersthene, occur in olivine 
 of certain gabbros so abundantly that the mineral appears black 
 macroscopically and in thin section it is scarcely transparent. 
 Glass and slag inclusions are not rare in basaltic rocks, and liquid 
 inclusions are found in peridotites as well as in the olivine fels 
 bombs of basalts. 
 
 In certain gabbros olivine is surrounded by a wreath of 
 amphibole arranged radial to the grain. If it borders on feld- 
 spar this wreath is composed of common hornblende, but other- 
 wise it is tremolite. All of the olivine may finally be replaced 
 by a confused fibrous aggregate of tremolite pilite. This is 
 frequently considered to be a result of dynamometamorphism 
 
 (100) 
 FIG. 285. Olivine, Basal Section. 
 
DESCRIPTIVE SECTION 
 
 275 
 
 FIG. 286. 
 
 Hyalosiderite Border around Olivine, Lim- 
 burgite. Limburg on Kaiserstuhl. 
 
 and is genetically entirely different from serpentinization. It 
 may be noted as contradictory to this hypothesis that tremolite 
 is the most frequent by-product in the formation of serpentine. 
 
 In basaltic rocks olivine ap- 
 pears to be generally richer in 
 iron and often shows a brilliant 
 brown border of hyalosiderite 
 with a stronger double refrac- 
 tion, Fig. 286. Similar borders 
 seem to be of secondary origin, 
 the olivine having been altered 
 into a yellow to reddish-brown 
 substance consisting of parallel 
 fibers with very high indices of 
 refraction. This has been de- 
 scribed as goethite, but does 
 not correspond to it in all of its 
 properties. It probably is 
 another ferric hydroxide. 
 
 Alteration into scaly aggregates of talc or granular aggregates of 
 carbonate, which is frequently very poor in magnesia, is noticed 
 especially in melaphyres. Now and then biotite also results. 
 By far the most frequent alteration of olivine is into serpentine, 
 which is generally accompanied by a separa- 
 tion of iron ores. It gives the rock a green 
 color, veined or streaked with various shades 
 of red, brown, black, etc. 
 
 Two kinds of serpentine formations are dis- 
 tinguished in olivine. One begins on the border 
 of the crystals and in the cleavage cracks and 
 forms parallel fibrous aggregates perpendicular 
 to them. It is called fibrous serpentine or 
 chrysotile. New cracks result from the in- 
 crease in volume accompanying the change 
 and these are filled with fine fibrous aggre- 
 gates of serpentine. This gives rise to the 
 typical mesh structure shown diagrammati- 
 cally in Fig. 287, and as it appears in thin section, Fig. 218, 
 page 199. Checked olivine may be present within the meshes 
 or this may be entirely changed to a compact aggregate of 
 serpentine. 
 
 FIG. 287. Olivine 
 Beginning Alteration 
 to Chrysotile, Mesh 
 Structure. 
 
276 
 
 PETROGRAPHIC METHODS 
 
 In other rocks, stubachite, one may note that the fresh olivine 
 is crossed by numerous lamellae of flaky serpentine anti- 
 gorite. These are mostly orientated parallel to {011} and form 
 a system of laths cutting each other at an angle of about 60, 
 
 Fig. 288. The angular parts of 
 the olivine lying between them 
 alter into irregular, scaly aggre- 
 gates of antigorite beginning at 
 the laths and progressing out- 
 ward. When serpentinization 
 is complete, the intersecting 
 lamellae produce the so-called 
 lattice structure. In both these 
 cases the process of serpentini- 
 zation cannot be looked upon 
 as a weathering process; see 
 Allgemeine Gesteinskunde, page 
 FlG A 2 88 -T u e ? nte 'f with 152. Sometimes alteration of 
 
 Antigorite. Lattice Structure. 
 
 olivine, very rich in iron, 
 
 produces a flaky aggregate of serpentine orientated in a simpler 
 manner. It has a rust brown color and is called Iddingsite, 
 see under serpentine. 
 
 The significance of fayalite as a rock constituent has not been investigated 
 very much, but it is known principally in druses and in mineral veins. 
 Possibly some of the formations called hyalosiderite belong to it. At any 
 rate olivine in lamprophyric rocks is often very rich in iron on the border 
 having brownish color and showing increase in the double refraction. 
 
 Titanium olivine is quite similar to it. It is macroscopically dark brown 
 and yellow to orange in thin section. It is probably monoclinic, but occurs 
 in grains laminated by twinning and these appear similar to chondrodite. 
 a = 1.669, /?-1.678, r = 1.702. r-a = 0.033, 2V = 63, p<v, a orange, 
 6 <= c light yellow. Sp. gr. = 3 . 25. They can only be distinguished by the 
 difference in extinction in sections showing twinning lamination and the 
 optical orientation of such a section, as shown in Figs. 289 and 290. 
 
 Forsterite has only been observed in contact-metamorphosed limestones 
 and dolomites. It is widely disseminated in them, but is generally not 
 macroscopically visible. When the carbonate predominates it forms 
 rounded, colorless crystals, often containing quite large inclusions of iron 
 spinel and calcite. If it or other silicates are the principal constituents, 
 it forms irregular grains with the properties of olivine. Eozoon results by 
 serpentinization which takes place in the ordinary manner and is quite 
 widespread. 
 
 Monticellite is analogous to forsterite in its distribution, but is much 
 rarer. The lower double refraction is especially noteworthy and distin- 
 
DESCRIPTIVE SECTION 
 
 277 
 
 guishes it from all other olivine minerals. It forms isolated, colorless 
 rounded crystals in contact limestones and these often show incipient 
 serpentinization. 
 
 Confusion of titanium olivine with chrome chondrodite has already been 
 mentioned. Forsterite, colorless olivine, and monticellite often appear 
 quite similar to diopside, but the latter shows sharper cleavage cracks, 
 larger extinction, and has the optical plane orientated parallel to the 
 principal zone. They can be distinguished from colorless humite only 
 by a somewhat stronger dispersion of the optic axes if the cleavage does 
 
 FIG. 289. Chondrodite, 
 
 FIG. 290. Titanium Olivine, 
 
 Optical Orientation with Respect to the Twinning. 
 
 not appear. If the cleavage shows distinctly the optical plane in humite 
 lies parallel to it, in the olivine minerals perpendicular. In other cases the 
 olivine is distinguished by its solubility in hydrochloric acid and by its 
 ability to assume a brown color, which increases with the content of iron, 
 when it is roasted in an oxidizing flame. This latter property can serve 
 also to distinguish olivine from forsterite and monticellite. 
 
 Pyroxene Group (10) 
 
 Orthorhombic and monoclinic pyroxenes frequently occur in 
 parallel or lamellar intergrowths. The triclinic members play 
 no r61e among rock-forming minerals. From this fact the ori- 
 entation of the orthorhombic pyroxenes in the table is so chosen 
 that the acute prism angle lies to the front and the transverse 
 axis is the shorter diagonal, although this is not the usual posi- 
 tion. Prismatic cleavage is nearly always distinct and if other 
 cleavages are present the one parallel to the front pinacoid 
 tends to be the most perfect. It occurs particularly in ortho- 
 rhombic pyroxenes and in diallage. Cleavage plates of the 
 former parallel to this face are parallel to the optic plane, 
 while those of the latter are almost perpendicular to an optic 
 axis showing quite strong dispersion. A large extinction angle 
 
278 
 
 PETROGRAPHIC METHODS 
 
 (001) 
 
 (001) 
 
 FIG. 291. Enstatite, 
 
 FIG. 292. Hypersthene, 
 
 Section Parallel (100). 
 
 (100) 
 
 (010) 
 (110) 
 
 (nor 
 
 
 I (010) 
 
 /(HO) 
 
 Fia.^293. Enstatite and Hypersthene, 
 
 (100) 
 FIG. 294. Diallage, 
 
 Cross section. 
 
 FIG. 295. Diopside (Diallage), 
 
 (001) 
 
 FIG. 296. Aegirine, 
 
 Section Parallel Plane of Symmetry. 
 
DESCRIPTIVE SECTION 279 
 
 characterizes the monoclinic pyroxenes with the exception of 
 those rich in sodium. They are further distinguished from the 
 orthorhombic members by a much higher double refraction and 
 by pleochroism, which is particularly typical for orthorhombic 
 pyroxenes. It is also noteworthy that, unlike the amphibole 
 group, the double refraction increases with the content of sodium. 
 The pyroxenes can be isolated quite easily from rocks by hydro- 
 fluoric acid because they are very slowly attacked by it. One 
 of the most important means for determining all pyroxenes is 
 the very good cleavage parallel to the prism forming an angle 
 of approximately 90. This distinguishes them from the am- 
 phiboles with a more perfect prismatic cleavage forming an 
 obtuse angle. 
 
 (a) Orthorhombic Pyroxenes 
 
 Enstatite and bronzite are low in iron and may show various 
 shades of brown macroscopically. Hypersthene is high in iron 
 and is black. They form large irregular individuals in the granu- 
 lar rocks of the gabbro-peridotite series and in the olivine fels 
 inclusions in basalt, but hypersthene is not so common as the 
 others. They show very perfect cleavage parallel to the macro- 
 pinacoid, but always in displaced cracks, and generally have a 
 metallic shimmer caused by parallel orientated inclusions of 
 brown tabular crystals and laths (ilmenite?). This passes over 
 into golden yellow upon beginning alteration into serpentine 
 bastite. Macroscopically, they are generally various shades of 
 brown, but in thin section only those members rich in iron are 
 colored, and then they show characteristic pleochroism, but no 
 distinct difference in the intensity of the colors. Very frequently 
 an indistinct twinning lamination occurs, which passes over into 
 the formation of fine fibers. Likewise lamellar intergrowths 
 with diallage can be distinctly discerned even in sections showing 
 parallel extinction, because of the different optical orientation 
 of orthorhombic and monoclinic pyroxenes. 
 
 Well developed crystals, invisible macroscopically, are widely 
 disseminated in porphyrites and andesites, and in these hyper- 
 sthene generally predominates. The crystals are short prismatic 
 with rounded pyramidal ends and in the vertical zone they are 
 principally bounded by the end faces, Fig. 293. They are often 
 rich in inclusions of light colored glass or slag. In these the 
 cleavage is only prismatic and pleochroism becomes a char- 
 
280 PETROGRAPHIC METHODS 
 
 acteristic property. Parallel intergrowths with monoclinic py- 
 roxenes are found in which the latter form a shell around a 
 center of the orthorhombic variety. Glass inclusions and the 
 rounding of the edges are frequent, but orientated inclusions 
 and fibrous structure are lacking. A second generation of hy- 
 persthene in microlitic development is observed now and then 
 in the ground mass of andesitic rocks. 
 
 Alteration is comparatively frequent, especially that into ser- 
 pentine which affects the orthorhombic pyroxenes, particularlv 
 when they occur as constituents of serpentinized olivine rocks. 
 An analogous alteration is also observed in the crystals in por- 
 phyries. A parallel, scaly aggregate of antigorite is formed 
 progressing from the transverse cracks outward, and the faces, 
 parallel to which antigorite possesses perfect cleavage, lie parallel 
 to the macropinacoid of the pyroxene. Cleavage plates of the 
 pseudomorphs bastite thus formed, are perpendicular to the 
 acute bisectrix and the optic angle varies as is characteristic for 
 serpentine. The names diaclasite, protobastite, etc., refer to 
 masses of serpentine with pyroxene which is still fresh. Horn- 
 blende is formed now and then at the expense of orthorhombic 
 pyroxene in granular rocks. 
 
 Aside from the parallel extinction, orthorhombic pyroxenes are 
 distinguished from the monoclinic members by a much lower 
 double refraction and from aegirine by the positive character 
 of the principal zone. They are distinguished from andalusite, 
 olivine, etc., by a more distinct development of the cleavage and 
 by the character of the principal zone, which is positive in ortho- 
 rhombic pyroxenes, negative in andalusite, and variable in 
 olivine. 
 
 (b) Monoclinic Pyroxenes 
 
 Monoclinic pyroxenes, free from sesquioxides, are called diop- 
 sfde normally. It is one of the most characteristic minerals in 
 contact rocks rich in lime and occurs in short prisms or grains, 
 which are macroscopically light grayish-green or less often sap- 
 green. It is found in eclogites, as omphazite containing a little 
 aluminium and in certain peridotites as chrome diopside con- 
 taining chromium. It is sometimes blackish-green in heden- 
 bergite rich in iron, which occurs only in the silicate aggregates 
 known as skarn. The mineral is generally colorless in thin sec- 
 tion, but may show a pale greenish tinge. The deeper colored 
 
DESCRIPTIVE SECTION 
 
 281 
 
 varieties are also green under the microscope and hedenbergite 
 is deep green, but shows no pleochroism. The presence of pleo- 
 chroism, except in the case of fassaite, seems to be connected 
 with the presence of alkali silicates. 
 
 Pyroxene similar to diopside is found in acid eruptive rocks 
 from granite to diorite malacolite and especially in the basic 
 differentiation products from minette to kersantite. In certain 
 varieties of trap, a diopside occurs along with common augite, 
 described below, but it is distinguished by a high content of 
 magnesia salite. A very small optic angle is observed in these 
 occurrences. 
 
 In general they form short prismatic crystals, Fig. 297, or 
 tabular crystals parallel to the orthopinacoid, Fig. 298. The 
 cleavage shows quite distinctly in thin section. They are more 
 or less crystallographically bounded even where the common 
 augite forms the interstitial material. The form of the pyrox- 
 
 \ 
 
 FIG. 297. Augite, 
 Short Prismatic. 
 
 FIG. 298. Augite, Tabular 
 Parallel (100). 
 
 enes in the extrusive rocks of the acid and intermediate series 
 is quite similar to the above. Pyroxenes are found as phenocrysts 
 especially in andesites. Diallage includes those varieties which 
 are distinguished from diopside by a low content of alumina. It 
 shows basal parting macroscopically as well as parting parallel 
 to the orthopinacoid, and also twinning laminations parallel to 
 it. It has a metallic chatoyancy and the individuals, which occur 
 particularly in gabbro and peridotite, are quite distinctly visible 
 macroscopically. Under the microscope it is often somewhat 
 pleochroic and aside from its occasional fibrous properties, is 
 marked by the regular arrangement of small, brown, tabular 
 inclusions, mentioned under bronzite. It is not to be considered 
 as an independent mineral species. 
 
 Fassaite, which is chemically quite closely related to diallage, 
 plays a peculiar role. It is found principally as a constituent of 
 
282 PETROGRAPHIC METHODS 
 
 contact rocks, but is also quite important in monzonites. It 
 corresponds to aegirine, described below, in its optical properties, 
 especially in the pleochroism and strong inclined dispersion, but 
 differs greatly in chemical constitution. The sharp pyramidal 
 habit exhibited by the crystals in an extraordinary manner is 
 distinctive for it. 
 
 There is a series of gradual transition members from diopside 
 to common or basaltic augite, the latter being characterized by a 
 higher content of alumina. The mineral is always black macro- 
 scopically, but under the microscope it generally has only a very 
 pale, brownish tint and is not normally pleochroic. Common 
 
 FIG. 299. Augite as Interstitial Material. FIG. 300. Simple Twinning 
 
 Diabase. Arran, Scotland. Lamellae in Augite. 
 
 augite is disseminated first of all in the basic eruptive rocks, and 
 in porphyric rocks it often occurs in two generations and the 
 first of these often forms phenocrysts of considerable size. The 
 occurrence of diabase augite as interstitial material, Fig. 299, 
 is especially distinctive. The formation of twins is quite common 
 in common augite, especially in the form of isolated lamellae. 
 Juxtaposition and penetration twins are also found and the 
 hourglass structure, which is frequently observed, is also looked 
 upon as complicated twinning. The twinning lamellae are ar- 
 ranged parallel to {lOO} and, therefore, in a pyramidal section 
 they lie oblique to the cleavage parallel to the prism faces, 
 Fig. 300. Many kinds of forms of growth, crystal skeletons, etc., 
 are found in glassy eruptive rocks. 
 
 A very large extinction angle is a characteristic of this augite 
 and it shows particularly in comparison with the hornblende, 
 
DESCRIPTIVE SECTION 283 
 
 which occurs as a border around such augite. If the small 
 content of titanium, which is usually present, increases a dis- 
 tinct violet color generally appears in thin section. Such titan- 
 ium augites are somewhat pleochroic. The ray vibrating parallel 
 to b is colored deep violet. Sections with the most oblique 
 extinction show the least pleochroism. Locally, reddish to 
 brownish-red colors are observed, and these can be referred to a 
 content of manganese. However, some occurrences of greenish 
 or brownish augites assume that color when roasted in air. 
 The color of augite is rarely yellow. 
 
 Alteration in diopside and augite is very widespread. In the 
 first case the decomposition gives rise to fibrous scaly aggregates, 
 which are generally considered to be serpentine, but which 
 scarcely belong to it. Augites rich in. aluminium are generally 
 changed to chlorite. Low doubly refracting aggregates of it 
 are congregated on the borders and in cleavage cracks of the 
 individuals, and give rise to the formation of the greenstone 
 facies of augite rocks. They are usually penetrated by grains of 
 epidote and calcite, and frequently also by small needles of 
 hornblende. They include small individuals of titanite, anatase, 
 and opaque ores. It is noteworthy that this alteration is every- 
 where accompanied by the formation of pyrite. Sometimes an 
 aggregate of quartz with carbonates and rust occurs instead of 
 the green minerals in a rock that is entirely decomposed and 
 changed to wache. This kind of alteration appears in its entire 
 distribution to be a direct antithesis to the first. 
 
 Uralite is an especially interesting pseudomorph. It is gen- 
 erally green, but sometimes bluish hornblende in the form of 
 augite. It was generally considered as a product of paramor- 
 phism, but it is a typical product of pneumatolytic hydatogenic 
 activity in which anhydrous augite passes over into hornblende 
 containing an hydroxyl. This is frequently accompanied by an 
 intensive alteration of its entire composition. In porphyric 
 rocks uralite porphyrite the form of the augite is perfectly 
 retained and the formation of fibrous hornblende attacks the 
 whole crystal without change of form. In granular rocks on 
 the other hand, e.g., in uralite gabbro, the newly developed 
 hornblende grows like brushes spreading out over the crystals 
 so that uralitization, which frequently goes hand in hand with 
 saussuritization, entirely obscures the structure of the rock. 
 
 Although the alteration first mentioned is interpreted as 
 
284 PETROGRAPHIC METHODS 
 
 weathering, the latter type is a typical result of dynamometamor- 
 phism, see Allgemeine Gesteinskunde page 151. Recrystallization 
 from magmatic waters may be the cause, and it cannot be doubted 
 in the alteration to seladonite where an augite rock free from 
 potassium is transformed into a green silicate rich in potassium. 
 This occurs in a very characteristic manner in the melaphyres 
 of the Fassatal. 
 
 Augite is itself not rare as a secondary product, and forms 
 the principal constituent of resorbed residues of hornblende or 
 mica in extrusive rocks. It is a typical product of crystallization 
 from a magma after its extrusion upon the surface and the dark 
 minerals formed during the intratelluric period of the magma are 
 aggregates of augite grains mixed with opaque ores. 
 
 Diopside is often uncommonly plastic when subjected to oro- 
 genic pressure, and its prismatic individuals are frequently 
 greatly bent. Augite and diallage are more inclined to the de- 
 velopment of cataclastic structure, which strangely enough occurs 
 just as perfectly developed in young eruptive rocks, which have 
 not suffered any kind of deformation, and is therefore a protoclase. 
 
 The members of the series containing sodium, leading from 
 augite or diopside to aegirine, are confined to the soda rocks, and 
 are characterized as a whole by distinct pleochroism, which in the 
 normal occurrences alternates between yellow and green shades, 
 and is deeper green the richer they are in sodium. If they 
 contain titanium, as is frequently the case in basalts, they show 
 a violet color principally in the direction of B. The titanium 
 augites containing sodium are strongly pleochroic and the hour- 
 glass structure, mentioned on page 188, is especially conspicuous 
 in them. The members containing sodium, as a whole, fre- 
 quently show fine zonal development in which the very narrow 
 zones alternate with each other in an extremely regular manner. 
 Sometimes a more or less intensely corroded and irregular grain 
 is surrounded by shells, which are crystallographically well 
 defined. In the soda rocks it may be observed that in the 
 poorly bounded pyroxenes, which are frequently tabular parallel 
 to the front pinacoid, the core is almost colorless, non-pleochroic, 
 and is manifestly closely related to diopside. Toward the 
 border it passes into aegirine augite and finally the outer zone 
 is aegirine. Intergrowths with hornblende are not at all rare 
 but then, unlike the augite, hornblende containing sodium is in 
 the center and the pyroxene grows around it. 
 
DESCRIPTIVE SECTION 
 
 285 
 
 With reference to extinction the monoclinic pyroxenes differ 
 from each other in a distinctive manner. From diopside to 
 common augite, and from this to aegirine, the angle between 
 the positive bisectrix and the vertical axis becomes constantly 
 greater, Fig. 301. 
 
 The soda pyroxenes show crystal form particularly in the 
 porphyric rocks in which they occur in two generations. The 
 large phenocrysts often show tabular development parallel to 
 {100} while the second generation is 
 always richer in sodium and is fine 
 acicular showing a radial development. 
 In other cases, the latter type forms 
 short irregular specks, which are often 
 so intergrown with small nepheline crys- 
 tals that they are scarcely transparent. 
 The soda pyroxenes, especially acmite, 
 which is generally spear-shaped, show 
 extremely irregular forms, but not to 
 the same degree as the corresponding 
 amphiboles. Manifold crystal skeltons 
 are found in glassy rocks. It cannot be 
 positively determined whether the forms 
 of growth illustrated on page 188 are aegirine or hornblende. 
 
 Sodium augite and the titanium augites, belonging to it, are 
 characterized by a strong inclined dispersion, as a result of which 
 sections parallel to the plane of symmetry give an extremely 
 incomplete extinction, and in sections perpendicular to the optic 
 axis very brilliant anomalous interference colors are seen. The 
 difference in this respect from diopside is notable. It has a very 
 small dispersion and, therefore, its extinction in the plane of 
 symmetry is very complete. Alteration of the soda pyroxenes 
 is rare. Sometimes aggregations of opaque ores, which are 
 difficultly determinable, occur in place of them in entirely 
 decomposed keratophyres. 
 
 Although in general a regular relationship between the optical 
 properties and the chemical composition of monoclinic pyroxenes 
 cannot be denied, yet investigations to determine the chemical 
 composition merely from the optical properties lead to rather 
 negative results. Minerals of the aegirine- augite series, which 
 are designated as fedorowite or violaite, are in a way not to be 
 considered as an independent species, but chemical analysis 
 
 FIG. 301. Extinction of 
 Monoclinic Pyroxenes. 
 
286 
 
 PETROGRAPHIC METHODS 
 
 shows great deviations from what would be expected from the 
 optical properties. The pyroxene group is built up in too com- 
 plicated a manner in contradistinction to the plagioclase group, 
 in which such conclusions appear to be well justified. 
 
 Colorless pyroxenes are most frequently confused with olivine 
 or epidote, but are distinguished from both by the position of the 
 optical plane with respect to the zone of cleavage. The colored 
 members are sometimes very similar to common hornblende and, 
 if the depth of color is the same in each, the absorption of the 
 latter in the principal zone is stronger. The extinction is less 
 with the exception of aegirine, which can be recognized however 
 by its sap green color, higher double refraction, and the negative 
 character of the principal zone. Under certain circumstances, 
 the cleavage angle of pyroxene approximates that of hornblende 
 in oblique sections. If these sections have parallel extinction 
 the optic plane lies in the bisector of the acute cleavage angle 
 in pyroxene and, in general, in the bisector of the obtuse cleavage 
 angle in hornblende. 
 
 Soda pyroxenes poor in iron or free from iron are comparatively rare as 
 rock constituents. They are macroscopically white to green, but under the 
 microscope generally colorless. Jadeite (NaAlSi 2 O 6 ) belongs especially to 
 this group. It occurs only in fine columnar masses with properties like 
 
 saussurite, in the serpentine from Burmah, 
 Turkestan, etc., but is widely known from the 
 stone age when it was worked up. It is easily 
 distinguished from the similar nephrite by its 
 low fusibility. Further, spodumene must be 
 mentioned. It occurs as a constituent of 
 pegmatite in clear beautifully colored crystals, 
 emerald green hiddenite and rose-red kuntzite, 
 and as a constituent of other rocks, of granites 
 and injected schists, in poorly developed gray- 
 ish-green needles, which are purely accessory. 
 
 Lawsonite (10) 
 
 -# (ooi) 
 
 FIG. 302. Lawsonite, Section 
 Parallel (010). 
 
 Lawsonite has been found as a constituent 
 of certain eclogites. It forms large lobated 
 particles in them without any characteristic 
 microstructure. The mineral is also suspected 
 in certain occurrences of saussurite, but the 
 
 determination does not seem to be reliable because of the lack of 
 characteristic properties of the mineral and its poor development, which 
 all the minerals in saussurite show. In such aggregates confusion with 
 hornblende or prehnite is natural, but it is distinguished from the other 
 minerals of the aggregate by much stronger double refraction. Lawsonite 
 
DESCRIPTIVE SECTION 
 
 287 
 
 is distinguished from hornblende by its constant parallel extinction and 
 the direction of the cleavage cracks in macropinacoidal sections and prehnite 
 is distinguished by the characteristic rosette arrangement in its cross sec- 
 tions. Stronger double refraction and positive character of the principal 
 zone distinguish it from andalusite and the position of the optical plane 
 parallel to the cleavage from wollastonite, while the distribution distin- 
 guishes it from both. 
 
 Amphibole Group (11) 
 
 The minerals jof the amphibole group have a very perfect 
 cleavage parallel to the prism making an angle of about 124. 
 They always show a prismatic development, Fig. 303, 
 but may be short, thick forms like the crystals of 
 pargasite or basaltic hornblende, or the grains of 
 common hornblende. They may be long, needle-like 
 and fibrous, as is often the case in the light green 
 hornblendes of green schists and the secondary aggre- 
 gates in diabase. The crystal form is nearly always 
 observed in the principal zone and consists of the 
 prism and clinopinacoid. The amphiboles have a FIG 303 
 tendency to show a defective development on the Hornblende, 
 ends, when they occur as phenocrysts in porphyries. 
 Compare with aegirine for the occurrence of very 
 delicate crystal skeletons in certain pitchstones. Zonal structure 
 and parallel growths of various members of .the series are ob- 
 served, although they are not numbered among the character- 
 
 FIG. 304. 
 Actinolite, 
 
 FIG. 305. 
 Basaltic Hornblende, 
 
 FIG. 306. 
 Glaucophane, 
 
 Section Parallel Plane of Symmetry. 
 
288 
 
 PETROGRAPHIC METHODS 
 
 istic features as is the case with the soda pyroxenes. See under 
 
 pyroxenes for parallel growth with them. 
 
 Unlike the pyroxenes, with which they can be confused in 
 
 color, index and double refraction, the amphiboles have a very 
 
 small extinction angle of 
 from to 25 with the 
 exception of the rare group 
 of cataphorites. This is 
 shown in Fig. 308 in which 
 the relationship of c: c is 
 diagrammed for the most 
 important members. This 
 
 FIG. 307. Cross Section of Hornblende. angle has a Very small 
 
 value in the amphiboles 
 
 free from alkali, but in soda hornblende the angle c : a is small. 
 The minerals of the amphibole group are generally deep colored 
 and show various colors even in thin section. With the ex- 
 ception mentioned above, the ray vibrating in the principal zone 
 is most strongly absorbed. The plane of the optic axes is nearly 
 always parallel to the plane of symmetry. Only in the case of 
 blue hornblendes rich in sodium are 
 these two planes perpendicular. The 
 chemical composition of the minerals 
 belonging in this group is quite varia- 
 ble and is in general analogous to that 
 of the pyroxenes, but differs from 
 them in the content of the hydroxyl, 
 which is never absent. In conse- 
 quence of this fact, the deep seated 
 rocks are the ones which contain the 
 hornblende, while augite takes its 
 place in the extrusive rocks. It is 
 further observed that hornblende is 
 changed to augite by the action of 
 heat, magmatic resorption, while hydro- 
 chemical processes have the opposite 
 effect and form uralite. The specific 
 gravity and indices of refraction are lower in hornblende than in 
 the pyroxenes, which have an analogous composition, but the 
 amphiboles are more strongly magnetic. 
 
 Pleochroic halos with brown colors are common. They are 
 
 FIG. 308. Extinction on Mono- 
 clinic Amphiboles. 
 
DESCRIPTIVE SECTION 
 
 289 
 
 also observed in uralite but not a trace of them can be found in 
 the original pyroxene. The index and double refraction are 
 higher in the halos. 
 
 The amphiboles are not difficultly fusible and those rich in 
 sodium even belong to the most easily fusible minerals. Green 
 hornblende turns brown upon roasting gently, and it assumes the 
 absorption and optical orientation of basaltic hornblende. 
 
 Anthophyllite and gedrite, which are considered orthorhombic 
 because of the parallel extinction, are found sporadically in 
 amphibolites and are predominant in anthophyllite schist. 
 They are also found in certain serpentines. They form flaky, 
 columnar or tufted aggregates, and less often small acicular 
 crystals. They possess a brownish color. Alteration into talc 
 and serpentine is known. 
 
 Tremolite usually appears in fine radiating needles with white 
 or pale greenish color. These occur in granular limestone and 
 dolomite, or in serpentine. In the latter rock there are all 
 possible transitions from trem- 
 olite to actinolite, which is 
 always distinctly green macro- 
 scopically, and in thin section 
 it is generally somewhat 
 colored, but is scarcely pleo- 
 chroic. Both types are some- 
 times found in larger prisms. 
 The occurrence in serpentine, 
 which is uncommonly wide- 
 spread, especially in the micro- 
 scopical development, must 
 be looked upon as a secondary 
 by-product in the formation 
 of serpentine. Sometimes the 
 transition from olivine direct to such aggregates of amphibole 
 with spherulitic structure or that similar to snow crystals is 
 observed pilite. An analogous development also occurs in 
 nephrite, which is usually light green, macroscopically, and 
 consists of a dense mat of actinolite needles. It represents the 
 toughest of all rocks. Its occurrence is also connected with 
 serpentine. Poorly developed individuals of tremolite and 
 actinolite are often bent by mechanical stresses and are crushed 
 into isolated columns, which show a parquetry appearance in 
 
 19 
 
 FIG. 309. Distorted Actinolite in Serpentine, 
 Rifflkees. Stubachtal, Salzburg. 
 
290 PETROGRAPHIC METHODS 
 
 cross section, Fig. 309. It is most similar to the normal occur- 
 rence of prehnite and is distinguished by the obtuse angle of the 
 fragments. These minerals also form fibrous individuals and 
 pass over into asbestos. 
 
 Macroscopic varieties, with a sap green color, which are often 
 extremely fine fibrous and generally contain some alumina and 
 occur as pseudomorphs after diallage, are called smaragdite. 
 In thin section it is colorless to light green and often shows the 
 structure and inclusions of diallage quite distinctly. 
 
 Among the amphiboles free from alumina, griinerite (Fe- 
 Mg)SiO 3 is remarkable because of its similarity to epidote. 
 n = l.7. f- a = 0.03-0. 056. tt = B colorless, c light yellow. 
 Sp. gr.=3.3. c:c=15. 2E = 95. Optically positive. Usu- 
 ally shows twinning lamination parallel to the orthopinacoid. 
 Brownish cummingtonite, (MgFe)Si0 3 , may also be mentioned. 
 a = B light yellow, t light brownish, c : c = 15 and 7- - a = . 022. 
 The petrographical importance of these is but little known. 
 
 Edenite, an amphibole poor in iron and rich in alumina, is 
 macroscopically light green, but generally colorless in thin section. 
 It cannot be distinguished from those already described. It 
 is found in isolated, rounded and greatly corroded crystals with 
 a short prismatic development in granular limestones and lime- 
 silicate fels together with pargasite, which is richer in iron and is 
 macroscopically darker, being green, or brown to black. It 
 forms the transition to common green hornblende rich in alumina 
 and ferrous iron. The latter appears blackish-green macro- 
 scopically. It is found in compact, rarely elongated, prismatic 
 grains in the acid and intermediate rocks of the normal series, 
 and shows well developed crystals in the porphyric forms of these. 
 Very strong absorption is always distinctly observed, and iso- 
 lated twinning lamellae parallel to the orthopinacoid are not 
 rare. In contact rocks the cross sections of hornblende often 
 show a bluish-green color and transitions to carinthine, which 
 is deep bluish-green in thin section, are observed especially in 
 the sheaf-like aggregates of the Garbenschiefer. Hornblende 
 showing a similar development is very widespread in amphi- 
 bolites in which it sometimes forms short shreds and sometimes 
 parallel columnar aggregates with frayed ends sedgy horn- 
 blende. The latter form of development is very similar to the 
 uralite pseudomorphs after augite found in numerous basic 
 eruptive rocks. In such rocks the mineral is often very light 
 
DESCRIPTIVE SECTION 291 
 
 .colored and is developed in long needles. It is then very difficult 
 to distinguish from similar occurrences of actinolite without mak- 
 ing a thorough chemical investigation. The stronger absorption 
 of the ray vibrating in the direction of elongation is nearly always 
 distinctly observable in common hornblende. Green hornblende 
 with such a development is, along with chlorite, the most ^fre- 
 quent green pigment of schistose rocks. 
 
 A brownish-green variety of hornblende occurs in place of the 
 green in porphyrites. The crystals are short prismatic and the 
 ray vibrating parallel to b is very distinctly brownish. This 
 passes over into pure brown varieties rich in ferric iron, which are 
 lighter or deeper colored corresponding to the content of iron. 
 The lightest colored varieties of common, brown hornblende are 
 macroscopically brownish-black, and they replace the green 
 entirely in deep seated basic rocks, e.g., in gabbro. When horn- 
 blende is primary in such rocks it belongs to the brown variety. 
 Secondary processes analogous to the formation of uralite alter 
 the brown, primary hornblende into secondary green, sedgy horn- 
 blende. Tonalite and certain granulites are the principal acid 
 rocks which contain brown hornblende. It is also found as 
 phenocrysts in numerous porphyric rocks and is especially char- 
 acteristic for certain lamprophyres in which small needles of 
 brown hornblende are the principal constituent of the ground 
 mass. They are always replaced by augite in the ground mass of 
 extrusive rocks. 
 
 If titanium dioxide also occurs in varieties rich in iron, the 
 mineral is macroscopically jet black and has a pitchy luster. 
 The cleavage faces are exceptionally brilliant and in thin section 
 the absorption in the principal zone is more complete, while the 
 extinction diminishes to 0. This series is designated as basaltic 
 hornblende and contains up to 5 per cent. Ti0 2 . It is widespread 
 in numerous extrusive rocks in which the crystals are long or 
 short prismatic, and are well developed. They are frequently 
 zonal with a light center and darker borders. They show twin- 
 ning parallel to the orthopinacoid most frequently and this some- 
 times appears as isolated lamellae or as two equal parts of twins. 
 The indices and double refraction are considerably higher than that 
 of the other amphiboles. On the border it often shows a zone 
 with stronger absorption. It has resulted from the action of the 
 magma on the mineral after it was formed magmatic border. 
 This leads to magmatic resorption in which pseudomorphs of 
 
292 PETROGRAPHIC METHODS 
 
 augite and magnetite after hornblende are formed. Finally 
 even the form is destroyed. Thus, in the tuffs, large crystals of 
 basaltic hornblende are present but in the extrusive rocks 
 specks very rich in iron are found as a residue from them. The 
 varieties richest in titanium, 6-7 per cent. TiO 2 , are called Kar- 
 sutite. 
 
 Amphiboles containing aluminium and ferric oxide suffer the 
 same alterations as common augite, but not to the same extent. 
 A distinction is made between chloritization, which is always 
 accompanied by the formation of epidote, carbonates, and titan- 
 ium minerals, and the alteration into quartz, carbonates, and 
 rust. Amphiboles, like the pyroxenes, are resistive to acids. All 
 green members assume a brown color and stronger absorption 
 when heated in air. Those members richer in iron show the op- 
 tical properties of basaltic hornblende after roasting. 
 
 There are many members of the amphibole group that are rich 
 in sodium. Two groups may be distinguished. The one is poor 
 in ferric iron and rich in alumina and occurs chiefly in contact 
 rocks, especially in eclogites. The other is rich in ferric iron and 
 is distributed principally in the rocks of the soda series. 
 
 There are transition members from common, green hornblende 
 to carinthine, which is macroscopically bluish-black, and in thin 
 section is deeply colored. The bluish-green color of the ray 
 vibrating in the direction of elongation is characteristic for it. 
 It passes over into glaucophane or gastaldite, which is a very 
 delicate blue in thin section. The latter minerals are character- 
 istic constituents of eclogites and rocks embedded in them. The 
 columnar individuals with a dark violet blue color are often 
 macroscopically visible. They are best recognized in thin section 
 by the blue color and the splendid pleochroism. Zonal structure 
 is found in them and some of the zones may have a very low 
 double refraction. It is not uncommon to see the center entirely 
 colorless with a low double refraction and small extinction, and 
 surrounded by a brilliant blue envelope. Crocidolite, which is 
 externally not unlike glaucophane, but is always fibrous and 
 richer in iron, often occurs as a secondary development in crevices 
 in amphibolites and related rocks. It is distinguished by a 
 comparatively high double refraction. It has a larger extinction 
 angle than the other soda hornblendes rich in iron, and the char- 
 acter of the principal zone is different. The ray vibrating in the 
 principal zone is the most strongly absorbed, as is the case in 
 
DESCRIPTIVE SECTION 293 
 
 nearly the whole hornblende group. It is remarkable that the 
 same phenomena are seen in crossite and in the other soda horn- 
 blendes in which the optical plane is transverse to the plane 
 of symmetry, and the optic normal forms an angle of 25 or less 
 with the vertical axis. Even in this case the principal zone 
 absorbs the rays most strongly. 
 
 Another series branches off from the basaltic hornblende. 
 It consists of soda amphiboles rich in ferric iron. Barkevikite, 
 which stands next to it, is quite rich in titanium. It is brown and 
 can only be distinguished by the larger extinction, c:c = about 
 14. It is found especially in nepheline syenites. A character- 
 istic series of amphibole minerals, cataphorite, seems to be 
 related to barkevikite. They are rarer, have larger extinction, 
 c:c = 30-60, and a peculiar reddish color. They are distin- 
 guished from all the other amphiboles by strong absorption 
 transverse to the principal zone, B deep brownish-red to violet, 
 c yellowish-green, a reddish-yellow, B>c>tt. Cataphorite is 
 very frequently surrounded by arfvedsonite, which is free from 
 titanium and distinguished by a bluish-green color. Like the 
 amphiboles free from sodium, it shows strong absorption in the 
 principal zone, but this corresponds to the direction a. Arfved- 
 sonite is likewise a widespread constituent of soda rocks and is 
 often surrounded by a border of aegirine. 
 
 Pure sodium iron silicate, riebeckite, occurs particularly in 
 soda granites. The absorption in the principal zone is almost 
 complete even in very thin sections. It forms irregular shreds 
 that look quite like tourmaline, and were formerly determined 
 as such. Aside from the cleavage, it is distinguished by the 
 fact that it absorbs the light in a different direction, i.e., rie- 
 beckite has stronger absorption in the principal zone. Altera- 
 tion of this amphibole, like that of the pyroxenes, leads to the 
 formation of blurred specks rich in opaque ores. 
 
 Anthophyllite, tremolite, and edenite, as well as very weakly 
 double refracting occurrences of glaucophane, are generally color- 
 less in thin section. Actinolite, pargasite, and common green 
 hornblende are green; carinthine and arfvedsonite are bluish- 
 green; glaucophane, crocidolite, crossite, and riebeckite are 
 blue; griinerite and cummingtonite are yellowish; common 
 brown hornblende, basaltic hornblende, and barkevikite are 
 brown, while cataphorite is reddish to violet. 
 
 The extinction angle is as follows, Fig 308, page 288; c:c for 
 
294 PETROGRAPHIC METHODS 
 
 anthophyllite and gedrite 0; basaltic hornblende 0-10; glau- 
 cophane 4-6; barkevikite and brown hornblende 12-14; 
 actinolite, pargasite 15-18; green hornblende 20 25; catapho- 
 rite 30-60; crocidolite 65-70; arfvedsonite and crossite 76; 
 riebeckite 85-86. Unlike the pyroxenes the double refraction 
 and the strong dispersion of the optic axes decrease in those 
 members rich in ferric iron and sodium. 
 
 The extraordinarily manifold properties of the minerals of the 
 amphibole group give rise to numerous confusions especially with 
 the pyroxenes. Aside from the difference in the cleavage angle, 
 by which they can be distinguished in basal sections, they vary 
 also in other properties. The pyroxenes have larger extinction, 
 and when they are deeply colored, show less absorption than the 
 amphiboles. Tremolite is similar to sillimanite, but the latter has 
 parallel extinction in all sections, a very small optic angle, and 
 simple cleavage in one direction transverse to the principal zone. 
 It is also similar to wollastonite and epidote in which the optical 
 plane is perpendicular to the principal zone, to diaspore which 
 has a negative principal zone, to cyanite which is distinguished 
 by the acute bisectrix being perpendicular to the cleavage, and 
 to lawsonite which always shows parallel extinction and becomes 
 cloudy upon heating and fuses quite easily. Cleavage plates of 
 the latter are parallel to the optical plane. Green hornblende is 
 quite like tourmaline, but the latter has a stronger absorption 
 perpendicular to the principal zone. It also resembles chlorite, 
 which has much lower indices and double refraction, and 
 chloritoid in which the optical plane lies transverse to the 
 principal zone. Brown hornblende frequently resembles tour- 
 maline; also orthite, staurolite, and acmite, but the last three 
 are characterized by higher indices of refraction and weaker 
 absorption. It is also like biotite which has lower indices of 
 refraction, and also has the optical plane transverse to the 
 principal zone and the interference figure is nearly uniaxial and 
 appears symmetrical in cleavage plates. Blue hornblende is 
 also often confused with tourmaline, and in rare instances it is 
 similar to chloritoid. 
 
 Triclinic aenigmatite, or cossyrite, is included in the amphibole group but 
 does not really belong to it. It forms prismatic crystals and grains showing 
 twinning laminations parallel to the brachypinacoid. It has a less perfect 
 cleavage parallel to the prism making an angle of 114. It occurs in erup- 
 tive rocks rich in sodium. Minute individuals are found in the ground 
 
DESCRIPTIVE SECTION 295 
 
 mass of basalts and are formed here by refusion of basaltic hornblende. 
 Hardness is 5 . 5 and sp. gr. 3 . 8. The axial plane is nearly parallel to the 
 brachypinacoid, and on it c:c is 45 f*. On the macropinacoid it is 4, 
 2E = 60; positive; t blackish-brown or almost opaque, B deep brown, a 
 lighter reddish-brown; very high indices of refraction. Aenigmatite is at- 
 tacked by hot hydrochloric acid and fuses easily, forming a black glass. 
 It is similar to lievrite in both these properties, but is more transparent. 
 
 Dumortierite (11) 
 
 Dumortierite is found in pegmatites and occurs also as minute needles 
 or tufts in certain contact rocks. The needles are recognized by their 
 intense color and strong pleochroism. It forms penetration twins according 
 to the aragonite law. It is surrounded by a pleochroic halo when it is 
 included in cordierite. It gives an opalescent bead with microcosmic 
 salt. Stronger absorption in the principal zone distinguishes it from 
 piemontite and tourmaline, and the direction of the cleavage in basal 
 sections differentiates it from hornblende. It is decolorized by heating, 
 whereas hornblende becomes brown and fuses. Its distribution has as 
 yet not been investigated to any great extent. 
 
 Axinite (12) 
 
 Axinite is a rare product of contact metamorphism or of pneumatolytic 
 activity, especially in coarse-grained dikes occurring in granite Hmurite. 
 If it has crystal form the cross sections are wedge-shaped. If crystal form 
 is lacking, it is very easily overlooked because it possesses few characteristic 
 properties. Color and pleochroism are generally very delicate. It can be 
 easily isolated and then it gives the greenish boron flame upon fusing. 
 
 Rinkite (12) 
 
 Rinkite is found as an accessory mineral in various soda rocks, especially 
 in nepheline syenite, and is associated with other rare minerals such as 
 mosandrite, lavenite, etc., and is generally penetrated by fluorite. It is 
 rapidly decomposed by hydrochloric acid even in a slide, and silica con- 
 taining titanium separates out. It fuses easily with intumescence and gives 
 off fluorine. The positive bisectrix is nearly perpendicular to the cleavage 
 plates, this being the best property to use for its determination. It is 
 often transformed into yellowish, clouded aggregates. 
 
 Sillimanite (12) 
 
 Sillimanite forms colorless needles without terminations, and 
 these are often finely shredded and have a silky luster macro- 
 scopically. Radial aggregates, that are only transparent in thin 
 section, are called fibrolite. It is found in contact rocks in heli- 
 coidal bands wound in various ways and penetrating through 
 
 *f.- front. 
 
296 
 
 PETROGRAPHIC METHODS 
 
 the other constituents of the rocks. Elongated sections are 
 narrow lath-shaped and often bent, or fractured transversely. 
 ^ Basal sections only show good forms 
 where the individuals are isolated. 
 They are then extremely characteristic, 
 Fig. 311. It is disseminated principally 
 in contact rocks rich in aluminium, in 
 certain gneisses, and as isolated needles 
 in granite. Alteration is not known. 
 
 \ 
 
 I 
 
 \ 
 
 /' 
 
 \ 
 
 / 
 
 \ 
 
 / 
 
 \ 
 
 / 
 
 \ 
 
 f 
 
 1 
 
 
 I 
 
 \ 
 
 1 
 
 \ 
 
 1 
 
 \ 
 
 I 
 
 \ 
 
 I 
 
 \ 
 
 (010? 
 
 s~ ( VH* 
 
 (}Jv v^> -X 
 
 (110) 
 
 FIG. 310. 
 
 (100)^ 
 FIG. 311. 
 
 Sillimanite, Section 
 Parallel (100). Parallel (001). 
 
 C M (001) 
 
 (100) 
 
 Concerning the microscopic distinction from apatite or trem- 
 olite, see those minerals. The positive character of the principal 
 zone distinguishes it from andalusite 
 and prismatine, parallel extinction from 
 chloritoid, as also the position of the 
 optical plane parallel to the cleavage, 
 and the absence of twinning laminations. 
 
 Datolite (12) 
 
 Datolite is only known as a secondary 
 formation, chiefly in cavities in basic eruptive 
 rocks. Its distribution as a rock constituent 
 has not been investigated, but it may occur in 
 certain contact rocks. It forms granular or 
 fibrous aggregates parallel to the 6 axis 
 botryolite. In the latter the axial plane lies 
 transverse to the fibers. It fuses before the 
 blowpipe to a clear glass and colors the flame 
 green. The powder has a strong alkaline 
 
 reaction even before fusing. Datolite is best determined by separating it 
 from the rock powder with heavy liquids, its specific gravity being 2.9, 
 
 FIG. 312. Datolite, 
 
 Section Parallel Plane of 
 
 Symmetry. 
 
DESCRIPTIVE SECTION 297 
 
 and then treating it with hydrochloric acid. Alcohol is then poured upon 
 the residue and if datolite was present it will burn with a greenish flame. 
 
 Mosandrite (12) 
 
 Mosandrite is found as an accessory constituent in nepheline syenite 
 in small tabular crystals associated with rinkite, etc., and penetrated 
 by fluorite. It is brittle and, therefore, generally checked. Lath-shaped 
 cross sections show twinning laminations. The mineral is often changed 
 into earthy brown to opaque aggregates. Johnstrupite is very similar to 
 it but has lower indices of refraction. Mosandrite is easily fusible and 
 forms a light colored bead. It gives a dark red solution with hydrochloric 
 acid and evolves chlorine. Strong inclined dispersion of the axes in cleavage 
 plates distinguishes it from rinkite. 
 
 Barite (Heavy Spar) (12) 
 
 Barite forms granular or platy, rarely fibrous, aggregates without dis- 
 tinct borders. They occur now and then filling crevices and cavities in 
 rocks, as cement in sandstone, and particularly as concretions in marl and 
 sand. These concretions are frequently filled with inclusions of sand, etc., 
 and consist of imperfect, tabular crystals arranged in rosettes. If it forms 
 the predominant cement of a rock, it is recognized by the great weight of 
 the same. Cleavage plates of the mineral are perpendicular to the negative 
 obtuse bisectrix. If it occurs in subordinate amounts, it is easily over- 
 looked. Its distribution as a rock constituent is, therefore, not accurately 
 known. Aside from a different flame reaction, it is distinguished from 
 celestite by a comparatively small optic angle and from quartz, feldspar, 
 and similar colorless rock constituents by higher indices of refraction and 
 insolubility in hydrofluoric acid. 
 
 Andalusite (12) 
 
 Andalusite is generally found in short, prismatic, rounded 
 crystals in which very frequently the cleavage cannot be recog- 
 nized. The individuals tend to arrange themselves in divergent, 
 radial groups or fingered aggregates. It is colorless in thin 
 section or speckled with a pale reddish color, and is then pleo- 
 chroic. It is found now and then in parallel intergrowths with 
 sillimanite. The arrangement of graphite inclusions is especially 
 characteristic chiastolite, Figs. 313 and 314, also Fig. 211, page 
 196. 
 
 Andalusite is very frequently altered to a compact aggregate 
 of mica, but these pseudomorphs can be recognized by the 
 arrangement of the graphite inclusions. If this arrangement 
 becomes more and more irregular and the outer form quite 
 imperfect, these elongated mica aggregates filled with graphite 
 
298 
 
 PETROGRAPHIC METHODS 
 
 dust are extremely difficult to determine. They are widely 
 distributed in contact rocks and form a large part of the knots 
 that appear macroscopically in such rocks. In most favorable 
 cases a few unaltered, but corroded residues point to the original 
 andalusite. Its chief distribution is in contact rocks rich in 
 aluminium. Andalusite is the lightest of the three modifications 
 
 (100) 
 
 Andalusite, Section Parallel to 
 (001) (010) 
 
 of aluminium silicate. It is, therefore, absent in rocks which 
 have been formed under high pressure, especially in the contact 
 rocks of the Central Alps. * 
 
 The negative character of the principal zone affords a dis- 
 tinction of colorless andalusite from sillimanite, and the lower 
 double refraction together with the orthorhombic properties of 
 all sections distinguishes it from diopside. Red andalusite may 
 appear similar to hypersthene but they have opposite characters 
 of the principal zone. It may also be confused with piemontite 
 in which the optical plane lies transverse to the cleavage. 
 
 Lazulite (12) 
 
 The petrographical importance of lazulite has been very little in- 
 vestigated. Good crystals of it have been found in quartz veins in phyllites 
 and in impregnations in granular limestone bluespar. High double re- 
 fraction, absence of cleavage, and its pyramidal form distinguish it from 
 other blue minerals. 
 
DESCRIPTIVE SECTION 299 
 
 Carpholite (12) 
 
 Carpholite is known as a fibrous, secondary formation in crevices in altered 
 rocks. Weaker absorption in the principal zone distinguishes it from 
 most other yellowish minerals except tourmaline, which is uniaxial, and 
 epidote in which the optical plane lies transverse to the cleavage. 
 
 Prehnite (12) 
 
 Prehnite belongs to those minerals whose petrographical im- 
 portance is but little known. It is found in radial, flaky aggre- 
 gates in cavities in eruptive rocks, or in compact aggregates 
 as a constituent of saussurite, lotrite. It is also quite wide- 
 spread as an alteration product of feldspar. No other mineral 
 shows such variable properties as prehnite, and it can only be 
 identified by the rosette structure in thin sections, which is ex- 
 tremely characteristic. Double refraction, normal and anom- 
 alous interference colors, position of the optical plane, size 
 of the optic angle, and the lattice structure vary in the dif- 
 ferent occurrences, so that even though the mineral can be de- 
 termined macroscopically it presents many difficulties under the 
 microscope. 
 
 Celestite (12) 
 
 Celestite is analogous to barite as a rock-forming mineral, except that it 
 rarely forms individuals so rich in inclusions. See under barite for its 
 distinction from that and other similar minerals. 
 
 Aragonite (13) 
 
 Aragonite is a rare constituent of rocks, but it occurs in columnar aggre- 
 gates which are very difficult to distinguish from fibrous calcite. The 
 principal characteristics by which it can be determined when accurate 
 observation of the interference figure is not possible, are the lack of cleavage 
 and the special reaction with cobalt solution, page 175. It is probably quite 
 widespread as an original constituent of rocks, but it has been very ex- 
 tensively transformed into calcite. Aragonite itself is found only in 
 secondary deposits from hot springs, particularly as onyx. 
 
 Wollastonite Group (13) 
 
 Wollastonite forms columnar to flaky individuals in which the axis of 
 symmetry is the axis of elongation. Cross sections parallel to it are lath- 
 shaped, and those perpendicular to it are somewhat elongated along the 
 orthopinacoid and are bounded by from six to eight faces. The cleavage 
 cracks are all parallel to the axis of symmetry, and in transverse sections 
 two systems of very perfect cleavage cracks nearly at right angles are seen 
 
300 
 
 PETROGRAPHIC METHODS 
 
 (101) 
 
 c (ooi) 
 
 (100) 
 
 along with another set that is less distinct, Fig. 315. Twinning parallel 
 to the orthopinacoid is very common. It is recognized in transverse sec- 
 tions by the extinction. Wollastonite is chiefly characteristic for granular 
 limestone. In feldspar-bearing rocks it is only found in the neighborhood 
 of such formations. When it occurs in eruptive rocks, it is probably the 
 result of resorbed lime inclusions or it may form pseudomorphs after an- 
 orthite. The moistened powder gives an alkaline reaction. It fuses be- 
 fore the blowpipe to a crystalline slag. The ease with which it is attacked 
 by hydrochloric acid distinguishes it from numerous other minerals with 
 
 which it might be confused if the cross sec- 
 tions are poorly developed. The mineral is 
 always colorless but is well characterized by 
 the position of the optical plane transverse 
 to the cleavage in sections showing parallel 
 extinction, the comparatively small optic 
 angle with distinct inclined dispersion, and 
 other optical and chemical properties. The 
 lower indices of refraction and the solubility 
 distinguish it from epidote minerals which 
 are similarly orientated. 
 
 Pectolite is related to wollastonite. It 
 occurs in fibrous, radial aggregates as a 
 Section Parallel Plane of Symmetry, secondary formation in crevices of eruptive 
 
 rocks rich in lime and in amphibolites 
 
 derived from them, and now and then as an alteration product of feldspar 
 in soda rocks. It is distinguished from wollastonite by the orientation 
 of the optical plane parallel to the positive principal zone and the higher 
 double refraction. Cleavage plates are almost perpendicular to the obtuse 
 bisectrix. 
 
 Rosenbuschite is still rarer. It forms colorless to yellowish, radial, fibrous 
 aggregates in nepheline syenites. In it the optical plane is parallel to the 
 principal zone which is negative. It is characteristically accompanied by 
 fluorite. It is distinguished from similar minerals by the negative character 
 of the principal zone and by its solubility. 
 
 FIG. 315. Wollastonite, 
 
 Humite Group (13) 
 
 The minerals of the humite group cannot be distinguished from one an- 
 other macroscopically and frequently also microscopically. They are found 
 mostly in contact limestones or dolomites in rounded grains in which the 
 cleavage is very poorly developed. They often show irregular cracks. 
 Twinning lamination parallel to the basal pinacoid is common in chondro- 
 dite and clinohumite. If the humites are colorless, they are very similar 
 to olivine from which they can only be distinguished by the extremely 
 low dispersion and by the position of the optical plane, which is parallel to 
 the cleavage in humite. The direction of extinction in the monoclinic 
 members, showing cleavage cracks or twinning lamination also serves to 
 distinguish them from olivine. See under olivine for the distinction between 
 colored varieties and titanium olivine. The humites often pass over into 
 
DESCRIPTIVE SECTION 
 
 301 
 
 serpentine with the formation of the mesh structure. Brucite also results 
 from such alterations. They become white, but do not fuse before the blow- 
 pipe. 
 
 (001) C =/ 
 
 (001) 
 
 FIG. 316. Humite, 
 
 FIG. 317. Clinohumite, 
 
 FIG. 318. Chondrodite, 
 
 Section Parallel (010). 
 
 Topaz (13) 
 
 Topaz forms short, prismatic crystals and grains which show 
 perfect cleavage in the elongated sections by the presence of a 
 few very sharp cracks. Cleavage plates are perpendicular to the 
 acute, positive bisectrix. The optic angle is generally quite large, 
 but is variable. The mineral is also found in columnar or radial 
 aggregates pycnite where it has been formed from other 
 minerals by the activity of fumaroles, as in the topaz quartz 
 porphyries, occurring in connection with tin ore veins. In these 
 the character of the principal zone is positive. The mineral is 
 known in the associated granites as a primary constituent, but 
 otherwise it is always secondary and points to very intensive 
 post-volcanic processes. 
 
 Topaz is always colorless in thin section and is distinguished 
 from quartz by higher relief. Distinction from sillimanite may 
 be very difficult, but the latter is characterized by higher indices 
 and double refraction, and by the cleavage parallel to the prin- 
 cipal zone. Topaz can be easily isolated from a rock on account 
 of its high specific gravity and resistance to acids. It is infusible 
 before the blowpipe, but turns white and at the same time 
 flakes up. 
 
 Anhydrite (14) 
 
 Anhydrite itself forms a rock consisting of granular, or less 
 often columnar aggregates, that change easily into gypsum. It 
 is often colored blue or red, macroscopically, but in thin section 
 
302 PETROGRAPHIC METHODS 
 
 it is always colorless and has three cleavages differently devel- 
 oped. Its solubility in water is characteristic, and gypsum 
 crystallizes out upon evaporation. It is very similar to musco- 
 vite in thin section but the latter has but one distinct cleavage. 
 The mineral is confined to sedimentary rocks and is often found 
 in very coarse-grained aggregates in the crevices of such rocks. 
 
 Mica Group (14) 
 
 A very large series of minerals is characterized by an excep- 
 tionally perfect cleavage in one direction, so that they have a 
 flaky or scaly appearance. The outer form of the small plates is 
 almost without exception hexagonal. A table of the members 
 of this group, that are important in rocks, can be found on page 
 270. The mica group is the most important of these. 
 
 The members of the mica group, which are important in rocks, 
 are potassium mica muscovite, magnesium micas phlogopite 
 and biotite, and the lithium micas, "which are classed 
 together under the name lithionite. The determination of the 
 crystal system of mica is generally not possible by optical 
 methods. Most of them show parallel extinction although they 
 are monoclinic, and apparently uniaxial varieties are very 
 common. If an optic angle can be determined distinctly, it 
 may be observed that the negative, acute bisectrix coincides 
 almost exactly with the vertical axis, while in the uniaxial 
 varieties this is the direction of the optic axis. 
 
 Cleavage plates of mica are perpendicular or almost perpen- 
 dicular to the acute, negative bisectrix. The true optic angle is 
 small to medium large, from approximately in most magnesium 
 micas to 30-40 in normal muscovite, but rarely larger. In con- 
 vergent light, therefore, the optic axes appear symmetrically 
 within the field of vision. Transverse sections give brilliant 
 interference colors in consequence of the high double refraction, 
 and they extinguish almost exactly parallel to the numerous, 
 sharp cleavage cracks. When such sections are colored, they 
 are distinctly pleochroic. Basal sections are entirely different, 
 making it difficult for the beginner to determine it. This varia- 
 tion aids the experienced observer, however, to' recognize it. 
 Such sections are either entirely dark between crossed nicols, 
 or they show low interference colors corresponding to the 
 small difference ;--/?, which never exceeds 0.008. In addition 
 to this, nothing is seen of the cleavage, and the colored members 
 
DESCRIPTIVE SECTION 
 
 303 
 
 of the group are rarely pleochroic. However, in convergent 
 polarized light a remarkably distinct interference figure is always 
 obtained. 
 
 The optical plane sometimes lies perpendicular to the plane of 
 symmetry mica of the first order, Fig. 319, and sometimes in 
 the plane of symmetry mica of the second order, Fig. 320. Musco- 
 vite belongs to the first type and most of the biotites to the second. 
 The orientation can be recognized by means of the percussion 
 figure which is shown in the figures for all the minerals belonging 
 in this group. Twinning is very widespread and generally 
 parallel to the basal pinacoid. It is not often observed in 
 
 Mica Orientated by Percussion Figure. 
 First Order. Second Order. 
 
 transverse sections because of the parallel extinction, but it can 
 be recognized in thick cleavage plates by means of the distorted 
 interference figure. Micas are elastic and are perfectly flexible 
 under the influence of orogenic stresses. They, therefore, 
 occur in variously bent forms, Fig. 204, page 192. Now and then 
 they are torn apart parallel to the gliding plane, which is trans- 
 verse to the cleavage, and the alteration to chlorite proceeds 
 outward preferably from such parting structures. 
 
 Colored micas always show great differences in absorption, and 
 the browns possess this to a higher degree than the rarer greens. 
 The rays vibrating in the cleavage plane are always the most 
 strongly absorbed, and generally show no difference among 
 themselves. For this reason cleavage plates are generally not 
 pleochroic. Biotites with entirely anomalous properties occur in 
 soda rocks, but they are very rare. Here, they have quite a large 
 optic angle and quite an appreciable angle of extinction and 
 also show a distinct difference in absorption between ft- and c. 
 
304 PETROGRAPHIC METHODS 
 
 In pleochroic halos, which are found in the micas in all rocks, 
 the direction of the stronger, often complete absorption, is like- 
 wise parallel to the cleavage cracks. Indices of refraction and 
 double refraction are higher in them. 
 
 The micas always contain the hydroxyl group. They are more 
 widespread among the intrusive rocks than among the extrusive. 
 In the latter they are often magmatically resorbed in a manner 
 analogous to that described for hornblende. The specific gravity 
 is high but they cannot be isolated by heavy liquids on account 
 of their flaky development. See page 155 for a method of sepa- 
 rating mica from other minerals. Members rich in iron are 
 easily attracted by an electro-magnet. Light colored micas are 
 only slowly attacked by hydrofluoric acid, but the darker ones 
 are decolorized by hydrochloric acid and gradually lose their 
 double refraction. The fusibility is quite variable from the 
 difficultly fusible muscovite to the easily fusible lithia micas. It 
 may also be noted that the micas like all aluminium silicates be- 
 come blue when roasted with cobalt nitrate, and this is the 
 only real positive reaction, without making thorough chemical 
 tests, for distinguishing between mica and talc. 
 
 Muscovite, Fig. 321, is rare as a primary constituent of eruptive 
 rocks. It occurs in-very large crystals in granitic .pegmatites 
 
 and forms irregular flakes in 
 \$P g '1 binary granites, which are recog- 
 
 nized macroscopically by the 
 brilliant luster. It is also found 
 abundantly in fresh plagioclase 
 in the central granite. Here it 
 forms minute, well-defined, and 
 ; FIG. 32i. Muscovite, Orthopinacoid. regularly arranged scales, which 
 
 can only be considered as primary 
 
 inclusions. The appearance ,. of secondary mica, which may 
 develop from any of the feldspars, is entirely different. It is 
 sometimes in larger scales, which follow the cleavage cracks, 
 etc., and form veins or bands with irregular properties. More 
 often it is a dense, confused, scaly formation imbedded in the 
 clouded substance of the feldspar or entirely replacing it. 
 Similar aggregates, that are macroscopically always dense, 
 occur as pseudomorphs after nepheline, scapolite, andalusite, 
 etc. Their properties indicate that they belong to the sericites 
 described below. Small flakes of muscovite are found also in 
 
DESCRIPTIVE SECTION 
 
 305 
 
 the ground mass of quartz porphyries that have been somewhat 
 altered and they can only be secondary. 
 
 Muscovite is far more widespread in contact rocks, especially 
 those formed by piezocontact metamorphism. It is shown in 
 the Allgemeine Gesteinskunde, page 138, how mica schist occurs 
 under these conditions in place of the normal hornfels. Mus- 
 covite is the most frequent cause of the schistose structure of the 
 crystalline schists formed by contact metamorphism. When 
 light mica occurs in isolated individuals that are poorly developed 
 but are macroscopically distinct, the schistosity is generally 
 very imperfect. This phenomenon is seen in many injected 
 schists, especially in amphibolite and eclogite, but in the latter 
 rock the mica does not appear to belong to muscovite, but is a 
 mica whose properties have not been accurately determined. 
 
 The mica in these rocks tends to bind itself into membranes 
 with a silvery luster, which wrap around all the other rock con- 
 stituents as in a mica schist. 
 In the phyllites these mem- 
 branes become finer and finer 
 and are folded in various 
 ways, Fig. 322. Then the 
 single flakes cannot be seen 
 with the naked eye and the 
 mica can only be recognized 
 by the silky luster of the 
 rock. Such fine, scaly aggre- 
 gates of mica, related by a 
 series of transition members 
 to true muscovite, are called 
 sericite. They have properties 
 different from muscovite, chief 
 among which are the smaller 
 optic angle and a greater susceptibility to the action of acids. 
 
 This mineral forms the principal constituent of a widespread 
 series of sericite schists, that have extremely thin layers, and 
 are mostly white with a silky luster. They are rocks in which a 
 few phenocrysts of corroded quartz can be distinctly seen, Fig. 
 199, page 189. These can only be looked upon as altered quartz 
 porphyry. Granite, gneiss, etc., suffer similar sericitization now 
 and then, especially in the neighborbood of faults and when 
 certain ore veins are formed in the crevices thus produced. 
 20 
 
 FIG. 322. Folded Sericite Membrane. 
 Quartz Phyllite, Sunk, Steiermark. 
 
306 PETROGRAPHIC METHODS 
 
 Rocks rich in sericite are apparently quite similar to fine, 
 scaly talc, and this is increased by the greasy feeling of them. 
 It is interesting to note that nearly all such occurrences were 
 formerly designated as talc schists, and even to-day, sericite 
 schists are generally included with talc schists. Aggregates of 
 soda mica paragonite are outwardly very similar to sericite. 
 They are well-known because of the content of staurolite and 
 cyanite. They replace the granite pegmatites in the Central 
 Alps. Nothing is known of a further distribution of this mineral. 
 Dense light green or yellowish aggregates of mica with properties 
 similar to soapstone are known in small masses in the schists of 
 the Central Alps and in other places under analogous conditions. 
 They are called onkosine, pragratite, cossaite, damourite, margar- 
 odite, etc., and consist of aggregates that are composed of various 
 sorts of minerals which cannot be distinguished from sericite. 
 
 Muscovite is always a mica of the first order but is often in 
 parallel intergrowth with biotite. Characteristic intergrowth 
 with quartz resembling palm leaves occurs in pegmatitic rocks 
 and called mica palme. Alteration of muscovite is not known. 
 It is therefore often found unaltered in 
 clastic rocks, but without accurate investi- 
 gation it can be confused with bleached 
 biotite. 
 
 Phlogopite, Fig. 323, is a typical mineral 
 of contact-metamorphosed limestones and 
 dolomites, and occurs principally in rounded 
 crystals or those elongated along the verti- 
 FIG. 323. cal axis. Macroscopically, it is usually 
 
 Biotite (Phlogopite), T i , i -r . i * 
 
 Cleavage Plate. Ver 7 lj g nt brown. Large tabular crystals 
 
 of commercial importance are found in the 
 
 granite pegmatites, which cut through carbonate rocks. It is 
 always colorless in thin section and distinguished from muscovite 
 by the fact that it is usually nearly uniaxial, and that the cleavage 
 cracks do not have a tendency to be so sharp. Oblique extinc- 
 tion is observed. Needle-like inclusions sillimanite and rutile 
 tend to arrange themselves diagonal to the rays of the per- 
 cussion figure and upon alteration, which occurs now and then, 
 sagenitic intergrowths of rutile are seen along with scaly aggre- 
 gates. The distribution of phlogopite in rocks is but little 
 known. 
 
 Biotite is much more widespread. It is macroscopically 
 
DESCRIPTIVE SECTION 307 
 
 brown to black and often has a bronzy luster. Dark green 
 varieties are rarer. Deep reddish-brown rubellane of the basic, 
 extrusive rocks is generally burned and strongly resorbed by the 
 magma. A few rarer varieties are the micas of the first order 
 anomite and these frequently have an extinction of 4-5, and 
 a somewhat larger optic angle. Most of the biotites are micas 
 of the second order meroxene and lepidomelane. The latter is 
 particularly rich in iron and titanium, and the absorption of the 
 rays vibrating parallel to the cleavage is almost complete in 
 thin sections. In rarer varieties, which occur especially in the 
 group of soda rocks, the optic angle is quite large, up to 35, 
 and at the same time an important extinction, up to 10, makes 
 its appearance. Transverse sections show distinct twinning 
 laminations and a difference between the two rays vibrating 
 nearly in the cleavage plane, B golden yellow, c reddish-brown. 
 
 Most of the biotites are brown in thin section with different 
 degrees of intensity. Less often they are colored yellowish- 
 brown, red brown or finally green, a yellowish, & = c green. 
 Biotite is widely disseminated in eruptive rocks and contact 
 rocks of all sorts. The best developed crystals, apparently 
 hexagonal, are found in glassy extrusive rocks while in other 
 extrusive rocks they are magmatically resorbed. They are the 
 more resorbed the more crystalline the rock. Inclusions of 
 apatite and zircon are frequent and the former are without, the 
 latter with pleochroic halos, Fig. 216, page 197. Sagenitic inter- 
 growths of rutile are also common. In granulitic rocks the cross 
 sections are often completely perforated by rounded grains of 
 quartz. The flakes themselves have rounded outline in contact 
 rocks, and skeletal development is occasionally observed as well. 
 They do not combine to form membranes as frequently as 
 sericite does. They never form such fine crystalline films. On 
 the other hand, biotite flakes pentrate the other constituents of a 
 hornfels in a helicoidal manner and in schistose rocks, the flakes 
 are often placed transverse to the schistosity. They are then 
 often filled with graphite, etc., and do not show a thin tabular de- 
 velopment, but often a distinct prismatic growth, Fig. 324, in 
 contradistinction to the biotite flakes which lie in the structure 
 plane of the rock. 
 
 It is found in parallel intergrowth with muscovite in binary 
 granites, gneisses, etc., and with chlorite in the central granites. 
 Perfect freshness of the biotite prevents confusion with the 
 
308 
 
 PETROGRAPHIC METHODS 
 
 alteration of mica to chlorite, which is so common. It is often 
 intergrown with hornblende and pyroxene, or a narrow homo- 
 geneous border surrounds an irregular grain of opaque ore, par- 
 ticularly in gabbro. 
 
 Alteration is very common. The simplest process is bleaching 
 in which the mineral loses its color and fresh appearance, and 
 passes over into a colorlesp mica with a somewhat lower double 
 
 refraction. Chloritization is 
 more frequent. Proceeding 
 inward from the edges and 
 along gliding planes, the min- 
 eral is changed to lamellar 
 chlorite. The separation of ti- 
 tanium minerals, then quartz, 
 carbonates, iron ores, epidote, 
 etc., allow the process of 
 alteration to be followed 
 closely. Sometimes rutile is 
 observed in the form of 
 sagenite, the crystals inter- 
 secting each other at an angle 
 of 60. This rutile is a by- 
 product of the formation of 
 chlorite or the bleaching process. In certain cases almandine 
 or olivine are altered into biotite. 
 
 The lithionites are sometimes light, and sometimes dark col- 
 ored. They are constituents of lithionite granite and can only 
 be distinguished from muscovite or biotite by chemical reactions, 
 especially the flame reaction, and by the greater fusibility. Dark 
 lithia micas have a variable optic angle up to about 65, and are 
 micas of the second order. They are observed first of all in the 
 lithionite granites, which occur in combination with tin ore veins, 
 and they are here the principal micas. The colorless, as well as the 
 brown, are often completely filled with pleochroic halos, which 
 occur around inclusions of rutile, zircon, and cassiterite. Mica 
 very rich in iron, macroscopically black raven mica is found 
 in certain soda rocks. 
 
 See the introduction to this section for a distinction between 
 the real micas and a series of minerals with a similar development. 
 Special attention is called to the use of the cleavage plates in 
 the investigation, for this is sometimes the only means of dis- 
 
 FIG. 324. Biotite, Transverse to Schistocity, 
 
 Prismatic. Graphite Schist, Maurertal, 
 
 Grosvenediger. 
 
DESCRIPTIVE SECTION 309 
 
 tinction, e.g., muscovite from pyrophyllite, although the latter 
 has a much larger optic angle. Therefore the phenomena, which 
 the cleavage plates of mica and micaceous minerals show, are 
 considered first in the diagrams; see also the table, page 270. 
 This method on the other hand does not give results m the case 
 of dense aggregates of talc. Here chemical reaction, page 168, 
 must find a place. Diaspore may appear similar to colorless 
 micas on account of the coincidence of the interference colors, 
 but it is characterized by higher indices of refraction. Also 
 anhydrite, which has more than one cleavage not all equal, is 
 similar to colorless mica. The most perfect cleavage planes of 
 that mineral are perpendicular to the obtuse negative bisectrix. 
 Minerals of the amphibole group, tourmaline, and orthite are 
 confused with colored micas, but all these minerals have higher 
 indices of refraction and poorer cleavages. The hydrofluosilicic 
 acid reaction, page 163, is valuable under all circumstances for 
 determining the micas. 
 
 Chrome micafuchsite must also be mentioned. It occurs in emerald 
 green flakes in contact rocks. It has a variable optic angle, a pale sky- 
 blue, B = c siskin green. 
 
 Seladonite is appended to the mica group. It is an alkali silicate similar 
 to mica and occurs as a rock constituent in basic eruptive rocks and 
 tuffs. It is sometimes in the veins in them, sometimes as pseudomorphs 
 after augite, and sometimes as an impregnation in the rock. It is dark green 
 in color, has dense, earthy properties green earth and is very soft. 
 Sp. gr. = 2.6-2.8. It is apparently uniaxial, negative, o pale yellowish- 
 green, c deep green. Double refraction is somewhat lower than that of 
 the micas. Glauconite with a similar composition is distinguished from it 
 by its occurrence, which appears to be confined to the sedimentary rocks 
 green sand. It is found in small dark green spheres, which appear macro- 
 scopically like grains of gun powder and is often collected into masses as an 
 agent of petrifaction, especially of foraminifera. Its structure is generally 
 confused even microscopically. Distinct flaky, radial aggregates, showing 
 marked pleochroism from light yellow to deep green, are rare. It is also 
 negative but is biaxial, 2E = 30-40. f a. = 0.020. Both are difficultly 
 soluble in hot hydrochloric acid. Chrome ocher appears very similar and is 
 likewise brilliant green. It is nearly uniaxial, negative, a yellowish to 
 colorless, B = c brilliant green. It is found particularly as a halo around 
 chrome spinel in serpentine. 
 
 It is not determined whether the small greenish-yellow, radial, fibrous 
 spheres with a perfect cleavage that occur in quartz schists and alum schists 
 and are called astrolite, belong here or not. Aside from a lower double re- 
 fraction, they possess properties very similar to mica. The number of 
 names, which micaceous aggregates have received, is so large that they 
 cannot be discussed in this text. 
 
310 PETROGRAPHIC METHODS 
 
 Chlorite Group and Serpentine (14) 
 
 The minerals of the chlorite group in the narrower sense can 
 but rarely be recognized macroscopically as rock constituents and 
 are generally dark green. Perfect micaceous cleavage, low 
 hardness and flexibility of the flakes are characteristic. They 
 are generally so fine scaly that the individual flakes cannot be 
 recognized distinctly. They have a somewhat parallel arrange- 
 ment giving to the rock a soft velvety luster. The normal 
 chlorites, i.e., those members of the series rich in iron, together 
 with sedgy hornblende, constitute the most frequent green pig- 
 ments of crystalline rocks, and numerous greenstones, green- 
 schists, etc., owe their color to it. Leuchtenbergite, which is free 
 from iron and colorless, is very rare and is scarcely ever visible 
 macroscopically while the dark red to violet members containing 
 chromium, Cotschubeyite and cdmmererite or rhodochrome are con- 
 fined to secondary formations in serpentines rich in chromite. 
 
 Distinction is made under the microscope between pennine, 
 which is sometimes positive and sometimes negative, uniaxial 
 with parallel extinction and frequently shows anomalous lavender 
 blue to rust brown interference colors, and clinochlore. The 
 latter has higher double refraction, is biaxial, and always positive. 
 It shows normal interference colors and on account of its not 
 insignificant oblique extinction it often shows twinning lamina- 
 tion according to the mica law in thin section. These minerals 
 are very light in thin section, but are generally distinctly colored 
 and, whether they are positive or negative, they show stronger 
 absorption of the rays vibrating in the cleavage plane. Colorless 
 cross sections of a mineral with properties similar to chlorite 
 and often with quite anomalous interference colors appear 
 occasionally in the rocks chiefly as a decomposition product of 
 silicates poor in iron. It has been identified as leuchtenbergite. 
 Cotschubeyite corresponds to clinochlore in its optical behavior 
 while cammererite frequently has anomalous interference colors 
 and is similar to pennine. Both are hyacinth red parallel to 
 c = c and perpendicular to c deep violet. Deep blue chlorite, 
 erinite, has been observed as a rock constituent. It is yellow- 
 ish parallel to c = a and cobalt blue perpendicular to c. 
 
 Pleochroic halos with brown colors are common in chlorites. 
 Scarcely a trace of an interference figure can be observed in cleav- 
 age plates on account of the deep color and the low double 
 refraction. When roasted in air the chlorites become deep brown 
 
DESCRIPTIVE SECTION 
 
 311 
 
 (010) 
 
 and show great differences in absorption, but they fuse with 
 difficulty. They can be etched out of rocks with hydrochloric 
 acid, but leave gelatinous silica behind. 
 
 Coarse, scaly aggregates of chlorite are found as secondary 
 veins in serpentine and as a facies of that rock. These aggregates 
 frequently contain large crystals of magnetite and occur as irregu- 
 lar non-schistose masses. They consist principally of anomalous 
 pennine which often has a radial flaky ap- 
 pearance, Fig. 325. The same species is 
 also common as an alteration product of 
 basic silicates and sometimes forms pseu- 
 domorphs after biotite. These are always 
 penetrated by titanium minerals and have 
 abundant pleochroic halos. Sometimes it 
 forms irregular scaly aggregates, which 
 have developed from hornblende or 
 pyroxene; also from almandine, feldspar, 
 
 etc. The same formation is found in greenstones and propyl- 
 lites as 'a fine pigment throughout the whole rock. It is not 
 definable optically and is called viridite. In piezocrystalline 
 rocks, especially in the central granites, large poorly developed 
 flakes of pennine are undoubtedly primary constituents. These 
 often grow parallel with biotite, but are distinguished from 
 secondary chlorites by a lack of by-products and by the perfect 
 clearness of the mica. 
 
 (001) 
 
 FIG. 325. 
 Pennine Cleavage Plate. 
 
 ffi, 
 
 FIG. 327. 
 
 Clinochlore, 
 
 Parallel Cleavage. 
 
 Parallel Plane of Symmetry. 
 
 The tabular individuals of chlorite occurring in green contact 
 rocks of all kinds, amphibolite, eclogite, green schist and chlorite 
 schist, belong chiefly to clinochlore, Figs. 326 and 327. It shows 
 twinning lamination and normal interference colors. In denser 
 
312 PETROGRAPHIC METHODS 
 
 formations approaching phyllites, a mineral similar to chlorite 
 occurs but is distinguished from clinochlore by higher indices of 
 refraction but they do not reach those of the brittle micas al- 
 though they approach them. 
 
 The table, page 270, shows the distinction between the chlorites and other 
 micaceous minerals. A series of dark green minerals, generally with low 
 but sometimes with high double refraction, f a = 0.02, is appended to 
 the chlorites. They are called delessite, ripidolite, etc., and some of them 
 at least are chlorites rich in iron. Likewise silicates very rich in iron with 
 the general habit of chlorite are found frequently as predominant constit- 
 uents of sediments particularly of oolite. They are called thuringite, 
 owenite, chamosite, etc., and form scaly cleavable aggregates with a specific 
 gravity of about 3 . 2. They are similar to chlorite in all the optical proper- 
 ties, being negative with a small optic angle and having a positive principal 
 zone. 
 
 The relations between the chlorite minerals and serpentine 
 have as yet not been thoroughly determined. There is no doubt 
 that flaky serpentine or antigorite is quite analogous to the 
 chlorites in its optical behavior and the chemical composition of 
 chlorite rich in magnesium has been explained by an isomorphous 
 mixture of magnesium aluminium silicate called amesite with 
 magnesium silicate, serpentine. Besides the serpentine minerals, 
 which belong here, there is a fibrous serpentine or chrysotile, that 
 has no relation whatever to the chlorite group. 
 
 The serpentine minerals are not known in individual crystals. 
 All occurrences so designated are pseudomorphs. Antigorite in 
 deep green, flaky masses and chrysotile in yellowish-green, fine, 
 fibrous masses with a silky luster and fine enough to spin, are 
 observed in crevices of serpentine consisting for the most part of 
 very fine aggregates of that mineral. The principal field of 
 distribution of the mineral is in those rocks, which are macro- 
 scopically compact with a fine splintery fracture and streaked 
 with dark green or red, rarely with honey yellow. 
 
 The appearance of the two varieties of serpentine under the 
 microscope is quite variable in spite of the similarity with respect 
 to index and double refraction. Both are generally colorless in 
 thin section and are therefore not pleochroic. If pleochroic 
 varieties are thought to have been found, mllarsite, chemical 
 observation will not confirm it and the mineral is undoubtedly 
 chlorite, which is so often mixed with serpentine. Figs. 328 and 
 329 show the optical constants of the two types of serpentine and 
 in these the difference in habit appears distinctly. However, the 
 
DESCRIPTIVE SECTION 
 
 313 
 
 A e=^ B 
 
 \ 
 
 / 
 
 
 / 
 
 \ 
 
 \ 
 
 r 
 
 /f 
 
 / 
 
 \ 
 
 / 
 
 \ 
 
 / 
 
 \ 
 
 
 / 
 
 \ 
 
 M B /r A. 
 
 FIG. 328. FIG. 329. 
 
 question is by no means clear in all respects and there are un- 
 doubtedly forms of development in genuine serpentine, which 
 could not be classified under either of the diagrams. 
 New names cannot be given to these dense aggregates simply 
 because they have different habit as has been done in the anti- 
 gorite varieties, picrosmine, metaxite, picrolite, etc., and likewise 
 the microscopic forms of aggregation, e.g., the radial, which has 
 lately given rise to the name radiotine, cannot 
 be recognized as sufficient ground for evolv- 
 ing a new name. 
 
 Serpentine is undoubtedly very rare as a 
 primary constituent of rocks. Antigorite as 
 such has been men- 
 tioned and shown 
 in Fig. 288, page 
 276. It occurs in 
 parallel growth 
 with olivine. If an 
 olivine rock is in- 
 tergrown with 
 antigorite, which 
 represents a typi- 
 cal formation of 
 
 piezocrystallization, stubachite, later suffers more or. less com- 
 plete serpentinization by thermal processes, a very fine, scaly, 
 irregular aggregate of secondary serpentine is developed 
 between the lath-shaped crystals of the primary antigorite. 
 The secondary serpentine is dense and compact and scarcely 
 acts upon polarized light while the laths of fresh antigorite 
 appear like a lattice in it, lattice structure, Fig. 330, and often 
 fine, granular residues of olivine that are scarcely transparent 
 are seen. This development is to be sure very typical, but by 
 no means constant. It gives way to radial fibrous and con- 
 fused, almost structureless masses. The various antigorite 
 serpentines are characteristic for regions in which the rocks 
 have been formed under the influence of piezocrystallization. 
 They are predominant under such circumstances, but are excep- 
 tional under normal conditions. 
 
 The appearance of crysotile serpentine is unlike that of anti- 
 gorite. It forms fibrous masses perpendicular to the edges and 
 filling out the cracks of olivine thus causing the mineral to crack 
 
 Optical Orientation of 
 Antigorite. Chrysotile. 
 
314 PETROGRAPHIC METHODS 
 
 further and these cracks are again filled with fine fibrous aggre- 
 gates. So the process goes on until the characteristic appearance 
 of the mesh structure is seen, Fig. 331. Small grains of more or 
 less clear olivine may be present in it or the whole may be altered 
 to serpentine. In the veins of such fibrous serpentine there 
 are generally two differents kinds of substances, which appear 
 quite similar. On the outer border the axes of the fibers are 
 positive and the middle portion is also fibrous with the crystals 
 perpendicular to the sides of the vein, but the principal zone is 
 negative. 
 
 Serpentine as a rock is always developed from peridotites. 
 Other rocks, e.g., pyroxenite and amphibolite, never turn to 
 serpentine. It is always a product of thermal activity, see 
 
 FIQ. 330. Serpentine with Lattice Struc- FIG. 331. Serpentine with Mesh Structure, 
 ture. Hackbrettl in Stubachtal. Trogen near Hof, Fichtelgebirge. 
 
 Allgemeine Gesteinskunde, page 152. Now and then pyroxenes 
 low in aluminium in such rocks are effected by this process of 
 alteration and a pseudomorph of antigorite is formed from it, 
 bastite. Generally, however, these minerals remain unchanged 
 and in the serpentines of the Central Alps a few individuals of 
 pyroxene constitute the only residue of the original rock and this 
 gave rise to the opinion that the whole rock was an altered 
 pyroxenite pyroxene serpentine. By-products are very wide- 
 spread in the formation of serpentine from olivine. Besides iron 
 ores, which always occur, chlorite, talc and actinolite are formed. 
 The latter material has been falsely considered as the parent 
 substance of serpentine, hornblende serpentine. 
 
DESCRIPTIVE SECTION 315 
 
 The pseudomorph formations are also formed in rocks in which 
 olivine is a subordinate constituent as in the contact-metamor- 
 phosed carbonate rocks where the serpentine is generally poor in 
 iron and is yellowish-green to sulphur yellow precious serpen- 
 tine but it is sometimes dark green to pure black. The latter 
 color is found especially in certain ophicalcites called eozoon. 
 Besides forsterite, humite may be the parent material in such 
 rocks and in both cases the serpentine shows the typical mesh 
 structure. More rarely it is formed from periclase and the 
 cubical cleavage shows even macroscopically. In the pseudo- 
 morphs the antigorite flakes lie parallel to the cube faces and 
 overlap each other in such a manner that no double refraction 
 can be observed in thin section. Fresh fragments of the original 
 mineral are sometimes seen as small rounded inclusions in calcite 
 grains. 
 
 Sometimes pseudomorphs after an olivine rich in iron are 
 found in olivine-bearing porphyric plagioclase rocks. These have 
 a cleavage like that of mica and are similar to bastite. They are 
 distinctly green in thin section and pleochroic, c green, B and a 
 yellow. They also have quite a strong double refraction, 
 f a = 0.025. They are serpentine very rich in iron. The 
 color and pleo^chroism become more intense upon oxidation and 
 when such pseudomorphs become brown they show very signifi- 
 cant difference in absorption from brownish-red to yellowish 
 c>fi>a, iddingsite or bowlingite. Similar substances are found 
 in the same rock occurring principally with the mesh structure 
 and when they are brown the serpentines rich in iron can be 
 made deeper brown and strongly pleochroic by roasting in the 
 air. The common serpentines remain unchanged by this treat- 
 ment so long as they do not lose their water. Serpentine can be 
 distinguished from chlorite, which appears very similar to it, by 
 roasting with cobalt solution when the latter becomes blue. 
 Sometimes it is necessary to dissolve the serpentine out with 
 hydrochloric acid in order to expose accessory minerals concealed 
 in it. 
 
 Black serpentine mentioned above must be briefly described here. It 
 occurs in many ophicalcites. In thin section, it generally forms a mass 
 like graphite, but a few individuals are prominent from their larger dimen- 
 sions. They show complete absorption even in the thinnest sections parallel 
 to the principal zone and perpendicular to it they are completely colorless. 
 Hydrosilicates containing nickel belong in this group. They occur as a 
 
316 
 
 PETROGRAPHIC METHODS 
 
 secondary development in crevices in serpentine and represent one of the 
 most important nickel ores, numeite, genthite, pimelite, etc. They are 
 fibrous scaly substances with brilliant green to yellowish-green color, sp. 
 gr. 2.6-2.9. Optically positive with medium double refraction and 
 positive principal zone. They turn black before the blowpipe and are 
 attacked by acids with difficulty. 
 
 Hydrargillite (14) 
 
 Hydrargillite is formed occasionally in the decomposition of various 
 feldspars especially in the formation of bauxite. It has also been found in 
 spreustein and a few occurrences of emery. The aggregates appear fibrous 
 in thin section and are extremely difficult to determine because various 
 occurrences have different optical properties. Strong double refraction, 
 negative principal zone and oblique extinction are characteristic and dis- 
 tinguish the mineral from brucite, kaolin, talc, muscovite, leuchtenbergite, 
 etc., which appear similar to it. Diaspore occurs with it and is recognized 
 by much higher indices of refraction. Chemical reactions, such as the blue 
 color upon roasting with cobalt solution and the lack of silica, are sometimes 
 employed to determine the diaspore. 
 
 Fio. 332. Hydrargillite, 
 Cleavage Plate. 
 
 FIG. 333. Talc, Cleavage Plate. 
 
 Talc (15) 
 
 Talc as a rock-forming mineral has, by no means, the signifi- 
 cance which geology in general ascribes to it. Most of the 
 occurrences so designated are scaly aggregates of sericite as 
 mentioned above. Talc has a much more greasy feel, but the 
 two cannot be differentiated under the microscope. Chemical 
 investigation alone furnishes a clue for the distinction and this 
 may be either the hydrofluosilicic acid reaction, page 163, or 
 roasting the mineral with cobalt solution, when sericite becomes 
 brilliant blue. 
 
 Talc rocks in general consist predominantly of fine, confused, 
 scaly aggregates of talc, such as talc schists, soapstone, potstone, 
 
DESCRIPTIVE SECTION 
 
 317 
 
 etc. They are very soft and can be carved or turned, but upon 
 roasting they attain a hardness greater than that of quartz. 
 They are always anomalous formations, which have resulted 
 from intense chemical processes sometimes in the neighborbood 
 of the most acid eruptive rocks, the granites, and sometimes they 
 have formed from the most basic rocks, the peridotites, in which 
 frequently the whole rock is changed to talc without any residue. 
 Rhombohedrons of magnesite, prisms of actinolite and micro- 
 lites of rutile are frequently by-products occurring in the yellow- 
 ish, grayish, or greenish aggregates of talc. Talc is found occa- 
 sionally as a subordinate constituent in occurrences that scarcely 
 deserve the name of rock, such as listwanite and duelho. It also 
 occurs in magnesite and numerous serpentines. 
 
 Pyrophyllite (15) 
 
 The distribution of pyrophyllite as a rock constituent cannot be estimated 
 because it is very difficult to distinguish it from muscovite in thin section 
 except that it has a larger optic angle. In cases of doubt a decision can be 
 reached by means of the hydrofluosilicic acid reaction. It is sometimes 
 formed as a by-product in the process of kaolinization and other similar 
 
 
 X^ 54 r 
 
 (001) 
 
 (010) 
 
 / 
 
 \ 
 
 FIG. 335. 
 
 Pyrophyllite, 
 Cleavage Plate. Macropinacoid. 
 
 replacement processes. It occurs in isolated scales and rarely in dense 
 aggregates of agalmatolite similar to soapstone. When roasted with cobalt 
 solution they become blue. It is more frequently found as a fine, scaly 
 coating on clay slates in which it often occurs in silvery aggregates covering 
 crevices or as an agent of petrifaction, of grapholites, gumbelite, for example. 
 
 Bertrandite (15) 
 
 Limpid, tabular crystals of bertrandite have been found in certain peg- 
 matites. It occurs also as a constituent of granites, aplites, etc., but is 
 very difficult to determine on account of its great similarity to muscovite. 
 
318 
 
 PETROGRAPHIC METHODS 
 
 It is very difficult to find in any case because its specific gravity is about 
 the same as that of the principal constituents occurring in the rocks and 
 it cannot be separated. The hydrofluosilicic acid reaction is of value to 
 distinguish it from muscovite. 
 
 Wagnerite (15) 
 
 Wagnerite is probably much more widespread than it is at present 
 supposed to be. It is only known as an associate of siderite in phyllitic 
 rocks. Its optical properties are not very characteristic, but it may be 
 isolated quite easily on account of its high specific gravity. The reaction 
 for phosphate is a good test for it. 
 
 Kaolin (15) 
 
 Feldspar rocks alter to kaolin by post-volcanic processes and 
 the alteration affects the plagioclase first and later the ortho- 
 clase, while imcrocline resists it almost entirely. Kaolin forms 
 extremely fine, scaly aggregates, which so far as is positively 
 
 
 FIG. 337. 
 
 Cleavage Plate. 
 
 Kaolin, 
 
 Plane of Symmetry. 
 
 known, are always porous and are scarcely ever observed in thin 
 section. The usual alteration products of feldspar, which cause 
 the cloudiness that is so widespread in them, do not belong to 
 kaolin as is commonly supposed, but are distinguished from it 
 even in the densest aggregates by a much more brilliant inter- 
 ference color. It is probably sericite. 
 
 When the presence of kaolin can be positively proved, the 
 rocks are so much altered that the kaolin, which is easily sus- 
 pended in water, can be isolated by simply washing and it can 
 be identified by chemical tests. The mineral is quite important 
 as a constituent of secondary deposits of kaolin sandstone and 
 
DESCRIPTIVE SECTION 319 
 
 kaolin clay. It is by no means positively determined to what 
 extent it occurs in plastic and non-plastic clays because the clay 
 substance, that is always present in these sediments and is 
 called kaolin, in many cases, does not prove to be kaolin by 
 chemical analysis. The same is true for its distribution in the 
 normal products of weathering and in soils, in which kaolin was 
 assumed as a constituent, without further investigation. It is 
 doubtful whether the scaly mineral in such occurrences can be 
 distinguished from kaolin or not, although it differs from kaolin 
 greatly in chemical composition, being an alkali mineral. Differ- 
 entiation may be accomplished by an application of the method 
 of Schroeder van der Kolk, Part I, page 38. 
 
 Nontronite (15) 
 
 Nontronite is not a rare constituent of greatly altered eruptive and con- 
 tact rocks. It may be formed from any mineral whether it bears ferric 
 oxide or not, and occurs especially in rocks that have been so shattered that 
 the inner texture is entirely porous. It sometimes forms a scaly sulphur 
 yellow coating in crevices of rocks or it penetrates the whole rock in a 
 regular manner and forms earthy pseudomorphs after it. Greenish-yellow 
 masses with a choncoidal fracture and impregnated with opal, chloropal, 
 are not rare in such cases. Nontronite is nearly always determined as 
 epidote on account of its color and double refraction but is distinguished 
 from it by much lower indices of refraction. 
 
 A mineral very similar to nontronite in appearance and optical properties 
 is very common as a crust in crevices especially on the surface of rocks con- 
 taining pyrite. This is copiapite, a basic ferrous sulphate, that is decom- 
 posed in water. 
 
 Hydromagnesite (15) 
 
 Hydromagnesite is found in dense masses as pseudomorphs after minerals 
 rich in magnesium and also in flaky or fibrous aggregates in crevices of rocks 
 rich in magnesium. It generally shows twinning lamination similar to 
 that of plagioclase. The aggregates are very fine, fibrous and irregular 
 and can be recognized with difficulty. Effervescence shows its presence in 
 a thin section if other carbonates are not present. 
 
 Cordierite (15) 
 
 Cordierite forms crystals with short prismatic habit that are 
 usually quite distinctly visible macroscopically in granites, 
 quartz porphyries and other extrusive rocks, but become micro- 
 lites in fritted sandstones. They are frequently in the form of 
 
320 
 
 PETROGRAPHIC METHODS 
 
 (01O) 
 
 (010) 
 
 FIG. 338. Cordierite, Section 
 through a Trilling. 
 
 penetration twins appearing in cross section as shown in Fig. 
 338. The edges are generally rounded and the crystals corroded 
 in varied manners. It is also found quite widespread in the 
 form of grains in clay slate hornfels, and these often show de- 
 cided twinning lamination. A portion of the knotty aggregates 
 in a spotted slate, Knotenschiefer, consists of poorly developed 
 individuals of cordierite very rich in in- 
 clusions. In these rocks it is colorless in 
 thin section and is frequently penetrated 
 by long slender needles of sillimanite, 
 Fig. 212, page 196. It also contains a 
 large number of inclusions of rutile and 
 zircon with typical yellow pleochroic 
 halos in which the double refraction is 
 lower, but the dispersion of the optic 
 axes and the indices of refraction are 
 higher and the ray vibrating parallel to c appears brilliant yellow. 
 If such characteristic inclusions are lacking in colorless cor- 
 dierite it is distinguished from quartz by observation in con- 
 vergent light, by special chemical reactions, page 174, and by 
 the beginning alteration of cordierite into greenish micaceous 
 aggregates, which begins on the borders or at the inclusions. 
 The pseudomorphs sometimes consist of parallel aggregates, 
 gigantolite, and sometimes of irregular masses similar to snow 
 crystals in polarized light, pinite. The pleochroic halos often 
 remain as brown specks after the alteration. Cordierite often 
 appears very similar to plagioclase on account of its twinning 
 lamination. If the characteristics mentioned above are lacking, 
 its determination in thin section may become very difficult. The 
 hydrofluosilicic acid reaction affords positive proof of the mineral. 
 When it occurs as a fresh constituent of extrusive rocks or in 
 tuffs it is generally decidedly blue in thin section and then shows 
 distinctive pleochroism. It becomes nontransparent before the 
 blowpipe and melts with difficulty. 
 
 Wavellite (15) 
 
 Wavellite is not rare in crevices of sedimentary rocks in which it forms 
 perfectly radial aggregates. It has apparently resulted from decom- 
 position of organic phosphates. Its distribution has never been deter- 
 mined microscopically, but the extraordinarily typical form of aggregation 
 together with the high double refraction afford some clues to the identity 
 of the mineral. 
 
DESCRIPTIVE SECTION 
 
 321 
 
 B ---J 
 
 Gypsum (15) 
 
 Gypsum is a prominent constituent of numerous members of 
 the halite formations, but it is also found in sedimentary rocks 
 other than these. Its origin from anhydrite can be determined 
 in many cases. It is also found as a product of solfataras and 
 sulphur springs and is sometimes formed from carbonate rocks or 
 occurs as impregnations in volcanic tuffs. In the latter case 
 it is generally accompanied by sulphur. 
 Gypsum is often found as a secondary 
 formation in the iron caps of sulphide ore 
 deposits and in calcareous rocks where 
 sulphides have been altered. 
 
 Rocks consisting predominantly of 
 gypsum are very soft and are generally 
 crossed by numerous veins of fibrous 
 gypsum. The color is white or gray, due (100) 
 to carbonaceous substance, or yellow to 
 red, due to ferric hydroxide. If gypsum 
 occurs as a subordinate constituent of 
 rocks it can only be recognized when 
 cleavage plates with pearly luster appear 
 distinctly. It is difficult to determine 
 under the microscope. Twinning lamina- 
 tion and fibrous fracture, which are distinctly visible in crystalline 
 aggregates, are rarely observed in the denser varieties. Positive 
 evidence of the presence of gypsum in rocks is obtained by leach- 
 ing the rock with water when small microscopic crystals of 
 gypsum are formed from the solution. 
 
 Feldspar Group (16) 
 
 The feldspars are the most widespread rock-forming minerals. 
 Accurate investigation of them is especially interesting on ac- 
 count of their importance because they form the basis for classi- 
 fying the eruptive rocks. Separation of monoclinic orthoclase 
 from triclinic plagioclase is far from sufficient for the modern 
 petrographical classification. It is also recognized that a natural 
 petrographical system founded on a chemical basis, is not possible 
 without determining the exact properties of the feldspar. For 
 these reasons many investigators have set about to determine 
 the composition of these minerals in an optical way without the 
 21 
 
 FIG. 339. Gypsum, Plane 
 of Symmetry. 
 
322 
 
 PETROGRAPHIC METHODS 
 
 aid of chemical analyses, which at best require much time and 
 are often not possible. Many methods, giving results in a more 
 or less direct manner, have been worked out so that now an 
 experienced petrographer can determine quite accurately with 
 the microscope, the chemical properties of a plagioclase in thin 
 section. 
 
 The dimensions of rock-forming feldspars are extremely 
 variable. Orthoclase crystals several cubic meters in size occur 
 in certain pegmatites, and crystals from the size of an egg to those 
 the size of a man's head occur in granite porphyries. All possible 
 dimensions are found from these down to the minutest microlites 
 of very glassy rocks. The microlites are often so small that even 
 in a thin section several individuals may overlap each other. 
 The plagioclases develop into large crystals much less than 
 orthoclase and in most instances they cannot be determined 
 macroscopically. 
 
 The crystallographic development of orthoclase is in the main 
 not very different from that of plagioclase. The combination 
 occurring most frequently on adularia crystals, viz., a prism with 
 
 FIG. 340. 
 
 FIG. 342. 
 
 Usual Types of Feldspar Crystals. 
 
 a rear hemidome, Fig. 340, appears to be the typical form of 
 an orthoclase and the cross sections are principally rhombic 
 rhombporphyry , Fig. 343. They are sometimes rounded and 
 sometimes elongated like a lancet. Another type of development 
 particularly common on orthoclase shows the above combination 
 modified by the addition of a side pinacoid which truncates the 
 sharp edges of the prism, Fig. 341. The basal pinacoid and a 
 number of other end faces are often present. Fig. 178, page 181, 
 shows the appearance of cross sections of the more isometric 
 individuals. The side pinacoid may become more and more 
 dominant in the combination and the crystals assume a thick 
 or thin tabular habit. This. is seen characteristically in cross 
 
DESCRIPTIVE SECTION 323 
 
 sections of plagioclase rich in lime, Fig. 344. Finally a type 
 elongated parallel to the a axis is quite widespread, Fig. 342. 
 Cross sections of it appear quadratic in orthoclase and nearly 
 quadratic in plagioclase, about 93, Fig. 371, page 338, because 
 of the equal development of the basal and side pinacoids. 
 
 The cleavage of the feldspars is about the same in the different 
 members, but in thin section it occurs in various ways, according 
 to the condition of the feldspar. A system of long sharp cracks, 
 parallel to the base and less perfect ones parallel to the side pina- 
 coid are seen in thin sections of the fresh unaltered occurrences 
 
 FIG. 343. Rhombporphyry. Kolsaas FIG. 344. Labradorite Porphyrite with 
 
 near Cristiania. Tabular Plagioclase. 
 
 of the Central Alpine granites and schists. Parting parallel to 
 the orthopinacoid often appears more distinctly in fresh sanidine. 
 Cleavage is less perfect in the clouded occurrences and is often 
 indicated in thin section more by the arrangement of decomposi- 
 tion products than by real cracks. When the rock is crushed, the 
 feldspars cleave very differently. In orthoclase cleavage plates 
 parallel to the base predominate, while in plagioclase plates 
 parallel to the side pinacoid are most common because of the 
 lamellar development parallel to it. Microscopic investigation 
 of these cleavage plates in parallel and convergent polarized 
 light affords considerable data for determining the feldspar. 
 
 Alteration is extremely common and those feldspars, that are 
 colorless when fresh, assume a clouded appearance and often be- 
 come deeply colored. In thin section the color can be seen to be 
 due to inclusions. Orthoclase usually appears with deep red 
 
324 
 
 PETROGRAPHIC METHODS 
 
 and brown colors while plagioclase is usually not so intensely 
 colored and is more apt to be yellow or greenish. In zonal 
 crystals the alteration often begins in the center and proceeds 
 outward along the cleavage cracks or it attacks the various zones 
 differently. The usual cloudiness is produced in orthoclase as 
 well as in plagioclase by fine, scaly aggregates of sericite. Kaolin, 
 on the other hand, is entirely independent genetically and is 
 wholly a local alteration. 
 
 Mechanical structures are not very widespread in the feld- 
 spars. The occurrence of fractured feldspar crystals in normal 
 extrusive rocks is characterized as protodase. The twinning 
 lamellae of plagioclase are often bent by orogenic processes while 
 orthoclase frequently assumes ' a lattice structure similar to 
 microcline. Both may be crushed and assume the cataclase 
 structure. 
 
 The feldspars are divided into two groups: 
 
 a. Alkali feldspars; orthoclase, microcline, anorthoclase. 
 
 b. Soda lime feldspars or plagioclase. 
 
 a. The Alkali Feldspars 
 
 The alkali feldspars occur as rock constituents in three forms 
 and two of these are monoclinic, orthoclase and sanidine. They 
 are distinguished by the size of the optic angle which in ortho- 
 
 FIG. 346. 
 
 Orthoclase Cleavage Plates. 
 Parallel Base. Parallel Plane of Symmetry. 
 
 clase is very large and in sanidine is always much smaller and is 
 often zero. Further, the plane of the optic axes in orthoclase is 
 always perpendicular to the plane of symmetry and in sanidine it 
 
DESCRIPTIVE SECTION 
 
 325 
 
 is sometimes parallel, Figs. 345 to 348. In the last case, strong 
 inclined dispersion of the optic axes is especially noticeable and 
 this is characteristic for sanidine. When orthoclase is slowly 
 heated, it may be noted that the optic angle gradually diminishes 
 to zero and at about 500 the axes separate again in a plane per- 
 pendicular to the first. When heated for a long time at a tem- 
 perature above 600 the orientation remains the same even upon 
 cooling. The geological conclusions concerning the temperature 
 
 FIG. 348. 
 
 Sanidine, Cleavage Plates, 
 Parallel Base. Parallel Plane of Symmetry. 
 
 at which the rocks are formed, drawn from this phenomenon, 
 have been shown to be entirely intenable. Triclinic microcline 
 is the third form of alkali feldspar. 
 
 Orthoclase sometimes develops in well outlined crystals which 
 are generally distinctly visible macroscopically and may attain 
 considerable size, especially in granite porphyry. Sometimes the 
 crystals are somewhat rounded. The small microlitic individuals 
 in the ground mass of trachytes, etc., may have perfect crystal- 
 lographic outline especially in rocks containing glass. The 
 crystals show the variable habits sketched above. The cross 
 sections are six-sided or rectangular to long lath-shaped. The 
 last form is common, especially in the minute individuals in the 
 ground mass of trachytes, etc. Fig. 183, p. 184, shows the lath- 
 shaped cross sections of this type with fluidal arrangement and 
 the brittleness of sanidine phenocrysts also appears distinctly in 
 this figure. 
 
 Twins are extremely widespread and generally two individuals 
 lie side by side separated by a more or less straight line. Twins 
 
326 
 
 PETROGRAPHIC METHODS 
 
 (001) 
 
 (QOl) 
 
 (100) 
 
 (100) 
 
 FIG. 349. Carlsbad Twin, Section 
 Parallel Clinopinacoid. 
 
 according to the Carlsbad law are most common. The ortho- 
 pinacoid is the twinning plane and the development may be such 
 
 that this is also the composition plane, 
 Fig. 349, or, as is more frequently the 
 case, the two individuals are grown 
 together with the clinopinacoid for 
 the composition plane, Fig. 350. The 
 Baveno is another, less common 
 twinning law. Here the clinodome 
 {011} is the twinning plane. This 
 type is found particularly in the crys- 
 tals developed prismatic along the a 
 axis. Fig. 351 shows a cross section 
 of it. The two individuals are almost 
 exactly at 90 to each other and 
 therefore extinguish simultaneously, 
 but equivalent vibration directions 
 are crossed in them. Twinning parallel to the base 
 and to other forms occur now and then but are gener- 
 ally very difficult to determine. Frequently the 
 crystals show zonal development. Zonal arrange- 
 ment of inclusions is often observed in fresh indi- 
 viduals. The zonal structure appears more distinctly 
 when the crystals are weathered because the different 
 zones alter differently. 
 
 Regular intergrowth of orthoclase with other min- 
 erals is frequent, above all with plagioclase. The 
 
 latter may penetrate orthoclase 
 throughout in parallel position so 
 that a fine network of acid plagioclase 
 with orthoclase results perthite Fig. 
 352, or the reverse may be true 
 antiperthite. A section perpendicular 
 to that shown in Fig. 352 appears 
 wholly fibrous even in ordinary light. 
 In other cases a few irregular elon- 
 gated spindles of plagioclase are reg- 
 ularly intergrown with orthoclase. 
 Such intergrowths are widespread in purely microscopic dimen- 
 sions microperthite and finally they become so fine that their 
 presence can be recognized only by the imperfect extinction of 
 
 (001) 
 
 (001) 
 
 (010) 
 
 (010) 
 
 FIG. 351. Baveno Twin, Section 
 Parallel Orthopinacoid. 
 
DESCRIPTIVE SECTION 327 
 
 the cross sections cryptoperthite. Orthoclase sometimes occurs 
 as a border around plagioclase crystals, but the reverse of this is 
 a rare occurence. 
 
 Graphic intergrowth of orthoclase and quartz is very impor- 
 tant. Long parallel rods of quartz penetrate through the feld- 
 spar crystals and the cross sections of the quartz show an angular 
 outline similar to writing, Fig. 353. This intergrowth is very 
 frequent in aplites and in the ground mass of quartz porphyry, 
 but often in microscopic dimensions. Then it is called micropeg- 
 matite. In many cases this becomes more and more indistinct. 
 
 FIG. 352. Perthitic Intergrowth of FIG. 353. Micropegmatite. Aplite, Col 
 
 Orthoclase and Plagioclase. de Tourmalet, Pyrenees. 
 
 It forms an extremely fine aggregate often appearing somewhat 
 radial granophyre until finally it passes over into a dense 
 aggregate of particles that are only transparent in the thinnest 
 sections and very little or no effect at all can be observed on 
 polarized light microfelsite. 
 
 In another kind of intergrowth of quartz with feldspar the 
 cross sections of the quartz rods are rounded and worm-like 
 myrmicoidal structure, Fig. 259, p. 248. This is beautifully seen 
 in rounded grains in granite, granulite, and quartz diorite and in 
 injected schists and it can frequently be positively proved that 
 in this intergrowth the feldspar is a plagioclase. 
 
 Inclusions are not very common in orthoclase, but all of the 
 other rock constituents may be found as inclusions in it. The 
 phenocrysts of a granite porphyry may be quite rich in such 
 foreign minerals. Biotite flakes and quartz crystals can be seen 
 with the naked eye and these sometimes show the zonal arrange- 
 
328 
 
 PETROGRAPHIC METHODS 
 
 ment. Liquid inclusions are comparatively rare. Glass and 
 slag inclusions are observed abundantly in glassy extrusive rocks. 
 The regularly arranged small red plates which cause the copper 
 red iridescence of sunstone are also to be mentioned. They 
 are generally considered to be hematite, but this has not been 
 definitely proved. 
 
 Orthoclase, as a constituent of granite and quartz porphyry, 
 is one of the most widespread minerals and is very important as 
 a source of potassium for the soils. It is also very common in 
 injected schists, but is rarer in contact rocks. It is only a subor- 
 dinate constituent of clastic rocks in which it is rather fresh. 
 
 In central granite the orthoclase is entirely fresh and is 
 characterized by a very perfect cleavage adularia. The form 
 known as sanidine is likewise often glassy, but it has a more 
 concoidal fracture. Ordinary orthoclase is in a state of altera- 
 tion, which sometimes begins on the edge, sometimes in the 
 center and sometimes in certain zones. It gives rise to cloudy, 
 
 (OO/) 
 
 (010) 
 
 FIG. 354. Microcline, Lattice Structure. 
 
 Fio. 355. Microcline, Explanation 
 of Lattice Structure. 
 
 scaly aggregates assembled chiefly along the cleavage cracks. 
 Ordinary alteration gives rise to sericitic substances, the double 
 refraction of which can be recognized even in the very fine, scaly 
 particles. Kaolinization is a very different process from this 
 and is entirely local. The aggregates resulting from it have 
 scarcely any double refraction at all and are generally so crumbly 
 that they are not observed in a thin section because they fall out 
 during the grinding. 
 
DESCRIPTIVE SECTION 
 
 329 
 
 Microcline is very similar to orthoclase macroscopically but 
 they can be differentiated under the microscope by different 
 behavior in polarized light. The former can always be recog- 
 nized beyond a doubt by the characteristic twinning lamination 
 known as lattice structure, Fig. 354. This distinctive property 
 appears to be the result of lamellar twinning according to the 
 albite law. The lamellae do not lie side by side as in the case of 
 the plagioclases, but cut each other in the manner shown in Fig. 
 355. The triclinic character of microcline appears on the cleav- 
 age plates which have an extinction of 15 and show an imsym- 
 metrical interference figure, Fig. 356. 
 
 doi) a 
 
 (101) 
 
 (no) 
 
 FIG. 356. Microcline, 
 Cleavage Plate Parallel Base. 
 
 (010) 
 
 FIG. 357. Anorthoclase. 
 
 The twinning mentioned for orthoclase is also observed on 
 microcline and likewise the intergrowth with albite microcline 
 perthite. It is sometimes found in very large crystals occurring 
 in pegmatites of the granite series or of the soda rocks. It often 
 occurs with a very brilliant red or green color amazonstone. 
 It is widespread as a rock constituent in granites and injected 
 schists but only the microscope reveals it as the last mineral 
 to separate out. It only shows crystal form in the neighborhood 
 of cavities. It is entirely lacking in extrusive rocks but the 
 orthoclase of these rocks sometimes shows the microcline struc- 
 ture as a result of pressure. Microcline is also very common in 
 granular soda rocks but its properties are much less regular. 
 
 Its great resistance to weathering is noteworthy. It is often 
 found unaltered in soil and in kaolinized granite when the other 
 feldspars have been entirely destroyed. 
 
 Soda orthoclase or anorthoclase is also triclinic, Fig. 357. It 
 
330 
 
 PETROGRAPHIC METHODS 
 
 is confined to the soda rocks in which it replaces orthoclase. In 
 some of these rocks it shows a characteristic rhombic form, 
 previously mentioned, while in others it shows the same develop- 
 ment as orthoclase. A brilliant bluish iridescence is quite com- 
 mon. It may be observed in thin section that anorthoclase is 
 united to cryptoperthite by all possible transition stages. Twin- 
 ning is exceptionally common. However, the lamellar structure 
 
 according'to the different laws 
 is so fine that it only appears 
 distinctly in thin section. 
 The laminated sections do 
 not have sharp boundaries 
 between the parts and these 
 finally grade over into homo- 
 geneous sections. The char- 
 acteristic denticulated laths 
 in the ground mass of certain 
 soda trachytes, Fig. 358, are 
 especially distinctive. Be- 
 sides the above phenomena, 
 
 FIG. 358. Soda Feldspar in Bostonite from . * , , _ 
 
 Conny island, Mass., u. s. A. anorthoclase is distinguished 
 
 from orthoclase by a smaller 
 
 extinction on the basal pinacoid and a larger extinction on the 
 side pinacoid running up to 10. It also has a smaller optic angle. 
 All alkali feldspars are not attacked by acids except hydro- 
 fluoric, but it dissolves them quite rapidly. They melt with 
 difficulty. They are distinguished from the plagioclases by 
 lower indices of refraction, even when the twinning lamination 
 is lacking in the latter. Investigation of the powder immersed 
 in benzonitrile (n =1.526) is very useful to distinguish the feld- 
 spars. Orthoclase has lower indices of refraction in all directions 
 than the liquid; anorthoclase and microcline are a little lower in 
 one direction and only a trifle higher in the other; all the plagio- 
 clases have higher indices in all directions. The alkali feldspars 
 are easily distinguished in thin section from quartz, cordierite, 
 scapolite, nephelite, etc., by their indices of refraction being 
 lower than that of Canada balsam. 
 
 b. Soda-lime Feldspars or Plagioclase 
 
 The plagioclase group consists of crystallized triclinic mixtures 
 of soda feldspar or albite, A b, with lime feldspar or anorthite, 
 
DESCRIPTIVE SECTION 
 
 331 
 
 An. It has as yet not been positively determined whether the 
 plagioclases represent a continuous isomorphous series in which 
 
 /;,.-:-. 7 . :: ^^ : ^- 
 
 ; i:,^:>-*. \; >;; :; : 
 
 FIG. 359. Albite. 
 
 FIG. 360. Oligoclase. FIG. 361. Andesine. 
 
 FIG. 362. Labradorite. FIG. 363. Bytownite. FIG. 364. Anorthite. 
 
 the two silicates may grow together in any proportions, or 
 whether only certain compounds with definite composition are 
 
332 
 
 PETROGRAPHIC METHODS 
 
 formed. The latter appears the more probable. At any rate, 
 certain types given in the table are much more common than 
 mixtures that might be placed between them. Phenocrysts in 
 porphyric rocks are well bounded crystallographically as are 
 often the microlitic individuals of the ground mass, especially if 
 the rock contains glass. The plagioclases show isometric forms 
 only in the more acid rocks. In other rocks they are partly 
 
 20 
 
 -jo 
 
 -20 
 
 -10 
 
 FIG. 365. Stereographic Projection of the Optical Constants of the Plagioclases. 
 
 prismatic along the a axis and partly tabular parallel to the side 
 pinacoid, particularly in the more basic varieties. Their cross 
 sections are predominantly lath-shaped, Fig. 366. 
 
 The basal and brachypinacoids, the two principal cleavage 
 directions, form an angle of about 93J. The base is grooved by 
 regular parallel striations produced by the ever present twinning 
 lamination parallel to the brachypinacoid albite law, Fig. 367. 
 These striations on the most perfect cleavage face are the only 
 useful macroscopic distinction from orthoclase, if the latter does 
 not have its characteristic red or brownish color, which does not 
 occur in plagioclase. The twinning lamination is also the best 
 
DESCRIPTIVE SECTION 
 
 333 
 
 characteristic under the microscope. It is rarely recognized in 
 thin sections by the reentrant angles, Fig. 368, but it shows up in 
 polarized light by the different extinction of the lamellae. If it is 
 lacking, as is frequently the case in basic plagioclase of eruptive 
 rocks and very often in contact 
 rocks, the determination of the 
 feldspar is very difficult. The 
 lamination is quite variable, 
 sometimes broad and some- 
 times narrow. The former ap- 
 pears to be the more common 
 in plagioclase rich in lime and 
 the latter in those consisting 
 predominantly of soda. A fine 
 system of lamellae may alter- 
 nate with one band which oc- 
 cupies most of the crystal. The 
 section is regularly laminated 
 in one case, and in the other, 
 
 there are a few fine lamellae, while the rest of the crystal ap- 
 pears homogeneous. Sometimes the stripes pass clear through 
 the individual and sometimes they narrow down and pinch out 
 entirely. Where orogenic pressure has acted on the plagioclase 
 the lamellae .may be bent or broken and may be displaced. 
 
 (00 f) 
 
 Fia. 366. Lath-shaped Plagioclaae in 
 Trap, Reykjavik, Iceland. 
 
 (010) 
 
 Fia. 367. Plagioclase, 
 
 Albite Twinning 
 
 Lamellae. 
 
 FIG. 368. Twin Lamination after the 
 
 Albite Law in a Section Parallel 
 
 to the Macropinacoid. 
 
 In addition to this most frequent twinning there is often another, 
 likewise lamellar, according to the pericline law. The b axis is 
 the twinning axis. This second system of lamellae crosses the 
 first under variable angles, but approximates a right angle in the 
 
334 PETROGRAPHIC METHODS 
 
 zone of the b axis, Fig. 371, page 338. Such polysynthetic twin- 
 ning may be combined with one of the laws for orthoclase, most 
 frequently the Carlsbad, and the appearance becomes quite com- 
 plicated. Globular and star-shaped intergrowths of many indi- 
 viduals are found in certain rocks, particularly andesites and 
 tephrites. By their regularity, they give the impression of a 
 very complicated growth of twins. 
 
 The occurrence of isomorphous layers is extraordinarily fre- 
 quent in intermediate eruptive rocks and is very troublesome in 
 the exact determination of the plagioclase. The different zones, 
 which are sometimes very narrow, extinguish differently. The 
 center always consists of a more basic plagioclase than the outer 
 zones. Twinning lamination continues undisturbed throughout 
 all the zones. Irregular penetration of different plagioclases 
 also occurs in eruptive rocks. Zonal structure is very wide- 
 spread in contact rocks, but here the order is reversed and the 
 center is more acid than the rest. See page 326 for intergrowths 
 with orthoclase. 
 
 The plagioclases are always colorless in thin section. They 
 are very fresh, particularly in unaltered extrusive rocks where 
 they have the appearance of sanidine. This has been distin- 
 guished as microtine. Abundant inclusions of dark glass, often 
 with zonal arrangement, are very common in glassy rocks. Most 
 of the acid plagioclases in certain Pentral Alpine granites and 
 tonalites are fresh and transparent, but they are often filled with 
 a large number of microlites so that they appear clouded macro- 
 scopically and become transparent only in very thin slides. The 
 microlites are well bounded crystallographically, Fig. 274, page 
 264, and generally they are orientated at random in the fresh 
 mass of the feldspar. Sometimes they are muscovite, sometimes 
 sillimanite or garnet, but in most cases they are members of the 
 epidote group poor in iron. These plagioclases are light reddish 
 in color when they contain garnet, but the epidote minerals pro- 
 duce a light greenish-yellow color. 
 
 Plagioclase is generally clouded in the eruptive rocks and its 
 alteration is quite analogous to that of orthoclase except that 
 plagioclase containing lime is more easily attacked, and epidote, 
 calcite, chlorite, etc., frequently occur as by-products of the 
 alteration. Pseudomorphs of hydrargillite after plagioclase have 
 been found locally in greatly altered rocks and replacement by 
 zeolite is not at all uncommon in soda rocks. Brown tabular 
 
DESCRIPTIVE SECTION 
 
 335 
 
 microlites with parallel orientation occur as inclusions, particu- 
 larly in basic plagioclase. These are often the cause of the cha- 
 toyancy. Such feldspars sometimes appear brownish-black mac- 
 roscopically, but under the microscope they seem to be filled with 
 dust. This appearance may also depend upon similar inclusions 
 minutely divided. 
 
 The two end members of the plagioclase series are the rarest 
 as rock constituents. Albite is the commonest representative 
 of the series in granite pegmatites, in which it is the last mineral 
 to crystallize. It occurs as a rock constituent in perthitic inter- 
 growth with orthoclase, but aside from this it is a rare exception 
 in granites. It occurs more frequently in acid members of the 
 soda series, in which it is often the only feldspar. Clouded indi- 
 viduals of it are present in the corresponding extrusive rocks, 
 especially in keratophyres. Oligoclase to andesine are the usual 
 plagioclases in acid and intermediate eruptive rocks. Labra- 
 dorite is sometimes quite widespread, for example, in monzo- 
 nite. It is the typical feldspar of gabbro and trap, sometimes 
 passing over into bytownite, but rarely into anorthite. In 
 general, phenocrysts in por- 
 phyrites and andesites ap- 
 proach labradorite, while more 
 acid plagioclase predominates 
 in the ground mass. 
 
 Plagioclase is much fresher 
 in contact rocks and is usually 
 clear and transparent. Its 
 lack of twinning lamellae has 
 been mentioned above. It 
 shows good crystal form only 
 in granular carbonate rocks in 
 which isolated microscopic in- 
 dividuals of albite are very 
 widespread, or in the lime- 
 silicate fels formed near them. Here anorthite often forms large 
 individuals. The Knoten in certain Alpine graphite schists are 
 often composed of rounded crystals of albite. These are more 
 commonly granular aggregates with beautifully developed mosaic 
 structure, Fig. 369. They are often very rich in inclusions. 
 
 Under the influence of piezocontact metamorphism, those 
 plagioclases rich in lime decompose into calcium-aluminium sili- 
 
 FIG. 369. Albite in Green Schist. Tummel- 
 bachtal, Grossvenediger. Mosaic Structure. 
 
336 PETROGRAPHIC METHODS 
 
 cates like clinozoisite, garnet, etc., and into a granular mosaic 
 of clear plagioclase low in lime. The higher the pressure during 
 the recrystallization the more the latter approaches albite. It 
 is, therefore, almost always pure albite in the schistose masses 
 of the Alps. Under lower pressure the plagioclase simply recrys- 
 tallizes so that in the normal contact metamorphism of diabase 
 the original lath-shaped crystals retain their outward form, but 
 consist of a granular aggregate of the same feldspar. 
 
 Another type of alteration, effecting especially basic plagio- 
 clase in eruptive rocks, is quite analogous in its entire character 
 to piezocontact metamorphism. This is saussuritization and is 
 usually accompanied by uralitization of the pyroxene. Macro- 
 scopically the saussurite appears to have the form of the feldspar, 
 but it has no trace of cleavage but consists of dense, hard, heavy 
 aggregates with splintery to conchoidal fracture and is character- 
 ized by great tenacity. Its color is greenish or yellowish, passing 
 over into almost pure white. It has a clouded appearance under 
 the microscope even in very thin slides because of the small size 
 of the irregular individuals with high indices of refraction. These 
 minerals are most frequently members of the epidote and garnet 
 groups poor in iron, vesuvianite, lawsonite, prehnite often 
 penetrated by hornblende needles or flakes of chlorite. The 
 host of all this aggregate seems to be plagioclase similar to albite. 
 A definite conclusion concerning the composition of saussurite 
 is very difficult because of the poor development of the various 
 constituents and the similarity of their optical properties. Pseu- 
 domorphs of chlorite or talc after different feldspars are found 
 locally as rare formations. 
 
 Acid plagioclase is effected by acids just as the alkali feldspars 
 are. Basic plagioclase is readily attacked by hot hydrochloric 
 acid and the richer in lime it is the more easily it is gelatinized. 
 Acid members are rather difficultly fusible before the blowpipe 
 and the basic are almost infusible. 
 
 In many cases it Jhklesirable to determine the plagioclase accurately 
 because of the importance of this mineral in the classification of the eruptive 
 rocks. The most reliable means for this purpose is quantitative chemical 
 analysis, but it requires a great deal of time and frequently enough pure 
 material cannot be obtained for it. Determination of the silica content 
 alone would be quite useful as will be seen from column 6, Table 16. The 
 difficulty here is the same as in the preceding case. 
 
 A number of qualitative chemical tests can be made on small fragments 
 that can be easily isolated in a pure condition from the rock, but they do 
 
DESCRIPTIVE SECTION 
 
 337 
 
 not give very definite results. Thus attempts have been made to estimate 
 the soda content of a feldspar by the color produced by the mineral in a 
 flame or by the relative amounts of sodium fluosilicate and calcium fluo- 
 silicate in the residue after the mineral has been decomposed by hydrofluoric 
 acid. The end members may be distinguished by their behavior toward 
 hydrochloric acid. Determination of the specific gravity, column 7, gives 
 better results than the above methods. A very pure minute grain can be 
 used for this purpose. 
 
 A few of the numerous optical methods for the determination of the plagio- 
 clase are much more reliable than all the above tests. The optical orienta- 
 tion of the plagioclases is known in a general way, although there is some 
 disagreement about minor details so that determinations by different 
 methods have given different results. This does not appear to be due to 
 the method used alone but is dependent, to a certain extent at least, upon 
 varying optical properties caused by uncontrollable contingencies. Optical 
 methods cannot replace quantitative chemical analysis if there is suitable 
 material for it. They often give better results than the latter, however, 
 under the conditions in which a petrographer finds them. 
 
 The optical properties of the plagioclases have been thoroughly in- 
 vestigated because of the importance of these minerals in the classification 
 of the eruptive rocks. Figs. 359 to 364, page 331, 
 and the projection hi Fig. 365 give a general survey 
 of these properties. A and B in these figures are 
 the optic axes and a, /?, 7* are the principal vibra- 
 tion directions. The index 1, Fig. 365, refers to 
 albite, 2 to oligoclase, etc., corresponding to the 
 six figures 359 to 364. In general, mean values 
 are used as the basis for construction so that there 
 are small deviations from the true relationships, but 
 a synopsis of the whole can be obtained from the 
 figure. 
 
 It is understood that in all the determinations 
 with these figures or with table 16 belonging to 
 them, the orientation of the section, if it is not a 
 cleavage piece, is determined much more accurately 
 in convergent polarized light than in parallel light, 
 but for other purposes only measurements in paral- 
 lel polarized light yield good results. The optic 
 angle of plagioclase is so large that its measure- 
 ment and the determination of the optical charac- 
 ter even with an immersion system give doubtful 
 results. The dispersion is so weak that it cannot 
 
 be positively determined in thin section. The double refraction is almost the 
 same for all members except the most basic. These properties are therefore 
 of no use to distinguish the different members of the plagioclase group. 
 
 The most important optical methods for determining them are the fol- 
 lowing : 
 
 1. The oldest and simplest is the method of determining the extinction 
 angle on cleavage plates parallel to P= {001} and M= {010} ; see table 16, 
 22 
 
 FIG. 370. Sign of the Ex- 
 tinction in Plagioclase. 
 
338 
 
 PETROGRAPHIC METHODS 
 
 (004) 
 
 fOJO) 
 
 Fia. 371. Orientation of a Plagioclase 
 Crystal Elongated along the a Axis. 
 
 columns 16 and 17. The significance of the sign in this determination is 
 explained by Fig. 370. This method gives results rapidly if the individuals 
 of plagioclase are large and fresh. It can also be used on very small in- 
 dividuals under certain circumstances. A small fragment of rock is crushed 
 between two object glasses and the resultant fine sand is used for the in- 
 vestigation. The best cleavage is parallel to the basal pinacoid, but the 
 cleavage plates are more often parallel to the brachypinacoid on account 
 of the lamellar development of the crystal parallel to this face. The 
 interference figures of such plates are not characteristic except for the end 
 members. 
 
 2. All sections perpendicular to the brachypinacoid, i.e., the twinning 
 plane of the albite law, show extinction of the individuals symmetrical to 
 the trace of the twinning plane. Such sections may be recognized in a slide 
 
 by the symmetrical extinction of the two 
 parts. The maximum extinction in this 
 zone, column 21, can be found approxi- 
 mately by measuring a large number of 
 sections showing symmetrical extinction 
 in a slide. These measurements give 
 valuable results, particularly with minute 
 individuals in the ground mass of ex- 
 trusive rocks, provided that all the 
 plagioclase cross sections do not have 
 about the same orientation on account of 
 flowage of the magma. tf ' "'M 
 
 Sections perpendicular to the a axis, column 20, also have a special signifi- 
 cance. These sections are rhombic, nearly quadratic, in outline on the 
 small individuals due to equal development of the basal and brachypina- 
 coids, Fig. 371. Cleavage parallel to the base and the albite twinning 
 lamellae are both perpendicular to these sections. The extinction on such 
 sections is a splendid means for determining plagioclase microlites that can 
 scarcely be determined by other methods. If the positive or negative 
 direction of the extinction cannot be determined the results may have a 
 two-fold meaning, at least with the acid plagioclases. 
 
 3. Fouque Method. Determination of the extinction angle in 
 sections in which the optical orientation can be easily and quite 
 accurately determined by investigation in convergent polarized 
 light, is very important. The best sections are those perpendic- 
 ular to one of the two bisectrices, columns 18 and 19. Sections, 
 orientated favorably enough, can be found quite readily with a 
 little practice, in almost any slide. The index of refraction of the 
 plagioclase is compared with that of the Canada balsam and then 
 by studying the figures 359 to 364, an idea of the direction of the 
 cleavage or twinning lamellae in the section in question is obtained. 
 Sections are then found corresponding to this conception, these 
 showing an interference color about half as high as the highest 
 
DESCRIPTIVE SECTION 339 
 
 color possible in plagioclase. When a section is found as nearly 
 perpendicular to a bisectrix as possible, its character is deter- 
 mined with a gypsum test plate, regardless of whether it is the 
 acute or obtuse bisectrix. On sections perpendicular to the 
 negative bisectrix, the angle between the optic plane and the 
 trace of the twinning plane can always be measured. This is 
 apparent in Figs. 359 to 364. Sections perpendicular to the 
 positive bisectrix give the angle between the cleavage parallel 
 to the base and the optic plane, while the twinning lamellae are 
 so oblique that they cannot be observed at all or are not very 
 sharp. Column 18 gives the angles between the optic plane and 
 the twinning lamellae of the albite law and the values in column 19 
 are the angles between the optic plane and the cleavage parallel 
 to the base. Both of these values together furnish a distinctive 
 characteristic for a feldspar, while only one of them is not suffi- 
 cient for the determination, in many cases. 
 
 4. In double twins where the albite and Carlsbad laws occur simultan- 
 eously, there are four conjugate extinction angles for the different parts in 
 each section where, these two laws appear distinctly. If 1 and 2 are the two 
 individuals twinned according to the Carlsbad law, each contains albite 
 lamellae I' and 2'. Each member of the pldgioclase series has a character- 
 istic value for 2 and 2' corresponding to a certain value for 1 and 1'. This 
 can be deduced from the stereographic projection. To take this up in 
 detail would lead too far for this text. 
 
 5. The mean value of a number of extinction angles, measured from the 
 twinning plane on several random sections in a slide, also furnishes some- 
 what of an indication of the kind of plagioclase. Thus more than half of 
 all sections of anorthite show an extinction between 31 and 50 and oligo- 
 clase between and 5. It follows from this that the twinning lamellae 
 are not distinctly seen in the latter case where the difference in extinction 
 of the two parts is so small, while in the former case the twinning appears 
 very distinctly. 
 
 6. As shown in Fig. 365, one of the optic axes in basic plagioclase is not 
 very much inclined to the vertical axis. The point at which this axis emerges 
 in each component of a double twin according to albite and Carlsbad laws 
 can be determined from the interference figure if the section is not greatly 
 inclined to the vertical axis. <p and X can be determined from this with 
 respect to the twinning plane and these values can be used to determine 
 the plagioclase from Fig. 365 in which the values are indicated. Such 
 sections of the members richer in sodium do not show the emergence of an 
 optic axis. This method therefore has very little practical significance. 
 
 7. Schroeder van der Kolk Method. One method, which often 
 gives rise to good results, is based upon the difference in the 
 indices of refraction for the different plagioclases. Whether the 
 
340 PETROGRAPHIC METHODS 
 
 plagioclase is acid, intermediate or basic can be very rapidly 
 determined by comparing its index with that of Canada balsam. 
 More accurate results can be obtained, especially with inter- 
 mediate members, if the different elasticity axes of the plagio- 
 clase are compared with the two vibration directions of the 
 quartz grains lying beside it, column 8, Table 16. This method is 
 especially valuable for the investigation of rock powders, sands, 
 and soils in which the minutest flecks of the feldspar are sufficient 
 to obtain perfect results. A series of liquids can be arranged in 
 well stoppered phials. The liquids must not decompose too 
 easily and they must not be of such a nature as to be impaired 
 very much by use. They are so chosen that the index of refrac- 
 tion of the liquid lies between those of two principal members of 
 the plagioclase series. A grain of the feldspar to be investigated 
 is placed upon an object glass in a drop of the liquid, the index of 
 refraction of which has been previously determined. The cover- 
 glass is laid on and the illuminating cone of light is narrowed 
 down as much as possible. Then the index of refraction of the 
 two vibration directions of the plagioclase is compared with that 
 of the liquid. 
 
 The liquids used are organic because solutions of inorganic 
 salts, described in Part I, page 37, change their indices rapidly 
 upon evaporation. The purity of the liquids and therefore the 
 indices of refraction are rather variable so that the indicators 
 must be tested with a Bertrand refractometer and the true index 
 determined before the series is made up. The values given below 
 were determined by Weinschenk for liquids obtained from Ben- 
 der and Hobein in Munich. The variations for different tests 
 from other sources amounted to about 0.002 to 0.004. 
 
 The following serve to distinguish : 
 
 Alkali feldspar from plagioclase: Benzonitrile n = 1.526 
 
 Albite from oligoclase : Eugenol n = 1 . 540 
 
 Bromtoluol n = l. 550 
 
 Nitrobenzol n = 1 . 552 
 
 Andesine from labradorite: Anethol n = l .558 
 
 Labradorite from bytownite: Monobrom benzol. . . .n = 1.560 
 Bytownite from anorthite: Orthotoluidine n = 1 . 575 
 
 &M?*W XX V/JLJ-I. \SAX^\S VA-*V- JLJ l&g 
 
 Oligoclase from andesine : < 
 
 It is necessary to carry out these investigations in sodium 
 light on account of the strong dispersion of the liquids and the 
 great difference in dispersion between them and the feldspars. 
 The above indices are given for sodium light. 
 
DESCRIPTIVE SECTION 341 
 
 8. The Fedorow universal stage was originally constructed for determining 
 the feldspars; see Part I, page 127. The universal stage is placed on the stage 
 of the microscope, with its principal axis of rotation exactly parallel to the 
 vibration direction of one of the nicols. The horizontal axis of the inner 
 stage will then lie parallel to the other vibration direction. The thin sec- 
 tion is placed on a small round object glass and fastened on the glass stage 
 with a drop of glycerine. The feldspar to be investigated is centered and a 
 plano-convex lens is placed over it and one under the stage, contact being 
 made with glycerine. Then the glass stage is rotated until the trace of the 
 twinning plane is parallel to the horizontal axis of the inner stage. Then 
 it is rotated on this axis until the two individuals of the albite law have the 
 same interference color, i.e., they extinguish symmetrically with respect to 
 the twinning plane. The angle of rotation is read and plotted in the stereo- 
 graphic projection. Then the inner stage is rotated on both of its axes 
 until one part of the twin remains dark when the nicols are rotated. The 
 pole of the axis which emerges perpendicularly is read. The pole of the 
 other optic axis is determined in a similar manner. The position of A and B 
 and of a, /? and f relative to the twinning plane in the albite law is thus 
 determined and the plagioclase can be determined directly from that. 
 
 9. A Wallerant total reflectometer, Part I, page 40, can also be used to 
 determine the plagioclase. The polarizer is pushed out and the critical 
 angle of total reflection is measured. 
 
 Biaxial Zeolites (17) 
 
 All zeolites are colorless of themselves, but they are sometimes colored 
 yellowish-brown to red by inclusions of ferric hydrates or brownish by 
 organic substances. They all have low indices of refraction, lower than 
 Canada balsam. The double refraction corresponds somewhat to that of 
 quartz. It is much lower in phillipsite and harmotome and unusually 
 high in thomsonite. The latter does not appear similar to the other 
 zeolites in thin section and can be determined as such only by its solubility 
 in hydrochloric acid and by its becoming cloudy when slightly heated. 
 Laumontite is another of the zeolites given in the table, which is somewhat 
 peculiar. It gives up a part of its water at ordinary temperature and is, 
 therefore, always cloudy. Some other zeolites show the same phenomenon. 
 
 The zeolites rarely form granular aggregates, but phillipsite and harmo- 
 tome may. Most of them appear tabular like heulandite or radial like 
 epistilbite and desmine. When they are macroscopically visible they nearly 
 always show the latter form of aggregates and this together with the perfect 
 cleavage is sufficient for the determination. 
 
 Since the indices and double refraction of the various zeolites are about 
 constant, the optical character of the principal zone can be used to distinguish 
 some of them, particularly natrolite from scolesite. The latter is also 
 characterized by a small optic angle and strong dispersion of the optic axes. 
 Mesolite stands between these two chemically and also takes an intermediate 
 position with respect to the optical properties. 
 
 Oblique extinction serves to distinguish monoclinic zeolites from the 
 orthorhombic. It is small in the minerals of the stilbite group and natrolite, 
 
342 PETROGRAPHIC METHODS 
 
 medium in scolesite, laumontite and phillipsite and quite large in harmo- 
 tome. Differentiation of the zeolites is, however, very difficult and their 
 significance as rock constituents has by no means been determined. 
 
 All zeolites are typical products of thermal activity. They occur chiefly 
 in amygdaloids, especially near the contacts of eruptive rocks and frequently 
 form beautiful crystals. They are not rare as infiltrations in tuffs and are 
 also found in sediments in the neighborhood of eruptive rocks. Whatever 
 their occurrence, the principal associate is chalcedony and the other modifi- 
 cations of silica. The difficulty qf distinguishing the zeolites from each other 
 is increased by the presence of these minerals, which are very similar opti- 
 cally. The determination is much easier when they occur as secondary de- 
 posits in cavities than when they occur in decomposed soda rocks. In the 
 latter case all kinds of zeolites occur in aggregates similar to ice flowers or in 
 dense pseudomorphs after nepheline, leucite, sodalite minerals and feldspar. 
 These are classed together as spreustein. Determination of the different 
 zeolites is impossible in such an occurrence. They can be shown to be zeo- 
 lites by their solubility in hydrochloric acid and by the cloudiness of the 
 section when it is heated very gently. 
 
DESCRIPTIVE SECTION 
 
 343 
 
 is 
 
 
 S "cij 
 SB a 
 
344 
 
 PETROGRAPHIC METHODS 
 
 
 ISOTROPIC 
 
 Page 
 
 Group 
 
 Minerals 
 
 Ciuemical 
 composition 
 
 Crys- 
 tal 
 system 
 
 Cleav- 
 age 
 
 Development 
 
 Sp. gr. B 
 
 213 
 
 
 Perovskite 
 
 CaTiO 3 
 
 C 
 
 (100) 
 
 Octahedral 
 Hexahedral 
 Granular 
 
 4.0 5 
 
 213 
 
 
 Sphalerite 
 
 ZnS 
 
 C 
 
 110 
 
 Granular 
 
 3.9 
 to 3 
 4.2 
 
 214 
 
 Garnet Group 
 
 Almandine 
 
 Fe 3 Al 2 Si 3 12 
 
 C 
 
 (110) 
 
 Dodecahedral 
 
 Less often 
 tetragonal 
 trisoctahedral 
 
 Granular 
 
 4.0 
 to 
 4.3 
 
 Pyrope 
 
 (MgFe) 3 Al 2 
 Si 3 12 
 
 3.75 
 
 Grossularite 
 
 Ca 3 Al 2 Si 3 O 12 
 
 3.45 
 to 
 4.1 
 
 Topazolite 
 
 Ca 3 Fe 2 Si 3 12 
 
 Melanite 
 
 Ca 3 Fe 2 
 (SiTi) 3 12 
 
 4.3 
 
 218 
 
 a 
 
 Chrome spinel 
 
 (Fe Mg) 
 (Al Cr) 2 4 
 
 C 
 
 
 Octahedral 
 Granular 
 
 4.1 
 to 1 
 4.5 
 
 Iron spinel 
 
 (Fe Mg) 
 (AlFe) 2 4 
 
 3.9 
 
 Spinel 
 
 MgAl 2 O 4 
 
 3.6 
 
 219 
 
 
 Periclase 
 
 MgO Contains iron 
 
 C 
 
 100 
 
 Octahedral 
 Granular 
 
 3.65 
 
 219 
 
 
 Boracite 
 
 Mg 7 Cl 2 B 16 30 
 
 C 
 
 
 Hexahedral 
 Tetrahedral 
 
 2.9 7 
 
 219 
 
 
 Leucite 
 
 KAlSi 2 O 6 
 
 C 
 
 
 Tetragonal 
 trisoctahedral 
 Rounded 
 
 2.5 c 
 
 220 
 
 
 | / acid 
 3 ^ basic 
 
 Variable 
 
 A 
 
 
 
 2.25 
 to 
 
 2.7 
 
 222 
 
 
 Analcite 
 
 NaAlSi,O 6 + aq. 
 
 C 
 
 100 
 
 Granular 
 Tetragonal 
 trisoctahedral 
 
 2.20 .1 
 
DESCRIPTIVE SECTION 
 
 MINERALS. 
 
 345 
 TABLE 1 
 
 (Color 
 
 n 
 
 Solubility 
 
 Remarks 
 
 Gray 
 Violet 
 Brown 
 
 2.38 
 
 Only soluble 
 in cone, sul- 
 phuric acid 
 
 Optical anoma- 
 lies 
 Lamination 
 
 Yellow 
 Brown 
 
 2.37 
 
 Soluble in 
 cone, nitric 
 acid 
 
 Zinc 
 coating on 
 charcoal 
 
 Almost 
 colorless to 
 reddish 
 
 1.76 
 
 to 
 1.81 
 
 Difficultly 
 soluble in hy- 
 drochloric acid 
 Easily 
 attacked 
 after melting 
 
 Often rich in 
 inclusions 
 Siebstructure 
 
 Light red 
 
 1.750 
 
 Kelyphite 
 border 
 
 Variable 
 Frequently 
 colorless 
 Reddish 
 Brownish 
 
 1.750 
 to 
 1.784 
 
 Optical anoma- 
 lies 
 Division into 
 segments 
 
 Brown 
 
 1.856 
 
 Zonal structure 
 
 Brown 
 
 approx. 
 2.0 
 
 Not attacked 
 by acids and 
 molten alkali 
 carbonate 
 Decomposed 
 by potassium 
 bisulphate 
 
 
 Green 
 
 1.750 
 
 Colorless 
 
 1.720 
 
 Colorless 
 . Light green 
 
 1.736 
 
 Difficultly 
 soluble 
 
 Often altered to 
 serpentine and 
 brucite 
 
 Colorless 
 
 1.667 
 
 Difficultly 
 soluble 
 
 Optical anomalies 
 Lattice structure 
 
 Colorless 
 
 1.509 
 
 Gives powdered 
 silica with hy- 
 drochloric acid 
 
 Optical anomalies 
 Lattice structure 
 Inclusions 
 
 Colorless 
 Brownish 
 
 1.49 
 to 
 1.63 
 
 Variable 
 
 Often fluidal 
 
 Colorless 
 
 1.488 
 
 Forms gelatin- 
 ous silica in hy- 
 drochloric acid 
 
 Optical 
 anomalies 
 Segments 
 
 Explanation of the Tables. 
 
 The first vertical column gives the 
 page in the book on which the mineral 
 is described. Under the heading 
 "Chemical Composition" frequently 
 only the principal constituents are 
 given and the formula is simplified so 
 that the chemical constitution of the 
 mineral is expressed only in a 
 general way. In the "Crystal Sys- 
 tem" column, A = amorphous, C = 
 cubic, H = hexagonal, T = tetragonal, 
 O = orthorhombic, M == monoclinic, Tr 
 = triclinic. The symbols for the cleav- 
 age forms are written differently from 
 the customary usage: a symbol in 
 parenthesis indicates imperfect cleav- 
 .age, without them only distinct cleav- 
 age, and underscored it is perfect 
 cleavage. Sp. gr. = specific gravity, and 
 H at the top of a column is for hard- 
 ness. 
 
 The values given for specific gravity, 
 index of refraction (n or a, /?, r) and 
 double refraction are only average 
 values and may vary greatly especi- 
 ally in mixed isomorphous groups. 
 When such values have been accurate- 
 ly measured, as in the case of scapolite 
 and epidote, the maximum and mini- 
 mum are given. 
 
 Color also may vary within wide 
 limits in one and the same mineral 
 Only the colors that are most import- 
 ant for rocks are given in that column 
 and then only those that can be ob- 
 served in thin section. Pleochroism is 
 even more variable than the regular 
 color of the mineral. When it is 
 characteristic and more or less con- 
 stant it is indicated under the indices 
 of refraction. Otherwise the absorp- 
 tion formula, which is much more 
 constant, is given. The character of 
 the double refraction of the mineral is 
 indicated under Chm and that of the 
 principal zone under Chz. 
 
 In the column "Optical Orienta- 
 tion" the 6 axis is always given in bi- 
 axial crystals. It can then be seen at a 
 glance whether the plane of the optic 
 axes is parallel or perpendicular to the 
 plane of symmetry in monoclinic min- 
 erals. The extinction angle is general- 
 ly measured from the vertical axis; / 
 after the number indicates that it is 
 
346 PETROGRAPHIC METHODS 
 ISOTROPIC 
 
 
 Page 
 
 Group 
 
 Minerals 
 
 Chemical 
 composition 
 
 Crys- 
 tal 
 system 
 
 Cleav- 
 age 
 
 Development 
 
 Sp.gr. 
 
 
 223 
 
 Sodalite Group 
 
 Sodalite 
 
 + NaCl 
 
 O 
 
 + Na 2 S0 4 
 
 03 
 
 
 
 eo 
 
 + (Na 2 Ca)SO 4 
 
 C 
 
 (110) 
 
 Dodecahedral 
 Granular 
 
 2.3 
 to 
 2.5 
 
 - 
 
 Noselite 
 
 Haiiyne 
 
 224 
 
 
 Opal 
 
 SiO 2 + z aq. 
 
 A 
 
 
 
 2.2 
 
 
 225 
 
 
 Fluorite 
 
 CaF 2 
 
 C 
 
 111 
 
 Granular 
 
 3.2 
 
DESCRIPTIVE SECTION 347 
 
 MINERALS. Continued. TABLE 2 
 
 Color 
 
 n 
 
 Solubility 
 
 Remarks 
 
 
 1.483 
 
 
 Regularly arrang- 
 ed rod-like in- 
 
 I 
 
 Colorless 
 
 
 Form gelatin- 
 ous silica 
 
 clusions 
 Black corrosion 
 
 1.495 
 
 Blue 
 (Yellow, Green) 
 
 
 with hydro- 
 chloric acid 
 
 borders 
 Often altered to 
 
 
 
 1.504 
 
 
 zeolite 
 
 
 
 
 Difficultly fusible 
 
 Colorless 
 
 1.46 
 
 Soluble in 
 potassium 
 hydroxide 
 
 Index of refrac- 
 tion variable 
 Often anomalous 
 double refraction 
 
 
 
 
 
 
 
 With sulphur- 
 
 
 Colorless 
 
 1 434 
 
 ic acid it gives 
 
 Phosphorescen t 
 
 Violet 
 
 
 off hydrofluoric 
 
 when heated 
 
 
 
 acid vapor 
 
 
 Explanation of the Tables Continued, 
 toward the front or in the obtuse angle 
 /?; r means to the rear. The expres- 
 sions "difficultly soluble" and "insol- 
 uble" refer to hydrochloric acid as a 
 solvent unless otherwise indicated. The 
 next column indicates the most impor- 
 tant phenomena that are sometimes 
 observed. Thus "optical anomalies" 
 only means that the mineral is some- 
 times anomalous and not that it al- 
 ways occurs that way. The meaning of 
 the various columns in table 15, which 
 includes the feldspars, may be detT- 
 mined from the text. Remarks in 
 parentheses refer to rarer and different- 
 ly developed varieties except in the 
 cleavage column as explained above 
 and in the absorption column where 
 the parentheses indicate that the ab- 
 sorption is not a characteristic ear mark 
 of the mineral, p^ti or u^o in the 
 dispersion column indicates that the 
 dispersion of the optic axes is extra- 
 ordinarily strong. 
 
348 PETROGRAPHIC METHODS 
 
 ROCK-FORMING MINERALS, OTHER THAN THE CUBIC, 
 INDEX OF REFRACTION, DOUBLE 
 
 Double refraction -> 
 Index of refraction | 
 
 < 0.005 
 
 to 0.010 
 
 to 0.015 
 
 to 0.020 
 
 + 
 
 ,;- I ETC 
 
 Tridymite 
 Zeolites -> 
 Apophyllite | 
 
 Quartz 
 Gypsum 
 
 <- Zeolites 
 
 Hydromagnesite 
 
 
 Nepheline 
 Apophyllite * 
 
 Chalcedony 
 Cordierite 
 Zeolites -> 
 
 <- Zeolites 
 
 
 + 
 1.55 
 
 Pennine 
 
 Quartz 
 Chrysotile 
 Clinochlore - 
 
 Wagnerite 
 <- Clinochlore 
 
 Alunite 
 Hydrargillite 
 
 to 
 1.60 
 
 
 Margarite 
 Beryl 
 Kaolin 
 Antigorite -> 
 
 Scapolite * 
 <- Antigorite 
 
 
 + 
 1.60 
 
 Melilite * 
 Eudialyte 
 
 Celestite 
 Topaz -* 
 
 Barite 
 *- Topaz 
 
 Prehnite * 
 Pargasite 
 
 1.65 
 
 Apatite 
 Melilite t 
 Eucolite 
 
 
 Andalusite 
 Wollastonite 
 
 Tourmaline \ 
 Monticellite ^ 
 
 + 
 1.65 
 
 Zoisite 
 Thulite 
 
 Enstatite 
 
 Mosandrite 
 
 Lawsonite 
 Rinkite 
 Spodumene 
 
 to 
 1.70 
 
 Riebeckite 
 Arfvedsonite 
 Orthite \ 
 
 Dumortierite 
 Gehlenite 
 Axinite 
 Margarite 
 
 Xanthophyllite 
 Prismatine 
 
 Monticellite t 
 Glaucophane 
 
 + 
 1.70 
 
 to 
 
 Clinozoisite -> 
 Chloritoid -> 
 
 Chrysoberyl 
 Staurolite 
 Serendibite 
 
 <- Clinozoisite 
 <- Chloritoid 
 
 
 1.80 
 
 Vesuvianite 
 Sapphirine 
 
 Corundum 
 
 Epidote' -> 
 Cyanite -> 
 Hypersthene 
 
 <- Cyanite 
 
 + 
 >1.80 
 
 
 Wurtzite 
 
 
 
 - 
 
 
 
 
 
DESCRIPTIVE SECTION 
 
 349 
 
 EXCEPT THE FELDSPARS, ARRANGED ACCORDING TO 
 REFRACTION, AND OPTICAL CHARACTER. TABLE 3 
 
 to 0.025 
 
 to 0.030 
 
 to 0.050 
 
 to 0.075 
 
 to 0.100 
 
 >0.100 
 
 Wavellite 
 
 Thomson! te 
 
 
 
 
 
 Cancrinite 
 
 
 
 
 
 
 
 Brucite 
 
 
 Anhydrite 
 
 
 
 
 , Nontronite 
 ; Bertrandite 
 
 * 
 
 <- Scapolite 
 Pyrophyllite 
 Mica | 
 
 Talc 
 
 
 
 Anthophyllite 
 i Crocidolite 
 
 Rosenbuschite 
 
 <- Prehnite 
 Pectolite 
 Humite 
 
 
 
 
 Gedrite 
 Hornblende 
 Carpholite 
 
 Tremolite 
 Actinolite 
 Lazulite 
 
 Mica * 
 
 
 
 Calcite 
 Dolomite 
 Magnesite 
 
 ! Diallage 
 Fassaite 
 Sillinaanite 
 
 Forsterite 
 Diopside 
 Jadeite 
 
 Oli vine 
 
 
 
 
 . 
 
 ] Augite 
 
 
 \ Tourmaline 
 Datolite 
 
 
 
 Aragonite 
 
 Aegirine augite 
 
 Diaspore 
 Monazite 
 
 Astrophyllite 
 
 Xenotime 
 
 
 
 Lavenite 
 \ Orthite 
 
 Piemontite 
 Aegirine 
 
 - Epidote 
 Basaltic 
 hornblende 
 
 
 
 
 
 
 Zircon 
 
 Cassiterite 
 Pseudobrookite 
 Titanite -> 
 
 Rutile 
 Brookite 
 + Titanite 
 
 
 
 Fayalite 
 
 Anatase 
 
 Goethite 
 Baddeleyite 
 
 Sulphur 
 
350 
 
 PETROGRAPHIC METHODS 
 
 UNIAXIAL 
 
 Page 
 
 p. 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 Crystal 
 system 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. gr. 
 
 H 
 
 Color 
 
 
 
 
 
 
 
 
 
 
 Brown 
 
 227 
 
 
 Rutile 
 
 TiO 2 
 
 T 
 
 110 
 (100) 
 
 Prismatic 
 Granular 
 
 4.25 
 
 6.5 
 
 Yellowish 
 Grayish- 
 
 
 
 
 
 
 
 
 
 
 violet 
 
 
 
 
 
 
 111 
 
 Granular 
 
 
 
 Colorless 
 
 229 
 
 
 Anatase 
 
 Ti0 2 
 
 T 
 
 
 Tabular 
 
 3.85 
 
 6 
 
 Blue 
 
 
 
 
 
 
 001 
 
 Pyramidal 
 
 
 
 Yellowisl 
 
 
 
 
 
 
 
 Granular 
 
 
 
 Yellowish 
 
 230 
 
 
 Cassiterite 
 
 SnC-2 
 
 T 
 
 (100) 
 
 Pyramidal 
 
 6.9 
 
 6.5 
 
 Red-brown 
 
 
 
 
 
 
 
 Prismatic 
 
 
 
 to colorlei-= 
 
 230 
 
 
 Wurtzite 
 
 ZnS 
 
 H 
 
 lolo 
 
 Fibrous 
 
 3.98 
 
 3.5 
 
 Yellow 
 Brownish 
 
 
 
 
 
 
 1 1 n 
 
 
 
 
 
 230 
 
 
 Zircon 
 
 ZrSiC-4 
 
 T 
 
 (100) 
 
 Prismatic 
 
 4.7 
 
 7.5 
 
 Colorles 
 
 231 
 
 
 Xenotime 
 
 Y 2 Os phosphate 
 containing SOs 
 
 T 
 
 110 
 
 Pyramidal 
 Prismatic 
 
 4.6 
 
 6.5 
 
 Colorles 
 Light 
 reddish 
 
 232 
 
 
 Corundum 
 
 A1 2 O 3 
 
 H 
 
 
 Granular 
 Pyramidal 
 
 4.0 
 
 9 
 
 Colorles* 
 Blue 
 
 
 
 
 
 
 
 Tabular 
 
 
 
 
 233 
 
 
 Vesuvianite 
 
 Calcium-aluminium 
 silicate 
 
 T 
 
 
 Short 
 prismatic 
 Granular 
 
 3.45 
 
 6.5 
 
 Colorless 
 
 (Rose) 
 
 
 
 
 
 
 
 Thick 
 
 
 
 
 
 o. 
 
 Gehlenite 
 
 Ca 3 Al 2 Si 2 Oio 
 
 
 
 tabular 
 
 3.0 
 
 
 
 
 2 
 
 
 
 
 
 Isometric 
 
 
 
 
 
 o 
 
 
 
 
 
 
 
 
 Colorle- 1 
 
 233 
 
 .- 
 
 
 
 T 
 
 001 
 
 
 
 5.5 
 
 (Yellowis: 
 
 
 1 
 
 Melilite 
 
 Ca 4 Si 3 Oio 
 
 
 
 Thin 
 tabular 
 
 2.9 
 
 
 
 
 
 Precious 
 
 
 
 
 
 
 
 Blue 
 
 
 <u 
 
 Tourmaline 
 
 
 
 
 
 3 
 
 
 Green j 
 
 234 
 
 "3 
 
 
 Aluminium silicate 
 containing B 2 Os 
 
 H 
 
 
 Prismatic 
 Acicular 
 
 to 
 3.25 
 
 7.5 
 
 Yellowis 
 Brown 
 Violet, 
 
 Schorl 
 
 ** 
 
 H 
 
 
 
 
 
 
 
 
 
DESCRIPTIVE SECTION 
 
 MINERALS. 
 
 351 
 TABLE 4 
 
 a 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 < 2.616 
 
 2.903 
 
 0.287 
 
 + 
 
 + 
 
 Insoluble 
 
 Adamantine luster 
 in reflected light 
 Frequently twinned 
 and laminated 
 Absorption > w 
 
 Rutile 
 
 2.493 
 
 2.554 
 
 0.061 
 
 - 
 
 ( + ) 
 
 Insoluble 
 
 Often adamantine 
 luster in reflected 
 light 
 Mottled color 
 Absorption w > e 
 
 Anatase 
 
 1.997 
 
 2.093 
 
 ; 0.096 
 
 -f 
 
 + 
 
 Insoluble 
 
 Adamantine luster 
 in reflected light 
 Zonal structure 
 Absorption >w 
 
 Cassiterite 
 
 >1 
 
 93 
 
 Low 
 
 + 
 
 + 
 
 Easily soluble in 
 cold hydrochlo- 
 ric acid 
 
 Absorption e > o> 
 
 Wurtzite 
 
 1.931 
 
 1.993 
 
 0.062 
 
 + 
 
 + 
 
 Insoluble 
 
 Sp. gr. = 4.0 in 
 some varieties 
 Not decomposed in 
 a bead of microcos- 
 mic salt 
 
 Zircon 
 
 1.721 
 
 1.816 
 
 0.095 
 
 + 
 
 + 
 
 Insoluble 
 
 Often decomposed in 
 the rocks, then H 
 = 4-5 
 Clouded 
 
 Xenotime 
 
 1.760 
 Light 
 bluish- 
 green 
 
 1.769 
 Blue 
 
 0.009 
 
 - 
 
 ( + ) 
 
 Insoluble 
 
 Mottled color 
 Absorption w> 
 Often zonal struc- 
 ture 
 
 Corundum 
 
 1.701 
 
 1.705 
 
 0.001 
 
 - 
 
 - 
 
 Difficultly solu- 
 
 Often anomalous in- 
 
 
 1.726 
 
 1.732 
 
 0.006 
 
 + 
 
 + 
 
 chloric acid 
 
 Optical anomalies 
 
 Vesuvianite 
 
 1.657 
 
 1.665 
 
 0.008 
 
 - 
 
 
 Form gelatinous 
 
 Anomalous 
 interference 
 color 
 
 Gehlenite 
 
 
 
 
 - 
 
 + 
 
 silica with hy- 
 drochloric acid 
 
 ^\. 
 
 
 1.629 
 
 1.631 
 
 0.003 
 
 + 
 
 - 
 
 
 Peg structure ^ 
 When colored > co 
 
 Melilite 
 
 1.620 
 
 1.640 
 
 0.017 
 
 
 
 
 Weak absorption 
 
 W> 
 
 Precious 
 Tourmaline 
 
 i to 
 1.651 
 
 to 
 1.685 
 
 to 
 0.034 
 
 
 
 Insoluble 
 
 Strong absorption 
 
 W> 
 
 Pleochroic halos 
 
 Schorl 
 
352 PETROGRAPHIC METHODS 
 UNIAXIAL 
 
 Page 
 
 a 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 Crystal 
 system 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. gr. 
 
 H 
 
 Color 
 
 236 
 
 
 Apatite 
 
 Ca 6 POi2 
 (C1.F) 
 
 H 
 
 
 Long 
 prismatic 
 Grains 
 
 3.16 
 
 5 
 
 Colorless 
 (Brown, 
 yellow) 
 
 238 
 
 Rhombohedral Carbonates 
 
 Calcite 
 
 CaCOj 
 
 H 
 
 1011 
 
 Granular 
 Radial 
 
 2.72 
 
 3 
 
 Colorles 
 
 Dolomite 
 
 CaMg(C03) 2 
 
 H 
 
 Rhombo- 
 hedral 
 Granular 
 
 2.95 
 
 4 
 
 Magnesite 
 
 MgCOs 
 
 H 
 
 3.0 
 
 4.5 
 
 Siderite 
 
 FeCOs 
 
 H 
 
 Granular 
 Rhombo- 
 hedral 
 
 3.9 
 
 4 
 
 ColorleM 
 Yellowkl 
 
 242 
 
 
 Eudialyte 
 (Eucolite) 
 
 Silicate containing 
 ZrO2 and Ce2Os 
 
 H 
 
 (0001) 
 
 Grains 
 
 3.0 
 
 5.5 
 5.5 
 
 Colorlass 
 (Reddisl 
 
 242 
 
 Scapolite Group 
 
 Marialite 
 
 Na4Ai3Si 9 024Cl 
 
 T 
 
 110 
 
 Prismatic 
 Granular 
 Columnar 
 
 2.55 
 to 
 2.75 
 
 Colorles, 
 
 Meionite 
 
 T 
 
 243 
 
 
 Alunite 
 
 KAl3(OH) 6 (SO 4 )2 
 
 H 
 
 0001 
 
 Rhombo- 
 hedral 
 
 2.7 
 
 4 
 
 Colorle 
 
 243 
 
 
 Beryl 
 
 BesAl2Si 6 Oi8 
 
 H 
 
 (0001) 
 
 Prismatic 
 
 2.7 
 
 8 
 
 Colorle 
 (Bluish 
 
 244 
 
 
 Brucite 
 
 Mg(OH) 2 
 
 H 
 
 0001 
 
 Tabular 
 
 2.35 
 
 2.5 
 
 Colorlw 
 
 244 
 
 1 
 
 Quartz 
 
 SiO 2 
 
 H 
 
 
 Granular 
 Pyramidal 
 
 2.65 
 
 7 
 
 
 Chalcedony 
 
 ' 
 
 
 Fine 
 fibrous 
 
 2.60 
 
 6.5 
 
 Colorles 
 
 Tridymite 
 
 H 
 
 
 Tabular 
 
 2.3 
 
 
DESCRIPTIVE SECTION 353 
 
 MINERALS. Confirmed. TABLE 5 
 
 a 
 
 r 
 
 f- a 
 
 Chm 
 
 Chz 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 1.634 
 
 1.637 
 
 0.003 
 
 - 
 
 - 
 
 Easily soluble 
 
 Absorption when 
 colored e>o> 
 
 Apatite 
 
 1.487 
 
 1.659 
 
 0.172 
 
 - 
 
 
 Effervesces in 
 cold acid 
 
 Twinning lamellae 
 parallel to JR 
 frequent 
 (Absorption u > ) 
 
 Calcite 
 
 1.503 
 
 1.682 
 
 0.179 
 
 - 
 
 
 
 (Absorption w>e) 
 Index and double re- 
 
 Dolomite 
 
 1.515 
 
 1.717 
 
 0.202 
 
 - 
 
 
 Effervesce in 
 warm acid 
 
 fraction increase 
 with the content of 
 iron 
 
 Magnesite 
 
 1.633 
 
 1.872 
 
 0.239 
 
 - 
 
 
 
 Frequently altered to 
 limonite 
 
 Siderite 
 
 1.608 
 (1.617) 
 
 1.610 
 (1.620) 
 
 0.002 
 (0.003) 
 
 <-> 
 
 
 Gelatinizes with 
 hydrochloric 
 acid 
 
 Absorption when 
 colored w>e 
 Easily fusible 
 
 Eudialyte 
 (Eucolite) 
 
 1.542 
 
 1.555 
 
 0.013 
 
 
 
 Not attacked by 
 hydrochloric 
 acid 
 
 Precipitate of silver 
 chloride when 
 
 Marialite 
 
 1.558 
 
 1.597 
 
 0.039 
 
 
 
 Easily decom- 
 posed by hydro- 
 chloric acid 
 
 treated with HF 
 and AgNOs 
 
 Meionite 
 
 1.572 
 
 1.592 
 
 0.020 
 
 + 
 
 
 Difficultly solu- 
 ble in sulphuric 
 acid 
 
 Soluble in water 
 after ignition 
 
 Alunite 
 
 1.572 
 
 1.577 
 
 0.005 
 
 - 
 
 - 
 
 Insoluble 
 
 Often apparently 
 biaxial 
 
 Beryl 
 
 1.560 
 
 1.581 
 
 0.021 
 
 + 
 
 -. 
 
 Easily soluble 
 
 Anomalous interfer- 
 ence colors 
 Brown with AgNOs 
 
 Brucite 
 
 1.544 
 
 1.553 
 
 0.009 
 
 - 
 
 + 
 
 
 Frequently c a t a- 
 clastic 
 
 Quartz 
 
 1.532 
 
 1.543 
 
 0.011 
 
 - 
 
 - 
 
 Entirely soluble 
 in hydrofluoric 
 
 Generally apparent- 
 ly biaxial 2V up to 
 40 
 
 Chalcedony 
 
 1.476 
 
 1.478 
 
 0.002 
 
 + 
 
 - 
 
 
 Frequently twins 
 Anomalous interfer- 
 ence, segments 
 2E about 65 
 
 Tridymite 
 
 23 
 
354 PETROGRAPHIC METHODS 
 UNIAXIAL 
 
 Page 
 
 1 
 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 Crystal 
 system 
 
 Cleav- 
 age 
 
 (1010) 
 (0001) 
 
 Develop- 
 ment 
 
 Sp. gr. 
 
 H 
 
 Colo 
 
 249 
 
 
 Nepheline 
 
 NaAlSiO 4 
 
 H 
 
 Short 
 prismatic 
 Granular 
 
 2.60 
 
 6 
 
 Colorle 
 
 250 
 
 
 Apophyllite 
 
 Ca, K silicate con- 
 taining water 
 
 T 
 
 001 
 
 Scaly 
 
 2.35 
 
 5 
 
 Colorle 
 
 250 
 
 
 Chabazite 
 
 CaAl 2 Si4O 12 + 6aq. 
 Contains Na 
 
 H 
 
 loll 
 
 Rhombo- 
 hedral 
 
 2.1 
 
 4 
 
 Colorle 
 
 250 
 
 
 Cancrinite 
 
 NaAlSiO 4 + CO 2 
 and aq. 
 
 H 
 
 loTo 
 
 (0001) 
 
 Columnar 
 
 2.45 
 
 5.5 
 
 Colorless I 
 
 251 
 
 
 Hydro- 
 nephelite 
 (Ranite) 
 
 HNa 2 Al3Si 3 Oi2 + 
 3 aq. 
 
 H 
 
 dolo) 
 
 Confused 
 columnar 
 
 2.25 
 
 5 
 
 Colorless 
 
DESCRIPTIVE SECTION 355 
 
 MINERALS. Confirmed. TABLE 6 
 
 a 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 1.538 
 
 1.542 
 
 0.004 
 
 
 ' - 
 
 Gelatinizes easily 
 with hydrochlo- 
 ric acid 
 
 Frequently altered 
 Optical anomalies 
 
 Nepheline 
 
 1.535 
 
 1 . 537 
 
 0.002 
 
 ~ 
 
 (-) 
 
 ( + ) 
 
 Forms powdered 
 silica when treat- 
 ed with hydro- 
 chloric acid 
 
 Cloudy after igni- 
 tion, Anomalous in- 
 terference colors, 
 Segments 
 
 Apophyllite 
 
 About 
 
 1.50 
 
 Low 
 
 (+> 
 
 
 Gelatinizes with 
 hydrochloric 
 acid 
 
 Often anomalous in- 
 terference Seg- 
 ments 
 
 Chabazite 
 
 1.496 
 
 1.522 
 
 0.025 
 
 - 
 
 - 
 
 Gelatinizes with 
 weak efferves-: 
 cence in hydro- 
 chloric acid 
 
 Cloudy after igni- 
 tion 
 Frequently altered 
 Pseudomorph after 
 nepheline 
 
 Cancrinite 
 
 1.484 
 
 1.496 
 
 0.012 
 
 + 
 
 + 
 
 Gelatinizes with 
 hydrochloric acid 
 
 Pseudomorph after 
 nepheline 
 (Spreustein) 
 
 Hydronephe- 
 lite 
 (Ranite) 
 
356 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 Page 
 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 Crys- 
 tal 
 
 sys- 
 tem 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a 
 
 
 
 254 
 
 
 Brookite 
 
 TiOi 
 
 
 
 010 
 
 Tabular || 
 {100} 
 
 4.0 
 
 6 
 
 Brownish- 
 red 
 Yellowish 
 
 (c>B=o) 
 
 2.583 
 
 2.586 
 
 254 
 
 
 Goethite 
 
 FeO 2 H 
 
 O 
 
 010 
 
 Fine acicu- 
 lar||{010} 
 
 4.4 
 
 5 
 
 . Brown 
 Yellowish 
 
 Above 
 
 255 
 
 
 Pseudo- 
 brookite 
 
 
 
 o 
 
 (010) 
 
 Tabular || 
 {100} 
 
 5.0 
 
 6 
 
 Brownish- 
 red 
 Hardly 
 transparent 
 
 
 Very 
 high 
 
 255 
 
 
 Sulphur 
 
 S 
 
 o 
 
 
 Pyramidal 
 Granular 
 
 2.05 
 
 2 
 
 Yellowish 
 
 1.950 
 
 2.038 
 
 255 
 
 
 Baddeley- 
 ite 
 
 Zr0 2 
 
 M 
 
 001 
 
 Prismatic 
 along the 
 baxis 
 
 6.0 
 
 6.5 
 
 Greenish to 
 brown 
 
 a=c>B 
 
 High 
 
 255 
 
 
 Titanite 
 
 Ca(SiTi)O 6 
 
 M 
 
 110 
 
 (134) 
 
 Flat pris- 
 matic |j 
 {123} or 
 {110} 
 Grains 
 
 3.5 
 
 6 
 
 Colorless 
 Yellowish 
 Reddish 
 
 ( t> B >o) 
 
 1.888 
 to 
 1.913 
 
 1.894 
 to 
 1.921 
 
 256 
 
 
 Lievrite 
 
 HCaFe2Fe 
 Si 2 Q 9 
 
 
 
 (010) 
 
 Prismatic 
 Columnar 
 Fibrous 
 
 4.0 
 
 6 
 
 Usually trans- 
 parent paral- 
 lel to a only 
 Brownish- 
 green 
 
 . About 
 
 257 
 
 
 Monazite 
 
 CePO4 
 
 M 
 
 001 
 
 Thick tab- 
 ular || 
 {100} 
 Grains 
 
 5.15 
 
 5.5 
 
 Colorless 
 Yellowish 
 Brownish 
 
 1.796 
 
 1.797 
 
 257 
 
 
 Lavenite 
 
 Silicate 
 containing 
 ZrC-2, Ti0 2 
 Nb 2 6 
 
 M 
 
 (100) 
 
 Prismatic 
 || c 
 Tabular || 
 {100} 
 
 3.5 
 
 6 
 
 Yellow to 
 colorless 
 
 Bright 
 yellow 
 
 About 
 1.75 
 Yellow- 
 ish- 
 green 
 
 257 
 
 
 Chryso- 
 beryl 
 
 BeAl 2 O4 
 
 
 
 (010) 
 
 Isometric 
 grains 
 
 3.73 
 
 8.5 
 
 Light green 
 Colorless 
 
 1.747 
 Reddish 
 
 1.748 
 Yellow- 
 ish 
 
DESCRIPTIVE SECTION 
 
 MINERALS. 
 
 357 
 TABLE 7 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical orien- 
 tation 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 2.741 
 
 0.158 
 
 + 
 
 + 
 
 Very 
 small 
 
 
 Crossed 
 optic 
 planes 
 
 a= C 
 
 b= aQ0) 
 b-B(w) 
 
 Insoluble 
 
 Pleochroism 
 Weak adaman- 
 tine luster 
 
 Brookite 
 
 2.5 
 
 High 
 
 - 
 
 Med- 
 ium 
 
 
 Crossed 
 optic 
 planes 
 
 b=a 
 
 c= t(p) 
 a= C(y) 
 
 DiflBcultly 
 soluble 
 
 Pleochroism 
 brown to yel- 
 low Ada- 
 mantine lus- 
 ter 
 
 Goethite 
 
 
 High 
 
 + 
 
 - 
 
 Large 
 
 
 u>p 
 
 a= t 
 b=0 
 
 Soluble in 
 hot H 2 SO4 
 
 Adamantine 
 luster 
 
 Pseudo- 
 brookite 
 
 2.240 
 
 0.290 
 
 - 
 
 
 70 
 
 
 
 a=0 
 b=B 
 
 Soluble in 
 KOH 
 
 Opaque to 
 Roentgen rays 
 
 Sulphur 
 
 
 High 
 
 - 
 
 + 
 
 70 
 
 
 
 c:0 = 13f 
 b=B 
 
 Insoluble 
 
 Always 
 twinned 
 
 Baddeley- 
 ite 
 
 1.978 
 to 
 2.054 
 
 0.090 
 to 
 0.141 
 
 . + 
 
 
 
 45 
 to 
 60 
 
 p^o 
 
 c:t = 51f 
 b=B 
 
 Soluble in 
 hot H 2 SO4 
 
 Twins . 
 Insect's eggs 
 
 Titanite 
 
 1.89 
 
 Un- 
 known 
 
 + 
 
 + 
 
 
 
 
 
 Gelatin- 
 izes with 
 HC1 
 
 Easily fusible 
 to black mag- 
 netic glass 
 
 Lievrite 
 
 1.841 
 
 0.045 
 
 + 
 
 + 
 
 
 25 
 
 u>p 
 
 c:C = 4 f 
 b-f 
 
 White 
 residue 
 with HC1 
 
 Spectroscopic 
 test 
 
 Monazite 
 
 Orange 
 
 About 
 0.03 
 
 - 
 
 - 
 
 80 
 
 
 
 c:0 = 20r 
 
 b=B 
 
 Attacked 
 with diffi- 
 culty 
 
 Easily fusible 
 Twins 
 
 Lavenite 
 
 1.756 
 Green 
 
 0.009 
 
 + 
 
 
 
 to 
 90 
 
 
 P>1> 
 
 u>/9 
 
 
 Insoluble 
 
 Dispersion ex- 
 tremely vari- 
 able 
 
 Chryso- 
 beryl 
 
358 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 ft 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 Page 
 
 E 
 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 ft 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 Zoisite ct 
 
 
 
 
 
 
 
 
 1.697 1.699 
 
 
 
 
 
 
 100 
 
 
 
 
 
 
 
 
 
 
 o 
 
 
 
 
 3 3 
 
 
 
 
 
 
 Zoisite /? 
 
 HCa 2 Al 3 Si 3 Ois 
 
 
 and 
 (001) 
 
 
 
 
 Colorless 
 Light red- 
 
 About 
 1.7 
 
 
 
 Clinozois- 
 
 
 
 
 
 3 35 
 
 
 
 
 
 
 ite 
 
 
 
 
 
 
 
 
 
 
 o, 
 
 
 
 
 
 
 
 
 
 1.718 1.720 
 
 258 
 
 I 
 
 8 
 
 Epidote 
 
 (Pistazite) 
 
 HCa 2 (AlFe) 3 
 Si 3 O i3 
 
 
 
 Prismatic 
 along the 
 b axis 
 
 
 7 
 
 Yellowish 
 Greenish 
 
 1.731 1.754 
 
 
 
 
 
 
 001 
 
 Granular 
 
 
 
 C 
 
 
 
 w 
 
 
 
 M 
 
 and 
 
 
 3.4 
 
 
 Red 
 
 ... 
 
 
 
 Piemont- 
 ite 
 
 HCa 2 (AlFeMn) 3 
 SisOis 
 
 
 (100) 
 115i 
 
 
 
 
 Violet 
 Yellow 
 
 Orange Violet 
 
 
 
 
 
 
 
 
 
 
 c< fe>a 
 
 
 
 
 
 
 
 
 
 
 
 Brown 
 
 
 
 
 Orthite 
 
 HCa2(AlFeCe> 
 
 
 
 
 3.6 
 
 
 (Pale red- 
 
 About 
 
 
 
 (Allanite) 
 
 SisOi 3 
 
 
 
 
 3.8 
 
 
 dish) 
 
 1.78 
 
 266 
 
 
 Staurolite 
 
 HFeAl 5 Si 2 Oi3 
 
 
 
 (010) 
 
 Prismatic 
 Prism angle 
 
 3.6 
 to 
 
 7.5 
 
 Yellow, Red- 
 dish brown 
 
 
 1.736 1.741 
 
 
 
 
 
 
 
 129 
 
 3.8 
 
 
 c>B = o 
 
 
 267 
 
 
 Diaspore 
 
 A1O 2 H 
 
 ,>; , 
 
 
 
 010 
 
 Platy || 
 {010} 
 
 3.5 
 
 6 
 
 Colorless 
 (Bluish) 
 
 1 . 702 1 . 722 
 
 
 
 
 
 
 100 
 
 Prismatic 
 
 
 
 
 ' 
 
 267 
 
 
 Cyanite 
 
 Al 2 SiO 5 
 
 Tr 
 
 (010) 
 001 
 
 lie 
 Tabular || 
 {100} 
 
 3.6 
 
 4-7 
 
 Colorless 
 Bluish 
 
 1.712 | 1.720 
 Color- | Pale 
 less violet 
 
 
 
 
 
 
 
 Tabular || 
 
 
 
 Very pale 
 
 1.706 
 
 268 
 
 
 Sapphirine 
 
 Mg 5 Ah 2 Si 2 O 2 7 
 
 M 
 
 
 {010} 
 
 3.5 
 
 7.5 
 
 blue to blu- 
 
 Color- 1 JS 
 
 
 
 
 
 
 
 Granular 
 
 
 
 ish green 
 
 less 
 
 
 
 
 Aluminium 
 
 
 
 
 
 
 
 
 269 
 
 
 Serendi- 
 bite 
 
 silicate con- 
 taining B 2 Os 
 
 Tr 
 
 
 Plates 
 Grains 
 
 3.4 
 
 7 
 
 Blue 
 Pleochroic 
 
 About 
 1.7 
 
 
 
 
 (CaMgFe)O 
 
 
 
 
 
 
 
 
 269 
 
 
 Prismatine 
 
 MgAl 2 SiO 6 
 
 O 
 
 100 
 
 (81) 
 
 Prismatic 
 Columnar 
 Radial 
 
 3.3 
 
 6.5 
 
 Colorless 
 Yellowish 
 
 1.669 1.680 
 
 
 
 
 
 
 
 
 
 
 
 
DESCRIPTIVE SECTION 
 
 Continued. 
 
 359 
 TABLE 8 
 
 r 
 
 f-a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical orien- 
 tation 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 1.702 0.005 
 
 -f 
 
 - 
 
 
 90 
 
 *j 
 
 c= C 
 b=0 
 
 
 Anomalous 
 interference 
 
 Zoisite a 
 
 
 Very 
 low 
 
 - 
 
 * 
 
 
 50 
 
 p>u 
 
 c= C 
 
 b=B 
 
 
 twinning 
 lamination 
 
 Zoisite /? 
 
 
 
 + 
 
 80 
 
 
 u^p 
 
 c:0 = 2 f 
 
 Gelatinize 
 
 \ 
 
 Clinozois- 
 
 \ 1.723 0.005 
 
 
 
 
 
 
 C:a about 30 
 
 in HC1 
 
 \ 
 
 ite 
 
 to 
 
 to 
 
 
 
 
 
 
 
 after 
 
 \ 
 
 
 
 
 
 
 
 
 
 1.768 
 
 0.061 
 
 - 
 
 + 
 
 70 
 
 
 p>u 
 
 c:0 = 3r 
 
 b=B 
 
 C:a about 30 
 
 melting 
 
 \ 
 
 \ 
 \ 
 
 Epidote 
 
 (Pistazite) 
 
 Carmine 
 
 
 I 
 
 * 
 
 Vari- 
 able 
 
 
 
 c: = 2 to 7 r 
 C :a about 30 
 
 
 Speckled \ 
 interfer- \ 
 ence colors \ 
 
 Piemont- 
 ite 
 
 
 0.002 
 to 
 0.030 
 
 - 
 
 t 
 
 70 
 
 
 
 c:0 = 36r 
 
 b=B 
 
 0:a about 30 
 
 Difficultly 
 soluble in 
 HCJ 
 
 Surrounded 
 by pleochroic 
 halo 
 
 Orthite 
 (Allanite) 
 
 1.746 
 
 0.010 
 
 ^ 
 
 - 
 
 85 
 to 
 90 
 
 
 Weak 
 p>u 
 
 c= C 
 
 b=0 
 
 Insoluble 
 
 Pleochroic 
 halos 
 Twins 
 
 Staurolite 
 
 1.750 
 
 0.048 
 
 + 
 
 - 
 
 85 
 
 
 u>f 
 
 a= C 
 
 b=B 
 
 Insoluble 
 
 
 Diaspore 
 
 
 
 
 
 
 
 
 
 
 Twinning j| 
 
 
 1.728 
 Pale 
 blue 
 
 0.012 
 
 to 
 0.016 
 
 - 
 
 - 
 
 82 
 
 
 p>u 
 
 about 1 {100} 
 c:C=+30 
 
 Insoluble 
 
 {100} 
 Fibrous frac- 
 ture || 
 
 Cyanite 
 
 
 
 
 
 
 
 
 
 
 {001} 
 
 
 1.711 
 light blue 
 
 0.005 
 
 - 
 
 
 69 
 
 
 ,,, 
 
 c:a = 81f 
 
 Insoluble 
 
 Infusible 
 
 Sapphirine 
 
 
 0.006 
 
 
 
 Large 
 
 
 
 c:0 about 40 
 
 Insoluble 
 
 Always twin 
 lamellae 
 
 Serendi- 
 
 
 
 
 
 
 
 
 
 
 Infusible 
 
 
 1 1.682 
 
 0.013 
 
 - 
 
 - 
 
 
 65 
 
 p>u 
 
 c = 
 
 b = C 
 
 Insoluble 
 
 Infusible 
 
 Prismatine 
 
360 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 a 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 2 
 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a 
 
 f 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 269 
 
 
 Astrophyl- 
 lite 
 
 Alkali iron 
 silicate con- 
 taining TiO 2 
 
 O 
 
 100 
 
 Tabular- 
 Prismatic 
 
 ilb 
 
 3.3 
 
 3.5 
 
 Orange 
 Brownish 
 Red 
 tt>fi>C 
 
 1.678 
 Deep 
 orange 
 
 Orange 
 
 269 
 
 i 
 
 a 
 
 Chloritoid 
 
 Basic alumi- 
 nium silicate 
 containing 
 (FeMgCa)O 
 
 Tr 
 
 001 
 (110) 
 
 Tabular || 
 {001} 
 Platy 
 
 3.5 
 
 6.5 
 
 Bluish-green 
 Colorless 
 
 1) > a > c 
 
 Olive- 
 green 
 
 1.741 
 Indigo 
 
 Xantho- 
 phyllite 
 
 M 
 
 3.1 
 
 5 
 
 Colorless 
 Light green 
 
 1.649 
 
 1.66" 
 
 272 
 
 
 Margarite 
 
 H 2 CaAU 
 
 Si 2 Oi 2 
 
 1C 
 
 001 
 
 Tabular 
 Flaky 
 
 3.0 
 
 4 
 
 Colorless 
 
 
 1.66, 
 
 
 
 Forsterite 
 
 M g2 Si0 4 
 
 
 
 
 
 
 
 
 1.657, 
 
 272 
 
 I 
 
 Olivine 
 
 (MgFe) 2 Si0 4 
 
 Q 
 
 010 
 
 Short pris- 
 matic || c 
 
 3.2 
 to 
 3.6 
 
 7 
 
 Colorless 
 
 1.661 
 
 1.678 
 
 
 Oii vine 
 
 Fayalite 
 
 FesSiOi 
 
 
 (100) 
 
 Grains 
 
 
 6.5 
 
 Yellow 
 Brownish 
 
 1.824 
 
 1.86 
 
 
 
 Monticel- 
 lite 
 
 CaMgSiO 4 
 
 
 
 
 3.2 
 
 5.5 
 
 Colorless 
 
 1.651 
 
 1.66, 
 
DESCRIPTIVE SECTION 
 
 MINERALS. Continued. 
 
 361 
 TABLE 9 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical orien- 
 tation 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 1.733 
 Light 
 yellow 
 
 0.055 
 
 - 
 
 + 
 
 Very 
 large 
 
 
 p>u 
 
 c=B 
 
 b= C 
 
 Difficultly 
 soluble 
 
 Easily fusible 
 Star-shaped 
 aggregates 
 
 Astro- 
 phyllite 
 
 Greenish- 
 yellow 
 
 0.003 
 to 
 0.015 
 
 + 
 
 - 
 
 
 65 
 to 
 120 
 
 , 
 
 c:C = 0-15 
 b:tt = 0-25 
 
 Insoluble 
 
 Twinning 
 after the mica 
 law 
 Scarcely 
 fusible 
 
 Chlori- 
 toid 
 
 1.661 
 
 0.012 
 
 - 
 
 + 
 
 
 
 
 to 
 25 
 
 u>p 
 
 c=0 
 
 Xantho- 
 phyllite 
 
 
 0.009 
 
 - 
 
 - 
 
 Large 
 
 
 
 b-C 
 
 Quite 
 difficultly 
 soluble 
 
 Twinning 
 lamellae 
 
 Margarite 
 
 1.697 
 
 About 
 0.03 
 
 + 
 
 + 
 
 85 
 
 
 Weak 
 
 a= C 
 b=0 
 
 Gelatinize 
 slowly 
 with cold 
 HC1 
 
 Alteration 
 into 
 serpentine 
 
 Forsterite 
 
 0.036 
 
 (-> 
 
 About 
 90 
 
 
 Distinct 
 u>p 
 
 Olivine 
 
 1.874 
 
 0.050 
 
 - 
 
 
 50 
 
 
 p>u 
 
 Fayalite 
 
 Monticel- 
 lite 
 
 1.668 
 
 0.017 
 
 
 
 38 
 
 
 ,>. 
 
362 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 3 
 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 ^ 
 
 Color 
 
 a. 
 
 ft 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 
 Enstatite 
 (Bronzite) 
 
 MgSiOs 
 
 
 110 
 
 
 
 
 Colorless 
 
 1.660 
 
 1.665 
 to 
 
 
 
 
 
 O 
 
 100 
 (010) 
 
 
 3.1 
 to 
 3.5 
 
 
 
 to 
 1.716 
 Red- 
 brown 
 
 1.725 
 Yellow 
 ish- 
 brown 
 
 Hyper- 
 sthene 
 
 (MgFe)SiOs 
 
 Brown 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ' 1 
 
 
 
 
 
 
 
 
 
 
 Diopside 
 
 (MgCaFe)SiO 3 
 
 
 
 
 
 5.5 
 
 Colorless 
 Green 
 Scarcely 
 pleochroic 
 
 1.671 
 to 
 1.699 
 
 1.678 
 to 
 1.706 
 
 
 a 
 
 Diallage 
 
 
 
 
 Short pris- 
 
 3.3 
 
 
 Greenish 
 Brownish 
 
 1.679 
 Green- 
 ish 
 
 1.681 
 Yellow- 
 ish 
 
 
 P 
 
 
 
 
 
 matic along 
 
 
 
 
 
 
 
 
 
 
 277 
 
 
 a> 
 
 Fassaite 
 
 
 
 
 the c axis 
 
 
 
 Green 
 
 Green 
 
 Yellow- 
 
 
 i 
 
 
 
 
 
 
 
 
 
 
 green 
 
 
 * 
 
 Augite 
 
 (MgFe) (AlFeh 
 SiO 6 
 
 M 
 
 110 
 
 87 
 
 
 
 
 Greenish 
 Brownish 
 
 1.698 
 to 
 1.706 
 
 1.704 
 to 
 1 .712 
 
 
 
 
 
 
 
 
 
 
 Green 
 
 
 
 
 
 Aegirine- 
 augite 
 
 
 
 
 
 3.4 
 
 
 Violet with 
 content of 
 
 Green 
 
 Bright 
 green 
 
 
 
 
 
 
 
 
 to 
 3.5 
 
 
 TiO 2 
 
 
 
 
 
 
 
 
 
 
 
 6 
 
 
 
 1 799 
 
 
 
 Aegirine 
 
 (Acmite) 
 
 NaFeSi 2 O 6 
 
 
 
 
 
 
 Sap green 
 (Brown) 
 
 1.763 
 
 Grass- 
 green 
 
 (Brown) 
 
 Light 
 green 
 
 (Light ! 
 brown) 
 
 
 
 Spodu- 
 
 LiAlSi 2 O 6 
 
 
 
 Prismatic 
 to tabular 
 
 3.1 
 
 6.5 
 
 Colorless 
 
 1.660 
 
 1.666 
 
 
 
 mene 
 
 
 
 
 || {100} 
 
 
 
 
 
 
 
 
 
 
 
 
 Tabular || 
 
 
 
 
 
 
 286 
 
 
 Lawsonite 
 
 H 4 CaAl 2 Si 2 Oio 
 
 O 
 
 
 {001} 
 Prismatic 
 
 3.1 
 
 8.5 
 
 Colorless 
 
 1.665 
 
 1.669 
 
 
 
 
 
 
 001 
 
 lie 
 
 
 
 
 
 

 DESCRIPTIVE SECTION 363 
 
 MINERALS. Contained. TABLE 10 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solu- 
 bility 
 
 Remarks 
 
 Minerals 
 
 
 
 
 
 77 
 to 
 
 
 
 
 
 \^ Infusible 
 
 Ensta- 
 tite 
 
 to 
 
 0.010 
 
 
 90 
 
 
 
 
 
 For 
 
 (Bronzite) 
 
 1.729 
 
 to 
 
 
 
 
 
 
 
 FeO = 10 % .; 
 
 
 Gray- 
 green 
 
 0.013 
 
 
 90 
 to 
 50 
 
 
 p>u 
 
 b= a 
 
 
 Xficultly 
 
 Hyper- 
 sthene 
 
 
 
 
 
 
 
 
 
 X X f usible 
 
 
 1 700 
 
 
 
 
 
 
 
 
 to 
 1 . 727 
 
 0.029 
 
 
 
 KOO 
 
 1 OQ 
 
 p>u 
 
 c:C = 39-44f 
 
 
 Difficultly Diopside 
 fusible 
 
 
 
 
 
 
 Twinning 
 
 1 . 703 
 j Greenish 
 
 0.024 
 
 
 
 
 c:C = 40f 
 
 
 lamellae and 
 parting i| {100} 
 
 Diallage 
 
 
 
 
 
 
 and {001} 
 
 
 \ Green 
 
 0.021 
 
 
 
 
 
 c:C = 45f 
 
 b=B 
 
 Difficultly 
 soluble 
 even in 
 
 Strong dis- 
 persion of the 
 bisectrices 
 
 Fassaite 
 
 
 
 
 
 
 
 1* i 
 
 
 1.723 
 to 
 
 0.022 
 to 
 
 
 
 Vari- 
 able 
 
 
 c:C = 54 f 
 
 
 Frequently a 
 few twinning 
 
 Augite 
 
 1.728 
 
 0.025 
 
 
 
 
 
 
 
 
 lamellse 
 
 
 Greenish- 
 
 
 
 
 
 
 
 c:C = 60 f 
 
 
 
 Aegirine- 
 
 yellow 
 
 
 
 
 
 b=B 
 
 
 Strong dis- 
 
 augite 
 
 
 
 
 
 
 
 
 
 
 persion of the 
 
 
 1.813 
 
 
 
 
 
 
 
 
 Yellow- 
 ish 
 (Green- 
 
 0.050 
 
 - 
 
 - 
 
 64 
 
 
 
 c:C = 94f 
 
 
 Easily 
 fusible 
 
 Aegirine 
 
 (Acmite) 
 
 ish) 
 
 
 
 
 I 
 
 
 
 
 
 1.676 
 
 0.016 + 
 
 + 
 
 60 
 
 
 u>p 
 
 c:C = 25 
 
 
 Easily fusible 
 with intumes- 
 
 Spodu- 
 mene 
 
 
 
 
 
 
 
 
 
 cence 
 
 
 
 
 
 
 
 
 
 
 Gelatin- 
 
 
 1.684 
 
 0.019 
 
 t 
 
 
 84 
 
 
 p>u 
 
 c= C 
 
 b=B 
 
 izes with Easily fusible 
 HC1 after Twins 
 
 Lawsonite 
 
 
 
 
 
 
 
 
 
 melting 
 
 
364 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 o. 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 2 
 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a. 
 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 
 Antho- 
 phyllite 
 (Gedrite) 
 
 (MgFe)- 
 SiOs con- 
 taining 
 A1 2 O 3 
 
 
 O 
 
 100 
 110 
 
 (010) 
 
 Columnar 
 {110} =124 
 Flaky 
 Fascicular 
 
 3.1 
 
 5.5 
 
 Colorless 
 Light brown- 
 ish 
 Reddish 
 ( C> fc > 0) 
 
 1.633 
 
 1.64 
 
 
 
 Actinolite 
 
 (Tremolite) 
 
 (MgFeCa) 
 SiOa 
 
 
 
 
 
 3.0 
 
 
 Colorless 
 Light green 
 
 1.607 
 
 
 
 
 Pargasite 
 
 (MgFeCa) 
 
 
 
 
 
 
 
 Light green 
 (Brownish) 
 
 1.616 
 
 1.62i 
 
 
 
 
 AloSiOa 
 
 
 
 
 
 
 
 
 
 
 
 
 Green 
 hornblende 
 
 
 
 
 
 
 3.1 
 
 to 
 
 
 Green, Bluish- 
 green, Brown- 
 ish-green 
 
 1.629 
 Yel- 
 lowish 
 
 a 
 
 lowisf 
 
 
 
 
 
 
 
 
 
 3.2 
 
 
 c>b > o 
 
 
 greet 
 
 
 I 
 
 Brown 
 
 (MgFeCa) 
 (AlFe) 2 - 
 
 
 
 
 
 Prismatic 
 along the 
 
 
 
 Brown 
 
 Yel- 
 
 1.6-1 
 
 
 i 
 
 hornblende 
 
 SiO 6 
 
 
 
 
 
 c axis 
 
 
 5.5 
 
 c=B>a 
 
 lowish 
 
 Brawii 
 
 
 
 
 
 r\ 
 
 
 
 
 
 to 
 
 
 
 
 287 
 
 I 
 
 
 
 M 
 
 
 
 
 
 6 
 
 
 
 
 
 1 
 
 Basaltic 
 hornblende 
 
 Contains 
 TiO 2 
 
 Containir 
 
 M 
 
 110 
 124 
 (001) 
 
 
 3.3 
 
 
 Red brown 
 Yellowish- 
 brown 
 Deep brown 
 
 1.680 
 Yel- 
 lowish- 
 brown 
 
 Brov>; 
 
 
 
 
 
 
 
 
 
 
 
 
 1.621 
 
 
 
 
 Glauco- 
 
 phane 
 
 NaAl- 
 Si 2 6 
 
 
 
 
 
 3.1 
 
 
 Light blue 
 
 Light 
 yellow- 
 
 Violo 
 
 
 
 
 
 
 
 
 
 
 
 
 ish 
 
 
 
 
 Crocido- 
 lite 
 
 
 
 
 
 Fibrous 
 Resembles 
 
 
 
 Light blue 
 
 Blue 
 
 Viol* 
 
 
 
 
 
 
 
 
 asbestos 
 
 
 
 
 
 
 
 
 Riebeckite 
 
 NaFe- 
 Si 2 0e 
 
 
 
 
 Short pris- 
 matic 
 Irregular 
 shreds 
 
 3.4 
 
 5 
 
 Deep blue 
 
 Black- 
 ish- 
 blue 
 
 Aboi 
 1.7 
 Blue 
 
 Arfved- 
 
 3.5 
 
 Bluish-green 
 
 Green- 
 ish- 
 
 Abo a 
 1.7 
 
 
 
 
 
 
 
 
 
 
 
 blue 
 
 Bluet, 
 
 295 
 
 
 Dumor- 
 tierite 
 
 AUSisOig con- 
 taining boron 
 
 O 
 
 100 
 
 Acicular || c 
 
 3.3 
 
 7 
 
 Violet 
 Blue 
 
 1.678 
 Bluish- 
 violet 
 
 1.68 
 
 Yel- 
 lowisi 
 
DESCRIPTIVE SECTION 
 
 MINERALS. Continued. 
 
 365 
 TABLE 11 
 
 r 
 
 r-a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solu- 
 bility 
 
 Remarks 
 
 Minerals 
 
 1.657 
 
 0.024 
 Varia- 
 ble 
 
 i 
 
 
 Very 
 large 
 
 
 u>p 
 
 (p>U) 
 
 c= t 
 
 
 Pleochroic 
 halos 
 
 Antho- 
 phyllite 
 (Gedrite) 
 
 1.634 
 
 0.027 
 
 
 
 80 
 
 
 u>p 
 
 c:C = 10-20f 
 b fi 
 
 
 Very weakly 
 pleochroic at 
 
 Actinolite 
 (Tremo- 
 
 
 
 
 
 
 
 
 
 
 best 
 
 lite) 
 
 1.635 
 
 0.019 
 
 - 
 
 
 55 
 
 97 
 
 p>u 
 
 c:C = 18f 
 
 b=B 
 
 
 Short thick 
 prisms 
 
 Pargasite 
 
 1.653 
 
 
 
 
 
 
 
 
 
 
 
 Blue- 
 brown 
 Deep 
 
 0.024 
 
 
 
 84 
 
 
 
 c:C = 12-20f 
 
 
 
 Green 
 hornblende 
 
 green 
 
 
 
 
 
 
 
 
 
 Pleochroic 
 
 
 Brown 
 
 About 
 0.025 
 
 
 
 
 
 
 c:C = 14f 
 
 Attacked 
 
 
 Brown 
 hornblende 
 
 
 
 
 
 
 
 
 
 
 with dif- 
 
 
 
 I 752 
 
 
 
 
 
 
 
 
 
 Magmatic 
 
 
 Deep 
 arown 
 
 0.072 
 
 
 
 80 
 
 
 
 c:C = 10-0f 
 
 
 resorption 
 Strong dis- 
 persion 
 
 Basaltic 
 hornblende 
 
 ; 1.639 
 
 ky blue 
 
 0.018 
 
 
 
 50 
 
 85 
 
 
 c:C = 4-6f 
 
 
 
 Glauco- 
 phane 
 
 Llmost 
 color- 
 less 
 
 0.025 
 
 + 
 
 
 
 95 
 
 
 c:0 = 18-20f 
 
 b=B 
 
 
 Easily fus- 
 ible 
 
 Crocido- 
 lite 
 
 Bluish- 
 green 
 
 0.003 
 
 - 
 
 - 
 
 
 Very 
 large 
 
 I) ^> p 
 
 c:0 = 5r 
 
 b=B 
 
 
 Strong dis- 
 persion of the 
 
 Riebeckite 
 
 
 
 
 
 
 
 
 
 
 K4 A/t-f f.1 rf\& 
 
 
 Irayish- 
 green 
 
 Low 
 
 ' 
 
 
 
 Very 
 large 
 
 
 c:0 = 14r 
 
 b=B 
 
 
 Very easily 
 fusible 
 
 Arfved- 
 sonite 
 
 1.689 
 
 
 
 
 
 
 
 
 
 Decolorized 
 
 
 Color- 
 less 
 
 0.011 
 
 - 
 
 - 
 
 30 
 
 54 
 
 p^o 
 
 b=B 
 
 Insoluble 
 
 by strong ig- 
 nition 
 
 Dumor- 
 
 tierite 
 
 
 
 
 
 
 
 
 
 
 Infusible 
 
 
 . 
 
 
 
 
 
 
 
 
 
 
366 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 Crys- 
 
 
 
 
 Page 
 
 a 
 
 S 
 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp ' H Color a 
 gr. 
 
 
 
 
 
 tem 
 
 
 
 
 295 
 
 
 Axinite 
 
 Aluminium 
 silicate con- 
 taining boron 
 
 Tr 
 
 (110) 
 
 Wedge shape 
 Granular 
 
 3.3 
 
 Colorless 1.672 
 Light violet Colorless 
 
 i 
 
 
 
 
 Cerium silicate 
 
 
 i nn 
 
 Thick tabu- 
 
 
 Colorless 
 
 
 
 Rinkite 
 
 containing 
 
 M 
 
 
 lar i| 
 
 3.46 
 
 5 Yellowish 1.665 1. 
 
 295 
 
 
 
 TiC-2 and Fl 
 
 
 
 {100} 
 
 
 c=fe>a 
 
 295 
 
 
 Sillimanite 
 
 Al 2 SiO 5 
 
 O 
 
 100 
 
 Acicular 
 
 3.24 
 
 6.5 Colorless ! 1.656 1. 
 
 
 
 
 
 
 
 II c 
 
 
 
 296 
 
 
 Datolite 
 
 HCaBSiOs 
 
 M 
 
 
 Granular 
 Fibrous || b 
 
 3.0 
 
 5 Colorless 1.626 1. 
 
 297 
 
 
 Mosan- 
 drite 
 
 Similar to 
 rinkite. Con- 
 tains ZrO 2 
 
 M 
 
 100 
 
 Tabular || 
 {100} 
 
 3.1 
 
 1.646 1.61 
 Colorless , 
 4 /-IT- 11 u\ i (Green- (Brov 
 (Yellowish) 
 ish) isli 
 
 
 
 
 
 
 oni 
 
 Granular 
 
 
 297 
 
 
 Barite 
 
 BaSC-4 
 
 O 
 
 
 Platy || 
 
 4.5 3.5 Colorless 1.637 !.(> 
 
 
 
 
 
 
 110 
 
 {001} 
 
 " 
 
 297 
 
 
 Andalusite 
 
 Al 2 Si0 5 
 
 
 
 110 
 91 
 
 Prismatic 
 
 lie 
 
 3.2 
 
 Colorless 1.622 l.t 
 Reddish Rose Cok 
 
 298 
 
 
 Lazulite 
 
 H 2 (FeMg) 
 A1 2 P 2 O 9 
 
 M 
 
 
 Pyramidal 
 
 3.0 
 
 1.603 . 
 5 Blue Color- "j 
 
 
 
 
 
 
 
 
 
 less 
 
 299 
 
 
 Carpholite 
 
 H 4 MnAl 2 Si 2 Oio 
 
 M 
 
 (010) 
 
 Acicular 
 Fine fibrous 
 
 2.9 
 
 5 Yellow 
 \ ellowist 
 
 i 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Tabular || 
 
 
 j i 
 
 299 
 
 
 Prehnite 
 
 H 2 Ca 2 Al 2 
 SisC-12 
 
 O 
 
 001 
 (001) 
 
 {001} 
 Fibrous 
 
 2.9 
 
 1.616 l.i 
 6.5 Colorless y ^ y 
 
 
 
 
 
 
 Rosettes 
 
 
 
 299 
 
 
 Celestite 
 
 SrSO4 
 
 
 
 001 
 (110) 
 
 Granular 
 Fibrous 
 
 3.95 
 
 3.5 Colorless 1.622 l. : 
 
 
 
 
 
 
 
 
 
 
DESCRIPTIVE SECTION 
 
 367 
 TABLE 12 
 
 r r- 
 
 Chm Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical Solu- 
 orientation bility 
 
 Remarks 
 
 Minerals 
 
 1.681 
 Color- 
 
 0.000 
 
 _ 
 
 
 72 
 
 x 
 
 u>p 
 
 Insoluble 
 
 Test for 
 boron 
 
 i Axinite 
 
 less 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Twinning 
 
 
 1.681 
 
 0.016 
 
 + 
 
 + 
 
 
 80 
 
 >P 
 
 c : b = 7 r Easily de- lamellae || 
 b=a composed {100} 
 
 Rinkite 
 
 
 
 
 
 
 
 
 Fuses easily 
 
 
 1.677 
 
 0.021 + 
 
 + 
 
 20 to 
 30 
 
 35 to 
 55 
 
 P>u 
 
 c= C Often very 
 i Insoluble -i_ . 
 b= fine fibrous 
 
 Sillimanite 
 
 1.670 
 
 0.044 
 
 _ 
 
 
 74 
 
 
 p>u 
 
 Gelatinizes , , 
 c:0 = 4r Test for 
 , *. readily in , 
 b = TT , boron 
 
 Datolite 
 
 
 
 
 
 
 
 
 HC1 
 
 
 
 1.658 
 
 (Light 
 yellow) 
 
 0.012 
 
 - 
 
 
 70 
 
 
 , 
 
 Twinning 
 c:0 = 2 Easily de- lamellae || 
 b=B composed {100} 
 Fuses easily 
 
 .Mosan- 
 drite 
 
 
 
 
 
 
 
 Soluble in 
 
 
 
 1.649 
 
 0.012 + 
 
 
 37 
 
 63 
 
 u>p 
 
 warm 
 
 
 Barite 
 
 
 
 
 
 
 H 2 SO4 
 
 
 1.643 
 less 
 
 0.011 
 
 - 
 
 - 
 
 84 
 
 
 
 Pleochroic 
 Insoluble j halos 
 Chiastolite 
 
 Andalu- 
 site 
 
 1.639 
 
 blue 
 
 0.036 
 
 - 
 
 69 
 
 
 * 
 
 Attacked 
 ~ T with dif- 
 ficulty 
 
 Decolorizes 
 giving off wa- 
 ter upon igni- 
 tion 
 
 Lazulite 
 
 Colorless 0.022 
 
 
 
 + 
 
 60 
 
 
 
 Attacked 
 
 b- o * dth dif " 
 
 
 Carpho- 
 lite 
 
 
 
 
 
 
 
 ficulty 
 
 
 
 1.649 
 variable 
 
 0.033 
 
 - 
 
 - 
 
 Variable 
 
 Gelatin- 
 c = C izes with 
 b=B HC1 after 
 melting 
 
 Optical 
 anomalies 
 Parquet forms 
 
 Prehnite 
 
 1.631 
 
 0.009 
 
 + 
 
 About 
 Q0 
 
 
 
 Soluble 
 c= C 
 fi only in 
 
 Fusible 
 Colors flame 
 
 Celestite 
 
 
 
 
 
 
 
 
 hot H 2 SO4 
 
 red 
 
 
368 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 | 
 
 2 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 Sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a 
 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 299 
 
 
 Aragonite 
 
 CaCOs 
 
 
 
 
 Columnar 
 
 lie 
 
 2.95 
 
 4 
 
 Colorless 
 
 1.530 
 
 1.682 
 
 
 
 
 Silicate con- 
 
 
 
 
 
 
 
 
 
 
 
 Rosen- 
 buschite 
 
 taining 
 ZrOz, TiO 2 
 LaaOs 
 
 
 (100) 
 
 Radial || 
 b axis 
 
 3.3 
 
 5.5 
 
 Yellowish 
 
 
 Abou 
 1.65 
 
 
 3 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 001 
 
 
 
 
 
 
 
 
 > 
 
 
 
 
 
 
 
 
 
 
 
 299 
 
 *3 
 
 WoUaston- 
 ite 
 
 CaSiOs 
 
 M 
 
 100 
 
 (102) 
 
 Columnar 
 || b axis 
 
 2.85 
 
 5 
 
 
 1.621 
 
 1.63:5 
 
 
 S3 
 
 
 
 
 (101) 
 
 
 
 
 
 
 
 
 !> 
 
 
 
 
 
 
 
 
 Colorless 
 
 
 
 
 
 Pectolite 
 
 (CaNa 2 H 2 ) 
 SiOs 
 
 
 100 
 001 
 
 Sheaf-like 
 
 || b axis 
 
 2.8 
 
 4.5 
 
 
 
 Abo 
 1.6 
 
 
 0. 
 
 Humite 
 
 Mg 7 (FOH) 2 
 
 Si 3 0l2 
 
 
 
 
 
 
 
 
 
 
 300 
 
 1 
 
 -2 
 
 Clino- 
 humite 
 
 Mg 9 (FOH) 2 
 Si 4 Oi6 
 
 
 001 
 
 Rounded 
 grains 
 
 3.1 
 
 to 
 
 39 
 
 6.5 
 
 Colorless 
 Yellowish 
 
 1.607 
 to 
 
 1.6H 
 to 
 1.67( 
 
 
 
 
 
 
 
 
 
 
 
 
 Yellow 
 
 Color 
 
 
 w 
 
 Chondro- 
 
 Mg 5 (FOH) 2 
 
 
 
 
 
 
 
 
 
 
 
 dite 
 
 Si 2 8 
 
 
 
 
 
 
 
 
 
 301 
 
 
 Topaz 
 
 Al 2 SiO4 
 (FOH) 2 
 
 O 
 
 001 
 
 Granular 
 Columnar 
 
 || caxis 
 
 3.4 
 to 
 3.6 
 
 8 
 
 Colorless 
 
 1.607 
 . to 
 1.629 
 
 1.61 
 
 to 
 1.61 
 
DESCRIPTIVE SECTION 
 
 MINERALS. Continued. 
 
 369 
 TABLE 13 
 
 r 
 
 f-Oi 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solu- 
 bility 
 
 Remarks 
 
 Minerals 
 
 1.686 
 
 0.156 
 
 - 
 
 - 
 
 18 
 
 31 
 
 U>p 
 
 c=0 
 
 b= t 
 
 Readily 
 soluble 
 inHCl 
 
 
 Aragonite 
 
 
 About 
 0.026 
 
 
 - 
 
 About 
 90 
 
 
 
 c:C = 13r 
 b=0 
 
 
 Easily fusible 
 
 Rosen- 
 buschite 
 
 1.635 
 
 0.014 
 
 - 
 
 ! 
 
 40 
 
 70 
 
 "- 
 
 c:C = 32r 
 
 b=B 
 
 Readily 
 soluble 
 inHCl 
 
 Powder re- / 
 acts alka-/ 
 lino / 
 
 Wollaston- 
 ite 
 
 
 About 
 0.038 
 
 + 
 
 + 
 
 60 
 
 
 
 c:0 = 5f 
 b=C 
 
 
 Twinning 
 lamellas par- 
 allel to {100} 
 
 Pectolite 
 
 
 
 
 
 
 
 
 c=B 
 
 b=C 
 
 
 
 Humite 
 
 
 
 
 
 
 
 
 
 
 T t "IO / 
 
 
 1.639 
 to 
 
 0.032 
 
 + 
 
 
 About 
 70 
 
 
 Very 
 
 c:|J=9 f 
 b=C 
 
 Gelatin- 
 ize readily 
 
 ran'+fi TTP1 
 
 
 Clinohum- 
 ite 
 
 less 
 
 
 
 
 
 
 
 b= C 
 
 
 / ning 
 / lamellae 
 
 Chondro- 
 dite 
 
 1.618 
 
 0.011 
 
 
 
 67 
 
 126 
 
 
 
 
 Chz formed 
 
 
 to 
 
 to 
 
 + 
 
 + 
 
 to 
 
 to 
 
 p>o 
 
 c= t 
 
 Insoluble 
 
 by cleavage, 
 
 Topaz 
 
 1.637 
 
 0.008 
 
 
 
 50 
 
 86 
 
 
 
 
 negative 
 
 
 24 
 
370 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a 
 
 ft 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 
 
 
 
 001 
 
 
 
 
 
 
 
 301 
 
 
 Anhydrite 
 
 CaSC-4 
 
 O 
 
 010 
 
 Granular 
 
 2.95 
 
 3 
 
 Colorless 
 
 1.570 
 
 1.57; 
 
 
 
 
 
 
 (100) 
 
 
 
 
 
 
 
 
 
 Muscovite 
 
 K 2 O 
 
 
 
 
 
 
 Colorless 
 
 1.562 
 
 1 . 593 
 
 
 
 
 "o 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 2.8 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 to 
 3 
 
 
 Colorless 
 
 
 
 
 a 
 
 Phlogopite 
 
 8 MgO 
 
 3 
 
 
 
 
 
 
 Light 
 brownish 
 
 1.541 
 
 1.573 
 
 
 
 
 
 t 
 
 
 
 Tabular || 
 
 
 
 c=B>o 
 
 
 
 302 
 
 O 
 
 
 1 
 
 M 
 
 001 
 
 {001} 
 
 
 2.5 
 
 
 
 
 
 
 
 | 
 
 
 
 Scaly 
 
 3.0 
 
 
 Brown 
 
 
 
 
 s 
 
 Biotite 
 
 FeO 
 
 
 
 
 to 
 
 
 Green 
 
 1.580 
 
 1.638 
 
 
 
 
 | 
 
 
 
 
 3.2 
 
 
 C == ^ 
 
 
 
 
 
 
 3 
 
 
 
 
 
 
 
 
 
 
 
 Lithionite 
 
 g Li 2 
 3 (FeO) 
 
 
 
 
 2.8 
 to 
 
 
 Colorless . 
 Brown 
 
 1.562 
 
 1.600 
 
 
 
 
 
 
 
 
 3.2 
 
 
 e-*>0 
 
 
 
 
 
 Pennine 
 
 H 4 (MgFe) 2 
 Al 2 SiO 9 
 
 
 
 
 
 
 
 
 
 
 
 
 
 M 
 
 
 Tabular || 
 {001} 
 Scaly to 
 radial 
 
 2.6 
 to 
 3.0 
 
 2.5 
 
 Green, || c 
 light yellow 
 _L c green 
 Colorless 
 
 1.576 
 to 
 1.585 
 
 1.576 
 to 
 1.58c 
 
 
 
 
 
 
 001 
 
 
 
 
 
 
 
 
 o. 
 
 Clino- 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 chlore 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 310 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Antigorite 
 
 
 
 
 Flaky 
 
 
 
 
 1.560 
 
 1.57C 
 
 
 
 
 
 
 
 
 
 
 Colorless 
 
 
 
 
 
 
 H4(MgFe) 3 
 Si 2 O 9 
 
 O? 
 
 
 
 2.5 
 
 to 
 
 2 7 
 
 3.5 
 
 Light green- 
 ish 
 
 
 
 
 
 
 
 
 
 
 Chrysotile 
 
 
 
 110 
 About 
 
 Fibrous 
 
 
 
 Rarely green 
 
 
 Aboi; 
 
 I C ". 
 
 
 
 
 
 
 130 
 
 
 
 
 
 
 
 316 
 
 
 Hydrar- 
 gillite 
 
 A1(OH) 3 
 
 M 
 
 001 
 
 Scaly 
 Fibrous 
 
 2.4 
 
 2.5 
 
 Colorless 
 
 1.535 
 
 , K 
 
DESCRIPTIVE SECTION 
 
 MINERALS. Continued. 
 
 371 
 TABLE 14 
 
 r 
 
 r -a 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solu- 
 bility 
 
 Remarks 
 
 Minerals 
 
 
 
 
 
 
 
 
 
 Difficultly 
 
 Readily al- 
 
 
 1.614 
 
 0.044 
 
 + 
 
 
 43 
 
 71 
 
 U>p 
 
 a C 
 
 soluble in 
 
 tered to gyp- 
 
 Anhydrite 
 
 
 
 
 
 
 
 
 
 water 
 
 sum 
 
 
 
 
 
 
 30 
 
 55 
 
 
 
 
 Twin lami- 
 
 
 1.603 
 
 0.041 
 
 
 
 to 
 50 
 
 to 
 90 
 
 > 
 
 c=0 
 
 b=C 
 
 
 nation paral- 
 lel {001] 
 
 Muscovite 
 
 1.575 
 
 0.034 
 
 
 
 
 
 to 
 25 
 
 
 c= a 
 b=B 
 
 Attacked 
 with diffi- 
 culty but 
 
 observed 
 Fuses with 
 difficulty 
 
 Phlogopite 
 
 
 
 
 
 
 
 
 
 more easi- 
 
 
 
 
 
 ! + 
 
 
 
 
 
 ly the 
 
 V 
 
 
 
 
 
 
 
 
 
 
 c: = to (10) 
 
 higher the 
 
 >v 
 
 
 1.638 
 
 0.058 
 
 
 
 
 to 
 
 
 b= I) 
 
 content of 
 
 \x 
 
 Biotite . 
 
 
 
 
 
 
 70 
 
 
 (b=C) 
 
 Mg and 
 
 >v 
 
 
 
 
 
 
 
 
 (p>u) 
 
 
 Fe 
 
 Pleochroic / 
 
 
 
 
 
 
 
 
 
 
 
 halos ,/ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1.606 
 
 0.044 
 
 
 
 
 to 
 
 p>u 
 
 c=0 
 
 b= C 
 
 
 / Readily 
 
 Lithionite 
 
 
 
 
 
 
 90 
 
 
 
 
 fusible 
 
 
 
 
 
 
 
 
 
 
 
 Very variable 
 
 
 
 
 
 
 
 
 
 
 
 optically 
 
 
 
 
 i 
 
 
 
 
 
 ^ 
 
 c C (0) 
 
 
 Anomalous 
 
 
 
 
 (-) 
 
 ( + ) 
 
 
 
 
 Oo>y) 
 
 
 
 interference 
 
 Pennine 
 
 
 
 
 
 
 
 
 
 
 colors 
 
 
 1.577 
 to 
 
 0.001 
 
 
 
 
 
 
 
 
 ^^^^ 
 
 
 1.596 
 
 0.011 
 
 
 
 
 
 
 
 Pleochroic / 
 
 
 
 
 
 
 
 
 
 
 halos / 
 
 
 
 
 + 
 
 
 Variable 
 
 > 
 
 c:C = 0-15f 
 
 Gelatin- 
 ize with 
 HCl 
 
 / Twin- 
 / ning lam- 
 /ellffi || {001} 
 
 Clinochlore 
 
 1.571 
 
 0.011 
 
 - 
 
 + 
 
 
 
 c=0 
 
 b=C 
 
 
 Pseudomorph 
 
 Antigorite 
 
 
 
 
 
 
 Very 
 
 
 
 after olivine 
 
 
 
 
 
 
 
 weak 
 
 
 
 Normal inter- 
 
 
 
 0.009 
 
 + 
 
 + 
 
 Quite small 
 
 p>u 
 
 c= C 
 b=tt 
 
 
 ference colors 
 Very diffi- 
 cultly fusible 
 
 Chrysotile 
 
 1.510 
 
 0.025 
 
 + 
 
 - 
 
 
 to 
 40 
 
 
 p>u 
 
 001:0 = 65 f 
 
 Soluble in 
 ho't H 2 SO4 
 
 Twinning 
 lamellee 
 II {001} 
 
 Hydrar- 
 gillite 
 
 
 
 
 
 
 
 
 
 
 Infusible 
 
 
372 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 ft 
 
 1 
 
 O 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 gr. 
 
 H 
 
 Color 
 
 a 
 
 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 316 
 
 
 Talc 
 
 H 2 Mg 3 Si4()i 2 
 
 M 
 
 001 
 
 Scaly 
 
 2.7 
 
 1 
 
 Colorless 
 
 1.539 
 
 1.589 
 
 317 
 
 
 Pyrophyl- 
 lite 
 
 HAlSi 2 O 8 
 
 O 
 
 001 
 
 Radial 
 Fibrous 
 
 2.8 
 
 1 
 
 Colorless 
 
 
 1.58 
 
 317 
 
 
 Bertrand- 
 ite 
 
 H 2 Be4Si 2 O 9 
 
 O 
 
 010 
 001 
 
 Tabular || 
 {001} or 
 
 2.6 
 
 6 
 
 Colorless 
 
 
 1.57 
 
 
 
 
 
 
 
 {010} 
 
 
 
 
 
 
 
 
 
 
 
 110 
 
 
 
 
 
 
 
 318 
 
 
 Wagnerite 
 
 Mg 2 FPO4 
 
 M 
 
 
 Prismatic 
 
 3.0 
 
 5.5 
 
 Colorless 
 
 1.569 
 
 1.570 
 
 
 | 
 
 Kaolin 
 
 H 4 Al 2 Si 2 O 9 
 
 
 001 
 
 
 2.5 
 
 2.5 
 
 Colorless 
 
 
 1.55 
 
 318 
 
 e! 
 
 
 3 
 
 Nontro- 
 nite 
 
 H 4 Fe 2 Si 2 O 
 
 M 
 
 001 
 
 (110) 
 
 Scaly 
 
 2.7 
 
 2 
 
 Yellow ' 
 
 Light 
 yellow 
 
 About 
 1.6 
 Greenish- 
 
 
 
 
 
 
 010 
 
 
 
 
 
 
 
 319 
 
 
 Hydro- 
 magnesite 
 
 Mg4(C0 3 ) 3 
 (OH) 2 + 3aq. 
 
 M 
 
 100 
 
 Columnar 
 
 2.15 
 
 2 
 
 Colorless 
 
 
 About 
 1.54 
 
 319 
 
 
 Cordierite 
 
 MgzAUSisOis 
 
 O 
 
 
 Granular 
 
 2.65 
 
 7 
 
 Colorless 
 Light blue 
 
 1.535 
 
 Colorless 
 
 1.540 
 Blue 
 
 320 
 
 
 Wavellite 
 
 AlsP 2 O8 + 12aq. 
 
 
 
 110 
 
 Acicular 
 Spherulitic 
 
 2.4 
 
 3.5 
 
 Colorless 
 
 
 1.526 
 
 
 
 
 
 
 010 
 
 
 
 
 
 
 
 321 
 
 
 Gypsum 
 
 CaSC-4 + 2aq. 
 
 M 
 
 (100) 
 
 Granular 
 Fibrous 
 
 2.3 
 
 2 
 
 Colorless 
 
 1.521 
 
 1.523 
 
 
 
 
 
 
 Ill- 
 
 
 
 
 
 
 
DESCRPITIVE SECTION 
 
 MINERALS Continued. 
 
 373 
 TABLE 15 
 
 r 
 
 r - 
 
 Chm 
 
 Chz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solubility 
 
 Remarks 
 
 Minerals 
 
 1.589 
 
 0.050 
 
 - 
 
 4- 
 
 
 to 
 17 
 
 Small 
 
 p>U 
 
 c==0 
 
 Attacked 
 with 
 difficulty 
 
 Similar to 
 muscovite 
 Distinguished 
 by cobalt 
 solution 
 
 Talc 
 
 
 0.041 
 
 - 
 
 + 
 
 62 
 
 109 
 
 p>o 
 
 c= 
 b=C 
 
 Soluble 
 in hot 
 H 2 SO4 
 
 Similar to 
 muscovite 
 Distinguished 
 by hydrofluo- 
 silicic acid 
 
 Pyrophyl- 
 lite 
 
 
 High 
 
 - 
 
 i 
 
 
 75 
 
 u>p 
 
 a= n 
 
 Insoluble 
 
 Infusible 
 
 Bertrand- 
 ite 
 
 1.582 
 
 0.013 
 
 * 
 
 + 
 
 
 26 
 
 p>o 
 
 c= C 
 
 b=B 
 
 Easily 
 soluble 
 
 Very diffi- 
 cultly fusible 
 
 Wagnerite 
 
 0.008 
 
 
 + 
 
 About 
 90 
 
 
 p>u 
 
 c:0 = 20r 
 b= C 
 
 Soluble in 
 hot H 2 SO4 
 
 Infusible 
 
 Kaolin 
 
 yellow 
 
 0.02 
 
 - 
 
 + 
 
 
 60 
 
 
 c:C = 6 
 
 Gelatin- 
 izes with 
 HC1 
 
 Fusible to 
 black slag 
 
 Nontronite 
 
 
 About 
 0.02 
 
 + 
 
 
 
 About 
 120 
 
 
 c:0 = 33 
 b= C 
 
 Easily sol- 
 uble with 
 efferves- 
 cence 
 
 Twinning 
 lamination 
 || {100} 
 
 Hydro- 
 
 magnesite 
 
 1.544 
 
 Colorless 
 
 0.009 
 
 - 
 
 
 Variable 
 
 o>p 
 
 c= a 
 
 b=C 
 
 Attacked 
 with diffi- 
 culty 
 
 Penetration 
 trilling 
 Pleochroic 
 halos 
 
 Cordierite 
 
 
 0.025 
 
 + 
 
 
 
 
 50 
 
 p>u 
 
 c= C 
 b=0 
 
 Soluble 
 
 
 Wavellite 
 
 1.531 
 
 0.010 
 
 - 
 
 
 60 
 
 104 
 
 Inclined 
 
 c:C = 54f 
 
 Difficultly 
 soluble in 
 water 
 
 {111) Fi- 
 brous fracture 
 Twinning 
 lamellae 
 
 Gypsum 
 
374 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 8 9 10 
 
 Page 
 
 1 
 
 
 
 Minerals 
 
 Chemical 
 composition 
 
 Ab:An 
 
 %Si0 2 
 
 Sp.gr. 
 
 Indices com- 
 pared with 
 quartz 
 
 a 
 
 /? 
 
 321 
 
 a 
 
 i 
 
 Orthoclase 
 
 (KNa)AlSi 3 O 8 
 
 
 65% 
 
 2.56 
 
 (w for quartz 
 = 1 . 544) 
 
 1.519 
 
 1.524 
 
 Microcline 
 
 
 a] 
 /? < a, 
 f\ 
 
 1.522 
 
 1.526 
 
 Anorthoclase 
 
 (NaK)AlSi 3 Os 
 
 
 
 2.58 
 
 1.523 
 
 1.529 
 
 Albite 
 
 NaAlSiaOs 
 1 
 
 t 
 CaAl 2 Si 2 O 8 
 
 Ab 
 
 68% 
 
 2.62 
 
 a] 
 
 P \< a, 
 
 r\ 
 
 1.532 
 
 1.534 
 
 Albite- 
 oligoclase 
 
 AbeAm 
 
 65% 
 
 2.64 
 
 1.534 
 
 1.538 
 
 Oligoclase 
 
 Ab<Am 
 
 64% 
 
 )->w; <x< w 
 
 1.537 
 
 1.541 
 
 Oligoclase- 
 andesine 
 
 Ab 2 Am 
 
 60% 
 
 2.65 
 
 Y=\ a = <i> 
 
 1.544 
 
 1.548 
 
 Andesine 
 
 AbsAn 2 
 
 58% 
 
 2.67 
 
 jf>e; a< e 
 
 1.549 
 
 1.553 
 
 Labradorite 
 
 AbiAm 
 
 55% 
 
 2.69 
 
 a] 
 P\>9 
 
 r\ 
 
 1.555 
 
 1.558 
 
 Labradorite- 
 bytownite 
 
 Ab 2 Ans 
 
 53% 
 
 2.70 
 
 1.560 
 
 1.563 
 
 Bytownite 
 
 AbiAn4 
 
 48% 
 
 2.72 
 
 1.564 
 
 1.569 
 
 Anorthite 
 
 An 
 
 44% 
 
 2.75 
 
 (s for quartz 
 = 1.553) 
 
 1.575 
 
 1.583 
 
DESCRIPTIVE SECTION 
 
 MINERALS. Continued. 
 11 12 13 14 15 16 
 
 17 
 
 375 
 TABLE 16 
 
 18 19 20 21 22 
 
 r 
 
 r -a 
 
 Chm 
 
 2V 
 
 Disper- 
 sion 
 about 
 
 a 
 
 Extinction Angles 
 
 Minerals 
 
 on 001 
 
 on 010 
 
 JL 
 
 _L C 
 
 _La 
 
 Zone 
 010 
 
 1.526 
 
 0.007 
 
 - 
 
 69 
 
 p>0 
 
 
 
 + 5 
 
 
 
 5 
 
 
 
 
 
 Orthoclase 
 
 1.529 
 1.530 
 
 - 
 
 88 
 
 p>o 
 
 + 15i 
 
 + 5* 
 
 88 
 
 10 
 
 
 
 Microcline 
 
 
 45 
 
 p>u 
 
 + 2 
 
 + 9 
 
 88* 
 
 9 
 
 
 
 Anorthoclase 
 
 1.540 
 
 0.008 
 
 + 
 
 77 
 
 o>p 
 
 + 4 
 
 + 19i 
 
 74 
 
 19i 
 
 -15 
 
 -16 
 
 Albite 
 
 1.542 
 
 + 
 
 84 
 
 1)>P 
 
 + 2i 
 
 + 11* 
 
 84i 
 
 13 
 
 -6 
 
 00 
 
 Albite- 
 oligoclase 
 
 1.545 
 
 - 
 
 88. 
 
 p>u 
 
 + 2 
 
 + 8 
 
 83 
 
 3 
 
 
 
 + 5 
 
 Oligoclase 
 
 1.552 
 
 1.557 
 1.563 
 
 1.568 
 
 - 
 
 86 
 
 p>u 
 
 
 
 + * 
 
 75 
 
 5 
 
 + 4 
 
 + 10 
 
 Oligoclase- 
 andesine 
 
 + 
 
 88 
 
 1)>P 
 
 -2* 
 
 -8 
 
 68 
 
 7 
 
 + 13 
 
 + 19 
 
 Andesine 
 
 + 
 
 77 
 
 1)>P 
 
 5 
 
 -18 
 
 65 
 
 22 
 
 + 27 
 
 + 28 
 
 Labradorite 
 
 + 
 
 77 
 
 u>p 
 
 -9 
 
 -25 
 
 58 
 
 32 
 
 + 33 
 
 + 38 
 
 Labradorite- 
 bytownite 
 
 1.574 
 
 0.010 
 
 - 
 
 80 
 
 p>u 
 
 -21 
 
 -33 
 
 57 
 
 42 
 
 + 37 
 
 + 45 
 
 Bytownite 
 
 1.588 
 
 0.013 
 
 - 
 
 77 
 
 p>u 
 
 -37 
 
 -36 
 
 55 
 
 48 
 
 + 43 
 
 + 56 
 
 Anorthite 
 
376 
 
 PETROGRAPHIC METHODS 
 
 BIAXIAL 
 
 
 
 
 
 Crys- 
 
 
 
 
 
 
 
 
 Page 
 
 2 
 
 o 
 
 Minerals 
 
 Chemical 
 composition 
 
 tal 
 
 sys- 
 
 Cleav- 
 age 
 
 Develop- 
 ment 
 
 Sp. 
 
 H 
 
 Color 
 
 a 
 
 
 
 
 
 
 
 tem 
 
 
 
 
 
 
 
 
 
 | 
 
 Phillipsite 
 
 (CaK 2 )Al 2 
 Si 5 Oi4 + 5 aq. 
 
 
 
 
 2.2 
 
 
 
 
 
 
 ~ 
 
 
 
 M 
 
 (001) 
 (010) 
 
 Spherulitic 
 
 
 4.5 
 
 Colorless 
 
 
 
 
 
 
 
 
 
 I 
 
 Harmo- 
 tome 
 
 (BaK 2 )Al 2 
 Si 5 Oi4 + 5 aq. 
 
 
 
 
 2.5 
 
 
 
 1.503 
 
 1.506 
 
 
 
 Laumont- 
 ite 
 
 CaAl 2 Si4Oi 2 
 + 4aq. 
 
 M 
 
 010 
 110 
 
 Prismatic 
 
 2.4 
 
 3.5 
 
 Colorless 
 
 1.513 
 
 1.524 
 
 
 
 Thomson- 
 ite 
 
 (CaNa 2 )Al 2 
 Si 2 Os + 2J aq. 
 
 
 
 010 
 (100) 
 
 Tabular || 
 {100} 
 
 2.3 
 
 5 
 
 Colorless 
 
 1.497 
 
 1.503 
 
 341 
 
 a 
 
 Heuland- 
 ite 
 
 CaAl 2 Si 6 Oi6 
 + 5aq. 
 
 
 
 Platy I) 
 {100} 
 
 2.2 
 
 5 
 
 
 1.498 
 
 1.499 
 
 
 1 
 
 
 
 
 
 Columnar 
 
 
 
 
 
 
 
 
 Desmine 
 
 (CaNa 2 )Al 2 
 Si 6 Oi6 + 6 aq. 
 
 M 
 
 010 
 
 II c 
 Tabular || 
 
 2.15 
 
 3.5 
 
 Colorless 
 
 1.494 
 
 1.498 
 
 
 1 
 
 
 
 
 
 {010} 
 
 
 
 
 
 
 
 
 Epistilbite 
 
 CaAl 2 Si 6 Oi6 
 + 5 aq. 
 
 
 
 Radial 
 Fibrous 
 
 2.25 
 
 4 
 
 
 1.502 
 
 1.510 
 
 
 
 Natrolite 
 
 Na 2 Al 2 SisOio 
 
 
 
 
 2.2 
 
 
 
 1.478 
 
 1.481 
 
 
 1 
 
 O 
 
 Mesolite 
 
 
 M 
 
 110 
 
 Columnar 
 
 II 
 
 
 5.5 
 
 Colorless 
 
 
 About 
 
 
 1 
 
 
 A 
 
 
 (89) 
 
 Radial 
 
 
 
 
 
 1 .5 
 
 
 1 
 
 Scolesite 
 
 CaAhSisOio 
 + 3 aq. 
 
 
 
 
 2.4 
 
 
 
 
 1.502 
 
DESCRIPTIVEISECTION 
 
 ZEOLITES. 
 
 377 
 TABLE 17 
 
 r 
 
 f-OL 
 
 Chm 
 
 Cbz 
 
 2V 
 
 2E 
 
 Disper- 
 sion 
 
 Optical 
 orientation 
 
 Solu- 
 bility 
 
 Remarks 
 
 Minerals 
 
 
 0.003 
 
 + 
 
 + 
 
 70 
 
 
 
 c:C = 20 
 b-fi 
 
 
 Readily 
 
 
 Phillipsite 
 
 
 
 
 
 
 
 
 
 Gelatin- 
 
 
 o 
 
 
 
 
 
 
 
 
 Very 
 
 
 
 
 2 
 
 
 
 
 
 
 
 
 weak 
 
 
 TTPl 
 
 
 
 
 
 1.508 
 
 0.005 
 
 + 
 
 
 85 
 
 
 
 c:0 = 60 
 
 b=B 
 
 
 Difficult- 
 
 
 Harmo- 
 tome 
 
 1.525 
 
 0.012 
 
 - 
 
 - 
 
 
 55 
 
 u^p 
 
 c:0 = 70f 
 
 b=B 
 
 Gelatinizes 
 with 
 HC1 
 
 Decomposes 
 in air with 
 loss of water 
 
 Laumont- 
 ite 
 
 1.525 
 
 0.028 
 
 
 * 
 
 
 About 
 90 
 
 u^p 
 
 c=0 
 
 b= C 
 
 Gelatinizes 
 with 
 HC1 
 
 Fuses with 
 intumes- 
 cence 
 
 Thomson- 
 ite 
 
 1.505 
 
 0.007 
 
 - 
 
 - 
 
 
 
 to 
 50 
 
 Crossed 
 
 c nearly = B 
 b=C 
 
 Gelatinizes 
 with 
 HC1 
 
 Parallel 
 aggregates 
 
 Heuland- 
 ite 
 
 1.500 
 
 0.006 
 
 - 
 
 - 
 
 33 
 
 52 
 
 u>p 
 
 c:0 = 8 f 
 
 Give 
 powdered 
 
 Often radial 
 aggregates 
 
 Desmine 
 
 
 
 
 
 
 
 
 
 silica 
 
 
 
 
 1.512 
 
 0.010 
 
 - 
 
 - 
 
 40 
 to 70 
 
 
 u>p 
 
 c:C = 9 r 
 
 with HC1 
 
 Twinned 
 || {100} 
 
 Epistil- 
 bite 
 
 1.490 
 
 0.012 
 
 - 
 
 + 
 
 60 
 
 96 
 
 u>p 
 
 c:C = 0-5 
 
 
 Radial 
 aggregates 
 
 Natrolite 
 
 
 About 
 0.01 
 
 
 
 
 
 
 
 Gelatinize 
 with 
 HC1 
 
 Twinning 
 
 Mesolite 
 
 
 
 
 
 
 
 
 
 
 lamellse 
 
 
 
 0.008 
 
 - 
 
 - 
 
 
 58 
 
 ">' 
 
 c:0 = 17f 
 b = C 
 
 
 || {100} 
 
 Scolesite 
 
378 PETROGRAPHIC METHODS 
 
 TABLE 18 
 
 ROCK MINERALS GROUPED ACCORDING TO COLOR. 
 
 Grayish-violet : Perovskite, tourmaline, rutile, titanium augite, cataphor- 
 ite, axinite. 
 
 Violet : Dumortierite, piemontite, cotschubeyite, csemmererite, fluorite. 
 
 Blue : Riebeckite, arfvedsonite, carinthine, dumortierite, chlorite (erinite), 
 lazurite, tourmaline, glaucophane, haiiyne-noselite, lazulite, chloritoid, 
 serendibite, crocidolite, corundum, fuchsite, cordierite, anatase, cyanite, 
 sapphirine, sodalite, beryl, diaspore. 
 
 Green: Green hornblende, aegirine, tourmaline, glauconite, seladonite, 
 biotite, aegirine-augite, fassaite, chrome epidote, iron spinel, pargasite, brittle 
 mica, chlorite, lime garnet, actinolite, diopside, baddeleyite, chrysoberyl, 
 serpentine. 
 
 Orange and Yellow : Brookite, wurtzite, sphalerite, astrophyllite, biotite, 
 brown hornblende, rutile, cassiterite, lavenite, humite group, staurolite, 
 chrome epidote, rosenbuschite, monazite, glass, carpholite, baddeleyite, 
 fayalite, nontronite, sulphur, epidote, griinerite, cummingtonite, tourma- 
 line, apatite, melilite, lime garnet, anatase, titanite, rinkite, mosandrite, 
 prismatine, siderite. 
 
 Red: Hematite, piemontite, chrome chlorite, thulite, hypersthene, cas- 
 siterite, cataphorite, titanite, andalusite, garnet, orthite, clinozoisite, man- 
 ganvesuvianite, eudialyte, xenotime. 
 
 Brown : Lievrite, cossyrite, ilmenite, chromite, pseudobrookite, brookite, 
 basaltic hornblende, biotite, lithionite, tourmaline, rutile, melanite, orthite, 
 chrome spinel, perovskite, wurtzite, sphalerite, barkievikite, hyalosiderite, 
 astrophyUite, fayalite, staurolite, brown hornblende, glass, baddeleyite, 
 hypersthene, lime garnet, monazite, diallage, titanite, pargasite, antho- 
 phyllite, augite, brittle mica. 
 
 N. B. In a general way the minerals are arranged above according to 
 diminishing intensity of color. Those at the beginning of a series are in 
 some instances not very transparent while those at the end scarcely show any 
 color at all in thin section. Rare varieties are introduced here when the 
 color is characteristic. 
 
00 
 
 o w 
 
 P5 ^ 
 
 3 PQ 
 
 Double 
 
 Refraction 
 
 0.05 mm. 
 
 0.04 mm. 
 
 0.03 mm. 
 
 0.02 mm. 
 
 0.01 mm 
 
 Retardation 200 400 
 
 First Order 
 
 600 800 1000 | 
 
 Second Order 
 
Talc, Fay all te 
 Astrophyllite 
 Epidote f 
 Biotite 
 Anatase, Zircon 
 
 Basaltic Hornblende 
 
 Titanite | 
 Xenotime 
 Cassiterite 
 
 Titanite f 
 
 Aragonite, Brookite 
 Calcite, Dolomite 
 Magnesite 
 Siderite 
 
 0,287 Rutile 
 0,290 Sulphur 
 
 2000 
 Fourth Order 
 
 2200 
 
 2400 11. 
 
DESCRIPTIVE SECTION 379 
 
 TABLE 18. Continued. 
 
 I. INTERFERENCE FIGURES ARE OBSERVED IN CLEAVAGE 
 
 PLATES. 
 
 1. Uniaxial and Nearly Uniaxial: 
 
 a. Perpendicular to the optic axis: Brittle mica ^p, eudialyte +, alunite 
 + , phlogopite , biotite , pennine , brucite +, talc , apophyllite +, 
 heulandite +. 
 
 b. Oblique to the optic axis: Calcite, dolomite, magnesite, siderite, hy- 
 drargillite. 
 
 c. Parallel to the optic axis: Rutile, wurtzite, zircon, xenotime, scapolite, 
 cancrinite. 
 
 2. Biaxial: 
 
 a. Perpendicular to the acute bisectrix, approximate optic angle in paren- 
 thesis: Brittle mica i (not very large), margarite (large), prehnite + 
 (quite variable), topaz + (quite large, variable), celestite + (about 90), 
 muscovite (about 70), sericite (about 25), phlogopite and biotite 
 (very small), lithionite (variable), clinochlore + (variable), antigorite 
 (variable), talc (about 25), pyrophyllite ( > 100), hydromagnesite 
 + (120), heulandite + (small), thomsonite + (about 90), goethite + 
 (crossed optic planes). 
 
 b. Perpendicular to the obtuse bisectrix: Zoisite /? , astrophyllite , 
 anthophyllite -, pectolite , barite -, anhydrite -. 
 
 c. Parallel to the optic plane: Zoisite a, diaspore, lawsonite, sillimanite, 
 humite, bertrandite, gypsum, laumontite, desmine, epistilbite. 
 
 d. Slightly inclined to the acute bisectrix but perpendicular to the optic 
 plane: Monazite + (25), baddeleyite (very large), rinkite + (77), 
 cyanite (very large), clinochlore + (variable). 
 
 e. Greatly inclined to the acute bisectrix and perpendicular to the optic plane: 
 Hydrargillite, clinozoisite. 
 
 f. Nearly perpendicular to an axis: Epidote, diallage, wollastonite. 
 
 g. Inclined to the optic plane: Brittle mica, amphibole, augite, wavellite, 
 kaolin. 
 
 II. TWINNING LAMINATIONS. 
 
 (Those indicated with * do not often show lamination on account of the 
 small extinction angle). Leucite, rutile, perovskite, calcite, dolomite, 
 baddeleyite, zoisite, *clinozoisite, *epidote, orthite, cyanite, chloritoid, 
 margarite, serendibite, diallage, rinkite, mosandrite, prehnite, wollaston- 
 ite, pectolite, humite group, *mica, clinochlore, hydrargillite, gypsum, 
 hydromagnesite, microcline, plagioclase, epistilbite, mesolite, scolesite. 
 
380 PETROGRAPHIC METHODS 
 
 TABLE 18. Continued. 
 
 HI. MINERALS THAT FREQUENTLY SHOW ANOMALOUS 
 INTERFERENCE COLOR. 
 
 Chrysoberyl, clinozoisite, zoisite a, pennine, gehlenite, melilite, vesuvian- 
 ite, brucite, dumortierite, prehnite, lime garnet, chloritoid, f assaite, titanium 
 augite, laumontite, sanidine. 
 
 IV. PLEOCHROIC HALOS. 
 
 Found in : Amphibole, andalusite, chlorite, mica, cordierite, brittle mica, 
 staurolite, tourmaline. 
 
 Found around: Dumortierite, orthite, rutile, titanite, xenotime, cassi- 
 terite, zircon. 
 
 V. USUAL CRYSTALLOGRAPHIC HABIT OF NON-CUBIC MINERALS. 
 
 1. Isometric (pyramidal, thick tabular, short prismatic, granular): 
 Anatase, cassiterite, xenotime, gehlenite, carbonates, scapolite, alunite, 
 chabazite, quartz, nepheline, titanite, sulphur, monazite, chrysoberyl, 
 diaspore, forsterite, olivine, fayalite, monticellite, axinite, rinkite, lazulite, 
 barite, humite group, celestite, topaz, anhydrite, cordierite. 
 
 2. Tabular : Anatase, corundum, melilite, brucite, tridymite, apophyllite; 
 brookite, pseudobrookite, serendibite, astrophyllite, cyanite, brittle mica, 
 margarite, sapphirine, spodumene, axinite, mosandrite, micas, chlorite 
 group, antigorite, bertrandite, hydrargillite, talc, kaolin, heulandite. 
 
 3. Prismatic : Rutile, cassiterite, zircon, xenotime, corundum, vesuvian- 
 ite, tourmaline, apatite, scapolite, beryl, baddeleyite, lievrite, titanite, epi- 
 dote group, lavenite, prismatine, staurolite, cyanite, pyroxene group, am- 
 phibole group, andalusite, aragonite, wollastonite group, wagnerite. 
 
 4. Columnar (acicular) : Tourmaline, apatite, cancrinite, hydronephelite, 
 gcethite, lievrite, acmite, tremolite, actinolite, sillimanite, datolite, dumor- 
 tierite, aragonite, wollastonite group, topaz, celestite, hydromagnesite, 
 desmine, natrolite. 
 
 5. Fibrous: Wurtzite, chalcedony, sillimanite, crocidolite, prehnite, 
 carpholite, datolite, chrysotile, wavellite, hydrargillite, pyrophyllite, gypsum, 
 zeolites. 
 
 VI. CLASSIFICATION OF A FEW MINERALS ACCORDING TO 
 DECREASING MAGNETISM. (AFTER DOELTER.) 
 
 Metallic iron, magnetite, pyrrhotite; ilmenite, lievrite, hematite; chro- 
 mite, siderite, almandine, limonite, augite rich in iron, iron spinel, arf ved- 
 sonite; hornblende, augite poor in iron, epidote, pyrope; tourmaline, bronz- 
 ite, vesuvianite; staurolite, actinolite, olivine, pyrite; biotite, chlorite, 
 rutile; haiiyne, diopside, muscovite, nepheline, leucite, dolomite, feldspars. 
 
DESCRIPTIVE SECTION 381 
 
 TABLE IS. Continued: 
 
 VII. CLASSIFICATION OF ROCK-FORMING MINERALS ACCORDING 
 
 TO SOLUBILITY. 
 
 1. Insoluble in hydrofluoric acid: Magnetite, chromite, hematite, 
 ilmenite, graphite, carbonaceous substance, perovskite, spinel group, pris- 
 matine, astrophyllite, fluorite, rutile, anatase, cassiterite, zircon, xenotime, 
 chrysoberyl, corundum, baddeleyite, tourmaline, serendibite, beryl, brookite, 
 staurolite, diaspore, cyanite, chloritoid, xanthophyllite, sapphirine, axinite, 
 sillimanite, dumortierite, andalusite, topaz, bertrandite. 
 
 2. Attacked by hydrochloric acid with difficulty: Hematite, ilmenite, 
 sphalerite, boracite, garnet, vesuvianite, pseudobrookite, goethite, titanite, 
 orthite, ottrelite, lavenite, carpholite, lazulite, hydrargillite, labradorite. 
 
 3. With hydrochloric acid give powdered silica: Leucite, meionite, 
 apophyllite, rinkite, mosandrite, anorthite, desmine, epistilbite. 
 
 4. With hydrochloric acid give gelatinous silica : Analcite, sodalite group, 
 gehlenite, melilite, eudialyte, nepheline, hydronephelite, lievrite, for- 
 sterite, olivine, fayalite, monticellite, datolite, rosenbuschite, wollastonite, 
 pectolite, humite group, chlorite group, serpentine, heulandite, natrolite, 
 chabazite, phillipsite, harmotome, laumontite, thomsonite, mesolite, scolesite. 
 
 5. Soluble in hydrochloric acid: Pyrrhotite, wurtzite, magnetite, peri- 
 clase, apatite, *calcite, *dolomite, *magnesite, *siderite, brucite, *cancrinite, 
 monazite (white residue), wagnerite, wavellite, *aragonite, *hydromagnesite. 
 
 Those marked * evolve carbonic acid with effervescence. 
 
 6. Soluble in caustic potash: Opal (quartz), chalcedony, tridymite, 
 sulphur, kaolin (white residue) . 
 
 VIII. CLASSIFICATION OF THE MINERALS ACCORDING TO THEIR 
 
 FUSIBILITY BEFORE THE BLOWPIPE. 
 
 1. Easily fusible (KobelPs scale 1-3) : Almandine, grossularite, lievrite, 
 sulphur, lavenite, aegirine, aegirine augite, spodumene, glaucophane, 
 riebeckite, arfvedsonite, crocidolite, axinite, rinkite, datolite, mosandrite, 
 rosenbuschite, pectolite, lithionite, phillipsite, scolesite. 
 
 2. Fairly easily fusible (3-4) : Pyrope, hessonite, topazolite, melanite, 
 analcite, tourmaline, eudialyte, scapolite, elaeolite, apophyllite, chabazite, 
 cancrinite, hydronephelite, epidote group, astrophyllite, augite, fassaite, 
 lawsonite, tremolite, actinolite, hornblende, prehnite, celestite, lepidome- 
 lane, anhydrite, nontronite, gypsum, anorthoclase, basic plagioclase, 
 heulandite, desmine, natrolite, thomsonite, epistilbite. 
 
 3. Difficultly fusible (4-5) : Boracite, sodalite, melilite, nepheline, mona- 
 zite, chloritoid, fayalite, hypersthene, diopside, diallage, carpholite, wollas- 
 tonite, muscovite, phlogopite, biotite, clinochlore, orthoclase, microcline, 
 acid plagioclase, harmotome. 
 
382 PETROGRAPHIC METHODS 
 
 TABLE IS. Continued. 
 
 4. Only fusible on the edges (5-6): Magnetite, sphalerite, wurtzite, 
 haiiyne, noselite, fluorite, gehlenite, apatite, beryl, pseudobrookite, baddel- 
 eyite, titanite, margarite, monticellite, hyalosiderite, enstatite, antho- 
 phyllite, barite, humite group, pennine, serpentine, cordierite, wagnerite. 
 
 5. Infusible : Perovskite, spinel, periclase, leucite, opal, rutile, anatase, 
 cassiterite, zircon, xenotime, corundum, carbonates, ahmite, brucite, quartz, 
 chalcedony, tridymite, brookite, chrysoberyl, goethite, serendibite, pris- 
 matine, staurolite, diaspore, cyanite, xanthophyllite, sapphirine, forsterite, 
 olivine, sillimanite, dumortierite, lazulite, andalusite, aragonite, topaz, 
 bertrandite, hydrargillite, talc, pyrophyllite, kaolin, hydromagnesite, 
 wavellite. 
 
 IX. FINE POWDER GIVES ALKALINE REACTION. 
 
 Albite, analcite, andesine, anorthite, apophyllite, axinite, biotite, bytown- 
 ite, datolite, desmine, epidote, heulandite, hydronephelite, lime garnet, 
 clinochlore, clinozoisite, labradorite, leucite, lithionite, margarite, micro- 
 cline, muscovite, natrolite, nepheline, oligoclase, orthoclase, pennine, prehn- 
 ite, serpentine, talc, tremolite, vesuvianite, wollastonite, zoisite. 
 
DESCRIPTIVE SECTION 
 
 383 
 
 TABLE 19. 
 
 X. ROCK-FORMING 
 
 MINERALS 
 SPECIFIC 
 
 Metallic iron 7.8 
 
 Galena 7.5 
 
 Cassiterite 6.9 
 
 Baddeleyite 5.8 
 
 Hematite 5.25 
 
 Magnetite 5.2 
 
 Ilmenite 5.2-4.8 
 
 Pyrite 5.0 
 
 Pseudobrookite 5.0 
 
 Zircon 4.7 
 
 Pyrrhotite 4.6 
 
 Xenotime 4.6 
 
 Chromite 4.5 
 
 Koppite 4.5 
 
 Barite . 4.5 
 
 Melanite 4.3 
 
 Almandine 4.3 
 
 Pyrochlore 4.3 
 
 Goethite 4.3 
 
 Rutile 4.25 
 
 Chalcopyrite 4.2 
 
 Chrome spinel 4.1 
 
 Topazolite 4.1 
 
 Dysanalyte 4.1 
 
 Brookite.. 4.1-3.9 
 
 Sphalerite 4.0 
 
 Wurtzite 4.0 
 
 Lievrite 4.0 
 
 Perovskite 4.0 
 
 Corundum 4.0 
 
 Celestite 3.95 
 
 Iron spinel 3.9 
 
 Siderite 3.9 
 
 Anatase 3.9 
 
 Hessonite 3.8 
 
 Staurolite 3 . 8-3 . 6 
 
 Pyrope 3.75 
 
 Chrysoberyl 3.7 
 
 Periclase 3.65 
 
 Cyanite 3.6 
 
 Ardennite 3.6 
 
 Orthite 3.6 
 
 Spinel 3.6 
 
 Lavenite 3.6 
 
 Fayalite 3.6 
 
 Topaz 3.6-3.4 
 
 Rohrbach solution 3-58 
 
 Diamond. . .3.5 
 
 ARRANGED ACCORDING TO 
 GRAVITY. 
 
 Chloritoid 3.5 
 
 Titanite 3.5 
 
 Sapphirine 3.5 
 
 Aegirine 3.5 
 
 Arf vedsonite 3.5 
 
 Triphyline 3.5 
 
 Hypersthene 3.5 
 
 Diaspore 3 . 45 
 
 Rinkite 3.45 
 
 Grossularite 3 . 45 
 
 Vesuvianite 3 . 45 
 
 Woehlerite 3.4 
 
 Barkievikite 3.4 
 
 Serendibite 3.4 
 
 Epidote 3.4 
 
 Augite 3.35 
 
 Astrophyllite 3 . 35 
 
 Prismatine 3 . 35 
 
 Jadeite 3.35 
 
 Clinozoisite 3 . 35 
 
 Methylene iodide 3.32 
 
 Zoisite 3.3 
 
 Diallage 3.3 
 
 Olivine 3.3 
 
 Bronzite 3.3 
 
 Basaltic hornblende 3.3 
 
 Axinite.. 3.3 
 
 Riebeckite 3.3 
 
 Dumortierite 3.3 
 
 Rosenbuschite 3.3 
 
 Diopside 3.3 
 
 Klein's solution 3.28 
 
 Cornerupine 3 . 25 
 
 Sillimanite 3 . 25 
 
 Forsterite 3 . 25 
 
 Tourmaline 3.25-3.0 
 
 Crocidolite 3.2 
 
 Fluorite.... 3.2 
 
 Andalusite 3.2 
 
 Monticellite 3.2 
 
 Fuggerite. 3.2 
 
 Common hornblende 3 . 2-3 . 1 
 
 Humite 3.2-3.1 
 
 Chondrodite 3 . 2-3 . 1 
 
 Clinohumite 3 . 2-3 . 1 
 
 Fassaite 3.2-3.0 
 
 Biotite. 3 . 2-3 . 
 
 Lithionite.. ..3.2-2.8 
 
384 
 
 PETROGRAPHIC METHODS 
 
 TABLE 19. Continued. 
 
 X. ROCK-FORMING MINERALS ARRANGED ACCORDING 
 SPECIFIC GRAVITY. Continued. 
 
 TO 
 
 Thoulet solution 3.18 
 
 Apatite 3.15 
 
 Enstatite 3 
 
 Lazulite 3 
 
 Magnesite 3 
 
 Spodumene 3 
 
 Lawsonite 3 
 
 Euclase 3 
 
 Xanthophyllite 3 
 
 Anthophyllite 3 
 
 Glaucophane 3 
 
 Mosandrite 3 
 
 Eucolite 3.05 
 
 Actinolite 3.0 
 
 Phlogopite 3.0 
 
 Wagnerite 3.0 
 
 Datolite 3.0 
 
 Boracite 3.0 
 
 Cryolite 3.0 
 
 Phenacite 3.0 
 
 Margarite 3.0 
 
 Acetylene bromide 3.0 
 
 Dolomite 2 . 95 
 
 Carpholite 2.95 
 
 Aragonite 2 . 95 
 
 Anhydrite 2.95 
 
 Melilite 2.9 
 
 Prehnite 2.9 
 
 Pectolite 2.9 
 
 Tremolite 2.9 
 
 Chlorite .2.9-2.6 
 
 Conchite 2 . 85 
 
 Wollastonite 2.85 
 
 Paragonite 2 . 85 
 
 Muscovite 2 . 85 
 
 Iddingsite 2.85 
 
 Pyrophyllite 2.8 
 
 Anorthite 2 . 75 
 
 Scapolite 2.75-2.6 
 
 Bytownite 2 . 72 
 
 Calcite 2.72 
 
 Alunite 2.7 
 
 Beryl 2.7 
 
 Talc 2.7 
 
 Pennine 2.7 
 
 Clinochlore 2.7 
 
 Nontronite. . . 2.7 
 
 Serpentine 2 . 7-2 . 5 
 
 Labradorite 2 . 69 
 
 Andesine 2 . 67 
 
 Quartz 2.65 
 
 Cordierite 2 . 65 
 
 Ktypeite 2.65 
 
 Glass. . 2.65-2.2 
 
 Oligoclase 2 . 64 
 
 Albite 2.60 
 
 Chalcedony 2.6 
 
 Bertrandite 2.6 
 
 Nepheline 2.6 
 
 Anorthoclase 2.58 
 
 Orthoclase 2 . 56 
 
 Microcline 2 . 56 
 
 Leucite. . . 2.5 
 
 Haiiyne. 
 
 2.5 
 
 Kaolin 2.5 
 
 Harmotome 2.5 
 
 Cancrinite 2.45 
 
 Hydrargillite 2.4 
 
 Wavellite 2.4 
 
 Brucite 2.4 
 
 Laumontite 2.4 
 
 Apophyllite 2.35 
 
 Sodalite 2.3 
 
 Graphite 2.3 
 
 Tridymite 2.3 
 
 Gypsum 2.3 
 
 Glauconite 2.3 
 
 Scolesite 2.3 
 
 Thomsonite 2.3 
 
 Epistilbite 2.25 
 
 Analcite 2.2 
 
 Hydronephelite 2.2 
 
 Natrolite 2.2 
 
 Opal 2.2 
 
 Heulandite 2.2 
 
 Phillipsite 2.2 
 
 Desmine 2.15 
 
 Hydromagnesite 2.15 
 
 Chabazite 2.1 
 
 Halite 2.1 
 
 Gmelmite 2.1 
 
 Sulphur 2.0 
 
 Meerschaum 2.0 
 
 Coals.. ....<2.0 
 
INDEX 
 
 a-dimethyl glyoxime, 167 
 
 Abbe, drawing apparatus, 134 
 
 test plate, 24 
 
 total reflect ometer, 39 
 Abnormal interference colors, 78 
 Accessory apparatus, 125 
 Achromatic, 3 
 Acmite, 285 
 Actinolite, 289 
 Acute bisectrix, 59 
 Adjustment of the cross hairs, 28 
 Adjustment of the nicols, 28 
 Adularia, 328 
 Aegirine, 284 
 Aegirine augite, 284 
 Aenigmatite, 294 
 Agalmatolite, 317 
 Aggregate, 185 
 Aggregate polarization, 186 
 Alaunschiefer, 205 
 Albite, 330 
 Albite law, 332 
 Alidade, 40 
 Alkali feldspar, 324 
 Allanite, 261 
 Allotriomorphic, 181 
 Almandite, 215 
 Alum, separation by, 151 
 Aluminium, chemical test for, 168 
 Aluminium fluosilicate, 164 
 Alunite, 243 
 Amazon stone, 329 
 Amesite, 312 
 
 Ammonium, chemical test for, 165 
 Ammonium metavanadinate, 170 
 Ammonium phosphomolybdate, 171 
 Amphibole, alteration of, 292 
 
 color of, in a slide, 293 
 
 extinction of, 293 
 
 group, 287 
 
 Analcite, 222 
 
 Analyses by washing, 152 
 
 Analyzer, 13, 51 
 
 Anastigmatic lens, 3 
 
 Anatase, 229 
 
 Andalusite, 297 
 
 Andesine, 335 
 
 Angle of aperture of objectives, 98 
 
 Anhydrite, 301 
 
 Anigmatite, 256 
 
 Anisotropic, 52 
 
 Anomite, 307 
 
 Anorthite, 330 
 
 Anorthoclase, 329 
 
 Anthophyllite, 289 
 
 Antigorite, 276, 312 
 
 Antiperthite, 326 
 
 Apatite, 236 
 
 Apertometer, Abbe, 25 
 
 Aperture, 6 
 
 Aperture and magnification, 25 
 
 Aphanitic, 183 
 
 Aplanatic, 3 
 
 Aplanatic and achromatic, test for, 
 24 
 
 Apochromatic, 5 
 
 Apophyllite, 250 
 
 Apparatus for separating rock pow- 
 der, 155 
 
 Apparent optic angle, 112 
 
 Aragonite, 299 
 
 Arfvedsonite, 293 
 
 Astrolite, 309 
 
 Astrophyllite, 269 
 
 Attractive crystals, 57 
 
 Augite, 282 
 
 Authigenetic, 228 
 
 Automorphic, 181 
 
 Awaruite, 206 
 
 Axinite, 295 
 
 Axiolites, 186 
 
 385 
 
386 
 
 INDEX 
 
 Babinet compensator, 91 
 
 Baculite, 44, 189 
 
 Baddeleyite, 255 
 
 Barite, 297 
 
 Barium, chemical test for, 166 
 
 Barium fluosilicate, 164 
 
 Barium oxalate, 166 
 
 Barkevikite, 293 
 
 Basaltic augite, 282 
 
 Basaltic hornblende, 287, 291 
 
 Bastite, 279, 314 
 
 Baveno law, 326 
 
 Becke's method, 33 
 
 Beckelite, 213 
 
 Belonite, 183 
 
 Bertrand lens, 98, 113 
 
 Bertrand plate, 72 
 
 Bertrandite, 317 
 
 Beryl, 243 
 
 Beryllium, chemical test for, 166 
 
 Biaxial crystals, 58 
 
 negative, 59 
 
 positive, 59 
 Biaxial interference figure, 97, 107, 
 
 et sequa 
 
 Biaxial zeolites, 341 
 Binocular microscope, 41 
 Biot, quartz plate, 73 
 
 rotating quartz, 92 
 Biotite, 302 
 Birefractometer, 91 
 Bisectrices, 59 
 Bituminous matter, 212 
 Blue spar, 298 
 Bohemian garnet, 216 
 Bone substance, 237 
 Boracite, 219 
 
 Boron, chemical test for, 169 
 Botryolite, 296 
 Bowlingite, 315 
 Brandisite, 271 
 Bravais double plate, 72 
 Breislakite, 256 
 Brewster's cross, 123 
 Brewster lens, 2 
 Brezina double plate, 71 
 Brilliant green, 175 
 Brittle mica group, 269 
 Bronzite, 279 
 
 Brookite, 254 
 Brown hornblende, 291 
 Brucite, 244 
 Bruecke lens, 3 
 Bytownite, 335 
 
 Caesium, chemical test for, 165 
 
 Caesium alum, 169 
 
 Calcination, 176 
 
 Calcite, 238 
 
 Calcium, chemical test for, 165 
 
 Calcium fluosilicate, 164 
 
 Caldron double plate, 71 
 
 Cammererite, 310 
 
 Canada balsam, 143 
 
 Cancrinite, 250 
 
 Carbon, chemical test for, 169 
 
 Carbonaceous matter, 210 
 
 distinction from graphite, 211 
 
 Carinthine, 290 
 
 Carlsbad law, 326 
 
 Carpholite, 299 
 
 Carrara, 239 
 
 Cassiterite, 230 
 
 Cataclase, 192 
 
 Cataphorite, 288, 293 
 
 Cedar oil, 143 
 
 Celestite, 299 
 
 Centering screws, 14 
 
 Centering the stage, 26 
 
 Cerium, chemical test for, 168 
 
 Cerium epidote, 261 
 
 Ceylon, 210 
 
 Chabazite, 250 
 
 Chalcedony, 248 
 
 Chalcopyiite, 205 
 
 Chamosite, 312 
 
 Character of the double refraction 
 in biaxial minerals, 115 
 in uniaxial minerals, 105 
 
 Character of the principal zone, 
 determination of, 93 
 
 Characteristic angles, measurement 
 of, 42 
 
 Characteristic colors, 49 
 
 Chatoyancy, 196 
 
 Chemical methods of investigation, 
 162 
 
 Chemical microscope, 130 
 
INDEX 
 
 387 
 
 Chemical reactions, aluminium fluo- 
 silicate, 164 
 
 barium fluosilicate, 164 
 
 calcium fluosilicate, 164 
 
 fluosilicic acid, 163 
 
 lithium fluosilicate, 164 
 
 magnesium fluosilicate, 164 
 
 potassium fluosilicate, 163 
 
 sodium fluosilicate, 163 
 
 strontium fluosilicate, 164 
 Chemical tests for aluminium, 168 
 
 ammonium, 165 
 
 barium, 166 
 
 beryllium, 166 
 
 boron, 169 
 
 caesium, 165 
 
 calcium, 165 
 
 carbon, 169 
 
 cerium, 168 
 
 chlorine, 171 
 
 chromium, 168 
 
 cobalt, 167 
 
 didymium, 168 
 
 erbium, 168 
 
 fluorine, 171 
 
 iron, 167 
 
 lanthanum, 168 
 
 lithium, 165 
 
 magnesium, 166 
 
 manganese, 168 
 
 molybdenum, 168 
 
 neodymium, 168 
 
 nickel, 167 
 
 niobium, 170 
 
 phosphorus, 171 
 
 potassium, 165 
 
 praseodymium, 168 
 
 rubidium, 165 
 
 silicon, 169 
 
 sodium, 164 
 
 strontium, 166 
 
 sulphur, 171 
 
 tantalum, 170 
 
 thorium, 168 
 
 tin, 170 
 
 titanium, 170 
 
 tungsten, 168 
 
 vanadium, 170 
 
 water, 172 
 
 Chemical tests for, yttrium, 168 
 
 zirconium, 170 
 Chiastolite, 297 
 
 Chlorine, chemical test for, 171 
 Chlorite group, 310 
 Chloritoid, 271 
 Chloropal, 319 
 
 Chlorous acid, separation by, 150 
 Chondrodite, 300 
 Chromatic aberration, 2 
 Chrome, diopside, 280 
 
 epidote, 261 
 
 mica, 309 
 
 ocher, 207, 309 
 
 spinel, 218 
 
 zoisite, 261 
 Chromite, 207 
 
 Chromium, chemical test for, 168 
 Chromoscope for interference colors, 
 
 84 
 
 Chrysoberyl, 257 
 Chrysotile, 275, 312 
 Cinnamon stone, 216 
 Circular polarization, 104 
 
 strength of, 104 
 Cleavage, 179, 190 
 
 determination of, 41 
 
 distinct, 44, 191 
 
 fibrous, 191 
 
 imperfect, 44, 191 
 
 perfect, 44, 191 
 Clinochlore, 310 
 Clinohumite, 300 
 Clinozoisite, 259 
 Clint onite, 271 
 Cobalt, chemical test for, 167 
 Coddington lens, 2 
 Color, 48 
 
 Colors of the axes, 61 
 Common, garnet, 215 
 
 hornblende, 287 
 
 spinel, 218 
 
 Comparator, quartz wedge, 81 
 Compensators, 88 
 
 use of, 93 
 Compound microscope, diagram of 
 
 rays through, 4 
 Concentric structure, 194 
 Conchite, 241 
 
388 
 
 INDEX 
 
 Condenser, 7, 19 
 Congo red, 173 
 Convergent light, 95 
 
 field of vision in, 96 
 
 methods of observation in, 97 
 Copiapite, 319 
 Cordierite, 319 
 Corundum, 232 
 Cossaite, 306 
 Cbssyrite, 294 
 Cotschubeyite, 310 
 Couseranite, 242 
 Cover-glass, 22 
 Crocidolite, 292 
 
 Cross sections of biaxial minerals, 
 252 
 
 of cubic minerals, 212 
 
 of uniaxial minerals, 225 
 Crossed dispersion, 112 
 Crossed nicols, 65 
 Crossite, 294 
 Cryptoperthite, 327 
 Crystal sandstone, 246 
 Crystallites, 44, 189 
 Crystallizing microscope, 130 
 Cummingtonite, 290 
 Cumulite, 44, 189 
 
 Cutting and polishing machine, 145 
 Cyanite, 267 
 
 Damourite, 306 
 
 Datolite, 296 
 
 Delessite, 312 
 
 Demantoid, 217 
 
 Desmine, 341 
 
 Determination of index of refraction 
 
 by raising the tube, 33 
 Development of rock constituents, 
 
 181 
 
 Diabase augite, 282 
 Diaclasite, 280 
 Diallage, 281 
 Diaspore, 267 
 Diatoms, 26 
 Dichroism, 62 
 Dichroscopic ocular, 61 
 Didymium, chemical test for, 168 
 Dilute colors, 49 
 Diphyre, 242 
 
 Dispersion, 60 
 
 colors, 79, 110 
 
 of the optic axes, 109 
 
 of ordinary light, 2 
 Disthene, 267 
 Dolomite, 238, 240 
 Dolomite ash, 241 
 Double refraction, 51 
 
 in calcite, 52 
 
 recognition of, 66 
 
 strength of, 73 
 
 of uniaxial crystals, 56 
 Drawing apparatus, Abbe, 134 
 
 Nachet, 135 
 Dry system, 6 
 Duelho, 317 
 
 Duke de Chaulnes' method, 46 
 Dumortierite, 295 
 Dysanalyte, 213 
 
 Edenite, 290 
 
 Eichstaedt apparatus, 152 
 
 Eighth undulation plate, 107 
 
 Elaeolite, 249 
 
 Elasticity, axes of, 58 
 
 direction of greatest, 57 
 direction of least, 57 
 direction of medium, 58 
 
 Electro-magnet, 160 
 
 Ellipsoid of rotation, 57 
 
 Emerald, 244 
 
 Emery, 232 
 
 Enstatite, 279 
 
 Eozoon, 276 
 
 Eozoon canadensis, 240 
 
 Epidosite, 264 
 
 Epidote, 261 
 
 Epidote fels, 264 
 
 Epidote group, 258 
 
 Epistilbite, 341 
 
 Erbium, chemical test for, 168 
 
 Erinite, 310 
 
 Eucolite, 242 
 
 Eudialyte, 242 
 
 Eugenol, 143 
 
 Eutaxite, 222 
 
 Eutectic mixture, 186 
 
 Exner refractometer, 38 
 
 Extinction, curve for diopside, 70 
 
INDEX 
 
 389 
 
 Extinction, directions, 67 
 
 oblique, 69 
 
 parallel, 69 
 
 position of, 67 
 
 symmetrical, 69 
 Extraordinary ray, 52 
 
 Fassaite, 281 
 Fayalite, 272 
 Fedorow, mica wedge, 90 
 
 universal stage, 127 
 Fedorowite, 285 
 Feldspar, alteration of, 323 
 
 cleavage of, 323 
 
 development of, 322 
 
 dimensions of, 322 
 
 group, 321 
 Felsite, 222 
 Ferrite, 208 
 Fibrolite, 295 
 
 Fluorine, chemical test for, 171 
 Fluorite, 225 
 Fluosilicic acid, chemical reactions 
 
 with, 163 
 Foraminifera, 187 
 Form, determination of, 41 
 Forsterite, 272, 276 
 Fouque method for determining the 
 
 plagioclase, 338 
 Fourlings, 120 
 Fraunhofer lens, 3 
 Fresnel's hypothesis, 54 
 Fritting, 220 
 Fuchsin, 175 
 Fuchsite, 309 
 Fuggerite, 234 
 
 Galena, 205 
 Garbenschiefer, 290 
 Garnet group, 214 
 Gas inclusions, 47 
 Gastaldite, 292 
 Gauss mirror, 43 
 Gedrite, 289 
 Gehlenite group, 233 
 Genthite, 316 
 Gieseckite, 250 
 Gigantolite, 320 
 
 Glass, 220 
 
 Glass inclusions, 48 
 
 Glauconite, 309 
 
 Glaucophane, 292 
 
 Globulite, 44, 189 
 
 Goethite, 254 
 
 Goniometer, reflection, 43 
 
 Granophyre, 248, 327 
 
 Granospherite, 187 
 
 Graphite, 209 
 
 Graphitic acid, 210 
 
 Graphitite, 209 
 
 Graphitoid, 209 
 
 Grapholite, 317 
 
 Green earth, 309 
 
 Green hornblende, 289 
 
 Green sand, 309 
 
 Greisen, 225 
 
 Grenough binocular microscope, 
 
 41 
 
 Grossularite, 217 
 Grothite, 256 
 Grunerite, 290 
 Giimbelite, 317 
 Gypsum, 321 
 
 Haidinger lens, 61 
 
 Hainite, 257 
 
 Half shadow polarizer, 73 
 
 Hardness, determination of, 179 
 
 Harmotome, 341 
 
 Haiiyne, 223 
 
 special test for, 174 
 Heating apparatus, 129 
 Heavy molten liquids, 159 
 Heavy organic liquids, 156 
 Heavy solutions, 157 
 Heavy spar, 297 
 Hedenbergite, 280 
 Helicoidal structure, 196 
 Hematite, 207 
 Hepar test, 171 
 Hercynite, 218 
 Hessonite, 217 
 Heulandite, 341 
 Hiddenite, 286 
 Horizontal dispersion, 111 
 Hourglass structure, 188 
 Humboldite, 233 
 
390 
 
 INDEX 
 
 Humite, 300 
 Humite group, 300 
 Hyalomelane, 221 
 Hyalosiderite, 273, 274 
 Hydatogenic activity, 283 
 Hydatopyrogenic, 194 
 Hydrargillite, 316 
 Hydrochloric acid, separation by, 
 
 150 
 Hydrofluoric acid, separation by, 
 
 149 
 Hydrofluosilicic acid, separation by, 
 
 150 
 
 Hydromagnesite, 319 
 Hydronephelite, 251 
 Hypersthene, 279 
 Hypidiomorphic development, 181 
 
 Iddingsite, 276, 315 
 Idiomorphic development, 181 
 Illuminating apparatus, 19 
 Ilmenite, 208 
 Ilvaite, 256 
 Immersion system, 6 
 Inclined dispersion, 111 
 Inclusions, 47, 194 
 
 carbon dioxide, 194 
 
 gas, 47 
 
 glass, 48 
 
 liquid, 48 
 
 Index of refraction, determination 
 by immersion method, 36 
 
 determination by the method of 
 Schroeder van der Kolk, 38 
 
 determination by using center 
 screen, 39 
 
 law of, 30 
 
 methods of determining, 30 
 Indol, 212 
 Interference colors, 74 
 
 abnormal, 78 
 
 measurement of the double 
 refraction by, 79 
 
 modification of, 77 
 
 order of, 76 
 
 subnormal, 78 
 
 supernormal, 78 
 
 with parallel nicols, 74 
 
 without using analyzer, 65 
 
 Interference figure, biaxial, 107, ct 
 sequa 
 
 of low double refracting min- 
 erals, 103 
 
 uniaxial, 100 
 
 Interior conical refraction, 83 
 Interstitial material, 182 
 Investigation, chemical methods of, 
 162 
 
 physical methods of, 177 
 lodocrase, 233 
 Iron cap, 321 
 Iron, 206 
 
 chemical test for, 167 
 Iron spinel, 218 
 Isotropic substances, 52 
 Itacolumite, 246 
 Ivaarite, 217 
 
 Jadeite, 286 
 Johnstrupite, 297 
 
 Kaolin, 318 
 Karsutite, 292 
 Kelyphite, 216 
 Klein's lens, 113 
 Klein's solution, 158 
 Knopite, 213 
 Knotenschiefer, 320 
 Kobell stauroscope, 71 
 Koppite, 213 
 Kornerupine, 269 
 Kreittonite, 218 
 Ktypeite, 241 
 Kuntzite, 286 
 
 Labradorite, 335 
 
 Lanthanum, chemical test for, 168 
 
 Lapis lazuli, 224 
 
 Lasaul x method, 97 
 
 Lattice lamination, 122, 185 
 
 Lattice structure, in microcline, 329 
 
 in olivine, 276 
 
 of serpentine, 313 
 Laumonite, 341 
 Lavenite, 257 
 Lawsonite, 286 
 Lazulite, 298 
 Lazurite, 224 
 
INDEX 
 
 391 
 
 Lead chloride, 159 
 
 Leeson's prism, 42 
 
 Left handed crystals, 104 
 
 Lens, 1 
 
 Lens stand, 161 
 
 Lepidomelane, 307 
 
 Leuchtenbergite, 243, 310 
 
 Leucite, 219 
 
 Leucite basalt, 219 
 
 Leucite syenite, 219 
 
 Leucite tephrite, 219 
 
 Leucitophyre, 219 
 
 Leucoxene, 208, 228 
 
 Lherzolite, 218 
 
 Liebenerite, 250 
 
 Lievrite, 256 
 
 Lime garnets, 217 
 
 Limonite, 254 
 
 Limurite, 295 
 
 Linck's solution, 175 
 
 Liquid inclusions, 48 
 
 .Listwanite, 317 
 
 Lithionite, 302 
 
 Lithium, chemical test for, 165 
 
 Lithium fluosilicate, 164 
 
 Lithium mica, 302 
 
 Lotrite, 299 
 
 Lussatite, 248 
 
 Lutezite, 248 
 
 Magmatic corrosion, 189 
 Magmatic resorption, 288 
 Magnesite, 241 
 
 Magnesium, chemical test for, 166 
 Magnesium ammonium phosphate, 
 
 167 
 
 Magnesium fluosilicate, 164 
 Magnesium mica, 302 
 Magnetic separation, 160 
 Magnetite, 206 
 Magnetites and, 206 
 Malachite green, 173 
 Malacolite, 281 
 Malacon, 231 
 Mallard law, 113 
 
 Manganese, chemical test for, 168 
 Margarite, 44, 189, 272 
 Margarodite, 306 
 Marialite, 242 
 
 Masonite, 271 
 
 Mastic, 146 
 
 Measurement of double refraction, 79 
 
 Mechanical stage, 18 
 
 Meionite, 242 
 
 Melanite, 217 
 
 Melilite, 233 
 
 Mercurous nitrate, 159 
 
 Meroxene, 307 
 
 Mesh structure in olivine, 275 
 
 Mesolite, 341 
 
 Mesostasis, 182 
 
 Metaxite, 313 
 
 Methods of investigation, 162 
 
 Methods of separatiod, 148 
 
 Methylene iodine, 156 
 
 Mica, alteration of, 308 
 
 first order, 303 
 
 group, 302 
 
 palme", 306 
 
 second order, 303 
 
 test plate, 90 
 
 wedge, 90 
 Microcline, 329 
 
 perthite, 329 
 
 Microfelsite, 222, 248, 327 
 Microlites, 183 
 Micrometer, object, 45 
 
 ocular, 45 
 
 Micropegmatite, 247, 327 
 Microperthite, 326 
 Microphotographic apparatus, 132 
 Microscope, electric heating, 131 
 Microscope goniometer, 125 
 Microtine, 334 
 Microtome, 21 
 Molybdenite, 206 
 
 Molybdenum, chemical test for, 168 
 Monazite, 257 
 Monticellite, 272, 276 
 Mortar structure, 191 
 Mortel structure, 191 
 Mosaic structure, 187 
 Mosandrite, 297 
 Muscovite, 302 
 Myrmecitic intergrowth, 248 
 Myrmecoidal, 327 
 
 Nachet drawing apparatus, 135 
 
392 
 
 INDEX 
 
 Natrolite, 341 
 Needle iron ore, 254 
 Negative crystals, 57, 105, 195 
 Neodymium, chemical test for, 168 
 Nepheline, 249 
 
 special test for, 174 
 Nephrite, 233, 286, 289 
 Nickel, chemical test for, 167 
 Nicol, 11 
 
 construction of, 10 
 Niobium, chemical test for, 170 
 Nontronite, 319 
 Noselite, 224 
 
 special test for, 174 
 Numeite, 316 
 Numerical aperture, 6 
 Nummulites, 187 
 Nutrition canals, 237 
 
 Object, clamp, 19 
 
 glasses, 23 
 
 marker, 19 
 
 micrometer, 45 
 Objective holders, 5 
 Obsidian, 220 
 Obtuse bisectrix, 59 
 Ocellary structure, 193 
 
 in leucite, 220 
 Ocular, diaphragm, 99 
 
 goniometer, 43 
 
 micrometer, 45 
 
 Planimeter, 46 
 
 Ramsden, 46 
 Oldhamite, 213 
 Oligoclase, 335 
 Olivine, 272 
 Olivine group, 272 
 Omphazite, 280 
 Onion structure, 221 
 Onkosine, 306 
 Onyx, 299 
 Oolite, 186 
 Opal, 224 
 
 Opal sandstone, 224 
 Opaque bodies, 49 
 Opazite, 207 
 Ophicalcite, 315 
 Optic angle, 59 
 
 apparent, 112 
 
 Optic angle, measurement of, 112 
 
 scale, 114 
 Optic axis, 55 
 Optic normal, 59 
 Optical anomalies, 123 
 Optical anomalies in garnet, 214 
 Optical sketch of a crystal, 138 
 Ordinary light, observations in, 30 
 Ordinary ray, 52 
 Organic acids, separation by, 150 
 Organic liquids, heavy, 156 
 Orthite, 261 
 Orthoclase, 324 
 Orthorhombic dispersion, 110 
 Orthoscope, Noerremberg, 9 
 Ottrelite, 270 
 Owenite, 312 
 
 Palimsest structure, 198 
 
 Paragonite, 306 
 
 Paragonite schist, 268 
 
 Parallel polarized light, observations 
 
 in, 51 
 
 Paramorph, 123, 198 
 Pargasite, 287, 290 
 Parting, 191 
 Pectolite, 300 
 Peg structure, 234 
 Pennine, 310 . 
 Periclase, 219 
 Pericline law, 333 
 Peridote, 272 
 Perimorph, 197 
 Peripheral development, 192 
 Periscopic ocular, 5 
 Perlite, 220 
 Perovskite, 213 
 Perthite, 326 
 Petrographic methods, summary of, 
 
 135 
 
 Phillipsite, 341; 
 Phlogopite, 302 
 Phosphorite, 237 
 
 Phosphorus, chemical test for, 171 
 Photographic camera, 133 
 Physical methods of investigation, 
 
 177 
 
 Picotite, 218 
 Picrolite, 313 
 
INDEX 
 
 393 
 
 Picrosmine, 313 
 Piemontite, 261 
 Pilite, 274, 289 
 Pimelite, 316 
 Pistazite, 261 
 Pitchstone, 220 
 Plagioclase, 330 
 Plane of the optic axes, 59 
 Plane of polarization, 7 
 Plane of vibration of polarized light, 8 
 Plane polarized light, 7 
 Planimeter ocular, 46 
 t Pleochroic halos, 64, 197 
 
 in hornblende, 288 
 Pleochroism, 61 
 
 artificial, 64 
 Pleonast, 218 
 
 Pleurosigma angulatum, 26 
 Poikilitic intergrowth, 192 
 Polariscope, Noerremberg, 9 
 Polarization, angle of, 9 
 
 by reflection, 9 
 
 by refraction, 9 
 Polarizer, 19, 51 
 Polarizing apparatus, 7 
 Polarizing microscope, Bausch & 
 Lomb, 18 
 
 Nachet, 14 
 
 polymeter, 17 
 
 Rei chert, 15 
 
 Seibert, 12 
 
 Voigt & Hochgesang, 16 
 Porfido rosso antico, 261 
 Positive crystals, 57 
 Potassium antimonate, 169 
 Potassium, chemical test for, 165 
 Potassium ferrocyanide, 167 
 Potassium fluoborate, 169 
 Potassium fluosilicate, 163 
 Potassium mercuric iodide, 37 
 Potassium mica, 302 
 Potassium platinochloride, 165 
 Potassium sulphocyanide, 167 
 Potstone, 316 
 Priigratite, 306 
 Praseodymium, chemical test for, 
 
 168 
 
 Precious garnet, 215 
 Precious serpentine, 315 
 
 Precious spinel, 218 
 Precipitates on thin section, 175 
 Predazzite, 219, 244 
 Prehnite, 299 
 Principal rays, 180 
 Principal section, 53 
 Principal zone, 63 
 Prismatine, 269 
 Projection apparatus, 133 
 Protobastite, 280 
 Protoclase, 191, 324 
 Prussian blue, 167 
 Pseudobrookite, 255 
 Pseudochalcedony, 249 
 Pseudochroism, 49 
 Pseudodichroism, 65 
 Pseudomorphs, 198 
 Pumice, 221 
 Pycnite, 301 
 Pyrenaite, 217 
 Pyrite, 204 
 Pyrochlore, 213 
 Pyrope, 216 
 Pyrophyllite, 317 
 Pyroxene group, 277 
 Pyroxenes, monoclinic, 280 
 
 orthorhombic, 279 
 Pyrrhite, 213 
 Pyrrhotite, 205 
 
 Quarter undulation plate, 90 
 Quartz, 244 
 
 au'gen, 245 
 
 vermicule, 248 
 
 wedge, 90 
 Quartzine, 248 
 
 Radde color scale, 49, 63 
 Radio active minerals, 64 
 Radiotine, 313 
 Ramsden ocular, 46, 99 
 Ranite, 251 
 Raven mica, 308 
 Real image, 1 
 Red I. test plate, 90 
 Red II. test plate, 90 
 Reflected light, observations in, 49 
 Reflection in the tube of the micro- 
 scope, 26 
 
394 
 
 INDEX 
 
 Refraction, 51 
 
 Refractometer, Exner, 38 
 
 Relation of optical character to 
 
 character of the principal 
 
 zone, 94 
 
 Repellant crystals, 57 
 Resolving power of objectives, 6 
 Rhaetizite, 267 
 Rhodochrome, 310 
 Rhomb ohedral carbonates, 238 
 Rhomb porphyry, 322 
 Riebeckite, 293 
 Right handed crystals, 104 
 Rinkite, 295 
 Ripidolite, 312 
 
 Rock constituents, size of, 183 
 Rock grinding machine, 21 
 Rock powder, 143 
 Rock salt, 219 
 Rohrbach solution, 159 
 Rosenbuschite, 300 
 Rotation apparatus, 125 
 
 C. Klein, 128 
 Rubellane, 307 
 
 Rubidium, chemical test for, 165 
 Ruby, 232 
 Ruby mica, 254 
 Ruin development, 188 
 Rutile, 227 
 
 Sagenite, 228, 308 
 
 Salite, 281 
 
 Sanidine, 324, 328 
 
 Saponlac, 146 
 
 Sapphire, 232 
 
 Sapphirine, 268 
 
 Saussurite, 263, 336 
 
 Saussuritization, 336 
 
 Scale for specific gravity, 154 
 
 Scapolite group, 242 
 
 Schaffgotsch, Count, 157 
 
 Schorl, 234 
 
 Schorlomite, 217 
 
 Schroeder van der Kolk method for 
 
 determining the plagio- 
 
 clase, 339 
 Schungite, 211 
 Schwarzmann's optic angle scale, 
 
 114 
 
 Scolesite, 341 
 
 Sedgy hornblende, 290 
 
 Seladonite, 284, 309 
 
 Sesnitive red tint, 76 
 
 Sensitive tint plate, 72 
 
 Separating funnel, 156 
 
 Separation according to specific 
 
 gravity, 153 
 
 Separation, chemical methods of, 
 148 
 
 magnetic, 160 
 
 physical methods of, 152 
 Separation of rock constituents by 
 alum, 151 
 
 chlorous acids, 150 
 
 hydrochloric acid, 150 
 
 hydroflouric acid, 149 
 
 hydrofluosilicic acid, 150 
 
 organic acid, 150 
 
 sodium hydroxide, 151 
 
 sulphuric acid, 151 
 Serendibite, 269 
 Sericite, 305 
 Sericite schist, 306 
 Serpentine, 312 
 Seyberite, 271 
 Siderite, 241 
 Sieve structure, 197 
 Silicon, chemical test for, 169 
 Silicon tetrafluoride, 169 
 Sillimanite, 295 
 Silver nitrate, 159 
 Sismondine, 271 
 Size, measurement of, 45 
 Skarn, 217, 280 
 Skatol, 212 
 Skeletal crystals, 44 
 Slag inclusions, 195 
 Smaragdite, 290 
 Soapstone, 316 
 Soda hornblende, 288 
 Soda-lime feldspar, 330 
 Sodalite, 223 
 Sodalite group, 223 
 Sodalite, special test for, 174 
 Soda mica, 306 
 Soda orthoclase, 329 
 Sodium, chemical test for, 164 
 Sodium fluosilicate, 163 
 
INDEX 
 
 395 
 
 Sodium hydroxide, separation by, 
 
 151 
 
 Sodium saltpeter nicols, 11 
 Sodium trithiocarbonate, 167 
 Sodium uranylacetate, 164 
 Solenhofener schist, 238 
 Sonnenstein, 208 
 
 Source of light for microscopic ob- 
 servations, 7 
 Special reactions, 172 
 Special tests for Hauyne, 174 
 
 Nepheline, 174 
 
 Noselite, 174 
 
 Scapolite, 174 
 
 Sodalite, 174 
 
 Specific gravity scale, 154 
 Spectro-polarizer, Abbe, 7 
 Spessartite, 216 
 Sphalerite, 213 
 Sphene, 256 
 Spherical aberration, 2 
 Spherosiderite, 241 
 Spherulites, 186 
 Spinel group, 218 
 Spodumene, 286 
 Spreustein, 223, 250, 251 
 Staining methods, 173 
 Stanhope lens, 2 
 Staurolite, 266 
 Stauroscopes, 71 
 Steinheil triplet, 3 
 Stereoscopic microscope, 41 
 Stereoscopic ocular, 41 
 Stilbite group, 341 
 Stink quartz, 246 
 Strontium, chemical test for, 166 
 Strontium fluosilicate, 164 
 Structure, concentric, 194 
 
 dodecahedral in garnet, 215 
 
 helicoidal, 196 
 
 hourglass, 188 
 
 mosaic, 187 
 
 onion-like, 221 
 
 palimsest, 198 
 
 peg in melilite, 234 
 
 sieb, 197 
 
 sieve, 197 
 
 Stubachite, 276, 313 
 Subnormal interference colors, 78 
 
 Sulphur, 255 
 
 chemical test for, 171 
 Sulphuric acid, separation by, 151 
 Summary of petrographic methods, 
 
 135 
 
 Sunstone, 208, 328 
 Supernormal interference colors, 78 
 Surface color, 60 
 Surirella gemma, 26 
 
 Table of interference colors, 75 
 
 Tachylite, 221 
 
 Talc, 316 
 
 Tantalum, chemical test for, 170 
 
 Tetrabrom acetylene, 156 
 
 Thallium mercurous nitrate, 159 
 
 Thallium silver nitrate, 159 
 
 Thickness, measurement of, 45 
 
 Thin sections, preparation of, 144 
 
 Thomsonite, 341 
 
 Thorium, chemical test for, 168 
 
 Thoulet solution, 157 
 
 Thulite, 261 
 
 Thuringite, 312 
 
 Tin, chemical test for, 170 
 
 Titanite, 255 
 
 Titanium, chemical test for, 170 
 
 Titanium magnetite, 206 
 
 Titanium olivine, 276 
 
 Tomlinson, W. H., 144 
 
 Topaz, 301 
 
 Topazolite, 217 
 
 Total reflection, 31 
 
 critical angle of , 31 
 Total refractometer, Abbe, 39 
 Total reflectometer, Wallerant, 40 
 Tourmaline, 234 
 
 hemimorphic development of, 
 235 
 
 suns, 235 
 
 tongs, 10 
 Tremolite, 289 
 Trichite, 44, 189 
 Trichroic, 62 
 Tridymite, 249 
 Trillings, 121 
 
 Tungsten, chemical test for, 168 
 Turner's test, 169 
 Twins, 120 
 
396 
 
 INDEX 
 
 Twins, cruciform, 184 
 
 juxtaposition, 184 
 
 penetration, 184 
 
 uniaxial with inclined axes, 120 
 Twin, compensator, 92 
 
 lamination, 122 
 
 polarizer, 73 
 Twinning, 184 
 
 lamination in leucite, 220 
 
 repeated, 185 
 
 Ultramicroscopy, 7 
 
 Ultraviolet rays, 7 
 
 Undulatory extinction, 191 
 
 Uniaxial crystals, 55 
 
 Uniaxial interference figure, 100 
 
 Uniaxial interference figure with 
 
 parallel nicols, 100 
 Uniaxial minerals in convergent 
 
 light, 99 
 Uralite, 283 
 Uralite gabbro, 283 
 Uralite porphyrite, 283 
 Uralitization, 283 
 Uwarowite, 217 
 
 Vanadium, chemical test for, 170 
 
 Velocity of light, 8 
 
 Verant lens, 3 
 
 Vertical illuminator, 49 
 
 Vesuvianite, 233 
 
 Vibration directions, determination 
 
 of position of, 67 
 Vibration of polarized light, 8 
 Villarsite, 312 
 Violaite, 285 
 
 Violet I, test plate, 72 
 Viridite, 311 
 Virtual image, 1 
 
 Wache, 283 
 
 Wagnerite, 318 
 
 Wallerant total reflectometer, 40 
 
 Water, test for, 172 
 
 Wavellite, 320 
 
 Wave surface of uniaxial crystals, 
 
 56 
 
 Wavy, extinction 191 
 Westphal balance, 177 
 White of the higher order, 77 
 Woehlerite, 257 
 Wollastonite, 299 
 Wollastonite group, 299 
 Wright wedge, 93 
 Wurtzite, 230 
 
 Xenimorphic, 181 
 Xenotime, 231 
 
 Yttrium, chemical test for, 168 
 
 Zeolites, biaxial, 341 
 
 Zinc chloride, 159 
 
 Zinc spinel, 218 
 
 Zircon, 230 
 
 Zircon syenite, 230 
 
 Zirconium, chemical test for, 170 
 
 Zirkelite, 213 
 
 Zoisitea, 260 
 
 Zoisite /?, 260 
 
 Zonal development, 188 
 
 Zonal structure in plagioclase, 334 
 
I &C\ f u- C/ a 
 

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 UNIVERSITY OF CALIFORNIA LIBRARY