THE 
 
 DETERMINATION 
 
 OF 
 
 ROCK-FORMING MINERALS. 
 
 BY 
 
 DR. EUGEN HUSSAK, 
 
 PRIVAT-DOCENT IN THE UNIVERSITY OF GRAZ. 
 
 WITH ONE HUNDRED AND THREE WOODCUTS. 
 
 AUTHORIZED TRANSLATION FROM THE FIRST GERMAN EDITION 
 
 BY 
 
 ERASTUS G. SMITH, PH.D., 
 
 PROFESSOR OF CHEMISTRY AND MINERALOGY, BELOIT COLLEGE, 
 BBLOIT, WISCONSIN. 
 
 SECOND EDITI&& ' ^ V"; V ; ; ; 
 
 '' |T ">,> 1 ""n t ^ ^ 3 
 
 NEW YORK : 
 JOHN WILEY & SONS. 
 
 1891. 
 
 J* X . 
 
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 Copyright, 1883, by 
 JOHN WILEY & SONS. 
 
PREFACE TO THE FIRST ENGLISH 
 TRANSLATION. 
 
 THE following authorized translation of Dr. Hussak's work 
 was undertaken with the view of supplying a want felt in our 
 colleges and universities. Though great progress has been 
 made in the sciences of mineralogy and lithology in later 
 years, through the study of the optical and the other physical 
 properties of minerals, few attempts have been made to con- 
 dense the exhaustive and original articles scattered through 
 the scientific periodicals and State and national publications, 
 and to put them in suitable shape for use in the laboratory 
 and the class-room. It has been the aim, therefore, to place 
 before' the American student a practical work which shall de- 
 scribe the methods and exhibit the results of such investiga- 
 tions. 
 
 The translation of a technical work of this character is beset 
 with difficulties appreciable only by those who have under- 
 taken a similar task. Few liberties have been taken with the 
 text, and the attempt has been made to reproduce the origi- 
 nal literally so far as possible ; at points, however, in order to 
 convey clearly the author's meaning, some recasting of sen- 
 tences was unavoidable. 
 
iv PREFACE TO THE FIRST ENGLISH TRANSLATION. 
 
 The translator will consider it a great favor if any errors 
 noticed either in statement or in translation are communi- 
 cated to him, in order that they may be eliminated in future 
 editions. 
 
 I would express my thanks to Dr. George Williams of Balti- 
 more for several such corrections to the original text already 
 received and incorporated in the translation. 
 
 ERASTUS G. SMITH. 
 
 BELOIT COLLEGE, BELOIT, WISCONSIN, October, 1885. 
 
PREFACE. 
 
 As the following Manual is designed especially for the use 
 of students, the cost of the work demanded as much abridg- 
 ment as possible. For this reason much of the knowledge of 
 minerals which belongs to mineralogy proper is passed over, and 
 in the bibliography of Part II. only those works are cited which 
 contain detailed communications concerning the microscopical 
 properties of the rock-forming minerals. 
 
 I must, at this time, express my gratitude to Professor Dr. 
 F. Zirkel for his many friendly suggestions ; nor am I under 
 less' obligations to Professor F. Fouqu, who has most cour- 
 teously allowed the reproduction of a large number of figures 
 from his well-known work, " Mineralogie Micrographique." 
 
 EUGEN HUSSAK. 
 
 GRAZ, November, 1884. 
 
TABLE OF CONTENTS. 
 
 PART I. 
 METHODS OF INVESTIGATION. 
 
 PAGE 
 
 Preparation of Microscopical Sections c 3 
 
 The Microscope provided with Polarization Apparatus suitable for Mine- 
 
 ralogical and Petrographical Investigation 7 
 
 A. Optical Methods of Investigation 16 
 
 1. Examination of Mineral Cross-sections in Parallel Polarized 
 
 Light 16 
 
 I. Single refracting Minerals 17 
 
 II. Double-refracting Minerals 17 
 
 2. Examination of Minerals in Convergent Polarized Light 29 
 
 3. Behavior of Twinned Crystals in Polarized Light 37 
 
 Twins of the Regular System 37 
 
 Twins of the Tetragonal and Hexagonal Systems 38 
 
 Twins of the Rhombic System 39 
 
 Twins of the Monoclinic System 40 
 
 Twins of the Triclinic System 43 
 
 4. Determination of the Index of Refraction 44 
 
 5. Pleochroism of Double-refracting Minerals 45 
 
 Determination of the Axial Colors 45 
 
 B. Chemical Methods of Investigation 50 
 
 Microchemical Methods 51 
 
 a. Boficky's Microchemical Method 55 
 
 b. Behrens's Microchemical Method 59 
 
 C. Mechanical Separation of the Rock-forming Minerals 66 
 
 I. Separation with the Solution of the Iodides of Potassium and 
 
 Mercury 67 
 
 II. Klein's Solution 73 
 
 III. Rohrbach's Solution of the Iodides of Barium and Mercury.. . . 75 
 
 IV. Methods of Separation based on the Different Action of Acids 
 
 on Minerals 76 
 
 V. Separation of the Rock-constituents by means of the Electro 
 
 magnet 79 
 
viii TABLE OF CONTENTS, 
 
 PAGE 
 
 D. Explanations of the Tables relating to the Morphological Properties of 
 
 the Rock forming Minerals.. 81 
 
 I. Mode of Occurrence of the Rock-constituents 81 
 
 II. St ructure of the Rock-forming Minerals 87 
 
 Shell formed Structure of Crystals 90 
 
 Interpenetration of the Rock-constituents 93 
 
 III. Inclosures of the Rock-forming Minerals 93 
 
 Gas pores. 94 
 
 Fluid Inclosures 95 
 
 Inclosures of Vitreous Particles 97 
 
 Inclosures of Foreign Minerals 99 
 
 IV. Decomposition of the Rock constituents 101 
 
 PART II. 
 TABLES FOR DETERMINING MINERALS. 
 
 Table for Determining the System of Crystallization of the Rock forming 
 
 Minerals , 106 
 
 A. Even in the thinnest Sections of Opaque Minerals 108 
 
 B. Minerals Transparent in Thin Sections 112 
 
 .1. Single refracting Minerals 112 
 
 a. Amorphous Minerals 112 
 
 b. Minerals Crystallizing in the Regular System 114 
 
 II. Double-refracting Minerals 122 
 
 a. Optically Uniaxial Minerals 122 
 
 1. Minerals Crystallizing in the Tetragonal System 122 
 
 a. Double-refraction Positive.. 122 
 
 ft. Double-refraction Negative 124 
 
 2. Minerals Crystallizing in the Hexagonal System 128 
 
 a. Double-refraction Positive 128 
 
 ft. Double-refraction Negative 132 
 
 b. Optically Biaxial Minerals 140 
 
 1. Minerals Crystallizing in the Rhombic System 140 
 
 2. Minerals Crystalling in the Monoclinic System 156 
 
 3. Minerals Crystallizing in the Triclinic System 178 
 
 C. Aggregates 189 
 
 Bibliography 197 
 
 Explanation of Cuts accompanying Part II 215 
 
 Cuts accompanying Part II 219 
 
 Index . 22Q 
 
ON THE DETERMINATION OF ROCK- 
 FORMING MINERALS. 
 
 PART I. 
 
 METHODS OF INVESTIGATION. 
 
 F. ZIRKEL. Die mikroskopische Beschaffenheit der Mineralien und Gesteine. 
 
 Leipzig, 1873. 
 H. ROSENBUSCH. Mikroskopische Physiographic der petrographisch wichtigsten 
 
 Mineralien. Stuttgart, 1873. 
 
 FOUQUB ET MICHEL LEVY. Mineralogie micrographique. Paris, 1878. 
 E. COHEN. Zusammenstellung petrographischer Untersuchungsmethoden.a' 
 
 Strassburg, 1884. 
 
 THERE are two methods of examining rocks, the macro- 
 scopical and the microscopical. 
 
 In the macroscopical investigation of rocks those parts of 
 the mineral mixture discernible with the naked eye can be 
 studied with reference to crystalline form, cleavage, color, 
 lustre, streak, hardness, solubility in acids, etc. For the more 
 exact optical investigation, however, cleavage sections exactly 
 oriented must be obtained, the cleavage angle, when possible, 
 measured in order to determine the plane of cleavage, and the 
 
2 tl ^ DETERMINATION OF ROCK-FORMING MINERALS. 
 
 section, if not already transparent, ground thin. Such an 
 investigation of the rock-forming minerals leads in most cases 
 to the goal, provided the particles have a certain magnitude, 
 at least 1-2 mm. Isolated particles of minerals can be 
 examined before the blow-pipe ; yet, because of their minute- 
 ness, such a purely macroscopical examination is insufficient 
 in most cases. This is especially true in porphyritic or very 
 fine-grained rocks, and therefore for these rocks the micro- 
 scopical examination is employed. It is necessary in such a 
 case that the pieces of rock under examination shall be ground 
 into thin transparent leaves. In such sections the single con- 
 stituents are cut in most varied directions. By these minute 
 cross-sections the crystals and the rock-forming minerals can 
 be determined by optical methods with the polarization-micro- 
 scope, and by combination of the optical with the crystallo- 
 graphical properties, i.e., with the form of the cross-section, 
 i.e., crystalline form, and cleavage. 
 
 This determination is more difficult if the minerals occur 
 only as grains. Of course here also the human eye has its 
 limitations: if the separate particles are so minute that they 
 cannot be observed in section, i.e., afford no cross-sections ; 
 or, when examined under the highest possible magnifying 
 power, they give no figures suitable i.e., large enough for 
 optical study, their determination by the polarization-micro- 
 scope is impossible. 
 
 In the following pages is given a description of the method 
 of producing preparations from rocks suitable for microscopi- 
 cal study ; of the application of the polarization-microscope 
 adapted to the complete exposition of the optical and chemical 
 methods of determination ; then follows the discussion of the 
 mechanical separation of the rock-constituents according to 
 their specific gravity and by the electro-magnet; and, finally, 
 a short chapter on the structure of the rock-forming minerals 
 and a systematic survey of them. 
 
METHODS OF INVESTIGATION. 
 
 The Preparation of Microscopical Sections. 
 
 In order to prepare a thin section from a rock, either a 
 suitable tablet is cut with a section-cutter from the rock, or a 
 convenient fragment, about 2 ccm. large, is broken off with a 
 hammer, and as even a face as possible is ground, using either 
 an emery-disk of a section-grinder or grinding by hand on an 
 iron plate with coarse emery-powder and water. The size of the 
 emery used depends entirely upon the hardness of the rock. 
 Evenness of the emery-powder, and an iron plate as smooth as 
 possible and free from furrows, are chief factors in obtaining 
 an even surface on the ground fragment. 
 
 If the face is sufficiently even, it is polished on a glass plate 
 with fine floated emery, or emery-flour, and water. The frag- 
 ment is then cemented by this face with boiled Canada balsam 
 to an ordinary glass plate (preferably one that is quadratic) 
 somewhat larger than the fragment, and rather thick, so that 
 it can be better grasped. 
 
 Certain precautionary measures must be observed. The 
 fragment must be first well cleaned and dried, the Canada bal- 
 sam sufficiently heated, boiled neither too much nor too little, 
 so that the emery-powder may not become distributed through 
 it or the balsam crack off from the glass. The balsam may be 
 boiled over an alcohol-lamp, either in an iron spoon or directly 
 on the object-glass. Care must be taken that the balsam does 
 not inflame. It is impossible to state the exact instant when 
 the balsam is sufficiently boiled, as this depends on its state of 
 dilution and must be determined by several experiments. The 
 balsam is sufficiently boiled if, after it has already begun to 
 fume rather strongly, large bubbles rise from the bottom, or 
 the balsam begins to evaporate from the edges of the glass 
 plate. The boiling of the balsam is conducted most safely in 
 
4 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 an oven with thermometer attachment, such as are sold by 
 Fuess of Berlin. If the balsam has been boiled in an iron 
 spoon, a small portion is placed on an object-glass, and this 
 gently warmed until the balsam becomes a thin liquid. 
 
 The evenly-ground rock-section is firmly pressed into the 
 boiled, still fluid, balsam, with the plane surface downward. In 
 the operation care must be taken that no bubbles of air remain 
 between the rock and glass, as often happens when the surface 
 ground on the rock is not perfectly even. The plate thus pre- 
 pared is allowed to thoroughly cool. If the balsam on the plate 
 about the rock-tablet receives no impression, or appears free 
 from fissures, it is sufficiently boiled. 
 
 The natural surface of the rock-fragment is next ground 
 with coarse emery-powder. This is continued until the 
 larger mineral particles or even the plate itself begins to be 
 translucent, i.e., until the thickness is about -J I mm. Here, 
 again, care must be exercised that the surface is as even as pos- 
 sible, and that the Canada balsam surrounding and protecting 
 the plate is not completely cut away. The grinding, as before, 
 is continued on glass plates with fine emery, and finally with 
 emery flour, until the tablet of rock becomes perfectly trans- 
 parent. It is then cleaned from the emery, the surrounding 
 Canada balsam is carefully scratched away, and is then dried. 
 For the final preparation a better object-glass is selected, one 
 well polished and freed from dust-particles or clinging threads, 
 dried, and a larger drop of Canada balsam placed upon it. The 
 balsam may be boiled directly on the object-glass or in a spoon, 
 and then transferred as in the previous case. 
 
 The thin rock-tablet, to which another small drop of balsam 
 has been added, is made movable by carefully and gently 
 heating the object-glass, and with a pointed bit of wood is 
 pushed over on to this second glass, which in turn is gently 
 warmed so that the balsam again becomes mobile and sur- 
 rounds the rock-section on every side ; the covering-glass, of 
 
METHODS OF INVESTIGATION. 5 
 
 course previously cleaned and warmed, is laid upon and care- 
 fully pressed down upon the rock-section so that the excess 
 of balsam and the air-bubbles escape. The preparation is 
 allowed to cool slowly until the balsam has solidified, and is 
 then cleaned by carefully scratching away the excess of balsam 
 with a knife and washing with alcohol. 
 
 As by scratching away the balsam the covering glass is 
 often liable to break away, owing to the overheating of the 
 balsam, it is advisable to shave away the balsam \\ith a 
 warmed knife, and then wash the preparation with alcohol. 
 
 Many rocks, especially those of a coarse granular structure, 
 exceedingly porous or decomposed, cannot thus be transferred, 
 and are shattered in the preparation. Sections from such 
 rocks therefore must be placed on a better object-glass at 
 once, and, after they are ground thin, must be finished on this 
 same glass by pouring boiling balsam on the dried and cleaned 
 section, and the rapid laying on and gentle pressing down of 
 the covering-glass. Here care must be taken neither to warm 
 the object-glass a second time, nor to press down the covering- 
 glass too firmly, as in either case the section is often broken ; 
 it is therefore necessary to finish the preparation as rapidly as 
 possible in order that the balsam upon the glass may not cool 
 and thus necessitate a second warming. 
 
 Such rocks as pumice-stone which are exceedingly porous 
 or full of cavities, or of a drossy character, or friable and 
 fragjle, as tufa, must be boiled in Canada balsam first, to make 
 possible the grinding of a plane surface, as the balsam forcing 
 its way into the cavities, and becoming solid on cooling, 
 imparts to the whole a greater degree of consistency. Such 
 thin sections must of course be finished according to the 
 method last described, upon the same object-glass on which 
 it was ground. 
 
 Sections easily shattered may be prepared most safely by 
 Canada balsam dissolved in ether or chloroform. The prepara- 
 
6 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 tion must not be heated, and must be allowed to dry very 
 slowly. It is advisable to use rather more balsam than is 
 ordinarily taken, as in the process of drying, i.e., the evapora- 
 tion of the ether, air-bubbles may enter the balsam beneath the 
 covering-glass. It is also advisable to avoid spotting the 
 covering-glass with balsam, as cleaning the preparation cannot 
 be undertaken for several weeks, until after the balsam is com- 
 pletely dried. 
 
 Thoulet has described a method of cutting isolated mineral 
 particles, sand, etc. 
 
 The powder to be examined is mixed with about ten times 
 its volume of zinc oxide, and the mixture is rubbed up to a 
 thick mud with potassium silicate (soluble glass). This is then 
 pressed into a mould conveniently made from. a short piece of 
 thick glass tubing, placed on an object-glass, and allowed to 
 stand several days and harden. When thoroughly dried, the 
 mass is easily slipped from the glass, is solid, and can be worked 
 into a thin section exactly as any rock fragment. 
 
 In order to grind friable rocks, or those become rotten 
 through advanced decomposition, according to A. Wichmann 
 (Tschermak's Min. u. petr. Mitth. V, 1882, 33) the best course 
 is the following : The fragment broken away is first shaved on 
 one side as even as possible with a khife, and this is polished 
 on a dry glass plate ; the fragment is then cemented to the 
 plate with Canada balsam, previously cooled so that the rock 
 may not be further changed by its high temperature, .and 
 again shaved on the opposite side until as thin a section as 
 possible remains, which is finally prepared with Canada balsam 
 dissolved in ether. 
 
 The Material for the Preparations. 
 
 The emery should be as pure as possible, i.e., unadulterated and rich in 
 corundum, the size of the coarser granules about 0.3-0.5 mm.; the fine emery 
 should be like flour. The coarser variety is known as " No. 70," the fine variety 
 as "emery flour." 
 
METHODS OF INVESTIGATION. J 
 
 The Canada balsam should be clear and rather liquid. 
 
 The object-glasses are not generally more than 18 mm. square. 
 
 Labels for microscopical preparations are to be had in book-form. 
 
 Thin rock-sections are prepared by Fuess, Berlin, S. W.. Alte Jakobstrasse, 
 108, and by Voigt & Hochgesang, Gottingen; large collections, also, of thin 
 sections, systematically arranged, can be obtained from the same firms. Both 
 houses supply excellent microscopes especially adapted to mineralogico- 
 petrographical investigations. 
 
 The Microscope provided with Polarization Apparatus 
 suitable for Mineralogical and Petrographical In- 
 vestigation. (Also often called the " polarization-micro- 
 scope.") 
 
 TH. LIEBISCH. Bericht uber die wissenschaftlichen Instrumente auf der Ber- 
 liner Gewerbeausstellung. Berlin, 1879. P- 34 2 - 
 
 H. ROSENBUSCH. N. Jahrb. f. Miner, u. Geol. 1876. p 504. 
 
 Ueber die Anvvendung der Condensorlinse bei Untersuchungen im con- 
 
 vergentpolarisirten Lichte: 
 
 v. LASAULX. N. Jahrb. f. Miner, u. Geol. 1878, p. 377. 
 
 E. BEUTRAND. Societe mineralogique de France. 1878, 9 Mai p. 22 and 
 14 Nov. p. 96. 
 
 C. KLEIN. Nachr. d. k. Ges. d. Wissensch. z. Gottingen. 1878, p. 461 
 Ueber stauroskopische Methoden: 
 
 H. LASPEYRES. Groth's Zeitschr. f. Krystallographie, VI. Bd. p. 429. 
 
 L. CALDERON. Groth's Zeitschr. f. Krystallographie, II. Bd. p. 68. 
 
 The completely equipped polarization-microscope (Figs. 
 I and 2) differs from the ordinary microscope by (i) the 
 presence of a graduated object-stage revolving horizontally 
 (Fig. I, c), with vernier attachment suitable for the determina- 
 tion of the directions of extinction, measurement of angles, 
 etc.; (2) two Nicol's prisms (Fig. 2, ss and rr) for investiga- 
 tions in parallel polarized light ; (3) a condenser (Fig. 2, TT) 
 for investigations in converging polarized light ; (4) a plate of 
 quartz (Fig. 2, ZZ} for determining feebly double-refracting 
 minerals, which is cut perpendicular to the chief axis, has 
 parallel planes, and can be introduced over the objective by 
 
8 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 a slit (Fig. 2, //) ; (5) a calcite plate for stauroscopic investi- 
 gations cut perpendicular to the chief axis, with parallel planes 
 that is, a Calderon's double-plate (Fig. 2, c] or a Brezina's 
 calcite plate set in an ocular; (6) a fourth undulation mica 
 plate and a Dove's quartz compensation-plate, i.e., a thin 
 wedge of quartz for the determination of the character of the 
 double -refraction, which either enters or is just below the 
 analyzer ; and finally (7) an apparatus for centring the ob- 
 ject-stage (Fig. I, m and ;/ ; Fig. 2, N, nn, mm), and various 
 minor pieces of apparatus, as the cross-threads in the ocular, 
 an ocular- and stage-micrometer, blende (Fig. 2, dd) for in- 
 vestigations in converging polarized light, the graduation of 
 the head of the micrometer-screw and of the plate on the stage. 
 
 For mineralogico-optical investigations one Nicol's prism, 
 the polarizer (Fig. 2, rr\ is fixedly adjusted beneath the 
 stage and above the reflector ; and the second, the analyzer 
 (Fig. 2, ss), is graduated and is above the ocular. For in- 
 vestigations in parallel polarized light it is very convenient if 
 the polarizer is fixed in such a position that the directions of 
 vibration of both nicols are at right angles to each other, 
 i.e., the nicols are crossed when the zero-point of the analyzer 
 coincides with a mark on the tube, and at the same time the 
 ocular with its cross-threads so adjusted in the tube that the 
 arms of the cross-threads are exactly parallel with the direc- 
 tions of vibration of both nicols. 
 
 If this is not the case, the nicols must always first be crossed 
 by turning the analyzer until complete darkness occurs and 
 this position of the analyzer is noted. Moreover, the arms of 
 the cross-threads must be parallel to the nicol chief sections. 
 This may be done in the following manner: We place on the 
 stage of the microscope, and between the crossed nicols, an 
 object-slide to which is firmly cemented either a small quartz- 
 crystal or a rock-section containing a longitudinal section of an 
 apatite crystal, and turn the stage until the quartz or the 
 
METHODS OF INVESTIGATION. 
 
 U 
 
 FIG. i. POLARIZATION-MICROSCOPE, BY R. FUESS. (New model.) 
 
IO DETERMINATION OF ROCK-FORMING MINERALS. 
 
 apatite crystal is completely darkened. The analyzer is now 
 removed from the ocular, and the ocular is revolved until one 
 arm of the cross-threads within the ocular is exactly parallel to 
 the prismatic edge of the quartz crystal or the longitudinal 
 edge of the apatite needle. In order to determine the direc- 
 tions of extinction in minerals, care must be taken that the 
 ocular carrying the cross threads, when correctly placed in the 
 manner described, is not displaced, as can easily occur in re- 
 moving the analyzer. 
 
 The Condenser (the Lasaulx-Bertrand lens) for producing 
 converging polarized light in the microscope is formed from 
 two plano-convex lenses. One of these is screwed directly 
 above the polarizer, and the second, in a suitable setting, laid 
 upon the first (Fig. 2, TT). In investigations in convergent 
 light, the ocular is removed and the nicols crossed. Objec- 
 tive 7 and ocular 3, Hartnack, is the best combination, though 
 a more acute objective system can often be advantageously 
 employed. In examining very diminutive crystalline cross- 
 sections, a blende (Fig. 2, dd) is placed above the analyzer for 
 the purpose of isolating the cross-section to be examined. The 
 Bertrand lens can be inserted within the tube in place of the 
 ocular (removed for the purpose), should an enlargement of the 
 interference-figures be required. 
 
 The Biot-Klein's Quartz Plate (Fig. 2, ZZ), about 2 mm. thick, 
 with parallel planes cut perpendicular to the optic axis, and 
 brass-mounted, is introduced through a suitable opening 
 directly above the objective (Fig. 2, //). In ord-r to. use this 
 quartz plate in examining feebly-refracting minerals or those 
 of marked zonal structure, the upper nicol is revolved, after 
 the quartz plate is introduced and the polarizer, objective, and 
 ocular are in suitable positions, until the extremely sensitive 
 red (the so-called u teinte-sensible"} of the circular polarizing 
 quartz appears. The mineral to be examined is then placed 
 beneath the objective. 
 
METHODS OF INVESTIGATION. 
 
 II 
 
 FIG. 2. POLARIZATION-MICROSCOPE, BY R. Fusss. (Older model. Cross-section.) 
 
12 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Minerals with feeble double-refraction, as leucite, or those 
 showing optic anomalies, as garnet, will induce a change of 
 color. 
 
 The quartz plate is also applied to the more exact determi- 
 nation of the position of the directions of vibration, as all 
 double-refracting minerals undergo a change of color, and this 
 remains unchanged only in isotropic sections or when an axis 
 of elasticity coincides with a nicol chief section. 
 
 The Calcite Plate, about 2 mm. thick, with parallel planes, 
 and cut perpendicular to the optic axis, is set in a cork ring, 
 and when in use is laid between the ocular and the analyzer. 
 The nicols are crossed, and the interference-figures of the 
 calcite plate then appear on the section under examination. 
 The arms of the cross-threads must again coincide with the 
 arms of the interference-cross of the calcite plate. More exact 
 stauroscopic investigations cannot be undertaken with this 
 plate except on the larger mineral sections. 
 
 For the microstauroscopical measurements the Calderon 
 Double-plate (Fig. 2, c) is peculiarly adapted. This is made 
 from a twin of calcite artificially formed (Fig. 3, abcdef) by 
 cutting a rhombohedron through the short 
 e diagonals, grinding away a wedge-shaped por- 
 tion from either half, and again cementing the 
 polished surfaces. If the projecting and re- 
 entrant angle of the twin thus formed be ground 
 
 CALDERON DOUBLE- 
 PLATE, away, a plane plate xyvw is obtained, divided 
 
 by the plane separating the two pieces of calcite c, d. This 
 plane appears from above as an extremely fine straight line. 
 This double-plate is so mounted in one of the oculars that the 
 boundary-line of the plate is parallel to the chief section of a 
 nicol ; .i.e., that both halves between crossed nicols show the 
 same degree of extinction. 
 
 A Fourth Undulation Mica Plate is employed to determine 
 the character of the double-refraction in uniaxial minerals; 
 
METHODS OF INVESTIGATION. 13 
 
 in biaxial minerals, either a plate of quartz about 2 mm. thick 
 and cut perpendicular to the optic axis, or a wedge of quartz 
 with one plane parallel to the optic axis and the other inclined 
 at an angle of about 5, is used. 
 
 In making use of the interference-figures obtained with the 
 condenser, to determine the character of the double-refraction 
 in optically-uniaxial minerals, the mica plate 'is laid on the 
 tube so that the plane of the optic axis of the mica, generally 
 indicated by a mark on the setting, makes an angle of 45 with 
 the planes of vibration of the nicols. 
 
 In investigating optically-biaxial minerals the quartz wedge 
 is inserted by an opening in the analyzer so that the chief axis 
 of the quartz forms an angle of 45 with the plane of vibration 
 of the analyzer. The interference-figures of the mineral under 
 examination are brought, by revolving the stage, into such a 
 position that the plane of the optic axis is at first parallel and 
 then perpendicular to the chief axis of the quartz wedge. 
 
 If but a single quartz plate cut perpendicular to the optic 
 axis is at hand, the analyzer must be raised with one hand 
 from the tube of the microscope, from which the ocular is 
 removed, so that the quartz plate can be used beneath it, 
 care being taken that both nicols remain exactly crossed. 
 Then with the other hand the quartz plate is turned a little 
 about' a horizontal axis so that the rays of light must pass 
 through a thicker layer of quartz, and so that the axis of 
 revolution is at first parallel to the plane of the optic axis of 
 the mineral and afterwards perpendicular to it. 
 
 In order to Centre exactly any particular point of an object 
 under examination, and revolve about its own centre, so often 
 necessary in the measurement of angles especially, either the 
 revolving-stage can be moved in two directions at right angles 
 to each other (Fig. I, ;, ^), or the tube acting within a socket 
 can be moved by two screws (Fig. 2, ;//;;/, nn). There must be 
 
14 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 a new centring of the stage or tube for each combination of 
 ocular and objective. 
 
 If the stage can be centred, one of the centring-screws 
 (Fig. I, m) can serve at the same time as Micrometer, Each 
 revolution of this screw, the total number being read off from 
 a circle (/) placed beside it, corresponds of course to a definite 
 magnitude of displacement of the stage, that is, of the object 
 lying upon it ; e.g., in the new microscope made by Fuess, one 
 interval of the micrometer-screw corresponds to a horizontal 
 movement of the stage of 0.002 mm. An ocular-micrometer 
 often accompanies the microscopes instead of this stage- 
 micrometer. Such a micrometer is made of glass, circular and 
 fitted to the ocular, with a fine millimetre-scale engraved on it. 
 
 The method of Due de Chaulnes is best adapted to deter- 
 mine the thickness of thin sections, i.e., the Index of Refraction, 
 in sections of minerals with parallel plane surfaces. The mi- 
 crometer-screw (Fig. 2, g) moving the tube in a vertical direc- 
 tion has a graduated circle attached, from which the revolutions 
 of the screw, and therefore the extent of vertical movement 
 of the tube, can be read. In Fuess's instrument, already men- 
 tioned, the tube micrometer-screw is divided into 500 degrees, 
 each of which corresponds to a vertical movement of o.ooi mm. 
 
 The index of refraction is determined according to the 
 
 formula n = -j , where d represents the thickness of the 
 
 mineral leaflet, and r the movement of the tube which is neces- 
 sary to see a point as clearly through the plate after it is 
 introduced as before its introduction. 
 
 In order to easily find a second time such places on the 
 preparation as may be desired, two scales are placed at right 
 angles to each other on the stage (Fig. I, c), which run from 
 the centre of the circular stage towards the o and 90 points 
 of the outer graduation of the same and are graduated into 
 whole or half millimetres. Then it is only necessary to place 
 
METHODS OF INVESTIGATION. 15 
 
 the object-slide upon the stage so that it lies directly over the 
 two scales with two of its sides parallel to the marks of gradua- 
 tion. By noting the numbers of these marks of graduation, 
 the position of the preparation as to right and left is fixed. 
 Should the object-glass be laid a second time on the stage in 
 the same position, the desired point will fall within the field. 
 
 Finally, those microscopes manufactured by Fuess or by 
 Voigt & Hochgesang are supplied with a Heating-stage, with 
 thermometer attached, to be placed upon the circular revolving- 
 stage. This can be heated by an alcohol-flame placed within 
 a mica chimney, and often does good service, e.g., in determin- 
 ing the fluid inclosures in minerals. 
 
 Different blendes are also added, suitable for placing either 
 upon the ocular, i.e., the analyzer, or of introduction in place 
 of the polarizer. 
 
 A heating-apparatus far more to the purpose than the one 
 just mentioned, and first suggested by Max Schultze, is described 
 by Vogelsang (Poggend, Ann. CXXXVII, p. 58). In it the 
 object is warmed by a platinum wire heated by means of a 
 galvanic current. With such an instrument a temperature of 
 200 C. can easily be attained, the rapidity of changes of tem- 
 perature regulated, and any degree of heat once reached con- 
 tinued quite constant. 
 
 The number of different ocular- and objective-lenses by 
 whose combination the object can undergo a varying enlarge- 
 ment is a matter of choice. In mineralogico-petrographical 
 investigations, oculars I, 2, 3, 4 and objectives 3, 5, 7, 9 of 
 Hartnack's system generally suffice. These are usually con- 
 sidered as the best, and are supplied with the Fuess instrument 
 as described. 
 
1 6 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 A. Optical Methods of Investigations. 
 
 i. EXAMINATION OF MINERAL CROSS-SECTIONS IN PARAL- 
 LEL POLARIZED LIGHT. 
 
 ROSENBUSCH. Mikr. Physiographic, etc., p. 55-107. 
 GROTH. Physikalische Krystallographie. Leipzig, 1876. 
 E. KALKOWSKY. Gr. Zeitschrift f. Kryst., IX, 486 
 
 For observations in parallel polarized light both nicols 
 are exactly crossed ; the short diagonals corresponding to the 
 direction of vibration in the nicols are thus perpendicular to 
 each other, total darkness of the field following ; the ocular 
 and objective for the desired magnifying power are inserted 
 in the tube, and the cross-section to be examined is so placed 
 that on revolving the stage it remains within the field, and its 
 behavior in polarized light throughout a total revolution of 
 the .stage noted. The gathering-lens, or condenser, above 
 the polarizer inducing converging polarized light can be left 
 in situ, as it does not impede the investigations because a 
 withdrawal of the ocular is unnecessary. 
 
 As is well known, a discrimination is made between single- 
 and double-refracting minerals ; the amorphous minerals and 
 those crystallizing in the regular system belonging to the first 
 class. The double-refracting minerals are further distinguished 
 
 c> o 
 
 according to the number of the optic axes and of the axes of 
 elasticity as optically-uniaxial and optically biaxial minerals. 
 Those minerals crystallizing in the tetragonal and hexagonal 
 systems belong to the optically-uniaxial, and those in the 
 rhombic, monoclinic, and triclinic systems to the optically- 
 biaxial minerals. 
 
 In the following pages the behavior of the minerals as 
 regards the different systems of crystallization to which they 
 belong will be discussed. 
 
METHODS OF INVESTIGATION. 
 
 I. Single-Refracting Minerals. 
 
 Amorphous and Regular. If such a mineral is placed under 
 the microscope with crossed nicols, all of its cross-sections 
 remain perfectly dark throughout a complete revolution of the 
 stage ; i.e., they are isotrope. 
 
 The darkness of the field induced by crossing the nicols is 
 not changed by introducing a section of an amorphous or 
 regularly crystallizing mineral, because isotrope bodies cause 
 no change in the direction of vibration of the penetrating 
 light, and the elasticity of the ether in such bodies is equal in 
 t every directiori. The index of refraction n is constant for all 
 directions. 
 
 In the stauroscope, with the calcite plate, no change of the 
 interference-figures occurs during a complete horizontal revolu- 
 tion, nor any change in the shading of either half of the 
 Calderon double-plate; as they remain equally dark, the sepa- 
 rating-line is invisible. 
 
 A series of amorphous and regular minerals, including opal, 
 garnet, analcime, perowskite, which occasionally appear as rock- 
 constituents, show often optical anomalies, in that thin sections 
 of them in parallel polarized light often brighten on revolving 
 the stage. The reason for these phenomena lies probably in 
 the internal tension produced during the growth of the crystal; 
 a detailed zonal structure is generally noticeable in such optical 
 anomalies. 
 
 1 1 . Double-Refracting Minerals. 
 
 A mineral is double-refracting when a part of its cross- 
 section exhibits color-phenomena during a complete revolution 
 in parallel polarized light, i.e., shows polarization-colors. Such 
 cross-sections become four times colored and dark, the latter 
 always occurring in turning from 90 to 90; i.e., it extin- 
 
1 8 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 guishes the ray so soon as one axis of elasticity coincides with 
 a chief section of a nicol. The double-refraction depends 
 upon the difference of the elasticity of the ether according to 
 definite directions within these minerals. The color-phenomena 
 are a consequence of the interference of the light-rays caused 
 by the double-refraction, and depend upon the magnitude of 
 the index of refraction, the direction of the section, and the 
 thickness of the mineral leaflet. 
 
 Byuniaxial minerals, embracing the tetragonal and hexago- 
 nal systems, are understood those in which the elasticity of 
 the ether differs in two directions, parallel or perpendicular to 
 the chief axis. Here a = the axis of greatest elasticity, and C 
 the least; and there is but one direction where no double- 
 refraction occurs, viz., in the direction of the optic axis, which 
 coincides with the chief axis. The index of refraction of the 
 ordinary ray (GO) vibrating perpendicularly to the optical 
 chief section (i.e., that plane which is parallel to the optic axis 
 and perpendicular to the entering face of the light) differs from 
 that (= ) of the extraordinary ray vibrating in the optical 
 chief section. If the chief axis, i.e. the optic axis, coincides 
 with the axis of greatest elasticity, c = a, and GO > e, and the 
 mineral is negative ; if c = c and co < f, the mineral is positive. 
 The greater the difference between the indices of refraction, 
 the more powerful is the double-refraction of the mineral. 
 
 A section of a tetragonal or hexagonal mineral, cut perpen- 
 dicular to the chief axis and parallel to oP, appears isotrope in 
 parallel polarized light throughout a complete horizontal revo- 
 lution, and as one of the single-refracting minerals; i.e., it re- 
 mains perfectly darkened. Sections parallel to the chief axis 
 and one of the prismatic faces are generally rectangular, and 
 between the crossed nicols are always dark when one of the 
 sides of the rectangle, i.e., one of the planes of cleavage paral- 
 lel to the chief axis, is parallel to one of the chief sections of a 
 nicol or an arm of the cross-wires. This occurs four times 
 
METHODS OF INVESTIGATION. 
 
 during one complete revolution. The longitudinal section is 
 then said to EXTINGUISH PARALLEL to the crystallographic axes. 
 
 Fig. 4 gives a clear idea of this parallel extinction in an 
 optically-uniaxial mineral cross-section abed, c is the chief 
 axis, and vw and xy are the cross- 
 sections of both crossed nicols, 
 whose optical chief sections coin- 
 cide with the short diagonals of 
 the rhombic transverse section. 
 
 So soon as the chief axis, i.e., 
 one of the sides, forms any angle 
 with the nicol chief section and the 
 cross-wires, the longitudinal sec- 
 tion shows the polarization-colors. 
 
 Sections inclined to the chief FIG. ^-PARALLEL EXTINCTION. 
 
 axis, e.g., parallel to a pyramidal plane, of course always ex- 
 tinguish parallel to the chief axis, but not always parallel to 
 the sides. Thus a triangular or pentagonal cross-section ex- 
 tinguishes parallel to one of the sides, as the chief axis in 
 such sections is perpendicular to the direction of one of these 
 sides, while a rhombic cross-section will extinguish parallel to 
 the diagonals of the figure. 
 
 The behavior of various cross-sections of a uniaxial crystal 
 in parallel polarized light can be easily demonstrated on a glass 
 crystal model in which the chief axis is marked, if one will 
 always bear in mind that the extinction occurs parallel to 
 the chief axis. 
 
 In the stauro-microscope (with calcite plate) transverse 
 sections of optically-uniaxial minerals always show the calcite 
 interference-figures. In longitudinal sections they are undis- 
 turbed only when the chief axis or one of the contour-lines of 
 the crystal parallel to it coincides with one of the arms of the 
 cross-wires already in conjunction with the nicol chief sections 
 in the microscope. 
 
20 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Transverse sections behave like isotrope cross-sections when 
 examined with the Calderon double-plate. Longitudinal sec- 
 tions always in(juce a different shading of both halves of the 
 plate when the chief axis is not parallel to the principal direc- 
 tion of vibration of the nicol, the arms of the cross-wires, or 
 the line of junction in the Calderon plate, three objects which 
 are exactly parallel to each other in the microscope. If the 
 chief axis is parallel to the line of junction, both halves of the 
 plate are equally dark with crossed nicols; if this is not the 
 case, then both halves are unequally shaded, the one dark and 
 the other light, or both are equally clear. 
 
 It is possible to determine whether a mineral under exam- 
 ination belongs to the tetragonal or hexagonal system only 
 from the character of the contour of the section cut at right 
 angles to the chief axis. If it is square or octagonal it belongs 
 to the tetragonal ; if hexagonal or dihexagonal, it belongs to 
 the hexagonal system. 
 
 In the optically-biaxial minerals there are two directions 
 wherein no double-refraction takes place, i.e., there are two 
 optic axes ; and further, we assume three axes of elasticity at 
 right angles to each other, i.e., three directions in which the 
 elasticity of the light-ether differs. The direction of the great- 
 est elasticity is designated by ft, that of middle value by b, and 
 that of the least by C. 
 
 The optic axes do not coincide with the crystallographic 
 axes, and form an angle with each other. The line dividing 
 equally the acute angle is called the first middle line, or acute 
 bisectrix; the line bisecting the more obtuse angle, the second 
 middle line, or obtuse bisectrix. The optic axes and both middle 
 lines lie in a single plane, THE PLANE OF THE OPTIC AXES 
 (A.P.) ; the optic normal lies perpendicular to the plane of the 
 optic axes. The axis of elasticity of middle value (b) always 
 coincides with the optic normal, while the axes of greatest and 
 least elasticity coincide with either the first or the second 
 
METHODS OF INVESTIGATION. 21 
 
 middle line. If a = I. M., then c = 2. M,, and the mineral is 
 negative; if c = i. M., and a = 2. M., the mineral is positive. 
 
 There are three different indices of refrafction, a', /?, y, 
 corresponding to these three axes of elasticity. 
 
 Minerals crystallizing in the rhombic, monoclinic, and tri- 
 clinic systems belong to the optically-biaxial minerals. 
 
 Rhombic Minerals. -^-In these minerals the three axes of 
 elasticity a > b > C coincide with the three crystallographic 
 axes a, b, c' ; a does not always equal a, etc., yet each of the 
 crystallographic axes can coincide with each of the axes of 
 elasticity, a and c are always middle lines, and the plane of 
 the optic axes (AP) is always parallel to one of the three 
 pinacoids. The following cases may therefore occur: . 
 
 If AP\\ oP, then d - a, b = c ) , __ ^ . 
 
 a c, b > a ) 
 If AP || oo Poo , then c' = a, d = C ) ^ = , . 
 
 c' = t, a = a ( 
 
 \IAP\\ oo P oo , then c f = a, b = C 
 
 c' = a, b = c -. 
 , I a = \). 
 
 c' c, b = a 1 
 
 Figs. 5 to 8 serve as examples of these cases. These are 
 schematic representations of the optic orientation of rhombic 
 augite and hornblende in sections parallel to the plane of the 
 optic axes. A and B represent the two optic axes ; the middle 
 lines or axes of elasticity are designated by German, the crys- 
 tallographic axes by italic letters. 
 
 Cross-sections parallel to the three planes of the pinacoids, 
 in general of rectangular figure, have a parallel extinction, i.e., 
 are dark between crossed nicols only when one of the sides of 
 the rectangle or one of the pinacoidal cleavage-fissures is 
 parallel to a chief section of a nicol. Darkness follows so soon 
 as one of the crystallographic axes coincides with a nicol 
 chief section. This occurs four times in a complete revolution, 
 
22 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 just as with the longitudinal sections of the uniaxial crystals. 
 The rhombic minerals can, however, be distinguished from 
 them in parallel polarized light, in that the sections parallel to 
 oP are not isotrope as in the uniaxial minerals. 
 
 Only those sections of rhombic minerals which are cut 
 
 FIG. 5. HYPERSTHENB. 
 
 || 00/00. 
 
 ^ 
 
 ^ 
 
 ^ 
 
 ^> 
 
 FIG. 6. BASTITE. 
 
 II oo /oo. 
 
 A c.*> 
 
 FIG. 7. ENSTATITE AND BRONZITE. 
 
 H 00/00. 
 
 \ 
 
 / 
 
 / 
 
 \ 
 
 IG. 8. ANTHOPHYLLITE 
 
 || 00 /CO. 
 
 exactly at right angles to one of the two optic axes remain per- 
 fectly dark throughout a complete revolution, i.e., are isotrope. 
 Such sections are parallel to Poo, /*oo, or a prismatic face, 
 according to the position of the optic axes. It is self-evident 
 that such isotrope sections are more rare in rhombic than in 
 optically-uniaxial crystals, and have, moreover, no such regular 
 forms. 
 
METHODS OF INVESTIGATION. 2$ 
 
 Just as in the pinacoidal sections, i.e., from the zones 
 oP : oo .Poo and oP : oo/^oo, so all longitudinal sections parallel 
 to the vertical axis (c r ) from the zone caPcn : oo/^co extinguish 
 parallel to the sides or one of the cleavage-fissures parallel to 
 the vertical axis. Symmetrical sections inclined to the ver- 
 tical axis, not belonging to any of the above zones, do not 
 extinguish for the most part according to their axial figures. 
 
 When examined with the stauroscope, the calcite inter- 
 ference-figures, i.e., the darkening of the Calderon double-plate, 
 appear undisturbed only when one of the crystallographic axes 
 coincides with a nicol chief section ; isotrope sections, of course, 
 exert no action on either plate during a complete revolution. 
 
 Monoclinic Minerals. In the monoclinic system only the 
 orthodiagonal axis b coincides with one of the axes of elas- 
 ticity ; both of the remaining axes of elasticity form an angle 
 with the crystallographic axes a and c ' . The plane of the 
 optic axes is either parallel oj* at right angles to the plane of 
 symmetry oo^Poo. In the monoclinic minerals there are the 
 following possibilities for optical orientation : 
 
 \IAP\\ oo Poo, then I. M. = c ) ^ _ * 
 or i. M. = a ) 
 
 and c aod a are inclined to c f and h. 
 
 If, on the contrary, AP1.& jPoo, then i. M. = b = ft, 
 
 1. M. = b = t- 
 or 2. M. = b = a, 
 
 2. M. = b = c. 
 
 In this case b and C, or b and a, are inclined to c' and ^. 
 
 In Figs. 9 to 14 are given schematic representations of 
 several rock-forming monoclinic minerals. The cross-sections 
 are parallel to the plane of the optic axes. A and B are the 
 
24 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 !958' 
 
 J. 
 
 FIG. 9. HORNBLENDE. 
 
 HoojPoo. 
 
 (After Fouque*.) 
 
 FIG. io. AUGITE. 
 
 || oo Poo. 
 (After Fouque'.) 
 
 FlG. II. WOLLASTONITB. 
 
 llooPoo. 
 
 (After Fouqu^.) 
 
 FIG. 12. EPIDOTE. 
 
 II ooPoo. 
 
 (After Fouque 1 .) 
 
METHODS OF INVESTIGATION. 
 
 optic axes, a and c middle lines, and c the vertical axis. In 
 titanite, Fig. 13, there is shown, in addition, the dispersion 
 of the optic axes v < p ; in orthoclase, Fig. 14, attention is 
 called to the case where the plane of the optic axes is 
 _L oojPoo. A l l are the optic axes where AP\\ oojPoo, A^B g 
 
 FlG. 13 TlTANITB. 
 ||coPe. 
 
 (After Fouque.) 
 
 F\c. 14. ORTHOCLASE. 
 
 || 00 f> 00. 
 
 (After Fouque.) 
 
 where AP is J_ oo;Poo ; in both cases a is at an angle of 5 to 
 the edge oP : oo Pec. 
 
 As a consequence of this inclination of the axes of elas- 
 ticity to the crystallographic axes, longitudinal sections are 
 not darkened during a complete revolution whenever the 
 crystallographic axes or the cleavage-fissures parallel to these 
 coiacide with a nicol chief section, as is the case with rhom- 
 bic minerals ; but in many cases extinction (i.e., the. section 
 becomes dark under crossed nicols) first takes place when the 
 crystallographic axes are inclined to the nicol chief section; 
 i.e., it extinguishes obliquely. 
 
 Fig. 15 represents the oblique extinction of an optically- 
 
26 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 &70" 
 
 biaxial crystal cross-section, abed, wherein C represents the 
 
 vertical axis, and vw and xy are 
 again the nicol cross-sections. 
 The crystal cross-section is in 
 the position where it completely 
 extinguishes the ray ; the incli- 
 *' nation of the axis of elasticity 
 lying in the direction xy to the 
 vertical axis is 50 in this case. 
 
 Extinction always follows, as 
 is well known, whenever one 
 
 180- 
 
 FIG. 15. OBLIQUE EXTINCTION. axis of elasticity coincides with 
 a nicol chief section ; in the monoclinic system, however, two 
 axes of elasticity are always inclined to the crystallographic 
 axes. This angle of inclination is exceedingly characteristic 
 for the various monoclinic crystals (comp. Figs. 9 to 14), and 
 can be readily determined in parallel polarized light. As a 
 consequence of the optical orientation this oblique extinction 
 can be accurately determined only in those sections parallel 
 to the plane of symmetry, ooPoo (comp. Fig. 15). A cleavage- 
 fissure parallel to the chief axis, or one of the edges parallel 
 to it, is placed in position parallel to an arm of the cross-wires 
 (i.e., a nicol chief section), and the degrees read on the circle 
 of the stage. In this position, between crossed nicols, the 
 cross-section is colored. The stage is then revolved until the 
 cross-section is perfectly darkened. The number of degrees 
 through which the stage must be revolved to cause total dark- 
 ness gives the angle of inclination of one axis of elasticity to 
 the vertical axis, the "angle of extinction;" e.g., on augite 
 the inclination C : c 38, therefore a : c 52. This angle 
 which one of the axes of elasticity forms with the vertical 
 axis of course equals that which the other axis of elasticity 
 makes with the normal to oo P oo. The angle of extinction can 
 also be measured in relation to another known edge in the 
 
METHODS OF INVESTIGATION. 
 
 section || ooPoo; e.g., to the edge oP : ooPoo, i.e., as 
 
 the same inclination as <&, the angle of inclination of the other 
 
 axis of elasticity to the clinodiagonal. 
 
 The application of the stauroscope is therefore clear from 
 what has just been stated. This is used, as it is very difficult 
 to determine with the eye alone the exact point of maximum 
 darkness; with the aid of an exceedingly sensitive Calderon 
 double plate, however, this is possible with accuracy to within 
 some few minutes; it is therefore peculiarly adapted to the 
 more exact determination of the position of the plane of the 
 axes of elasticity. Equality of shading in the double-plate of 
 course follows when an axis of elasticity is parallel to the line 
 of junction in the plate. 
 
 All sections of monoclinic crystals from the zone oP : 
 oo P oo extinguish parallel, as in these the orthodiagonal 
 always coincides with one of the axes of elasticity; extinc- 
 tion follows, therefore, always when one of the edges parallel 
 to the vertical axis or one of the cleavage-fissures parallel to 
 this coincides with a chief section of a nicol. The shading of 
 the Calderon double-plate will therefore be undisturbed only 
 when the orthodiagonal coincides with a nicol chief section, 
 i.e.,. the line of junction. 
 
 Sections from the zones oP : oo^Poo and oo^oo : ooPoo 
 always extinguish at an angle. The angle of extinction finally 
 reaches o when the section is parallel to oP or oo/'oo. 
 
 Thus, according to Michel Lvy, the value of the oblique 
 extinction varies in augite and hornblende with the direction 
 of the section in the following manner: 
 
28 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Direction of the 
 section in the zone. 
 
 Augite. 
 For zv = 58 59'. 
 
 Hornblende. 
 For zv = 79 24'. 
 
 oP : oo Poo 
 
 c = e = 38 4< j. Maximum, 
 o = a= 22 55 fr 
 In sections parallel to ooP oo, 
 a :rt 22 55'; with more 
 acute inclination of the sec- 
 tion the value increases and 
 reaches its maximum on the 
 plane which makes an angle 
 of 67 14' 6" with co P co; 
 it then lessens and becomes 
 o in sections parallel to oP. 
 
 Maximum of 29 58' to 14 
 58' parallel to ooP GO, accord- 
 ing to the species of horn- 
 blende, then decreases and 
 becomes o parallel to oP. 
 
 ooP oo : oof oo 
 
 Maximum of extinction ob- 
 liquity on co Poo, c : c 38 
 44'. According to the in- 
 clination towards coPoo the 
 angle decreases and becomes 
 o parallel co P oo. 
 
 Minimum parallel oo^Pco, be- 
 tween 15 (for hornblende) 
 and o (actinolite); increases 
 and reaches the maximum 
 (actinolite = 15 15' 20") in 
 the plane which forms an 
 angle of 38 18' 25" with 
 co P oo; then decreases and 
 equals o parallel coP oo. 
 
 oP : oo p oo 
 
 All sections possess parallel extinction. 
 
 Triclinic Minerals. In the triclinic minerals no one of the 
 ^ OP three axes of elasticity coincides with 
 
 the crystallographic axes. 
 
 Fig. 1 6 is an example of the opti- 
 cal orientation of a triclinic rock- 
 ?. forming mineral, a, b, and c are the 
 three axes of elasticity ; the angle of 
 inclination of C to the vertical axis 
 in disthene is 30 measured in sections 
 parallel to ooPoo; a is exactly per- 
 pendicular to oo Poo. 
 
 All sections, therefore, parallel to 
 the three pinacoidal faces have the oblique extinction. 
 The obliquity of extinction to the faces oP and oo P oo 
 
 FIG. 16. DTSTHENE. 
 
 II oo Poo. 
 (After Fouque".) 
 
METHODS OF INVESTIGATION 2Q 
 
 is known in most of the rock-forming minerals, and gives, 
 therefore, an excellent means of determining the minerals 
 of this system. In the thin sections one can in most cases 
 determine from the shape of the cross-section whether it is 
 parallel to one of these pinacoids. If now an oblique extinc- 
 tion is proved on both of these pinacoids, it is sufficient for 
 assignment of the mineral to the triclinic system, as in the 
 monoclinic system the oblique extinction obtains only parallel 
 to the plane ooPco. Exact measurement of the extinction- 
 obliquity must, however, be made on cleavage-lamellae parallel 
 oo/^oo and oP. 
 
 In the stauroscope the calcite interference figures, i.e., the 
 shading of the Calderon double-plate, will be disturbed when- 
 ever one of the crystallographic axes or a cleavage line or edge 
 parallel to them is parallel to a nicol chief section. 
 
 2. EXAMINATION OF MINERALS IN CONVERGENT 
 POLARIZED LIGHT. 
 
 In order to produce convergent polarized light the con- 
 denser is placed above the polarizer, and, after the cross- 
 section has been adjusted and centred in the microscope, the 
 oculaivis removed and the nicols crossed. If the cross-section 
 is very small, and high magnifying powers must be used, 
 thereby decreasing the interference-figures, the Bertrand lens, 
 for the necessary enlargement, is inserted within the tube in 
 place of the ocular, and the nicols of course are again crossed. 
 
 The interference-figures observed with the condenser in 
 different sections of the double-refracting minerals are exactly 
 the same as those obtained on such sections with the Norrem- 
 berg instrument. The interference-figures, however, are not 
 so clear and large in the microscope, as the mineral cross- 
 sections are very small, and in the slides are exceedingly 
 
30 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 thin. The great advantage gained through the application 
 of the condenser to microscopical petrography, as first recom- 
 mended by Lasaulx and Bertrand, is evident ; e.g., we can 
 determine whether a mineral is a single-refracting, optically 
 uniaxial or biaxial, if but a single isotrope cross-section of the 
 mineral is at hand. The following observations will demon- 
 strate this. Of course the examination in parallel polarized 
 light must always precede that in convergent light. 
 
 Clear interference-figures can be obtained on using objec- 
 tive 9, Hartnack, and the Bertrand magnifying-lens in con- 
 vergent light, if the mineral cross-section is not less than 0.05 
 mm. ; if the cross-sections are less, their determination in con- 
 vergent light is impossible in most cases. In such cases the 
 examination in parallel polarized light is all the more important. 
 The behavior of mineral cross-sections in convergent light for 
 the different systems of crystallization is the following : 
 
 Regular and Amorphous Minerals. The amorphous miner- 
 als and those crystallizing in the regular system remain dark 
 throughout a complete revolution in all cross-sections, and 
 show no interference-phenomena. 
 
 Optically-TJniaxial Minerals (Fig. 17, I and II). The iso- 
 trope transverse sections of tetragonal and hexagonal miner- 
 als show, in case the section is exactly at right angles to the 
 chief axis (Fig. 17, I), a fixed dark interference-cross with 
 several colored concentric rings. The presence and number of 
 the rings in the cross-section depend on its thickness and the 
 power of double-refraction of the mineral. If the section is 
 not exactly at right angles to the chief axis, as is evident in 
 ordinary light from the irregular transverse section (e.g., dis- 
 torted rectangles or hexagons), or the confirmation of an 
 imperfect depolarization in parallel polarized light, the inter- 
 ference-cross in convergent polarized light remains undisturbed 
 throughout a complete horizontal revolution ; i.e., it does not 
 open, but moves according to the lesser or greater inclination 
 
METHODS OF INVESTIGATION. 31 
 
 of the section to the chief axis either within the field or 
 without on the circumference, and in the same direction as 
 the stage is revolved. 
 
 If the section is so inclined that the axial point of the inter- 
 ference-figure falls without the field of the microscope (Fig. 17, 
 II), it will not appear in parallel light as isotrope (show polari- 
 zation-colors and become darkened four times during a revolu- 
 tion) ; in this case, by revolving the stage from 90 to 90 only 
 one part of the interference-cross will appear as a straight black 
 cloud in the field. The cloud moves, during a revolution of 
 the stage through 90, from one side of the microscope-stage, 
 i.e., the field of the microscope, to the other in the same plane. 
 As will be shown later, similar pictures will be obtained in sec- 
 tions of optically-biaxial minerals which are cut at right angles 
 to one of the optic axes, yet the black cloud in these moves 
 about an axial point situate within. 
 
 For the Determination of the Character of the Double Refrac- 
 tion (in sections at right angles to the chief axis) a fourth undu- 
 lation mica plate is most advantageously employed. As already 
 stated, this is laid on the tube from which the ocular has been 
 removed, and the analyzer placed upon it, with the nicols 
 crossed, and with the plane of the optic axes of the mica in- 
 clined at an angle of 45 to a nicol chief section. The black 
 interference cross of a uniaxial crystal diminishes until only 
 two dark points remain and the colored rings are disturbed. 
 
 If the two dark points are so situated that the line joining 
 them is perpendicular to the plane of the optic axes of the 
 mica (generally indicated by a mark on the plate), then the 
 mineral under examination is optically festttve ; if the joining 
 line of both the dark points concides with the direction of the 
 axial plane of the mica, the mineral is optically negative. 
 
 Optically-Biaxial Minerals (Fig. 17, III, IV, and V). If 
 an optically-biaxial mineral be cut at right angles to one of the 
 middle lines bisecting the angle which the two optic axes make 
 
32 DETERMINATION OF ROCK-FORMING MINERALS. 
 OPTICALLY-UNIAXIAL CRYSTALS. 
 
 OPTICALLY-BIAXIAL CRYSTALS. 
 
 FIG. 17. INTERFERENCE-FIGURES OF DOUBLE-REFRACTING MINERALS ON USING THE CONDENSER 
 IN THE POLARIZATION-MICROSCOPE. (After Fouqud.) 
 
 4 is the angle which a vertical plane passing through an optic axis A forms with the optic 
 chief section of the polarizer. 
 
METHODS OF INVESTIGATION. 33 
 
 with each other (Fig. 17, V), and be examined in convergent 
 polarized light, an interference-figure is seen, in case the plane 
 of the optic axes coincides with a nicol chief section, which 
 is formed from two closed systems of curvature correspond- 
 ing to the two axial points ; these in turn are surrounded 
 by a larger system of curvatures, the lemniscates, and are trav- 
 ersed by a black cross of which one arm, the narrower, passes 
 through the two axial points and thus shows the position of 
 the plane of the optic axes, and whose second arm, much 
 broader, is at right angles to it. 
 
 The number of the colored curves depends, again, on the 
 thickness of the mineral leaflet ; if this is very thin, as may be 
 expected in rock-sections, only the black cross is visible, thus 
 resembling the interference-figure of optically-uniaxial crystals. 
 The difference is immediately seen on revolving the mineral 
 section on the stage (Fig. 17, V, cp > 45); in the optically- 
 biaxial minerals the cross does not remain fixed, but opens 
 and divides into two hyperbolas which move about either axial 
 point and by revolving 90 again close into the cross. 
 
 The distance between the two points, or the hyperbolas 
 passing through them, gives both the position of the plane of 
 the optic axes and the magnitude of the axial angle ; if this 
 angle is large, then each of the hyperbolas lies without the 
 field, so soon as the plane of the axes forms an angle of 45 
 with a nicol chief section (Fig. 17, V, cp 45). It can gen- 
 erally be determined from the proximate estimation of the 
 magnitude of the axial angle whether the section is made per- 
 pendicular to the first or second middle line. There are cases, 
 as in the rhombic pyroxenes, where the acute axial angle differs 
 but little from the obtuse ; in such cases it is impossible to de- 
 termine by the microscope which axes of elasticity coincide 
 with the first and second middle lines. 
 
 If it is known whether the section is'at right angles to the 
 first or second middle line, then it can be determined which of 
 
34 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 the axes of elasticity a or c coincides with the same, i.e., the 
 optical orientation. If the axial angle is very small, the inter- 
 ference-figure will be similar to the optically-uniaxial mineral 
 and the cross apparently remains closed. 
 
 The Determination of the Character of Double-refraction in the 
 optically-biaxial crystals is effected in the following manner: 
 The axial figure is placed in such a position that the plane of the 
 optic axis is at an angle of 45 with a nicol chief section, i.e., 
 the cross seems merged into the hyperbolas ; the quartz plate 
 described on page 10, or the quartz wedge, is so used beneath 
 the analyzer that the axis of revolution of the quartz plate or 
 quartz wedge is at first parallel and then perpendicular to the 
 plane of the optic axes. In any case, a change of the inter- 
 ference-figure is visible on revolving the quartz plate or on 
 pushing in the quartz wedge ; the inner rings move from the 
 circumference of the field towards the centre, the outer lem- 
 niscates, on the other hand, in the opposite direction. If this 
 enlargement and movement of the rings occur when the axis 
 of revolution of the quartz plate or the quartz wedge is perpen- 
 dicular to the plane of the optic axis, the mineral is positive 
 double-refracting ; under reversed conditions, negative. 
 
 If the mineral was proved positive double-refracting on sec- 
 tions at right angles to the first middle line, then .the axis of 
 the least elasticity coincides with it and the plan is the fol- 
 lowing: 
 
 First middle line = c (positive); 
 Second middle line = a; 
 Optic normals always = fc. 
 
 The reverse is true in case the second middle line is positive : 
 
 First middle line = a (negative); 
 Second middle line == c; 
 Optic normals b. 
 
METHODS OF INVESTIGATION. 35 
 
 Sections at right angles to one of the two optic axes appear 
 as isotrope in parallel polarized light, and show in convergent 
 polarized light a spherical or elliptical colored ring-system 
 traversed by a dark cloud (Fig. 17, III). If the section is 
 exactly at right angles to the optic axis, on revolving the prep- 
 aration the cloud moves in a contrary direction about the axial 
 point lying in the centre of the ring-system ; on sections more 
 or less inclined to the optic axes (Fig. 17, IV) a movement of 
 the whole axis-figure is observed concordant with the revolv- 
 ing of the object-stage. 
 
 If the section is so oblique to the optic axis (Fig. 17, IV) 
 that the axial point falls without the field, only a part of the 
 cloud ever lies in the centre of the field on revolving from 90 to 
 90, just as with the optically-uniaxial minerals cut inclined to 
 the axis ; the difference consists, however, in the movement of 
 the cloud itself about the axis-point in the direction opposite 
 to that of the revolving-stage. Sections parallel to the plane 
 of the optic axes,' at right angles to b, show no interference- 
 figures in convergent polarized light, become colored as in 
 parallel polarized light, and appear dark whenever an axis of 
 elasticity coincides with a nicol chief section. 
 
 Rhombic Minerals, Sections at right angles to the crystallo- 
 graphic axes, consequently parallel to the pinacoidal planes, 
 show perfectly the optical orientation. According to the posi- 
 tion of the plane of the optic axes (see page 22), either the 
 vertical axis, the brachy- or macro-diagonal will be the first 
 middle line. One of the pinacoidal sections will show perpen- 
 dicular to it the first middle line with the smaller angle of the 
 optic axes, a second the appearance of the second middle line 
 with the larger axial angle, and the third parallel to the plane of 
 the axes will show no interference-figures. The transverse sec- 
 tions are the most favorable (at right angles to <:'); as on the one 
 hand but few of the rock-forming minerals, e.g., olivine, have 
 the axial planes parallel oP, consequently in these, at any rate, 
 
36 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 an interference-figure is seen ; and on the other hand, as the 
 predominating cleavage is prismatic or pinacoidal, it can be 
 controlled as to whether the section is made exactly at right 
 angles to the vertical axis. 
 
 As a consequence of the dispersion of the optic axes the in- 
 terference-figure develops in white light a varying color-distri- 
 bution according as the axial angle for red is greater or smaller 
 
 than for blue (p ^ v) ; in the rhombic system the distribution 
 
 is symmetrical to the middle lines. Where p > v, in the posi- 
 tion : axial plane parallel to the nicol chief section, the inner 
 closed curves are blue on the inner limb, and red on the outer ; 
 in the position : axial plane inclined 45 to the nicol chief sec- 
 tion, the hyperbolas become red on the inner, the convex sur- 
 face, and blue on the outer, the concave surface. Where p < v 
 the reverse holds true. The phenomena of dispersion, when 
 not too weak, can be studied in convergent light very well in 
 the rock-constituents, e.g., zoisite, etc. Often the simple obser- 
 vation of an hyperbola in relation to the colored edges suffices 
 for the determination of the form of axial dispersion; it is not 
 absolutely necessary, therefore, that the sections should be at 
 right angles to the middle lines. 
 
 Monoclinic Minerals. If the plane of the optic axes in mono- 
 clinic minerals is parallel oo Poo, the sections at right angles to 
 the vertical axis and parallel ooPoo will not show a perpendicu- 
 lar development of a middle line, as is the case with the cor- 
 responding pinacoidal sections of rhombic crystals, but a dis- 
 placed axial picture (AP parallel to the edge oP\ ooPoo or 
 oo Poo : co Poo) ; or simply an appearance of one of the optic 
 axes according to the degree of inclination of the middle line 
 to the crystallographic axes. Sections at right angles to the 
 middle lines obtain only accidentally and are extremely rare 
 (compare with the rhombic minerals) ; such, of course, spring 
 from the zone 0P:ooPoo. In prismatic sections the displaced 
 
METHODS OF INVESTIGATION. 
 
 37 
 
 axial picture or appearance of one axis is not visible in the 
 middle of the mineral leaflet, but at one side. If the inclina- 
 tion of the axes of elasticity to the crystallographic axes is 
 very small, as, e.g., from a : c in mica, the mineral is apparently 
 
 FIG. 18. MUSCOVITE. || oP. 
 
 Mica I. Class. 
 (After Fouqud.) 
 
 FIG. 19. BIOTITE. || oP. 
 Mica II. Class. 
 (After Fouque".) 
 
 rhombic (Figs. 18 and 19). In the mica minerals the first mid- 
 dle line a differs but little from the normals tooP;A and B are 
 the two optic axes ; a, b, and c, the axes of elasticity. 
 
 If the plane of the optic axes be at right angles to oo Poo, an 
 appearance of one middle line perpendicular to oojPoo may al- 
 ways be observed ; yet such an appearance is not shown on 
 sections parallel oP or oo Pec ; on these a distorted axial pic- 
 ture is again visible, AP parallel to the edge oP : coPoo. 
 
 In the Triclinic Minerals a perpendicular appearance of a 
 middle line obtains in none of the pinacoidal sections, the plane 
 of the optic axes is neither parallel nor at right angles to a 
 pinacoid, and only portions of the interference-figure can be 
 described in the pinacoidal sections. . 
 
 The phenomena of dispersion in monoclinic and triclinic 
 minerals cannot be established with great precision by the mi- 
 croscope or prove of value in determining the minerals; in 
 
 general it can only be determined whether p ^ v. 
 
 3. BEHAVIOR OF TWINNED CRYSTALS IN POLARIZED LIGHT. 
 
 Twins of the Regular System cannot be recognized as such 
 either in parallel or convergent polarized light, as both indi- 
 viduals remain equally dark between crossed nicols ; therefore 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 According to SPoo. According to /*<. 
 FIG. 20. RUTILE TWINS. 
 
 the form of the cross-section and the cleavage must be solely 
 regarded in the determination of the law of twinning. 
 
 Twins of the Tetragonal and Hexagonal Systems. a. With 
 parallel axial systems. These also, for the same reason as the 
 regular minerals, cannot be recognized in polarized light. 
 b. On the other hand, twins with inclined axial systems can 
 
 be recognized easily in parallel 
 polarized light. In these min- 
 erals the chief axes and axes of 
 elasticity form an angle with each 
 other, and the twinned crystal will 
 not, therefore, act as a unit in ex- 
 tinguishing the light ; e.g., rutile 
 C\C l = 1 14 26' (Fig. 20). Cj C, are 
 the chief axes of both individuals, and N is the twinning-seam. 
 When one individual appears dark between crossed nicols, 
 the second becomes colored. The angle between the two 
 chief axes can therefore be determined, if an edge of one in- 
 dividual parallel to the chief axis be first placed on the centred 
 stage parallel to the nicol chief section so that it is darkened, 
 the stage revolved until the second is darkened, and the num- 
 ber of degrees read through which it was necessary to revolve 
 the stage. 
 
 If several individuals are twinned (polysynthetic twins), 
 these are wont to occur in laminations, as, e.g., in calcite, 
 twinning-plane ^R (Fig. 21): in these, 
 in sections inclined to the twinning- 
 plane, the axes of elasticity of the 
 lamellae I, 3, 5, etc., have a similar 
 position, i.e., they extinguish at the 
 same instant. In sections parallel to 
 the twinning-plane no twinning stria- 
 tions can be observed, as in this case but a single individual is 
 met with. 
 
 FIG. 21. CALCITE TWIN. 
 
 According to R. 
 
 H tf-face. 
 
METHODS OF INVESTIGATION'. 
 
 39 
 
 If the twinning-plane in calcite is the 7^-face, although 
 never occurring in the rock-forming individuals, the chief axes 
 form nearly a right angle with each other, C : C l = 89 8' ; 
 both individuals therefore extinguish the ray at nearly the 
 same instant. 
 
 Twins of the Rhombic System, The most common examples 
 of this system are : 
 
 1. Twinning-plane a face of a brachydome. 
 
 2. " " pyramid. 
 
 3. " " prism. 
 
 In the first two cases the crystallographic axes form an 
 angle with each other ; in longitudinal sections of such twins, 
 therefore, no unit-extinction between crossed nicols can 
 occur. In staurolite, for example, the vertical axes c : c l9 
 which in this case coincide with the axis of elasticity c, form 
 with each other an angle of 60 according to the law f Pf ; but 
 
 FIG. 22. STAUROLITE TWINS ACCORDING TO 
 
 an angle of 90 according to the law f P oo ; i.e., in the latter 
 case both individuals extinguish together (Fig. 22). 
 
 A further point of recognition for the twinning develop- 
 ment in colored minerals lies in the pleochroitic behavior, as 
 both individuals, by virtue of their different position with 
 reference to the chief direction of vibration of the polarizer, 
 will be differently colored. 
 
40 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 If one of the prismatic faces is the twinning-plane, a law 
 exemplified, e.g., often on aragonite, rarely on cordierite, etc. 
 (Fig. 23), the longitudinal sections parallel to the vertical axis, 
 when in parallel polarized light, show no difference in the 
 direction of extinction, as the axes of elasticity of both in- 
 dividuals coinciding with the <r'-axis are again parallel. The 
 two individuals in such sections, however, can be accurately 
 distinguished in convergent polarized light, as the same inter- 
 ference-figure does not appear on both members ; but the ap- 
 pearance of a middle line on one side and only one of the 
 
 J[ 
 
 FIG. 23. CORDIERITE TWINS. 
 (According to v. Lasaulx.) 
 
 optic axes on the other, etc., will be observed, the phenomena 
 depending on the direction of the section. 
 
 Penetration twins or trillings after this law often imitate 
 the form of an hexagonal prism. Cross-sections of such twins, 
 however, divide into six sectors in parallel polarized light, two 
 of which in opposite positions will extinguish at the same 
 instant. The axes of elasticity of these three individuals are 
 inclined 60 to each other ; an equal inclination of the plane 
 of the optic axes in the individuals can therefore be observed 
 on such twins by convergent polarized light, provided they are 
 not of a mineral with the plane of the optic axes parallel to oP. 
 
 Twins of the Monoclinic System. The most commonly-oc- 
 curring twinnings are according to the law : twinning-plane 
 oo Poo. Twinnings according to a prismatic face seldom occur. 
 
METHODS OF INVESTIGATION. 
 
 Augite, amphibole, epidote, and gypsum may be brought for- 
 ward as examples of the rock-forming minerals with repeated 
 twinning according to coPco. Sections perpendicular to the 
 twinning-plane and parallel to oo jPoo will show, in parallel polar- 
 ized light, in both individuals, an oblique extinction equally 
 inclined to the vertical axis, i.e., the twinning-seam or line 
 of development, but in opposite directions ; e.g., on augite 
 c : c = c l : Cj = 38 (Fig. 24). Such sections in convergent 
 light show no difference ; nor can interference-figures be recog- 
 nized, as in these minerals the plane of symmetry is at the 
 
 t, 
 
 FIG. 24. AUGITE TWIN 
 according to oo foo. \\ccfoo. 
 
 FlG. 25. POLYSYNTHETIC AUGITE TWIN 
 
 according to <x>P<v>. Section _L c'-axis. 
 
 same time the plane of the optic axes. Such twins, with 
 parallel vertical axes, can be easily recognized in parallel 
 polarized light as belonging to a monoclinic mineral, as both 
 individuals, if rhombic, would extinguish at the same instant. 
 As already stated, several twinning lamellae are often inter- 
 polated according to this law (Fig. 25) ; therefore in parallel 
 polarized light, especially in sections at right angles to the 
 vertical axis, there is often observed an interchange of bril- 
 liantly-colored lines, all parallel to a boundary-line of the 
 apparently simple crystal. Twins occur but rarely after the 
 plane of a dome or pyramid, as in augite according to P2, 
 and more rarely still penetration-twins according to Poo. In 
 the latter case we are vigorously reminded of the staurolite 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 twinning ; but the extinction in these augite twins, so commonly 
 occurring in certain basaltic rocks, does not occur parallel to 
 the vertical axes of both individuals on epidote (Fig. 26). 
 One or more narrow interpolated twinning-lamellae are often 
 
 OP 
 
 FIG. 26. EPIDOTE TWIN 
 according to oo^oo. Section || oojPc 
 
 FIG. 27. TITANITE TWIN 
 according to oP. Section HcojPoo. 
 
 noticed in the hexagonal sections parallel to the plane of sym- 
 metry, which is the same as the plane of the optic axes parallel 
 to oo Poo. 
 
 In titanite (Fig. 27), contact-twins often occur after the 
 law : twinning-plane oP. In this case sections at right angles 
 to the twinning-plane, where they are not parallel to ooPoo, 
 also develop on either side an angle of extinction equally in- 
 
 FIG. 28. ORTHOCLASE TWIN 
 according to the Carlsbad and Baveno laws. 
 
 clined to the vertical axis. Extinction follows here nearly 
 parallel to the face ^Pco, as the first middle line is nearly per- 
 pendicular to it. Sections parallel oo Poo develop in convergent 
 polarized light one optic axis in each individual, but in opposite 
 directions. 
 
METHODS OF INVESTIGATION. 
 
 43 
 
 The most varied twinning development occurs on ortho- 
 clase(Fig. 28). These will be described more exactly in Part II. 
 
 Twins of the Triclinic System. The triclinic rock-forming 
 minerals, especially plagioclase and disthene, are quite com- 
 monly polysynthetically twinned ; i.e., several parallel twin- 
 lamellae are interpolated in the crystal. Such twins can be 
 recognized easily also in parallel polarized light, in that the 
 separate twinning-lamellae appear with varying polarization- 
 colors, and the directions of extinction do not have the same 
 position in two adjoining lamellae. 
 
 In plagioclase the " Albite law" is the most common : 
 twinning-plane co^oo (Fig. 29). Sections at right angles to this 
 plane from the zone oP : ooPoo will always develop in parallel 
 polarized light the polysynthetic twinning-striations. Such 
 
 FIG. 29. POLYSYNTHETIC PLAGIOCLASE TWIN. 
 
 || oo/>oo. 
 
 FIG. 30. PLAGIOCLASE TWINNED 
 according to the Albite and Pericline laws. 
 
 twins were not possible in the monoclinic system, as the plane 
 in the monoclinic system oo;Poo corresponding to the plane 
 oc/>oo is at the same time a plane of symmetry, and such a 
 symmetrical development gives no twins. Such polysynthetic 
 twins are wanting in orthoclase. It is easy, therefore, to dis- 
 tinguish orthoclase from plagioclase, although it is not impos- 
 sible for the latter also to occur as a simple twin. 
 
 A second less common twinning-law of plagioclase, appear- 
 ing also combined with the Albite law, is the " Pericline law:" 
 twinning-plane at right angles to the zone oP : oo Poo, developed 
 after a plane which, with the prismatic faces, gives a rhombic 
 section. 
 
44 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 The twinned developments of plagioclase also will be dis- 
 cussed again at the proper point in Part II. If the Albite and 
 Pericline laws are combined (Fig. 30), one will observe in sec- 
 tions cut approximately parallel to coPoo a double system of 
 twinning-striations cutting each other at nearly right angles. 
 
 Disthene occurs, though in rocks more rarely, as t.wins, 
 according to the following laws : 
 I. Twinning-plane oo Pec. 
 II. " at right angles to the <:'-axis. 
 
 III. " " " " -axis. 
 
 IV. " parallel oP. This form of twinning is 
 often repeated also, and is commonly observed in the disthene 
 occurring in rocks. 
 
 And, finally, it may be mentioned that two twins after one 
 definite law can combine according to another law; e.g., as 
 often occurs in plagioclase, where two plagioclase species 
 twinned after the Albite law (twinning-plane oo/>oo) are united 
 with each other according to the so-called Carlsbad law, com- 
 mon on orthoclase (twinning-plane 
 
 4. DETERMINATION OF THE INDEX OF REFRACTION. 
 
 H. CLIFTON SORBY. On a new method for determining the index of double- 
 
 refraction in thin sections of mineral substances. Miner. Mag. 1877, 
 
 No. 6. 
 H. CLIFTON SORBY. Determination of minerals in thin sections by means of 
 
 their refractive indices. Miner. Mag. 1878, No. 8. 
 J. THOULET. Contributions a 1'etude des proprietes phys. et chim. des miner. 
 
 microsc. Bull. Soc. miner. 1880, III, 62 et 1883, VI, 184. 
 MICHEL LEVY. Bull Soc. miner. 1883, VI, 143 et 1884, VII, 43. 
 
 One method of determining the index of refraction in mi- 
 croscopic mineral particles has been mentioned before, under 
 the description of the polarization-microscope (page 14). The 
 following method, applicable in many cases, was proposed first 
 by Thoulet : 
 
 Certain minerals, as olivine, different augites, titanite, etc., 
 
METHODS OF INVESTIGATION. 45 
 
 show on their sections a rough shagreenous surface, considered 
 as almost characteristic for olivine ; this is much plainer if the 
 rock-section is not covered with Canada balsam and a cover- 
 ing-glass. This phenomenon is a result of imperfect polishing 
 of the slide, i.e., of the particular mineral, and disappears on a 
 more perfect finishing. By immersion of the mineral showing 
 such a rough upper surface in different liquids whose index 
 of refraction is known, it is possible to determine the index of 
 refraction of the mineral, as the rough upper surface will dis- 
 appear as soon as a liquid is used whose index of refraction 
 equals or is very near that of the mineral. The liquid filling 
 the depressions in the mineral on immersion thus removes all 
 difference between depression and elevation in the mineral leaf- 
 lets. Examples of such liquids where n = index of refraction 
 are: 
 
 Water, n 1.34 
 
 Alcohol, n = 1.36 
 
 Glycerine, n 1.41. 
 
 Olive-oil, n 1.47 
 
 Beech-oil, n = 1.50. 
 
 Clove-oil, n 1.54 
 
 Cinnamon-oil, n = 1.58. 
 
 Bitter-almond-oil, n = 1.60. 
 
 Carbon disulphide, n = 1.63. 
 
 5. PLEOCHROISM OF DOUBLE-REFRACTING MINERALS. 
 Determination of the Axial Colors. 
 
 GROTH and ROSENBUSCH, 1. c. 
 
 TSCHERMAK, Sitzungsber. d. k. Akad. d. Wissensch., math.-naturw. Cl., Wien. 
 
 1869, 59. Bd., Mai-Heft. 
 LASPEYRES. Groth's Zeitscbr. f. Krystallographie. 1880, IV, p. 454. 
 
 We understand by pleochroism that property of double- 
 refracting minerals whereby the light penetrating in different 
 
46 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 directions shows different colors. Of course only such double- 
 refracting minerals as are colored could show the phenomenon, 
 as it depends upon the varying refraction and partial absorption 
 of the light penetrating in the different directions. Pleochro- 
 ism, or absorption, stands in closest relationship to double- 
 refraction. Optically-uniaxial colored minerals show differ- 
 ences of absorption in two directions ; the optically-biaxial in 
 three directions at right angles to each other and corresponding 
 to the different axes of elasticity. 
 
 Optieally-Uniaxial Minerals. On looking through such a 
 mineral in a direction at first parallel and then perpendicular 
 to the chief axis, a difference in color will be noted. The color 
 observed on looking through parallel to the chief axis is called 
 the " basal color" (Basis-farbe), the color at right angles to it 
 the "axial color" (A. r en-far be). If a transverse section of such 
 a mineral be examined in a polarization-microscope, with only 
 the polarizer in position, the single nicol thus performing the 
 duty of a dichroscope, and the section be turned through one 
 complete revolution, no difference in color is noticed, as only 
 rays vibrating at right angles to the optic axis are in the field, 
 and there is no double-refraction in the direction parallel to 
 the chief axis. If, on the other hand, a longitudinal section be 
 placed in the microscope, a change of color is noticed on re- 
 volving the stage. The greatest difference in absorption is 
 noticed first when the chief axis is parallel to the nicol chief 
 section, and secondly when it is at right angles to it. Thus, 
 e.g., tourmaline (Fig. 31; xy denoting the optical chief section 
 of the polarizer, c the chief axis, a and c the two axes of elas- 
 ticity), which, as is well known, absorbs the ordinary ray much 
 more powerfully than the extraordinary, appears nearly black 
 when its r-axis is at right angles to the shorter diagonal of the 
 polarizer, but shows a light gray or blue -color when the ^-axis 
 is parallel to the nicol chief section. Consequently the or- 
 dinary ray (p) vibrating at right angles to the chief axis is 
 
METHODS OF INVESTIGATION. 
 
 47 
 
 transmitted with darker colors; the extraordinary ray (e) vi- 
 brating parallel to the chief axis with a lighter gray or bluer 
 color. As the double-refraction of tourmaline is negative, 
 therefore a = c and c _L c. We can thus express the axial 
 colors : a = light gray (t), c = black (o). And in general, in 
 minerals with negative double-refraction the ordinary ray is 
 more strongly absorbed; in those with positive double-refraction, 
 
 FIG. 31. DICHROISM. (Tourmaline. Section I! c-axis.) 
 (After Fouque.) 
 
 the extraordinary ray. The color with which o is transmitted 
 corresponds with the basal color, while the axial color is com- 
 pounded from both of the colors for o and f. In order to 
 observe the colors of the faces, the under nicol, the polarizer, 
 is of course also removed, and the investigation carried on by 
 ordinary light. 
 
 Optically-Biaxial Minerals. The differences of absorption are 
 developed in optically-biaxial minerals in three directions at 
 right angles to each other and coinciding with the three axes 
 of elasticity for the most part. We discriminate here, there- 
 fore, between three face-colors and three axial colors. The 
 three axial colors corresponding to the axes of elasticity are 
 designated by a, b, and C ; each face-color is composed of two 
 axial colors. If one looks through a cordierite crystal, e.g., 
 through the plane oP, i.e., in the direction of the vertical axis, 
 here coinciding with the axis of elasticity a, it appears blue ; 
 that is to say, the face-color A which is composed of the axial 
 
4 8 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 colors b and c is blue; parallel oo^oo (c) the face-color C is 
 yellowish white from the composition of a and b ; and parallel 
 to oo Pvz (b) the face-color. is a bluish white from composition 
 of the axial colors a and c. On the other hand, the axial colors 
 for cordierite are: a, yellowish white; b, light Prussian blue; c, 
 dark Prussian blue. The determination is effected in the fol- 
 lowing manner : If a cross-section of a crystal whose optical 
 orientation is known be selected, e.g., a section of hypersthene 
 (Fig. 32) at right angles to the <r'-axis (parallel oP), the axial colors 
 b and c can be determined in it with the polarizer; on turning 
 the stage, first the brachy-axis (a = a) and then the macro-axis 
 
 FIG. 32. TRICHROISM. (Hypersthene. Sections -L<r'-axis and || oo/oo.) 
 
 (b = b) is parallel to the nicol chief section, and above the 
 polarizer. Another cross-section of mineral is needed in order 
 to determine the axial color for c. This section in the case 
 cited can be either parallel ooPoo or oo/^oo. Parallel to oo^oo 
 two axial colors, a and c, can again be determined ; the axial 
 color c is observed so soon as the vertical axis (c r = c) coincides 
 with a nicol chief section, that for a so soon as the brachy-axis 
 coincides. The axial color a, therefore, was determined twice, 
 and must correspond in both cases if the sections were of equal 
 thickness. 
 
 If pleochroitic minerals are examined in extremely thin 
 sections, as is always the case in rock thin sections, the differ- 
 ences of absorption are often imperceptible. This is true of 
 cordierite and andalusite, while tourmaline, e.g., shows the 
 
METHODS OF INVESTIGATION. 49 
 
 most marked dichroism even in the thinnest needles. For this 
 reason it is advisable to prepare a somewhat thicker section 
 from the rock under examination for the investigation of the 
 optical properties of the larger mineral constituents. 
 
 The power of absorption in different directions in any mineral 
 is represented by an > or < annexed to the axes of elasticity ; 
 in tourmaline, e.g., o > or C > a, i.e., the ordinary ray is more 
 powerfully absorbed than the extraordinary. In cordierite 
 C > b > d, or, as the axes of elasticity in the rhombic minerals 
 coincide with the crystallographic axes, in these according to 
 the optical orientation corresponding b > a > I ; i.e., the absorp- 
 tion in cordierite is greatest- in the direction of the macro-axis. 
 
 In tetragonal and hexagonal minerals the directions in which 
 the greatest color-difference can be recognized Laspeyres 
 calls them " axes of absorption" coincide with the two axes 
 of elasticity, i.e., are parallel and at right angles to the chief 
 axis ; in the rhombic, with the three axes of elasticity, i.e., the 
 crystallographic axes ; in the monoclinic and triclinic minerals, 
 however, according to the latest investigations of Laspeyres, 
 the three axes of absorption do not coincide with axes of elas- 
 ticity, but yet are at right angles to each other. 
 
 In. the monoclinic minerals there appears to be but one 
 coincidence of an absorption-axis, and that with the ortho- 
 diagonal'; while each of the others, lying in the plane of sym- 
 metry, forms an angle with the axes of elasticity. Colorless 
 double-refracting minerals, e.g., apatite, often show pleochroism 
 as a consequence of the regular inclosures of colored particles 
 or other mineral fragments. 
 
 And, finally, it may be mentioned that the axial colors of 
 pleochroitic minerals do not remain constant, in that often on 
 cross-sections of one and the same mineral now C > a > b and 
 now c > b > d, and so on, are observed ; or one and the same 
 mineral can be now feebly and now powerfully pleochroitic. 
 Nevertheless pleochroism is a characteristic for certain min- 
 
5O DETERMINATION OF ROCK-FORMING MINERALS. 
 
 erals, as andalusite, cordierite, tourmaline, hypersthene, horn- 
 blende, biotite, and others, and thus lends its aid to their 
 determination. 
 
 B. Chemical Methods of Investigation. 
 
 The chemical examination of rocks should go hand in 
 hand with the microscopical investigation ; a quantitative 
 analysis will always give a welcome explanation of the min- 
 eralogical composition, or at least will confirm the microscopi- 
 cal examination to a greater or lesser degree. It is, however, 
 impossible to determine the component minerals or to give 
 their individual- chemical composition from such a rock-analy- 
 sis alone. In order that the rock-forming minerals may be 
 separately analyzed and their chemical composition correctly 
 determined it is necessary to separate them from each other. 
 Such a mechanical separation can be simply effected with a 
 needle beneath a microscope, when only a small fragment may 
 be needed for a qualitative chemical test of the minerals ; or it 
 may be effected by solutions of high specific gravity, thus tak- 
 ing advantage of the differing specific gravities of the minerals 
 for obtaining larger quantities of mineral for the quantitative 
 chemical investigation. There is also another advantage in this 
 latter method, as the specific gravities of the separate mineral 
 components are thus known. 
 
 If the rock under examination is coarsely granular, the sep- 
 arate components often can be distinguished with the naked 
 eye and the different cleavage-leaflets be examined optically 
 as well as by chemical qualitative and quantitative analysis. 
 In this separation it is, e.g., impossible to separate several feld- 
 spars from each other in case they occur in the same rock. Such 
 a separation of the components is also impossible in the fine- 
 grained rocks. In order to examine chemically the rock-con- 
 stituents in such cases and thus obtain a clue in the determina- 
 
METHODS OF INVESTIGATION. 51 
 
 tion, the microchemical reactions are applied ; the component 
 under examination beneath the microscope is dissolved either 
 directly on the rock-section or on detached granules, and 
 treated with such reagents as give exceptionally characteristic 
 precipitates. Sometimes a more exact and careful mechanical 
 separation of the mineral components is attempted by treat- 
 ing the powdered rocks with solutions of high specific gravity. 
 A partial analysis of the portion of rock soluble or insoluble 
 in hydrochloric acid in many cases gives valuable conclusions 
 and simplifies the determination of the constituents. 
 
 MICROCHEMICAL METHODS. 
 
 H. ROSENBUSCH, 1. c., p. ioy, und N. Jahrb. fiir Min. und Geol. 1871, p. 914. 
 
 F. ZIRKEL. Basaltgesteine und Lehrb. d. Petrographie. 
 
 A. STRENG. Ueber die mikroskopische Unterscheidung von Nephelin und 
 
 Apatit. Tschermak's Miner. Mitth. 1876, p. 167. 
 E. BOKICKY. Elemente einer neuen chemisch-mikroskopischen Mineral- und 
 
 Gesteinsanalyse. Archiv d. naturvv. Landesdurchforsch. Bohmens. 
 
 III. Bd., V. Abthlg., Prag, 1877. 
 SZAB6. Ueber eine neue Methode, die Feldspathe auch in Gesteinen zu 
 
 bestimmen. Budapest, 1876. 
 TH. H. BEHRENS. Mikrochemische Methoden zur Mineralanalyse. Verslagen 
 
 en Mededeelingen der k. ^Academic v. Wetenschappen. Amsterdam, 
 
 1881. Afdeeling Natuurkunde. 2. Reeks, XVII. Deel. p. 27 73. 
 A. STRENG. XXII. Ber. der oberhesisschen Ges. f. Nat. u. Heilkunde. 1883, 
 
 p. 258 u. 260. 
 
 E. BORIC,KY. N. Jahrb. i. Min. u. Geol. 1879. P- 5 6 4- 
 MICHEL LEVY et L. BOURGEOIS. Compt. rendus 1882, 20 mars, and Bull. Soc. 
 
 miner. 1882, V. p. 136 (Reaction auf Zirkonerde). 
 SCHONN. Zeitscbr. fur analyt. Chemie. 1870. IX. p. 41 (Reaction auf Titan- 
 
 saure). 
 A. KNOP. N. Jahr. f. Min. u. Geol. 1875, p. 74. 
 
 Hydrochloric acid has been applied for a long period as a 
 microscopical reagent in investigations of rocks. Zirkel (comp. 
 Petrogaphie, II. p. 293, 1870) applied it most advantageously 
 in discriminating between the varieties of plagioclase allied to 
 anorthite and those related to albite, and between magnetite 
 
52 DETERMINATION OF ROCK-FORMING MINERALS 
 
 and ilmenite. The application of hydrochloric acid for the 
 determination of calcite in rocks has been known for a much 
 longer period; also in the recognition of silicates soluble in 
 this acid, as nepheline, members of the meionite group, etc. 
 Thus Roth (1865) rightly conjectured the presence of melilite 
 in the basaltic lavas from Eifel because of the large amount 
 of calcium dissolved in the acid. 
 
 In such a testing of the rock-constituents regard is had first 
 of all for the solubility, and secondly for the products of the 
 decomposition effected by the acid ; as the evolution of CO 2 
 from calcite, the deposition of the NaCl-cubes on evaporating 
 a drop of the test for nepheline, the appearance of the gelatin- 
 ous SiO 2 on treating olivine with hydrochloric acid, etc. 
 
 In such examinations the testing is made with powdered 
 rock by examining microscopically the rock-section or powder 
 both before treatment with acid and also afterward if a residue 
 remains. In the second case the testing is undertaken directly 
 on the slide without a glass cover. There are great evils in 
 either case ; in the one in that it is difficult to recognize the 
 minerals in powdered condition and thus determine what has 
 been dissolved away, and in the other in that in treating the 
 section with acids the whole section crumbles away and is 
 destroyed. 
 
 A. Streng has recommended a method of isolating the 
 minerals of a thin section for microchemical study which is to 
 be recommended in many cases. If a mineral granule in a thin 
 section is treated with acid, it is almost always unavoidable 
 that the drop of solvent may touch also the other neighboring 
 particles, react on them, and thus render the chemical reactions 
 questionable. This evil can be remedied by first covering the 
 section with a perforated covering-glass which is coated on the 
 under side with fluid boiled Canada balsam, so that the open- 
 ing of about ^-i millimetre in diameter is opposite the mineral 
 particle to be tested. The Canada balsam filling the opening 
 
METHODS OF INVESTIGATION. 53 
 
 may be easily removed by alcohol. Such perforated covering- 
 glasses can be easily prepared by treatment with hydrofluoric 
 acid. An ordinary covering-glass is first dipped in melted wax 
 and allowed to cool; a hole \-\ mm. diameter is then made 
 through the wax, and concentrated hydrofluoric acid dropped 
 on the bared opening until a hole is eaten through the glass at 
 this point. The wax is then removed from the covering-glass. 
 
 The reaction for distinguishing between nepheline and 
 apatite, first proposed by Streng (1876), deserves special men- 
 tion as one nearly always accomplishing its purpose. Both 
 minerals occur in rocks very commonly, and are remarkably 
 similar hexagonal (coP.oP.P), optically negative, and color- 
 less. 
 
 The microchemical reactions for apatite are : 
 
 (a) Reaction for phosphoric acid. A drop of concentrated 
 nitric-acid solution of ammonium molybdate is transferred with 
 a glass rod to the apatite crystal lying exposed, i.e., not covered 
 by other minerals of the section ; the whole of the thin section 
 within the field of the microscope not protected by glass is 
 thus covered. A muscovite or glass leaflet is often cemented 
 with glycerine to the objective to protect the lens, which in 
 such experiments is easily attacked by the acid vapors. The 
 apatite dissolves slowly in the nitric acid of the reagent, form- 
 ing beautiful yellow grains and small octahedra of the ammo- 
 nium phospho-molybdate (ioMoO 3 + PO 4 (NH 4 ) 3 + iJH 2 O). 
 These yellow crystals are wreathed about the apatite and not in 
 the former position of the apatite crystal, as here the excess of 
 phosphoric acid prevents the formation of a precipitate. 
 
 (b) Reaction for lime. A crystal of apatite in the thin 
 section is dissolved in hydrochloric or nitric acid and a drop 
 of sulphuric acid added : fine white feathery aggregates of 
 gypsum are formed round about the point previously occupied 
 by the apatite. If a crystal of apatite is treated with sulphuric 
 acid alone it is not dissolved, as a thin coating of gypsum is 
 
54 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 formed which prevents the further action of the acid on the 
 apatite. 
 
 The reaction of Streng for phosphoric acid is the surest and 
 most exact if it is carried out, not on the thin section directly, 
 but on an isolated granule : or if the thin section be treated 
 with dilute nitric acid, the solution taken up with a capillary 
 pipette, evaporated, again dissolved in dilute nitric acid, and 
 the reaction completed on an ordinary object-glass. 
 
 Nepheline can be recognized from the negative results to 
 the reactions given above for apatite, as well as by a reaction 
 with hydrochloric acid ; if a drop of the acid be deposited on 
 a crystal under examination it is easily decomposed, i.e., dis- 
 solved. After some time numbers of minute colorless cubes 
 of sodium chloride, easily recognized, are formed in the cavity 
 formerly occupied by the crystal. They are formed by the 
 action of the hydrochloric acid on sodium silicate, and are 
 difficultly soluble in the concentrated acid. 
 
 A. Streng has recently found acetate of uranium to be an 
 excellent reagent for sodium. If a drop of concentrated solu- 
 tion of acetate of uranium be added to the residue from the 
 solution of a silicate in hydrochloric acid, clearly defined, bright 
 
 (O O O \ 
 
 yellow tetrahedra I . or . oo O) of sodium uranate, 
 
 7 \2 2 2 / 
 
 difficultly soluble in water, are formed. More rarely penetra- 
 tion-twins after a tetrahedral face occur, and in polarized light 
 can be easily distinguished from the double-refracting, rhom- 
 bic, nearly cubical crystals of the acetate of uranium. 
 
 A. Knop has recommended a reaction for the recognition 
 of members of the hauyn group, which when colorless are 
 difficultly distinguishable from apatite or nepheline sections. 
 The thin section of the rock bearing the hauyn is carefully 
 loosened from the object-glass by warming, and is washed clean 
 with alcohol. The clean section is introduced into a platinum 
 crucible, and as much flowers of sulphur as can be taken up on 
 
METHODS OF INVESTIGATION. 55 
 
 the point of a knife added. If now the crucible is heated to 
 glowing for some minutes, whereby the sulphur vaporizes and 
 fills the crucible, and then, still covered, is allowed to cool, all 
 ferrous compounds appear blackened, while the hauyn is con- 
 spicuous among the rock-components by the beautiful azure- 
 blue color. The other rock-forming minerals do not become 
 blue on heating in sulphur-vapor. Knop does not state, how- 
 ever, whether sodalite, like hauyn, becomes blue. 
 
 These few characteristic microreactions have reference, 
 however, to an extremely limited number of the rock-forming 
 minerals nepheline, apatite, and hauyn. The necessity for a 
 method of complete microchemical qualitative analysis of 
 the rock-constituents has been remedied by Boricky and 
 Behrens. 
 
 Boricky s Microchemical Method. 
 
 Chemically pure hydrofluosilicic acid is the only reagent 
 required. It should contain 13 per cent acid, and must be 
 absolutely pure ; i.e., when allowed to dry on a layer of balsam 
 on an object-glass it must leave no residue of silico-fluoride 
 crystals. It cannot, therefore, be prepared or stored in glass 
 bottles. Almost all of the rock-forming minerals are attacked 
 more or less by strong hydrofluosilicic acid. It is therefore 
 available for the formation of the silico-fluorides, which dis- 
 solve in the solution of hydrofluosilicic acid, and after evapo- 
 ration of this solution appear as beautifully-developed crystals, 
 characteristic for the different elements or groups of elements. 
 
 The microchemical tests with this acid can be carried out 
 either directly on the rock-section without a glass cover, or, 
 better yet, on minute particles of the minerals of about the size 
 of a pin's head, on an object-glass coated with Canada balsam. 
 One or two drops of the hydrofluosilicic acid are transferred 
 with a caoutchouc rod to the mineral granule under examina- 
 
56 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 tion, and the preparation is allowed to rest quietly in a place 
 free from dust, preferably at a temperature of about 18 C., 
 until the drop has dried away. 
 
 If the mineral is easily attacked by the acid, all of the 
 metals are generally found after evaporating the solution in 
 their several peculiar crystalline forms, and in about the same 
 proportion as in the mineral. If the mineral is but slightly 
 attacked, only those metals most easily soluble can be proven, 
 and the same mineral fragment must be treated again with the 
 acid ; in the latter case it is often of advantage to treat, in a 
 small platinum dish, first with hydrofluoric acid and then with 
 hydrofluosilicic acid, evaporate to dryness, redissolve in water, 
 and allow a drop to evaporate on an object-glass. 
 
 Thin sections are more easily attacked than granules or 
 cleavage-pieces, and must be exceedingly thin. It is better if 
 the test is taken from carefully-selected mineral particles, as 
 sections become coated with a dull white crust. The silico- 
 fluorides crystallize most perfectly when lixiviated with boiling 
 water, and the solution 'allowed to cool on another object- 
 glass. The silico-fluorides are always in minute crystals, and 
 are best observed under 200-400 diameters. They are distin- 
 guished by their crystalline forms, and there appears : 
 
 1. Potassium Silico-fluoride in skeleton groups of small crys- 
 tals of the regular system clearly defined, generally oo O oo, also 
 often with O and oo O. Yet potassium silico-fluoride often 
 crystallizes in larger, apparently rhombic crystals of the form 
 oo Pn . mP oo, if the acid was in excess or the evaporation 
 occurred at lower temperatures (12 C.), or in presence of a 
 large amount of sodium. 
 
 2. Sodium Silico-fluoride (Fig. 33) in short hexagonal columns 
 with oP. P, also oo Pz ; imperfect crystals are barrel-shaped. 
 The more calcium silico-fluoride present the larger the crystals. 
 Easily soluble in water. 
 
 3. Calcium Silico-fluoride (Fig. 34) in peculiar, long, pointed, 
 
METHODS OF INVESTIGATION. 
 
 57 
 
 spindle-shaped crystals, often grouped in rosettes; the combina- 
 tion of parallel straight lines and planes is characteristic for 
 
 FIG. 33. SODIUM SILICO-FLUORIDE. 
 (After Bofrcky.) 
 
 FIG. 34. CALCIUM SILICO-FLUORIDE. 
 . (After Bofrcky.) 
 
 this compound. It crystallizes in monoclinic crystals, and is 
 easily soluble in water. 
 
 4. Magnesium Silico-fluoride (Fig. 35) appears in rhombohe- 
 dra with polar edges truncated by 
 
 oR and combinations of R . oo/^ or 
 R . oo P^ . oR ; all of the crystals have 
 well-defined edges and faces. It 
 often appears also in rhombohedra 
 elongated in one direction, or in 
 cruciform, hook-shaped, or feathery 
 figures. It is easily soluble in water. 
 
 5. Iron Silico-fluoride cannot be 
 distinguished from magnesium sili- 
 co-fluoride; the same with manga- 
 nese silico-fluoride ; while strontium 
 
 silico-fluoride can scarcely be distinguished from calcium silico- 
 fluoride. 
 
 Lithium Silico-fluoride appears generally in regular flat hexag- 
 onal pyramids, where one pair of faces is sometimes remarkably 
 
 FIG. 35. MAGNF.SIUM STLICO- 
 
 FLUORIDE. 
 (After Boricky.) 
 
58 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 developed ; barium silico-fluoride in extremely minute, short, 
 pointed needles. 
 
 Distinction between the Silico-fluorides of Calcium and Stron- 
 tium. If a drop of sulphuric acid diluted with an equal bulk 
 of water is added to the silico-fluorides, the crystals of calcium 
 are immediately surrounded with a thick fringe of mono- 
 clinic gypsum crystals, while those of strontium change but 
 slowly. 
 
 Distinction between the Silico-fluorides of Iron, Manganese, 
 and Magnesia. These can be distinguished either by subject- 
 ing to the action of chlorine gas for about twenty minutes, 
 when the magnesium silico-fluoride remains colorless, the iron 
 becomes yellow and the manganese red ; or these silico- 
 fluorides can be distinguished by the reaction with ammonium 
 sulphide, when the silico-fluoride of magnesium remains color- 
 less, while the iron is blackened and the manganese becomes 
 reddish-gray and granular. 
 
 The fluorides of Fe, Mn, Co, Ni, and Cu can be distinguished 
 also by their reaction with potassium ferrocyanide. If this 
 solution be dropped on the silico-fluorides the corresponding 
 ferrocyanides will be formed, which can be recognized from 
 the characteristic color : Fe is blue, Mn brown, Cu red, Co dark 
 green, and Ni light green. 
 
 This method has many disadvantages ; e.g., it is impossible 
 to prove by it the presence of alumina ; the distinction be- 
 tween the silico-fluorides of iron and magnesium is difficult 
 and detailed; the calcium silico-fluoride crystals are also in- 
 sufficiently characteristic. Nevertheless it is advantageously 
 employed, especially in testing for the alkalies. 
 
 Th. A. Behrens has proposed another complete system of 
 microchemical methods for use in petrography. In this 
 method also a series of new and admirable microreactions are 
 introduced. If a combination of these two methods that of 
 Boricky for the determination of the alkalies, and of Behrens 
 
METHODS OF INVESTIGATION. 59 
 
 be effected, a complete qualitative analysis in many cases 
 can be carried out with the microscope. In this latter method, 
 however, the operation cannot be carried out on the rock- 
 section itself. 
 
 Behrenss Micrcchemical Method. 
 
 Preparation of the Mineral. The minerals to be examined 
 must always be separated from the mass of the rock. In the 
 coarse-grained rocks this is easily done by picking out the 
 pieces from the coarse rock-powder either under the micro- 
 scope or with a pocket-lens. In the fine-grained rocks, where 
 the rock-constituents can no longer be distinguished in the 
 powder, the mineral particle is removed from the slide by 
 aid of the microscope and a lance-shaped needle ; the section 
 is ground until the desired mineral granule is transparent 
 and partly polished. The isolation of the desired mineral is 
 effected by gradually breaking away the section from the edge. 
 The isolation of the mineral is lightened if the object-glass is 
 first warmed, and the Canada balsam under the rock-leaflet 
 thus.softened. The isolated mineral particle, of at least 0.3 
 mm. diameter and o.i mg. in weight, is cleaned and pulverized 
 in an agate mortar beneath a piece of filter-paper to prevent 
 loss. 
 
 The Testing. The tests are made in a hemispherical plati- 
 num dish about I cm. in diameter, closed by a concave platinum 
 cover ; the reagent employed is chemically pure hydrofluoric 
 acid, or ammonium fluoride, or concentrated hydrochloric acid. 
 Two or three drops of either acid are transferred to the small 
 dish, and the mineral, finely powdered, added. The mixture 
 is heated, and, if necessary, hydrofluoric acid added a second 
 time, and the evaporation repeated. The dried fluorides are 
 then evaporated with concentrated sulphuric acid until volu- 
 
60 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 minous clouds of the gray acid-vapors appear. The sulphuric 
 acid, however, must not be completely volatilized ; it is advis- 
 able, therefore, to repeat the evaporation with a drop of sul- 
 phuric acid. The decomposed mass is then dissolved in water, 
 the platinum capsule being about half filled, and the contents 
 evaporated by gentle heat until each centigram of solution 
 contains about o.i mg. substance. 
 
 A drop of this solution is transferred by a capillary pipette 
 to a slide without a covering-glass to facilitate evaporation, 
 and is placed beneath the microscope. Two hundred diameters 
 is the best magnifying power. The objective here also 
 must be protected by a leaf of muscovite 'cemented with 
 glycerine. 
 
 This drop is examined first for 
 
 Calcium. If the mineral was calciferous, free crystals of 
 gypsum (Fig. 36) separate on evaporation ; the columns are 
 
 thin, of oo P. oo jPoo . P, generally lying 
 on ooPco or arranged in rosettes. 
 Often larger crystals of the well- 
 known swalfow-tail twins are discern- 
 ible in the outer edge of the drop. 
 The presence of 0.0005 m g- CaO 
 can be demonstrated by this reaction. 
 If a smaller amount of lime is present, 
 or the gypsum separates too slowly, 
 the slide with the drop is moistened 
 
 FIG. 36. GYPSUM. 111 rr>i 
 
 (After Behrens.) with alcohol. The crystals then 
 
 formed are, however, smaller and less distinct, but the sensi- 
 tiveness of the reaction is quadrupled. 
 The same drop is searched for 
 
 Potassium. A drop of concentrated platinum chloride is 
 added by means of a platinum wire to the drop to be tested. 
 Crystals of the double . chloride of platinum and potassium 
 ig- 37> 0) are formed within a few minutes, and generally on 
 
METHODS OF INVESTIGATION. 6 1 
 
 the outer edge of the drop. They are sharply-defined octa- 
 
 hedra of high refractive power and of a bright yellow color. 
 
 If a concentrated solution was employed, clover-leaved trillings 
 
 and fourlings also appear. The 
 
 crystals are formed more rapidly 
 
 in chloride solution than in sulphate 
 
 solution, and are smaller. A large 
 
 excess of sulphuric acid prevents 
 
 their formation. 0.0006 mg. K 2 O 
 
 .can be demonstrated by this re- 
 
 action. 
 
 Sodium is proved with cerium 
 sulphate. A drop of the concen- 
 trated solution of this reagent, and . POTASBTW-TIIWM CHLORIDE. 
 
 .1 j f ,, i , . f b. POTASSIUM FLUOBOKATE. 
 
 another drop of the solution from (After 
 
 the decomposed mineral, are placed on a slide about 5 mm. 
 apart, and joined by a thread of glass. Tufts of cerium 
 sulphate appear in the drop of reagent, and on the edge an 
 opaque brown zone of the sodium double-salt, which permeates 
 the whole drop if the percentage of sodium is large ; with 
 600 diameters this zone is shown to be composed of minute 
 white transparent granules. If the mineral contains potassium 
 also,jBi coarsely granular gray zone of the potassium double-salt 
 is formed in the centre of the drop, which is made up from 
 granules and fragments similar to potato-starch. In lower 
 percentages of the alkali metals in the mineral the phenomena 
 are more easily observed. Lumps and short rhombs of the 
 potassium double-salt, and acute prisms and spindle-formed 
 crystals of the sodium double-salt, appear. A large excess of 
 sulphuric acid retards the reaction. 
 
 This reaction can be first applied for both alkali-metals, 
 and then that with platinum chloride for potassium on the 
 same slide, and finally the test for sodium with liydrofluosilicic 
 acid after the slide has been prepared with balsam. At any 
 
62 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 rate the Boricky test for sodium is to be preferred, as also the 
 test for potassium with platinum chloride. 
 
 Magnesium is shown by hydrogen-sodium-ammonium phos- 
 phate (microcosmic salt). The drop already searched for Al 
 or K is saturated with ammonia, a drop of water placed at 
 a distance of about I cm., a grain of microcosmic salt dissolved 
 in it, and the two drops connected by a thread of glass. There 
 are immediately formed either double, forked crystalloids 
 similar to the microlites in the natural glasses, or, if the 
 solution is quite dilute, well-defined twins of hemimorphous 
 crystals of ammonium-magnesium phosphate (Fig. 38). The 
 reaction for magnesium often does not appear or is ill-defined, 
 
 FIG. 38. AMMONIUM-*AGNESIUM PHOSPHATE. 
 (After Behrens.) 
 
 FIG. 39. CAESIUM ALUM. 
 (After Behrens.) 
 
 owing to an insufficiency of ammonium salts ; it is advisable, 
 therefore, to add a little hydrochloric acid or ammonium 
 chloride before saturation with ammonia, o.ooi ing. MgO can 
 be proved by this reaction. 
 
 Behrens found caesium chloride an excellent reagent for 
 Aluminium. A minute portion of the deliquescent salt is placed 
 with the point of a platinum wire on the edge of the test-drop. 
 Large transparent octahedra(more rarely oo <9co . (?) of caesium 
 alum are immediately formed (Fig. 39). If the solution of the 
 
METHODS OF INVESTIGATION. 63 
 
 mineral is concentrated, only a dendritic mass is formed, 
 and a small drop of water must be placed beside that of the 
 reagent. A large amount of sulphuric acid interferes with the 
 formation of the alum crystals, o.oi mg. A1 2 O 3 can be clearly 
 proved by this reaction. 
 
 Iron is rarely searched for with the microscope. The color 
 of the flocculent, fine-grained precipitate obtained with potas- 
 sium ferrocyanide from iron solutions is sufficiently character- 
 istic and intense when examined macroscopically. 
 
 Manganese is proved by fusing with soda. The character- 
 istic green color is obtained with the smallest amount. A 
 microscopical examination is therefore superfluous. 
 
 Lithium is precipitated by an alkaline carbonate from the 
 solution in sulphuric acid, and gives well-developed monoclinic 
 crystals of lithium carbonate with rectangular cross-section. 
 These crystals can be distinguished from gypsum by their rect- 
 angular form and solubility in dilute sulphuric acid ; from 
 magnesium double-salt by the property that they are formed 
 in every proportion of potassium carbonate and lithium sul- 
 phate, and remain constant ; while the crystals of magnesium 
 double-salt are formed only in large excess of the alkaline car- 
 bonates and in close proximity, and soon become granular. 
 Phosphoric acid seriously retards the formation of the lithium- 
 carbonate crystals. 
 
 Barium and Strontium. These are found, together with cal- 
 cium and gypsum, in the residue after lixiviation of the 
 mass in the platinum capsule with water. This residue is dis- 
 solved in hot concentrated sulphuric acid, is allowed to cool, 
 and is extracted with water. From a drop there is first a 
 separation of barium sulphate in small lenticular, crossed 
 crystals ; then of strontium sulphate, at first in matted tufts 
 and fine needles, then in larger, often rhombic, cruciform, 
 twinned crystals ; last of all gypsum separates. 
 
 Metalloids. Of the remaining reactions proposed by Behrens 
 
64 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 for use with the rock-forming minerals, the following are of 
 importance : 
 
 Chlorine. The mineral granule to be examined for chlorine 
 is fused with soda and decomposed ; a large quantity of con- 
 centrated sulphuric acid is added to the fused mass in the 
 platinum capsule, and the escaping hydrochloric-acid gas is 
 absorbed by a drop of water adhering to the under side of a 
 glass covering the capsule. This cover is kept cool by a few 
 drops of water on the upper surface. At the close of the pro- 
 cess the water is wiped from the upper surface with filter-paper, 
 and the covering-glass inverted and placed on the stage of the 
 microscope. A granule of thallium sulphate is then laid in the 
 centre of the drop of adhering water. Colorless octahedra as 
 well as O . oo O of thallium chloride are rapidly formed. These 
 refract light powerfully, and are often combined into clover-leaf 
 trillings and f curlings. 0.004 m g- NaCl can be thus proved. 
 
 Phosphorus and Sulphur can be proved by reversing the 
 reactions already described for aluminium (for S) and for mag- 
 nesium (for P). Insoluble sulphates and phosphates must be 
 fused with soda, and the pulverized fused mass lixiviated with 
 water. For proving the presence of sulphur a drop of this 
 solution is placed near a drop of solution of aluminium 
 chloride and hydrochloric acid with a little caesium chloride. 
 The two drops are then united by a glass thread, when the 
 caesium-alum octahedra are developed, as before, near the ex- 
 tremity. A concentrated solution of ammonium chloride and 
 magnesium sulphate is used as a reagent for the detection of 
 phosphorus. 
 
 Fluorine. The mineral containing fluorine is dissolved in 
 concentrated sulphuric acid, and the escaping gas is absorbed by 
 dilute sulphuric acid. Such minerals as topaz or tourmaline must 
 be first fused with twice their volume of soda in order to change 
 the fluorine into hydrofluosilicic acid ; powdered sand is some- 
 times added. A drop of the sulphuric acid is placed on the 
 
METHODS OF INVESTIGATION. 65 
 
 convex surface of the platinum cover, and is then laid on the 
 platinum capsule with the drop downward,' the upper surface 
 being cooled as in the chlorine reaction. The capsule is gently 
 warmed, and at the close of the distillation the water used for 
 cooling is removed by filter-paper, and the drop of acid con- 
 taining the fluorine is transferred to a slide coated with Canada 
 balsam, or a leaflet of barite. (In order to avoid spurting it is 
 advisable to heat the test, first fused with soda, with acetic 
 acid, and evaporate before using the sulphuric acid.) A grain 
 of sodium chloride is then added to the transferred drop. At 
 first six-leaved rosettes, and later hexagonal tablets, oo P . oP, 
 and short columns, oo P . P, of sodium silico-fluoride will appear. 
 0.0036 mg. fluorine can be thus detected. 
 
 Silicon and Boron. Tneir determination is precisely the 
 same as fluorine, except that hydrofluoric acid must be used 
 with the sulphuric. If only one of the two elements is to be 
 proved, sodium chloride is again used as the reagent ; the hexag- 
 onal tablets already mentioned are again formed. If, however, 
 boron as well as silicon is to be detected, potassium chloride is 
 used. Potassium silico-fluoride crystallizes in the regular system, 
 as O and O . oo O oo ; while potassium boro-fluoride (Fig. 37, b) 
 appears in lance-shaped leaves and in rhombs with obtuse an- 
 gles, often replaced by edges. The silico-fluorides separate 
 first. If the mineral under examination is rich in silicon, the 
 greater part of the silicon must be removed before the pres- 
 ence of boron can be accurately proved. The mineral powder 
 mixed with hydrofluoric and sulphuric acids must be warmed 
 until the greater part of the silico-fluoride is driven out, which 
 is absorbed by the diluted sulphuric acid and tested for silicon 
 with sodium chloride. Hydrofluoric acid is again added to the 
 mineral test and again heated until the white fumes of sulphu- 
 ric acid appear. The distillate is warmed to about 120 C., and a 
 drop of water is added to the residue, which is transferred to a 
 slide and tested for boron with potassium chloride. The rhom- 
 
66 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 bic crystals of potassium boro-fluoride are formed only when 
 the drop has dried. 
 
 \Vater. The water-determination is carried out with mi- 
 nute mineral particles in the same way as in blowpipe analysis. 
 Behrens recommends the following small apparatus for this 
 purpose : A small tube about 10 mm. long and 3 mm. in diame- 
 ter is drawn out at one end to a thread about 2 cm. in length 
 and 0.5 mm. in diameter; after a gentle heating of the whole 
 tube and drawing through of air it is closed. While the tube 
 is yet warm, the mineral granule is introduced and the tube 
 drawn out to about half its length and melted at the other end 
 also, making it blunt. The capillary end is then cooled by 
 alcohol, or is heated to glowing if no deposition has taken 
 place. Such a deposition of water then generally occurs, which 
 collects in the capillary portion without artificial cooling. 
 
 By the application of the delicate method of Behrens we 
 are in position to determine immediately with ease and per- 
 fect accuracy those most important elements of the rock-form- 
 ing minerals, potassium, calcium, magnesium, and aluminium ; 
 the Boricky method appears to be more characteristic and ac- 
 curate for sodium. Rosenbusch recommends the flame-reac- 
 tion when the amount of sodium is very small. 
 
 C. Mechanical Separation of the Rock-forming 
 Minerals. 
 
 THOULET. Bull, de la Soc. mineralog. de France, 1879, II. p. 17 and 189. 
 FOUQUE ET MICHEL LEVY. Mineralogie micrographique, p. 114. 
 GOLDSCHMIDT. N. Jahrb. f. Mineralogie und Geologic, 1881, i. Beilagebd. 
 
 P- 179- 
 
 K. OEBBEKE. Ebenda, p. 454. 
 E. COHEN u. L. v. WERVEKE. N. Jahrb. f. min. u. Geol., 1883, II. Bd. p. 
 
 86-89. 
 D. KLEIN. Bull, de la Soc. miner, de France, Juin 1881, 4. p. 149, and Zeitschr. 
 
 f. Krystallographie und Mineralogie v. Groth, VI. 1882, p. 306, or N. 
 
 Jahrb. f. Min. u. Geol. 1882, II. Bd. Ref. p. 189. 
 
METHODS OF INVESTIGATION. 67 
 
 P. GISEVIUS. Beitrage z. Methode d. Bestimmung d. spec. Gew. v. Min. u. d. 
 
 mechanischen Trennung von Mineralgemengen. Inaug.-Diss. Univ. 
 
 Bonn, 1883. 
 C. ROHRBACH. N. Jahrb. f. Min. u. Geol. 1883, II. Bd. p. 186, and Wiede- 
 
 mann's Annalen f. Physik u. Chemie. 
 P. MANN. N. Jahrb. f. Min. u. Geol. 1884, II. p. 175. 
 
 In order to institute a quantitative chemical analysis of the 
 several rock-forming minerals, they must be separated as per- 
 fectly as possible from each other ; a partial separation of the 
 minerals, as already stated, is possible by treatment with dif- 
 ferent acids and with the magnet ; but the separation is best 
 effected by taking advantage of the relative specific gravities 
 of the minerals. Solutions of high specific gravities are best 
 adapted to this purpose, as by dilution of the solution it can 
 be lowered easily. This method of the mechanical separation 
 of the rock-constituents has the additional advantage that their 
 specific gravities can be exactly determined at the same time, 
 and thus a further vantage-ground for the determination of the 
 mineral be won. 
 
 The solutions at present known and universally applied to 
 the mechanical separation and determination of the specific 
 gravities are : 
 
 .1. The solution of iodides of potassium and mercury with 
 a highest specific gravity of 3.196 (Thoulet-Goldschmidt). 
 
 II.. 'The solution of cadmium boro-tungstate with a specific 
 gravity of 3.6 (Klein). 
 
 III. The solution of iodides of barium and mercury with a 
 specific gravity of 3.588 (Rohrbach). 
 
 I. SEPARATION WITH THE SOLUTION OF THE IODIDES OF 
 POTASSIUM AND MERCURY. 
 
 Preparation and Properties of the Solution. Potassium iodide 
 and mercuric iodide are weighed out in proportion of I : 1.239 ; 
 both portions are thrown into a large evaporating-dish, mixed, 
 
68 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 and dissolved in as little water as possible. The solution is 
 then evaporated on the water-bath until a piece of mineral, 
 tourmaline e.g., sp. gr. 3.1, floats upon it ; the dish is then re- 
 moved from the water-bath and allowed to cool, when the mass 
 thickens and the maximum of specific gravity is reached. Gen- 
 erally acicular crystals of a hydrous double iodide of potassium 
 and mercury separate from the concentrated solution during 
 the process of cooling ; this precipitate can be dissolved in a 
 few drops of water, or can be filtered off if there is an abun- 
 dance of the solution. The salt thus removed by filtration can 
 be redissolved in water and evaporated to the required specific 
 gravity. If too much potassium iodide was used, crystals of 
 the salt of the combination oo O co . O will separate on the sur- 
 face of the liquid ; if, on the other hand, there is an excess of 
 mercuric iodide, a thick felt of yellow needles is formed which 
 is decomposed on dissolving in water, with the deposition of a 
 red crystalline powder HgI 2 , but which dissolves in potassium- 
 iodide solution without decomposition. The concentrated so- 
 lution is often decomposed on adding water with deposition of 
 the red powder, which is, however, again redissolved on agitat- 
 ing the solution. The specific gravity of the solution changes 
 on long standing ; this depends on the temperature and moisture 
 of the atmosphere ; the solution is also decomposed by organic 
 substances, as filter-paper, etc. The highest attainable specific 
 gravity of the solution is 3.196 (Goldschmidt). 
 
 Determination of the Specific Gravity of Minerals and Rocks 
 by the Solution. The specific gravity of all those minerals un- 
 der 3.196 can be determined by means of this solution in the 
 following manner: The fragments of trie mineral or rock, 
 washed in pure water and dried, are thrown into a tall slim 
 beaker-glass filled with the solution at its maximum density ; 
 the liquid is then diluted with water, or diluted solution, until 
 the mineral is completely suspended in the solution, i.e. neither 
 sinks nor rises. The solution is then poured into a 25-cc. flask 
 
METHODS OF INVESTIGATION. 69 
 
 accurately calibrated, and filled exactly to the mark the mark 
 had best be on the under side of the meniscus. The excess of 
 liquid is removed either with a capillary pipette or filter-paper. 
 The filled flask is weighed and then emptied back into the 
 beaker-glass and the solution tested with the fragment of min- 
 eral ; the flask is refilled to the mark and weighed, and the 
 operation repeated for a third time. A mean is taken of these 
 three weighings. The weighings need not be perfectly exact 
 (i.e. to a few milligrams), varying often between 10 and 20 milli- 
 grams, as the error is lessened by the triple weighing. Deter- 
 minations of specific gravity by this method are carried with 
 accuracy to the third decimal place. E.g., quartz and a 25-cc. 
 flask gave : 
 
 First weighing 77.981 grams. 
 Second " = 77.919 " 
 Third -77-973 " 
 
 Mean = 77.957 " 
 Flask = 11.682 " 
 
 66.275 
 66.275 -T- 25 = 2.654 specific gravity. 
 
 Such determinations can be made much more rapidly and 
 as accurately, according to the principle of Mohr, on a balance 
 constructed by G. Wcstphal of Celle (price, 45 marks). By 
 this method the specific gravity is read directly on the balance- 
 beam after a single weighing and with weights in rider form. 
 
 It must be noted that specific-gravity determinations of 
 mineral powder cannot be made with the solution, and, as is 
 well known, that decompositions or inclosures may lower or 
 raise the specific gravity of minerals. 
 
 Separation of the Rock-components by means of the Solution. 
 In order to separate the components of a rock from each 
 other, the rock must be pulverized ; this should be preceded 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 by an orientation concerning its probable mineralogical com- 
 position by an investigation of a thin section. The powder is 
 then passed through sieves of different mesh, and that part se- 
 lected for separation which the microscopical examination has 
 demonstrated to be homogeneous, i.e., wherein several minerals 
 do not cohere. The very finest flour-like powder cannot be used 
 for the separation, as it mixes with the syrupy solution to form 
 a thin mud ; the minute mineral particles, grains, and crystals, 
 therefore, which constitute for the most part the micro-crystal- 
 line or porphyritic rocks cannot be separated by this method. 
 
 If the rock is very coarsely granular, it is often of great 
 advantage if the minerals distinguished by the pocket-lens, 
 broken away and dissociated, are separated by 
 means of the solution ; e.g., the white feldspars 
 or the black bisilicates. Mica can be obtained 
 pure by allowing the mineral powder simply to 
 slide over rough paper. 
 
 The granular powder obtained in this manner 
 is poured into an apparatus filled with the po- 
 tassium-mercury-iodide solution at its highest 
 specific gravity. 
 
 Apparatus. As the very simplest piece of 
 apparatus and one especially adapted to the pur- 
 pose, an ordinary large glass separating-funnel, 
 or the pear-shaped vessel described by T. Harada, 
 is to be recommended. This latter apparatus is 
 closed above with a ground-glass stopper, and 
 terminates in a narrow tube below, also provided 
 with a ground-glass stop-cock (Fig. 40). The so- 
 *(cSJJ A fro T m S ' lution after the powder is added to it is well 
 K. Oebbeke.) s ti r red with a glass rod and allowed to settle ; 
 those minerals possessing a higher specific gravity than the solu- 
 tion sink to the bottom, and can be removed by carefully open- 
 ing the lower glass cock after the solution has had time to clear. 
 
METHODS OF INVESTIGATION. Jl 
 
 The potassium-mercury solution is then diluted by care- 
 fully dropping distilled water accompanied by constant agita- 
 tion of the liquid with a stirring-rod until another portion of 
 the powder has either settled to the bottom or is suspended 
 in the liquid. Care must be taken that particles of the powder 
 do not cling to the rod itself or the walls of the funnel. 
 
 In order to establish the specific gravity of. the solution, and 
 consequently of the precipitated mineral granules, either a 
 direct specific-gravity determination is made with the Mohr- 
 Westphal balance, whereby a short and broad glass tube is 
 thrust into the solution and the plunger of the balance sunk 
 inside of it (a device preventing a large loss of mineral by ad- 
 hesion), or the so-called indicators are employed. A series of 
 larger mineral fragments of a known specific gravity and vary- 
 ing from I to 3.2 is used for this purpose. A large number of 
 such minerals, especially of those with a specific gravity 2-3.2, 
 should be at hand. By the use of several of, these indicators, 
 selected according to the mineral composition of the rock as 
 established by the microscope, the specific gravity of the solu- 
 tion can be determined easily. 
 
 After removing the powder which has fallen to the bottom, 
 the solution is again diluted, and the operation repeated. The 
 separated powder is well washed with water. The washings can 
 be evaporated with the diluted solution on the water-bath until 
 the maximum density is again reached. 
 
 The rock-powder can thus be divided into portions of 
 different known specific gravity which are partly pure, i.e., 
 contain fragments made up of one and the same mineral, or, 
 if the rock was too coarsely pulverized, show the so-called 
 intermediary products, impurities resulting from the inter- 
 penetrations of several minerals. In the latter case these por- 
 tions must be more finely pulverized and again separated. 
 
 Example. Tonalite. The microscopical examination of this 
 coarse-grained rock developed as components : plagioclase 
 
72 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 predominating, orthoclase subordinate, much quartz, green 
 hornblende, brown biotite, and magnetite, ilmenite, and garnet 
 as accessory. 
 
 The biotite is first of all slid off on rough paper, and thus 
 obtained quite pure. The magnetite can be withdrawn by a 
 magnet. The residual powder is then thrown into the solution, 
 when the garnets and titanic iron sink to the bottom. Horn- 
 blende, orthoclase, and quartz fragments are selected as indi- 
 cators, as the specific gravities of the minerals to be separated 
 lie between them. By slow dilution of the potassium-mercury 
 solution the hornblende will be first precipitated, and only a 
 white powder will remain. The plagioclase will first precipitate 
 from this white powder, then quartz, and finally the orthoclase. 
 
 If the specific gravity of the solution was determined while 
 the plagioclase was suspended, and it was found to be 2.67, 
 the value shows that the plagioclase is an andesine. 
 
 Finally, optical investigations can be instituted on cleavage- 
 fragments of andesine and hornblende selected from the 
 separated mineral particles. The plagioclase also can be sub- 
 jected to a quantitative chemical analysis after the purity of 
 the powder is established. 
 
 Precautionary Rules in Working with the Potassium-Mercury 
 Solution. I. A large loss of the solution should be guarded 
 against, because of the cost of preparation. All scattered 
 drops, residues, and washings from the apparatus should there- 
 fore be gathered, and this dilute solution again evaporated on 
 the water-bath. 2. The solution is very poisonous and attacks 
 the skin. 
 
 Regeneration of the Solution. The solution changes to a 
 dark or reddish brown after long usage, owing to the separa- 
 tion of free iodine. The iodine is removed, as L. v. Werveke 
 has recommended, by addition of mercury and agitation of the 
 cold solution ; or, better, by concentrating the solution on the 
 water-bath with constant agitation, and consequent division of 
 
METHODS OF INVESTIGATION. 73 
 
 the mercury. The solution again assumes a honey-yellow color, 
 and can be raised to its highest specific gravity without injury. 
 The free iodine combines with the mercury to form the sub- 
 iodide, which precipitates as a dirty-green dust on the mercury. 
 This is again changed to metallic mercury and mercuric iodide 
 on concentrating the solution, and is dissolved by the excess 
 of potassium iodide which caused the separation of the free 
 iodine. 
 
 II. KLEIN'S SOLUTION. 
 
 D. Klein has recommended a solution* of boro-tungstate of 
 cadmium (9WO 3 . B 2 O 3 . 2CdO, 2H 2 O + l & a q-) f r the separa- 
 tion of the rock-forming minerals. Although the preparation 
 of this solution is far more complicated than that of the 
 potassium-mercury iodides, yet it is to be preferred, as nearly 
 all of the rock-forming minerals can be separated by it, owing 
 to its high specific gravity 3.6 ; while many minerals, and the 
 most important, as augite, hornblende, olivine, etc., whose 
 specific gravity lies above 3.19, cannot be separated by the 
 solution of the iodides. 
 
 The process of separation with Klein's solution is exactly 
 analogous to that with the iodide solution. It must be re- 
 membered, however, to dissolve out with acids all carbonates, 
 such as calcite, etc., from the rock-powder, as they decompose 
 the solution. The apparatus, either separating-funnel or 
 Harada's vessel, must be surrounded by hot water or otherwise 
 warmed, as the salt must be melted at 75 if a solution with a 
 specific gravity of 3.5-3.6 is desired. 
 
 The Preparation of the solution is as follows : A solution of 
 Na 2 WO 4 in five parts water is first prepared and then boiled 
 
 * According to the author's experience, Klein's solution is the best and the most durable of 
 all the solutions of high specific gravity used for the mechanical separation of the rock-com- 
 ponents. 
 
74 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 with 1.5 parts B(OH) 3 until the whole is dissolved. On cooling 
 and agitating the solution, crystals of borax and sodium poly- 
 borates separate, which must be removed. The decanted liquid 
 is again evaporated, and the newly-formed crystals removed, 
 and this process is repeated until glass floats on the surface. A 
 boiling solution of BaCl 2 is then added (iBaCl, : 3Na 2 WO 4 ). 
 A thick white precipitate is formed, which is filtered off, well 
 washed with water, and finally dissolved in dilute HC1 
 (iHCl sp. gr. 1. 1 8 : ioH 2 O). Hydrochloric acid is added in 
 excess to the solution, and the whole evaporated to dryness, 
 when H 2 WO 4 separates. The dried mass is again dissolved in 
 hot water, boiled for about two hours, water being added from 
 time to time, and the H 2 WO 4 filtered off. Tetragonal crystals 
 of the compound 9WO 3 . B 2 O 3 . 2BaO, 2H 2 O + 19 aq. separate 
 from the solution, and these are purified by recrystallization. 
 Finally, CdSO 4 is added to a boiling solution of these crystals, 
 when the soluble cadmium boro-tungstate 9\VO 3 . B 2 O 3 . 2CdO, 
 2 H 2 O + 16 aq. is formed and filtered from the insoluble BaSO 4 . 
 
 Cadmium boro-tungstate dissolves in less than ^ of its 
 weight of water; it crystallizes on evaporation on the water- 
 bath and cooling. The solution of these crystals has a specific 
 gravity of 3.28 at 15 C. 
 
 Evaporation of the solution must be done always on the 
 water-bath ; if a specific gravity of 3.6 is desired, the solution 
 is evaporated until olivine floats, and is then allowed to stand 
 24 hours. Crystalline masses are deposited which are removed 
 from the solution, purified and melted at 75 in the separat- 
 ing-apparatus, placed either over the water-bath or in a jacket 
 filled with hot water. Spinel floats on this fused mass. 
 
 Cadmium boro-tungstate solution can be obtained from 
 chemical depots ready for use. 
 
 In addition to its higher specific gravity this solution has 
 these further advantages over the potassium-mercury solution : 
 it is non-poisonous, does not attack the skin, and remains at a 
 
METHODS OF INVESTIGATIOb r . 75 
 
 constant specific gravity; carbonates and metallic iron, how- 
 ever, decompose it. 
 
 III. ROHRBACH'S SOLUTION OF THE IODIDES OF BARIUM 
 AND MERCURY. 
 
 This is even more valuable than Klein's solution for the 
 separations. The specific gravity of the concentrated barium- 
 mercury solution is nearly the same as Klein's, but the prep- 
 aration is not so complicated ; moreover no decomposition is 
 effected by the carbonates. 
 
 The solution of the Iodides of Mercury and Barium is pre- 
 pared in the following manner : 100 parts of barium iodide and 
 130 parts of mercuric iodide are weighed as rapidly as possible ; 
 both in powdered form are transferred to a dry kolben over 
 an oil-bath heated to about 200 C, are well shaken together, 
 and dissolved in about 20 kcm. of water. The solution is 
 hastened by whirling the contents with a glass rod bent at the 
 lower extremity. If all is dissolved, the solution is allowed to 
 boil a little, and is then transferred to a water-bath, where it is 
 evaporated until a fragment of epidote floats. On allowing the 
 solution to cool, the specific gravity increases until olivine 
 floats ; a double-salt, however, is deposited, wh'ich is allowed to 
 settle at the bottom of a tall beaker-glass, and is removed by 
 careful decantation of the clear solution. Filtration is not ad- 
 visable, as filter-paper cannot be used. The solution thus pre- 
 pared attains a specific gravity of 3-575-3-588. 
 
 The method of operation with this solution is exactly the 
 same as with the potassium-mercury solution, except that 
 the barium-mercury solution must not be diluted with water, 
 but always with dilute solution. This latter solution can be 
 obtained easily by allowing a layer of water to stand for about 
 24 hours on the concentrated solution in a beaker-glass, when 
 
76 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 the mixing will follow by diffusion. Red mercuric iodide gen- 
 erally deposits on diluting with water. The powder to be 
 separated must be perfectly dry ; iodide of potassium must be 
 used at first in washing, which redissolves any precipitated 
 mercuric iodide. 
 
 Rohrbach recommended also that the separation of all 
 minerals below 3.1 should be carried out with the potassium- 
 mercury solution, and that the further separation of the 
 heavier minerals of sp. gr. 3.1-3.58 should be prosecuted with 
 the barium-mercury solution ; closed apparatus for separation, 
 as Harada's, is also advisable. On continued standing (i.e., for 
 several months) the solution becomes specifically lighter, ow- 
 ing to the deposition of mercuric iodide ; it cannot, therefore, 
 be employed in separating minerals of sp. gr. 3.2-3.6. 
 
 IV. METHODS OF SEPARATION BASED ON THE DIFFERENT 
 ACTION OF ACIDS ON MINERALS. 
 
 ZlRKEL Und ROSENBUSCH, 1. C. 
 
 F. FOUQUE et MICHEL LEVY. Mineralogie micrographique. Paris, 1879, P- II6 - 
 F. FOUQUE. Nouveaux precedes d'analyse mediate des roches et leur applica- 
 tion aux laves de la derniere eruption de Santorin. Mem. savants 
 etrangeres de 1' Academic des sciences. Paris, XXII. p. n, and Compt. 
 rend. 1874, p. 869. 
 
 K. OEBBEKE. N. Jahrb. f. Min. u. Geol. 1881, I. Beilagebd. p. 455. 
 A. CATHREIN. Ebenda, 1881. I. Bd. p. 172. 
 
 
 
 It has been hinted already that a basis for the more exact 
 determination of many minerals can be obtained in many cases 
 by simple treatment of the powdered rock with various acids. 
 With this in view, a thin section of the rock is first examined 
 in order to gain some idea of its mineralogical composition. 
 Small fragments of the rock are then finely powdered and 
 treated with concentrated hot hydrochloric acid in a beaker- 
 glass. Any evolution of gas must be carefully noted, or forma- 
 
METHODS OF INVESTIGATION. 77 
 
 tion of any precipitate, especially separation of sulphur or 
 silicic acid. The acid is generally allowed to act on the powder 
 for some hours, and is then filtered. The sulphur is then dis- 
 solved from the dried powder on the paper with carbon disul- 
 phide or ether, and the silicic acid by boiling in sodium car- 
 bonate. The powder is then thoroughly washed, dried, mixed 
 with Canada balsam, and suitably prepared on a slide for a 
 microscopical examination. If it is evident that one or more 
 of the rock-forming minerals has dissolved, the ordinary quali- 
 tative chemical analysis of the filtrate is set in course. 
 
 The following rock-forming minerals are soluble in hydro- 
 chloric acid : 
 
 I. Soluble without evolution of gas or separation of a 
 precipitate : 
 
 Magnetite, Hematite, Apatite (P a O B ), Titaniferoiis 
 
 Magnetite (difficultly soluble). 
 II. Soluble with evolution of CQ 2 : 
 
 Calcite, Aragonite (Ca), Dolomite (CaMg), Magnesite 
 (difficultly soluble), Sidcrite (Fe). 
 
 III. Soluble with separation of S : 
 
 Pyrrhotite, Pyrite (difficultly soluble). 
 
 IV. Soluble with separation of pulverulent SiO 2 : 
 
 Leucite (K), Meionite (Ca), Scapolite (Ca, Na), Labra- 
 dorite and Bytownite (more difficultly soluble, 
 Ca, Na), Anorthite (Ca). , 
 V. Soluble with separation of gelatinous SiO 2 : 
 
 Sodalite (Cl), Hauyn and Nosean (SO a ), Ncplieline 
 (Na), Wollastonite (Ca), Olivine (Mg), Melilite (Ca), 
 nearly all Zeolites, Serpentine, then Chlorite and 
 Epidote (difficultly soluble). 
 
 Exact determinations cannot be carried out by this method, 
 and all the less because many minerals, and those too the 
 
/8 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 most commonly-occurring silicates, possess a similar chemical 
 composition ; e.g., scapolite or meionite, with the species of 
 plagioclase closely related to anorthite. Such minerals as the 
 carbonates, apatite or sodalite, can be more easily demonstrated, 
 as they give characteristic reactions. If hydrochloric acid of 
 different degrees of concentration be used, more exact results 
 are obtained, as the solubility of the minerals depends upon 
 the size of the granule, temperature, duration of action, and 
 degree of concentration of the acid. Unfortunately no careful, 
 systematic investigations have been made in this direction ; 
 e.g., nepheline and olivine occurring together in a nepheline 
 basalt can be separated by treatment with hydrochloric acid. 
 
 Fouque has proposed another method of separation which 
 depends upon the application of hydrofluoric acid of different 
 degrees of strength. 
 
 Pure concentrated hydrofluoric acid is poured into a plati- 
 num dish, and about 30 grams of the powdered rock is slowly 
 added and stirred with a platinum spatula. Nearly all min- 
 erals except those containing Fe and Mg are dissolved, form- 
 ing fluorides and silico-fluorides and a thick jelly of silicic acid 
 and alumina. The different minerals can be separated accord- 
 ing to the duration of the reaction ; the amorphous minerals 
 being decomposed first, then the feldspars, then quartz, and 
 finally the iron silicates and magnetite. If the action of the 
 acid on a mineral has been studied sufficiently and its arrest is 
 desired, a strong fine stream of water may be directed into the 
 dish, and the acid thus be diluted until it ceases to act on the 
 powder. The gelatinous mass is pressed together, and washed 
 with water ; the unattacked mineral remaining at the bottom 
 of the dish. 
 
 In this manner feldspar, e.g., can be separated from a vit- 
 reous mass, or augite and hornblende from other components. 
 
METHODS OF INVESTIGATION. 79 
 
 V. SEPARATION OF THE ROCK-CONSTITUENTS BY MEANS OF 
 THE ELECTRO-MAGNET. 
 
 F. FOUQUE. Santorin. Paris, 1879. 
 
 F. FOUQUE. Mem. Acad. des sciences, 1874, XXII, No. n. 
 
 C. DOELTER. Sitzungsb. d. k. Akad. d. Wiss. in Wien. LXXXV. Bd. I. Abth. 
 
 1882. p. 47 and 442. 
 
 C. DOELTER. Die Vulcane der Capverden. Graz, 1882. 
 P. MANN. N. Jahrb. f. Min. u. Geol. 1884. II. p. 181. 
 
 It has been noted already that for a long period the ex- 
 traction of magnetite from the rock-powder has been effected 
 by means of an ordinary powerful magnet ; more recently the 
 electro-magnet has been applied to the separation of the fer- 
 riferous minerals from those containing no iron. 
 
 The credit for its application to petrographical investiga- 
 tions is due to Fouque, and especially that he called attention 
 to its value in the mechanical analysis of rocks. 
 
 It is impossible to separate the components of a rock by 
 use of the electro-magnet alone ; several methods must always 
 be combined in order that the minerals may be separated as 
 pure- as possible. Therefore the solution of the iodides of 
 potassium and mercury is first advantageously employed, 
 then Klein's or Rohrbach's solution, and finally the mineral 
 portions separated by means of these solutions are completely 
 purified with the electro-magnet. E.g., it is required to separate 
 the components of a phonolite magnetite, sanidine, nepheline, 
 and augite. The magnetite is removed with the magnetic 
 needle. In the residue, sanidine and nepheline are separated 
 from the augite by means of the potassium-mercury solution 
 of specific gravity about 3, when the augite is obtained very 
 pure. The sanidine and nepheline can be purified by means 
 of the electro-magnet, and the nepheline separated from the 
 sanidine (and augite accidentally present) again by means of 
 
8O DETERMINATION OF ROCK-FORMING MINERALS. 
 
 the potassium-mercury solution ; or the nepheline can be dis- 
 solved in hydrochloric acid. 
 
 If, on the other hand, the components of a vitreous augite- 
 andesite are to be separated, the vitreous base may be removed 
 by means of hydrofluoric acid, the augite separated from the 
 plagioclase by the electro-magnet, and the varieties of plagio- 
 clase, in case several species are present, isolated by the potas- 
 sium-mercury solution. 
 
 The powder must be dry and free from the very finest dust 
 when the electro-magnet is used. The size of the grains 
 depends upon the density of the rock. 
 
 If several ferriferous mineral species occur in the rock to be 
 examined, e.g., magnetite, ilmenite, augite, biotite, olivine, etc., 
 they can be separated from each other by varying the strength 
 of current passing through the electro-magnet. At first two 
 elements are used, then four, six, eight, and finally ten. 
 Doelter has shown that the minerals can be arranged in the 
 following series according to their different powers ^ of being 
 attracted : 
 
 Magnetite, 
 
 Hematite, Ilmenite, 
 
 Chvomite, Siderite, Almandine, 
 
 Hedenbergite, Anker it e, Limonite, 
 
 Augite (rich in iron), Pleonaste, Arfvedsonite, 
 
 Hornblende, Augite (light-colored), Epidote, Pyrope, 
 
 Tourmaline, Bronzite, Idocrase, 
 
 Staurolite, Actinolite, 
 
 Olivine, Pyrite, C hale opy rite, 
 
 Biotite, Chlorite, Rutile, 
 
 Hauyn, Diopside, Muscovite, 
 
 Nepheline, Leucite, Dolomite. 
 
 Doelter has also described a piece of apparatus suitable for 
 such separations. In this the distance between the powder 
 
METHODS OF INVESTIGATION. 8 1 
 
 lying on a glass plate and the hook-shaped poles of the horse- 
 shoe magnet can be measured. He also advised the prepara- 
 tion of a scale of minerals for each apparatus with its varying 
 power of the current, analogous to the indicators used in the 
 separation by solutions of high specific gravity, in order to 
 determine the individual power of attraction with the different 
 strength of current. The mineral granules to be separated 
 should be from 0.14 to 0.18 mm. in diameter, v. Pebal states 
 that powder suspended in water is preferable to the dry. 
 
 D. Explanations of the Tables relating to the Mor- 
 phological Properties of the Rock-forming Minerals. 
 
 ZIRKEL. Mikr. Beschaff. d. Min. u. Gesteine. Leipzig, 1873. 
 ROSENBUSCH. Mikr. Physiogr. d. petrogr. wicht. Miner. Stuttgart, 1873. 
 
 E. COHEN. Sammlung von Mikrophotographien zur Veranschaulichung der 
 
 mikroskopischen Structur von Mineralien und Gesteinen. Stuttgart, 
 
 1883. 
 
 FOUQU& ET MICHEL L6vv. Mineralogie micrographique. Paris, 1879. 
 THOULET. Contributions a 1'etude des proprietes physiques et chimiques des 
 
 mineraux microscopiques. Paris, 1880. 
 v. PEBAL. Sitzungsber. d. k. k. Akad. der Wiss. math. nat. Cl. 1882. p. 193. 
 
 I. MODE OF OCCURRENCE OF THE ROCK-CONSTITUENTS. 
 
 The mineral constituents of a rock occur either in perfectly- 
 developed crystals, often sharply defined, in crystalline grains, 
 or as microlites or crystallites. 
 
 It is seldom, however, that the crystals appearing in the 
 rocks are so large that the system of crystallization can be de- 
 termined by the macroscopical examination or measurement 
 of the angles alone. In order, therefore, to determine the 
 mineralogical composition of a rock, a thin section must be 
 prepared wherein the constituents, appearing in the forms just 
 mentioned, are in sections in every possible direction. In this 
 
82 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 case the determination of the crystalline form is rendered 
 much more difficult, and is impossible simply from the form of 
 the cross-section. By suitable combination of the form of 
 cross-section, optical properties, cleavage, and finally by meas- 
 urement of the angles, it can be determined in most cases to 
 which system of crystallization the mineral belongs. E.g., a 
 mineral appears whose cross-sections are octagonal, with cleav- 
 age at nearly right angles ; or are elongated, rectangular, or 
 hexagonal, with cleavage-fissures parallel to the longest axis. 
 The mineral could belong to the tetragonal as well as the 
 rhombic or monoclinic system. The section must be examined, 
 therefore, in parallel and convergent polarized light. The 
 form of the cross-sections shows that the mineral is developed 
 in long eight-sided prisms with prismatic cleavage ; the octago- 
 nal sections are the transverse sections at right angles to the 
 -axis. If they appear as isotrope in parallel polarized light 
 and show in convergent polarized light a fixed axial cross, the 
 mineral is tetragonal, possibly belonging to the meionite group. 
 If, on the contrary, the transverse sections as well as the lon- 
 gitudinal are anisotrope and develop a middle line in con- 
 verging polarized light, it is rhombic ; and if, finally, one optic 
 axis is visible, it is monoclinic and the mineral may be, e.g., 
 from the augite group. 
 
 By measurement of the angles it can, in the latter case, still 
 be determined which faces belong to the prism oo P and the 
 pinacoids, and to which faces the cleavage-fissures are parallel. 
 
 In measuring the angle of cleavage the direction of the sec- 
 tion must always be carefully noted, as the value of the angle 
 of cleavage varies within wide limits, according to the inclina- 
 tion of the section to the chief or vertical axis. E.g., augite 
 cannot be distinguished from hornblende by the angle of cleav- 
 age alone, as augite prisms cut at an angle of 40 to the verti- 
 cal axis, following 2^00 in the zone oP : oo^oo, will show 
 a cleavage-angle of 124 2', which lies very near the angle of 
 
METHODS OF INVESTIGATION. 83 
 
 a section of hornblende cut perpendicularly to the vertical 
 axis. 
 
 Thoulet (1. c., p. 28) has determined the value of the cleav- 
 age-angle of augite, hornblende, orthoclase, and labradorite for 
 the different directions of the sections and according to the am- 
 plitude of its inclination to the vertical axis. The determina- 
 tion for the first two of these minerals is given in the table on 
 the following page. 
 
 It is therefore impossible by observation of a single cross- 
 section with nearly rectangular cleavage to determine with ac- 
 curacy, for example, whether the observed monoclinic green 
 or brown mineral is augite or hornblende. Nor less by simply 
 proving the presence or absence of pleochroism. It is therefore 
 necessary to examine a series of cross-sections of the particular 
 mineral, and it can only be settled with any great accuracy 
 whether a mineral is augite or hornblende when several trans- 
 verse sections show a cleavage-angle approaching 87 or 124. 
 
 Often the shape of the crystal outline shows that the plane 
 of the section is inclined to the vertical axis, and gives ap- 
 proximately its angle of inclination ; if the constituents are in 
 a granular condition, this mark of recognition is wanting, and 
 hence complicates the determination. The direction of the 
 section can also be approximately determined by comparison 
 of the -optical relations (according to examinations in converg- 
 ent polarized light). 
 
 The simple proof of parallel extinction on one or a few sec- 
 tions can give no safe conclusions as to whether the mineral is 
 rhombic or monoclinic ; e.g., the determination of c: c to about 
 20 in augite and hornblende. As many observations as pos- 
 sible, therefore, must be made on sections optically oriented. 
 In the cases mentioned this is done most easily on prismatic 
 cleavage-leaflets. 
 
 Microscopical Measurements of Angles are made with the 
 polarization-microscope in the same manner as the determina- 
 
84 DETERMINATION OF ROCK-FORMING MINERALS. 
 
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METHCIDS OF INVESTIGATION. 85 
 
 tion of the direction of extinction. The instrument is ac- 
 curately centred, one leg of the angle to be measured so dis- 
 posed that it coincides exactly with one arm of the cross- 
 threads, the apex of the angle reaching exactly to the junction 
 of the cross-threads of the ocular. The position of the stage is 
 read and the stage revolved until the other leg of the angle 
 coincides with the same arm of the cross-threads, and its posi- 
 tion again read : the difference of the two readings gives the 
 magnitude of the angle measured. 
 
 If the rock-components are granular, their determinations 
 are greatly co'mplicated, as one can neither draw any satisfac- 
 tory conclusion as to the crystalline form from the character of 
 the outline merely, nor can it be determined to which faces the 
 cleavage-fissures are parallel ; one is therefore restricted to 
 the determination of the color, direction of cleavage, magni- 
 tude of the cleavage-angle, and especially to the 'optical prop- 
 erties of the mineral granules. 
 
 The Microlite is another form of development of the rock- 
 forming minerals. E.Cohen has designated as "microlites" 
 all those crystals which cannot be prepared in sections in suit- 
 able positions, generally horizontal, the micas, however, vertical, 
 appearing in the thin section as perfectly developed individu- 
 als ; it makes no difference whether the mineral species can 
 be det'ermined or not. Vogelsang (Phil. d. Geol. 1867, p. 139) 
 has recommended that the term "microlite " be used only with 
 the acicular microscopical mineral forms without any regard as 
 to whether it can or cannot be determined to which mineral 
 the microlite belongs. Many rock forming minerals, as augite, 
 v hornblende, and the feldspars, appear as microlites ; in the 
 porphyritic rocks these occur with larger crystals or grains, and 
 thus chronicle their different stages of formation or separation. 
 The large crystals and grains the so-called " springlings" 
 (Einsprenglinge) (components of the first class) were formed 
 sooner than the microlites (components of the second class) 
 
86 
 
 DETER MUTATION OF ROCK-FORMING MINERALS. 
 
 of the same mineral species forming the principal ground-mass 
 
 of the porphyritic rocks. 
 
 As microlites, and nearly always as such, appear sillimanite 
 
 (comp. Fig. 71), rutile, zircon, commonly tourmaline, etc.; while 
 
 other minerals, as olivine, titanite, etc., never or rarely thus 
 
 appear. 
 
 The Crystallites (see Fig. 41) form a transition-stage to the 
 microlites, i.e., lie between the amor- 
 phous and crystalline condition. 
 Vogelsang designates by this term 
 "all inorganic products which show 
 some systematic arrangement, but 
 not the general character of crystal- 
 lized bodies, i.e., no polyhedral out- 
 line." The crystallites exert no in- 
 fluence on polarized light. 
 
 Crystallites occur frequently in 
 vitreous or semi-vitreous rocks. The 
 
 simplest forms are the Globulites, as those exceedingly minute, 
 
 isotrope, for the most part globular, forms which have separated 
 
 in the vitreous ground-mass of such 
 
 rocks are designated. If several 
 
 such globulites are chained together, 
 
 the Margarites are formed. If the 
 
 members of this chain-like aggregate 
 
 of globulites are fused together into 
 
 a long needle, the Longulites 
 
 formed. 
 
 The Crystalloids form yet another 
 
 stage of transition to the microlites; 
 
 " these are more of a unit, act also 
 
 on polarized light, but do not yet 
 
 show the polyhedral outline of the microlite." 
 
 The genesis of the rock-forming minerals is therefore 
 
 FIG. 41. 
 
 CRYSTALLITES AND MICROLITHS. 
 
 are v-r 
 
 FIG. 42. 
 
 MICRO-FLUCTUATION STRUCTURE. 
 BELONITES AND TRICHITES. 
 
METHODS OF INVESTIGATION. 8/ 
 
 briefly as follows : The crystallites are the primitive form, 
 the glob u lit es being first in order ; the crystalloids mark a 
 further progress in development ; these form a transition to 
 the microlites, which in turn only differ in size from the crystals. 
 Vogelsang has proposed a further subdivision of the 
 crystallites and crystalloids, resting upon their pellucidity. 
 A pellucid species may be called a Belonite ; a non-pellucid, a 
 Trichite. (Fig. 42.) 
 
 II. STRUCTURE OF THE ROCK-FORMING MINERALS. 
 The following should be especially noted concerning the 
 microscopical relations of the rock-components: 
 
 1. The disturbances in crystallization. 
 
 2. The destruction of crystals already formed. 
 
 3. The concentric structure of crystals. 
 Disturbances in Crystalization are not common, and are 
 
 manifested in the imperfect development of the crystal at one 
 end or in the sunken faces whereby the crystals take on an 
 " etched appearance ;" the phenomenon so often noticed in 
 magnetite is also to be mentioned the regular grouping of 
 several small crystals in three directions at right angles to each 
 other corresponding to the axes, thus forming the outline to 
 a larger crystal. 
 
 Imperfectly-developed crystals occur on one termination ; 
 e.g., on hematite, where hexagonal tablets are notched and 
 lapped on one or two sides, or on the crystals of hornblende, 
 augite, etc., which are often covered at one end with several sub- 
 individuals and thus acquire an appearance resembling a ruin! 
 
 On olivine, leucite, etc., often occur crystals with faces de- 
 pressed in consequence of the interrupted development. In a 
 word, exactly the same phenomena of growth and disturbance 
 are noticed in the crystals separated from the molten rock- 
 magma as can be perceived on crystals formed from a solution. 
 The destruction, fracture, and bruising of crystals already fully 
 
88 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 formed can be commonly observed on the microscopic con- 
 stituents of the more recent and vit- 
 reous rocks, just as the same phe- 
 nomena are observed on the macro- 
 scopic individuals ; e.g., of tourma- 
 line, epidote, etc. The larger mineral 
 components wjiich first separated 
 show such fractures especially. These 
 are a direct consequence of the pres- 
 sure which the molten, fluctuating 
 rock-magma exerted on the crystals 
 CORRODED Q& CRYSTAL. , already formed, if any change in the 
 (After Fouqu<5.) rapidity of fluctuation was induced 
 
 by any obstruction ; e.g., another opposing large crystal lying 
 in the immediate vicinity. The corresponding fragments of 
 the crystal, as well as the crystal or other matter causing the 
 fracture, can be observed very often lying close together. Such 
 fractures are for the most part restricted to the thin tabular or 
 long acicular crystal individuals ; they are observed, therefore, 
 most commonly on the feldspars, augite or hornblende crystals, 
 while the micas because of their elasticity show only a bending 
 or exfoliation. However, quartz grains and crystals often ap- 
 pear shattered into small splinters and plates. 
 
 The Destruction of Crystals already Formed. 
 
 The larger crystalline components undergo further changes 
 through the caustic action of the liquid magma, as manifested 
 in the corrosion, partial fusion, and even total destruction of 
 the crystal. Thus quartz occurring in the porphyritic eruptive 
 rocks often shows a sinus-like penetration of the ground-mass. 
 (Fig. 43.) Leucite and olivine as well as augite crystals or grains 
 often show an etched surface, sometimes covered with regular 
 depressions, probably caused by the caustic action of the mag- 
 ma on the crystals for a long period, similar to the figures and 
 
METHODS OF INVESTIGATION. 89 
 
 depressions often formed on artificial crystals by action of the 
 mother-liquor. 
 
 If action of magma on the crystals, already formed was 
 more powerful, a partial fusion ensued, as may be observed 
 very often on crystals of feldspar or augite of the eruptive 
 rocks, where some faces are yet more or less evident. 
 
 The resolution of the edges into minute crystals and grains 
 as is often observed on the larger olivine, augite, and feldspar 
 crystals is another remarkable corrosion-phenomenon, depend- 
 ing upon this action of the magma. The minute crystals are 
 to be regarded as newly-deposited crystals of the same mineral, 
 and the grains as separated particles. The diopside, bronzite, 
 and olivine grains of the so-called " olivine lumps" in the 
 basalts often show such changes. More remarkable yet is that 
 on the omphacite of eclogite, a rock classed, however, accord- 
 ing to its formation with the crystalline schists. 
 
 Another change also ascribed to the action of the molten 
 magma, and commonly observed on hornblende and biotite 
 crystals of the more recent eruptive rocks richer in 
 iron, consists of the appearance of an opaque margin 
 (Fig. 44). The crystals are surrounded by a border, 
 or narrow, dense, opaque black hem, formed from 
 exceedingly minute granules of an unknown iron 
 compound the so-called " opacite." Often the 
 whole crystal has undergone such an igneous meta- HORNBLENDE. 
 morphosis and only remnants of the fresh, brown, original 
 mineral are to be found. 
 
 This opaque bounding of hornblende and biotite must not 
 be confounded with the decompositions effected by water, 
 whereby such a marginal hem is formed, proved to be of 
 magnetite. In this case the hornblende is not perfectly fresh, 
 but is partially changed to chlorite, and the opaque hem is 
 not so dense as those crystals metamorphosed by fire. 
 
 Finally, the occurrence of the so-called " Pseudo-crystals" of 
 
9 o 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 hornblende, augite, and biotite must be briefly noticed. In 
 the younger eruptive rocks bearing these minerals, aggregates 
 of minute augitic granules, feldspathic grains, and especially 
 magnetite or hematite leaflets often occur, which assume their 
 crystalline forms ; often a fresh, irregular, partially-fused kernel 
 of hornblende or biotite or augite is seen within. It is very 
 probable that these aggregates occurring in the eruptive rocks 
 have been formed by the action of the liquid rock-magma on 
 the unchanged hornblende, biotite, or augite crystals, the form 
 of the crystal being meanwhile preserved. These pseudo- 
 crystals can be formed experimentally by dipping hornblende, 
 etc., crystals in fused rock-magma and allowing to cool. 
 
 The Shell-formed Structure of Crystals. 
 
 A macroscopical examination of many crystals shows a 
 zonal structure, e.g. barite, tourmaline, epidote, garnet, etc.; the 
 shell-structure proves a repeated interrupted growth of the 
 crystal, each layer or coat corresponding to a period of growth. 
 This shell-structure may be easily shown in artificial crystals by 
 suspending a crystal successively in different mother-liquors ; 
 e.g., an octahedron of alum in a solution of chrome-alum. 
 
 In the same way an exceedingly detailed laminated forma- 
 tion may be observed often in the microscopical crystal individ- 
 uals occurring as rock-constituents. Among 
 these, the feldspars, augite, hornblende, mel- 
 anite, tourmaline, more rarely epidote, titan- 
 ite, disthene, andalusite, corundum, hauyn, 
 nepheline, etc., must be mentioned particu- 
 larly. 
 
 The different layers are often very nu- 
 merous and exceedingly thin, and can be 
 distinguished from each other easily, es- 
 pecially if multicolored, as is so commonly 
 the case with augite (Fig. 45) or hornblende, where a green 
 
 FIG. 45. 
 
 ZoNALLY-DEVItLOPED 
 
 AUGITE. 
 Section II oojPoo. 
 
METHODS OF INVESTIGATION. 9! 
 
 centre is surrounded by a brown layer, or green and brown or 
 nearly colorless layers alternate. In melanite dark-brown lay- 
 ers alternate with lighter ; in andalusite often a red centre, in 
 disthene and corundum a blue centre, is enveloped by a colorless 
 coating. 
 
 In many cases the shell-formed structure of crystals, as in the 
 feldspars, augite, and hornblende, is first evident in polarized 
 light ; the different layers thus show different polarization-col- 
 ors, and also the direction of extinction varies somewhat in 
 them, due, it appears, to the slight variation in chemical 
 constitution of the successive layers. These lines of growth 
 run undisturbed through the twinnings of the feldspars, etc.; 
 this would indicate that the laminated development was 
 synchronous with the formation of the twins. 
 
 The single layers often can be distinguished from each 
 other more sharply by the inclosures of fluids, glass, or micro- 
 lites lying between them ; the successive layers have a course 
 nearly parallel with the central crystal (see Fig. 45). Now anfl 
 then, however, crystals are observed, especially of feldspar and 
 augite, where the edges and angles of the kernel-crystal are 
 replaced by faces of the enveloping layers. 
 
 .As already mentioned, a very common and prominent de- 
 velopment of crystals from two zones of different optical orien- 
 tation' is noticed in the feldspars, in sanidine, as well as in 
 some species of plagioclase. In these latter species it can be 
 proved often that the kernel-crystal is a plagioclase of more basic 
 composition ; but the envelopes, on the other hand, belong to a 
 plagioclase richer in silicic acid and sodium. Hoepfner (N. 
 Jahrb. f. Min. u. Geo-L, 1881, II. p. 883) first called attention 
 to these relations by showing that the plagioclase of andesite 
 from Monte Tajumbina often has an anorthite centre sur- 
 rounded by an envelope of oligoclase. Becke confirmed 
 these observations on the feldspars in kersantite from the 
 lower Austrian forest (Tscher. Min. Mitth., 1882, V. p. 161). 
 
9 2 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 The change from kernel to envelope is quite gradual, as each 
 successive layer deposits a feldspar richer In sodium. The 
 observation of Rosenbusch that the decomposition of a feld- 
 spar is generally from the centre outward is quite in harmony. 
 The hypothesis already proposed by the same investigator, 
 that the kernel of these species of plagioclase possesses a 
 more basic constitution, and therefore undergoes an alteration 
 first, is confirmed by the observations of Hoepfner and Becke. 
 A peculiar structure of crystal is the so-called " hour-glass 
 structure" as seen not rarely in monoclinic augite of many 
 basaltic rocks (Figs. 46 and 47), especially of limburgite, 
 
 V 
 
 FIG. 46. AUGITE WITH " HOUR-GLASS 
 STRUCTURE." Section || oofoo. 
 (After L. v. Werveke.) 
 
 FIG. 47. SCHFMATIC REPRESENTATION 
 OF THE " HOUR-GLASS AUGITE." 
 
 more rarely in hornblende, and also in andalusite and stauro- 
 lite. Sections parallel to the plane of symmetry divide into 
 four fields in polarized light, any two of which lying opposite 
 each other show the same colors and the same optical orienta- 
 tion. The deviation in optical orientation is generally slight. 
 The sections parallel oo^oo are similar. 
 
 While sections perpendicular to the vertical axis show the 
 
METHODS OF INVESTIGATION. 93 
 
 ordinary zonal structure. At first a crystal-skeleton shaped like 
 an hour-glass appears to have been formed, both of whose 
 conical spaces were filled subsequently with an augitic sub- 
 stance varying somewhat in chemical composition. 
 
 Interpenetration of the Rock-constituents. 
 
 Graphic granite or pegmatyte serves as one of the best- 
 known examples of a regular interpenetration of two rock- 
 constituents. In pegmatyte numberless macroscopic quartz 
 individuals all showing the same optical orientation are formed 
 within large orthoclase individuals. The same penetration 
 precisely is found commonly among the microscopic individu- 
 als of the rock-constituents, and is called the " micro-pegmatitic 
 structure." This proves a nearly simultaneous formation of 
 both mutually-developed individuals, and occurs very -com- 
 monly in the granites and crystalline schists. In the latter 
 case, however, not only is the orthoclase regularly developed 
 with quartz, but also other constituents, as garnet or augite 
 with quartz, plagioclase with augite, etc. Their development 
 is often irregular, in that the augite grains penetrating the 
 plagioclase individuals, e.g., do not show throughout the same 
 optical orientation. Regular interpenetrations commonly oc- 
 cur also between the augites and hornblendes, where either 
 monoclinic augite, especially diallage or omphacite, also pos- 
 sessing the brachy-pinacoidal separation (Absonderung), is 
 grown into the monoclinic hornblende so that the ortho-pin- 
 acoidal faces of both lie parallel ; or rhombic and monoclinic 
 augite are interpenetrated so that both lie with the ortho- or 
 macro-pinacoids adjoining. 
 
 III. INCLOSURES OF THE ROCK-FORMING MINERALS. 
 
 Macroscopical inclosures have been observed in many 
 crystalline minerals for a long period ; quartz is especially rich 
 
94 DETERMINATION OF ItOCK-FORMING MINERALS. 
 
 in them. The microscopic constituents of the rocks also con- 
 tain inclosures many of which may be regarded as character- 
 istic for certain minerals. Among these inclosures of the rock- 
 components are gas-pores, fluids, vitreous particles (of the 
 rock-mass), and, finally, inclosures of Other minerals also shar- 
 ing in the composition of the rock. 
 
 Gas-pores (Fig. 48). 
 
 During the development of a crystal minute bubbles of air 
 often cling fast to the faces, which afterward are surrounded 
 
 and finally inclosed by the crystal- 
 line material during the succeeding 
 growth ; this phenomenon can be 
 best observed with artificial crystals 
 on removal from the solution. Inex- 
 actly the same manner bubbles of gas 
 which were absorbed by the mother- 
 liquor and are of such common occur- 
 rence in the vitreous ground-mass of 
 rocks were inclosed by the rock-form- 
 GAS-PORES AND FLU.D INCLOSUKES. j ng minerals during their separation 
 from the molten magma: these are the so-called Gas-pores. It 
 is difficult to determine what gases are inclosed "within the 
 minute, generally egg-shaped or irregularly-defined spaces ; it is 
 very probable that gaseous (i.e. condensed) carbon dioxide is 
 of common occurrence. The gas-pores are often regularly dis- 
 tributed through the crystals; being sometimes zonal, parallel 
 to the crystal faces if they are inclosed between two successive 
 concentric layers, or forming an elongated series. 
 
 The minerals of the hauyn group among the rock-con- 
 stituents are especially rich in inclosures of gas-pores ; apatite, 
 the feldspars, augite, etc., also contain them. Cavities empty, 
 or filled with gas, often occur, especially in quartz, which ex- 
 
METHODS OF INVESTIGATION. 95 
 
 hibit the form of the mineral in which they occur the so-called 
 " negative crystals." Such regular pores filled with air occur 
 in artificial crystals ; e.g., the cube-shaped cavities in rock-salt. 
 During the development of this mineral regular cubical de- 
 pressions are formed ; an air-bubble forces its way into the 
 depression, which becomes covered afterward by succeeding 
 depositions of the crystalline material. 
 
 Fluid Inclosures (Fig. 48). 
 
 If the mother-liquor is forced into the irregular or cubical 
 cavity mentioned in the last example, instead of air or other 
 gases absorbed by the mother-liquor, fluid inclosures are formed 
 which contain a small air- or gas-bubble, sometimes called the 
 " libella," which by turning the piece of salt vibrates along the 
 sides of the cavity. 
 
 In just the same way the fluid inclosures commonly occur- 
 ring, especially in quartz, are formed in the rock-forming min- 
 erals. The fluid inclosures occur more rarely in the younger 
 and recent eruptive rocks, and are for the most part inclosures 
 of liquid carbon dioxide a proof that these rocks were formed 
 under immense pressure. Inclosures of aqueous solutions also 
 occur in the constituents of the volcanic rocks ; it is probable 
 that these liquids were inclosed in a fluid condition. The 
 formation of the bubble within the fluid inclosures can be 
 accounted for most easily by supposing that the crystals 
 separated at a high temperature and under heavy pressure ; 
 on subsequent cooling the inclosed liquid contracted and thus 
 left an empty space the bubble. The bubble in the micro- 
 scopic fluid inclosures commonly shows a perfect freedom of 
 motion, at one time slow and again exceedingly rapi-d. 
 
 If the inclosed liquid was a concentrated salt solution, 
 minute crystals have been deposited since the cooling, and 
 liquid, crystals, and bubble can be distinguished within the 
 cavity. The form of the fluid inclosure is generally an irregu- 
 
96 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 larone ; the egg-shaped and spherical are more rare ; and more 
 rare yet those assuming the form of the inclosing mineral, as 
 occasionally in quartz, gypsum, etc. The inclosure is com- 
 monly very small and does not generally exceed some hun- 
 dredths or thousandths of a millimetre. What has been said 
 already concerning the distribution of the gas-pores is equally 
 applicable to these fluid inclosures. 
 
 As regards the chemical constitution of the inclosed liquids, 
 all determinations up to the present time have shown them to 
 be either water, liquid carbon dioxide, or some salt solution, es- 
 pecially of sodium chloride. The majority of the simply 
 aqueous inclosures have a quiescent or feebly-vibrating bubble, 
 which does not disappear on heating to about 100 C. The in- 
 closures of liquid carbon dioxide, on the other hand, have gen- 
 erally a very mobile bubble which disappears on heating to 
 about 32 C. If the bubble in such an inclosure is very large, 
 i.e. but little liquid is present, the liquid CO 2 is changed into 
 the gaseous condition when the bubble disappears ; if, on the 
 other hand, the bubble is so minute that the whole space is filled 
 through the expansion of the liquid carbon dioxide, the gaseous 
 bubble disappears. 
 
 Inclosures of liquids of two kinds commonly occur where 
 liquid carbon dioxide is present together with a purely aqueous 
 inclosure in one and the same mineral grain ; also, but rarely 
 in quartz, two different liquids are inclosed in one and the 
 same cavity, without commingling ; in this case the inner liquid, 
 generally carbon dioxide, possesses a bubble. 
 
 Inclosures of concentrated salt solutions have, for the most 
 part, an immovable bubble, or at least one moving but slowly or 
 after warming, and minute crystals deposited from the inclosed 
 mother-liquor; sodium-chloride crystals are the most common. 
 The bubble, as well as the minute cube, does not disappear on 
 warming the preparation, or disappears first at higher tempera- 
 tures. 
 
METHODS OF INVESl^IGATION. 
 
 97 
 
 The bubble is wanting in many fluid inclosures, as the 
 cavities are completely filled with liquid. Such microscopic 
 cavities can be distinguished from gas-pores only with great 
 difficulty. They are surrounded in transmitted light with a 
 broad dark border, in consequence partly of a total reflection 
 of the rays ; the two can be distinguished only by the presence 
 of the bubble. This dark border of the gas and fluid inclos- 
 ures will be the stronger the greater the indices of refraction of 
 the inclosed gas and the inclosing mineral. For this reason 
 the gas-pores have always a darker border than the fluid 
 inclosures. 
 
 Inclosures of Vitreous Particles. 
 
 Particles of the ground-mass, either purely vitreous or semi- 
 individualized, become inclosed during the process of crystal- 
 lization from the fused magma, just 
 as fluids are inclosed within crystals 
 deposited from solution. These very 
 minute and irregular, egg-shaped, or 
 spherical glassy particles, the vitreous 
 inclosures (Fig. 49), solidified during 
 or after their inclosure, generally 
 have one or several gas-bubbles in- 
 closed with them. This gas-bubble is 
 of course immovable, and, unlike the 
 bubble of the fluid inclosures, cannot 
 be moved by heating. 
 
 The vitreous inclosures in minerals are colorless or brown 
 according as the vitreous matrix of the rock is light or dark- 
 colored (the acidic lavas have generally only a colorless, basic, 
 light-colored or brown glass) ; both varieties very commonly 
 occur together, the coloration of the glass depending meVely 
 on the amount of iron present. 
 
 FIG. 
 
 VITREOUS INCLOSURES. 
 
98 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 The distribution of the vitreous inclosures is either an 
 irregular one or is in zones corresponding to the shell-formed 
 structure of the crystal. Sometimes the kernel of the crystal 
 is filled with these inclosures, and the enveloping layers poor 
 in them, or the reverse. 
 
 The vitreous inclosures are especially common in the feld- 
 spars of the younger and recent eruptive rocks, common also 
 in quartz and augite. 
 
 Vitreous inclosures of dihexahedral form are occasionally 
 found in quartz, corresponding to its crystalline form. Such 
 regular inclosures are formed in the same manner as the 
 dihexahedral gaseous or fluid inclosures in quartz, with this 
 difference, that the substance filling the regular cavities is in 
 this case a vitreous mass. A jagged bubble is often seen in 
 such vitreous inclosures, or a gas-bubble partially freed from 
 the inclosure, which was prevented from escaping by rapid 
 deposition of crystalline matter. The presence of such an 
 escaping bubble, as well as that of several bubbles within the 
 inclosures, is a proof of their solid vitreous character: such 
 phenomena could not occur in fluid inclosures. 
 
 Minute crystals, magnetite octahedra, augite microlites, 
 trichites, etc., have often separated during solidification of vit- 
 reous, inclosed particles in the same manner as crystals are de- 
 posited from inclosures of saturated solutions ; i.e., the glass is 
 "devitrified" (entglasf). The magnitude of the gas-bubble 
 has absolutely no genetic connection with the magnitude of 
 the inclosure. The vitreous inclosures will show in transmit- 
 ted light no such dark border as the gas-pores and fluid inclos- 
 ures, as the index of refraction of the glass is rather high and 
 differs less than air or water from that of the mineral. The 
 vitreous portion of the inclosure has consequently a less 
 marked border, although the gas-bubble shows all the darker 
 broati band. 
 
 The presence of a gas-bubble in the vitreous inclosure cut 
 
METHODS OF INVESTIGATION. 99 
 
 through in the process of grinding the mineral section is an 
 additional means of discriminating between a vitreous and a 
 fluid inclosure. As the gas-bubble is an empty cavity, fixed in 
 the solid vitreous body, it is cut through during the process of 
 preparation, becomes filled with Canada balsam, and the vit- 
 reous inclosure appears in the preparation as only a feebly out- 
 lined circle ; a fluid inclosure, on the other hand, thus cut 
 through would become completely filled with Canada balsam, 
 as the liquid escapes during the process of cutting and the bub- 
 ble in this case is completely dissipated. 
 
 Often large, irregular particles of non- or but poorly-indi- 
 vidualized vitreous masses with no inclosed gas-bubbles occur 
 in the rock-forming minerals ; as, e.g., between the layers or in 
 the kernel of feldspar, olivine, etc. They, as well as the vitreous 
 inclosures containing gas-bubbles, are a proof of the formation 
 of the rock (i.e. the minerals) from a molten magma. 
 
 In the quartz grains of rock of undoubted sedimentary ori- 
 gin which were solidified from confined and metamorphosed 
 eruptive rocks, vitreous inclosures are also discovered, but of a 
 secondary character, being first formed through the action 
 of eruptive magma heated to redness on the inclosed rock ; this 
 can -be proved by experiment. The way, however, in which 
 such secondary vitreous inclosures could be made is at present 
 unexplained. (See Chrustschoff, Tschermak's Min. Mitth. 
 1882, IV. p. 473.) 
 
 Inclosures of Foreign Minerals. 
 
 Macroscopical inclosures of other minerals have been ob- 
 served commonly in quartz (prase, etc.). Among the micro- 
 scopical constituents also, quartz, as well as many other min- 
 erals, as staurolite, etc., is especially rich in inclosures. The 
 granules or crystals thus inclosed within the rock-constituents 
 
IOO DETERMINATION OF ROCK-FORMING MINERALS. 
 
 are of those minerals making up the composition of the par- 
 ticular rock, and are for the most part very minute and often 
 regularly distributed through the inclosing mineral. In augite, 
 e.g., long, narrow indeterminable microlites (augite?) together 
 with vitreous inclosures are commonly arranged in zones 
 these were inclosed in the same manner as the vitreous parti 
 cles, during separation of the crystal from the vitreous, semi 
 individualized magma. In other minerals the mineral inclos- 
 ures are regularly distributed parallel to cer 
 tain faces, as the opaque to brownish trans- 
 lucent rectangular tablets parallel to oo^oo 
 in hypersthene and bronzite (Fig. 50), or the 
 opaque microlites and tablets parallel to the 
 *:'-axis in labradorite. 
 
 The zonally arranged inclosures of small 
 quartz granules in the garnet and staurolite 
 INCLOSURES OF' BROOK- of the crystalline schists; the inclosures of 
 
 ITE (?) TABLETS IN 
 
 HYPERSTHENE. minute elongated needles of rutile, regular 
 and crossed at an angle of 60, occurring in some species of 
 magnesian micas in certain eruptive rocks ; and, finally, the in- 
 closures of sillimanite microlites in cordierite and quartz of 
 crystalline schists, etc., are also especially worthy of mention. 
 
 Comparison of the inclosures of rock-constituents often 
 proves of importance in determining the order of separation, i.e. 
 the formation ; thus magnetite, menaccanite, spinel, rutile, zir- 
 conite, are generally the minerals first formed in the crystalline 
 rocks, as they are always found included within all the min- 
 erals occurring in one and the same rock. 
 
 In the eruptive rocks the magnesian silicates generally 
 followed these in order of separation (augite, hornblende, bio- 
 tite, and olivine), then the feldspars, and finally quartz. Never- 
 theless no universal law can be formulated. Still less possible 
 is it to formulate a law for the crystalline schists. Quartz, and 
 also orthoclase, are found included within hornblende and gar- 
 
METHODS OF INVESTIGATION*. !<>I 
 
 net i.e., they were first formed ; or quartz and orthoclase are 
 interpenetrated (micro-pegmatitic, graphic-granitic) i.e., both 
 were developed at the same time. 
 
 The chemical and physical properties of minerals are of 
 course changed by these inclosures. Specimens as free as 
 possible from inclosures must therefore be selected for ex- 
 amination. 
 
 IV. DECOMPOSITION OF THE ROCK-CONSTITUENTS. 
 
 J. ROTH. Allgemeine und chemische Geologic. Berlin, 1879. I. Bd. 
 
 The rock-forming minerals are far more exposed to the 
 decomposing and solvent influences of filtrating waters than 
 the larger developed minerals. In the volcanic rocks a further 
 change of the rock-constituents is induced by the action of 
 the gaseous emanations accompanying the eruptions. For 
 these reasons, therefore, different minerals are found in the 
 rock-preparations in different stages of decomposition. The 
 metamorphosis in most cases can be studied and followed on 
 the thin sections. It begins almost always from without and 
 advances inwards, especially on the cleavage-fissures of crystals 
 or grains ; the crystal-kernel, as in the feldspars, though rarely, 
 first urfdergoes decomposition. 
 
 Olivine, orthoclase, and magnetite, of the rock-forming 
 minerals, most commonly occur thus metamorphosed. 
 
 In the metamorphosis of olivine into serpentine, fine green- 
 ish or brown thread-like aggregates appear along the fissures. 
 These gradually broaden, whereby the cross-section of olivine 
 on the slide seems drawn into a net of serpentine, in whose 
 meshes lie the fresh olivine residues. These also finally under- 
 go decomposition, and a complete pseudomorphosis of serpen- 
 tine after olivine results. 
 
 Serpentine is generally tinged red by freshly-formed iron 
 
F ROCK-FORMING MINERALS. 
 
 hydroxide. Clino-chlore is deposited in many cases in olivine- 
 fels by the metamorphosis of magnetite. In these cases water 
 is taken up and magnetite and iron silicates are deposited. 
 If the silicates are removed so that only the ferrous oxide 
 separated from olivine remains as ferric oxide and hydroxide, 
 pure pseudomorphs of ferric oxide and hydroxide after olivine 
 are often formed. 
 
 Grayish to brownish opaque pseudomorphoses after olivine 
 are often found in the picrites, consisting principally of calcite 
 and showing a mesh-like structure. The meshes themselves 
 are formed from calcium silicate, while the spaces between 
 are filled with calcite. In this case silicic acid and magnesia 
 are removed, and alumina, lime, carbon dioxide, and alkalies 
 are taken up. Similar pseudomorphs of calcite after augite 
 also occur. 
 
 In the metamorphosis of feldspar into kaolin no such regu- 
 lar progress of decomposition beginning with the cleavage- 
 fissures, as a rule, can be determined ; they become spotted 
 and opaque, and metamorphosed into an aggregate of minute 
 gray or white grains. The alumina remains constant, silicic 
 acid is partially removed, water and potassium are taken up. 
 In the zonally-developed feldspars the layers rich in inclosures 
 first undergo decomposition. 
 
 Potassium micas, in minute brilliantly-polarizing tablets, 
 are also commonly formed by the decomposition of the feld- 
 spars ; quite perfect pseudomorphs of muscovite, after ortho- 
 clase, are often found. In this case the greater part of the 
 alkali remains; the rest is removed together with silicic acid, 
 which often separates as quartz (SiO 2 ). 
 
 Menaccanite becomes coated with a gray opaque coating 
 (leucoxene), and is finally metamorphosed into transparent 
 titanite ; lime must be added. More rarely menaccanite meta- 
 morphoses into rutile with separation of ferric oxide, which de- 
 posits as a reddish border about the decomposed mineral. 
 
METHODS OF INVESTIGATION. 1 03 
 
 Finally, mention must be made of the metamorphosis of 
 minerals of the hauyn group, and of nepheline into the zeolites, 
 especially natrolite, wherein calcite often separates ; the meta- 
 morphosis of garnet into chlorite ; of biotite, hornblende, and 
 augite into chlorite and epidote, with elimination of quartz, 
 ferric hydroxide, and calcite ; the decomposition of rhombic 
 augite in bastite, etc. 
 
 END OF PART T. 
 
PART II. 
 
 TABLES FOR DETERMINING MINERALS. 
 
 ABBREVIATIONS USED IN THE TABLES. 
 
 Under the heading "Optical Properties:" 
 
 AP = Plane of the optic axes. 
 
 1 M. = First middle line. 
 
 2 M. = Second middle line. 
 
 a = Axis of greatest elasticity. 
 b = Axis of middle elasticity = optic normal, 
 c = Axis of least elasticity. 
 I = Parallel. 
 JL = At right angles. 
 
 w = Index of refraction. 
 
 For optically- ( GO = Index of refraction for the ordinary ray. 
 uniaxial minerals. ( = Index of refraction for the extraordinary ray. 
 
 For optically- j ft Index of refraction of middle value, 
 -biaxial minerals. ( p = For red light. 
 
 i. c. p. 1. = In convergent polarized light, 
 i. p. p. 1. = In parallel polarized light. 
 For the crystallographic axes : 
 
 c = Chief, i.e. vertical, axis. 
 In rhombic or i a = Brachydiagonal axis, 
 triclinic minerals. ( ~b = Macrodiagonal axis. 
 In monoclinic c a = Clinodiagonal axis, 
 minerals. ( b' = Orthodiagonal axis. 
 Under the heading "Structure:" 
 
 I. O. = Components first in order of separation. 
 II. O. = Components second in order of separation. 
 
1 06 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 rk 
 n. 
 
 ROPE 
 
 and da 
 
 .2 > 
 
 
 
 tX ^ 
 
 it 
 
 -s.s 
 
 posi 
 
 lac 
 ion 
 op 
 
 w 
 
 v 
 
 .- o.'o-j 
 *2- 
 
 
 
 u rt^i 
 i2^ . 
 
 iillll 
 
 ??..S < 9 
 
 
 
 Ilillfilll 
 
 "lo 5 -- 8 =8 = 
 
 & 
 
 U X 
 
 nli 
 
 
 111 
 
 K- O 
 
 M a -a 
 
 
 rt <S : J3 $ > 
 
 
 . 
 
 
 iii 
 
 
 sli 
 
 .sB-^ e 
 
 < 
 
 ill 
 
 syjSuv fifSi^ fv suot 
 
 S*iip 3 iff of 
 
 in 
 
TABLES FOR DETERMINING MINERALS. 
 
 107 
 
 il 
 
 - g 
 
 1~* " o S 8 3 *. o =-S- s S 
 
 ** 
 
 ropic sections at right angles to one of the optic axes show in con- 
 ing polarized light a black cloud with or without colored rings, 
 rding to the power of double-refraction. According as the section 
 ore or less perpendicular to the middle line, the axial point appears 
 out or within the field. On revolving the stage this cloud revolves 
 contrary direction. If this cloud, i.e., hyperbola, is red on the 
 ex side and blue on the concave, then the dispersion of the axes 
 > v\ if the reverse. 71 > p. The angle of inclination of the axes of 
 ticity to the crystallographic axes in sections parallel oojPoo in 
 oclinic minerals, and parallel oP and oo A in the triclinic, gives 
 dmirable means of determining them. 
 
 a 
 
 Sjrf -a-S , v 
 
 5 ux: <u o ix: <L> . 
 
 liil 1 
 
 o be 3 5-= 
 
 
 
 s 
 
 
 a- > 
 
 ' -oS 
 
 S9j3uV 
 
io8 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 A. Even in the thinnest Sections 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 System 
 of 
 crystalliza- 
 tion. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the 
 cross-section. 
 
 Twins. 
 
 1. Magnetite. 
 
 Fe 3 4 
 (FeO -|- Ft- 2 O 3 ). 
 
 4.9-5.2. 
 
 
 According 
 toO. 
 
 Grains and 
 octaliedra. 
 
 According 
 toO. 
 
 eisen.) 
 
 Easily soluble 
 in HC1. 
 
 
 
 
 Squares and 
 
 
 
 
 
 
 
 equilateral 
 
 
 
 
 
 
 
 triangles. 
 
 
 2. Titaniferous 
 Magnetite. 
 (Tittin- 
 magneteisen . ) 
 
 JTp O 
 
 Distinguished 
 from 
 magnetite only 
 by chemical 
 analysis. 
 {Reaction for 
 titanium?) 
 
 4.8-5.1. 
 
 1 
 K 
 
 
 Octahedra 
 and 
 grains. 
 
 > 
 
 3. Pyrite. 
 
 FeS 2 . 
 Easily soluble 
 in HN0 3 , 
 with 
 separation of S. 
 
 4 9-5.2- 
 
 
 According 
 to oo O oo. 
 
 2L& 
 
 2 
 
 Regular 
 hexagons and 
 pentagons. 
 
 Penetration- 
 twins of 
 
 2 
 
TABLES FOR DETERMINING MINERALS. 
 
 of Opaque Minerals. 
 
 1 09 
 
 Color and 
 lustre. 
 
 Structure. 
 
 Association. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 Iron-black; 
 in reflected 
 
 Often in 
 beautiful 
 
 With nearly 
 all of the 
 
 Commonly 
 into iron 
 
 i. As primary 
 essential 
 
 
 light bluish- 
 black metallic 
 lustre. 
 
 crucitorm 
 aggregates ; 
 o.' as product 
 of decomposi- 
 tion wreathed 
 about the 
 minerals; 
 also deposited 
 upon the 
 
 rock-forming 
 minerals; 
 especially with 
 augite, 
 olivine, 
 plagioclase, 
 nepheline, 
 and leucite. 
 
 hydroxide. 
 A reddish- 
 brown circle 
 about the 
 magnetite 
 crystals. 
 
 constituent of the 
 basic eruptive 
 rocks; accessory 
 in nearly all of 
 the crystalline 
 rocks. 
 2. As decomposi- 
 tion-product of 
 olivine, augite, 
 
 
 
 cleavage- 
 
 
 
 hornblende, 
 
 
 
 fissures. 
 
 
 
 and biotite. 
 
 
 Ditto. 
 
 
 
 Into titanite, 
 leucoxene, 
 and 
 iron hydroxide. 
 
 Primary; in 
 basaltic rocks and 
 crystalline schists. 
 
 Forms at the 
 same time 
 the transition- 
 products to 
 
 
 
 
 
 
 
 ilmenite. 
 
 In reflected 
 light 
 bra ss-yello iv . 
 Metallic 
 lustre. 
 
 
 
 
 Into iron 
 hydroxide. 
 
 Rarely as 
 accessory 
 secondary 
 constituents in 
 decomposed basic 
 eruptive rocks, 
 and (also primary) 
 in ciystalline 
 schists. 
 
 
110 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 System 
 of 
 crystalliza- 
 tion. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the 
 cross-section. 
 
 Twins. 
 
 4. Ilmenite. 
 (Titan-eisen ) 
 
 FeTiOg + 
 X (Fe a 6 8 ). 
 Difficultly 
 soluble in HC1. 
 Ti-reaction 
 with 
 
 4.56-5.21. 
 
 
 R and oR; 
 conchoidal 
 separation 
 (absonde- 
 rung). 
 
 Tabular 
 R . oK 
 also 
 -W, ~2/?, 
 and grains 
 which are not 
 
 With parallel 
 axial 
 systems. 
 Polysynthetic 
 twins 
 af er R. 
 
 
 microcosmic 
 
 
 
 
 spherical 
 
 
 
 salt. 
 
 
 
 
 but for the 
 
 
 
 
 
 
 
 most part 
 
 
 
 
 
 
 
 long rods. 
 
 
 
 
 
 
 
 Cross-sections 
 
 
 
 
 
 
 
 generally 
 
 
 
 
 
 
 
 hexagonal, 
 
 
 
 
 
 
 
 long 
 
 
 
 
 
 
 
 threaalike, 
 
 
 
 
 
 
 
 jagged, or 
 
 
 
 
 
 
 
 netted forms. 
 
 
 
 
 
 73 
 
 
 
 
 
 
 
 1 
 
 
 
 
 5. Graphite 
 
 (and 
 bitumen). 
 
 C. 
 
 Bituminous 
 black rocks, 
 
 1.9-2.3. 
 
 1 
 
 <u 
 
 flP. 
 
 Rarely in thin 
 hexagonal 
 tablets and 
 
 
 
 becoming' 
 
 
 
 
 irregular 
 
 . 
 
 
 grayish-white 
 
 
 
 
 leaves. 
 
 
 
 on heating. 
 
 
 
 
 
 
 6. Pyrrhotite. 
 
 (Magnet kies.) 
 
 F eS n +l. 
 
 4.54-4.64. 
 
 
 
 Irregular 
 grains. 
 
 
 MINERALS RENDERED TRANSPARENT 
 
 
 1. Ohromite. 
 
 1 
 
 
 
 
 
 
 
 r See page 14. 
 
 
 Regular. 
 
 1 
 
 Grains and 
 octahedra. 
 
 
 2. _ ; ieonaste. 
 
 J 
 
 
 
 
 
 
 3. Haematite. 
 
 See page 32. 
 
 
 Hexagonal . 
 
 
 Tablets. 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Ill 
 
 Color and 
 
 lustre. 
 
 Structure. 
 
 Association. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 Black-brown; 
 
 
 With 
 
 Into titanite 
 
 In basic 
 
 Can be 
 
 metallic 
 lustre. 
 In reflected 
 
 
 plagioclase, 
 augire, 
 hornblende, 
 
 (leticoxene) and 
 rutile with, 
 heematite. Ilmenite 
 
 eruptive rocks 
 (especially the 
 granular 
 
 distinguished 
 from magnetite 
 by the form 
 
 light gray, 
 
 
 and 
 
 is metamorphosed 
 
 diabases, 
 
 of the 
 
 if decomposed. 
 
 
 olivine. 
 
 by decomposition 
 
 gabbros, 
 
 cross-section. 
 
 
 
 
 first into a 
 
 basalts, 
 
 and especially 
 
 
 
 
 grayish, opaque, 
 pulverulent mine- 
 ral (leucoxene), 
 changing gradually 
 
 picrites); 
 also in 
 crystalline 
 schists. 
 
 by the. 
 phenomena 
 attending 
 decomposition. 
 
 
 
 
 into one brown and 
 
 
 
 
 
 
 transparent, which 
 
 
 
 
 
 
 can be determined 
 
 
 
 
 
 
 as titanite; often 
 
 
 
 
 
 
 thin decomposed 
 
 
 
 
 
 
 threads of 
 
 
 
 
 
 
 ilmenite remain. 
 
 
 
 
 
 
 (Comp. Fig. 51.) 
 
 
 
 Iron-black ; 
 
 
 
 
 Rare, in 
 
 Distinguished 
 
 metallic 
 
 . 
 
 
 
 crystalline 
 
 from haematite 
 
 lustre. 
 
 
 
 
 schists, clay 
 and clay-mica 
 
 by its opacity 
 or decoloration 
 
 
 
 
 
 schists, gneiss, 
 limestone, and 
 
 by heating. 
 
 
 
 
 
 as an inclosure 
 
 
 
 
 
 
 in staurolite, 
 
 
 
 
 
 
 andalusite, 
 
 
 
 
 
 
 chiastolite, 
 
 
 
 
 
 
 dipyre, and 
 
 
 
 
 
 
 couzeranite. 
 
 
 Bronze-yellow 
 and 
 copper-red. 
 
 
 
 
 Rarely in 
 crystalline 
 schists, 
 contact-schists. 
 
 Can be 
 
 distinguished 
 from pyrite 
 easily by the 
 
 
 
 
 
 
 lustre in 
 
 
 
 
 
 
 reflected light. 
 
 ONLY WITH DIFFICULTY. 
 
 Black: 
 
 
 
 
 
 
 metallic lustre; 
 
 
 
 
 
 
 if transparent, 
 broivn. 
 
 
 
 
 Rare in olivine 
 
 
 Green. 
 
 
 
 
 rocks. 
 Contact rocks 
 
 Similarity 
 with 
 
 
 
 
 
 and 
 schistose rocks. 
 
 magnetite. 
 
 Red. 
 
 
 
 
 Generally as 
 
 Similar to 
 
 
 
 
 
 accessory 
 constituent and 
 
 graphite, etc. 
 
 
 
 
 
 product of 
 
 
 
 
 
 
 decomposition. 
 
 
112 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 B. Minerals Transparent 
 I. SINGLE-REFRACTING MINERALS. 
 
 a. Amorphous 
 
 
 Chemical 
 
 
 
 
 NAME. 
 
 composition and 
 
 Specific gravity. 
 
 Color. 
 
 Structure. 
 
 
 reactions. 
 
 
 
 
 1. Opal, 
 (Porodinamorph.) 
 
 Essentially SiOo 
 (H 2 O. traces of 
 Fe, Ca, Al, Mg, 
 
 1.9-2.3- 
 
 Colorless 
 (white, yellowish), 
 often colored 
 
 a. Homogeneous 
 and devoid 
 of structure. 
 
 
 and the alkaloids). 
 Soluble in 
 
 
 red and brown 
 by ferric oxide 
 
 b. Concentric- 
 conchoidal, and 
 
 
 KOH. 
 
 
 and hydroxide. 
 
 then showing 
 
 
 
 
 n = 1.455. 
 
 often in 
 
 
 
 
 
 parallel polarized 
 light feeble 
 
 
 
 
 
 double-refraction 
 
 
 
 
 
 (interference- 
 
 
 
 
 
 cross). 
 
 
 
 
 
 In clusters, 
 
 
 
 
 
 crust-like. 
 
 
 
 
 
 (See Fig. 52.) 
 
 2. Hyalite. 
 (Glasmasse.) 
 
 Always a 
 complicated 
 silicate 
 (Si, Al, Fe, Ca, 
 Mg, alkalies). 
 Acid vitreous 
 
 Acidic 
 = 2.2-2.4 
 (obsidian). 
 Basic 
 = 2 .5 r 
 (trachylyte). 
 
 Colorless, 
 or colored gray, 
 brown, or red. 
 The basic hyalite 
 is generally dark, 
 the acidic light. 
 
 a. Obsidians 
 absolutely devoid 
 of structure, 
 with separation of 
 free glasses. 
 b. Pitchstone, 
 
 
 mass containing 
 about 70 per cent 
 SiO a insoluble m 
 
 
 (for obsidian) 
 = 1.488. 
 
 with macroscopic 
 separations. 
 c. Pumice-stone, 
 
 , 
 
 HC1. 
 
 
 
 fibrous and filled 
 
 
 Basic vitreous 
 mass with 
 
 
 
 with gas-pores. 
 d. Perlite. 
 
 
 about 40 per cent 
 SiO 2 , for the most 
 
 
 
 spherical with 
 concentric- 
 
 
 part soluble in 
 
 
 
 conchoidal 
 
 
 HC1. 
 
 
 
 structure. 
 
TABLES FOR DETERMINING MINERALS, 
 
 in Thin Sections. 
 
 (Isotropic in all cross-sections.) 
 
 Minerals. 
 
 Inclosures. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 a. Brownish -red, 
 dust-like inclosures of 
 
 ferric hydrate. 
 
 b. Aggregates of 
 hexagonal tablets of 
 
 tridymite. 
 
 c. Fluid inclosures 
 
 and gas-pores. 
 
 Always secondary ; 
 
 decomposition- 
 product of the rock- 
 constituents 
 feldspar, augite, 
 hornblende, biotite, 
 and then deposited 
 either in the primary 
 
 position, i.e., as 
 pseudomorphs after 
 
 these minerals, 
 
 or in some secondary 
 
 position lining the 
 
 walls of cavities ; 
 
 especially in the 
 
 acidic younger 
 
 eruptive "rocks, the 
 
 rhyolites and 
 
 andesites, but also in 
 
 the basic basalts. 
 
 The ground-mass of 
 
 many decomposed 
 
 eruptive rocks is 
 
 almost completely 
 
 metamorphosed into 
 
 opal (semi-opal). 
 
 Inclosures, i.e., 
 separations, are 
 
 gas-fores, 
 
 crystallites, and 
 
 microliths very 
 
 commonly ; also 
 
 crystals and 
 
 sphaeroliths. 
 
 (Compare Figs. 41 
 
 and, 42.) 
 
 Into viridite in the basic 
 rocks, basalt ; into 
 opal in the acidic, 
 
 rhyolite. 
 
 Basic glasses are often 
 decomposed into a 
 
 yellowish, 
 
 do- .ble-refracting, 
 
 fibrous substance 
 
 (pelagonite). 
 
 The natural glasses 
 
 are only one method 
 
 of solidification of the 
 
 eruptive rocks. 
 
 Hyalite occurs more 
 
 or less commonly in 
 
 often apparently 
 
 purely crystalline 
 
 eruptive rocks, and 
 
 only in them.' 
 
 Rock glasses 
 
 (vitrophyre) are known 
 
 in the following 
 
 eruptive rocks: 
 
 a. Acidic = vitreous 
 rhyolith, trachyte, 
 
 dacite, andesite, 
 
 porphyries; rarely 
 
 porphyrites and 
 
 phonoliths. 
 
 b. Basic = vitreous 
 diabase, melaphyr. 
 
 augite-andesite, and 
 basalts (trachylyte. 
 hyalomelane, sidero- 
 melane, palagonite, 
 
 hydrotachylyte). 
 
 Frequently pure hyalite 
 
 can be distinguished 
 
 from opal or.ly with 
 
 difficulty; the only 
 
 surety lies in the 
 
 ttt icro- ch em ica I 
 
 analysis (preferably 
 
 corrosion with hydro- 
 
 Jluosilicic acid). The 
 
 glasses are mentioned 
 
 here only because of 
 
 their differences 
 
 from opal. 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 b. Minerals Crystallizing 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the 
 cross-section. 
 
 Twins. 
 
 Color and power 
 refracting light. 
 
 
 1 
 
 
 
 
 
 1. Hauyn 
 Group. 
 
 
 
 
 
 
 
 a. Sodalite. 
 
 3(Na 2 Al 2 Si 2 O e ) 
 
 2.13-2.29. 
 
 ooO. 
 
 Grains and 
 
 Penetra- 
 
 Colorless; 
 
 
 {- 2 N aCJ 1 
 
 
 
 ooO (rarely 
 
 tion twins 
 
 colored by 
 
 
 Cl-reaction \ 
 
 
 
 O.ooOoo); 
 
 after a 
 
 Ke 2 O 3 
 
 
 easily soluble 
 in HC1; 
 gelatinous 
 
 
 
 cross-sections 
 rectangular 
 and hexagonal. 
 
 trigonal 
 secondary 
 axis. 
 
 red, green, 
 and blue 
 mostly blue. 
 
 
 SiO 2 ; 
 
 
 
 
 
 
 
 NaCl-cubes on 
 
 
 
 
 
 
 
 evaporation. 
 
 
 
 
 
 
 |3. Hauyn 
 and 
 y. Nosean. 
 
 2Na 2 CaAl a 
 
 Na 2 (Ca)S0 4 . 
 Reaction for 
 Caand H 2 SO 4 . 
 
 2.4-2.5. 
 2279-2.399 
 
 ooO. 
 
 Crystals 
 oo O and O, 
 like sodalite. 
 
 Commonly 
 distorted or 
 
 Twins 
 after O 
 and like 
 sodalite. 
 
 Colorless, blue 
 or black. 
 
 Colorless^ 
 brown or 
 
 
 Reaction lor 
 
 
 
 corroded 
 
 
 black. 
 
 
 H 2 S0 4 . 
 
 
 
 cryrtals. 
 
 
 
 
 Both soluble 
 
 
 
 
 
 
 
 in HC1 with 
 
 
 
 
 
 
 
 separation of 
 
 
 
 
 
 
 
 gelatinous 
 
 
 
 
 
 
 
 SiO 3 . 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS, 
 in the Regular System. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 Interpenetrated 
 
 With micro- 
 
 Fluid 
 
 Becomes 
 
 As primary 
 
 
 with teldspar 
 and horn- 
 blende; the 
 
 cline, augite, 
 and mica in 
 syenites. 
 
 Enclosures and 
 gas-pores. 
 
 opaque by 
 decomposition 
 into zeolites. 
 
 constituent in 
 syenites 
 (elseolith- 
 
 
 centre often 
 
 
 Vitreous 
 
 
 syenite), and 
 
 
 colorless, and 
 
 With sanidine 
 
 inclosures and 
 
 
 rarely in 
 
 
 outer layers 
 
 and augite 
 
 augite needles. 
 
 
 augitic 
 
 
 blue or 
 
 in trachytes. 
 
 
 
 trachytes. 
 
 
 colored red by 
 
 
 
 
 
 
 ferric oxide. 
 
 
 
 
 Secondary in 
 
 The three minerals 
 
 
 
 
 
 cavities of the 
 latter. 
 
 can be accurately 
 distinguished 
 
 
 
 
 
 
 only by the 
 
 
 
 
 
 
 m icro-ch em ical 
 
 
 
 
 
 
 qualitative 
 
 
 
 
 
 
 
 analysis. 
 
 The outer 
 coats colored. 
 
 Generally with 
 leucite, 
 
 Numberless 
 gas pores 
 
 Into a felt-like 
 aggregate of 
 
 Primary 
 constituent. 
 
 Hauyn is 
 distinguished from 
 sodalite by 
 presence of the 
 
 with opaque 
 
 nefiheline^ 
 
 and vitreous 
 
 colorless, 
 
 
 characteristic 
 
 or dark core; 
 often colored 
 red bv 
 
 and augite. 
 
 inclosures 
 arranged in 
 streamers. 
 
 double- 
 refracting 
 needles and 
 
 In \htyounger 
 eruptive rocks 
 and the 
 
 gypsum needles 
 on evaporating a 
 drop of the 
 
 iron oxide 
 at the cleavage- 
 fissures. 
 
 
 Black 
 minute grains 
 and needles, 
 
 filaments of 
 zeolites and 
 calcite. A 
 
 sanidine and 
 plagioclase 
 rocks, as 
 
 hydrochloric acid 
 solution (on 
 account of the 
 
 Regularly 
 disposed 
 inclosures, 
 and systems of 
 dark streaks 
 at right angles 
 
 
 like dust, 
 often at regular 
 spaces. 
 
 Pyrite tablets. 
 
 decoloration of 
 the hauyn 
 thus occurs ; 
 a yellowish, 
 secondary 
 coloration 
 
 trachyte 
 (rarely), 
 phonolite, 
 leucttophyr, 
 tephhtes, 
 nepheline 
 
 calcium present 
 in hauyn); 
 sodalite is 
 characterized by 
 the chlorine. 
 
 to each other. 
 
 
 
 of the 
 
 and leucite 
 
 
 (See Fig. 53.) 
 
 
 
 decomposition- 
 
 basalts. 
 
 
 
 
 
 product by 
 
 
 
 
 
 
 ferric 
 
 Very common 
 
 It is difficult also 
 
 
 
 
 hydroxide. 
 
 in the 
 
 to distinguish 
 
 
 
 
 
 trachytic 
 
 hauyn from nosean 
 
 
 
 
 
 volcanic lavas. 
 
 chemically. 
 
 
 
 
 
 
 Mineralogically 
 
 
 
 
 
 
 they are one. 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Color and 
 power of 
 refracting light. 
 
 2. Garnet 
 Group, 
 a. Almandine. 
 
 Fe 3 Al a Si 3 O ia . 
 
 3.78 
 (3.1-4.2). 
 
 Imperfect 
 
 00 O. 
 
 oo0.202 
 and grains. 
 Cross- 
 sections 
 quadratic, 
 hexagonal, 
 or 
 octagonal. 
 
 
 Red, in very 
 thin sections 
 nearly 
 colorless. 
 ftp = 1.772. 
 
 ft. Pyrope. 
 
 (CaO, MgO, 
 FeO, MnO) 
 Al a 3 3SiO a . 
 Contains Cr. 
 
 3-7-3 8. 
 
 Imperfect 
 
 00 O. 
 
 Grains. 
 
 
 Blood-red. 
 
 y. Melanite. 
 
 Ca 3 Fe a Si 8 12 . 
 
 3.6-4.3. 
 
 Imperfect 
 
 00 O. 
 
 Crystals 
 
 00 O. 
 
 
 Black; 
 dark brown 
 in sections. 
 Transparent 
 only with 
 difficulty. 
 
 
 All garnets are 
 insoluble in acids. 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 117 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 Commonly 
 disseminated 
 through 
 micro-pegmatic 
 quartz and 
 feldspar. 
 
 Generally with 
 quartz, 
 orthoclase, 
 biotite, and 
 hornblende. 
 
 Cavities of 
 the form of 
 garnet 
 (= negative 
 crystals). 
 Fluid 
 inclosures, 
 
 Commonly 
 metamorphosed 
 into chlorite 
 tablets on the 
 upper surfaces 
 and cleavage- 
 fissures. More 
 
 Primary 
 constituent ; 
 in many 
 crystalline 
 schists, 
 common in 
 granite, rare 
 
 
 
 
 quartz- 
 
 rarely, as in 
 
 in trachytic 
 
 
 
 
 granules. 
 
 pyrope, 
 
 rocks. 
 
 
 
 
 Rutile; often 
 
 metamorphosed 
 
 
 
 
 
 zonally 
 
 about the 
 
 
 
 
 
 disposed. 
 (See Fig. 54.) 
 
 edges into a 
 fibrous 
 hornblende, 
 
 
 
 
 
 
 or augite- zone. 
 
 
 
 
 
 
 
 
 The garnets 
 
 
 
 
 
 
 can be easily 
 
 
 
 
 
 
 distinguished 
 
 
 
 
 
 
 from the 
 
 
 
 
 
 
 hauyn by the 
 color and 
 
 
 With olivine 
 and augite. 
 
 Very poor. 
 
 Augitic fibrous 
 tufts shooting 
 out in marginal 
 
 Primary 
 constituent. 
 In serpentines. 
 
 insolubility 
 in acids. 
 
 
 
 
 zones, 
 
 
 
 
 
 
 perpendicular to 
 the surface of 
 
 
 
 
 
 
 the grains 
 
 
 
 
 
 
 are very common 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 characteristic. 
 
 
 
 ' 
 
 
 
 (See Fig. 55.) 
 
 
 
 Very 
 commonly 
 beautiful 
 zonal 
 
 With augite, 
 saniditte^ 
 nepheline^ 
 hauyn, and 
 
 Very poor. 
 Augite and 
 apatite 
 needles; 
 
 
 Primary 
 constituent. 
 In phonoliths, 
 leucitophyr, 
 
 Compare with 
 chromite. 
 
 structure, 
 
 teucite. 
 
 vitreous 
 
 
 and volcanic 
 
 
 then 
 
 
 inclosures. 
 
 
 lavas. 
 
 
 generally 
 showing 
 
 
 
 
 
 / 
 
 double 
 
 
 
 
 
 
 refraction. 
 
 
 
 
 
 
 (See Fig. 54.) 
 
 
 
 
 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 
 
 
 
 Ordinary 
 
 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 combinations 
 and form 
 of the 
 
 Twins. 
 
 Color and 
 power or 
 refracting light. 
 
 
 
 
 
 cross-section. 
 
 
 
 3. Spinel Group, 
 a. Lhromite. 
 
 FeO,Cr 2 3 . 
 
 4.4-4.6. 
 
 Imperfect 
 
 Grains and 
 octahedra. 
 
 
 Becomes 
 translucent 
 
 
 
 
 
 
 
 only with 
 difficulty ; 
 
 i 
 
 
 
 
 
 
 dark brown^ 
 
 
 
 
 
 
 
 reddish brown, 
 
 
 
 
 
 
 
 metallic 
 
 
 
 
 
 
 
 lustre. 
 
 j8. Picotite. 
 
 MgO) A1 2 O 3 I 
 FeO f Fe 2 3 f 
 
 4.08. 
 
 
 Octahedra. 
 Very minute 
 
 Twins 
 according 
 
 ditto. 
 
 
 
 
 
 grains. 
 
 toO. 
 
 
 y. Pleonaste. 
 
 FeO I A1 2 O 3 } 
 MgO f Fe 2 3 f 
 
 Above 3.65. 
 (3-8-4.1). 
 
 
 ditto. 
 
 
 Dark green. 
 
 8. Hercynite. 
 
 FeO, A1 2 O 3 . 
 
 3.91-3.95. 
 
 
 Octahedra. 
 
 
 ditto. 
 
 
 Insoluble in 
 
 
 
 
 
 
 
 acids; 
 
 
 
 
 
 
 
 unattacked 
 
 
 
 
 
 
 
 by HF1. 
 
 
 
 
 
 
 4. Analcime. 
 
 Na 2 Al 2 SLO 12 
 Soluble in HC1 
 
 2.1-2.28. 
 
 'Imperfect); 
 according 
 to 
 
 Generally 
 compact grains: 
 in cavities 
 
 
 Colorless, 
 -white, 
 np = 1.4874. 
 
 
 with 
 
 
 Tschermak, 
 
 2O2. 
 
 
 
 
 separation 
 of gelatinous 
 
 
 clearly 
 ooOoo. 
 
 
 
 
 
 SiO a . 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Many 
 
 With olivine 
 
 
 
 I 
 
 Great similarity 
 
 individual 
 
 and augite. 
 
 
 
 Primary 
 
 with melanite; if 
 
 grains 
 
 
 
 
 accessory 
 
 in grains, can 
 
 occurring in 
 
 
 
 
 constituent. 
 
 be distinguished 
 
 basalts show 
 
 
 
 
 In olivine rocks, 
 
 only by chemical 
 
 a broad, 
 
 
 
 
 serpentines, 
 
 tests. Melanite is 
 
 opaque border. 
 
 
 
 
 and in basalts. 
 Picotite 
 
 attacked by 
 concentrated HF1, 
 
 
 
 
 
 commonly 
 inclosed in 
 
 and is free from 
 chromium. 
 
 
 
 
 
 olivine. 
 
 Melanite is 
 
 
 
 
 
 J 
 
 almost always 
 
 
 
 
 
 
 crystallized, and 
 
 
 
 
 
 
 hence can be 
 
 
 
 
 
 
 easily distinguished 
 
 
 
 
 
 
 from chromite. 
 
 
 
 
 
 
 Chromite and 
 
 
 Rarely with 
 olivine 
 
 
 
 
 picotite can be 
 distinguished only 
 by chemical means. 
 
 
 and augite. 
 
 
 
 
 The spinels are 
 distinguished from 
 
 
 Common 
 with quartz, 
 orthoclase, 
 
 
 
 The same, but 
 rare. More 
 common in 
 granulites 
 and in 
 metamorphic 
 (contact) rocks. 
 
 magnetite by 
 their transparency 
 (in very thin 
 sections) and 
 insolubility in acids. 
 Pleonaste and 
 hercynite can be 
 distinguished only 
 
 
 and mica. 
 
 
 
 
 by chemical 
 
 
 
 
 
 
 analysis. 
 
 * 
 
 
 
 
 J 
 
 
 Often showing 
 double- 
 refraction and 
 remarkable 
 
 With 
 plagioclase, 
 augite, or 
 hornblende. 
 
 Poor in 
 inclosures. 
 Fluid 
 inclosures. 
 
 
 Either primary (?) 
 or decomposition 
 product of 
 nepheline (?) 
 
 Can be determined 
 accurately only 
 by chemical tests. 
 
 zonal structure; 
 
 
 Apatite 
 
 
 Rare in the 
 
 
 generally 
 opaque: 
 
 
 needles. 
 
 
 younger basic 
 eruptive rocks, 
 
 
 interpenetrated 
 
 
 
 
 the teschenites. 
 
 
 with . 
 plagioclase. 
 
 
 
 
 As decomposition- 
 products in 
 
 
 
 
 
 
 cavities (secondary) 
 in phonoliths, 
 
 
 
 
 
 
 trachytes, 
 
 
 
 , 
 
 
 
 andesites, basalts. 
 
 
120 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 
 
 
 
 Ordinary 
 
 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 combinations 
 and form 
 of the 
 
 Twins. 
 
 Color and power 
 of refracting 
 light. 
 
 
 
 
 
 cross-section. 
 
 
 
 5 Fluorite. 
 
 CaFl 2 . 
 Decomposed 
 
 3-I-3-2- 
 
 Perfect 
 after O. 
 
 In rocks 
 only in form 
 
 
 Blue, 
 transparent. 
 
 
 by 
 
 
 
 of minute 
 
 
 n = 1.435. 
 
 
 concentrated 
 
 
 
 angular 
 
 
 
 
 H 2 SO 4 with 
 
 
 
 granules. 
 
 
 
 
 evolution 
 
 
 
 
 
 
 
 of HF1. 
 
 
 
 
 
 
 6. FerowsMte. 
 
 CaTiOg, 
 not attacked 
 by HC1 ; 
 decomposed 
 by concen. 
 H a S0 4 . 
 
 4.0-4.1. 
 
 ooQoo. 
 
 In irregular^ 
 arborescent, 
 and 
 jagged fortns^ 
 often in 
 sharp 
 
 Rare; 
 penetra- 
 tion- 
 twins. 
 
 Grayish-violet; 
 grayish- 
 reddish 
 brown. 
 Relief 
 well marked. 
 
 
 
 
 
 octahadra. 
 
 
 
 
 
 
 
 (See Fig. 56.) 
 
 
 
 MINERALS APPARENTLY CRYSTALLIZING 
 
 
 Leucite. 
 
 
 
 
 Apparently 
 2O2. 
 
 
 
 
 
 
 
 Compare 
 with minerals 
 
 
 
 
 
 
 
 of the 
 
 
 
 
 
 
 
 tetragonal 
 
 
 
 
 
 
 
 system. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 121 
 
 
 
 
 
 
 Structure. 
 
 Association. 
 
 Inclosures. , Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 
 
 
 
 
 Developed in 
 feldspar and 
 irregularly 
 distributed 
 through the 
 
 With quartz, 
 orthoclase, 
 biotite. 
 
 Fluid 
 inclosures. 
 
 
 Very rare ; 
 secondary in 
 quartzose 
 porphyries. 
 
 
 ground-mass. 
 
 
 
 
 
 
 The rough 
 surface 
 of the slide very 
 characteristic. 
 
 With 
 nepheline, 
 melilite, 
 augite^ 
 
 Very poor. 
 
 
 In nepheline, 
 leucite, and 
 melilite 
 basalts. 
 
 Can be 
 distinguished 
 from spinel 
 by its 
 
 Grouped 
 
 and olivine. 
 
 
 
 
 color and 
 
 as inclosure 
 in melilite and 
 
 
 
 
 
 optical 
 anomalies ; 
 
 also in olivine, 
 
 
 
 
 
 and from 
 
 often showing 
 double refraction. 
 Polarization- 
 
 
 
 
 
 garnet by the 
 crystalline 
 form. 
 
 colors very 
 
 
 
 
 
 
 faint. 
 
 
 
 
 
 
 IN THE kEGULAR SYSTEM. 
 
 Twinning 
 striations^ double- 
 refracting. 
 
122 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 II. DOUBLE-REFRACTING 
 a. Optically- Uniaxial 
 
 i. MINERALS CRYSTALLIZING IN 
 
 A. DOUBLE-REFRACTION 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Character 
 and strenj?th Polarization- 
 of double- color, 
 refraction. 
 
 1. Leucite. 
 
 Soluble in 
 HCl. 
 Separation of 
 pulverulent 
 Si0 2 . 
 K-reaction 
 
 2.45-2.5. 
 
 Imperfectly 
 prismatic, 
 oo Pea and 
 oP. 
 
 Grains, but 
 (mostly) 
 crystals 
 P. 4 P2. 
 Apparently 
 a regular 
 form, 20-2. 
 
 AfterSPoo; 
 polysyn- 
 thetic 
 twinning- 
 striations 
 after these 
 faces, 
 
 The small \ Not very 
 individuals, brilliant 
 without bluish-gray, 
 twinning- 
 striations. 
 apparently 
 isotrope. 
 
 
 with hydro- 
 
 
 
 Crystal 
 
 crossing at 
 
 No clear 
 
 
 
 fluosilicic 
 
 
 
 cross- 
 
 right or 
 
 axial 
 
 
 
 acid. 
 
 
 
 section 
 generally 
 
 oblique 
 angles. 
 
 picture 
 evident in 
 
 
 
 
 
 
 octagonal, 
 
 (See Fig. S7 .) 
 
 c. p. 1. 
 
 
 
 
 
 
 often nearly 
 
 
 Double- 
 
 
 
 
 
 
 circular. 
 
 
 refraction 
 
 
 
 
 
 
 more rarely 
 rectangular 
 
 ' 
 
 positive; 
 very weak. 
 
 
 
 
 
 
 or 
 
 
 
 
 
 
 
 
 hexagonal. 
 
 
 
 
 2. Rutile. 
 
 (Nigrine, 
 Sagenite.) 
 
 Ti0 3 . 
 Ti-reaction 
 with micro- 
 cosmic bead. 
 
 4.2-4.3 
 (4-277). 
 
 oo P and 
 
 oo Poo. 
 
 co/> . ooPoo. 
 P. 
 Grains; 
 often, 
 
 Very 
 
 common 
 and charac- 
 teristic 
 
 The crystals 
 are gene- 
 rally too 
 minute to 
 
 None 
 especially 
 bright. 
 
 
 
 
 
 however, in 
 
 after .Poo. 
 
 examine 
 
 
 
 
 
 
 minute, 
 
 Bent at an 
 
 with the 
 
 
 
 
 
 
 very long 
 and narrow 
 
 angle of 
 114 25'. 
 
 condenser. 
 Double- 
 
 
 
 
 
 
 needles and 
 
 Also, a web 
 
 refraction 
 
 
 
 
 
 
 crystals. 
 The prisms 
 
 of needles 
 which cut 
 
 strongly 
 positive. 
 
 
 
 
 
 
 show a 
 
 each other 
 
 
 
 
 
 
 
 striation 
 
 at an angle 
 
 
 
 
 
 
 
 parallel to 
 
 of about 
 
 
 
 
 
 
 
 the oaxis. 
 
 60. 
 
 
 
 
 
 
 
 
 Heart- 
 
 
 
 
 
 
 
 
 shaped 
 
 
 
 
 
 
 
 
 twins 
 
 
 
 
 
 
 
 
 according 
 
 
 
 
 
 
 
 
 to 3 Poo 
 
 
 
 
 
 
 
 
 are very 
 
 
 
 
 
 
 
 
 common. 
 
 
 
 
 
 
 
 
 (Comp. 
 Figs. 20 
 
 
 
 
 
 
 
 
 and 59.) 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 123 
 
 MINERALS. 
 
 Minerals, 
 
 THE TETRAGONAL SYSTEM. 
 
 POSITIVE. 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Colorless, 
 clear as 
 
 
 Aggregates 
 of spherical 
 
 With 
 augite, 
 
 Inclosures 
 of minute 
 
 Into an 
 aggregate 
 
 Primary 
 essential 
 
 Easily distin- 
 guished from 
 
 water, 
 w = 1.508. 
 
 
 crystals 
 into a large 
 
 olivine, 
 nepheline, 
 
 vitreous 
 particles, 
 
 of colorless 
 or yellow- 
 
 constituent. 
 With 
 
 other minerals by 
 crystalline form, 
 
 e = 1.509. 
 
 
 crystal. 
 Zonal and 
 
 plagioclase, 
 and 
 
 gas-pores, 
 needles of 
 
 ish, fine 
 radial 
 
 sanidine, 
 etc., in the 
 
 twinning-stria- 
 tions, and inclo- 
 
 
 
 radial 
 disposition 
 of the 
 
 sanidine. 
 
 augite, etc., 
 arranged in 
 zones and 
 
 filaments or 
 grains of 
 zeolites. 
 
 leucitophyrs, 
 leucite- 
 tephrites, 
 
 sures, i.e., their 
 regular arrange- 
 ment. If the 
 
 
 
 inclosures. 
 
 
 rays, or 
 
 Rarely 
 
 and basalts; 
 
 leucite occurs in 
 
 
 
 (See Fig. s 8.) 
 Large 
 corroded 
 
 
 gathered 
 together at 
 the centre, 
 
 pseud o- 
 morphs of 
 analcime 
 
 also with 
 nepheline 
 and 
 
 very minute 
 grains through 
 the ground-mass, 
 
 
 
 crystals 
 
 
 are charac- 
 
 after 
 
 plagioclase. 
 
 it is often difficult 
 
 
 
 I. 6. and 
 
 
 teristic. 
 
 leucite. 
 
 Especially 
 
 to distinguish 
 
 
 
 minute 
 
 * 
 
 Also rich in 
 
 
 only in the 
 
 from the colorless 
 
 
 
 often sharp- 
 ly formed 
 
 
 inclosures 
 of other 
 
 
 younger 
 basic 
 
 vitreous base 
 lying between; 
 
 
 
 II. O., 
 
 
 minerals, as 
 
 
 eruptive 
 
 in such cases 
 
 
 
 the latter 
 
 
 hauyn, 
 
 
 rocks. 
 
 recognized only 
 
 
 
 also 
 developed 
 
 
 augite, 
 apatite, 
 
 
 
 by micro-chemical 
 reactions. 
 
 
 
 in augite. 
 
 
 melanite. 
 
 
 
 
 Honey- 
 
 Not es- 
 
 Rutile, as 
 
 With 
 
 Very poor 
 
 
 As primary 
 
 Easily distin- 
 
 yellow to 
 reddish 
 brown. 
 In grains* 
 
 pecially 
 notice- 
 able. 
 
 sagenite, 
 often occurs 
 regularly 
 developed 
 
 quartz, 
 potassium 
 feldspar, 
 garnet, 
 
 
 
 accessory 
 constituent 
 very 
 common in 
 
 guished from 
 zircon by polar- 
 ization-colors, 
 color, and 
 
 often 
 
 
 in biotite; 
 
 hornblende, 
 
 
 
 nearly all 
 
 common twinned 
 
 opaque or 
 only trans- 
 
 , 
 
 also inter- 
 penetrated 
 
 omphacite. 
 
 
 
 crystalline 
 schists, 
 
 formation. 
 
 lucent 
 
 
 with 
 
 
 
 
 especially 
 
 
 (nigrine), 
 
 
 ilmenite. 
 
 
 
 
 those 
 
 
 then with 
 
 
 Very 
 
 
 
 
 containing 
 
 
 metallic 
 
 
 common as 
 
 
 
 
 hornblende 
 
 
 lustre; 
 
 
 inclosure 
 
 
 
 
 and augite, 
 
 
 occurring in 
 
 
 in the 
 
 
 
 
 as the horn- 
 
 
 
 this form 
 
 
 minerals 
 
 
 
 
 alenditesand 
 
 
 but rarely. 
 
 
 accompany- 
 
 
 
 
 eclogites. 
 
 
 
 
 ing it, 
 
 
 
 
 As decom- 
 
 
 
 
 especially 
 
 
 
 
 position- 
 
 
 
 
 in garnet 
 
 
 
 
 product of 
 
 
 
 
 and 
 
 
 
 
 ilmenite 
 
 
 
 
 omphacite. 
 
 
 
 
 secondary. 
 
 
 
 
 
 
 
 
 Very 
 
 
 
 
 
 
 
 
 common in 
 
 
 
 
 
 
 
 
 aluminious 
 
 
 
 
 
 
 
 
 schists, as 
 
 
 
 
 
 
 
 
 "aluminous- 
 
 
 
 
 
 
 
 
 schist 
 
 
 
 
 
 
 
 
 needles." 
 
 
124 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Character 
 and strength 
 of double 
 refraction. 
 
 Polarization- 
 colors. 
 
 3. Zircon. 
 
 ZrO 2 -f SiO 2 . 
 Acids have 
 
 4.4-4.7. 
 
 Imperfect, 
 Pand oo P. 
 
 P.ooPoo, 
 
 Rarely 
 after P. 
 
 Double- 
 refraction; 
 
 Exceedingly 
 brilliant, 
 
 
 no action, 
 
 
 
 rich in com- 
 
 
 very 
 
 emerald- 
 
 
 except 
 H 2 S0 4 , 
 which decom- 
 
 
 
 binations; 
 nearly 
 always in 
 
 
 "trongly 
 positive. 
 
 green, 
 hyacinth- 
 red, and 
 
 
 poses it. 
 
 
 
 minute 
 
 
 
 iridescent. 
 
 
 
 
 
 but sharply- 
 
 
 
 
 
 
 
 
 defined 
 
 
 
 
 
 
 
 
 crystals. 
 
 
 
 
 
 
 
 
 
 (See Fig. 
 
 
 
 
 
 
 
 
 60.) 
 
 
 
 
 B. DOUBLE-REFRACTION 
 
 4. Anatase. 
 
 Like rutile. 
 
 3-S3-3-93- 
 
 oP and P. 
 
 Sharp />. 
 
 
 Like rutile. 
 
 Like rutile. 
 
 5. Meionite 
 
 Group. 
 a. Meionite. 
 
 0.Scapolite. 
 
 Ca 6 (Al a )Si 9 O 3fl 
 
 R 8 Al. 2 Si 6 21 
 R = predomi- 
 nating Ca, some 
 Mg, Na a soluble 
 in HCl, with 
 separation of 
 pulverulent 
 Si0 2 . 
 
 2.734-2.737 
 2.63-2.79. 
 
 Perfect 
 
 oo Poo. 
 
 (See Fig. 
 6k.) 
 
 Crystals 
 after 
 ooP. wPoo . 
 P 
 and larger 
 grains , or 
 elongated 
 Prismatic 
 
 
 Double- 
 refraction; 
 rather 
 strongly 
 negative. 
 
 Brilliant 
 like quartz. 
 
 
 
 
 
 individuals. 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 125 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Colorless, 
 
 Not 
 
 Like rutile, 
 
 With 
 
 Fluid 
 
 
 Primary 
 
 Well 
 
 wine- 
 
 notice- 
 
 one of the 
 
 quartz, 
 
 inclosures, 
 
 
 accessory 
 
 characterized by 
 
 yelloii>; 
 
 able. 
 
 first-formed 
 
 orthoclase, 
 
 acicular 
 
 
 constituent 
 
 crystalline form, 
 
 very strong 
 refraction ; 
 
 
 rock con- 
 stituents; 
 
 plagioclase, 
 biotite, 
 
 cavities, 
 and 
 
 
 in garnet ', 
 syenite, 
 
 polarization- 
 colors, and 
 
 therefore 
 the con- 
 
 
 therefore 
 common as 
 
 hornblende, 
 augite. 
 
 elongated 
 undeter- 
 
 
 quartz, 
 porphyry, 
 
 powerful 
 refraction of 
 
 tours have 
 
 
 inclosure 
 
 
 minable 
 
 
 trachytes, 
 
 light. 
 
 black 
 
 
 in the 
 
 
 needles. 
 
 
 and many 
 
 
 borders (by 
 
 
 others. 
 
 
 
 
 other 
 
 
 the total 
 
 
 
 
 
 
 eruptive 
 
 
 reflection 
 
 
 
 
 
 
 rocks, but 
 
 
 of the 
 
 
 
 
 
 
 rare; 
 
 
 incident 
 
 
 
 
 
 
 more 
 
 
 light). 
 
 
 
 
 
 
 commonly 
 
 
 w = 1.92 
 
 
 
 
 
 
 accompany- 
 
 
 = 1.97. 
 
 
 
 
 
 
 ing rutile 
 
 
 
 
 
 
 
 
 in 
 
 
 
 
 
 
 
 
 chrystalline 
 
 
 
 
 
 
 
 
 schists. 
 
 
 NEGATIVE. 
 
 Dark 
 
 Like 
 
 
 With 
 
 
 
 Very rare 
 
 
 lavender- 
 
 rutile. 
 
 
 quartz. 
 
 
 
 in granite, 
 
 
 blue. 
 
 
 
 orthoclase, 
 and 
 
 
 
 quartzose 
 porphyries, 
 
 
 
 
 
 biotite. 
 
 
 
 and 
 
 
 
 
 
 
 
 
 crystalline 
 
 
 
 
 
 
 
 
 
 schists. 
 
 
 <a = 1.594 
 
 -' 
 
 
 With 
 
 
 
 Primary 
 
 Scapolite can 
 
 1-597 
 = 1.558 
 
 
 
 sanidine, 
 sodalite, 
 
 
 
 accessory 
 constituent; 
 
 be easily 
 distinguished 
 
 I.56I 
 
 Colorless. 
 
 
 
 augite. 
 
 
 
 very rare in 
 trachytic 
 rocks. 
 
 from orthoclase 
 and calcite by 
 the optical 
 
 
 
 
 
 
 
 
 properties and 
 
 White. 
 
 
 
 
 
 
 
 cleavage; 
 
 u* = 1.566 
 = 1-545- 
 
 
 Scapolite 
 appears 
 often to 
 replace 
 
 With 
 quartz, 
 plagioclase, 
 calcite, 
 
 Poor; 
 fluid in- 
 closures; 
 rutile in 
 
 Opaque, 
 fibrous ; de- 
 composed 
 into 
 
 Rare in 
 
 crystalline 
 schists, 
 with 
 
 meionite is 
 recognizable by 
 the crystalline 
 form. 
 
 
 
 plagioclase 
 
 augite. 
 
 scapolite. 
 
 calcite. 
 
 plagioclase 
 
 
 
 
 and to be 
 
 garnet, 
 
 
 
 secondary 
 
 
 
 
 a decom- 
 
 rutile. and 
 
 
 
 accessory 
 
 
 
 
 position- 
 
 titanite. 
 
 
 
 constituent. 
 
 
 
 
 product 
 
 
 
 
 
 
 
 
 from tt. 
 
 
 
 
 
 
126 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Polarization- 
 colors. 
 
 y. Oouse- 
 ^ ranite nnd 
 
 Similar to 
 scapolite; 
 rich in the 
 alkalies, 
 
 2.69-2.76 
 (2.613). 
 2.62-2.68. 
 
 According 
 to oo/>oo. 
 According 
 to oP. 
 
 Long 
 prisms 
 
 00/>. C0/>00 
 
 with 
 
 
 As in 
 scapolite. 
 Rather 
 energetic. 
 
 Rather 
 brilliant. 
 
 
 H a O. 
 
 
 
 termina- 
 
 
 
 
 
 Not attacked 
 
 
 
 tions 
 
 
 
 
 
 by acids 
 (HC1), or at 
 
 
 
 either 
 rounded or 
 
 
 
 
 
 least only 
 
 
 
 fibrous. 
 
 
 
 
 
 with 
 
 
 
 
 
 
 
 
 difficulty. 
 
 
 
 1 
 
 
 
 
 xi 5. Melilite. 
 
 (Huinbold- 
 ite.) 
 
 (Al 2 Fe a ) a , 12> 
 Si 9 36 . 
 Easily soluble in 
 HC1 with 
 separation of 
 gelatinous SiO 2 . 
 
 2.90-2.95. 
 
 Parallel to 
 
 Nearly 
 always in 
 crystals; 
 thin tablets 
 predomi- 
 nating, 
 oP . oo P . 
 
 oo Poo. 
 
 Irregular 
 
 Rarely 
 penetra- 
 tion 
 twins, with 
 chief axes 
 at right 
 angles to 
 each other. 
 
 Double- 
 refraction 
 feebly 
 negative. 
 
 Not very 
 brilliant ; 
 if yellow 
 and 
 fibrous, 
 shows 
 aggregate 
 polariza- 
 tion- 
 
 
 
 
 
 grains. 
 Cross- 
 
 
 
 colors ; if 
 colorless, 
 
 
 
 
 
 sections 
 for the 
 
 
 
 bluish-gray 
 polariza- 
 
 
 
 
 
 most part 
 
 
 
 tion- 
 
 
 
 
 
 rectan- 
 
 
 
 colors. 
 
 
 
 
 
 gular, 
 
 
 
 
 
 
 
 
 more 
 
 
 
 
 
 
 
 
 rarely 
 
 
 
 
 " *' r 
 
 
 
 
 circular. 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 127 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 retracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Bluish, 
 
 
 Crystals 
 
 With 
 
 Very rich; 
 
 Fibrous 
 
 As contact- 
 
 Can be 
 
 colorless 
 in thin 
 
 
 developed 
 in 
 
 calcite, 
 actinohte, 
 
 particles 
 of carbon, 
 
 decomposi- 
 tion, with 
 
 in in era I in 
 metamor- 
 
 distinguished 
 from chiastolite 
 
 section. 
 
 
 limestone^ 
 
 and mica. 
 
 quartz- 
 
 formation 
 
 phosed 
 
 by the 
 
 clear as 
 
 
 often rich 
 
 
 grains, 
 
 of calcite 
 
 limestone. 
 
 structure ; 
 
 water. 
 
 
 in 
 
 
 and 
 
 on the 
 
 Very rare. 
 
 from 
 
 black 
 
 
 inclosures. 
 
 
 leaflets of 
 
 crevices. 
 
 
 andalusite 
 
 from 
 
 
 
 
 muscovite 
 
 
 
 by the 
 
 inclor.ures. 
 
 
 
 
 distributed 
 
 
 
 optical 
 
 u> = 1.558. 
 
 
 
 
 at random. 
 
 
 
 properties. 
 
 e = 1.543. 
 
 
 
 
 
 
 
 
 Generally 
 lemon- 
 
 Longi- 
 tudinal 
 
 Rectangu- 
 lar longi- 
 
 With 
 nepheline, 
 
 Poor. 
 
 The 
 formation 
 
 As 
 
 primary 
 
 Easily 
 recognizable 
 
 yellow, 
 
 sec- 
 
 tudinal 
 
 leucite, 
 
 
 of fibres 
 
 constituent 
 
 by the 
 
 honey- 
 yellow^ 
 colorless to 
 yellowish 
 white. 
 
 tions ; 
 rect- 
 angles 
 show a 
 
 very 
 
 sections 
 show a 
 striation 
 and 
 fibrous 
 
 augite, 
 and 
 olivine. 
 
 
 is a 
 result of 
 the 
 decomposi- 
 tion into 
 
 often 
 replacing' 
 nepheline 
 in the 
 nepholine 
 
 crystalline form, 
 color, and fibrous 
 tendency. 
 If colorless, 
 easily confounded 
 
 
 feeble 
 
 tendency 
 
 
 
 zeolitic 
 
 and 
 
 with nepheline, 
 
 
 dichro- 
 
 parallel 
 
 
 
 substances. 
 
 leucite 
 
 although the 
 
 
 ism. 
 
 to the 
 short 
 
 
 
 (Compare 
 with 
 
 basalts 
 and 
 
 hexagonal 
 isotropic sections 
 
 
 
 sides, 
 
 
 
 "struc- 
 
 lavas. 
 
 are wanting 
 
 
 
 i.e., the 
 
 
 
 ture.") 
 
 
 in melilite. 
 
 
 
 chief 
 
 
 
 
 
 Can be 
 
 
 
 axis c\ 
 
 
 
 
 
 distinguished 
 
 
 
 there are 
 
 
 
 
 
 from serpentine 
 
 
 
 also 
 
 
 
 
 
 often only by 
 
 
 
 very fine 
 
 
 
 
 
 the paler color 
 
 
 
 spindle- 
 
 
 
 
 
 and the 
 
 
 
 shaped 
 
 
 
 
 
 interlacing 
 
 
 
 cavities 
 
 
 
 
 
 of olivine 
 
 . 
 
 
 which 
 
 
 
 
 
 particles ; 
 
 
 
 appear as 
 minute 
 
 
 
 
 
 from biotite 
 leaflets by the 
 
 
 
 circles 
 
 
 
 
 
 paler color 
 
 
 
 within the 
 
 
 
 
 
 and the want of 
 
 
 
 rounded 
 
 
 
 
 
 dichroitic 
 
 
 
 sections 
 
 
 
 
 
 longitudinal 
 
 
 
 cut at 
 
 
 
 
 
 sections. 
 
 
 
 right angles 
 
 
 
 
 
 
 
 
 to the 
 
 
 
 
 
 
 
 
 chief axis 
 
 
 
 
 
 
 
 
 the 
 
 
 
 
 
 
 
 
 so-called 
 
 
 
 
 
 
 
 
 " PJlock- 
 
 
 
 
 
 
 
 
 structure" 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 Fig. 62 ) 
 
 
 
 
 
 
 
 
 Developed 
 
 
 
 
 
 
 
 
 with 
 
 
 
 
 
 
 
 
 leucite, 
 
 
 
 
 
 
 
 
 i.e., 
 
 
 
 
 
 
 
 
 interpene- 
 
 
 
 
 
 
 
 
 trated 
 
 
 
 
 
 
 
 
 with its 
 
 
 
 
 
 
 
 
 crystal . 
 
 
 
 
 
 
128 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 2. MINERALS CRYSTALLIZING IN 
 A. DOUBLE REFRACTION 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reaction. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Polariza- 
 tion-colors. 
 
 1 Quartz. 
 
 SiO 9 . 
 
 Unattacked 
 by HC1 and 
 H a S0 4 : 
 dissolved by 
 HF1. 
 
 2-65 
 average 
 (2.651). 
 
 Imperfect 
 according 
 to-ff. 
 The 
 sections 
 uneven 
 owing to 
 
 Grains or 
 crystals 
 R . - R or 
 &R.R.-R. 
 Generally 
 in large 
 individuals; 
 
 With 
 parallel 
 axial- 
 ry stems. 
 As a rock- 
 constituent 
 never or 
 
 Double- 
 refraction 
 positive. 
 Strong. 
 
 Brilliant 
 yet weak 
 in very 
 thin 
 sections; 
 bluish-gray, 
 like 
 
 
 
 
 the 
 conchoidal 
 
 regular 
 hexagons, 
 
 rarely 
 twinned. 
 
 
 feldspar. 
 
 
 
 
 fracture. 
 
 rhombs, 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 hexagons 
 
 
 
 
 
 
 
 
 with two 
 
 
 
 
 
 
 
 
 parallel 
 
 
 
 
 
 
 
 
 longer 
 
 
 
 
 
 
 
 
 sides. 
 
 
 
 
 
 
 
 
 Never as 
 
 
 
 
 
 
 
 
 microlites. 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 I2 9 
 
 THE HEXAGONAL SYSTEM. 
 POSITIVE. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Colorless; 
 
 
 Often colored 
 
 With 
 
 Poor in 
 
 None. 
 
 [. As primary com- 
 
 In grains' 
 
 clear as 
 
 
 by Fe 2 3 
 
 orthoclase 
 
 mineral 
 
 Changes 
 
 ponent : 
 
 often 
 
 water; 
 vitreous 
 
 
 entering 
 fissures ; 
 
 (and 
 sanidine), 
 
 inclosures. 
 Apatite 
 
 resulting 
 from the 
 
 (a) In eruptive 
 rocks as component 
 
 similar to 1 
 sanidine, 
 
 lustre. 
 
 
 cloudy from 
 
 nore rarely 
 
 prisms. 
 
 action of 
 
 I. O. As macro- 
 
 but can 
 
 w = i 548. 
 < = 1-558. 
 
 
 numberless 
 inclosures, as 
 
 alagioclase, 
 biotite. 
 
 In clastic 
 schists and 
 
 melted 
 magma are 
 
 scopic constituent 
 in grains and 
 
 be easily- 
 distin- 
 
 
 
 n the granites 
 
 lornblende, 
 
 granites 
 
 not un- 
 
 crystals with fluid 
 
 guished 
 
 
 
 and the clastic 
 ocks. Grains 
 
 and augite. 
 Never as 
 
 very rich 
 in fluid 
 
 common 
 among the 
 
 inclosures in 
 granites, quartz- 
 
 optically 
 from it. 
 
 
 
 and crystals. 
 
 primary 
 
 inclosures 
 
 quartz of 
 
 aorphyries, quartz- 
 
 Distin- 
 
 
 
 Quartz from 
 
 component 
 
 and long 
 
 the eruptive 
 
 trachytes, and their 
 
 guished 
 
 
 
 the eruptive 
 
 in augite- 
 
 brown or 
 
 rocks, or 
 
 glasses; in dacite 
 
 from 
 
 
 
 rocks give 
 
 olivine 
 
 ilack, often 
 
 from the 
 
 with vitreous 
 
 nephe- 
 
 
 
 evidence of 
 
 rocks ; as 
 
 bent, 
 
 rocks 
 
 inclosures as an 
 
 line and 
 
 
 
 corrosion. 
 
 also in 
 
 needles. 
 
 contained 
 
 essential con- 
 
 apatite by 
 
 
 
 seing rounded 
 
 nepheline 
 
 In the 
 
 in it as 
 
 stituent, and ac- 
 
 insolu- 
 
 
 
 and shattered; 
 the ground- 
 
 or leucite 
 rocks. 
 
 porphyries, 
 trachytes, 
 
 inclosures. 
 Compare 
 
 cessory in many 
 other eruptive 
 
 bility in 
 HC1; 
 
 
 
 mass is forced 
 
 
 dacites, 
 
 corrosion- 
 
 rocks ; as compo- 
 
 from 
 
 
 
 between as 
 
 
 and other 
 
 phenomena 
 
 nent of the 11. O. in 
 
 corun- 
 
 
 
 leaves of a 
 
 
 eruptive 
 
 and 
 
 the ground-mass of 
 
 dum by 
 
 
 
 book (see 
 
 
 rocks rich 
 
 secondary 
 
 those rocks in 
 
 the 
 
 
 
 Fig. 43)- 
 
 
 in primary 
 
 glassy 
 
 minute irregular 
 
 character 
 
 
 
 In granites 
 
 
 glassy 
 
 inclosures. 
 
 grains, never as 
 
 of 
 
 
 
 commonly 
 
 
 inclosures 
 
 (See 
 
 crystals. 
 
 double- 
 
 
 
 developed 
 with ortho- 
 
 
 and 
 gas- pores. 
 
 Fig. 43-) 
 
 () In nearly all 
 crystalline schists in 
 
 refrac- 
 tionj from 
 
 
 
 clase. as 
 
 
 
 
 irregular grains 
 
 calcile by 
 
 
 
 graphic- 
 
 
 
 
 with fluid inclo- 
 
 cleavage 
 
 
 
 granite 
 
 
 
 
 sures, as with the 
 
 and 
 
 
 
 = micro- 
 
 
 
 
 feldspars; especially 
 
 twinning. 
 
 
 
 pegmatite. 
 
 
 
 
 in gneiss, mica- 
 
 
 
 
 (Comp. Fig. 
 
 
 
 
 schist; in minute 
 
 
 
 
 63.) 
 
 
 
 
 grains in clay 
 
 
 
 
 In the 
 
 
 
 
 schists. 
 
 
 
 
 porphyritic 
 
 
 
 
 II. As secondary 
 
 
 
 
 eruptive 
 rocks also 
 
 
 
 
 product through 
 decomposition of 
 
 
 
 
 with radial 
 
 
 
 
 silicates, especially 
 
 
 
 
 structure as 
 
 
 
 
 of augite, horn- 
 
 
 
 
 spherulites. 
 
 
 
 
 blende, and biotite; 
 
 
 
 
 
 
 
 
 in diabases, as 
 
 
 
 
 
 
 
 
 granular aggre- 
 
 
 
 
 
 
 
 
 gates; on fissures 
 
 
 
 
 
 
 
 
 in lines and 
 
 
 
 
 
 
 
 
 filaments in many 
 
 
 
 
 
 
 
 
 rocks. 
 
 
 
 
 
 
 
 
 III. In clastic rocks 
 
 
 
 
 
 
 
 
 as flattened grains; 
 
 
 
 
 
 
 
 
 fluid inilosures, 
 
 
 
 
 
 
 
 
 joined together, 
 
 
 
 
 
 
 
 
 reaching to the very 
 
 
 
 
 
 With 
 
 Inter- 
 
 
 edges. 
 IV. As simple rock, 
 
 
 
 
 
 muscovite 
 
 twined 
 
 
 building qnartzite 
 
 
 
 
 
 and 
 biotite. 
 
 fluid 
 inclosures. 
 
 
 and quartz 
 schists. 
 
 
130 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of 
 cross-section. 
 
 Twins. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Polarization- 
 colors. 
 
 2. Tridy- 
 mite. 
 
 Like 
 quartz. 
 
 2.282-2.326. 
 
 Imperfectly 
 \\oP. 
 
 In very 
 minute 
 thin tablets, 
 
 Very 
 common. 
 Twinning- 
 
 Positive. 
 Very 
 feeble. 
 
 Not very 
 brilliant. 
 Gray. 
 
 
 
 
 
 pred. oP 
 
 plane 
 
 
 
 
 
 
 
 and oo P. 
 
 a face of 
 
 
 
 
 
 
 
 
 
 \P and \P. 
 
 According 
 
 
 
 
 
 
 
 
 to 
 
 
 
 
 
 
 
 
 v. Lasaulx 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 Schuster, 
 
 
 
 
 
 
 
 
 tridymite 
 
 
 
 
 
 
 
 
 crystallizes 
 
 
 
 
 
 
 
 
 in the 
 
 
 
 
 
 
 
 
 trielinic 
 
 
 
 
 
 
 
 
 system. 
 
 
 
 
 
 
 
 (According 
 
 
 
 
 
 
 
 
 to 
 
 
 
 
 
 
 
 
 v. Lasaulx 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 Schuster, 
 
 
 
 
 
 
 
 
 twins 
 
 
 
 
 
 
 
 
 according 
 
 
 
 
 
 
 
 
 to a plane 
 
 
 
 
 
 
 
 
 of oo/.) 
 
 A.P. 
 
 
 
 
 
 
 
 
 differing 
 
 
 
 
 
 
 
 
 but little 
 
 
 
 
 
 
 
 
 from the 
 
 
 
 
 
 
 
 
 normals to 
 
 
 
 
 
 
 
 
 oP. 
 
 
 
 
 
 
 
 
 i.M. =c 
 
 
 
 
 
 
 
 
 nearly 
 
 
 
 
 
 
 
 
 -LoP. 
 
 
 
 
 
 
 
 
 Axial angle 
 
 
 
 
 
 
 
 
 65-70. 
 
 
 
 
 
 
 A more exact crystallographical and 
 optical determination of tridymite 
 is generally impossible, owing to the 
 minuteness of the crystals occurring 
 in the rocks. Microscopical tridy- 
 mite behaves like an hexagonal min- 
 
 
 
 
 
 
 
 eral in p.p.l. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Colorltss, 
 clear as 
 water. 
 , i.e. 
 ft = 1-4285. 
 
 
 Generally 
 in 
 aggregates 
 of minute 
 thin tablets, 
 either 
 hexagonal 
 or 
 
 With 
 quartz, 
 sanidine, 
 plagioclase, 
 augite, 
 biotite, 
 and 
 horn- 
 
 Fluid 
 inclosures. 
 
 
 Primary as 
 accessory 
 constituent, 
 and 
 secondary 
 as decom- 
 position- 
 product in 
 
 The tendency to 
 form aggregates 
 is very 
 characteristic for 
 microscopic 
 tridymite; the 
 optical properties 
 and the 
 
 
 
 irregular 
 tablets^ 
 lapping 
 over 
 each other 
 
 blende. 
 
 Secondary 
 with opal 
 and chalce 
 
 
 
 rhyolites, 
 trachytes, 
 hornblende- 
 and augite- 
 andesites. 
 
 twinnings for 
 the larger 
 crystals and 
 those suitable 
 for optical 
 
 
 
 like 
 shingles. 
 Often in 
 
 dony. 
 
 
 
 Exceed- 
 ingly 
 common 
 
 investigations. 
 
 
 
 the 
 
 
 
 
 in the 
 
 
 
 
 neighbor- 
 hood of 
 
 
 
 
 younger 
 acidic 
 
 
 
 
 feldspars, 
 
 
 
 
 rocks ; 
 
 
 
 
 or in large 
 
 
 
 
 rare in the 
 
 
 
 
 groups 
 
 
 
 
 basic older 
 
 
 / 
 
 
 in the 
 
 
 
 
 rocks. 
 
 
 
 
 ground- 
 
 
 
 
 Secondary 
 
 
 
 
 mass. 
 
 
 
 
 in cavities 
 
 
 
 
 (See 
 
 
 
 
 of these 
 
 
 
 
 Fig. 64.) 
 
 
 
 
 rocks, and 
 
 
 
 
 
 
 
 
 then 
 
 
 
 
 
 
 
 
 generally 
 
 
 
 
 
 
 
 
 in larger 
 
 
 
 ' 
 
 
 
 
 
 tablets. 
 
 
132 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 B. DOUBLE-REFRACTION 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of 
 cross-section. 
 
 Twins. 
 
 Character 
 and 
 
 strength of 
 double- 
 refraction. 
 
 Polarization- 
 colors. 
 
 1 Calcite. 
 
 CaCO 3 . 
 
 Easily 
 soluble 
 in HCI, 
 with rapid 
 evolution 
 of CO a . 
 Easily 
 
 2.6-2.8 
 
 (2.72). 
 
 Perfect 
 R. 
 (See 
 Fig. 65.) 
 
 Only in 
 regular 
 grains 
 and 
 crystalline 
 aggregates. 
 
 Poly- 
 synthetic 
 twinning- 
 striation 
 after 
 
 (Sel *Figs. 
 21 and 65.) 
 
 Double- 
 refraction 
 very 
 strongly 
 negative. 
 
 Generally 
 weak; gray; 
 yet often 
 brilliantly 
 iridescent 
 like talc, 
 especially 
 in those cases 
 
 
 soluble in 
 
 
 
 
 
 
 where 
 
 
 acetic acid. 
 
 
 
 
 
 
 the calcite 
 
 
 
 
 
 
 
 
 occurs in 
 
 
 
 
 
 
 
 
 minute 
 
 
 
 
 
 
 
 
 granules. 
 
 2. Dolomite. 
 
 (CaHM g , 
 
 More 
 difficultly 
 soluble in 
 acids than 
 calcite. 
 
 2.85-2.95. 
 
 ditto. 
 
 Grains and 
 R. 
 
 See " Re- 
 marks." ' 
 
 
 ditto. 
 
 3. Mag- 
 nesite. 
 
 MgC0 3 . 
 More 
 difficultly 
 soluble in 
 HCI. 
 
 2.9-3.1. 
 
 ditto. 
 
 ditto. 
 
 
 
 ditto. 
 
 4. Sideritc. 
 
 FeCO 3 . 
 
 Soluble in 
 
 3-7-3-9- 
 
 ditto. 
 
 ditto. 
 
 
 
 ditto. 
 
 
 HCI with 
 
 
 
 
 
 
 
 
 evolution of 
 
 
 
 
 
 
 
 
 gas. 
 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 133 
 
 NEGATIVE. 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Colorless, 
 white, 
 
 Feeble 
 absorption. 
 
 Generally 
 in granular 
 
 In nearly 
 all rocks 
 
 Fluid 
 inclosures ; 
 
 None. 
 
 As primary 
 constituent, 
 
 Well 
 character- 
 
 grayish. 
 <o = 1.6543 
 e = 1.4833 
 
 
 aggregates, 
 in cavities; 
 in threads 
 
 bearing 
 augite, 
 tiorn blende. 
 
 very poor 
 in 
 mineral 
 
 
 building by 
 itself a 
 simple rock, 
 
 ized by 
 therhombo- 
 hedral 
 
 
 
 and fibres. 
 
 biotite, 
 
 inclosures. 
 
 
 limestone; 
 
 cleavage 
 
 
 
 In irregular 
 
 and 
 
 
 
 not yet 
 
 and 
 
 
 
 grains, 
 
 plagioclase. 
 
 
 
 assuredly 
 
 twinning- 
 
 
 
 single 
 
 
 
 
 proved as 
 
 striation. 
 
 
 
 individuals, 
 
 
 
 
 such in 
 
 If occurring 
 
 
 
 between 
 
 
 
 
 eruptive 
 
 in minute 
 
 
 
 the other 
 
 
 
 
 rocks. 
 
 granules, 
 
 
 
 con- 
 
 
 
 
 Here as 
 
 often 
 
 
 
 stituents. 
 
 
 
 
 dec o in posit io n 
 
 difficult to 
 
 
 
 
 
 
 
 product^ 
 
 accurately 
 
 
 
 
 
 
 
 especially of 
 
 determine; 
 
 
 
 
 
 
 
 the bisilicates 
 
 the 
 
 
 
 
 
 
 
 and of mica, 
 
 solubility 
 
 
 
 
 
 
 
 where it 
 
 and 
 
 
 
 
 
 
 
 occurs in 
 
 polarization 
 
 
 
 
 
 
 
 lenticular 
 
 colors 
 
 
 
 
 
 
 
 forms 
 
 remain as 
 
 
 
 
 
 
 
 between the 
 
 means of 
 
 
 
 
 
 
 
 laminae. 
 
 recognition. 
 
 
 
 
 
 
 
 Primary and 
 
 
 
 
 
 
 
 
 secondary in 
 crystalline 
 
 
 
 
 
 
 
 
 schists. 
 
 
 ditto. 
 
 
 ditto. 
 
 
 
 
 
 The poly- 
 
 
 
 
 
 
 
 
 synthetic 
 
 
 
 
 
 
 
 
 twinning- 
 
 
 
 
 
 
 
 
 striations 
 
 
 
 
 
 
 
 
 after \R 
 
 
 
 
 
 
 
 
 are wanting 
 on dolomite. 
 
 
 
 
 
 
 
 With olivine 
 
 
 ditto. 
 
 
 ditto. 
 
 With 
 
 
 
 and 
 
 
 
 
 
 serpentine. 
 
 
 
 serpentine as 
 
 
 
 
 
 
 
 
 decomposi- 
 
 
 
 
 
 
 
 
 tion-product. 
 
 
 
 
 
 
 
 
 
 Magnesite 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 siderite 
 
 
 
 
 
 
 
 
 can be dis- 
 
 Yellowish 
 to brown. 
 
 
 ditto. 
 
 See 
 Calcite. 
 
 
 
 As decomposi- 
 tion-product 
 
 tinguished 
 from 
 
 
 
 
 
 
 
 in small 
 
 calcite 
 
 
 
 
 
 
 
 balls of con- 
 
 only by 
 
 
 
 
 
 
 
 centrically- 
 
 chemical 
 
 
 
 
 
 
 
 arranged 
 
 means. 
 
 
 
 
 
 
 
 layers and 
 
 
 
 
 
 
 
 
 with radial 
 
 
 
 
 
 
 
 
 fibres; in 
 
 
 
 
 
 
 
 
 andesites, etc 
 
 
134 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Polarization 
 color. 
 
 S.Nepheline 
 
 
 
 
 
 
 
 
 Group, 
 a. Elseolite. 
 
 
 2.65 (2.591). 
 
 Imperfect; 
 coarse 
 fissures. 
 
 Only in 
 larger 
 grains. 
 
 
 
 Generally 
 bluish gray, 
 not very 
 brilliant. 
 
 
 (Na, K) 3 
 Al ? Si a O 8 . 
 
 
 
 
 
 
 
 
 Easily soluble 
 inHCl 
 with 
 separation of 
 gelatinous 
 SiO a ; on 
 evaporation 
 cubes of 
 NaCl. 
 
 
 
 
 
 Double- 
 refraction 
 negative, 
 not 
 energetic. 
 
 Similar to 
 the feldspar 
 in very 
 thin sections. 
 
 0. Nephe- 
 line. 
 
 (2.58-2.65) 
 2.56. 
 
 Imperfect 
 oPand 
 />. 
 
 Hexagons 
 and 
 rectangles; 
 in minute 
 crystals 
 
 
 
 
 
 
 00 P. Of, 
 
 
 
 
 
 
 
 
 in short 
 
 
 
 
 
 
 
 
 prisms, 
 
 
 
 
 
 
 
 
 and in 
 
 
 
 
 
 
 
 
 minute 
 
 
 
 
 
 
 
 
 irregular 
 
 
 
 
 
 
 
 
 granules. 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 Fig. 66.) 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 135 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 St-ucture. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Gray, 
 reddish 
 
 
 Irregular 
 grains 
 
 With 
 sodalite, 
 
 Poor; 
 often 
 
 Fibrous, 
 zeolitic 
 
 As primary 
 essential 
 
 Well recognizable 
 macroscopically. 
 
 brown, 
 fatty lustre; 
 colorless 
 
 
 inter- 
 penetrated 
 with 
 
 microline, 
 lornblende, 
 titanite. 
 
 colored 
 green by 
 hornblende. 
 
 meta- 
 morphosis. 
 
 constituent 
 in the older 
 eruptive 
 
 The solubility 
 and Na-reaction 
 are characteristic. 
 
 in thin 
 
 
 other con- 
 
 
 
 
 rocks; in the 
 
 
 section. 
 
 
 stituents. 
 
 
 
 
 elaeolite- 
 
 
 
 
 
 
 
 
 syenites. 
 
 
 Colorless, 
 clear as 
 water. 
 W = 1.539 
 
 
 In crystals 
 or in 
 aggregates 
 of minute 
 
 With 
 leucite, 
 augite, 
 olivine, 
 
 Inclosures 
 of augite 
 very 
 commonly 
 
 Generally 
 fresh, in 
 phonolites 
 more often 
 
 As primary 
 essential 
 constituent 
 in the 
 
 Distinguished 
 from: apatite by 
 the imperfect 
 cleavage, 
 
 = 1 -534 
 I -537 
 
 
 irregular 
 granules. 
 
 or with 
 sanidine 
 and augite, 
 
 arranged 
 parallel 
 to the faces. 
 
 opaque 
 and meta- 
 morphosed 
 
 younger 
 eruptive 
 rocks: 
 
 microchemical 
 reactions, and 
 crystalline forms, 
 
 
 
 
 or with 
 
 (See 
 
 into 
 
 nephelinites, 
 
 as apatite 
 
 
 
 
 hornblende 
 and 
 
 Fig. 66.) 
 
 zeolites; 
 then 
 
 nepheline- 
 and leucite- 
 
 commonly 
 shows /", 
 
 
 
 
 titanite. 
 
 
 polarizing 
 
 basalts. 
 
 besides </>, oF, 
 
 
 
 
 
 
 like an 
 
 phonolites 
 
 and occurs 
 
 
 
 
 
 
 aggregate. 
 
 and 
 tephrites. 
 
 in long prisms; 
 quartz never 
 
 
 
 
 
 
 
 
 occurs in such 
 
 
 
 
 
 
 
 
 minute crystals 
 
 
 
 
 
 
 
 
 as nepheline; 
 
 
 
 
 
 
 
 
 feldspar-threads 
 
 
 
 
 
 
 
 
 are long 
 
 
 
 
 
 
 
 
 and twinned; 
 
 
 
 
 
 
 
 
 tnelilite has no 
 
 ^' 
 
 
 
 
 
 
 
 hexagonal 
 
 
 
 
 
 
 
 
 transverse 
 
 
 
 
 
 
 
 
 sections; 
 
 
 
 
 
 
 
 
 zeolites generally 
 
 
 
 
 
 
 
 
 evidence their 
 
 
 
 
 
 
 
 
 secondary 
 
 
 
 
 
 
 
 
 character. 
 
 
 
 
 
 
 
 
 The short 
 
 
 
 
 
 
 
 
 rectangular 
 
 
 
 
 
 
 
 
 longitudinal 
 
 
 
 
 
 
 
 
 sections are 
 
 
 
 
 
 
 
 
 wanting in 
 
 
 
 
 
 
 
 
 tridymite. 
 
 
 
 
 
 
 
 
 If nepheline 
 
 
 
 
 
 
 
 
 occurs in 
 
 
 
 
 
 ' 
 
 
 
 aggregates of 
 
 
 
 
 
 
 
 
 minute granules, 
 
 
 
 
 
 
 
 
 tlien it is 
 
 
 
 
 
 
 
 
 similar to a 
 
 
 f 
 
 
 
 
 
 
 colorless vitreous 
 
 
 
 
 
 
 
 
 mass or quartz 
 
 
 
 
 
 
 
 
 and orthoclasc 
 
 
 
 
 
 
 
 
 aggregates, 
 
 
 
 
 
 
 
 
 and can be 
 
 
 
 
 
 
 
 
 proven only by 
 microchemical 
 
 
 
 
 
 
 
 
 tests. 
 
136 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Character 
 and ntrength 
 of double- 
 refractioii. 
 
 Polarization- 
 colors. 
 
 y. Cancri- 
 
 
 
 
 
 
 
 
 nite. 
 
 Composition 
 like a 
 
 2.448-2.454. 
 
 Imperfect 
 
 00 P. 
 
 Larger 
 irregular 
 
 
 Double- 
 refraction 
 
 Rather 
 brilliant; 
 
 
 nepheline, 
 
 
 
 grains. 
 
 
 negative. 
 
 aggregate 
 
 
 poor in 
 
 
 
 
 
 
 polariza- 
 
 
 potassium, 
 
 
 
 
 
 
 tion. 
 
 
 together with 
 
 
 
 
 
 
 
 
 CaO, C0 2 , 
 
 
 
 
 
 
 
 
 and H 2 0. 
 
 
 
 
 
 
 
 
 Soluble in 
 
 
 
 
 
 
 
 
 HC1 with 
 
 
 
 
 
 
 
 
 effervescence, 
 
 
 
 
 
 
 
 
 with 
 
 
 
 
 
 
 
 
 separation of 
 
 
 
 
 
 
 
 
 gelatinous 
 
 
 
 
 
 
 
 
 SiO 2 . 
 
 
 
 
 
 
 
 S. Lieben- 
 
 
 
 
 
 
 
 
 erite. 
 
 Potassium- 
 aluminium 
 
 2.799-2.814. 
 
 Very- 
 imperfect 
 
 Only larger 
 crystals 
 
 
 Double- 
 refraction 
 
 Remark- 
 able 
 
 
 silicate. 
 
 
 OOP. 
 
 00 P . ffP. 
 
 
 negative (?^ 
 
 agg> egate 
 
 
 H 2 O, traces 
 of Fe, Mg. 
 
 
 
 
 
 Can,not be 
 confirmed, 
 
 polariza- 
 tion, very 
 
 
 Similar to 
 
 
 
 
 
 as the 
 
 brilliant. 
 
 
 pinite, 
 incompletely 
 
 
 
 
 
 crystals 
 are always 
 
 
 
 decomposed 
 
 
 
 
 
 completely 
 
 
 
 by HC1. 
 
 
 
 
 
 decom- 
 
 
 
 
 
 
 
 
 posed. 
 
 
 6. Apatite. 
 
 Ca 6 P 3 12 Cl. 
 Ca 5 P 3 12 Fl. 
 Soluble in 
 
 3.16-3.22. 
 
 Crystals, 
 more rarely 
 grains. 
 
 ooP.P 
 and more 
 rarely oP. 
 
 
 Double- 
 refraction 
 negative, 
 
 General! y 
 not very 
 brilliant, 
 
 
 acids. 
 Reaction for 
 phosphoric 
 acid. 
 
 
 Imperfect 
 cleavage 
 parallel oP 
 and oo P. 
 Remarkable 
 
 Generally 
 long prisms. 
 
 Fig^V) 
 
 
 feeble. 
 
 as with 
 nepheline. 
 
 
 
 
 "separa- 
 
 
 
 
 
 
 
 
 tion 1 '' 
 
 
 
 
 
 
 
 
 {abson- 
 
 
 
 
 
 
 
 
 derung) 
 
 
 
 
 
 
 
 
 || oP, the 
 
 
 
 
 
 
 
 
 acicular 
 
 
 
 
 
 * 
 
 
 
 crystals 
 
 
 
 
 
 
 
 
 thereby 
 
 
 
 
 
 
 
 
 separating 
 
 
 
 
 
 
 
 
 into several 
 
 
 
 
 
 
 
 
 members. 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 137 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Lemon- 
 
 
 Fibrous, 
 
 Like 
 
 Poor. 
 
 Fibrous 
 
 Like 
 
 Can be 
 
 yello-w^ 
 
 
 inter- 
 
 elaeolite. 
 
 Leaflets of 
 
 decomposi- 
 
 elaeolite; 
 
 dis- 
 
 nearly 
 colorless, 
 
 
 penetrated 
 with 
 
 
 ferric oxide, 
 etc., like 
 
 tion with 
 formation of 
 
 rare. 
 
 tinguished 
 from 
 
 in sections. 
 
 
 nepheline 
 
 
 elaeolite. 
 
 calcite. 
 
 
 elaeolite 
 
 
 
 and 
 
 
 
 (Cancrinite 
 
 
 only 
 
 
 
 orthoclase. 
 
 
 
 appears to be 
 
 
 macro- 
 
 
 
 
 
 
 only a 
 
 
 scopically 
 
 
 
 
 * 
 
 
 decomposed 
 
 
 and by the 
 
 
 
 
 
 
 nepheline '.) 
 
 
 content of 
 
 
 
 
 
 
 
 
 
 CaCO 8 . 
 
 Oil-green; 
 in sections 
 colorless; 
 white. 
 
 
 Only as 
 "spring- 
 
 in macro- 
 scopic 
 
 With 
 flesh-red 
 orthoclase 
 and mica. 
 
 
 It is probably 
 a completely 
 decomposed 
 nepheline(J) or 
 cordierite (?); 
 
 Rare. 
 In 
 
 orthoclase- 
 porphyry. 
 
 Easily 
 recog- 
 nizable 
 by the 
 crystalline 
 
 
 
 crystals. 
 
 
 
 consists 
 
 
 form and 
 
 
 
 
 
 
 principally of 
 
 
 decom- 
 
 
 
 
 
 
 minute 
 
 
 position. 
 
 
 
 
 
 
 Muscovite 
 
 
 
 
 
 
 
 
 leaflets, which 
 
 
 
 
 
 
 
 
 i.p.l. appear 
 
 
 
 
 
 
 
 
 very clearly. 
 
 
 
 Colorless, 
 w.iiite; 
 
 If 
 colored, 
 
 [n irregular 
 grains, or 
 
 Proven 
 in nearly 
 
 Vitreous 
 inclosures, 
 
 Always fresh. 
 
 As 
 accessory 
 
 Dis- 
 tinguished 
 
 colored 
 reddish, 
 
 plainly 
 di- 
 
 inelongated 
 often very 
 
 all rocks. 
 
 ?as-pores. Very 
 characteristic 
 
 
 constituent 
 in nearly 
 
 from: 
 nepheline 
 
 brown, 
 
 chroitic. 
 
 narrow 
 
 
 are inclosures 
 
 
 all eruptive 
 
 especially 
 
 black, 
 
 
 columns, 
 
 
 of black or 
 
 
 rocks and 
 
 by the 
 
 from 
 
 
 broken as a 
 
 
 brown needles, 
 
 
 crystalline 
 
 micro- 
 
 numberless 
 
 
 con- 
 
 
 or minute dust- 
 
 
 schists. 
 
 chemical 
 
 inclosures; 
 
 
 sequence of 
 
 
 like granules. 
 
 
 One of the 
 
 reactions 
 
 not water- 
 
 
 the basic 
 
 
 which permeate 
 
 
 minerals 
 
 (com p. 
 
 clear like 
 
 
 separation. 
 
 
 the whole 
 
 
 first 
 
 nepheline), 
 
 nepheline; 
 
 
 Inclosures ! 
 
 
 crystal; in this 
 
 
 eliminated 
 
 inclosures 
 
 more 
 
 
 (See 
 
 
 respect, in the 
 
 
 in 
 
 very char- 
 
 prominent 
 
 
 Fig. 67.) 
 
 
 transverse 
 
 
 formation 
 
 acteristic; 
 
 among the 
 
 
 Only 
 
 
 sections 
 
 
 of the 
 
 hauyn in 
 
 rock-con- 
 
 
 accessory 
 
 ' 
 
 especially, 
 
 
 rocks. 
 
 the longi- 
 
 stituents. 
 
 
 constituent. 
 
 
 showing ;i great 
 
 
 
 tudinal 
 
 = 1.657. 
 
 
 As 
 
 
 similarity to 
 
 
 
 sections 
 
 
 
 inclosure 
 
 
 many hauyns 
 
 
 . 
 
 and basic 
 
 
 
 especially 
 
 
 also rich in 
 
 
 
 separations: 
 
 
 
 common 
 
 
 inclosures. 
 
 
 
 olivine by 
 
 
 
 in the 
 
 
 Central 
 
 
 
 the optical 
 
 
 
 bisilicates 
 
 
 inclosures of 
 
 
 
 properties 
 
 
 
 hornblende 
 
 
 glass, etc. .often 
 
 
 
 and the 
 
 
 
 and 
 
 
 with the form 
 
 
 
 separation. 
 
 
 
 biotite. 
 
 
 of apatile. 
 
 
 
 
133 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Polarization- 
 colors. 
 
 7. Corun- 
 dum. 
 
 A1 2 O 3 . 
 Insoluble 
 
 3-9-4- 
 
 R and oR. 
 
 oo Pf . oR. 
 R and grains. 
 
 In rocks 
 seldom (?) 
 
 Double- 
 refraction 
 
 Very 
 brilliant. 
 
 
 in acids. 
 
 
 
 Hexagons 
 
 
 strongly 
 
 
 
 
 
 
 and 
 
 
 negative. 
 
 
 
 
 
 
 rectangles 
 
 
 
 
 
 
 
 
 whose angles 
 
 
 
 
 
 
 
 
 are 
 
 
 
 
 
 
 
 
 truncated 
 
 
 
 
 8. Tourma- 
 line. 
 
 (Schorl. j 
 
 Very 
 complicated. 
 
 R 6 Al 2 B a 
 S( 4 O 20 
 
 +R 3 (A1 2 ) 9 .B 2 
 S: 4 20 
 
 R = Na 
 predominat- 
 ing and 
 
 3.059- 
 
 Imperfect 
 R and oo/? 2 . 
 Separation 
 \\oR. 
 Very 
 perfect. 
 
 Almost only 
 in crystals. 
 
 Transverse 
 sections 
 three-, six-, 
 and nine-sided 
 when observed 
 parallel to 
 the chief axis; 
 often showing 
 
 * 
 
 D >uble- 
 refraction 
 negative, 
 energetic. 
 
 Rather 
 brilliant; 
 between 
 brown and 
 red. 
 
 
 R = Mg, Fe. 
 
 
 
 good hetni- 
 morphism, 
 
 
 
 
 
 Contains 
 
 
 
 oR below 
 
 
 
 
 
 boric acid. 
 Not attacked 
 by acids. 
 
 
 
 R above. 
 (Comp. Figs. 
 68 and 69.) 
 
 
 
 
 9. Haema- 
 tite. 
 (Eisen- 
 glanz.) 
 
 Fe 2 3 . 
 
 Easily soluble 
 in HC1. 
 
 5.19-5.28. 
 
 R.oR, 
 
 Not charac- 
 teristic in 
 microscopic 
 individuals. 
 
 Mostly tabular 
 thin leaflets, 
 
 and irregular 
 leaflets. 
 
 With 
 parallel 
 axial 
 systems. 
 Penetration 
 
 Indetermin- 
 able, as 
 occurring 
 always in 
 exceedingly 
 
 Not very 
 brilliant. 
 
 
 
 
 
 
 twins 
 
 thin 
 
 
 
 
 
 
 
 with re- 
 
 leaflets in 
 
 
 
 
 
 
 
 entrant 
 
 crevices 
 
 
 
 
 
 
 
 angles. 
 
 in minerals. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 139 
 
 Color and 
 
 
 
 
 
 
 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decom- 
 >osition. 
 
 Occurrence. 
 
 Remarks. 
 
 light. 
 
 
 
 
 
 
 
 
 Colorless, 
 
 azure-blue, 
 spotted, 
 also brown 
 from 
 inclosures; 
 
 If 
 colored, 
 >ery strong. 
 w = azure- 
 blue, 
 e = sea- 
 
 Rough section- 
 surface. 
 In rounded 
 grains or 
 short prisms. 
 One of the 
 
 With quartz, 
 orthoclase, 
 and biotite; 
 pleonaste, 
 andalusite. 
 
 Very poor; 
 fluid and 
 vitreous 
 inclosures; 
 zircon. 
 Brown 
 
 
 Very rare. 
 Contact- 
 mineral, in 
 metamorphic 
 schists and 
 in trachytes. 
 
 When 
 in grains 
 similar to 
 apatite, yet 
 recognizable 
 by the 
 
 power fu lly 
 refracting 
 
 green. 
 
 minerals first 
 separated. 
 
 
 dust-like 
 inclosures 
 
 
 
 brilliant 
 polarization- 
 
 light. 
 
 
 Zonal 
 
 
 (so-called 
 
 
 
 colors. 
 
 appearing 
 well in 
 
 
 structure, 
 blue core. 
 
 
 ordinary 
 corundum). 
 
 
 
 istinguished 
 rom : quartz 
 
 sections. 
 
 
 and colorless 
 
 
 
 
 
 by the rough 
 
 w = 1.768 
 
 
 layers. 
 
 
 
 
 
 surface and 
 
 e = 1.760. 
 
 
 
 
 
 
 
 character of 
 
 
 
 
 
 
 
 
 the double- 
 
 
 
 
 
 
 
 
 refraction, 
 
 
 
 
 
 
 
 
 also not so 
 
 
 
 
 
 
 
 
 clear as 
 
 
 
 
 
 
 
 
 quartz; 
 
 
 
 
 
 
 
 
 divine by 
 
 % 
 
 
 
 
 
 
 
 the optical 
 
 
 
 
 
 
 
 
 properties. 
 
 Mostly 
 grayish 
 blue, 
 
 Very 
 marked 
 dichroistn. 
 
 In 
 macroscopic 
 and in 
 
 With quartz, 
 orthoclase, 
 and 
 
 Very poor. 
 Fluid 
 inclosures. 
 
 
 Common 
 as primary 
 constituent. 
 
 Easily 
 recognizable 
 by crystalline 
 
 brown. 
 
 to = dark 
 
 exceedingly 
 
 muscovite 
 
 
 
 In certain 
 
 form and 
 
 w = 1.64 
 
 blue, brown 
 
 minute 
 
 in granite. 
 
 
 
 granites in 
 
 pleochroism. 
 
 e = 1.62. 
 
 to black. 
 
 crystals, 
 
 With quartz, 
 
 
 
 grains. 
 
 Distinguished 
 
 
 e = light 
 
 rarely in 
 
 orthoclase. 
 
 
 
 Accessory in 
 
 from : 
 
 
 l 'H llt - 
 
 blue to 
 light gray 
 
 granules, as 
 in certain 
 
 mica, and 
 other 
 
 
 
 many crystal- 
 line schists, 
 
 hornblende by 
 the optical 
 
 
 and brown. 
 
 metamorphic 
 rocks. In 
 
 accessory 
 minerals, as 
 
 
 
 especially 
 clay-schists, 
 
 properties; 
 biotite by the 
 
 , 
 
 
 granites in 
 grains inter- 
 
 staurolite 
 and garnet. 
 
 
 
 also in 
 clastic rocks. 
 
 repeated 
 formation of 
 
 
 
 penetrating 
 quartz. The 
 
 
 
 
 Finally, 
 common and 
 
 laminae. 
 
 / 
 
 
 larger 
 
 
 
 
 characteristic 
 
 
 
 
 
 individuals 
 
 
 
 
 in schists 
 
 
 
 
 often in 
 
 
 
 
 metamor- 
 
 
 
 
 radial fibres 
 
 
 
 
 phosed by 
 
 
 
 
 at the 
 
 
 
 
 contact with 
 
 
 
 
 terminations. 
 
 
 
 
 eruptive 
 
 
 
 
 
 
 
 
 rocks. 
 
 
 
 
 
 
 
 
 especially 
 
 
 
 
 
 
 
 
 granite. 
 
 
 Iron-black 
 with 
 
 
 Occurring 
 only in leaves 
 
 In nearly 
 all rocks 
 
 
 Into 
 brown- 
 
 Compare 
 association. 
 
 Easily 
 recognizable 
 
 metallic 
 
 
 infiltrated on 
 
 especially 
 
 
 ish-red 
 
 A ccessory 
 
 by the lorm 
 
 lustre; in 
 
 
 the fissure 
 
 with 
 
 
 and 
 
 secondary 
 
 of occurrence 
 
 thin leaves 
 
 
 of minerals. 
 
 decomposed 
 
 
 brown 
 
 mineral. 
 
 and 
 
 blood-red, 
 also 
 
 
 Is always a 
 secondary 
 
 biotite, 
 hornblende, 
 
 
 pulver- 
 ulent 
 
 
 blood -red 
 color. 
 
 yellowish 
 
 
 mineral. 
 
 augite, 
 
 
 ferric 
 
 
 
 red to 
 
 
 Only in some 
 
 magnetite, 
 
 
 hydrox 
 
 
 
 brownish 
 
 
 basalts also 
 
 as decom- 
 
 
 ide. 
 
 
 
 violet. 
 
 
 appearing as 
 
 position- 
 
 
 
 
 
 
 
 primary. 
 
 product. 
 
 
 
 
 
I4O DETERMINATION OF ROCK-FORMING MINERALS. 
 
 C. MINERALS APPARENTLY 
 
 1. Biotite; in brown, apparently isotrope (optically -uniaxial), 
 
 2. Chlorite; in green, apparently isotrope (optically-uniaxial), 
 
 II. b. Optically-Biaxial 
 
 i. MINERALS CRYSTALLIZING IN 
 
 a. No INTERFERENCE-FIGURE VISIBLE 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 O dinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 i-efraction. 
 
 Direction 
 of 
 extinction. 
 
 1. Olivine. 
 (Chryso- 
 lite.) 
 
 MgoSiO 4 
 + FeS,0 4 4 
 Rather 
 easily 
 soluble in 
 HC1, with 
 separation 
 
 gelatinous 
 Si0 2 . 
 
 3-2-3-4- 
 
 Perfect 
 II oo/oo, 
 
 impejfect 
 
 00/00. 
 
 Con- 
 choidal 
 fracture 
 not so 
 evident 
 in 
 
 Tabular 
 crystals, 
 oo/>. Poo . 
 
 00/00. 
 
 Hexagonal 
 cross- 
 sections 
 
 00/ > =I 3 02 / . 
 
 / O o = 7 65 4 ' 
 or large 
 rounded 
 
 Very 
 rare 
 according 
 to 
 
 also pene- 
 tration- 
 twins. 
 
 i. M. -L 
 
 00/00 
 
 a = C. 
 
 ~b = <a. 
 c' = b. 
 Feeble 
 dispersion 
 of the 
 axes. 
 
 Double- 
 refraction 
 very 
 strongly 
 positive. 
 
 
 
 
 
 sections. 
 Gene- 
 rally 
 twisted 
 
 gr.ins. 
 (See 
 Fig. 71.) 
 
 
 P <?', 
 large 
 axial 
 angle. 
 
 
 
 
 
 
 
 crevices. 
 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 141 
 
 HEXAGONAL. 
 
 hexagonal leaflets; small axial angle. See Monoclinic System. 
 
 hexagonal or irregular leaflets ; small axial angle. See Monoclinic System. 
 
 Minerals. 
 
 THE RHOMBIC SYSTEM. 
 
 (opt. A. P. \\oP) IN SECTIONS \\oP. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decomposition. 
 
 Occurrence. 
 
 Remarks. 
 
 Exceed- 
 
 Colorless ; 
 
 
 Possesses 
 
 Princi- 
 
 Rather 
 
 Most 
 
 As primary 
 
 Distin- 
 
 ingly 
 
 in 
 
 
 a rough 
 
 pally with 
 
 poor; 
 
 commonly 
 
 essential 
 
 guished 
 
 brilliant, 
 
 sections 
 
 
 section- 
 
 augite, 
 
 besides 
 
 into 
 
 constituent 
 
 Irom : 
 
 stronger 
 
 rarely 
 
 
 surface ; 
 
 plagio- 
 
 the 
 
 serpentine 
 
 in all 
 
 quartz in 
 
 than in 
 augite. 
 
 greenish ; 
 rough 
 
 
 the manner 
 of decom- 
 
 clase, 
 nephe- 
 
 minute 
 bn>wn 
 
 (compare 
 aggregates), 
 
 basaltic 
 rocks, in 
 
 isotrope 
 sections 
 
 
 section- 
 sur -faces. 
 Relief 
 
 
 position 
 into 
 yellowish- 
 
 line, 
 leucite. 
 Also with 
 
 picotite 
 octa- 
 hedra, 
 
 whereby the 
 picotite 
 inclosures 
 
 olivine- 
 fels and 
 picrite, 
 
 easily ; 
 zoisite by 
 the 
 
 
 marked. 
 
 
 red, brown 
 
 horn- 
 
 vitreous 
 
 remain. 
 
 melaphyr, 
 
 crystalline 
 
 
 5 = 1.678. 
 
 
 or greenish 
 serpentine, 
 
 blende 
 and 
 
 and 
 rarely 
 
 Also into a 
 brown fibrous 
 
 olivine- 
 gabbro, 
 
 form (never 
 in long 
 
 
 
 
 beginning 
 
 biotite. 
 
 fluid 
 
 aggregate ; 
 
 olivine- 
 
 needles) 
 
 
 
 
 at the 
 
 Almost 
 
 inclo- 
 
 into picrites 
 
 norite, 
 
 and polari- 
 
 
 
 
 crevices, is 
 
 never 
 
 sures, 
 
 and pseud o- 
 
 olivine- 
 
 zation- 
 
 
 
 
 charac- 
 
 with 
 
 magne- 
 
 morphs of 
 
 diabase. 
 
 colors ; 
 
 
 
 
 teristic ; 
 
 primary 
 
 tite. Very 
 
 calcite after 
 
 (In 
 
 colorless 
 
 
 
 
 also the 
 inclosures 
 
 quartz or 
 ortho- 
 
 rarely 
 other 
 
 olivine. 
 By the de- 
 
 crystalline 
 schists ?) 
 
 augite 
 by the 
 
 
 
 
 of picotite 
 
 clase. 
 
 mineral 
 
 composition, 
 
 Also in 
 
 cleavage in 
 
 
 
 
 (see 
 
 
 inclo- 
 
 elimination 
 
 certain 
 
 sections 
 
 
 
 
 Fig. 70). 
 
 
 sures. 
 
 of ferric 
 
 mica- 
 
 at right 
 
 ' 
 
 
 
 Partly in 
 sharp 
 
 
 
 hydroxide, 
 and magne- 
 
 porphyries. 
 
 angles to 
 the oaxis ; 
 
 
 
 
 crystals, 
 
 
 
 tite in the 
 
 
 sanidine 
 
 
 
 
 partly in 
 
 
 
 crevices. 
 
 
 by the 
 
 
 
 
 fragments 
 
 
 
 Totally 
 
 
 rough 
 
 
 
 
 of them, 
 
 
 
 decomposed 
 
 
 surface and 
 
 
 
 
 or in 
 
 
 
 olivine, 
 
 
 the ex- 
 
 
 
 
 irregular 
 
 
 
 very rich in 
 
 
 ceedingly 
 
 
 
 
 grains. 
 
 
 
 iron, always 
 
 
 brilliant 
 
 
 
 
 Constituent 
 
 
 
 in sharp 
 
 
 polariza- 
 
 
 
 
 1.0 in 
 
 
 
 tabular 
 
 
 tion-colors. 
 
 
 
 
 vitreous 
 
 
 
 crystals in 
 
 
 
 
 
 
 eruptive 
 
 
 
 limburgite 
 
 
 
 
 
 
 rocks ; 
 
 
 
 from 
 
 
 
 
 
 
 also in 
 
 
 
 Sasbach, 
 
 
 
 
 
 
 minute 
 
 
 
 was called 
 
 
 
 
 
 
 crystals, 
 
 
 
 hyalosiderite. 
 
 
 
 
 
 
 otherwise 
 
 
 
 
 
 
 
 
 
 only as 
 
 
 
 
 
 
 
 
 
 "spring- 
 ling." 
 
 
 
 
 
 
142 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 ft. AXIAL PICTURE VISIBLE IN SECTIONS || oP. 
 aa. Appearance of the i (-(-) 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 re ctions. 
 
 > 
 
 Sueciflc 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 tie cross- 
 section. 
 
 Twins. 
 
 Optical 
 
 orientation. 
 
 Character 
 and st.ength 
 of doubie- 
 refraction. 
 
 Direc- 
 tion of 
 extinc- 
 tion. 
 
 1. Silli- 
 
 
 
 
 
 
 
 
 
 manite. 
 
 AlSiO 6 . 
 
 Not 
 
 3.23-3.24. 
 
 || oo .Poo 
 Separa- 
 
 Prismatic 
 individuals 
 
 
 A. P. || 00 ^00 
 
 i.M. -L oP. 
 
 Double- 
 retraction 
 
 
 
 attacked 
 by acids. 
 
 
 tion of 
 the thin 
 needles 
 
 without 
 denned 
 termina- 
 
 
 c' = C. 
 
 J. 
 
 a = b. 
 
 positive. 
 
 
 
 
 
 \\oP. 
 
 tions 
 oo /> = in . 
 (See 
 
 
 Small" 
 axial angle 
 
 
 
 
 
 
 
 Fig. 72.) 
 
 
 p = 44. 
 Strong 
 
 
 
 
 
 
 
 
 
 
 dispersion. 
 
 
 
 2. Stau- 
 rolite. 
 
 H 2 R 8 (A1 2 ) 6 
 
 Si e O34- 
 
 R = 3Fe + 
 iMg. 
 Insoluble 
 in acids. 
 
 3- 34-3- 77- 
 
 Perfect 
 oofoo, 
 imperfect 
 
 00 P. 
 
 Separa- 
 tion 
 \\oP. 
 
 Rarely 
 irregular 
 grains. 
 Crystals 
 
 00/>. 00^00 
 
 oP. 
 oo/> = 128 
 
 42'. 
 
 Penetra- 
 tion- 
 twins, 
 wherein 
 the c-axes 
 cut each 
 other 
 at right 
 or nearly 
 
 A.P. || CO/ 5 *, 
 
 i. M. X oP. 
 c 1 = C. 
 
 ~b = a. 
 rf = b. 
 (Dispersion 
 feeble 
 P> v.) 
 
 Double- 
 refraction 
 positive, 
 strong. 
 
 
 
 
 
 
 
 right. 
 
 
 
 
 
 
 
 
 
 angles; 
 
 * 
 
 
 
 
 
 
 
 
 rarely 
 
 
 
 
 
 
 
 
 
 micro- 
 
 
 
 
 
 
 
 
 
 scopic. 
 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 
 Figs. 22 
 and 73.) 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 DOUBLE-REFRACTION IN THEM POSITIVE. 
 Middle Line on oP. 
 
 143 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decora- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 
 light. 
 
 
 
 
 
 
 
 
 Very 
 brilliant, 
 some- 
 
 Colorless, 
 often 
 colored 
 
 
 In 
 exceedingly 
 long, thin 
 
 With 
 quartz, 
 orthoclase. 
 
 Very poor. 
 Fluid 
 inclosures. 
 
 
 As primary 
 accessory 
 constituent 
 
 Distin- 
 guished 
 from : 
 
 what 
 
 red by 
 
 
 needles, 
 
 biotite, and 
 
 
 
 in 
 
 zoisite 
 
 like 
 musco- 
 
 Fe,0, 
 
 on 
 
 
 generally 
 in large 
 
 muscovite. 
 
 
 
 crystalline 
 schists; 
 
 by the 
 character 
 
 vite. 
 
 'ractures. 
 
 
 numbers, 
 
 
 
 
 rare. 
 
 of the 
 
 
 
 
 often 
 
 
 
 
 
 double- 
 
 
 
 
 finely 
 
 
 
 
 
 refraction 
 
 
 
 
 fibrous 
 
 
 
 
 
 and polar- 
 
 
 
 
 and ar- 
 
 
 
 
 
 ization- 
 
 
 
 
 ranged 
 
 
 
 
 
 colors; 
 
 
 
 
 parallel; 
 
 
 
 
 
 andalusite 
 
 
 
 
 developed 
 in quartz, 
 
 
 
 
 
 by the 
 character 
 
 
 
 
 cordierite, 
 
 
 
 
 
 of the 
 
 
 
 
 and other 
 
 
 
 
 
 double- 
 
 
 
 
 minerals. 
 
 
 
 
 
 refraction 
 
 
 
 
 (See 
 
 
 
 
 
 and 
 
 
 
 
 Fig. 77.) 
 
 
 
 
 
 cleavage. 
 
 Ve/y 
 
 brilliant. 
 
 Light or 
 deep 
 yellowish 
 brown. 
 Relief 
 
 Easily 
 recogniz- 
 able, 
 especially 
 in the 
 
 In large 
 and small 
 crystals 
 the num- 
 berless in- 
 
 With 
 quartz, 
 orthoclase, 
 mica, and 
 garnet. 
 
 Inclosures 
 of minute 
 quartz 
 grains, 
 bitumen, 
 
 
 As primary 
 accessory 
 constituent 
 in 
 crystalline 
 
 Well 
 character- 
 ized by the 
 color and 
 pleochro- 
 
 
 very 
 marked. 
 
 longitu- 
 dinal 
 
 closures are 
 character- 
 
 
 hematite, 
 are 
 
 
 sch ists, 
 especially 
 
 ism. 
 
 
 0p=i.75a6 
 
 sections. 
 
 istic. 
 
 
 common. 
 
 
 mica- 
 
 
 
 
 c = dark 
 
 
 
 
 
 schists. 
 
 
 
 
 brown. 
 
 
 
 
 
 
 
 
 
 a =b 
 
 
 
 
 
 
 
 
 
 with 
 
 
 
 
 
 
 
 
 
 slight 
 
 
 
 
 
 
 
 
 
 difference 
 
 
 
 
 
 
 
 
 
 = light 
 
 
 
 
 
 
 
 
 
 yellow. 
 
 
 
 
 
 
 
144 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direc- 
 tion of 
 extinc- 
 tion. 
 
 3, Ensta- 
 
 
 
 
 
 
 
 
 
 tite. 
 
 MgSiOg. 
 
 Only with 
 difficulty 
 attacked 
 by acids. 
 
 (3-153). 
 
 Perfect 
 
 Longi- 
 tudinal 
 
 Long 
 prismatic 
 
 * r> fifyr. 
 
 Octagonal 
 transverse 
 
 
 N.B That 
 position is 
 selected 
 where 
 oo /> = 92 
 lies to the 
 
 Double- 
 refraction 
 positive, 
 rather 
 strong. 
 Weaker (?) 
 
 
 
 
 
 sections 
 inclining 
 
 sections 
 with two 
 
 
 front : 
 
 A.P. || 00/>00 
 
 than in 
 monoclinic 
 
 
 
 
 
 to 
 fibrous. 
 
 controlling 
 pairs ff 
 
 
 i. M. -L oP 
 
 c' = C. 
 
 augite. 
 
 
 
 
 
 oo P about 
 
 faces. 
 
 
 
 
 
 
 
 
 92. 
 
 similar to 
 
 
 = b. 
 
 
 
 
 
 
 Separa- 
 tion 
 II ol\ 
 
 monoclinic 
 augite. 
 
 
 a. = tt. 
 (See 
 
 Fig 7.) 
 Like 
 
 
 
 
 
 
 
 
 
 bronzite. 
 
 
 
 
 
 
 
 
 
 The 
 
 
 
 
 
 
 
 
 
 positive 
 
 
 
 
 
 
 
 
 
 axial angle 
 
 
 
 
 
 
 
 
 increasing i 
 
 
 
 
 
 
 
 
 with iron 
 
 
 
 
 
 
 
 
 present. 
 
 
 
 
 
 
 
 
 
 Dispersion 
 
 
 
 
 
 
 
 
 
 not strong. 
 
 
 
 
 
 
 
 
 
 P > 7> 
 
 
 
 
 
 
 
 
 
 and clear. 
 
 
 
 
 
 
 
 
 
 [Comp. 
 Zirkel.Min. 
 
 
 
 
 
 
 
 
 
 p. 597. Ac- 
 
 
 
 
 
 
 
 
 cording to 
 
 
 
 
 
 
 
 
 
 Tscher- 
 
 
 
 
 
 
 
 
 
 mak's 
 
 
 
 
 
 
 
 
 
 position the 
 
 
 
 
 
 
 
 
 
 optical 
 
 
 
 
 
 
 
 
 
 orientation 
 
 
 
 
 
 
 
 
 
 in enstatite 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 bronzite is 
 
 
 
 
 
 
 
 
 
 the 
 
 
 
 
 
 
 
 
 
 following: 
 
 
 
 
 
 
 
 
 
 A.P. || oo Poo 
 
 
 
 
 
 
 
 
 
 c' = c. i. M. 
 
 
 
 
 
 
 
 
 
 (See' 
 
 
 
 
 
 
 
 
 
 Fig. 74-) 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 145 
 
 Polari- 
 zation- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Very 
 bril- 
 liant. 
 
 Colorless 
 to 
 greenish. 
 Relief 
 
 
 In irregular 
 elongated 
 prismatic 
 individuals 
 
 With 
 plagioclase, 
 olivine, 
 and 
 
 Very poor. 
 
 Into 
 serpentine 
 with 
 formation 
 
 As essential 
 and 
 accessory 
 consiituent 
 
 Distin- 
 guished 
 from : 
 olivine 
 
 
 marked. 
 
 
 which 
 
 monoclinic 
 
 
 of talc. 
 
 in basic 
 
 by the 
 
 
 
 
 show 
 lU-a^ 
 longi- 
 tudinal 
 
 augite. 
 
 
 Into bastite 
 (compare). 
 Decompo- 
 sition 
 
 porphy- 
 ritic 
 eruptive 
 rocks. 
 
 fibrous 
 tendency 
 lit; 
 zoisite 
 
 
 
 
 striation 
 
 
 
 resembles 
 
 With 
 
 by the 
 
 
 
 
 like 
 
 
 
 the meta- 
 
 olivine in 
 
 character 
 
 
 
 
 fibres. 
 (See 
 
 
 
 morphosis 
 of olivine 
 
 olivine-fels. 
 Rare in 
 
 of the 
 double- 
 
 
 
 
 Fig. 75-) 
 
 
 
 into 
 
 quartzose 
 
 refraction 
 
 
 
 
 More 
 
 
 
 serpentine, 
 
 rocks as 
 
 and 
 
 
 
 
 rarely in 
 crystals, 
 generally 
 decom- 
 
 
 
 yet mostly 
 crystalline 
 outlines 
 obtained. 
 
 quartz- 
 porphy- 
 rites; in 
 porphy- 
 
 polariza- 
 tion-colors; 
 silliman- 
 ite by the 
 
 
 
 
 posed. 
 
 
 
 
 rites, 
 
 form 
 
 
 
 
 More often 
 
 
 
 
 diabase- 
 
 (never in 
 
 
 
 
 interpene- 
 
 
 
 
 porphy- 
 
 so minute 
 
 
 
 
 trated || c 
 
 
 
 
 rites. 
 
 needles) 
 
 
 
 
 with 
 monoclinic 
 
 
 
 
 melaphyrs ; 
 also in 
 
 and 
 cleavage; 
 
 
 
 
 augite. 
 
 
 
 
 gabbro 
 
 the follow- 
 
 
 
 
 
 
 
 
 and norite. 
 
 ing 
 
 
 
 
 
 
 
 
 
 minerals 
 
 
 
 
 
 
 
 
 
 only by the 
 
 
 
 
 
 
 
 
 
 variation 
 
 
 
 
 
 
 
 
 
 of the 
 
 
 
 
 
 
 
 
 
 contained 
 
 
 
 
 
 
 
 
 
 iron. 
 
146 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 4. Bronzite. 
 
 ;(MgSiO 8 ) 
 
 3-3-5 
 
 (3.12- 
 
 Perfect 
 
 OOP, 
 
 Elongated 
 acicular. 
 
 Not 
 rarely the 
 
 Like 
 
 enstatite. 
 
 Double- 
 refraction 
 
 
 
 n (FeSiO 3 ). 
 Not 
 
 3-25). 
 
 and 
 separates 
 
 CO P. 00/00 
 00/00 
 
 large 
 bronzite 
 
 [According 
 to 
 
 positive, 
 
 like 
 
 
 
 attacked 
 by acids. 
 
 
 according 
 
 00/00. 
 
 predomi- 
 nating; 
 
 cleavage- 
 leaves 
 
 || 00/00 
 
 Tschermak 
 or 
 
 emtatite 
 In sections 
 at right 
 
 
 
 
 
 
 very 
 similar 
 
 bent, 
 wavy, 
 
 c = a-] 
 Large axial 
 
 angles to 
 the middle 
 
 
 
 
 
 
 to the 
 
 through 
 
 angle, 
 
 lines (i.e., 
 
 
 
 
 
 
 monoclinic 
 augite. 
 
 repeated 
 twinning 
 
 6o-oo. 
 Inter- 
 
 II^P and 
 oo/oo) only 
 
 
 
 
 
 
 
 after 
 
 mediary 
 
 an opening 
 
 
 
 
 
 
 
 J4/00. 
 
 product 
 
 cross, with 
 
 
 
 
 
 
 
 Rarely in 
 
 between 
 
 traces of 
 
 
 
 
 
 
 
 por- 
 
 enstatite 
 
 the lemnis- 
 
 
 
 
 
 
 
 phyntic 
 
 and hyper- 
 
 cates. is 
 
 
 
 
 
 
 
 eruptive 
 
 sthene. 
 
 visible. 
 
 
 
 
 
 
 
 rocks 
 
 
 In sections 
 
 
 
 
 
 
 
 knee- 
 
 
 at right 
 
 
 
 
 
 
 
 shapeJ 
 
 
 angles to 
 
 
 
 
 
 
 
 twins 
 
 
 an optic 
 
 
 
 
 
 
 
 after 
 
 
 axis one or 
 
 
 
 
 
 
 
 m /oo in 
 
 
 no ring is 
 
 
 
 
 
 
 
 asteroid 
 
 
 visible. 
 
 
 
 
 
 
 
 crystal- 
 
 
 
 
 
 
 
 
 
 line 
 
 
 
 
 
 
 
 
 
 groups. 
 
 
 
 
 5. Antho- 
 phyllite. 
 
 * (MgSi0 3 ) 
 4- FeSi0 3 . 
 
 3.1*7- 
 
 3 22 5- 
 
 || oo Poo 
 
 00 P, 
 
 In leaf-like 
 masses. 
 
 
 A. P. = 
 
 co pea 
 
 Double- 
 refraction 
 
 
 
 Not 
 attacked 
 by acids. 
 
 
 CO/00 
 
 imper- 
 feet 
 
 very rarelv 
 in crystals. 
 Transverse 
 
 
 i.M. oP 
 
 c' = c 
 /> = 6 
 
 strongly 
 positive. 
 
 
 
 
 
 [Accord- 
 
 sections 
 
 
 d = ft. 
 
 
 
 
 
 
 ing to 
 Tscher- 
 mak 
 imperfect 
 
 like those 
 of 
 monnrlinic 
 hornblende. 
 
 
 Dispersion 
 clear about 
 C = 71 > p ; 
 large axial 
 
 
 
 
 
 
 II /, 
 
 
 
 ancle. 
 
 
 
 
 
 
 con- 
 choidal 
 
 
 
 (See Fig. 8.) 
 
 
 
 
 
 
 separa- 
 
 
 
 
 
 
 
 
 
 tion after 
 
 
 
 
 
 
 
 
 
 00/00.] 
 
 
 
 
 
 
 
 
 
 OOP 
 
 
 
 
 
 
 
 
 
 = about 
 
 
 
 
 
 
 - 
 
 
 
 125 or 
 
 
 
 
 
 
 
 
 
 55- 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Inter- 
 ference- 
 tigures 
 7ess 
 brilliant 
 by far 
 than in 
 mono- 
 
 = 1.639 
 Dark 
 brown. 
 
 Very 
 slight, 
 
 Partly in 
 large 
 irregular 
 grains in 
 coarsely- 
 granular 
 rocks; 
 partly in 
 
 With 
 olivine, 
 plagio- 
 clase, 
 mono- 
 clinic 
 augite, 
 magnet- 
 
 Inclosures 
 of brown 
 rectangular 
 leaflets, or 
 opaque 
 needles 
 distributed 
 
 Similar 
 to 
 bastite, 
 into a 
 green 
 fibrous 
 aggre- 
 gate, will 
 
 Like 
 
 enstatite, 
 accessory 
 primary 
 constituent 
 Often 
 replacing 
 monoclinic 
 
 Can be dis- 
 tinguished 
 from : 
 monoclinic 
 augite only 
 by examin- 
 ing the 
 transverse 
 
 clinic 
 
 
 
 sharply- 
 
 ite ; like 
 
 (or, after 
 
 elimira- 
 
 augite as 
 
 sections and 
 
 augite. 
 
 
 
 defined 
 crystals in 
 
 enstatite. 
 
 Tschermak 
 
 Hoodoo). 
 
 tion of 
 Fe 3 4 
 
 essential 
 constituent 
 
 the exactly 
 parallel 
 
 
 
 
 the por- 
 
 
 Vitreous 
 
 or 
 
 Also in the 
 
 direction of 
 
 
 
 
 phyritic 
 
 
 inclosures. 
 
 Fe 2 3 . 
 
 younger 
 
 extinction 
 
 
 
 
 eruptive 
 
 
 
 
 basic 
 
 of the longi- 
 
 
 
 
 rocks. 
 
 
 
 
 eruptive 
 
 tudinal 
 
 
 
 
 
 
 
 
 rocks 
 
 sections ; 
 
 
 
 
 
 
 
 
 and the 
 
 hyper- 
 
 
 
 
 
 
 
 
 coarse- 
 
 sthene by 
 
 
 
 
 
 
 
 
 grained 
 
 pleochroism 
 
 
 
 
 
 
 
 
 older ones. 
 
 and charac- 
 
 
 
 
 
 
 
 
 
 ter of the 
 
 
 
 
 
 
 
 
 
 double- 
 
 
 
 
 
 
 
 
 refraction; 
 
 
 
 
 
 
 
 
 
 hornblende 
 
 
 
 
 
 
 
 
 
 and biotite 
 
 
 
 
 
 
 
 
 
 by the want 
 
 
 
 
 
 
 
 
 
 of powerful 
 
 
 
 
 
 
 
 
 
 pleochro- 
 
 
 
 
 
 
 
 
 
 isra. 
 
 Bril- 
 
 Dark 
 
 Strong 
 
 Inclosures 
 
 With 
 
 Inclosures 
 
 
 Very rarely 
 
 Distin- 
 
 liant. 
 
 broivn. 
 0p =1.636. 
 
 pleo- 
 chroism. 
 
 similar to 
 bronzite. 
 
 olivine, 
 plagio- 
 
 of minute 
 brown and 
 
 
 accessory 
 as 
 
 guished 
 from : 
 
 
 Relief 
 not 
 marked. 
 
 Greenish 
 yellow 
 parallel 
 
 The longi- 
 tudinal 
 sections 
 
 clase, 
 augite, 
 and horn- 
 
 greenish 
 mica-like 
 leaves, 
 
 
 secondary 
 constituent. 
 Decom- 
 
 biotite by 
 cleavage, 
 strength of 
 
 
 
 to the 
 striations 
 
 generally 
 fibrous as 
 
 blende. 
 
 often 
 regularly 
 
 
 position- 
 product of 
 
 pleo- 
 chroism, 
 
 
 
 ( il c). 
 
 a conse- 
 
 
 arranged ; 
 
 
 olivine in 
 
 and 
 
 
 
 reddish 
 brown 
 
 quence of 
 cleavage. 
 
 
 otherwise 
 very poor 
 
 
 gabbro and 
 olivine- 
 
 magnitude 
 of axial 
 
 
 
 at right 
 
 
 
 in 
 
 
 fels. 
 
 angle; 
 
 
 
 angles to 
 
 
 
 inclosures. 
 
 
 
 hornblende 
 
 
 
 them. 
 
 
 
 Magnetite. 
 
 
 
 by the 
 
 
 
 
 . 
 
 
 
 
 
 optical 
 
 
 
 
 
 
 
 
 
 orientation ; 
 
 
 
 
 
 
 
 
 
 bronzite 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 hyper- 
 
 
 
 
 
 
 
 
 
 sthene by 
 
 
 
 
 
 
 
 
 
 the pleo- 
 
 
 
 
 
 
 
 
 
 chroism 
 
 
 
 
 
 
 
 
 
 (axial 
 
 
 
 
 
 
 
 
 
 colors) 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 cleavage. 
 
148 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 ranee of the 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form Twins, 
 of the cross- 1 
 section. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direc- 
 tion of 
 i extinc- 
 tion. 
 
 1. Hyper- 
 sthene. 
 
 Like 
 bronzite, 
 yet far 
 richer in 
 iron. 
 
 3-3-34- 
 (3-34-) 
 
 oo /> 
 perfect. 
 
 00/00 
 
 con- 
 choidal 
 
 Large 
 irregular 
 grains and 
 minute 
 columns 
 
 Knee- 
 shaped 
 twins 
 in the 
 crystals 
 
 2.Sl."j-/'. 
 
 I. .\l. -Loo/oo 
 c' = C 
 
 positive. 
 
 || 00/00 
 
 negative. 
 Feebler 
 
 
 
 
 
 separa 
 tion. 
 
 CO/00 
 
 iniper- 
 
 Sf = 
 
 about 92. 
 
 of 
 oo P ._oo P<x> 
 
 00 P<X>. 
 
 also: J/co 
 
 2/00 . 3/g. 
 
 as in 
 bronz- 
 ite. 
 
 d = 
 (See 
 Fig. 5.) 
 Large 
 axial angle. 
 Dispersion 
 about a 
 
 than in the 
 monoclinic 
 augites. 
 (Compare 
 bronzite.) 
 
 
 
 
 
 
 
 
 P> ?', 
 
 
 
 
 
 
 
 
 
 feeble. 
 
 
 
 
 
 
 
 
 
 [According 
 
 
 
 
 
 
 
 
 
 to Tscher- 
 
 
 
 f 
 
 
 
 
 
 
 mak the 
 
 
 
 
 
 
 
 
 
 acute oo/ > - 
 
 
 
 
 
 
 
 
 
 angle lies 
 
 
 
 
 
 
 
 
 
 to the front. 
 
 
 
 
 
 
 
 
 
 then 
 
 
 
 
 
 
 
 
 
 A.P=00/00 
 
 
 
 
 
 
 
 
 
 c' = c. 
 
 
 
 
 
 
 
 
 
 7> = a. 
 
 
 
 
 
 
 
 
 
 d = b.] 
 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 
 Fig. 74.) 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 149 
 
 Secona Middle Line (-}-) on oP. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color 
 and 
 power of 
 reflect- 
 ing light. 
 
 Pleo- 
 chruism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Rather 
 bril 
 
 liant. 
 
 Light 
 to dark 
 brown. 
 
 Strong, 
 especially 
 in the 
 
 In large 
 irregular 
 grains in 
 
 With 
 plagioclase, 
 olivine, 
 
 Nu m berless 
 inclosures 
 of brown 
 
 Hyper- 
 stheneoften 
 decomposes 
 
 \n grains 
 in gabbro, 
 norites in 
 
 Distin- 
 guished 
 from : 
 
 Com- 
 pare 
 
 bronz- 
 ite. 
 
 Black 
 from 
 iron in- 
 closures 
 
 1.639. 
 
 longitudi- 
 nal 
 sections 
 and the 
 thicker 
 sections. 
 Axial 
 
 the granu- 
 lar older, 
 and in 
 small, 
 augite-like 
 crystals in 
 the 
 
 and 
 monoclinic 
 augite. 
 
 or violet 
 rectangular 
 leaflets 
 often in the 
 large 
 grains with 
 marked 
 
 into a 
 dirty- 
 brown 
 or 
 greenish 
 fibrous 
 aggregate 
 
 younger 
 eruptive 
 rocks ; 
 especially 
 in augite- 
 andesites, 
 and feld- 
 
 bronzite by 
 the 
 character 
 of the 
 double- 
 refraction 
 and power- 
 
 
 
 colors; 
 a = 
 
 younger 
 porphyritic 
 
 
 separation 
 \ oo/*oo 
 
 parallel to 
 the <r'-axis 
 
 spathic 
 basalts 
 
 lul pleo- 
 chroism ; 
 
 
 
 hyacinth 
 red, b = 
 reddish 
 brown, 
 
 eruptive 
 rocks. 
 Primary 
 constituent 
 
 
 whereby 
 they appear 
 striated. 
 (See 
 
 and similar 
 to that of 
 enstatite; 
 the 
 
 poor in 
 olivine. As 
 primary 
 essential 
 
 monoclinic 
 a ugite 
 often only 
 by exami- 
 
 
 
 c = gray- 
 ish green. 
 
 I. 0. 
 
 
 Fig. 50.) 
 Inclosures 
 
 decomposi- 
 tion begins 
 
 constituent 
 and 
 
 nation 
 with the 
 
 
 
 Absorp- 
 tion 
 
 
 
 of opaque 
 needles 
 
 here also, 
 first on the 
 
 together 
 with 
 
 condenser, 
 especially 
 
 
 
 c'>a>~b. 
 
 
 
 regularly 
 distributed 
 
 issures and 
 especially 
 
 monoclinic 
 augite. 
 
 in 
 transverse 
 
 
 
 
 
 
 often occur 
 
 on those 
 
 
 sections. 
 
 
 
 
 
 
 in the 
 
 at right 
 
 
 and by the 
 
 
 
 
 
 
 crystals. 
 
 angles to 
 
 
 feebler 
 
 
 
 
 
 
 Otherwise 
 
 the c'-axis. 
 
 
 double- 
 
 
 
 
 
 
 poor in 
 
 It is a 
 
 
 refraction ; 
 
 
 
 
 
 
 inclosures. 
 
 44 bast it e- 
 
 
 biotite by 
 
 
 
 
 
 
 Vitreous 
 
 like'" 
 
 
 absence 
 
 
 
 
 
 
 particles. 
 
 decom- 
 
 
 of the 
 
 
 
 
 
 
 
 position. 
 
 
 marked 
 
 
 
 
 
 
 
 
 
 cleavage ; 
 hornblende 
 
 
 
 
 
 
 
 
 
 by optical 
 
 
 
 
 
 
 
 
 
 orientation 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 prismatic 
 
 
 
 
 
 
 
 
 
 angle. 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 
 extinction. 
 
 2. a. Proto- 
 
 
 3054 
 
 
 
 
 
 
 
 bastite. 
 
 
 
 
 
 
 
 
 
 (Dia- 
 
 
 
 
 
 
 
 
 
 clasite.) 
 
 .- 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 In sections 
 
 
 
 
 
 
 
 
 
 \0f 
 
 
 
 
 
 
 Partly in 
 rather 
 
 
 
 positive; 
 conse- 
 
 
 
 
 
 
 sharp ly- 
 
 
 
 quently 
 
 
 /3. Bastite. 
 
 Like 
 
 bronzite. 
 Contains 
 H 2 O. 
 
 a.6-2.8. 
 
 00/>. ' 
 
 defined 
 crystals, 
 partly in 
 grains of 
 columnar 
 form, 
 and in 
 irregular 
 leafy in- 
 dividuals. 
 
 Very 
 rare. 
 Penetra- 
 . lion- 
 twins. 
 
 A. P --= 
 j| oo Pv>. 
 2. M. -L &P. 
 
 i. M. -L 
 
 00/00. 
 
 negative 
 c' = C 
 
 ?= a 
 a. = b 
 Dispersion 
 p > ?/ 
 about o. 
 Rather 
 strongly 
 doubly- 
 
 
 
 
 
 
 00 * = 93 
 
 
 
 refracting, 
 
 
 
 
 
 
 
 
 
 like hyper- 
 
 
 
 
 
 
 
 
 
 sthene. 
 
 
 DISTINGUISHING OF THE RHOMBIC AUGITES 
 
 The three rhombic augites, enstatite, bronzite, and hypersthene, are in general distinguished 
 only by the amount of iron present, together with the magnitude of the optic axial angle lying in the 
 plane II oo/oo; in those poor in iron, i. M. (= c') -L oP\ in those rich in iron is c' to 2. M. (-L oP). A 
 positive double-refraction is observable in the transverse sections of both varieties, only the magni- 
 tude of the axial angle determining whether the acute angle is visible || oP or II <x>Pv>. 
 
 Pleochroism also, in general, allows no conclusion, as only hypersthenes which are very rich in 
 iron seem to show the mentioned powerful pleochroism. Protobastite and its decomposition product, 
 bastite, however, have the optic axial plane II <x>P<x>. The rhombic augites mentioned above also show 
 a tendency to metamorphose into bastite. 
 
 The rhombic augites, as regards optical orientation, are distinguished from the monoclinic by 
 the much feebler double-refraction and inferior brilliancy of the polarization-colors. The isotrope 
 
TABLES FOR DETERMINING MINERALS. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Rather 
 
 Light 
 
 
 Proto- 
 
 
 
 Into 
 
 As 
 
 Distinguish- 
 
 brilliant. 
 
 yellow- 
 
 
 bastite, 
 
 
 
 bastite. 
 
 primary 
 
 able Irom 
 
 
 ish. 
 
 
 fresh and 
 
 
 
 
 essential or 
 
 enstatite and 
 
 
 
 
 free from 
 
 
 
 
 accessory 
 
 bronzite 
 
 
 
 
 inclosures. 
 
 
 
 
 mineral in 
 
 only by the 
 
 
 
 
 often 
 
 
 
 
 certain 
 
 optical 
 
 
 
 
 shows the 
 
 
 
 
 gabbros 
 
 orientation 
 
 
 
 
 beginning 
 
 
 
 
 and por- 
 
 (position of 
 
 
 
 
 of a 
 
 
 
 
 phyritic 
 
 i. M.). 
 
 
 
 
 fibrous 
 
 
 
 
 augite- 
 
 
 
 
 
 dr com posi- 
 
 
 
 
 plagioclase 
 
 
 
 
 
 tion into 
 
 
 
 
 rocks. 
 
 
 
 
 
 bnstite, in 
 
 
 Many 
 
 
 
 
 
 
 
 that the 
 
 
 times in- 
 
 
 
 
 
 
 
 formation is 
 
 With 
 
 closures 
 
 
 
 
 
 
 
 of fibres 
 
 plagio- 
 
 like 
 
 
 
 
 
 
 
 \\c'. 
 
 clase, 
 
 hyper- 
 
 
 
 
 
 
 
 
 olivine, 
 
 sthene 
 
 
 
 
 
 
 
 
 mono- 
 
 and 
 
 
 
 
 Not very 
 brilliant. 
 
 Dirty 
 pale 
 green. 
 
 Very 
 
 weak. 
 Absorp 
 
 Commonly 
 interpene- 
 trated with 
 
 clinic 
 augite, 
 magnet- 
 
 bronzite. 
 Inclos- 
 ures of 
 
 Bastite 
 itself is 
 always a 
 
 As 
 secondary 
 decom- 
 
 Distinguished 
 from : 
 serpentine 
 
 
 
 tlOll 
 
 olivine, i.e., 
 
 ite. 
 
 picotite 
 
 decom- 
 
 position- 
 
 by the stria- 
 
 
 
 c > a 
 and b. 
 
 serpentine. 
 A metallic 
 
 
 and 
 chromite. 
 
 position- 
 product 
 
 product of 
 rhombic 
 
 lion parallel 
 to the vertical 
 
 
 
 
 lustre on 
 
 
 
 of 
 
 augite in 
 
 axis; 
 
 
 
 
 00 A. 
 
 
 
 rhombic 
 
 olivine-fels, 
 
 chlorite by 
 
 
 
 
 Finely 
 
 
 
 augite. 
 
 gabbro, 
 
 less perfect 
 
 
 
 
 striated 
 
 
 
 
 norite, 
 
 cleavage, and 
 
 
 
 
 parallel to 
 
 
 
 
 andesites, 
 
 not running 
 
 
 
 
 the vertical 
 
 
 
 
 rarely in 
 
 II oP, by the 
 
 
 
 
 axis. Often 
 shows a 
 
 
 
 
 melaphyrs, 
 and 
 
 feebler pleo- 
 chroism. and, 
 
 
 
 
 remnant of 
 
 
 
 
 diabase- 
 
 finally, 
 
 
 
 
 fresh 
 
 
 
 
 porphyries. 
 
 almost always 
 
 
 
 
 enstatite or 
 
 
 
 
 
 by the 
 
 
 
 
 protobastite 
 mineral. 
 
 
 
 
 
 pseudo- 
 morphs after 
 
 
 
 
 
 
 
 
 
 augite crys- 
 
 
 
 
 
 
 
 
 
 
 tals. 
 
 FROM EACH OTHER AND FROM THE MONOCUNIC. 
 
 sections of monoclinic augiie, cut at right angles to one of the optic axes, show two to three rings and 
 clouds ; those of rhombic augite of the same thickness, none or at most one ring. 
 
 The polarization colors of rhombic avgite in very thin sections are generally yellowish white 
 I. O., II 0/>and oo .Poo (in bronzite and hypersthene, enstatite shows more brilliant polarization-colors) ; 
 in these sections also a biaxial interference-figure is discernible; if monoclinic augite, a blue to red, 
 green, and side appearance of one of the optic axes. 
 
 Finally, all sections of rhombic augites parallel to the c'-axis have a parallel extinction i. p. p. 1. ; 
 of monoclinic augites, an extinction with a varying obliquity (to 45) to the c-axis. The common poly- 
 synthetic twins of monoclinic augites \\oofoo are wanting in the rhombic (seen especially well on 
 transverse sections). 
 
152 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 y. AXIAL PICTURE VISIBLE IN SECTIONS \\cP.\ 
 aa. Appearance of the i . M. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 1 a. Anda- 
 
 Al a Si0 5 . 
 
 3.10 
 
 Prismatic 
 
 Rarely 
 
 
 
 
 
 lusite. 
 
 Not acted 
 
 3.17. 
 
 oo/ 
 
 grains; 
 
 
 
 
 
 
 on by acids. 
 
 
 imperfect 
 after 
 
 long 
 columns 
 
 
 
 
 
 
 
 
 ooPoo, 
 
 00 /', 
 
 Quadratic 
 
 
 
 
 
 
 
 
 and 
 
 transverse 
 
 
 
 
 
 
 
 
 
 sections. 
 
 
 
 
 
 
 
 
 CO/= 
 
 longi- 
 
 
 
 
 
 
 
 
 90 5'- 
 Separa- 
 tion 
 
 tudinal 
 sections, 
 elongated 
 
 
 
 
 
 
 
 
 \\oP. 
 
 rectangles. 
 
 
 
 
 
 
 
 
 
 
 
 A. P. || oo Pv> 
 
 
 
 
 
 
 
 
 
 i.M. oP. 
 
 Double- 
 
 
 
 
 
 
 
 
 c' = a 
 
 refraction 
 
 
 |3. Variety 
 of anda- 
 lusite: 
 
 
 
 
 
 
 I =b 
 
 d = C. 
 Large 
 axial angle. 
 
 strongly 
 negative. 
 
 
 Oliiastolite. 
 
 ditto. 
 
 2.9-3.1. 
 
 Perfect 
 
 00 P. 
 
 columns 
 
 
 
 
 
 
 
 
 <x>P = ooP.oP; 
 91 40'. not in form 
 
 
 
 
 
 
 
 
 
 of 
 
 
 
 
 
 
 
 
 
 microlites. 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 153 
 
 IN THESE DOUBLE-REFRACTION NEGATIVE. 
 on oP Negative. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decompo- 
 sition. 
 
 Occurrence. 
 
 Remarks. 
 
 Very 
 
 'bril- 
 
 Colorless ; 
 tinged 
 
 Very 
 strong; 
 
 Rarely in 
 grains, almost 
 
 With 
 quartz 
 
 Some- 
 times 
 
 Com- 
 monly 
 
 As primary- 
 accessory 
 
 Distinguished 
 from: colorless 
 
 liant. 
 
 flesh-red. 
 Relief 
 
 a = 
 dark 
 
 always in 
 columnar 
 
 ortho- 
 clase, 
 
 very poor, 
 and again 
 
 decom- 
 posed 
 
 constituent 
 in granite 
 
 augite by 
 the commonly oc- 
 
 
 very 
 
 blood- 
 
 individuals. 
 
 biotite, 
 
 in the 
 
 into a 
 
 and in 
 
 curring pleochro- 
 
 
 marked. 
 
 red, 
 
 Like 
 
 and 
 
 meta- 
 
 greenish 
 
 crystalline 
 
 ism (reddish 
 
 
 (3p =1.638. 
 
 b = oil- 
 green, 
 
 staurolite, 
 often so filled 
 
 mus- 
 covite. 
 
 morphic 
 schists 
 
 fibrous 
 aggregate. 
 
 schists, as 
 mica schist, 
 
 green), and by the 
 always parallel 
 
 
 
 c = 
 
 with 
 
 
 very rich 
 
 which has 
 
 granulite. 
 
 extinction; ensta- 
 
 
 
 olive- 
 green. 
 
 Inclosures 
 hat but little 
 
 
 granules, 
 in 
 
 a certain 
 similarity 
 
 As meta- 
 morphic 
 
 tite bylhe charac- 
 ter of the double- 
 
 
 
 
 of the 
 
 
 quartz 
 
 to the 
 
 mineral 
 
 refraction; hyper- 
 
 
 
 absorp- 
 tion 
 
 andalusite 
 substance is 
 
 
 granules, 
 bitumi- 
 
 decom- 
 position- 
 
 in the 
 contact- 
 
 sthene by the color 
 and character of 
 
 
 
 / - - 
 
 to be seen. 
 
 
 nous in- 
 
 product 
 
 schists. 
 
 double refraction; 
 
 
 
 > t>>a 
 
 Often in long 
 
 
 closures, 
 
 of cor- 
 
 hornfels. 
 
 zoisite by thepleo- 
 
 
 
 
 thin needles 
 
 
 and 
 
 dierite. 
 
 
 chroism, form of 
 
 
 
 
 in radial 
 
 
 leaves of 
 
 
 
 cross-section, pris- 
 
 
 
 
 aggregates. 
 
 
 biotite. 
 
 
 
 matic angle, and 
 
 
 
 
 
 
 
 
 
 position of the 
 
 
 
 
 
 
 
 
 
 i . M . ; sillima n ite 
 
 
 
 
 
 
 
 
 
 only with diffi- 
 
 
 
 
 
 
 
 
 
 culty if occurring 
 
 
 
 
 
 
 
 
 
 in minute needles. 
 
 
 
 
 
 
 
 
 
 Sillimanite gen- 
 
 
 
 
 
 
 
 
 
 erally occurs in 
 
 
 
 
 
 
 
 
 
 minute needles, 
 
 
 
 
 
 
 
 
 
 andalusite in large 
 
 
 
 
 
 
 
 
 
 columns or grains. 
 
 
 
 
 
 
 
 
 
 They differ, more- 
 
 
 
 
 
 
 
 
 
 over, in pleochro- 
 
 
 
 
 
 
 
 
 
 ism, prismatic 
 
 
 
 
 
 
 
 
 
 angle, and 
 
 
 
 
 
 
 
 
 
 cleavage. 
 
 
 
 
 The trans- 
 
 
 Bitumen. 
 
 Similar to 
 
 Rarely in 
 
 Characterized 
 
 
 
 
 
 verse sections 
 
 
 Leaves 
 
 anda- 
 
 me ta- 
 
 by the regular 
 
 
 
 
 present a 
 
 
 of mus- 
 
 lusite; 
 
 ut orphic 
 
 inclosures. 
 
 
 
 
 peculiar 
 
 
 covite 
 
 whole 
 
 schists 
 
 
 
 
 
 structure. 
 
 
 and 
 
 pseud o- 
 
 (contact on 
 
 
 
 
 
 Quadratic 
 
 
 biotite. 
 
 morphsof 
 
 granite). 
 
 
 
 
 
 cores, 
 
 
 
 quartz, 
 
 
 
 
 
 
 inclosures of a 
 
 
 
 mus- 
 
 
 
 
 
 
 bituminous 
 
 
 
 covite, 
 
 
 
 
 
 
 substance, are 
 
 
 
 and 
 
 
 
 
 
 
 arranged 
 
 
 
 chloritic 
 
 
 
 
 
 
 at t^e centre 
 
 
 
 substance 
 
 
 
 
 
 
 and on the 
 
 
 
 after 
 
 
 
 
 
 
 four corners; 
 
 
 
 chi as to- 
 
 
 
 
 
 
 very common 
 
 
 
 lite. 
 
 
 
 
 
 
 are the 
 
 
 
 
 
 
 
 
 
 remarkable 
 
 
 
 
 
 
 
 
 
 regularly- 
 
 
 
 
 
 
 
 
 
 disposed 
 
 
 
 
 
 
 
 
 
 inclosures of 
 
 
 
 
 
 
 
 
 
 carbonaceous 
 
 
 
 
 
 
 
 
 
 particles. 
 
 
 
 
 
 
 
 
 
 (See Fig. 76.) 
 
 
 
 
 
 
154 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 Cordierite. 
 (Dichro- 
 Ue) 
 
 Mg.R, 
 
 Rfi 8 
 
 2.59-2.66. 
 
 00/00, 
 
 imperfect 
 
 In large 
 grains and 
 small 
 
 If in 
 crystals 
 so often 
 
 A. P| 
 
 <x> P oo. 
 I. M. JLnP. 
 
 Negative, 
 not very 
 energetic. 
 
 
 
 AI 2 . Fe 2 . 
 Scarcely 
 
 
 00/> = 
 
 119 10'. 
 
 crystals. 
 
 OO/*. 00/00. 
 
 pene- 
 tration 
 
 c' = a 
 J = c 
 
 
 
 
 attacked 
 
 
 
 oP. 
 
 twins 
 
 * . 
 
 
 
 
 by acids. 
 
 
 
 Hexagonal 
 transverse 
 
 and 
 
 four- 
 
 ft U. 
 
 Axial angle 
 rather 
 
 
 
 
 
 
 
 sections 
 and rect- 
 angular 
 longitu- 
 dinal 
 
 lings 
 after 
 oo/', 
 rarely 
 after 
 
 large. 
 Dispersion 
 feeble 
 p < v. 
 
 
 
 
 
 
 
 sections. 
 
 00/3. 
 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 
 Figs. 
 
 
 
 
 
 
 
 
 
 23 and 
 
 
 
 
 
 
 
 
 
 78.) 
 
 
 
 
 Finite. 
 
 
 
 
 Large 
 
 
 
 
 
 (Decom- 
 
 
 
 
 crystals = 
 
 
 
 
 
 position- 
 
 
 
 
 oo/. oo/oo. 
 
 
 
 
 
 product 
 
 
 
 
 00/00 . oP. 
 
 
 
 
 
 of cor- 
 
 
 
 
 
 
 
 
 
 dierite.) 
 
 
 
 
 
 
 
 
 
 bb. Appearance of 2 M. 
 
 Zoizite. 
 
 H 2 Ca 4 
 (AI 2 ) 3 
 Si 6 26 . 
 Attacked 
 by acids 
 only with 
 difficulty; 
 * after 
 ignition, 
 however, 
 
 3.22-3.36. 
 
 00/00 
 
 very 
 perfect. 
 Separa- 
 tion 
 
 Elongated 
 grains and 
 long 
 transverse- 
 limbed 
 columns. 
 
 00/ > . 00/00 
 
 (see Fig. 
 Hexagonal 
 
 
 A. P. || 
 
 00/00 
 
 always 
 d = c = 
 
 I = b 
 c' = a. 
 Or if 
 A. P. || oP, 
 
 InoP 
 appearance 
 of 2. 
 negative M. 
 positive. 
 Feeble 
 double- 
 refraction. 
 
 
 
 soluble 
 
 
 
 transverse 
 
 
 7 
 
 
 
 
 with 
 
 
 
 sections. 
 
 
 D = a 
 ../ n . 
 
 
 
 
 separation 
 of 
 amorphous 
 SiO 2 . 
 
 
 
 oo/- = 
 116 26'. 
 
 
 c D, 
 very 
 powerful 
 dispersion 
 P < v. 
 
 
 
 
 
 
 
 
 
 About a! 
 
 
 
 
 
 
 
 
 
 (if A. P. 
 
 
 
 
 
 
 
 
 
 || oP, then 
 
 
 
 
 
 
 
 
 
 dispersion 
 
 
 
 
 
 
 
 
 
 P > P.) 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 155 
 
 Polari- 
 zation- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclos- 
 ures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Rather 
 
 Violet- 
 
 Very 
 
 Always in 
 
 With 
 
 Fluid 
 
 Very 
 
 Rare, as 
 
 In thin sections 
 
 bril- 
 liant, 
 like 
 
 blue. 
 In very 
 thin 
 
 marked. 
 In thicker 
 sections 
 
 larger 
 individuals: 
 never in 
 
 quartz, 
 ortho- 
 clase, 
 
 inclos- 
 ures, 
 silli- 
 
 common, 
 especially 
 if occurring 
 
 accessory 
 primary con- 
 stituent in 
 
 and in grains 
 often very 
 similar to 
 
 quartz. 
 
 sections, 
 
 well 
 
 microlites. 
 In rounded 
 
 and 
 
 manite 
 
 in grains or 
 
 granite, 
 
 quartz, yet 
 
 
 = 1-54- 
 
 nizable. 
 
 larger 
 
 biotite. 
 With 
 
 needles, 
 pleo- 
 
 large crys- 
 tals, on the 
 
 quartz-por- 
 phyry(pinite), 
 
 easily distin- 
 guished by the 
 
 
 1.56. 
 
 a = yel- 
 lowish 
 white, 
 b = pale 
 
 grains, or 
 in small 
 crystals ; 
 the latter in 
 
 plagio- 
 clase, 
 quartz, 
 sani- 
 
 naste 
 crystals, 
 zircon, 
 vitreous 
 
 crevices, or 
 completely 
 decomposed 
 (pinite) into 
 
 and in grains 
 in gneiss. 
 Rarely in 
 crystals in 
 
 phenomena of 
 decomposition 
 on the crevices 
 in c. p. 1. 
 
 
 
 to prus- 
 sian blue, 
 c = dark 
 
 eruptive 
 rocks. 
 Meta- 
 
 dine, 
 augite, 
 pleo- 
 
 inclos- 
 ures. 
 
 a greenish 
 fibrous 
 aggregate, 
 
 trachytes 
 (twins !), and 
 in the 
 
 If in crystals, 
 recognized by, 
 the color and 
 
 
 
 Prussian 
 
 morphic 
 
 naste, 
 
 
 similar to 
 
 trachytic 
 
 pleochroism. 
 
 
 
 blue 
 
 mineral. 
 
 corun- 
 
 
 andalusite. 
 
 volcanic 
 
 
 
 
 Absorp- 
 
 
 dum. 
 
 
 (See Fig. 
 
 overflows. 
 
 
 
 
 tion 
 
 
 
 
 77-) 
 
 
 
 Aggre- 
 
 Green. 
 
 7>> d>c'. 
 
 Wholly 
 
 With 
 
 
 
 
 Easily recog- 
 
 gate 
 
 Colorless. 
 
 
 composed 
 
 quartz, 
 
 
 
 
 nized macro-. 
 
 polari- 
 
 
 
 of minute 
 
 ortho- 
 
 
 
 
 scopically by 
 
 zation. 
 
 
 
 threads and 
 
 clase, 
 
 
 
 
 the crystalline 
 
 
 
 
 leaflets. 
 
 and 
 
 
 
 
 form, and from 
 
 
 
 
 
 biotite. 
 
 
 
 
 the decom- 
 
 
 
 
 
 
 
 
 
 position. 
 
 (negative) on oP. 
 
 Gene- 
 
 Colorless 
 
 
 The 
 
 With 
 
 Fluid 
 
 Often 
 
 Common in 
 
 Distinguished 
 
 rally 
 feebly 
 bril- 
 
 white. 
 0p = 1.70. 
 Relief 
 
 
 transverse 
 crevices on 
 the long 
 
 quartz, 
 ompha- 
 cite, 
 
 inclos- 
 ures 
 are 
 
 opaque on 
 the border. 
 
 crystalline 
 schists as 
 eclogites and 
 
 from: 
 apatite easily 
 by the optical 
 
 liant 
 blue- 
 green. 
 
 very 
 marked. 
 
 
 columns 
 and the 
 inclosures 
 
 garnet, 
 mica, 
 horn- 
 
 very 
 com- 
 mon. 
 
 
 especially 
 amphibolites. 
 
 properties ; 
 andalusite by 
 the cleavage in 
 
 
 
 
 are charac- 
 
 blende. 
 
 
 
 
 sections parallel 
 
 
 
 
 teristic. 
 
 
 
 
 
 oP, moreover 
 
 
 ( 
 
 
 (See Fig. 
 
 
 
 
 
 by the pleochro- 
 
 
 ' 
 
 
 79-) 
 
 
 
 
 
 ism and polari- 
 
 
 
 
 
 
 
 
 
 zation-colors ; 
 
 
 
 
 
 
 
 
 
 sillitnanite by 
 the polarization- 
 
 
 
 
 
 
 
 
 
 colors and the 
 
 
 
 
 
 
 
 
 
 optical orienta- 
 
 
 
 
 
 
 
 
 
 tion (never 
 
 
 
 
 
 
 
 
 
 sinks, like silli- 
 
 
 
 
 
 
 
 
 
 manite. to the 
 
 
 
 
 
 
 
 
 
 microliticform); 
 
 
 
 
 
 
 
 
 
 
 olivine by the 
 
 
 
 
 
 
 
 
 
 crystalline form 
 
 
 
 
 
 
 
 
 
 and polariza- 
 
 
 
 
 
 
 
 
 
 tion-colors 
 
 
 
 
 
 
 
 
 
 (optical investi- 
 
 
 
 
 
 
 
 
 
 gation, power of 
 
 
 
 
 
 
 
 
 
 dispersion); 
 enstatite by the 
 
 
 
 
 
 
 
 
 
 optical orienta- 
 
 
 
 
 
 
 
 
 
 tion, the polari- 
 
 
 
 
 
 
 
 
 
 zation-colors, 
 
 
 
 
 
 
 
 
 
 cleavage, and 
 
 
 
 
 
 
 
 
 
 the ooAangle. 
 
I 5 6 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 II. b. 2. Minerals Crystallizing 
 a. MINERALS APPARENTLY CRYSTALLIZING 
 
 CLEAVAGE MOST 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 
 of doul.lf- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 1. Mica 
 
 
 
 
 
 
 
 
 Group. 
 
 
 
 
 
 
 
 
 a. Mer 
 
 I 
 
 
 
 
 
 
 
 
 oxene 
 (Btotite). 
 
 > R 4 SiO 4 
 it 
 
 2.8-3.2. 
 
 Highly 
 eminent 
 
 oo P quite 
 
 120, 
 
 Rare. 
 T win- 
 
 A. P. 1 oo Poo 
 ' (mica 
 
 As 
 hexagonal 
 
 The 
 
 trans- 
 
 
 n R 2 SiO 4 
 
 
 II oP. 
 
 00/>. OOJPOO . 
 
 ning- 
 
 second 
 
 as a conse- 
 
 verse 
 
 
 VI 
 
 v R 2 Si 8 O 12 
 
 R = K, Na. 
 H, 
 
 
 Separa- 
 tions 
 corre 
 spending 
 to the 
 
 oP. Thin 
 tablets or 
 short 
 columns. 
 Transverse 
 
 plane, 
 >/>; 
 both in- 
 dividuals, 
 however, 
 
 class). 
 A.P. paral- 
 lel to two 
 opposite 
 
 sides(coPoo) 
 
 quence 
 of the 
 apparently 
 constant 
 parallel 
 
 sections 
 generally 
 apparent- 
 ly 
 isotrope; 
 
 
 ii 
 
 
 pressure- 
 
 sections 
 
 forced 
 
 of a 
 
 extinction 
 
 the lon- 
 
 
 R= Fe, 
 
 
 surfaces 
 
 ( II oP) 
 
 over each 
 
 hexagon 
 
 and of the 
 
 gitudinal 
 
 
 Mg, 
 
 
 or 
 
 hexagonal 
 
 other in a 
 
 and 
 
 small 
 
 sections 
 
 
 R a = A1 2 , 
 Fe 2 . 
 Slightly 
 
 
 " sliding- 
 planes" 
 (gleit- 
 
 tablets 
 without 
 cleavage- 
 
 plane 
 quite || oP\ 
 also with 
 
 coinciding 
 with a 
 " fracture- 
 
 axiil angle, 
 or, as the 
 i: M. differs 
 
 with 
 parallel 
 extinc- 
 
 
 attacked by 
 
 
 flaolic) 
 
 cracks. 
 
 several 
 
 line" 
 
 hut little 
 
 tion. 
 
 
 HCI, 
 
 
 - f> 9 and 
 
 More often 
 
 lamellae 
 
 (schlag- 
 
 from the 
 
 therefore 
 
 
 but 
 
 
 i/co. 
 
 with 
 
 inter- 
 
 linie). 
 
 normal 
 
 cannot 
 
 
 completely 
 
 
 
 "sliding 
 
 polated. 
 
 x.M = a 
 
 to oP, 
 
 be 
 
 
 decom- 
 posed by 
 
 
 
 plane" 
 (gleit- 
 
 Only the 
 latter 
 
 vary but 
 little from 
 
 apparently 
 rhotnbic. 
 
 studied in 
 c. p. 1. 
 
 
 H a S0 4 
 
 with 
 
 
 
 flache), 
 three line- 
 
 recogniz- 
 able 
 
 the normal 
 
 tO 0P. 
 
 Negative. 
 
 Ap- 
 
 parently 
 
 
 separation 
 
 
 
 systems 
 
 i. p. 1. 
 
 Axial 
 
 
 hexago- 
 
 
 ot a silica- 
 
 
 
 crossing 
 
 
 angle 
 
 
 nal. 
 
 
 skeleton. 
 
 
 
 each other 
 
 
 generally 
 
 
 
 
 
 
 
 at an 
 
 
 very small 
 
 
 
 
 
 
 
 angle of 
 
 
 = 5-i3, 
 
 
 
 
 
 
 
 60, and 
 
 
 but 
 
 
 
 
 
 
 
 rectangular 
 
 
 variable, 
 
 
 
 + 
 
 
 
 
 longitudi- 
 
 
 being 
 
 ' 
 
 
 
 
 
 
 nal 
 
 
 sometimes 
 
 
 
 
 
 
 
 sections 
 
 
 very large. 
 
 
 
 
 
 
 
 ( II to the 
 
 
 Dispersion 
 
 
 \ 
 
 
 
 
 
 t'-axis), 
 
 
 p < "v. 
 
 
 
 
 
 
 
 with 
 
 
 (Set 
 
 
 
 
 
 
 
 numberless 
 
 
 Fig. 19 ) 
 
 
 
 
 
 
 
 cleavage- 
 
 
 
 
 
 
 
 
 
 lines 
 
 
 
 
 
 
 
 
 
 parallel to 
 
 
 
 
 
 
 According 
 
 
 
 the longer 
 sides. 
 
 
 
 
 
 
 to 
 Tschermak 
 
 
 
 (See 
 Figs. 80 
 
 
 
 
 
 
 mixtures 
 of 
 
 
 
 and 81.) 
 
 
 
 
 
 
 H 8 K 3 (A1 2 ) 3 
 
 
 
 
 
 
 
 
 
 Si e O 24 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 The axial 
 
 
 
 
 Mg 12 Si 6 24 
 
 
 
 
 
 angle 
 
 
 
 
 in 
 
 
 
 
 
 lessens with 
 
 
 
 
 proportion 
 
 
 
 
 
 decrease 
 
 
 
 
 of 
 
 
 
 
 
 of iron 
 
 
 
 
 T : i or 2 : r. 
 
 
 
 
 
 present. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 157 
 
 in the Monoclinic System. 
 
 IN THE HEXAGONAL (OR RHOMBIC} SYSTEM. 
 
 PERFECT 1 1 oP. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 light. 
 
 Pleochroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occur- 
 rence. 
 
 Remarks. 
 
 Not 
 particu- 
 larly 
 
 Brown- 
 black, 
 dark 
 
 Transverse 
 sections 
 show 
 
 Primary 
 constituent 
 I. 0. 
 
 Gen- 
 erally 
 with 
 
 Generally 
 free from 
 inclosures; 
 
 Into 
 chloritic 
 minerals 
 
 In nearly 
 all rocks. 
 In many 
 
 Easily re- 
 cognizable 
 by eminent 
 
 brilliant, 
 brownish 
 
 green. 
 |8 = 1.61. 
 
 almost no 
 pleochro- 
 
 Partly in 
 large 
 
 quartz 
 and 
 
 yet often 
 numbers 
 
 very 
 commonly 
 
 as 
 essential 
 
 cleavage 
 and exceed- 
 
 tints ; 
 
 
 ism. 
 
 crystals 
 
 ortho- 
 
 of epidote 
 
 with 
 
 primary 
 
 ingly 
 
 in very 
 
 
 In longi- 
 
 (in eruptive 
 
 clase ; 
 
 needles 
 
 epidote and 
 
 constitu- 
 
 powerful 
 
 thin 
 
 
 tudinal 
 
 rocks) 
 
 horn- 
 
 arranged 
 
 calcite. 
 
 ent. 
 
 pleochro- 
 
 leaflets 
 
 
 sections 
 
 often 
 
 blende, 
 
 in tufts 
 
 In this 
 
 One 
 
 ism. Dis- 
 
 irides- 
 
 
 very 
 
 cracked 
 
 more 
 
 (comp. 
 
 decompo- 
 
 of the 
 
 tinguished 
 
 cent, 
 
 
 strong. 
 
 or 
 
 rarely 
 
 decomposi- 
 
 sition the 
 
 first- 
 
 from : 
 
 carmine- 
 red. 
 
 
 stronger 
 than 
 
 shattered, 
 or with 
 
 augite 
 and 
 
 tion), 
 or long 
 
 biotite 
 loses its 
 
 formed of 
 minerals. 
 
 hornblende 
 in that the 
 
 
 
 hornblende. 
 
 broad 
 
 olivine. 
 
 thin rutile 
 
 brown 
 
 As decom- 
 
 transverse 
 
 
 
 a and b 
 
 opaque 
 
 
 needles 
 
 color, and 
 
 position- 
 
 sections 
 
 
 
 differing 
 but. 
 
 border; 
 or in 
 
 
 arranged 
 very 
 
 becomes 
 green; 
 
 product 
 of augite. 
 
 are not 
 pleochroitic 
 
 
 
 slightly. 
 In 
 
 minute 
 irregular 
 
 
 regularly 
 at 60; 
 
 lenticular 
 calcite 
 
 horn- 
 blende, 
 
 (and by the 
 investiga- 
 
 
 
 longitudi- 
 nal 
 
 leaflets, 
 especially 
 
 
 apatite, 
 zircon. 
 
 separates 
 between 
 
 rarely of 
 olivine. 
 
 tion 
 i.e. p. 1.) 
 
 
 
 sections 
 
 in the 
 
 
 
 the leaves, 
 
 As 
 
 the longi- 
 
 
 
 parallel 
 
 crystalline 
 
 
 
 and 
 
 contact- 
 
 tudinal 
 
 
 
 to the 
 
 schists ; 
 
 
 
 needles of 
 
 mineral 
 
 sections 
 
 
 
 <r'-axis 
 
 or dis- 
 
 
 
 epidote 
 
 in meta- 
 
 cari be 
 
 
 
 (= the 
 shorter 
 
 tributed 
 through the 
 
 
 
 appear. 
 (See 
 
 morphic 
 schists. 
 
 distin- 
 guished 
 
 ., 
 
 
 sides of 
 
 ground- 
 
 
 
 Figs. 80 
 
 
 by the 
 
 
 
 the 
 
 mass 
 
 
 
 and 81.) 
 
 
 oblique 
 
 
 
 rectangle). 
 
 constituent 
 
 
 
 In the 
 
 
 extinction 
 
 
 ' 
 
 a = yellow 
 
 II. O., 
 
 
 
 decomposi- 
 
 
 to be 
 
 
 
 pale 
 brown; 
 
 as in the 
 basalts and 
 
 
 
 tion of 
 biotite also 
 
 
 proven on 
 horn- 
 
 
 
 Perpen- 
 
 other rocks. 
 
 
 
 a separa- 
 
 
 blende; 
 
 
 
 dicular to 
 
 
 
 
 tion of 
 
 
 chlorite 
 
 
 
 the c'-axis 
 
 
 
 
 ferric 
 
 
 in that 
 
 
 
 (parallel 
 
 
 
 
 hydroxide 
 
 
 chlorite is 
 
 
 
 to the 
 
 
 
 
 or magnet- 
 
 
 always 
 
 
 
 longer 
 
 
 
 
 ite about 
 
 
 green, 
 
 
 
 sides). 
 
 
 
 
 the 
 
 
 never so 
 
 
 
 C = dark 
 
 
 
 
 periphery. 
 
 
 well 
 
 
 
 brown to 
 
 
 
 
 
 
 crystal- 
 
 
 
 black. 
 
 
 
 
 
 
 lized, 
 
 
 
 Absorption 
 
 
 
 
 
 
 feebler 
 
 
 
 c > b > a. 
 
 
 
 
 
 
 pleochro- 
 
 
 
 
 
 
 
 
 
 itic, and 
 
 
 
 
 
 
 
 
 
 generally 
 
 
 
 
 
 
 
 
 
 arranged 
 
 
 
 
 
 
 
 
 
 in tufts. 
 
158 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 b. Rubellan. 
 
 Like 
 
 See 
 
 See mrr- 
 
 La rye 
 
 
 Large 
 
 See meroxene. 
 
 
 biotite 
 
 mer- 
 
 oxene. 
 
 thin 
 
 
 axial angle. 
 
 
 
 (meroxene); 
 rich in 
 
 oxene. 
 
 
 hexagonal 
 tables. 
 
 
 
 
 
 
 iron. 
 
 
 
 
 
 
 
 
 c. Phlogo- 
 pite. 
 
 A 
 
 magnesian 
 mica, 
 nearly free 
 front iron. 
 According 
 to 
 Tschermak, 
 a mixture 
 of 
 K 6 (A1 2 ) 3 
 Si 6 O 24 , 
 H 8 Si 10 U 24 , 
 
 2-75- 
 2.97. 
 
 See mer- 
 oxene. 
 
 See 
 meroxene. 
 
 See mer- 
 oxene; 
 also twins 
 after oo / 
 with indi- 
 viduals 
 lying 
 near one 
 another. 
 
 A. P. || oojPoo 
 (X nearly 
 -Lfff. 
 c:a = z\. 
 Dispersion 
 P <v. 
 Axial angle 
 about 15. 
 
 Negative 
 like 
 meroxene. 
 
 Like 
 mer- 
 oxene, 
 
 alwa- 3 
 parallel 
 extinc- 
 tion. 
 
 
 and 
 
 
 
 
 
 
 
 
 
 Mg 12 Si 6 O 24 
 
 
 
 
 
 
 
 
 
 in the 
 
 
 
 
 
 
 
 
 
 proportion 
 
 
 
 
 
 
 
 
 
 of nearly 
 
 
 
 
 
 
 
 
 
 3:1:4. 
 
 
 
 
 
 
 
 
 d. Anomite. 
 
 According 
 to 
 Tschermak, 
 
 
 See mer- 
 oxene; 
 also here 
 
 See 
 meroxene. 
 
 
 A.P.JLoojPoo 
 
 (mica 
 I class). 
 
 Negative. 
 
 ditto. 
 
 
 a mixture 
 
 
 the 
 
 
 
 a nearly 
 
 
 
 
 of 
 
 
 " gliding- 
 
 
 
 _!_<>/>. 
 
 
 
 
 H 2 K 4 (A1 2 ) 3 
 Si 6 O 2 4 
 
 
 planes" 
 very com- 
 
 
 
 Axial angle 
 about 
 
 
 
 
 and 
 
 
 monly 
 
 
 
 12-16 
 
 
 
 
 Mgi 2 Si 6 O 24 
 
 
 dis- 
 
 
 
 and less. 
 
 
 
 
 in 
 
 
 cernible. 
 
 
 
 Dispersion 
 
 
 
 - 
 
 proportion 
 
 
 One of 
 
 
 
 p> v. 
 
 
 
 
 of i : -i 
 
 
 the 
 
 
 
 
 
 
 
 or 2 : i. 
 
 
 gliding- 
 
 
 
 
 
 
 
 
 
 planes 
 
 
 
 
 
 
 
 
 
 parallel 
 
 
 
 
 
 
 
 
 
 oojPoo. 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 159 
 
 
 Color and 
 
 
 
 
 
 
 
 
 Polariza- 
 tion-colors 
 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occur- 
 rence. 
 
 Remarks. 
 
 
 light. 
 
 
 
 
 
 
 
 
 
 Brown- 
 ish red, 
 brick-red. 
 
 
 Often 
 appears as 
 a foreign 
 inclosure; 
 
 With 
 augite, 
 olivine 
 plagio- 
 
 Augitic 
 needles, 
 ferric 
 hydrate, 
 
 Depositing 
 ferric 
 hydroxide. 
 
 In 
 basalts 
 and 
 lavas. 
 
 Dis- 
 tinguished 
 from 
 biotite 
 
 
 
 
 only 
 primary 
 
 clase, 
 uephe- 
 
 and 
 microlites 
 
 
 
 only by the 
 color. 
 
 
 
 
 constituent 
 1.0. Is 
 
 line, or 
 leucite. 
 
 regularly 
 arranged 
 
 
 
 
 
 
 
 only an 
 
 
 at 60 
 
 
 
 
 
 
 
 altered 
 
 
 as in 
 
 
 
 
 
 
 
 (fiyrogene?) 
 
 
 meroxene. 
 
 
 
 
 
 
 
 biotite. 
 
 
 
 
 
 
 See mer- 
 oxene. 
 
 Yellow, 
 pale 
 brown, 
 red- 
 brown, 
 like mer- 
 
 Very 
 strong, 
 yet 
 weaker 
 than 
 mer- 
 
 Mostly 
 in thin 
 irregular 
 . leaflets. 
 
 With 
 calcite 
 and 
 ser- 
 pentine. 
 
 Very poor; 
 as in 
 rubellan, 
 regular 
 layers of 
 thread-like 
 
 Becoming 
 green, 
 like 
 meroxene. 
 
 In 
 granular 
 rarely 
 compact 
 lime- 
 stones, 
 
 Differs 
 from 
 meroxene 
 only in 
 chemical 
 composition 
 
 
 oxene. 
 
 oxene. 
 
 
 
 needles. 
 
 
 dolo- 
 
 and color. 
 
 
 Relief 
 
 
 
 
 
 
 mites, 
 
 
 
 marked. 
 
 
 
 
 
 
 and in 
 
 
 
 
 
 
 
 
 
 serpen- 
 
 
 
 
 
 
 
 
 
 tine 
 
 
 
 
 
 
 
 
 
 rocks'. 
 
 
 ditto. 
 
 Red- 
 brown. 
 
 ditto. 
 
 ditto. 
 
 With 
 olivine, 
 augite, 
 and 
 actino- 
 lite. 
 
 
 Becoming 
 green to 
 colorless 
 as above. 
 At the 
 beginning 
 of the 
 decomposi 
 tion it 
 becomes 
 
 Rare in 
 olivine- 
 fels. 
 
 
 
 
 
 
 
 
 opaque, and 
 
 
 
 
 
 
 
 
 
 contains 
 
 
 
 
 
 
 
 
 
 numbers 
 
 
 
 
 
 
 
 
 
 of brown 
 
 
 
 
 
 
 
 
 
 grains 
 
 
 
 
 
 
 
 
 
 inclosed. 
 
 
 
i6o 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross- section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and stiviitfth 
 of double- 
 refraction. 
 
 Direction 
 ot 
 extinction. 
 
 e. Musco 
 
 
 
 
 
 
 
 
 
 vite 
 (and 
 sericite). 
 
 H 4 K 2 (A1 2 ) 3 
 Si0 24 . 
 Noi 
 attacked by 
 acids. 
 
 2.76-3.1. 
 
 Very 
 perfect 
 11 oP; 
 " sliding- 
 planes 1 ' 
 (gleit- 
 
 Rarely 
 crystallized 
 in rocks; 
 hexagonal 
 tables. 
 
 See 
 merox- 
 ene. 
 
 A. P. j_ oo Poo 
 (mica 1. 
 class), 
 fl differing 
 but little 
 from c' 
 
 Strongly 
 negative. 
 
 Like 
 magnesia- 
 mica 
 with 
 parallel 
 extinc- 
 
 
 
 
 flache) 
 
 
 
 nearlyj.tf.P. 
 
 
 tion ; ap- 
 
 
 
 
 as in 
 mer- 
 
 
 
 Dispersion 
 p > ?'. 
 
 
 parently 
 rhombic. 
 
 
 
 
 oxene. 
 
 
 
 Axial angle 
 
 
 
 
 
 
 
 
 
 generally 
 
 
 
 
 
 
 
 
 
 large. 
 
 
 
 
 
 
 
 
 
 60-70. 
 (See 
 
 
 
 
 
 
 
 
 
 Fig. ,8.) 
 
 
 
 2. Taic. 
 
 U 2 M ?3 
 S. 4 ia . 
 Not 
 attacked by 
 acids. 
 Al- 
 reaction. 
 
 2.69-2.8. 
 
 Eminent 
 \\oP 
 (imper- 
 fect 00 P). 
 
 Never in 
 crystals; 
 in rocks 
 mostly in 
 minute 
 irregular 
 leaflets like 
 mica. 
 
 
 A.P.||oojPoo 
 
 II to a 
 fracture- 
 line 
 (schlag- 
 linie). 
 nearly 
 oP. 
 (According 
 to 
 Tschermak 
 axial angle 
 about 17.) 
 
 Feebly 
 negative. 
 
 Ap- 
 parent ly 
 rhombic ? 
 
TABLES FOR DETERMINING MINERALS. 
 
 161 
 
 Polariza- 
 
 Color and 
 
 
 
 
 
 
 
 tion- 
 colors. 
 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. p^S^. 
 
 Occurrence. 
 
 Remarks. 
 
 
 
 
 
 
 
 
 
 
 Exceed- 
 
 Colorless, 
 
 
 As 
 
 With 
 
 Very poor; 
 
 
 As 
 
 Easily recog- 
 
 ingly 
 brilliant, 
 iritfes- 
 
 light 
 green, 
 oil-green. 
 
 
 primary 
 constituent 
 I. O. in 
 
 quartz, 
 orthoclase, 
 biotite. 
 
 rarely 
 ruiile 
 needles, 
 
 
 primary 
 constituent 
 in granites, 
 
 nizable by 
 the highly 
 eminent 
 
 ccnt 
 (red to 
 
 
 
 large 
 leaves and 
 
 tourmaline. 
 
 hematite 
 tablets, or 
 
 
 especially 
 tourmaline 
 
 cleavage and 
 brilliant po- 
 
 yellow 
 
 
 
 tables, in 
 
 
 tourmaline 
 
 
 granites, 
 
 larization col- 
 
 colors). 
 
 
 
 tufted and 
 
 
 columns. 
 
 
 and in 
 
 ors; yet diffi- 
 
 
 
 
 stellate 
 
 
 Zircon. 
 
 
 crystalline 
 
 cult to distin- 
 
 
 
 
 aggregates. 
 
 
 
 
 schists; 
 
 guish with the 
 
 
 
 
 As 
 
 
 
 
 especially 
 
 microscope 
 
 
 
 
 secondary 
 
 
 
 
 prominent 
 
 from talc. 
 
 
 
 
 product 
 in 
 
 
 
 
 in gneiss, 
 mica-schist, 
 
 Sericite is 
 only a musco- 
 
 
 
 
 aggregates 
 of minute 
 
 
 
 
 and clay 
 schists. 
 
 vite. appear- 
 ing like talc, 
 
 
 
 
 irregular 
 
 
 
 
 As primary 
 
 soft, 
 
 
 
 
 leaflets. 
 
 
 
 
 constituent 
 
 greasy to the 
 
 
 
 
 In 
 
 
 
 
 nowhere 
 
 touch, non- 
 
 
 
 
 crystalline 
 
 
 
 
 else in 
 
 elastic and 
 
 
 
 
 schists in 
 
 1 
 
 
 
 eruptive 
 
 occurring in 
 
 
 
 
 minute 
 
 
 
 
 rocks. As 
 
 compact 
 
 
 
 
 irregular 
 
 
 
 
 decomposi- 
 
 aggregates of 
 
 
 
 
 leaflets. 
 
 
 
 
 tion-product 
 
 minute irreg- 
 
 
 
 
 
 
 
 
 in the 
 
 ular leaflets, 
 
 
 
 
 
 
 
 
 feldspars, 
 
 in certain 
 
 
 
 
 
 
 
 
 chiastolite. 
 
 semi-crystal- 
 
 
 
 
 1 
 
 
 
 
 tiebenerite, 
 
 line schists. 
 
 
 
 
 
 
 
 
 etc. 
 
 
 
 
 
 
 
 
 
 
 
 See 
 
 Colorless, 
 
 
 Mostly in 
 
 With 
 
 Very poor. 
 
 
 As 
 
 Difference be- 
 
 musco- 
 
 white, 
 
 
 irregularly 
 
 quartz, 
 
 Biotite, 
 
 
 primary 
 
 tween musco- 
 
 vite. 
 
 light 
 green. 
 
 
 disposed 
 interlaced 
 
 orthoclase, 
 mica, 
 
 actinolite. 
 Like 
 
 
 constituent 
 in many 
 
 vite and talc : 
 Muscovite oc- 
 
 
 
 
 or 
 rosette- 
 
 or with 
 augite and 
 
 muscovite. 
 
 
 crystalline 
 schists. 
 
 curs general- 
 ly in large in- 
 
 
 / 
 
 
 shnped 
 
 olivine. 
 
 
 
 Not 
 
 dividuals 
 
 
 
 
 stellate 
 
 
 
 
 common. 
 
 remarkable 
 
 
 
 
 aggregates 
 
 
 
 
 As second- 
 
 for the basal 
 
 
 
 
 of minute 
 leaflets. 
 
 
 
 
 ary product 
 in the de- 
 
 cleavage, or 
 in separate 
 
 
 
 
 
 
 
 
 composition 
 
 leaves. 
 
 
 
 
 
 
 
 
 of augites 
 
 Talc, how- 
 
 
 
 
 
 
 
 
 and horn- 
 
 ever, occurs 
 
 
 
 
 
 
 
 
 blendes 
 
 generally in 
 
 
 
 
 
 
 
 
 poor in 
 
 aggregates of 
 
 
 
 
 
 
 
 
 iron, 
 especially 
 
 compact inter- 
 twined mi- 
 
 
 
 
 
 
 
 
 enstatite 
 
 nute leaflets 
 
 
 
 
 
 
 
 
 before oc- 
 
 arranged in 
 
 
 
 
 
 
 
 
 curring in 
 
 stellate 
 
 
 
 
 
 
 
 
 olivine-fels 
 
 groups. 
 
 
 
 
 
 
 
 
 and 
 
 The micro- 
 
 
 
 
 
 
 
 
 serpentines. 
 
 chemical in- 
 
 
 
 
 
 
 
 
 
 vestigation of 
 
 
 
 
 
 
 
 
 
 isolated leaf- 
 
 
 
 
 
 
 
 
 
 lets with hy- 
 
 
 
 
 
 
 
 
 
 drofluo-silicic 
 
 
 
 
 
 
 
 
 
 acid is the 
 
 
 
 
 
 
 
 
 
 only safe one. 
 
1 62 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 i strength of 
 double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 3. Ohlonte 
 
 
 
 
 
 
 
 
 
 Group. 
 
 
 
 
 
 
 
 
 
 a. Ripido- 
 lite. 
 (Chlorite in 
 restricted 
 sense.) 
 
 Mixture ff/ 
 
 3MgO.' 
 2SiO a ) + 
 q (2H a O . 
 
 2.78- 
 2-95- 
 
 Very 
 Perfect 
 \\oP. 
 
 Leaflets 
 and six- 
 sided 
 tablets 
 ooP. oP 
 
 
 Apparently 
 hexagonal 
 (optically- 
 
 uniaxial), 
 often 
 
 Feebly 
 negative. 
 
 Cleavage- 
 leaflets 
 like 
 isotrope. 
 Longi- 
 
 
 2MgO . 
 A1 2 O 8 . 
 SioJ. 
 
 
 
 like hexag- 
 onal. If 
 monoclmic, 
 
 
 clearly 
 optically- 
 biaxial, 
 
 
 tudinal 
 sections 
 with 
 
 
 p : q = i : 2. 
 
 
 
 then ooP. 
 
 
 with, very 
 
 
 parallel 
 
 
 
 
 
 oo Poo . oP. 
 
 
 small axial 
 
 
 extinc- 
 
 b. Hel- 
 
 Decom- 
 
 
 ditto. 
 
 Lon^f ver- 
 
 Six-sided 
 
 angle. 
 
 
 tion. 
 
 minth. 
 
 posed by 
 
 
 
 micular 
 
 leaflets 
 
 -L0P. 
 
 
 
 
 H 3 SO 4 . 
 
 
 
 curled 
 
 with re- 
 
 
 
 
 
 
 
 
 columns. 
 
 entrant 
 
 
 
 
 
 
 
 
 
 angles. 
 
 
 
 
 c. Penum- 
 ite. 
 
 P'- 9 = 3:2. 
 Decom- 
 posed by 
 HC1. 
 
 2.61- 
 2.77. 
 
 ditto. 
 
 Crystals 
 like 
 rhombo- 
 hedra oP. 
 R or UK 
 
 Pene- 
 tration 
 three- 
 lings. 
 (Biaxial 
 
 Often 
 clearly 
 optically- 
 biaxial. 
 
 Sometimes 
 positive, 
 sometimes 
 negative ; 
 very feeble. 
 
 Longi- 
 tudinal 
 sections 
 with 
 parallel 
 
 
 
 
 
 or oo /? . R. 
 
 parts. 
 
 
 
 extinc- 
 
 
 
 
 
 
 Visible in 
 
 
 
 tion. 
 
 
 
 
 
 
 three 
 
 
 
 Cleavage- 
 
 . .. ^ 
 
 d. Kacm- 
 mererhe. 
 
 Contains 
 Cr a 8 . 
 
 2.617- 
 2.76. 
 
 ditto. 
 
 Irregular 
 leaflets 
 
 positions 
 differing 
 
 Clearly 
 optically- 
 
 
 leaflets 
 some- 
 
 
 
 
 
 apparently 
 P . oP. ' 
 
 by 120 
 in leaves 
 
 biaxial. 
 
 
 times 
 isotrope, 
 
 
 
 
 
 
 II oP.) 
 
 
 
 some- 
 
 
 
 
 
 
 
 
 
 times 
 
 
 
 
 
 
 
 
 
 double- 
 
 
 
 
 
 
 
 
 
 refract- 
 
 
 
 
 
 
 
 
 
 ing. 
 
 e. Chno- 
 chlore. 
 
 p : a = 2 : 3. 
 More 
 difficultly 
 
 2.6s- 
 2.78. 
 
 ditto. 
 "Sliding 
 planes" 
 
 Crystals of 
 monoclinic 
 habit 
 
 Com- 
 monly in 
 twins and 
 
 4. P. || oo Poo, 
 often also 
 
 Generally 
 positive. 
 
 c : c = 
 12-15. 
 
 
 decom- 
 
 
 (eleit- 
 
 00 P. 00 POO . 
 
 three- 
 
 c quite -L0P. 
 
 
 
 
 posed by 
 
 
 flache) 
 
 oP, etc. 
 
 lings. 
 
 Varying 
 
 
 
 
 acids than 
 the above. 
 
 
 similar 
 to mica. 
 
 oo P quite 
 120. In 
 
 Twinning 
 plane a 
 
 about 12-15 
 from the 
 
 
 
 
 
 
 
 large 
 
 face of 
 
 normal to 
 
 
 
 
 
 
 
 leaves. 
 
 the hemi- 
 
 oP. Large 
 
 
 
 
 
 
 
 
 pyramid 
 
 axial angle. 
 
 
 
 
 
 
 
 
 3P. 
 
 Dispersion 
 
 
 
 
 
 
 
 
 
 p < v. 
 
 
 
 f. Chlori- 
 toid 
 
 H a R(Ai a , 
 SiO 7 ; 
 R = FeO 
 
 3-52- 
 356. 
 
 II o P. 
 Not so 
 perfect 
 
 ditto. 
 Tablets. 
 
 Common- 
 ly tablets 
 of thin 
 
 A. P. 
 
 II oo Poo. 
 
 r. M. differs 
 
 Negative. 
 
 a : c = 
 5-12. 
 
 and 
 
 and some 
 
 
 as in the 
 
 
 eaves de- 
 
 about 12 
 
 
 
 
 MgO. 
 Decom- 
 
 
 others. 
 
 
 veloped 
 twin-like, 
 
 from the 
 
 
 
 g. Sismon- 
 
 posed by 
 
 
 
 
 which are 
 
 
 Feeble. 
 
 
 dine. 
 
 concen- 
 
 
 
 
 placed at 
 
 
 
 
 
 trated 
 
 
 
 
 120 to 
 
 
 
 
 
 H a S0 4 . 
 
 
 
 
 each 
 
 
 
 
 
 
 
 
 
 other. 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 163 
 
 
 Color and 
 
 
 
 
 1 
 
 
 
 Polari- 
 zation- 
 colors. 
 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclos- 
 ures. 
 
 1 Decom 
 position. 
 
 Occi rrence. 
 
 Remarks. 
 
 
 
 
 
 
 
 
 Primary. 
 More com- 
 
 
 Feebly 
 bril- 
 
 Light 
 to dark 
 
 Very 
 feeble. 
 
 The chlorites 
 do not for the 
 
 As 
 primary 
 
 Very 
 
 poor. 
 
 
 monly in 
 leaflets in 
 
 
 liant. 
 
 green. 
 
 
 most part 
 
 con- 
 
 Hema- 
 
 
 chloritic 
 
 
 bluish, 
 
 /a. = 1.575. 
 
 
 occur as rock- 
 
 stituent 
 
 tite and 
 
 
 schists, as de- 
 
 
 blue to 
 
 
 
 constituents 
 
 with 
 
 hydrated 
 
 
 composition- 
 
 
 green. 
 
 
 
 in large lamel- 
 
 quartz, 
 
 ferric 
 
 
 p'roduct of 
 
 
 
 
 
 lary hexag- 
 
 ortho- 
 
 oxide, 
 
 
 mica, augite, 
 
 
 
 
 
 onal tablets 
 
 clase, 
 
 and 
 
 
 hornblende, 
 
 
 
 
 
 like the micas, 
 
 biotite, 
 
 needles ol 
 
 
 and garnet. 
 
 Difficult to 
 
 
 
 
 but like talc 
 in aggregates 
 of minute ir- 
 regular leaf- 
 lets either 
 
 and 
 musco- 
 vite. 
 
 rutile and 
 actinolite. 
 
 
 As decomposi- 
 tion-product 
 after mica and 
 hornblende, 
 and inter- 
 
 distinguish 
 from 
 decomposed 
 or green- 
 colored mica. 
 
 
 
 
 singly or 
 disposed in 
 radial groups. 
 
 
 
 
 penetrated in 
 minerals of 
 the crystal- 
 line schists. 
 
 The chlorites 
 as rock- 
 constituents 
 are extremely 
 
 
 
 
 
 
 
 
 Rare 
 
 difficult to 
 
 
 
 
 
 
 
 
 
 distinguish 
 from each 
 
 See 
 ripido- 
 
 Leek to 
 bluish 
 
 Feeble. 
 Green 
 
 ditto. 
 
 ditto. 
 
 Rare as rock- 
 constituent 
 
 lite. 
 
 green. 
 
 shades. 
 
 
 
 
 
 as above, in 
 
 Clinochlore 
 
 
 
 
 
 
 
 
 leaves. 
 
 alone (also 
 
 
 
 
 
 
 
 
 
 ottrelite) is 
 
 
 
 
 
 
 
 
 
 well charac- 
 
 
 
 
 
 
 
 
 
 terized 
 
 
 
 
 
 
 
 
 
 through the 
 
 
 Peach to 
 blood-red. 
 
 
 Often inter- 
 penetrated 
 
 With 
 olivine, 
 
 
 By 
 
 decom- 
 
 Rarely in 
 serpentines. 
 
 pronounced 
 pleochroistn 
 
 
 
 
 with clino- 
 
 augite, 
 
 
 position 
 
 
 as well as the 
 
 
 
 
 chlore. 
 
 and 
 
 
 is de- 
 
 
 common 
 
 
 
 
 
 chromite. 
 
 
 color- 
 
 
 twinning; 
 
 
 
 
 
 
 
 ized and 
 
 
 more easily 
 
 
 
 
 
 
 
 resem- 
 
 
 determined 
 
 . 
 
 
 
 
 
 
 bles 
 
 
 by optical 
 
 
 
 
 
 
 
 talc. 
 
 
 examination. 
 
 More 
 brilliant 
 than in 
 the 
 
 Dark oil 
 U bluish 
 green. 
 
 Often 
 very 
 strong. In 
 sections 
 
 In larger 
 leaves, yet 
 not so 
 marked by 
 
 With 
 quartz, 
 orlho- 
 clase, and 
 
 ditto. 
 
 
 Primary. 
 Common in 
 crystalline 
 schists, as 
 
 Ottrelite is 
 marked by 
 the greater 
 lardness, less 
 
 other 
 
 
 
 lamellae as 
 
 mica. 
 
 
 
 chloritic 
 
 perfect 
 
 chlo- 
 rites. 
 Indigo- 
 yellow. 
 
 
 yellow ; 
 II flight 
 green, 
 yellowish 
 green. 
 -Lc blue- 
 green, 
 
 mica. 
 
 With 
 augite, 
 horn- 
 blende, 
 olivine, 
 or ser- 
 pentine. 
 
 
 
 schist, and 
 secondary in 
 serpentine. 
 
 cleavage, 
 absence of 
 laminations, 
 and richness 
 of inclosures ; 
 also dis- 
 tinguished by 
 chemical 
 
 
 
 green. 
 
 
 
 
 
 
 quantitative 
 analysis. 
 
 See 
 
 Dark 
 
 See 
 
 ditto. 
 
 With 
 
 Fluid m- 
 
 Chloritoid in 
 
 clino- 
 
 green. 
 
 clino- 
 
 
 quartz, 
 
 closnres 
 
 
 certain semi- 
 
 
 chlore. 
 
 
 chlore. 
 
 
 ortho- 
 
 very 
 
 
 crystalline 
 
 
 
 
 II c 
 
 
 clase. and 
 
 common. 
 
 
 schists. 
 
 
 
 
 yellowish 
 
 
 mica. 
 
 Rutile 
 
 
 Sismondine 
 
 
 
 
 green. 
 
 
 With 
 
 needles. 
 
 
 rarely in 
 
 
 
 
 -L c 
 
 
 augiie, 
 
 
 
 glaucophane- 
 
 
 
 
 greenish 
 
 
 rutile, 
 
 
 
 eclogite. 
 
 
 
 
 blue. 
 
 
 titanite, 
 
 
 
 
 
 
 
 
 
 glauco- 
 
 
 
 
 
 
 
 
 
 phane. 
 
 
 
 
 
i6 4 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 strength of 
 double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 h. Ottrelite. 
 
 H,R 3 (Al 2 ) a 
 SigO^. 
 R = Fe,Mn. 
 Attacked 
 by H a S0 4 
 only with 
 difficulty. 
 
 4-4 (?) 
 
 oP very 
 perfect. 
 Besides, 
 according 
 to oo P, 
 with an 
 angle of 
 
 110-120 
 
 (Becke). 
 
 Thin 
 spherical 
 tablets; 
 rounded 
 cross- 
 sections 
 || ffP rare. 
 Elongated 
 rectangles 
 
 See 
 
 sismondine; 
 Poly 
 
 synthetic 
 tivinning- 
 striations 
 \oP. 
 (Fig. 82.) 
 
 Optically 
 biaxial ; 
 i.M. rather 
 sharply 
 inclined to 
 the perfect 
 cleavage- 
 planes; 
 small axial 
 
 Very 
 feebly 
 negative. 
 
 Com- 
 monly 
 parallel 
 to the 
 longer 
 axis of 
 (he cross- 
 section. 
 
 
 
 
 
 if the 
 
 
 angle. 
 
 
 
 
 
 
 
 sections are 
 
 
 
 
 
 
 
 
 
 inclined 
 
 
 
 
 
 
 
 
 
 tooP. 
 
 
 
 
 
 2. MONOCL1N1C 
 
 aa. PLANE OF OPTIC AXES GENERALLY J_copco; PERFECT 
 
 1. a. Ortho- 
 
 
 
 
 
 
 
 
 
 clase. 
 
 K a Al a 
 Sie0 16 . 
 Not 
 attacked 
 by acids. 
 Small 
 
 2.50- 
 2-59 
 (2o7). 
 
 Eminent 
 HoPand 
 oo Poo. 
 Cleavage- 
 angle 
 90. 
 
 In grains, 
 or partly 
 columnar: 
 
 OP. oo Poo. 
 
 00 P. 2 POO. 
 2POO.P, 
 
 Very 
 common, 
 especially 
 after the 
 following 
 three laws: 
 
 A.P. 
 
 generally 
 -L ooPoo, 
 equally 
 inclined 
 with 0Pand 
 
 Rather 
 feebly 
 negative. 
 
 In 
 sections 
 or 
 cleavage- 
 leaflets i| 
 ooPoo a 
 
 
 amount of 
 
 
 
 and partly 
 
 most 
 
 forms with 
 
 
 direction 
 
 
 Na, Ca, Fe, 
 
 
 
 of large or 
 
 commonly 
 
 the vertical 
 
 
 of ex- 
 
 
 Mg. 
 
 
 
 more rarely 
 
 i. The 
 
 axis an 
 
 
 tinction 
 
 
 
 
 
 minute 
 
 Carlsbad 
 
 angle of 
 
 
 varies ' 
 
 
 
 
 
 tabular 
 
 laiv. 
 
 6q n'. 
 
 
 from the 
 
 
 
 
 
 crystals. 
 
 Twinning- 
 
 C = ^ 
 
 
 edge oP : 
 
 
 
 \ 
 
 
 oo Poo. oo P. 
 
 plane oopoo 
 
 fl: a = 5. 
 
 
 ooPoo = 
 
 
 
 
 
 oP 2 Poo. 
 
 combined 
 
 True axial 
 
 
 a : a about 
 
 
 
 
 
 Poo. 
 
 or inter- 
 
 angle = 69 
 
 
 5 18'. 
 
 
 
 
 
 
 penetrated 
 in the direc- 
 
 (SeeFigT 4 .) 
 Axial 
 
 
 Sections 
 
 
 
 
 
 
 tion of the 
 
 dispersion 
 
 
 parallel b, 
 
 
 
 
 
 
 <-axis. 2. 
 
 = p > v. 
 
 
 that is, 
 
 
 
 
 
 
 Raveno law. 
 
 In sections 
 
 
 from the 
 
 
 
 
 
 
 Twinmng- 
 
 parallel 
 
 
 zone oP: 
 
 ~ 
 
 
 
 
 
 plane2Poo, 
 especially 
 in the 
 
 oo Poo or 
 
 oo Poo 
 
 i. cond. a 
 
 
 oo Poo. 
 
 of course 
 have a 
 
 
 
 
 
 
 columnar 
 
 distorted 
 
 
 parallel 
 
 
 
 
 
 
 examples. 
 3. Rarely 
 
 biaxial in- 
 terference- 
 
 
 extinc- 
 tion. 
 
 
 
 
 
 
 the 
 
 figure 
 
 
 
 
 
 
 
 
 Mane bach 
 
 visible. 
 
 
 
 
 
 
 
 
 law. 
 
 A.P. is rare. 
 
 
 
TABLES FOR DETERMIAUNG MINERALS. 
 
 i6 5 
 
 Poiariza- 
 
 tlOIl- 
 
 colors. 
 
 Color and 
 power of 
 reiractmg 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Not 
 brilliant, 
 
 Greenish 
 black; in 
 
 Rather 
 power- 
 
 Compare with 
 "Inclosures." 
 
 With 
 quartz, 
 
 Generally 
 exceed- 
 
 
 Rare, in 
 semi- 
 
 Ottr elite is 
 triclinic 
 
 similar 
 to 
 
 sections 
 light to 
 
 ful; 
 \oP lav- 
 
 In large 
 tablets of a 
 
 mica 
 (musco- 
 
 ingly rich 
 in in- 
 
 
 crystalline 
 and 
 
 (Renard). 
 Cleavage after 
 
 ripidolite. 
 
 grayish 
 green. 
 
 ender- 
 blue. 
 
 black color 
 (rather hard). 
 
 vite), 
 rutile 
 
 closures 
 of color- 
 
 
 metamorphic 
 schists. 
 
 oP\ also not 
 after oo P, but 
 
 
 
 bluish 
 
 Cleavage )| oP 
 
 needles, 
 
 less 
 
 
 
 in two directions 
 
 
 
 green, 
 
 never so 
 
 garnet. 
 
 quartz 
 
 
 
 cutting each 
 
 
 
 II e 
 
 very perfect 
 
 
 granules. 
 
 
 
 other at an 
 
 
 
 green- 
 
 as in the other 
 
 
 rutile 
 
 
 
 angle of about 
 
 
 
 ish 
 
 chlorites; 
 
 
 needles, 
 
 
 
 130. and in a 
 
 
 
 blue, 
 
 besides this, 
 
 
 and 
 
 
 
 third direction 
 
 
 
 yellow- 
 
 however, 
 
 
 earthy 
 
 
 
 at right angles 
 
 
 
 ish 
 
 always a 
 
 
 particles. 
 
 
 
 to one of these. 
 
 
 
 green. 
 
 cleavage || to 
 
 
 
 
 
 
 
 
 
 the c-axis. 
 
 
 
 
 
 
 CRYSTALS. 
 
 CLEAVAGE || oP AND 
 
 , ANGLE NEARLY 90. 
 
 Rather 
 
 Rarely 
 
 
 Orthoclase in 
 
 With 
 
 As a rule 
 
 Mostly 
 
 One of the 
 
 The large 
 
 brilliant, 
 not, 
 
 colorless, 
 clear as 
 
 
 large crystals 
 or grains I.O. 
 
 quartz, 
 biotite, 
 
 very poor. 
 Hema- 
 
 perfect- 
 ly de- 
 
 most common 
 constituents 
 
 crystals can 
 be easily dis- 
 
 however, 
 
 water: 
 
 
 and smaller 
 
 musco- 
 
 tite, 
 
 com- 
 
 of the 
 
 tinguished 
 
 so bright 
 as in 
 quartz. 
 
 generally 
 white or 
 opaque 
 
 
 granules, 
 rarely fila- 
 ments, II. O. 
 
 vite, 
 and 
 horn- 
 
 biotite 
 leaflets, 
 fluid in- 
 
 posed, 
 the 
 crystals 
 
 granular and 
 porphyritic 
 older eruptive 
 
 from the 
 other colorless 
 optically-biaxial 
 
 In ver-y 
 
 from 
 
 
 in eruptive 
 
 blende, 
 
 closures, 
 
 opaque, 
 
 rocks. Essen- 
 
 minerals by 
 
 thin 
 
 decom- 
 
 
 rocks; always 
 
 rarely 
 
 apatite 
 
 and 
 
 tial primary 
 
 the twinnings 
 
 sections 
 
 position^ 
 
 
 in grains in 
 
 augite, 
 
 needles, 
 
 non- 
 
 constituent in 
 
 in sections 
 
 and in 
 
 gray; 
 
 
 crystalline 
 
 plagio- 
 
 zircon. 
 
 trans- 
 
 granite, 
 
 || ffPand 90^00, 
 
 micro- 
 
 " tinged 
 
 
 schists. 
 
 clase, 
 
 
 parent; 
 
 syenite, 
 
 and by the 
 
 lites the 
 
 red from 
 
 
 Penetrations 
 
 elaeo- 
 
 
 into 
 
 quartzose 
 
 oblique 
 
 polariza- 
 
 ferric 
 
 
 with plagio- 
 
 lite. 
 
 
 kaolin 
 
 porphyry, and 
 
 extinction 
 
 tion- 
 
 oxide or 
 
 
 clase are 
 
 
 
 with 
 
 accessory in 
 
 parallel oojPoo. 
 
 colors of 
 
 hydrox- 
 
 
 common, 
 
 
 
 forma- 
 
 nearly all 
 
 The threadlets 
 
 ortho- 
 
 ide. 
 
 
 generally 
 
 
 
 tion of 
 
 plagioclase 
 
 of orthoclase 
 
 clase 
 
 
 
 
 
 
 musco- 
 
 rocks; more- 
 
 and sanidtne, 
 
 are dull. 
 
 fa k et&~f 
 
 
 graphic- 
 
 
 
 vite 
 
 over, in the 
 
 so often 
 
 generally 
 
 / 
 
 
 granite-like, 
 
 
 
 or 
 
 crystalline 
 
 appearing in the 
 
 blue- 
 
 j^&r 
 
 
 with quartz 
 
 
 
 epidote. 
 
 schists, 
 
 ground-mass 
 
 as, e.g., 
 
 
 
 (micro-peg- 
 matite). (See 
 
 
 
 
 especially the 
 gneisses; 
 
 of rocks, have 
 often a marked 
 
 nephe- 
 
 
 
 Fig. 63, e.) 
 
 
 
 
 here often 
 
 similarity to 
 
 hne. 
 
 
 
 Zonal struc- 
 
 
 
 
 glassy, as 
 
 nepheline. 
 
 
 
 
 ture rare; also 
 
 
 
 
 sanidine. 
 
 
 
 
 
 inclosures 
 
 
 
 
 
 
 
 
 
 zonal ly 
 
 
 
 
 
 
 
 
 
 disposed. 
 
 
 
 
 
 
i66 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composi- 
 tion and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 b, Sani- 
 
 
 
 
 
 
 
 
 
 dine 
 
 
 
 As above; 
 
 Sanidine in 
 
 Twinning- 
 
 Parallel 
 
 
 
 
 
 
 crystals 
 
 minute, 
 
 plane = oP. 
 
 ooPoo; 
 
 
 
 
 
 
 fur- 
 rowed. 
 
 long, 
 narrow 
 
 (See Fig. 28.) 
 Cross- 
 
 a : a 
 
 
 
 
 
 
 
 threads, as 
 microlites, 
 
 sections of 
 twins: 
 
 equals 5. 
 
 
 
 
 
 
 
 or large 
 crystals, 
 
 a. In the 
 Carlsbad 
 
 
 
 
 
 
 
 
 never in 
 
 twins the 
 
 
 
 
 
 
 
 
 grains. 
 
 rectangular 
 
 
 
 
 
 
 
 
 Form of 
 
 sections at 
 
 
 
 
 
 
 
 
 cross- 
 
 right angles 
 
 
 
 
 
 
 
 
 sections 
 
 ooPoo 
 
 
 
 
 
 
 
 
 parallel oP 
 and 00 Poo 
 
 divided into 
 halves 
 
 
 
 
 
 
 
 
 long and 
 thread-like; 
 
 parallel to 
 the edges 
 
 
 
 
 
 
 
 
 parallel 
 
 0P/ooPoo 
 
 
 
 
 
 
 
 
 oo Poo 
 
 and 
 
 
 
 
 
 
 
 
 distorted 
 
 00 P/ 00 POO. 
 
 
 
 
 
 
 
 
 hexagons 
 whose sides 
 
 b. In the 
 Baveno 
 
 
 
 
 
 
 
 
 correspond 
 to 
 
 twins the 
 quadratic 
 
 
 
 
 
 
 
 
 oP P . 
 
 sections 
 
 
 
 
 
 
 
 
 oo Poo. 
 In columnar 
 types of 
 the crystals: 
 rectangular 
 sections if 
 
 at right 
 angles ooPoo 
 are divided 
 into halves 
 by the 
 diagonals. 
 
 
 
 
 
 
 
 
 at right 
 
 
 
 
 
 
 
 
 
 angles to 
 
 
 
 
 
 
 
 
 
 octagonal if 
 
 
 
 
 
 
 
 
 
 besides 
 
 
 
 
 
 
 
 
 
 these also 
 
 
 
 
 
 
 
 
 
 2Poo is 
 
 
 
 
 
 
 
 
 
 present. 
 (See Fig. 83.) 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 I6 7 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleochro- 
 ism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inciosures. 
 
 Decompo- 
 sition. 
 
 Occurrence. 
 
 Remarks. 
 
 
 Sanidine 
 
 
 Sanidine 
 
 Sanidine 
 
 Sanidine 
 
 Almost 
 
 Essential 
 
 and certain 
 
 
 is always 
 colorless. 
 
 
 in large 
 crystals 
 
 like 
 ortho- 
 
 generally 
 very rich, in 
 
 always 
 fresh, 
 
 primary 
 constitu- 
 
 melilites. 
 The isotrope 
 
 
 clear as 
 
 
 I. O and 
 
 clase; 
 
 inclosures. 
 
 rarely 
 
 ent of the 
 
 hexagonal 
 
 
 water. 
 /3p = 
 
 
 minute 
 threads 11. 0. 
 
 besides 
 with 
 
 especially 
 vitreous 
 
 opaque. 
 In 
 
 trachytes, 
 rhyolites, 
 
 transverse 
 sections are 
 
 
 I-5237- 
 
 
 in eruptive 
 rocks. 
 The large 
 crystals 
 
 augite, 
 nephe- 
 line, and 
 leucite, 
 
 inclosures, 
 generally 
 zonally 
 disposed, 
 
 andesites 
 and 
 trachytes 
 a decom 
 
 phono- 
 lites, 
 and the 
 glasses of 
 
 wanting on 
 orthoclase. 
 Grains of 
 orthoclase 
 
 
 
 
 are often 
 crumbled or 
 fused, and 
 
 never 
 with mus- 
 covite. 
 
 augite- 
 microlites, 
 apatite 
 
 position 
 into opal. 
 
 the ortho- 
 clase 
 rocks; 
 
 in isotrope 
 sections 
 are 
 
 
 
 
 with an 
 exceedingly 
 beautiful 
 
 
 needles. 
 
 
 accessory 
 in nearly 
 all of the 
 
 easily 
 distinguish- 
 ed from 
 
 
 
 
 zonal 
 structure, 
 
 
 
 
 younger 
 plagio- 
 
 quartz by 
 the 
 
 
 
 
 seen 
 
 
 
 
 clase 
 
 condenser, 
 
 
 
 
 particularly 
 well. 
 
 
 
 
 rocks, 
 
 as in 
 orthoclase 
 
 
 
 
 i. p. p. I. 
 
 
 
 
 
 cross- 
 
 
 
 
 Inciosures 
 
 
 
 
 
 sections 
 
 
 
 
 are common, 
 
 
 
 
 
 one of the 
 
 
 
 
 arranged 
 
 
 
 
 
 optic axes 
 
 
 
 
 in zones. 
 
 
 
 
 
 is visible. 
 
 
 
 
 
 
 
 
 
 Orthoclase 
 
 
 
 
 
 
 
 
 
 is distin- 
 
 
 
 
 
 
 
 
 
 guished from 
 
 
 
 
 
 
 
 
 
 plagioclase 
 
 
 
 
 
 
 
 
 
 by the 
 
 
 
 
 
 
 
 
 
 optical 
 
 
 
 
 
 
 
 
 
 orientation 
 
 
 
 
 
 
 
 
 
 and absence 
 
 
 
 
 
 
 
 
 
 of the 
 
 
 
 
 
 
 
 
 
 polysynthet- 
 ic stnation. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 | 
 
1 68 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 bb. PLANE OF OPTIC AXES 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleav- 
 age. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 1. Mono- 
 
 (3.17-3 41) 
 
 
 
 
 
 
 
 clinic 
 
 
 
 
 
 
 
 
 Augite | 
 
 
 
 
 
 
 
 
 Group. 
 
 
 
 
 
 
 
 
 a.Ordinary RSiO 3 
 
 3.34-3.38. 
 
 Emi- 
 
 Rarely in 
 
 Very 
 
 A.PJooPoo; 
 
 Positive, 
 
 In 
 
 and 
 basaltic 
 Augite. 
 
 R = Mg, 
 Ca, Fe, 
 and Fe 2 O 3 
 and A1 2 O 3 
 
 
 nent 
 
 00 P. 
 
 grains; in 
 crystals; 
 oo/> ooPoo, 
 
 OOJ?00./, 
 
 common. 
 T win- 
 ning- 
 plane 
 
 the 
 r. M. = c is 
 wanting 
 in the 
 
 strong. 
 
 sections 
 parallel 
 oo Poo 
 
 c : c = 
 
 
 Mixture of 
 
 
 
 and 
 
 ,Poo, 
 
 obtuse 
 
 
 about 39. 
 
 
 CaMg 
 
 
 
 sometimes 
 
 also in 
 
 tingle /3, 
 
 
 Varies 
 
 
 SiaOe 4. Ca 
 FeSiTOe 
 + MV 
 A1 a Si0 6 . 
 (Tscher- 
 mak.) 
 Fe-rich 
 augites. 
 Not 
 attacked by 
 acids. 
 
 
 
 Pn.oP 
 and -P. 
 oo J P = 876 / . 
 Sections at 
 right angles 
 to the 
 r-axis are 
 octagonal, 
 with 
 evident 
 prismatic 
 
 poly- 
 svnthetic 
 twins. 
 (See Figs. 
 24 and 25.) 
 More 
 rarely 
 penetra- 
 tion- 
 twins 
 after: 
 
 b = 
 (See 
 Fig. 10.) 
 The 
 positive 
 axial angle, 
 as in the 
 rhombic 
 augites, 
 decreases 
 with the 
 
 
 from 39 
 to 54. 
 a : to edge 
 
 o/yoojpoo 
 = a : & = 
 about 22. 
 Sections 
 II b have 
 parallel 
 extinc- 
 tion. In 
 
 
 
 
 
 cleavage. 
 The longi- 
 tudinal 
 
 twinning- 
 pbuie a 
 face 
 
 iron 
 present, 
 about 60. 
 
 
 sections 
 inclined 
 to oo Poo 
 
 
 
 
 
 sections 
 distorted 
 hexago- 
 nally 
 with the 
 
 Poo; or 
 after: 
 twinning- 
 plane a 
 face f>2. 
 
 Sections 
 at right 
 angles to 
 the c-axis 
 and 
 
 
 c : c de- 
 creases to 
 
 parallel 
 
 oo^oo. 
 
 
 
 
 
 c-axis 
 
 
 parallel 
 
 
 
 
 
 
 
 parallel to 
 
 
 oooo show 
 
 
 
 
 
 
 
 the 
 
 
 
 
 
 
 
 
 
 cleavage- 
 fissures. 
 
 
 condenser 
 one outic 
 
 
 
 
 
 
 
 Also rect- 
 angular 
 
 
 axis exactly 
 in the 
 
 
 
 
 
 
 
 parallel 
 
 
 centre of 
 
 
 
 
 
 
 
 OOjPoo; 
 
 
 the field. 
 
 
 
 
 
 
 
 often a 
 
 
 
 
 
 
 
 
 
 rhomb. 
 
 
 
 
 
 
 
 
 
 (See 
 
 
 
 
 
 
 
 
 
 Fig. 84.) 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 169 
 
 EMINENT CLEAVAGE AFTER co/ > 87. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 
 chroism 
 
 Structure. 
 
 Associa- 
 tion. 
 
 In- 
 
 closures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 Very 
 brilliant, 
 especially 
 in the 
 
 In 
 sections 
 
 green to 
 broivn, 
 
 Gen- 
 erally 
 very 
 feeble, 
 
 In large crystals 
 I.O. and col- 
 umns in micro- 
 litesll. O. The 
 
 Princi- 
 pally 
 with pla- 
 gioclase, 
 
 Vitre- 
 ous in- 
 closures 
 are 
 
 Augite 
 crystals 
 are com- 
 monly de- 
 
 As essential 
 primary 
 constituent 
 in many 
 
 Easily dis- 
 tinguished 
 from other 
 optically- 
 
 light- 
 colored, 
 yellow to 
 
 often 
 violet to 
 brown in 
 
 yet 
 augite 
 is 
 
 first very com- 
 monly show a 
 zonal structure, 
 
 nephe- 
 line, 
 leucite, 
 
 com- 
 mon, as 
 are 
 
 composed 
 into a pro- 
 duct of 
 
 younger 
 prophyritic 
 eruptive 
 
 biaxial 
 minerals 
 by the 
 
 red. 
 
 the 
 
 strong- 
 
 a green core, 
 
 with or 
 
 also 
 
 chloritic 
 
 rocks: 
 
 important 
 
 
 basalts. 
 
 ly pleo- 
 
 e.g. with brown 
 
 without 
 
 gas- 
 
 material, 
 
 diabases, 
 
 oblique 
 
 
 The 
 
 chroitic 
 
 ayers which in 
 
 olivine 
 
 pores 
 
 calcite, 
 
 melaphyrs, 
 
 extinction 
 
 
 same 
 
 as in 
 
 turn are often 
 
 and 
 
 and 
 
 ferric 
 
 augite- 
 
 c * c 
 
 
 crystal 
 
 the 
 
 again composed 
 
 biotite. 
 
 apatite 
 
 hydrate, 
 
 andesites, 
 
 and 
 
 
 often 
 
 phono- 
 
 of numberless 
 
 Rarely 
 
 needles. 
 
 epidote, 
 
 and all 
 
 prismatic 
 
 
 shows 
 several 
 
 lites 
 and 
 
 thin layers. As 
 a consequence 
 
 with 
 ortho- 
 
 Mag- 
 netite. 
 
 and quartz. 
 Perfect 
 
 basaltic 
 rocks; also 
 
 clearage 
 with angle 
 
 
 colors. 
 
 then 
 
 ol this varying 
 
 clase, 
 
 
 pseudo- 
 
 common in 
 
 of 87; 
 
 
 (Comp. 
 
 resem- 
 
 constitution 
 
 horn- 
 
 
 morphs of 
 
 andesites, 
 
 especially 
 
 
 "struc- 
 
 bles 
 
 of both core and 
 
 blende, 
 
 
 one or more 
 
 trachytes, 
 
 in trans- 
 
 
 ture.") 
 
 horn- 
 
 layers, opti- 
 
 and 
 
 
 of these 
 
 phonolites. 
 
 verse sec- 
 
 
 /3p = 1.69. 
 
 blende. 
 Absorp- 
 
 cal differences, 
 as in directions 
 
 quartz. 
 
 
 minerals 
 after augite 
 
 Rare and in 
 larger 
 
 tions ; more 
 difficult 
 
 
 
 tion 
 
 of extinction 
 
 
 
 are 
 
 grains in 
 
 when gran- 
 
 
 
 feeble 
 
 and polariza- 
 
 
 
 common. 
 
 the older 
 
 ular; in sec- 
 
 
 
 c>a>b. 
 
 tion-colors, are 
 
 
 
 Into opal. 
 
 granular 
 
 tions in- 
 
 
 
 a about 
 
 = b. 
 
 'requent. (See 
 Fig. 45.) As 
 
 
 
 More 
 rarely the 
 
 eruptive 
 rocks, and 
 
 clined to 
 the c-axis 
 
 
 
 
 with orthoclase, 
 
 
 
 metamor- 
 
 in 
 
 the 
 
 
 
 
 the successive 
 layers in twins 
 
 . 
 
 
 phosis into 
 hornblende 
 
 crystalline 
 schists. 
 
 cleavage- 
 angle ap- 
 
 
 
 
 of augite 
 
 
 
 uralitizing) 
 
 
 proaches 
 
 .- 
 
 
 
 run equally and 
 unimpeded 
 
 
 
 wherein 
 the form of 
 
 
 that of 
 nornblendo. 
 
 
 
 
 through both in- 
 
 
 
 augite re- 
 
 
 Easily dis- 
 
 
 
 
 dividuals. Au- 
 
 
 
 mains with 
 
 
 tinguished 
 
 
 ' 
 
 
 gites often show 
 
 
 
 lornblendic 
 
 
 from epi- 
 
 
 
 
 the so-called 
 
 
 
 cleavage. 
 
 
 dote by the 
 
 
 
 
 " hour-glass " 
 
 
 
 Finally the 
 
 
 color, direc- 
 
 
 
 
 formation 
 
 
 
 rare meta- 
 
 
 tion of ex- 
 
 
 
 
 where sections 
 
 
 
 morphosis 
 
 
 tinction, re- 
 
 
 
 
 II oo/'oo divide 
 
 
 
 into 
 
 
 lief, and 
 
 
 
 
 into four fields 
 
 
 
 serpentine, 
 
 
 polariza- 
 
 
 
 
 of which each 
 
 
 
 with forma- 
 
 
 tion-colors. 
 
 
 
 
 two lying oppo- 
 site extinguish 
 
 
 
 tion of 
 talc and 
 
 
 If augite is 
 perfectly 
 
 
 
 
 at the same in 
 
 
 
 chlorite. 
 
 
 colorless, 
 
 
 
 
 slant. (See 
 
 
 
 
 
 the polari- 
 
 
 
 
 Figs. 46 and 47.) 
 
 
 
 
 
 zation- 
 
 
 
 
 Augite crystals 
 
 
 
 
 
 colors are 
 
 
 
 
 ire often fused. 
 
 
 
 
 
 very bril- 
 
 
 
 
 also commonly 
 
 
 
 
 
 liant and 
 
 
 
 
 separated into 
 
 
 
 
 
 resemble 
 
 
 
 
 large aggre- 
 
 
 
 
 
 olivine. 
 
 
 
 
 gates, the so- 
 
 
 
 
 
 
 
 
 
 called " augite- 
 
 
 
 
 
 
 
 
 
 eyes," or 
 
 
 
 
 
 
 
 
 
 needles radially 
 
 
 
 
 
 
 
 
 
 grouped. 
 
 
 
 
 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direc- 
 tion of 
 extinc- 
 tion. 
 
 b, Diallage. 
 
 See augite. 
 
 3-23- 
 3-34- 
 
 co/>(8 7 ) 
 concen- 
 trically 
 arranged 
 after 
 
 00^*00. 
 
 Rarely in 
 clearly-defined 
 crystals, mostly 
 in large tabular 
 or granular 
 individuals, 
 
 || 00^*00 
 
 poly- 
 synthet- 
 ic; not 
 rarely 
 after oP. 
 
 s 
 
 ee augite. 
 
 
 
 
 
 
 fibrous parallel 
 
 
 
 
 
 
 
 
 
 to the oaxis. 
 
 
 
 
 
 c. Ompha- 
 cite. 
 
 d. Diopside. 
 
 See .-tugite. 
 Rich in 
 A! 2 3 . 
 
 More CaO 
 than MgO, 
 
 3-3- 
 3-3- 
 
 See 
 augite, 
 also sepa- 
 ration 
 I oo/>oo, 
 
 yet not so 
 perfect 
 as in 
 diallage. 
 
 ditto. 
 
 Only in grains 
 ditto. 
 
 Rare, 
 ditto. 
 
 s 
 
 ditto. 
 
 ee augite. 
 ditto. 
 
 ditto. 
 
 
 poor in 
 
 
 
 
 
 
 
 
 
 AI 2 O 3 . 
 
 
 
 
 
 
 
 
 
 Mixture of 
 
 
 
 
 
 
 
 
 
 CaMgSi 2 8 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 CaFeSi 2 O 6 . 
 
 
 
 
 
 
 
 
 
 (Tscher- 
 
 
 
 
 
 
 
 
 
 mak.) 
 
 
 
 
 
 
 
 
 <r. Sahlite. 
 
 Pale green 
 augite. 
 
 3-2-3-3- 
 
 Separa- 
 tion after 
 
 In grains and 
 long columns 
 
 ditto. 
 
 ditto. 
 
 duto. 
 
 ditto. 
 
 
 poor in Fe. 
 
 
 oP 
 together 
 with 
 
 with separation 
 at right angles 
 to the longest 
 
 
 
 
 
 
 
 
 cleavage 
 after </> 
 
 axis; generally 
 without 
 
 
 
 
 
 
 
 
 and 
 
 terminal planes. 
 
 
 
 
 
 
 
 
 00^00. 
 
 Cross-sections 
 
 
 
 
 
 
 
 
 
 resemble 
 
 
 
 
 
 
 
 
 
 augite. 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 171 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 light. 
 
 Pleo- 
 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 See 
 auyite. 
 
 Greenish 
 brown. 
 
 Very 
 feeble. 
 
 Occurs only 
 in large ir- 
 
 With 
 plagio- 
 
 As in 
 bronzite. 
 
 Formation 
 of uralite 
 
 Primary 
 
 constituent. 
 
 Often 
 resembles 
 
 
 
 
 regular 
 
 clase, 
 
 Inclosures 
 
 common, in 
 
 Common in 
 
 bronzite. 
 
 
 
 
 grains. A 
 great simi- 
 larity in 
 
 ordinary 
 augite, 
 olivine, 
 
 of brown 
 leaflets 
 of gothite 
 
 that 
 diallage 
 changes at 
 
 gabbro, 
 norite, 
 rare in 
 
 Easily dis- 
 tinguished 
 from it on 
 
 
 
 
 structure to 
 bronzite es- 
 pecially as 
 
 horn- 
 blende. 
 Rarely 
 
 parallel 
 
 00^*00, 
 
 otherwise 
 
 the ends 
 into dark 
 green, 
 
 porphyritic, 
 eruptive 
 rocks. 
 
 sections or 
 cleavage- 
 leaves 
 
 
 
 
 regards in- 
 
 with 
 
 poor in 
 
 strongly 
 
 In serpen- 
 
 || oo ^oo; 
 
 
 
 
 closures, 
 
 quartz. 
 
 inclosures. 
 
 pleo- 
 
 tine and 
 
 i. c. p. 1. 
 
 
 
 
 separating 
 into fibres, 
 
 
 
 chroitic, 
 hornblende 
 
 olivine-fels. 
 Rare in 
 
 appearance 
 of one optic- 
 
 
 
 
 and twin- 
 
 
 
 fibres. 
 
 crystalline 
 
 axis. 
 
 
 
 
 nings. 
 
 
 
 Into 
 
 schists. 
 
 
 
 
 
 Often inter- 
 
 
 
 viridite; 
 
 
 
 
 
 
 penetrated 
 
 
 
 into serpen- 
 
 
 
 
 
 
 with ordi- 
 
 
 
 tine with 
 
 
 
 
 
 
 nary augite, 
 
 
 
 formation 
 
 
 
 
 
 
 lornblende. 
 
 
 
 of chlorite 
 
 
 
 
 
 
 or mica. 
 
 
 
 and talc. 
 
 
 
 
 
 
 Rare in 
 
 
 
 
 
 
 
 
 
 crystals. 
 
 
 
 
 
 ( 
 
 ditto. 
 
 Grass- 
 green. 
 
 See 
 augite. 
 
 Only 
 known in 
 fresh grains 
 
 With 
 quartz, 
 horn- 
 
 Rare. Fluid 
 inclosures 
 and needles 
 
 
 In eclogites 
 and amphi- 
 bolites. 
 
 See augite. 
 They are 
 distinguish- 
 
 
 
 
 poor in in- 
 
 blende, 
 
 of rutile. 
 
 
 
 ed from : 
 
 
 
 
 closures: 
 
 garnet, 
 
 
 
 
 augite by 
 
 
 
 
 often inter- 
 
 zoisite, 
 
 
 
 
 the paler 
 
 
 
 
 penetrated 
 
 disthene. 
 
 
 
 
 color 
 
 
 
 
 with 
 
 rutile. 
 
 
 
 
 (small 
 
 
 
 
 inrnblende. 
 
 
 
 
 
 amount of 
 
 
 
 
 Often en- 
 
 
 
 
 
 Fe) and by 
 
 
 
 
 veloped by 
 
 
 
 
 
 the 
 
 
 
 
 surround- 
 
 
 
 
 
 crystalline 
 
 
 
 
 ing grains. 
 
 
 
 
 
 form; 
 
 ditto. 
 
 ditto. 
 
 ditto. 
 
 
 With 
 olivine, 
 
 Very rare. 
 Vitreous 
 
 
 As primary 
 constituent 
 
 diallage by 
 the lack of 
 
 
 
 
 
 chromite, 
 
 inclosures. 
 
 
 in olivine- 
 
 the perfect 
 
 
 
 
 
 diallage, 
 and the 
 
 
 
 fels (so- 
 called chro- 
 
 separation 
 after oo^oo. 
 
 
 
 
 
 rhombic 
 
 
 
 mium diop- 
 
 
 ' 
 
 
 
 
 augites. 
 
 
 
 side). 
 
 
 
 
 
 
 
 
 
 Rarely sec- 
 
 
 
 
 
 
 
 
 
 ondary as 
 
 
 
 
 
 
 
 
 
 metamor- 
 
 
 
 
 
 
 
 
 
 phic pro- 
 
 
 
 
 
 
 
 
 
 duct of 
 
 
 
 
 
 
 
 
 
 garnet. 
 (Pyrope.) 
 
 
 Verv 
 
 Pale 
 
 
 
 With 
 
 
 Rarely 
 
 In 
 
 
 brilliant. 
 
 green to 
 colorless. 
 
 
 
 quartz, 
 horn- 
 
 
 changed to 
 uralite. 
 
 crystalline 
 schists. 
 
 
 
 Relief 
 
 
 
 blende. 
 
 
 
 
 
 
 marked as 
 
 
 
 garnet, 
 
 
 
 
 
 
 a conse- 
 
 
 
 sea polite, 
 
 
 
 
 
 
 quence ol 
 
 
 
 plagio- 
 
 
 
 
 
 
 the 
 
 
 
 clase. 
 
 
 
 
 
 
 powerful 
 
 
 
 titanite. 
 
 
 
 
 
 
 refraction 
 
 
 
 
 
 
 
 
 
 of light. 
 
 
 
 
 
 
 
 
172 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 
 strength of 
 double- 
 
 Direction 
 of 
 extinction. 
 
 
 re ctions. 
 
 
 
 section. 
 
 
 
 refraction. 
 
 
 f. Acmite. 
 
 Na 2 Fe 2 
 Si 4 O 12 . 
 
 3-53- 
 3 55- 
 
 Eminent 
 
 00 />, 
 
 In grains or 
 columns 
 
 00^00 
 
 com- 
 
 S 
 A. P. NooPoo. 
 
 se augite. 
 Positive. 
 
 C ; c = very 
 
 
 
 
 87: 
 
 OOP. 00^00, 
 
 mon. 
 
 Large axial 
 
 
 small 
 
 
 
 
 imperfect 
 
 oojPoo, 
 
 
 angle. 
 
 
 angle 
 
 
 
 
 <X>f>03. 
 
 elongated 
 
 
 Sections 
 
 
 = 2-7*. 
 
 
 
 
 
 by pre- 
 
 
 or leaves 
 
 
 
 
 
 
 
 dominance 
 
 
 || oo jPco show 
 
 
 
 
 
 
 
 of faces 
 
 
 a distorted 
 
 
 
 
 
 
 
 oo Poo, 
 
 
 axial picture 
 
 
 
 
 
 
 
 
 
 of a biaxial 
 
 
 
 
 
 
 
 
 
 mineral. 
 
 
 
 g. Wollas- 
 tonite. 
 
 CaSi0 3 
 by HC1 
 perfectly 
 
 2.78- 
 2.91. 
 
 Parallel 
 
 00^00. oP, 
 
 and ^oo. 
 
 OOP =8 7 . 
 
 Only in 
 prisms, 
 
 ditto. 
 
 A.P. HoojPoo. 
 Apparent 
 axial angle 
 
 Positive, 
 strongly. 
 
 c forms 
 whjj oP 
 towards 
 
 
 decomposed 
 with. 
 
 
 
 irregular, 
 elongated, 
 
 
 = about 70. 
 (Compare 
 
 
 the front, 
 
 3 2 12'. 
 
 
 separation 
 of 
 
 
 
 fibrous. 
 following 
 
 
 Fig. ii.) 
 
 
 a : c = 12. 
 
 
 amorphous 
 SiO 2 . 
 
 
 
 the ^-ortho- 
 
 
 
 
 
 
 
 
 
 axis. 
 
 
 
 
 
 cc. PERFECT CLEAVAGE 
 
 2. Horn- 
 
 
 
 
 
 
 
 
 
 blende 
 
 
 
 
 
 
 
 
 
 Group. 
 a. Ordinary 
 and 
 
 basaltic 
 
 m RSiO 3 
 
 R a O 3 . 
 R=Ca, Mg, 
 
 3- 1 - 
 3-3- 
 
 Highly 
 eminent 
 /. 
 
 124 n'; 
 
 oo/>. ooiPoo, 
 co/'oo, and 
 oP.Porfoo 
 almost 
 
 ditto. 
 
 A.P. NooPoo. 
 The 
 i. M. = a 
 falls in the 
 
 Strongly 
 negative, 
 yet some- 
 what 
 
 c : c = about 
 *S* 
 
 Varies 
 from 2-18. 
 
 Horn- 
 blende. 
 
 Fe. 
 R 2 = Al a , 
 
 
 imperfect 
 
 00^00 
 
 always in 
 crystals. 
 
 
 obtuse 
 angle /3, 
 
 feebler 
 than 
 
 d:c = 75. 
 d:a = 
 
 
 Fe 2 . 
 Only those 
 rich in Fe 
 
 
 and 
 ooiPoo. 
 
 rarely in 
 grains. 
 Transverse 
 
 
 b = b 
 (see Fig. 9). 
 True axial 
 
 augite. 
 
 29 58'. 
 C : c = 
 13 15 
 
 
 partially 
 
 
 
 sections 
 
 
 angle about 
 
 
 * j 
 in green 
 
 
 attacked by 
 acids. 
 
 
 
 generally 
 hexagonal, 
 
 
 79, the posi- 
 tive axial 
 
 
 horn- 
 blendes, 
 
 
 
 
 
 also 
 
 
 angle becom- 
 
 
 - and = 
 
 
 
 
 
 octagonal, 
 
 
 ing larger 
 
 
 11-13 
 
 
 
 
 
 longitu- 
 
 
 with in- 
 
 
 and less in 
 
 
 
 
 
 dinal 
 
 
 creased per- 
 
 
 brown. 
 
 
 
 
 
 sections, as 
 
 
 centage of 
 
 
 
 
 
 
 
 in augite. 
 (Fig. 86.) 
 
 
 iron. 
 Parallel oP 
 
 
 
 
 
 
 
 
 
 and oo^oo 
 
 
 
 
 
 
 
 
 
 side appear- 
 
 
 
 
 
 
 
 
 
 ance of one 
 
 
 
 
 
 
 
 
 
 optic axis on 
 
 
 
 
 
 
 
 
 
 the circum- 
 
 
 
 
 
 
 
 
 
 ference of 
 
 
 
 
 
 
 
 
 
 field. Feeble 
 
 
 
 
 
 
 i 
 
 
 
 dispersion 
 
 
 
 
 
 
 
 
 
 P < v. 
 
 
 
 b. Smarag- 
 
 
 
 
 
 
 
 
 
 dite. 
 
 See 
 
 
 
 
 
 
 
 
 
 uralite. 
 
 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 173 
 
 
 Color 
 
 
 
 
 
 
 
 
 Polariza- 
 tion- 
 colors. 
 
 and 
 power of 
 refract- 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclos- 
 ures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 
 ing light. 
 
 
 
 
 
 
 
 
 See 
 
 augite. 
 
 Dark 
 brown, 
 dark 
 
 Rather 
 strong, 
 c dark 
 
 In large crys- 
 tals in the 
 syenites, often 
 
 With 
 elseolite, 
 sodalite, 
 
 Earthy 
 par- 
 ticles. 
 
 
 Not rare in 
 elceolite- 
 syenites, 
 
 
 
 green. 
 
 brown; 
 
 with fibrous 
 
 micro- 
 
 
 
 phonolites, 
 
 
 
 /3 above 
 
 a brown- 
 
 terminations. 
 
 el in e, 
 
 
 
 and 
 
 
 
 1.7. 
 
 ish 
 
 In minute crys- 
 
 and 
 
 
 
 trachytes. 
 
 
 
 
 green. 
 
 tals of yellow 
 
 biotite. 
 
 
 
 
 
 
 
 Absorp- 
 
 and dark green 
 
 
 
 
 
 
 
 
 tion 
 
 color in the 
 
 
 
 
 
 
 
 
 c> b > a. 
 
 trachytes and 
 
 
 
 
 
 
 
 
 
 phonolites. 
 
 
 
 
 
 
 Very 
 
 Color- 
 
 
 In aggregates 
 
 With 
 
 Fluid 
 
 
 As decom- 
 
 Resembles 
 
 bril- 
 
 less, 
 
 
 of fibrous 
 
 calcite, 
 
 inclos- 
 
 
 position- 
 
 tremolite, but 
 
 liant. 
 
 yellow- 
 ish 
 
 
 individuals in 
 tufts or radially 
 
 green 
 augite, 
 
 ures. 
 
 
 product or 
 contact-mine- 
 
 distinguish- 
 able by the 
 
 
 white. 
 
 
 disposed. 
 
 granite. 
 
 
 
 ral rare in 
 
 prismatic 
 
 
 Relief 
 
 
 
 
 
 
 granular 
 
 angle, solu- 
 
 
 marked 
 
 
 
 
 
 
 chalks meta- 
 
 bility in acids, 
 
 
 
 
 
 
 
 
 morphosed 
 from eruptive 
 
 and gelatin- 
 izing; difficult 
 
 
 
 
 
 
 
 
 rocks. Rjire 
 
 to distinguish 
 
 
 
 
 
 
 
 
 in elaeolite- 
 
 from zeolites 
 
 
 
 
 
 
 
 
 syenites and 
 
 as scolecite, 
 
 
 
 
 
 
 
 
 phonoliies. 
 
 e.g. 
 
 oo P = 124. 
 
 Less 
 bril- 
 
 Green 
 
 to 
 
 Generally 
 very 
 
 In large crys- 
 tals or grains 
 
 With 
 ortho- 
 
 Poor in 
 
 inclos- 
 
 Becomes 
 finely 
 
 Primary 
 essential con- 
 
 Easily dis- 
 tinguished 
 
 liant 
 
 brown 
 
 strong. 
 
 I. O. More 
 
 clase, 
 
 ures. 
 
 fibrous 
 
 stituent. In 
 
 from: 
 
 than in 
 
 
 a = 
 
 rarely in small 
 
 plagio- 
 
 Fluid 
 
 and 
 
 granular and 
 
 augite by the 
 
 augite; 
 
 1.62. 
 
 yellow- 
 
 crystals and 
 
 clase, 
 
 inclos- 
 
 bleached 
 
 porphyritic 
 
 prismatic 
 
 yellow 
 to 
 
 
 green or 
 honey- 
 
 microlites II. O. 
 The green 
 
 quartz, 
 biotite; 
 
 ures, 
 glass, 
 
 through 
 decom- 
 
 eruptive 
 r ocks : 
 
 cleavage- 
 angle, slight 
 
 green-- 
 
 
 yellow; 
 
 hornblendes are 
 
 more 
 
 gas- 
 
 position 
 
 syenite, dio- 
 
 inclination of 
 
 ish 
 
 
 
 often fibrous; 
 
 rarely 
 
 pores, 
 
 into 
 
 rite (green 
 
 C : <r, and 
 
 brown. 
 
 
 yellow- 
 
 the brown often 
 
 with 
 
 earthy 
 
 epidote, 
 
 hornblende), 
 
 powerful 
 
 
 
 brown; 
 c = 
 
 beautifully de- 
 veloped in 
 
 augite 
 and 
 
 par- 
 ticles, 
 
 calcite, 
 ferric hy- 
 
 porphyrite, 
 trachyte 
 
 pleochroism; 
 biotite on 
 
 
 
 black or 
 
 zones. The 
 
 olivine. 
 
 apatite 
 
 droxide, 
 
 (brown, more 
 
 sections at 
 
 
 
 greenish 
 brown. 
 
 brown horn- 
 blende of the 
 
 
 needles. 
 
 then 
 often sur- 
 
 rarely green, 
 hornblende). 
 
 right angles 
 to the vertical 
 
 
 
 Absorp- 
 tion 
 
 younger erup- 
 tive rocks often 
 
 
 
 rounded 
 by a 
 
 Accessory in 
 basalts(brown 
 
 axis. 
 In biotite 
 
 
 
 c > b> a. 
 
 shows a broad 
 
 
 
 wreath 
 
 H.), rare and 
 
 the cleavage 
 
 
 
 
 opaque margin 
 (see Fig. 44), or 
 
 
 
 of 
 mag- 
 
 in olivine-fels 
 (green H.). 
 
 and powerful 
 dichroism is 
 
 
 
 
 pseudomorphs 
 
 
 
 netite; 
 
 Common in 
 
 wanting in 
 
 
 
 
 of augite and 
 
 
 
 always as 
 
 crystalline 
 
 sucl) sec- 
 
 
 
 
 magnetite after 
 
 
 
 augite. 
 
 schists (green, 
 
 tions ( [I oP). 
 
 
 
 
 hornblende 
 
 
 
 Meta- 
 
 more rarely 
 
 
 
 
 
 occur. The 
 
 
 
 mor- 
 
 brown, H.). 
 
 
 
 
 
 green horn- 
 
 
 
 phoses 
 
 As essential 
 
 
 
 
 
 blendes are 
 
 
 
 into 
 
 constituent in 
 
 
 
 
 
 often inter- 
 
 
 
 biotite, 
 
 amphibolite, 
 
 
 
 
 
 penetrated wi'h 
 
 With 
 
 
 chlorite. 
 
 hornblendic 
 
 
 
 
 
 augite. 
 
 ompha- 
 
 
 
 schists.certam 
 
 
 
 
 
 
 cite. 
 
 
 
 gneisses, ec- 
 
 
 
 
 
 
 garnet, 
 
 
 
 logite (so- 
 
 
 
 
 
 
 zoisite. 
 
 
 
 called smar- 
 
 
 
 
 
 
 rutile. 
 
 
 
 aeditei. 
 
 
174 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form of 
 the cross- 
 section. 
 
 Twins. 
 
 Optical 
 orienta- 
 tion. 
 
 Character 
 and 
 strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 f-y c. Actinolite 
 
 CaMg 8 Si 4 
 12 + Ca 
 Fe 3 Si 4 O ia 
 
 3.026- 
 3.160 
 
 As above; 
 separation 
 at right 
 
 Long prisms, 
 generally 
 without 
 
 Rare. 
 
 See Hor 
 
 nblendc. 
 
 l:c 
 
 generally 
 
 
 Al-free 
 
 
 angles to 
 
 terminations 
 
 
 
 
 
 
 Fe-poor 
 
 
 the c-axis. 
 
 oo/\cojPoo. 
 
 
 
 
 
 
 (Tschermak). 
 
 
 
 
 
 
 
 
 
 RSi0 8 
 
 
 
 
 
 
 
 
 and 
 
 R = predomi- 
 
 
 
 
 
 
 
 
 
 nating Mg, 
 
 
 
 
 
 
 
 
 
 less Ca, and a 
 
 
 
 
 
 
 
 
 
 little Fe. 
 
 
 
 
 
 
 
 
 d. Tremo- 
 lite. 
 
 3 MgSiO 3 
 -f CaSi0 8 . 
 MgO pre- 
 
 2-93-3- 
 
 00 P 
 
 like 
 hornblende. 
 
 oo P. oofoo 
 generally in 
 long narrow 
 
 Rare. 
 Like 
 horn- 
 
 See hor 
 
 iblende. 
 
 C:r = i 5 . 
 
 
 dominating. 
 
 
 Separation 
 
 prisms. 
 
 blende. 
 
 
 
 
 
 Unattached 
 
 
 at right 
 
 
 
 
 
 
 
 by acids. 
 
 
 angles to 
 
 
 
 
 
 
 
 
 
 the <r-axis. 
 
 
 
 
 
 
 e. Arfved- 
 sonile, 
 
 Na 2 (Fe) 2 
 
 Insoluble in 
 acids. 
 
 3-33- 
 3-59- 
 
 oo /> like 
 hornblende. 
 
 In large 
 grains. 
 
 
 Se 
 
 2 hornblen 
 
 de. 
 
 f. Glauco- 
 phane 
 
 Na 2 (Al) a 
 Si 4 12 . 
 
 3-1- 
 
 Like 
 hornblende. 
 
 Elongated 
 prisms, 
 
 
 See hor 
 
 iblende. 
 
 c : c - 6 
 
 (Gastal- 
 dite). 
 
 Contains 
 Ca, Mg, Fe. 
 
 
 Separation 
 at right 
 
 generally 
 without 
 
 
 
 
 
 
 Nearly 
 
 
 angles to 
 
 terminal 
 
 
 
 
 
 
 unattacked 
 
 
 the c-axis 
 
 planes. 
 
 
 
 
 
 
 by acids. 
 
 
 
 
 
 
 
 
 g. Uralite 
 (Sm* rag- 
 el ite in part). 
 
 Like 
 ordinary 
 green 
 hornblendes. 
 
 3-I-3-3- 
 
 Like 
 hornblende; 
 often, 
 however, 
 
 See 
 " Structure;" 
 single fibres 
 show oo P 
 
 
 Se 
 
 j hornblen 
 
 de. 
 
 
 
 
 showing 
 
 = about 124. 
 
 
 
 
 
 
 
 
 in addition 
 
 Part in the 
 
 
 
 
 
 
 
 
 theaugite- 
 cleavage 
 
 form of 
 augite or in 
 
 
 
 
 
 
 
 
 quite 
 
 irregular 
 
 
 
 
 
 
 
 
 perfectly. 
 
 large grains. 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 1/5 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclos- 
 ures. 
 
 Decompo- 
 sition. 
 
 Occurrence. 
 
 Remarks. 
 
 See 
 horn- 
 blende. 
 
 Light to 
 dark 
 green. 
 
 cdark 
 green, a 
 yellowish 
 
 Generally 
 occurring in 
 long narrow 
 
 With 
 quartz, 
 mica, 
 
 Very 
 pour. 
 
 Often 
 perlect 
 pseudo- 
 
 Rather 
 common in 
 certain non- 
 
 Dis- 
 tinguished 
 from 
 
 
 
 green, 
 feebler 
 than in 
 
 needles or 
 grains, often 
 fibrous at the 
 
 chlorite, 
 rutile. 
 
 
 morphs of 
 biotite, 
 chlorite, 
 
 feldspathic 
 crystalline 
 schists. 
 
 ordinary 
 green horn- 
 blende by 
 
 
 
 the horn- 
 
 termination. 
 
 
 
 and ferric 
 
 in talcose, 
 
 chemical 
 
 
 
 blendes, 
 
 
 
 
 hydrox- 
 
 mica- 
 
 means; 
 
 
 
 generally 
 
 
 
 
 ide after 
 
 chloritic 
 
 actinolite 
 
 
 
 only in 
 
 
 
 
 actinolite 
 
 schists, in 
 
 always oc- 
 
 
 
 green 
 
 
 
 
 are 
 
 serpentines. 
 
 curs in long 
 
 
 
 tints. 
 
 
 
 
 observed. 
 
 
 columns, 
 
 
 
 c > b > a. 
 
 
 
 
 
 
 not like 
 
 
 
 
 
 
 
 
 
 hornblende 
 
 
 
 
 
 
 
 
 
 in short 
 
 
 
 
 
 
 
 
 
 crystals. 
 
 Very 
 brilliant. 
 
 Colorless, 
 relief 
 
 
 In long 
 columns, the 
 
 With 
 calcite; 
 
 Very 
 poor. 
 
 Into 
 calcite 
 
 As contact- 
 mineral in 
 
 Compare 
 wollaston- 
 
 
 marked. 
 
 
 termination 
 
 with 
 
 
 and talc. 
 
 limestones; 
 
 ite. 
 
 
 
 
 often in sheaf- 
 
 olivine, 
 
 
 
 as primary 
 
 
 
 
 
 like fibres; in 
 
 horn- 
 
 
 
 constituent 
 
 
 
 
 
 tufted 
 
 blende, 
 
 
 
 (also rarely 
 
 
 
 
 
 aggregates, 
 
 diallage. 
 
 
 
 secondary) 
 
 
 
 
 
 rarely in grains. 
 
 
 
 
 in crystal- 
 
 
 
 
 
 
 
 
 
 line schists 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 serpentines. 
 
 
 See 
 
 Blue- 
 
 Very 
 
 In irregular, 
 
 With or- 
 
 
 
 Rarely in 
 
 Dis- 
 
 horn- 
 blende. 
 
 green. 
 
 strong. 
 
 often fibrous 
 grains and long 
 
 thoclase, 
 micro- 
 
 
 
 elseolite 
 rocks. 
 
 tinguished 
 from horn- 
 
 
 
 
 columnar 
 
 cline,elae- 
 
 
 
 
 blende by 
 
 
 
 
 individuals. 
 
 olite, 
 
 
 
 
 chemical 
 
 
 
 
 
 sodalite. 
 
 
 
 
 composition 
 
 
 
 
 
 
 
 
 
 and color. 
 
 See 
 horn 
 blende. 
 
 Indigo-, 
 lavender- 
 blue. 
 
 Very 
 strong. 
 a= white, 
 
 Mostly in long 
 fibrous needles, 
 often 
 
 With 
 quartz, 
 horn- 
 
 Rutile 
 needles 
 and gas- 
 
 
 Rare in 
 crystalline 
 schists, 
 
 
 
 
 b= violet- 
 
 interpenetrated 
 
 blende, 
 
 pores 
 
 
 eclogites, 
 
 
 
 
 blue, 
 c = dark 
 
 with green 
 hornblende. 
 
 garnet, 
 zoisite, 
 
 are 
 com- 
 
 
 amphibo- 
 lites, mica 
 
 
 
 
 blue. 
 
 
 chlorite, 
 
 mon. 
 
 
 and 
 
 
 
 
 Absorp- 
 
 
 ompha- 
 
 
 
 chlorite 
 
 
 
 
 tion 
 
 
 cite, 
 
 
 
 schists. 
 
 
 
 
 c > b > a. 
 
 
 rutile, 
 
 
 
 
 
 
 
 
 
 titanite. 
 
 
 
 
 
 Generally 
 aggregate 
 
 Dark to 
 light 
 
 Partly 
 strong, 
 
 Finely-fibrous 
 decomposition- 
 
 With 
 plagio- 
 
 
 
 In gabbros 
 and 
 
 Compare 
 with dial- 
 
 polariza- 
 tion, as 
 
 green. 
 
 partly 
 weak. 
 
 product of 
 augite and 
 
 clase, 
 olivine, 
 
 
 
 serpentines; 
 in augitic 
 
 lage and 
 ordinary 
 
 the 
 
 
 
 diallagt, often 
 
 diallage, 
 
 
 
 porphyries. 
 
 hornblende. 
 
 separate 
 horn- 
 
 
 
 of the form of 
 augite and with 
 
 augite. 
 
 
 * 
 
 
 An ordinary 
 green 
 
 blende 
 
 
 
 remnants of the 
 
 
 
 
 
 hornblende 
 
 threads 
 
 
 
 augite or dial- 
 
 
 
 
 
 occurring 
 
 have not 
 the same 
 
 
 
 lage yet fresh. 
 The fibres show 
 
 
 
 
 
 in eclogite 
 was also 
 
 optical 
 
 
 
 the prismatic 
 
 
 
 
 
 called 
 
 orienta- 
 
 
 
 angle of horn- 
 
 
 
 
 
 stnaragdite* 
 
 tion. 
 
 
 
 blende. (See 
 
 
 
 
 
 
 
 
 
 Fig. 85.) 
 
 
 
 
 
 
1 7 6 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 dd. CLEAVAGE 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Sueciflc 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the 
 cross-section 
 
 Twins. 
 
 Character 
 Optical -uSgLj, 
 orientation. O f jouble- 
 refraction. 
 
 Direc- 
 tion of 
 extinc- 
 tion. 
 
 Epidote. 
 
 H^JR* 
 
 3 32-3-5- 
 
 Highly 
 eminent 
 
 Generally very 
 small prisms. 
 
 Rare 
 micro- 
 
 A.P. || ooPoo 
 at right 
 
 Strongly 
 negative. 
 
 : c = 
 
 2 io' 
 
 
 (R 2 ) = (AI 2 ) 
 
 Slightly 
 attacked by 
 HC1. 
 
 
 \\oP, 
 and per- 
 fect 00 POO 
 
 forming 
 an angle 
 
 elongated in 
 the direction of 
 ortho-diagonal 
 axis, the com- 
 bination oo P. 
 
 scopi- 
 cally. 
 Twinning 
 plane 
 
 angles to 
 the elonga- 
 tion of the 
 crystal, 
 b = b, i. M. 
 
 
 C: a = 
 27 47' 
 -f.oP. 
 
 
 
 
 Of II 5 24 / . 
 
 oP. Poo . oopco 
 predominating. 
 (See Fig. 89.) 
 
 (See Figs. 
 26 and 88.) 
 
 = nearly 
 coinciding 
 with c. 
 
 
 
 
 
 
 
 The longitudi- 
 
 
 
 
 
 
 
 
 
 nal sections 
 
 
 Sections || 
 
 
 
 
 
 
 
 parallel oojpco 
 
 
 oo PCD show 
 
 
 
 
 
 
 
 are hexagonal. 
 
 
 a biaxial 
 
 
 
 
 
 
 
 The transverse 
 
 
 interfer- 
 
 
 
 
 
 
 
 sections at right 
 angles to c and 
 
 
 ence-figure, 
 as the 2. M. 
 
 
 
 
 
 
 
 sections parallel 
 oP : oo poo are 
 
 
 is at nearly 
 right 
 
 
 
 
 
 
 
 long and nar- 
 
 
 angles. 
 
 
 
 
 
 
 
 row, rectangu- 
 lar or 
 
 
 (See Fig. 
 8? ) 
 
 
 
 
 
 
 
 hexagonal, 
 
 
 Q-J.) 
 
 
 
 
 
 
 
 with one pair 
 
 
 
 
 
 
 
 
 
 of sides longer ; 
 
 
 
 
 
 
 
 
 
 in grains. 
 
 
 
 
 
 ee. CLEAVAGE IMPERFECT 
 
 Titanite. 
 
 CaSiTiO 6 ; 
 contains 
 FeO. 
 Decom- 
 posed by 
 HnS0 4 ; 
 Ti0 2 
 dissolved 
 with forma- 
 
 3-4-3 6. 
 
 00 P 
 
 133 52', 
 
 Poo 
 
 113 150', 
 imperfect. 
 
 Mostly crystals: 
 cc P. oP.UPw; 
 i^Poo or %P t 
 prominent with 
 
 Poo, or in acute 
 'wedge-sh aped 
 grains. Such 
 are character- 
 
 Rather 
 common; 
 contact- 
 or pene- 
 tration- 
 twins; 
 twmning- 
 plane 
 = oP. 
 
 A.P. || ooPoo 
 i. M. = c at 
 nearly right 
 angles to 
 
 very strong 
 dispersion 
 of the axes. 
 
 P > 7'. 
 
 Strongly 
 positive. 
 
 0: c = 
 a 9 a' 7 ' 
 
 2ir 
 
 
 tion of 
 
 
 
 istic crystal 
 
 (See Fig. 
 
 (See Fig. 
 
 
 
 
 gypsum. 
 
 
 
 cross-sfctions. 
 
 27.) 
 
 13.) 
 
 
 
 
 
 
 
 (See Fig. 90.) 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 177 
 
 115' 
 
 Polari- 
 zation- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chivism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclos- 
 ures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Very 
 
 bril- 
 
 Lemon- 
 yellow. 
 
 Rather 
 Powerful 
 
 Generally 
 in long 
 
 With 
 quartz, 
 
 Very 
 poor. 
 
 
 Secondary min- 
 eral. Common 
 
 Similar to 
 augite, distin- 
 
 liant, 
 yellow 
 to red. 
 
 yellowish 
 green 
 Relief 
 very 
 
 in the 
 thicker 
 prisms. 
 o = very 
 
 minute 
 prisms, 
 lying in 
 chloritic 
 
 ortho- 
 clase, 
 plagio- 
 clase, 
 
 Fluid 
 inclos- 
 ures. 
 
 
 as decomposi- 
 tion product of 
 the feldspars, 
 hornblende, 
 
 guished from it 
 by the even 
 parallel extinc- 
 tion in sections 
 
 
 marked. 
 
 pale 
 
 matter, 
 
 horn- 
 
 
 
 biotite, more 
 
 parallel to the 
 
 
 /3 = 1.72- 
 
 yellow, 
 
 or in 
 
 blende, 
 
 
 
 rarely of augite, 
 
 longest 
 
 
 1-75- 
 
 b= brown 
 
 pseudo- 
 
 biotite, 
 
 
 
 in eruptive 
 
 development 
 
 
 
 to 
 yellowish 
 green, 
 C = green 
 to lemon- 
 yellow. 
 Absorp- 
 tion 
 6 > C > ft. 
 
 morphs, 
 more rarely 
 in grains. 
 
 augite 
 with 
 chlorite. 
 
 
 
 rocks and crys- 
 talline schists 
 bearing these 
 minerals, also 
 often as 
 primary con- 
 stituent in the 
 latter. 
 
 (= <5-axis) and 
 slight inclina- 
 tion of a : c. 
 The yellow 
 color, powerful 
 refraction of 
 light, and 
 brilliant polari- 
 
 
 
 
 
 
 
 
 
 zation-colors 
 
 
 
 
 
 
 
 
 
 are character- 
 
 
 
 
 
 
 
 
 ' . ' 
 
 istic forepidote. 
 
 poo; ACUTE WEDGE-SHAPED CROSS-SECTIONS. 
 
 Feeble, 
 gray 
 
 Pale 
 
 yellow, 
 reddish 
 
 Rather 
 strong in 
 the darker 
 
 Rough sur- 
 face of sec- 
 tion is 
 
 With 
 ortho- 
 clase, 
 
 Very 
 poor. 
 
 Rarely 
 aseudo- 
 morphs 
 
 As primary 
 accessory 
 constituent in 
 
 Easily 
 recognizable 
 by the almost 
 
 original 
 color; 
 
 brown 
 
 colored 
 varieties. 
 
 characters 
 tic for 
 
 plagio- 
 clase, 
 
 
 of 
 calcite 
 
 eruptive rocks. 
 Granite 
 
 constant wedge- 
 shaped cross- 
 
 much 
 
 colorless. 
 
 a = red- 
 
 titanite. 
 
 horn- 
 
 
 after 
 
 (rarely), 
 
 sections, 
 
 weaker 
 
 
 dish 
 
 Commonly 
 
 blende, 
 
 
 titanite. 
 
 syenite, 
 
 powerful refrac- 
 
 than 
 
 
 brown. 
 
 associated 
 
 augite, 
 
 
 
 phonolite, 
 
 tion of light, 
 
 augite 
 and 
 
 
 C = green- 
 ish 
 
 and inter- 
 penetrated 
 
 biotite, 
 chlorite, 
 
 
 
 leucitophyr, 
 elaeolite 
 
 and rough 
 surface. 
 
 horn- 
 
 
 yellow. 
 
 with augite 
 
 quartz, 
 
 
 
 syenite, 
 
 
 blende. 
 
 
 c> b> a. 
 
 and horn- 
 
 and other 
 
 
 
 trachyte, mica- 
 
 
 /3p = 
 
 
 Weaker 
 
 blende. 
 
 accessory 
 
 
 
 andhornblende- 
 
 
 1.005. 
 
 
 than in 
 
 One of the 
 
 minerals. 
 
 
 
 andesite, 
 
 
 Relief 
 
 
 the horn- 
 
 minerals 
 
 
 
 
 diorite and 
 
 
 very 
 marked. 
 
 
 blendes. 
 
 first formed 
 in the 
 erupiive 
 
 
 
 
 in crystalline, 
 especially 
 hornblendic 
 
 
 
 
 
 rocks. 
 
 
 
 
 schists. 
 
 
 
 
 
 
 
 
 
 Secondary 
 
 
 
 
 
 
 
 
 
 decomposition- 
 
 
 
 
 
 
 
 
 
 product of 
 
 
 
 
 
 
 
 
 
 ilmenite and 
 
 
 
 
 
 
 
 
 
 titaniferous 
 
 
 
 
 
 
 
 
 
 magnetite. 
 
 
178 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combinations 
 and form 
 of the cross- 
 section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Charactei 
 and 
 stivntrth 
 of double- 
 refraction. 
 
 Direc- 
 tion of 
 extinc- 
 tion. 
 
 Gypsum. 
 
 2H 3 6. 
 Difficultly 
 soluble in 
 
 2.2-2.4. 
 
 Highly 
 eminent 
 clino- 
 diagonal, 
 
 In granules 
 or elongated 
 prismatic 
 indii'idu- 
 
 Very rare 
 in micro- 
 scopic 
 individuals. 
 
 A. P. UoojPco 
 
 i. M. = o. 
 One optic 
 axis 
 
 Strongly 
 negative. 
 
 a : c = 
 52 3'. 
 C: c 
 37 3'- 
 
 
 acids. 
 
 
 perfect als, crystals 
 
 
 nearly 
 
 
 
 
 
 
 according 
 to -P. 
 
 00/>.OOJPOO. 
 
 -P. 
 
 
 One forms 
 
 
 
 
 
 
 
 
 
 83 with c, 
 
 
 
 
 
 
 
 
 
 the other 
 
 
 
 
 
 
 
 
 
 22. 
 
 
 
 II. b. 3. Minerals Crystallizing 
 a. LONG COLUMNAR CRYSTALS, COLORLESS OR OF A BLUE COLOR. 
 
 Disthene. 
 
 (Cyanite.) 
 
 Al a SiO 5 
 Acids have 
 no action. 
 
 3.48-3-68. 
 
 Highly 
 
 Grains, or 
 
 Common; 
 
 A. P. forms 
 with the 
 edge 
 oo/>oo : P 
 
 Rather 
 strongly 
 negative. 
 
 In 
 sections 
 parallel 
 
 00/>00 
 
 II oo Ao, 
 
 perfect 
 
 prisms, 
 co Poo 
 
 on micro- 
 scopic 
 
 
 
 
 *ooA, 
 
 predomi- 
 
 individuals. 
 
 an angle of 
 
 
 C: c = 
 
 
 
 
 and 
 parallel 
 oP. 
 (Gleit- 
 
 nating, 
 oo^oo with 
 an anule <>f 
 106 15'. 
 
 Twinning 
 plane 
 either: 
 i. oof co 
 
 30; with 
 oo /oo : P 
 an angle 
 60 15', and 
 
 
 30. 
 
 
 
 
 flache.*) 
 
 rarely with 
 
 repeated ; 
 
 like the 
 
 
 
 
 
 
 
 terminal 
 
 2. At right 
 
 i. M. = a 
 
 
 
 
 
 
 
 planes. 
 Transverse 
 
 angles to 
 the r-uxis; 
 
 is at right 
 angl.es 
 
 
 
 
 
 
 
 sections 
 
 3. At right 
 
 to ooPco. 
 
 
 
 
 
 
 
 rectangular 
 
 angles to 
 
 (See Fig. 
 
 
 
 
 
 
 
 or 
 hexagonal 
 if oo'/>or 
 
 oo /"is 
 
 added to 
 the above 
 combina- 
 
 the ^-axis; 
 4. Parallel 
 oP, caused 
 by pressure, 
 and re- 
 peated. 
 
 16.) Large 
 axial angle, 
 about 80. 
 Feeble dis- 
 persion of 
 the axes, 
 i> < p. 
 
 
 
 
 
 
 
 tion. 
 
 
 In sections 
 
 
 
 
 
 
 
 
 
 parallel 
 
 
 
 
 
 
 
 
 
 oo Poo a bi- 
 
 
 
 
 
 
 
 
 
 axial inter- 
 
 
 
 
 
 
 
 
 
 ference- 
 
 
 
 
 
 
 
 
 
 figure with 
 
 
 
 
 
 
 
 
 
 negative 
 
 
 
 
 
 
 
 
 
 middle line 
 
 
 
 
 
 
 
 
 
 is visible. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 179 
 
 Polari- 
 zation- 
 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 luclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 
 
 
 
 
 
 
 
 
 Very 
 
 bril- 
 liant. 
 
 Colorless, 
 secondary, 
 often 
 
 
 In 
 minute 
 granules, 
 
 Rarely 
 with clastic 
 constitu- 
 
 Fluid 
 inclos- 
 ures. 
 
 
 As simple 
 rock, granu- 
 lar or 
 
 
 I rides 
 
 colored by 
 
 
 and tangled 
 
 ents as 
 
 
 
 compact. 
 
 
 cent. 
 
 iron 
 
 
 or parallel quartz gran- 
 
 
 
 
 
 
 compounds. 
 
 
 fibrous 
 
 ules or 
 
 
 
 
 
 
 
 
 aggregates 
 of needles. 
 
 mica 
 leaflets. 
 
 
 
 
 
 
 
 
 Rarely in 
 
 
 
 
 
 
 
 
 
 crystals. 
 
 
 
 
 
 
 in the Triclinic System. 
 
 OR GRAINS. CLEAVAGE <x>Pa* . <x>Pv> AND oP. 
 
 Exceed- 
 
 Colorless, 
 
 If blue 
 
 In long 
 
 With 
 
 Very 
 
 Rare. 
 
 Rare. 
 
 If 
 
 ingly 
 bril- 
 liant. 
 
 azure-blue, 
 often 
 spotted. 
 /3p = 1.72. 
 Relief 
 
 rather 
 strong- 
 ly 
 pleo- 
 chro- 
 
 prisms or 
 irregular 
 grains, 
 traversed 
 by number 
 
 quartz, 
 mica, 
 garnet, 
 omphacite, 
 hornblende. 
 
 poor; 
 fluid 
 inclos- 
 ures. 
 
 Surrounded 
 by a 
 marginal 
 zone of a 
 brownish, 
 
 Primary 
 accessory 
 constituent 
 in 
 crystalline 
 
 colorless, 
 it is often 
 difficult to 
 distinguish 
 from 
 
 
 marked. 
 
 itic, 
 
 less fissures 
 
 rarely with 
 
 
 finely 
 
 schists. 
 
 sillimanite, 
 
 
 
 es- 
 pecially 
 parallel 
 
 00^00. 
 
 parallel or 
 at right 
 angles to 
 the chief 
 
 orthoclase. 
 
 
 fibrous, 
 felt-like 
 decomposi- 
 tion- 
 
 granuhte, 
 eclogite, 
 and 
 especially 
 
 with which 
 it 
 commonly 
 occurs; 
 
 
 
 c = 
 
 axis, often 
 
 
 
 product. 
 
 in 
 
 only possi- 
 
 
 
 blue. 
 
 = 
 
 irregularly 
 or com- 
 
 
 
 
 many 
 micaceous 
 
 ble by the 
 determina- 
 
 
 
 white. 
 
 pletely 
 
 
 
 
 schists. 
 
 tion of the 
 
 .." 
 
 
 
 colored 
 
 
 
 
 
 position of 
 
 
 
 
 blue. 
 
 
 
 
 
 the axes of 
 
 
 
 
 Rarely in 
 
 
 
 
 
 elasticity. 
 
 
 * 
 
 . 
 
 of thin 
 
 
 
 
 
 
 
 
 
 needles or 
 
 
 
 
 
 
 
 
 
 filaments; 
 
 
 
 
 
 
 
 
 
 the needles 
 
 
 
 
 
 
 
 
 
 cracked and 
 
 
 
 
 
 
 
 
 
 broken at 
 
 
 
 
 
 
 
 
 
 right angles 
 
 
 
 
 
 
 
 
 
 to the 
 
 
 
 
 
 
 
 
 
 chief axis. 
 
 
 
 
 
 
 
1 8o 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 ft. BROAD TABULAR CRYSTALS OR GRAINS, 
 
 NAME, 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 
 strength of 
 double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 Triclinic 
 
 
 
 
 
 
 
 
 
 Feldspars. 
 
 
 
 
 
 
 
 
 
 1. Potassium 
 
 
 
 
 
 
 
 
 
 Feldspar. 
 Microcline. 
 (Microper- 
 thite, 
 so-called 
 
 See ortho- 
 clase. 
 
 2.54- 
 2-57 
 
 (2.56). 
 
 Highly 
 eminent 
 \\oP. 
 Eminent 
 
 Very 
 similar to 
 orthoclase, 
 
 00/00.^. 
 
 Rare. 
 Countless 
 thin lamel- 
 la of ortho- 
 
 A. P. at 
 right angles 
 \ooP\ 
 its cross- 
 
 Rather 
 strongly 
 negative. 
 In leaflets 
 
 c with the 
 normal to 
 
 00/00 = 
 
 15 26'; 
 
 fibrous 
 orthoclase.) 
 
 
 
 parallel 
 
 00/00 
 
 oo'/>, 
 
 oo'/>. oo/ 
 
 predomi- 
 nating. 
 
 clase are 
 developed 
 parallel to 
 
 section with 
 
 00/00 
 
 forms with 
 
 parallel 
 
 oo/oo; 
 
 positive 
 
 a cleav- 
 age- 
 leaflet 
 
 
 
 
 CO/". 
 
 
 oo/oo and 
 
 the obtuse 
 
 double- 
 
 parallel 
 
 
 
 
 
 
 at right 
 
 edge 
 
 refrac- 
 
 oP 
 
 
 
 
 
 
 angles to 
 
 oP . oo /oo 
 
 tion. 
 
 does not 
 
 
 
 
 
 
 it, so that 
 
 5-6 in 
 
 
 therefore 
 
 
 
 
 
 
 in sections 
 
 the obtuse 
 
 
 extin- 
 
 
 
 
 
 
 parallel oP 
 a latticed 
 
 angle dc. 
 
 
 guish 
 Parallel 
 
 
 
 
 
 
 inter- 
 
 Cleavage- 
 
 
 like 
 
 
 
 
 
 
 penetration 
 
 leaflets 
 
 
 ortho- 
 
 
 
 
 
 
 of two 
 
 parallel 
 
 
 clase^ 
 
 
 
 
 
 
 systems of 
 
 00/00 
 
 
 3ut gives 
 
 
 
 
 
 
 striations 
 
 show one 
 
 
 an ex- 
 
 
 
 
 
 
 is visible, 
 
 of the optic 
 
 
 tinction 
 
 
 
 
 
 
 which is 
 
 axes more 
 
 
 to the 
 
 
 
 
 
 
 exceedingly 
 character- 
 
 clearly; the 
 axial plane 
 
 
 edge 
 oP: oo/oo 
 
 
 
 
 
 
 istic for 
 
 is some- 
 
 
 EB 
 
 
 
 
 
 
 microcline. 
 
 what in- 
 
 
 + I5-I6 ; 
 
 
 
 
 
 
 Besides, 
 
 clined to 
 
 
 parallel 
 
 
 
 
 
 
 lenticular 
 
 the plane 
 
 
 00/00 
 
 
 
 
 
 
 amellae and 
 
 00/00. 
 
 
 
 
 
 
 
 
 
 irregular 
 
 
 
 + 4-5. 
 
 
 
 
 
 
 lines of 
 
 
 
 
 
 
 
 
 
 polysyn- 
 
 
 
 
 
 
 
 
 
 thetic 
 
 
 
 
 
 
 
 
 
 twinned 
 
 
 
 
 
 - '. 
 
 
 
 
 albite are 
 
 
 
 
 
 
 
 
 
 so inter- 
 
 
 
 
 
 
 
 
 
 penetrated 
 
 
 
 
 
 
 
 
 
 that the 
 
 
 
 
 
 
 
 
 0^-planes 
 
 
 
 
 
 \ 
 
 
 
 of both 
 
 
 
 
 
 
 
 
 
 species of 
 
 
 
 
 
 
 
 
 
 plagioclase 
 
 
 
 
 
 
 
 
 
 fall in one 
 
 
 
 
 
 
 
 
 
 plane. 
 
 
 
 
 
 
 
 
 
 (See Figs. 
 
 
 
 
 
 
 
 
 
 9I-93-) 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 181 
 
 COLORLESS. CLEAVAGE PARALLEL cP AND oooo. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decompo- 
 sition. 
 
 Occurrence. 
 
 Remarks. 
 
 Exceed- 
 ingly 
 bril- 
 liant. 
 
 Colorless. 
 Relief 
 not so 
 marked 
 
 
 In rocks only 
 in grains ; 
 commonly 
 interpene- 
 
 a. With 
 orthoclase, 
 elseolite, 
 sodalite, 
 
 Generally 
 very 
 poor; 
 of mine- 
 
 Fibrous 
 decom- 
 position 
 with 
 
 As 
 primary 
 essential 
 constituent 
 
 Distinguished 
 from: 
 orthoclase 
 by the 
 
 
 as in 
 ortho- 
 clase. 
 
 
 trated with 
 quartz, like 
 graphic- 
 
 augite, 
 and 
 hornblende. 
 
 rals; 
 horn- 
 blende, 
 
 opacity 
 as in 
 ortho- 
 
 with 
 orthoclase 
 in: 
 
 oblique 
 extinction on 
 oP^ and the 
 
 
 
 
 granite, 
 also with 
 
 b. With 
 
 biotite, 
 zircon, 
 
 clase. 
 
 a. Elseolite- 
 syenite; 
 
 interpene- 
 tration 
 
 
 
 
 sodalite and 
 elaeolite. 
 
 quartz, 
 orthoclase, 
 
 apatite. 
 
 
 b. In 
 
 of twins; 
 the other 
 
 
 
 
 Compare the 
 twinning 
 development. 
 An ortho- 
 
 biotite, 
 hornblende, 
 muscovite. 
 
 
 
 different 
 granites, 
 especiaHy 
 graphic- 
 
 triclinic 
 feldspars 
 by the 
 latticed 
 
 
 
 
 clase or 
 
 c. With 
 
 
 
 granite; 
 
 structure 
 
 
 
 
 feldspar 
 
 these and 
 
 
 
 and 
 
 (interpene- 
 
 
 
 
 correspond- 
 
 garnet, 
 
 
 
 
 tration of 
 
 
 
 
 ing to 
 
 cyanite. 
 
 
 
 c. In 
 
 twins) 
 
 
 
 
 microcline 
 was called 
 
 
 
 
 crystalline 
 schists (as 
 
 parallel oP 
 and optical 
 
 
 
 
 microper- 
 
 
 
 
 micro- 
 
 properties. 
 
 
 
 
 thite; 
 
 
 
 
 perthite, 
 
 
 
 
 
 this contains 
 
 
 
 
 also 
 
 
 
 
 
 countless 
 
 
 
 
 called 
 
 
 
 
 
 exceedingly 
 thin lamellae 
 
 
 
 
 fibrous 
 orthoclase), 
 
 
 
 
 
 of a triclinic 
 
 
 
 
 especially 
 
 
 
 
 
 feldspar 
 
 
 
 
 in 
 
 
 
 
 
 closely 
 
 
 
 
 granulite 
 
 
 i* 
 
 
 
 related to 
 
 
 
 
 and 
 
 
 
 
 
 albite, which 
 
 
 
 
 gneisses. 
 
 
 
 
 
 can be 
 
 
 
 
 
 
 
 ,' 
 
 
 especially 
 
 
 
 
 
 
 
 
 
 well observed 
 
 
 
 
 
 
 
 
 
 in sections 
 
 
 
 
 
 
 
 
 
 parallel 
 
 
 
 
 
 
 
 
 
 oo ./^oo or 
 
 
 
 
 
 
 
 
 
 oojToo, as 
 
 
 
 
 
 
 
 
 
 rpindle- 
 
 
 
 
 
 
 
 
 
 shaped cross- 
 
 
 
 
 
 
 
 
 
 sections. 
 
 
 
 
 
 
 
 
 
 (See Fig. 93 ) 
 
 
 
 
 
 
182 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composi- 
 tion and 
 reactions. 
 
 Specific 
 
 gravity. 
 
 Cleav- 
 age. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 strength of 
 double- 
 refraction. 
 
 Direc- 
 tion of 
 l extinc- 
 tion. 
 
 2. Plagio- 
 
 
 
 
 
 
 
 
 
 clase, 
 
 
 
 
 
 
 
 
 
 Calcium- 
 
 
 
 
 
 
 
 
 
 Sodium 
 
 
 
 
 
 
 
 
 
 Feld- 
 
 
 
 
 
 
 
 
 
 spars : 
 
 
 
 
 
 
 
 
 
 a. Albite 
 
 Ab. 
 
 Na 2 Al 2 
 Si a O ]6 
 
 2.6 1- 
 
 2.63 
 (2.62). 
 
 Emi- 
 nent 
 paralle 
 
 co Pco . oP. 
 
 00 '/>/. 
 
 P,.Pt 
 
 Almost always 
 twinned, 
 i. Albite law. 
 
 A. P. forms 
 with the 
 c-axis an 
 
 Rather 
 strongly 
 positive. 
 
 On 
 cleav- 
 age 
 
 (and 
 
 with 
 traces of 
 CaandK 
 
 1-2*. 
 
 Not at- 
 tacked by 
 acids. 
 Si0 2 = 
 
 68%. 
 
 
 0/>and 
 
 oo^oo; 
 
 imper 
 feet 
 
 00 P 
 
 and P,. 
 Right 
 edges 
 oP: 
 oofoo = 
 
 very similar 
 to 
 orthoclase. 
 (See 
 Fig. 94 ) 
 
 Tiv inn ing -plane 
 <x>f<x and generally 
 polysynthetic; there- 
 fore in sections from 
 the zone of: <x>Poo 
 i. p. p. 1. the single 
 individuals appear 
 as fine lamellte with 
 varied polariza- 
 tion-colors. Only 
 
 angle of 
 96 16', with 
 the normal 
 to oo Poo an 
 angle of 
 16 17'. 
 i.M. = c. 
 Dispersion 
 feeble 
 P < v\ 
 
 
 pieces : 
 parallel 
 oPthe 
 oblique- 
 ness of 
 extinc- 
 tion to 
 the 
 edge 
 oP -. 
 
 oligo- 
 
 Ab 6 Anj. 
 
 
 93 36'. 
 
 
 those sections 
 
 large axial 
 
 
 00^00 = 
 
 clase 
 albite). 
 
 
 
 
 
 Parallel coP<x> show 
 no twinning-stria- 
 
 angle. 
 Cleavage- 
 
 
 + 3 54' 
 to 
 
 
 
 
 
 
 tions. Two such 
 
 leaflets 
 
 
 + 4 5i' 
 
 
 
 
 
 
 polysynthetically- 
 twinned albite indi- 
 
 parallel 
 oo ^oo show 
 
 
 (+4 
 30'); 
 
 
 
 
 
 
 viduals are often 
 
 quite com- 
 
 
 parallel 
 
 
 
 
 
 
 again combined 
 
 plete dis- 
 
 
 co Pco 
 
 
 
 
 
 
 according to the 
 Carlsbad orthoclase 
 
 torted inter- 
 ference- 
 
 
 also = 
 -f I 5 33' 
 
 
 
 
 
 
 twinning-law. 
 2. Pericline law. 
 
 figure (ap- 
 pearance of 
 
 
 to -f- 20 
 (+ 19) 
 
 
 
 
 
 
 Twins according to 
 the law : axis of 
 
 the positive 
 middle line 
 
 
 
 
 
 
 
 
 rotation the T>-axis. 
 
 perpendicu- 
 
 
 
 
 
 
 
 
 composition pla ne 
 the rhombic section, 
 i.e., the plane so 
 cutting the rhom- 
 boidal prism oo 'P. 
 oo P 1 that the plane 
 angles which these 
 
 oc^co); yet, 
 because of 
 the large 
 axial angle, 
 in the 
 position of 
 
 
 
 
 
 
 
 
 planes form with 
 co Poo are equal to 
 each other. The 
 
 4S the 
 hyperbolas 
 do not lie 
 
 
 
 
 
 
 
 
 twinning-edge 
 hereby forms with 
 
 n the field. 
 c inclined 
 to the 
 
 
 
 
 
 
 
 
 the edge oP. oopco 
 an angle of 13-22. 
 
 sharp edge 
 oP : <x>fioo. 
 
 
 
 
 
 
 
 
 Such twins are 
 
 /c- 
 
 
 
 
 
 
 
 
 often again united 
 after the Manebach 
 
 (.see 
 Fig. 97.) 
 
 
 
 
 
 
 
 
 orthoclase law. 
 
 
 
 
 
 
 
 
 
 Also oP as composi- 
 
 
 
 
 
 
 
 
 
 tion plane. By com- 
 
 
 
 
 
 
 
 
 
 bining both laws (i 
 
 
 
 
 
 
 
 
 
 and 2) a latticed 
 
 
 
 
 
 
 
 
 
 structure i. p. p. 1. 
 
 
 
 
 
 
 
 
 
 is observed in sec- 
 
 
 
 
 
 
 
 
 
 tions inclined to 
 
 
 
 
 
 
 
 
 
 oo/*oo, recalling 
 thut of microcline. 
 
 
 
 
 
 
 
 
 
 Compare 
 
 
 
 
 
 
 
 
 
 Figs. 29 and 30. 
 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chro- 
 ism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 llemarks. 
 
 For the 
 most part 
 very 
 brilliant. 
 
 Colorless, 
 clear as 
 water. 
 Relief 
 
 
 In large 
 grains, 
 rarely in 
 crystals; 
 
 With 
 calcite, 
 quartz, 
 mica, and 
 
 Very 
 
 poor; 
 fluid in- 
 closures. 
 
 Rarely 
 decom- 
 posed. 
 Fibrous, 
 
 Common in 
 granular 
 limestones. 
 In 
 
 The 
 
 polysynthetic 
 twinning after 
 oo^oo is peculiar 
 
 Not so feebly 
 powerful defined, 
 as in 0p=i. 5J7 . 
 quartz; in 
 very thin i 
 sections ! 
 feeble, : 
 blue- 
 green. 
 
 
 often inter- 
 penetrated 
 with 
 orthoclase 
 and quartz. 
 Compare 
 microcline. 
 In 
 eruptive 
 
 ortho- 
 clase ; 
 chlorite, 
 more 
 rarely 
 with 
 horn- 
 blende. 
 
 
 opaque 
 decom- 
 position. 
 See oligo- 
 clase. 
 
 crystalline 
 schists, in 
 many semi- 
 crystalline 
 gneisses, 
 phyllites, 
 sericite- 
 schists. 
 Rare in 
 
 to all plagio- 
 clase, and is 
 exceedingly 
 characteristic 
 for them. 
 The triclinic 
 feldspars can be 
 distinguished 
 from each other 
 
 
 
 
 rocks as 
 thin fibres. 
 
 
 
 
 eruptive 
 rocks, in 
 
 accurately only 
 by chemical 
 
 
 
 
 
 
 
 
 grains in 
 diorite. 
 
 analysis or by 
 determination 
 
 
 
 
 
 
 
 
 in fibres 
 
 of the oblique- 
 
 
 
 
 
 
 
 
 in many 
 
 ness of extinc- 
 
 
 
 
 
 
 
 
 andesites 
 
 tion on oPand 
 
 
 
 
 
 
 
 
 and 
 
 cop co on cleav- 
 
 
 
 
 
 
 
 
 porphyries . 
 
 age-pieces from 
 
 
 
 
 
 
 
 
 
 grains or larger 
 
 
 
 
 
 
 
 
 
 crystals. It 
 
 
 
 
 
 
 
 
 
 is therefore 
 
 
 
 
 
 
 
 
 
 impossible to 
 
 
 
 
 
 
 
 
 
 specify with 
 
 
 
 
 
 
 
 
 
 accuracy the 
 
 
 
 
 
 
 
 
 
 minute plagio- 
 
 
 
 
 
 
 
 
 
 clase threads 
 
 4 
 
 
 
 
 
 
 
 
 occurring in the 
 
 
 
 
 
 
 
 
 
 ground-mass of 
 
 
 
 
 
 
 
 
 
 the eruptive 
 
 
 
 
 
 
 
 
 
 rocks; one can at 
 
 
 ' 
 
 
 
 
 
 
 
 best determine, 
 
 
 
 
 
 
 
 
 
 by measuring 
 
 
 
 
 
 
 
 
 
 the obliqueness 
 
 
 
 
 
 
 
 
 
 of extinction 
 
 
 
 
 
 
 
 
 in sections, 
 
 
 
 
 
 
 
 
 
 whether they 
 
 
 
 
 
 
 
 
 
 belong to a 
 
 
 
 
 
 
 
 
 
 plagioclase 
 
 
 
 
 
 
 
 
 
 approximating 
 
 
 
 
 
 
 
 
 
 albite or 
 
 
 
 
 
 
 
 
 
 anorthite in 
 
 
 
 
 
 
 
 
 
 composition. 
 
 
 
 
 1 
 
 
 
 
 
 
1 84 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 
 and 
 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleav- 
 age. 
 
 Ordinary 
 combinations 
 and form of 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 strength 
 of double- 
 refraction. 
 
 Direction of 
 extinction. 
 
 b. Oligo- 
 clase. 
 
 Si0 2 = 
 62-66. 
 per cent. 
 But little 
 K. 
 = Ab 6 An x 
 to 
 
 2.62- 
 2.65. 
 (2.63.) 
 
 Most 
 perfect 
 II oP, 
 also 
 after 
 
 00/00 
 
 like 
 
 See albite. 
 
 Always 
 polysyn- 
 thetic 
 twinning 
 according 
 to the 
 Albite law; 
 
 Very 
 similar to 
 albite. In 
 cleavage- 
 planes 
 parallel 
 
 00/00 
 
 See 
 
 albite. 
 
 Parallel oP 
 to the edge 
 oP: oo/oo 
 
 = + 1 10' 
 
 (Ab 3 Anj =: 
 parallel 
 
 
 
 
 albite. 
 
 
 also 
 
 the axial 
 
 
 00/00 
 
 
 
 
 oP. 
 
 00/00 
 
 
 according 
 to the 
 
 points lie 
 still farther 
 
 
 to the edge 
 oP: oo/oo 
 
 
 
 
 
 right = 
 
 
 Pericline 
 
 beyond the 
 
 
 = 2-4. 
 
 
 
 
 93 28'. 
 
 
 law. 
 
 field than 
 
 
 
 
 
 
 
 
 
 in albite. 
 
 
 4 3 36'/. 
 
 
 
 
 
 
 
 c is inclined 
 
 
 
 
 
 
 
 
 
 to the 
 
 
 
 
 
 
 
 
 
 obtuse edge 
 
 
 
 
 
 
 
 
 
 oP: oo/oo. 
 
 
 
 
 
 
 
 
 
 (See Fig. 
 
 
 
 
 
 
 
 
 
 98.) 
 
 
 
 c. An- 
 desine. 
 
 Ab 3 Ani to 
 
 2.65. 
 
 ditto. 
 
 ditto. 
 
 See albite. 
 
 Similar to 
 oligoclase, 
 yet with the 
 axial plane 
 more 
 strongly 
 inclined 
 (above 15) 
 to the 
 obtuse 
 edge 
 oP: oo/oo. 
 Dispersion 
 p < v. 
 
 ditto. 
 
 Parallel oP 
 to the edge 
 oP: oo/oo 
 - i 57' to 
 
 2 19'; 
 
 parallel 
 
 00/00 
 
 - 4 50' to 
 -8. 
 
TABLES FOR DETERMINING MINERALS. 
 
 I8 5 
 
 Polari- 
 zation- 
 colors. 
 
 Color and 
 power of 
 refracting 
 
 Pleo- 
 chroism. 
 
 Structure. 
 
 Association. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occurrence. 
 
 Remarks. 
 
 
 light. 
 
 
 
 
 
 
 
 
 See 
 'albite. 
 
 Colorless, 
 clear as 
 
 
 In large 
 grains or 
 
 With 
 orthoclase, 
 
 Fluid 
 inclosures 
 
 Generally 
 fresh in the 
 
 As primary 
 essential or 
 
 Set- albite. 
 
 
 water or 
 clouded, 
 
 
 crystals 
 I. O. and as 
 
 quartz, 
 hornblende, 
 
 rare, and 
 vitreous 
 
 younger 
 eruptive 
 
 accessory 
 constituent 
 
 
 
 white, 
 grayish 
 white. 
 
 
 minute, 
 elongated, 
 and narrow 
 
 biotite, 
 augite, 
 olivine. 
 
 inclosures 
 common 
 in the 
 
 rocks, 
 in the 
 older 
 
 in eruptive 
 rocks, 
 granite, 
 
 
 
 
 
 threads 
 (cross- 
 sections of 
 
 
 younger 
 eruptive 
 rocks, 
 
 fibrous and 
 clouded. 
 Metamor- 
 
 diorite, 
 diabase, 
 gabbro, 
 
 
 
 
 
 thin 
 tablets). 
 Zonal 
 
 
 augite- anc 
 apatite- 
 microlites. 
 
 phosis into 
 epidote 
 called 
 
 trachyte, 
 andesiie, 
 also basalts, 
 
 
 
 
 
 develop- 
 
 
 
 saussurite; 
 
 and in 
 
 
 
 
 
 ment (see 
 
 
 
 into mus- 
 
 crystalline 
 
 
 
 
 
 Fig. 102) 
 
 
 
 covite 
 
 scliists, 
 
 
 
 
 
 and zonally 
 
 
 
 similar to 
 
 e y. 
 
 
 
 
 
 disposed 
 inclosures. 
 
 
 
 orthoclase; 
 observed 
 
 gneiss. 
 
 
 
 
 
 Almost 
 
 
 
 also in 
 
 
 
 
 
 
 always 
 
 
 
 nearly all 
 
 
 
 
 
 
 twinned 
 
 
 
 )lagioclase. 
 
 
 
 
 
 
 polysyn- 
 thetically. 
 
 
 
 
 
 
 
 
 
 Twinning 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 concentric 
 
 
 
 
 
 
 
 
 
 develop- 
 
 
 
 
 
 
 
 
 
 ment 
 
 
 
 
 
 
 
 
 
 occurred 
 
 
 
 
 
 
 
 
 
 simulta- 
 
 
 
 
 
 
 
 
 
 neously as 
 
 
 
 
 
 
 
 
 
 in ortho- 
 
 
 
 
 
 
 
 
 
 
 clase. 
 
 
 
 
 
 
 ditto. 
 
 ditto. 
 
 
 See 
 oligoclase. 
 
 With 
 sanidine, 
 
 ditto. 
 
 Mostly 
 fresh. 
 
 As primary 
 essential 
 
 ditto. 
 
 
 
 
 (Comp. 
 
 orthoclase, 
 
 
 
 constituent 
 
 
 
 
 
 Fig. 102.) 
 
 augite, 
 hornblende. 
 
 
 
 in tonalite. 
 (quartz- 
 
 
 
 
 
 
 biotite, 
 
 
 
 diorite), in 
 
 
 
 
 
 
 quartz. 
 
 
 
 andesites, 
 
 
 
 
 
 
 
 
 
 especially 
 dacites and 
 
 
 
 
 
 
 
 
 
 augite- 
 
 
 
 
 
 
 
 
 
 andesites, 
 
 
 
 
 
 
 
 
 
 porphy- 
 
 
 
 
 
 
 
 
 
 rites, 
 
 
 
 
 
 
 
 
 
 syenites, 
 
 
 
 
 
 
 
 
 
 also in 
 
 
 
 
 
 
 
 
 
 crystalline 
 
 
 
 
 
 
 
 
 
 schists. 
 
 
1 86 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleav- 
 age. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and 
 strength of 
 double- 
 refraction. 
 
 Direction 
 of 
 extinction. 
 
 d. Labra- 
 dorite. 
 
 AbiAtij to 
 AbiAno. 
 Si0 2 = 
 
 2.68 
 -2.70 
 (2.69). 
 
 See 
 oriho- 
 clase; 
 
 Mostly 
 in large 
 grains; 
 
 The albite 
 and 
 pericline 
 
 In planes 
 || oo/oo (right) 
 a side appear- 
 
 Like 
 
 ortho- 
 clase. 
 
 OnoP 
 - 4 30' 
 to 
 
 
 55-5 49 
 
 
 often 
 
 rarely in 
 
 laws 
 
 ance of one 
 
 
 - 6 54' 
 
 
 per cent. 
 De- 
 
 
 change 
 of 
 
 crystals; as 
 orthoclase. 
 
 combined 
 are 
 
 optic axis and 
 indication of 
 
 
 (AbjAn! 
 
 = 5 
 
 
 composable 
 byHCl. 
 
 
 color 
 on 
 
 
 common. 
 The 
 
 the lemniscates; 
 axial point 
 
 
 ioj); on 
 
 00/00 = 
 
 
 
 
 00/00. 
 
 
 individuals 
 
 invisible; 
 
 
 - 16 40' 
 
 
 
 
 
 
 twinned 
 
 parallel oP 
 
 
 to 
 
 
 
 
 
 
 1 CO/ 5 00 
 
 again 
 
 side appearance 
 of the other 
 
 
 21 12' 
 
 (Ab,An, 
 
 
 
 
 
 
 twinned 
 
 axis, the axial 
 
 
 = - 16;. 
 
 
 
 
 
 
 after the 
 Carlsbad 
 
 point also 
 invisible. 
 
 
 
 
 
 
 
 
 law or 
 
 Dispersion 
 
 
 
 
 
 
 
 
 according 
 
 p > i>. 
 
 
 
 
 
 
 
 
 to oo/oo 
 
 (See Fig. 99, a 
 
 
 
 
 
 
 
 
 or oP. 
 
 and b.} 
 
 
 
 
 
 
 
 
 See 
 
 
 
 
 
 
 
 
 
 structure. 
 
 
 
 
 *. By- 
 
 towmte. 
 
 AbiAn 3 to 
 An. 
 SiO 2 = 
 49-45 
 per cent. 
 More 
 easily 
 soluble in 
 HClthan^. 
 
 2.70 
 
 -2-73 
 (2.71). 
 
 
 Like labrado 
 
 rice. 
 
 Similar to labra- 
 dorite. Cleav- 
 age-leaflets 
 | oP and oo Poo 
 show the side 
 appearance of 
 one optic axis, 
 the axial point 
 not falling 
 within the field. 
 Dispersion 
 p > ?'. 
 (See Fig. 100, a 
 and b.) 
 
 Like 
 labrador- 
 ite. 
 
 Parallel 
 oP = 
 -H5 to 
 
 20, 
 
 (AbiAna 
 = - 17 
 4'); 
 parallel 
 
 00/00 = 
 
 27 to 
 -32. 
 Ab,An 3 = 
 -2 9 3 8'. 
 
TABLES FOR DETERMINING MINERALS. 
 
 I8 7 
 
 Polari- 
 zation- 
 colors. 
 
 Color and 
 power of 
 refracting 
 light. 
 
 Pleo- 
 chro- 
 isin. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decom- 
 position. 
 
 Occurrence. 
 
 Remarks. 
 
 Gene- 
 rally 
 very 
 
 Like 
 ortho- 
 clase. 
 
 
 In grains 
 and large 
 crystals I.O. 
 
 With 
 diallage, 
 hyper- 
 
 Hornblende, 
 olivine, 
 diallage, 
 
 Like 
 
 ortho- 
 clase. 
 
 Primary 
 essential 
 constituent 
 
 Like 
 
 albite. 
 
 bril- 
 
 
 
 and microlites 
 
 sthene, 
 
 magnetite, 
 
 Com- 
 
 in norite, 
 
 
 liant. 
 
 
 
 II. O. 
 
 olivine, 
 
 ilmeniie. 
 
 monly 
 
 gabbro, 
 
 
 
 
 
 Compare 
 
 also with 
 
 Especially 
 
 into 
 
 dolerite, 
 
 
 
 
 
 inclosures 
 and decom- 
 
 quartz, 
 augite, 
 
 prominent are 
 the countless 
 
 epidote 
 and 
 
 especially 
 in dacite, 
 
 
 
 
 
 position. 
 
 horn- 
 
 inclosures of 
 
 musco- 
 
 basalts, 
 
 
 
 
 
 If labradorite 
 
 blende, 
 
 long acicular 
 
 vite. 
 
 and 
 
 
 
 
 
 is twinned 
 according to 
 the Albite 
 
 biotite. 
 
 opaque micro- 
 lites disposed 
 parallel to the 
 
 
 diorites. 
 
 
 
 
 
 and Pericline 
 
 
 vertical axis or 
 
 
 
 
 
 
 
 laws, a 
 
 
 also to the edge 
 
 
 
 
 
 
 
 latticed 
 
 
 OP . OO ^00" 
 
 
 
 
 
 
 
 structure 
 
 
 also brownish 
 
 
 
 
 
 
 
 similar to 
 
 
 tablets (ferric 
 
 
 
 
 
 
 
 that of 
 
 
 oxide? 
 
 
 
 
 
 
 
 microcline 
 
 
 brookite?) 
 
 
 
 
 
 
 
 appears i.p. 1., 
 
 
 which lie with 
 
 
 
 
 
 
 
 
 yet the 
 
 
 their longer 
 
 
 
 
 
 ' 
 
 
 twinning 
 filaments in 
 
 
 direction per- 
 pendicular 
 
 
 
 
 
 
 
 labradorite 
 
 
 to the 
 
 
 
 
 
 
 
 are clearly 
 
 
 microlites, 
 
 
 
 
 
 
 
 distinguish- 
 
 
 or countless 
 
 
 
 
 
 
 
 able. 
 
 
 minute 
 
 
 
 
 
 
 
 
 
 colorless to 
 
 
 
 
 
 
 
 
 
 greenish 
 
 
 
 
 
 
 
 
 
 granules, so 
 
 
 
 
 
 
 
 
 
 that the 
 
 
 
 
 
 
 
 
 
 labradorite 
 
 
 
 
 
 
 
 
 
 
 appears opaque. 
 
 
 
 
 
 Like 1, 
 
 ibradori 
 
 e. 
 
 With 
 horn- 
 blende, 
 augite, 
 biotite, 
 diallage, 
 hyper- 
 sthene, 
 etc. 
 
 Like labrac 
 yet with no 
 microlites and 
 leaflets. 
 
 orite, 
 
 Primary 
 essential 
 constituent 
 in eruptive 
 rocks, 
 diorite, 
 gabbro, 
 andesites. 
 
 
188 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific 
 gravity. 
 
 Cleavage. 
 
 Ordinary 
 combina- 
 tions and 
 form of the 
 cross-section. 
 
 Twins. 
 
 Optical 
 orientation. 
 
 Character 
 and strength 
 of double- 
 refraction. 
 
 Direction 
 of 
 
 extinction. 
 
 /. Anor- 
 
 thite. 
 
 raAl 2 Si 3 
 O 8 . A n. 
 
 2 7:5- 
 
 2 75 
 
 Perfect 
 0/'nnd 
 
 Like a 
 
 [bite. 
 
 i. M. = c 
 nearly 
 
 Like 
 
 albite. 
 
 Parallel 
 oP = 
 
 
 Si0 2 = 
 
 (a-75). 
 
 00/>00. 
 
 
 
 perpen- 
 
 
 36 to 
 
 
 45-43 
 per cent. 
 Easily 
 
 
 P : M 
 right = 
 94 10'. 
 
 
 
 dicular 
 to 2, f 1 oo. 
 Dispersion 
 
 
 -42. 
 
 An = 
 
 - 37- 
 
 
 soluble in 
 HC1 
 
 
 
 
 
 p > v. 
 Leaflets 
 
 
 Parallel 
 
 00/00 
 
 
 without 
 formation 
 
 
 
 
 
 || oP and 
 
 00/00 
 
 
 = -37 
 to 43 . 
 
 
 of amor- 
 
 
 
 
 
 show a 
 
 
 An = 
 
 
 phous 
 
 
 
 
 
 side ap- 
 
 
 -36. 
 
 
 SiO a . 
 
 
 
 
 
 pearance of 
 
 
 
 
 
 
 
 
 
 one or the 
 
 
 
 
 
 
 
 
 
 other 
 
 
 
 
 
 
 
 
 
 of the 
 
 
 
 
 
 
 
 
 
 optic axes. 
 
 
 
 
 
 
 
 
 
 Axial 
 
 
 
 
 
 
 
 
 
 point on 
 
 
 
 
 
 
 
 
 
 margin of 
 
 
 
 
 
 
 
 
 
 the field. 
 
 
 
 
 
 
 
 
 
 (Comp. 
 
 
 
 
 
 
 
 
 
 Fig. 101, 
 
 
 
 
 
 
 
 
 
 a and t.) 
 
 
 
 DISTINCTION BETWEEN 
 
 The plagioclases from b to e inclusive are, as is well known, isomorphous mixtures of the 
 terminal members, albite (Ab) and anorthite (An). As there are all possible intermediary stages 
 between these two in chemical composition (oligoclase, andesine, labradorite, bytownite being only 
 names for such members), transitions in the physical properties, specific gravity, and especially the 
 optical orientation, are also shown. 
 
 As has been demonstrated by M. Schuster, one can directly determine the proportional mixture 
 ,of the feldspar to be determined, i.e., the plagioclase itself, by observing the directions of extinction 
 i'n cleavage-leaflets parallel oP and oo/w. 
 
 C. Aggre- 
 
 never show a simultaneous extinction, i.e., total darkness, i. p. p. 1. '(between 
 crossed nicols during a complete revolution), as the axes of elasticity of the exceedingly minute 
 individuals forming the aggregate are irregularly distributed. In a complete horizontal revolution of 
 the stage, therefore, the separate individuals extinguish in succession, and the entire aggregate does 
 not extinguish as a unit in revolving from 90 to 90. If the aggregates are radially fibrous, an inter- 
 ference-cross is visible i. p. p. 1. 
 
 In the following pages something will be given concerning the most difficultly determinable 
 
TABLES FOR DETERMINING MINERALS. 
 
 189 
 
 Polariza- 
 tion- 
 colors. 
 
 Color and 
 
 powi-i- of 
 
 refracting 
 
 light. 
 
 Pleochroism. 
 
 Structure. 
 
 Associa- 
 tion. 
 
 Inclosures. 
 
 Decomposi- 
 tion. 
 
 Occur- 
 rence. 
 
 Remarks. 
 
 See 
 
 labrador- 
 ite. 
 
 Color- 
 less, ' 
 clear as 
 water, 
 like 
 labrador- 
 
 
 Like 
 labradorite. 
 
 With 
 labra- 
 dorite, 
 augite, 
 hyper- 
 sthene, 
 
 Like 
 
 oligoclase. 
 
 Generally 
 fresh, 
 as in other 
 plagioclase. 
 
 Rather 
 rare. 
 Primary 
 essential 
 con- 
 stituent 
 
 
 
 ite. 
 
 
 
 olivine. 
 
 
 
 in 
 
 
 
 
 
 
 
 
 
 eruptive 
 
 
 
 
 
 
 
 
 
 rocks. 
 
 
 
 
 
 
 
 
 
 In 
 
 
 
 
 
 
 
 
 
 basaltic 
 
 
 
 
 
 
 
 
 
 rocks 
 
 
 
 
 
 
 
 
 
 and 
 
 
 
 
 
 
 
 
 
 augite- 
 
 
 
 
 
 
 
 
 
 andesites, 
 
 
 
 
 
 
 
 
 
 gabbro, 
 
 
 
 
 
 
 
 
 
 norite. 
 
 
 
 
 
 
 
 
 
 In crys- 
 
 
 
 
 
 
 
 
 
 talline 
 
 
 
 
 
 
 
 
 
 schists, 
 
 
 
 
 
 
 
 
 
 amphibo- 
 
 
 
 
 
 
 
 
 
 lites, 
 
 
 
 
 
 
 
 
 
 gneiss. 
 
 
 THE SPECIES OF PLAGIOCLASE. 
 
 The oblique extinctions given have reference to the usual setting up of a plagioclase (the oP- 
 plane falling. Yrom above forward and inclined from left to right) and always to the obtuse edge 
 6P : <x>P<, i.e., the plane oo^oo lying to the right. The symbol -f- prefixed indicates, on cleavage- 
 leaflets parallel oP, that the direction of extinction as regards the right prismatic edge is inclined 
 towards the obtuse oP : oo/^oo; on cleavage- leaflets parallel oo/^oo, that it is inclined towards the edge 
 oP : <x>P<x>, the same as the section of the plane //*/< with oo^oo. The symbol indicates in both cases 
 the opposite direction. 
 
 gates. 
 
 crypto-crystalline aggregates. Their determination is rendered unusually difficult by the minuteness 
 of the separate individuals; often the chemical investigation is the only safe means of determination. 
 All aggregates here introduced are secondary minerals, decomposition-products, and often inclose 
 fresh remnants of the original mineral. From those minerals already studied aggregates (crypto-crys- 
 talline also) are often formed; so. e.g., from talc, muscovite, tridymite, siderite : these have been 
 discussed already under the appropriate headings. 
 
DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition 
 and 
 reactions. 
 
 Specific gravity. 
 
 Color and 
 power of 
 refracting light. 
 
 Optical properties. 
 
 1. Serpentine. 
 
 H 2 Mg 8 Si 2 8 
 + aq. 
 Completely 
 decomposed by 
 HC1. 
 
 2.5-2.7. 
 
 Green, more 
 rarely yellow, 
 brown, reddish- 
 brown, black. 
 = 1-574- 
 
 Partly 
 amorphous, 
 partly showing 
 aggregate 
 polarization. 
 The variety 
 antigorite 
 rhombic (?). 
 (See "structure.") 
 
 Polarization- 
 colors feeble. 
 A ntigorite 
 negative 
 double-refracting:, 
 feebly 
 pleochroitic. 
 Dispersion clear, 
 but feeble. 
 
 
 
 
 
 a = i. M. oP\ 
 
 p> v. 
 
 
 
 
 
 i.e., 
 perpendicular 
 to the direction 
 
 
 
 
 
 
 
 of perfect 
 cleavage. 
 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 Structure. 
 
 Association. 
 
 Occurrence. 
 
 Decomposition- 
 product. 
 
 Remarks. 
 
 The mesh-structure 
 
 is characteristic. 
 The decomposition 
 begins on the 
 walls of the 
 olivine fissures; 
 generally yellowish- 
 green threads shoot 
 out at right angles; 
 thus a sort of 
 net is formed 
 embracing within 
 its meshes particles 
 of fresh olivine, 
 
 which are 
 subject to the lurther 
 
 decomposition. 
 
 The interior of the 
 
 meshes generally 
 
 appears filled with 
 
 tutted serpentine 
 
 threads. The 
 mesh-structure is 
 
 yet further 
 
 advanced, in that 
 
 between the single 
 
 fields earthy 
 
 particles are 
 
 deposited. In 
 
 other serpentines 
 
 the serpentine 
 
 substance is 
 
 arranged in form of 
 
 large often very 
 
 regular leaflets 
 
 lying at nearly right 
 
 angled, showing 
 the optical behavior 
 
 of the so-called 
 
 antigorite; here the 
 
 mesh-structure 
 
 is wanting. 
 In decomposition 
 
 magnetite is 
 
 separated, also 
 
 ferric oxide and 
 
 hydroxide. The 
 
 serpentines are 
 
 often impregnated 
 
 with amorphous 
 
 silicic acid or 
 
 chalcedony. 
 
 With olivine, 
 
 rhombic or 
 
 monoclinic augite, 
 
 hornblende, 
 
 garnet, magnetite, 
 
 chromite, chlorite, 
 
 magnesite. 
 
 Massive, as 
 decomposition- 
 product of 
 olivine-fels; in 
 pseudomorphs 
 after olivine, in 
 olivine-bearing 
 eruptive rocks, 
 
 and schists. 
 As decomposition- 
 product of 
 olivine, Al-free 
 augite, and 
 hornblende. 
 
 For the most part 
 
 of olivine and 
 augite free from 
 
 alumina 
 and hornblende. 
 
 Difficult to 
 distinguish from 
 the bastite and 
 
 chloritic 
 decomposition- 
 products of 
 augite. 
 
1 9 2 
 
 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical 
 composition and 
 reactions. 
 
 Specific gravity. 
 
 Color and power 
 of refracting 
 light. 
 
 Optical properties. 
 
 2. Viridite. 
 Partly chlorine, 
 partly serpen- 
 tine-like aggre- 
 gates, as; 
 a. Delessite ; 
 b. Chlorophae- 
 ite ; 
 c. Green earth 
 (Grunerde). 
 
 The augites es 
 pose into dirty to 
 called by the com 
 scope is impossibl 
 viriditic aggregate 
 sometimes they a 
 ceedingly fine-gra 
 
 pecially and the 
 arownish-green i 
 Drehensive term 
 on account of t 
 s show aggregat 
 r e finely radial a 
 ned or more or 
 
 hornblendes, als 
 ibrous aggregate 
 viridite. An ex 
 he minuteness < 
 e polarization, i 
 nd concentric o 
 ess laminated a 
 
 a garnet and biotite, often decom- 
 s, or, as in green earth, granular, 
 act specification with the micro- 
 >f the threads and grains. The 
 ind often a feeble pleochroism ; 
 r tangled fibrous, and again ex- 
 ggregates. The three minerals, 
 
 (Grunerde). 
 
 
 
 
 
 3. Bastite. 
 
 Green. Comp 
 rhombic pyroxen 
 very like that of J 
 especially the sepa 
 
 are with these 
 e crystals or gr 
 .erpentine. Her 
 ration-clefts par 
 
 the rhombic py 
 ains into bastit 
 e also the decoi 
 allel oP, and pro 
 
 roxenes. The decomposition of 
 e or a bastite-like aggregate is 
 nposition begins on the fissures, 
 presses into a threading parallel 
 
 4. Chalcedony. 
 
 Si0 3 . 
 Snail percentage 
 of H 2 0. 
 
 See quartz. 
 
 Colorless, 
 transparent, 
 often colored 
 
 See quartz. 
 
 
 
 
 by ferric oxide 
 
 
 
 
 
 or hydroxide. 
 
 
 
 
 
 n = 1.547. 
 
 
TABLES FOR DETERMINING MINERALS. 
 
 193 
 
 Structure. 
 
 Association. 
 
 Occurrence. 
 
 Decomposition- 
 products. 
 
 Remarks. 
 
 rt, , c occurring in su 
 very commonly in ps 
 
 ch crypto-crystallin< 
 eudomorphs after a 
 
 ; aggregates occur 
 agile. According 
 
 For the most 
 
 
 to the chemical composition, a is a hydrous FeMg alumina 
 
 part from 
 
 
 silicate, and from the high percentage of alumina resem- 
 bles the chlorites ; b and c are iron-magnesium silicates, 
 
 monoclinic augite 
 and hornblende, 
 
 
 hydrous and poor in 
 the decomposed basic 
 
 alumina. Very wid 
 : eruptive rocks and 
 
 ely distributed in 
 :rystalline schists. 
 
 garnet, biotite, etc. 
 
 
 to the c-axis. As a c 
 of these threads it i 
 bastite by studying i 
 under " Bastite" (pa^ 
 
 ansequence of the r 
 s often possible to ( 
 c. p. 1. Compare c 
 e 150). 
 
 egular disposition 
 letermine them as 
 ptical orientation 
 
 From the 
 rhombic pyroxenes. 
 
 
 Chalcedony is for 
 the most part a 
 mixture of 
 amorphous and 
 micro- or crypto- 
 ctystalline silicic 
 acid. The 
 aggregates are 
 either fine-grained 
 or tangled fibrous ; 
 also often radial. 
 
 Especially in 
 quartz-orthoclase- 
 biotite rocks, 
 with opal and 
 tridymite. 
 
 Secondary 
 mineral, common 
 in the acidic 
 eruptive rocks, 
 especially rhyolite, 
 dacite. quartz- 
 porphyries; 
 also in other 
 decomposed 
 eruptive rocks, as 
 basalt, andesite, 
 
 A long series of 
 minerals, 
 especially the 
 feldspars and 
 augite. yield on 
 decomposition 
 chalcedony, 
 together with 
 other products. 
 
 The primary 
 radial quartz 
 sphserulites are to 
 be distinguished 
 from the 
 chalcedony always 
 appearing as 
 decomposition- 
 product ; these 
 are direct 
 eliminations from 
 
 In the last case. 
 
 
 melapliyr, and 
 
 
 the eruptive 
 
 quartz individuals 
 elongated according 
 to the chi f axis are 
 
 
 porphyrite, in 
 cavities, clefts, 
 and irregular 
 
 
 magma, and can 
 be recognized as 
 primary products 
 
 combined to form 
 
 
 parts in the 
 
 
 from the nature 
 
 a ball, and such 
 aggregates 
 brilliantly polarizing 
 
 
 ground-mass. 
 
 
 of the limitations 
 (Abgrenzung). 
 
 show the 
 
 
 
 
 
 interference-dross 
 
 
 
 
 
 between 
 
 
 
 
 
 crossed nicols. 
 
 
 
 
 
 
 
 * 
 
 
 
194 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 NAME. 
 
 Chemical composition and 
 reactions. 
 
 Specific gravity. 
 
 Color and power 
 of refracting 
 light. 
 
 Optical properties. 
 
 5. Zeolites. 
 
 Of these, analcime has 
 
 been already sti 
 
 died. (Compar 
 
 Regular Minerals.) 
 
 a. Natrolite. 
 
 Na 2 Al a Si 3 O 10 + 2H 2 O. 
 
 2.172.26. 
 
 Colorless, clear 
 as water. 
 Relief not 
 marked. 
 
 Rhombic. 
 
 b. Scolecite. 
 
 CaAl 2 Si 3 O 10 -i-3HaO. 
 
 2.2-2.39. 
 
 ditto. 
 
 Moaoclinic or triclinic. 
 
 c. Stilbite. 
 
 H 4 CaAl 2 Si 6 O 18 + 3H a O. 
 
 2.1-2.2. 
 
 ditto. 
 
 Monoclinic. 
 
 d. Desmine. 
 
 CaAl 2 Si 6 16 + 6H 2 0. 
 
 2.1-2.2. 
 
 ditto. 
 
 ditto. 
 
 e. Chabasite. 
 
 R ' C R A ,'= S '?g'^K H ' a 
 
 All, a to e, are easily 
 soluble in HC1, with sepa- 
 ration of gelatinous silica. 
 
 2.07-2.15. 
 
 ditto. 
 
 Rhombohedral or 
 triclinic. Rhombohe- 
 dral cleavage. 
 
 6". Carbonates. 
 
 Of these, calcite, dol 
 " Hexagonal Minerals." ' 
 gates. 
 
 omite, magnesit 
 "hese occur also 
 
 ;, siderite, have 
 in extremely fine 
 
 been studied unde" the 
 -grained or radial aggre- 
 
 Aragonite. 
 
 CaC0 3 . 
 Easily soluble in HC1 with 
 effervescence. 
 
 2-9-3- 
 
 Colorless, 
 transparent. 
 
 Rhombic. Polarization- 
 colors as in calcite, 
 often iridescent. 
 Cleavage parallel 
 oo^oo and oo P . 
 A.P. U ooPoo; a = C. 
 
TABLES FOR DETERMINING MINERALS. 
 
 195 
 
 Structure. 
 
 Association. 
 
 Occurrence. 
 
 Decomposition- 
 product. 
 
 Remarks. 
 
 
 
 
 
 
 Besides analcime the following often occur as> decomposition-products : 
 
 Almost always in aggre- 
 gates of long acicular 
 
 Compare 
 occurrence. 
 
 Secondary 
 minerals, 
 
 The zeolites 
 occur 
 
 The distinctions are best 
 effected by the micro- 
 
 crystals, generally radially 
 disposed, -with brilliant 
 pola rizat ion-colors, 
 
 A.P. i| 00/00 ; C - C. 
 
 With augite, 
 olivine, 
 magnetite, 
 hornblende, 
 biotite, 
 
 especially 
 prominent in 
 the bubble- 
 cavities (see 
 Fig. 103) of 
 
 generally as 
 decomposition- 
 products of the 
 feldspars, of 
 nepheline, 
 
 chemical examination. 
 b to d inclusive can be 
 accurately distinguished 
 only by determining 
 the relation of the 
 
 Ditto. Generally in needles 
 
 feldspar, etc., 
 i.e. their 
 decomposition- 
 
 feldspar-, 
 nepheline-, and 
 leucite-basalts; 
 
 leucite, and 
 hauyn. 
 
 axes of elasticity to the 
 crystallographic axes 
 
 radially disposed. 
 
 product; with 
 
 the basanites, 
 
 
 
 tt : c = 11-12. 
 
 calcite and 
 
 tephrites, 
 
 
 
 
 aragonite. 
 
 phonolites. and 
 
 
 
 
 
 also in trachytic 
 
 
 
 
 
 and andesitic 
 
 
 
 Tabular crystals in radial 
 
 
 eruptive rocks. 
 
 
 
 groups, i. M. = c = 6. 
 
 
 
 
 \ 
 
 As above. A. P. |j oo j?oo. 
 
 
 
 
 
 b : c = 34. 
 
 
 
 
 
 i. M. with a about 5. 
 
 
 
 
 
 In rhombohedra. 
 
 
 
 
 
 Polarization-colors like 
 
 
 
 
 
 feldspar.. Forms more 
 
 
 
 
 
 granular aggregates. 
 
 
 
 
 
 
 
 
 
 
 Siderite is very common in spherical, radial, and concentric aggregates. Besides these rhombo- 
 hedral carbonates very commonly as decomposition-product occur : 
 
 Partly in large grains or in 
 radial fibrous tufts of long 
 needles. 
 
 With calcite 
 and zeolites. 
 See occurrence. 
 
 Common in 
 basic eruptive 
 rocks in cavities 
 
 Decomposition- 
 product of 
 calcareous 
 
 Characterized by 
 solubility with evolution 
 of CO a , and by 
 
 
 
 and geodes. 
 
 silicates. 
 
 crystalline form; easily 
 distinguished from 
 
 
 
 
 
 calcite by the latter 
 
 
 
 
 
 property. 
 
BIBLIOGRAPHY TO PART II. 
 
 The following larger text-books and treatises are not 
 embraced in this bibliography: 
 
 E. COHEN. Sammlung von Mikrophotographien zur Veranschaulichung der 
 
 mikroskopischen Structur von Mineralien und Gesteinen, aufgenommen 
 von J. Grimm in Offenburg. Stuttgart, Schvveizerbart'sche Verlagshand- 
 lung. 1883. 80 Tafeln. 
 
 FISCHER. Kritische mikroskop.-mineralogische Studien. 3 Hfte. Freiburg i. 
 Br. 1869-1873. 
 
 F. FOUQUE et A. MICHEL LEVY. Mineralogie micrographique roches eruptives 
 
 fran$aises. Paris, 1879. a. Atlas LV PI. 
 
 H. ROSENBUSCH. Mikroskopische Physiographic der petrographisch wichtigen 
 Mineralien. Stuttgart, Schweizerbart'sche Verlagshandlung. 1873. Mit 
 10 Tafeln. 
 
 Mikroskopische Physiographic der massigen Gesteine. Stuttgart, Schweizer- 
 
 bart'sche Verlagshandlung. 1877. 
 
 F. ZIRKEL. Die mikroskopische Beschaffenheit der Mineralien und Gesteine. 
 Leipzig, W. Engelmann. 1873. 
 
 Microscopical Petrography. Washington, 1876. w. XII PL 
 
 * 
 
 Acmite and Aegirine. 
 
 TSCHERMAK. Tschertnak's Mineral. Mitth. 1871. 33. 
 
 BECKE. Tschermak's Mineral, u. petr. Mitth. N. F. I. 1878. 554. 
 
 KOCH. N. Jahrbuch f. Min. u. Geol. 1881. I. Beil.-Bd. 156. 
 
 TORNEBOHM. Fo'rh. geol. Foren. i Stockholm. 1883. VI. 383 and 542. Comp. 
 
 Ref. N. Jahrb. f. Min. u. Geol. 1883. II. 370. 
 M T GGE. N. Jahrb. f. Min. u. Geol. 1883. II. 189. 
 MANN, N. Jahrb. f. Min. u. Geol. 1884. II. 172. 
 
198 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Actinolite (Smaragdite, Kar in thine). 
 
 TSCHERMAK. Tschermak's Min. Mitth. 1871, 37 and 44. 
 
 v. DRASCHE. Tsch. Min. Mitth. 1871. 85. 
 
 RIESS. Tsch. Min. u. petr. Mitth. N. F. 1878. I. 185, 192. 
 
 CH. WHITMAN CROSS. Tsch. Min. u. petr. Mitth. 1881. III. 386. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 234, 360. 
 
 Tsch. Min. u. petr. Mitth. 1882. V. 157. 
 
 Albite. 
 
 LOSSEN. Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 509 and 1879. XXXI. 441. 
 SCHUSTER. Tsch. Min. und petr. Mitth. N. F. 1881. III. 153. 
 BOHM. Tsch. Min. und petr. Mitth. N. F. 1883. V. 202. 
 
 Almandine (ordinary Garnet). 
 
 DRASCHE. Tsch. Min. Mitth. 1872. 2. 85. 
 
 WICHMANN. Pogg. Ann. f. Phys. u. Chem. 1876. CLVII. 282. 
 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1877. XXIX. 274. 
 
 RIESS. Tsch. Min. u. petr. Mitth. 1878. I. 186. 
 
 SZABO. N. Jahrb. f. Min. u. Geol. 1880. I. Beil.-Bd. 302. 
 
 SCHRAUF. Groth's Zeitschr. f. Kryst. 1882. 323. 
 
 RENARD. Bull, du Musee royal d'hist. nat. de Belgique. 1882. I. 
 
 v. LASAULX. Sitzungsber. d. niederrhein. Ges. in Bonn. 1883. 
 
 Analcime. 
 
 TSCHERMAK. Sitzungsber. Wien. Akad. d. Wiss. 1866. LIII. 260. 
 
 Andalusite. 
 
 JEREMEJEFF. N. Jahrb. f. Min. u. Geol. 1866. 724. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 68. 180. 
 
 ROSENBUSCH. Die Steiger Schiefer. Strassburg, 1877. 
 
 POHLIG. Zeitschr. d. deutsch. geol. Ges. 1877. XXIX. 560, and Tsch. Min. u. 
 
 petr. Mitth. 1881. III. 344. 
 
 TELLER u. JOHN. Jahrb. d. kk. geol. R.-Anst. Wien, 1882. XXXII. 589. 
 MULLER. N. Jahrb. f. Min. u. Geol. 1882. II. 205. 
 
 Andesine. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1864. XVI. 294. 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 173. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. V. 149, 160. 
 
BIBLIOGRAPHY TO PART II. 199 
 
 Anomite. 
 
 TSCHERMAK. Gr. Zeitschr. f. Kryst. 1878. 31. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 331. V. 151. 
 
 Anorthite. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 246. 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 208. 
 
 Anthophyllite. 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 37. 
 
 CH. WHITMAN CROSS. Tsch. Min. u. petr. Mitth. 1881. III. 388. 
 
 BECKE. Tsch. Min. u. petr. Mitth. N. F. 1882. IV. 331. 450. 
 
 SJOGREN (on Gedrit). Comp. Ref. N. Jahrb. f. Min. u. Geol. 1883. II. 366. 
 
 Apatite. 
 
 ROSENBUSCH. Nephelinit v. Katzenbuckel. Freiburg i. Br. 1869. 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 72. 
 
 N. Jahrb. f. Min. u. Geol. 1870. 806, 821. 
 HAGGE. Ueber Gabbro. In.-Diss. Kiel, 1871. 58. 
 KREUTZ. Tsch. Min. u. petr. Mitth. N. F. 1884. VI. 149. 
 
 Arfvedsonite. 
 
 KO'ENIG. Gr. Zeitschr. f. Krystall. 1877. 423. 
 
 Augite (ordinary and basaltic). 
 
 WEDDING. Zeitschr. d. deutsch. geol. Ges. 1858. 380. 
 BUTSCHLY. N. Jahrb. f. Min. u. Geol. 1867. 700. 
 TSCHERMAK. Shzungsber. d. Wien. Akad. d. Wiss. 1869. LIX. 
 
 Tsch. Min. Mitth. 1871. 28. 
 ROSENBUSCH. Neph. v. Katzenbuckel. 1869. 
 ZIRKEL. Basaltgesteine. 1870. 8. 
 
 VRBA. Zeitschr. " Lotos" Prag. Jahrg. 1870. 
 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1874. XXVI. i. 
 
 LAGORIO. Andesite d. Kaukasus. Dorpat, 1878. Ref. N. Jahrb. f. Min. u. Geol. 
 
 1880. I. 209. 
 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1879. 482. 822. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 365. 
 KREUTZ. Tsch. Min. u. petr. Mitth. 1884. VI. 141. 
 
200 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Bastite. 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871, 20. 
 HAGGE. Ueber Gabbro. Kiel, 1871. 27. 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 261. 
 DRASCHE. Tsch. Min. Mitth. 1873. 5. 
 
 Bronzite (comp. with Serpentine). 
 
 TSCHERMAK. Sitzungsber. d. Wien. Akad. d. Wiss. 1869. LIX. i. I. 
 
 Tsch. Min. Mitth. 1871. 17. 
 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 273. 
 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 321. 
 
 BACKING. Gr. Zeitschr. f. Kryst. 1883. VII. 502. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1883. V. 527. 
 
 ROSENBUSCH. N. Jahrb. f. Min. u. Geol. 1884. I. 197. 
 
 Bytownite. 
 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 202. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. V. 168. 
 
 RENARD. Bull. d. Musee roy. d'hist. nat. belgique. 1884. III. 10. 
 
 Calcite. 
 
 OSCHATZ. Zeitschr. d. deutsch. geol. Ges. 1855. VII. 5. 
 
 STELZNER. Ueber Gesteine v. Altai. Leipzig, 1871. Aus Cotta: D. Altai, p. 57. 
 INOSTRANZEFF. Tsch. Min. Mitth. 1872. I. 45. 
 ROSENBUSCH. N. Jahrb. f. Min. u. Geol. 1872. 64. 
 LEMBERG. Zeitschr. d. deutsch. geol. Ges. 1872. 226. 1876. 519. 
 LAGORIO. Mikrosk. An. ostbaltischer Gebirgsarten. Dorpat, 1876. 
 O. MEYER. Zeitschr. d. deutsch. geol. Ges. 1879. XXXI. 445. 
 RENARD. Bull. Acad. royal des Sciences belg. 1879. XLVII. Nr. 5. Comp. 
 Ref. N. Jahrb. f. Min. u. Geol. 1880. II. 146. 
 
 Cancrinite. 
 
 A. KOCH. N. Jahrb. f. Min. u. Geol. 1881. I. Beil.-Bd. 144. 
 TORNEBOHM. Geol. Foren. i Stockholm Forh. 1883. VI. 383. Comp. Ref. N. 
 Jahrb. f. Min. u. Geol. 1883. II. 370. 542. 
 
 Chalcedony. 
 
 REUSCH. Pogg. Ann. f. Ph. u. Chem. 1864. CXXIII. 94. 
 
 BEHRENS. Sitzungsber. d. Wien. Akad. d. Wiss. 1871. LXIV. Dec. I. 
 
BIBLIOGRAPHY TO PART II. 2OI 
 
 Chiastolite. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. 68. 
 
 POHLIG. Zeitschr. d. deutsch. geol. Ges. 1877. XXIX. 545. 563. 
 
 Tsch. Min. u. petr. Mitth. 1881. III. 348. 
 
 CH. WHITMAN CROSS. Tsch. Min, u. petr. Mitth. 1881. III. 381. 
 
 MULLER. N. Jahrb. f. Min. u. Geol. 1882. II. 205. 
 
 Chloritoid (Sismondine). 
 
 TSCHERMAK u. Sipocz. Gr. Zeitschr. f. Kryst. 1879. 506 and 509. 
 V. FOULLON. Jahrb. d. kk. geol. R.-Anst. Wien, 1883. XXXIII. 207. 
 BARROIS. Ann. de la Soc. geol. du Nord. Lille, 1883. XI. 18. Comp. Ref, N. 
 Jahrb. f. Min. u. Geol. 1884. II. 68. 
 
 Chromite. 
 
 DATHE. J. Jahrb. f. Min. u. Geol. 1876. 247. 
 THOULET. Bull. Soc. miner. Paris, 1879. 34. 
 
 Cordierite. 
 
 WICHMANN. Zeitschr. d. deutsch. geol. Ges. 1874. XXVI. 675. 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1872. 831. 
 - Gr. Zeitschr. f. Kryst. 1883. VIII. 76. 
 SZABO. N. Jahrb. f. Min. u. Geol. 1880. I. Beil.-Bd. 308. 
 HUSSAK. Sitzungsber. d. Wien. Akad. d. Wiss. 1883. April. 
 CALL-ERON y ARANA. Bal. d.l. Comis. d. Mapa. geolog. Madrid, 1882. 
 
 Corundum. 
 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1870. 822. 
 
 TELLER u. JOHN. Jahrb. d. kk. geol. R.-Anst. Wien, 1882. XXXII. 589. 
 
 WICHMANN. Verhandl. d. kk. geol. R.-Anst. Wien, 1884. 150. 
 
 Couseranite (Dipyr). 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 202. 
 GOLDSCHMIDT. N. Jahr. f. Min. u. Geol. 1881. I. Beil.-Bd. 225. 
 
 DiaUage. 
 
 G. ROSE. Zeitschr. d. deutsch. geol. Ges. 1867. 280, 294. 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 25, and Sitzungsber. d. Wien. Akad. d. 
 Wiss. 1869. LIX. i. i. 
 
202 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 v. DRASCHE. Tsch. Min. Mitth. 1871. i. 
 
 HAGGE. N. Jahrb. f. Min. u. Geol. 1871. 946. 
 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 377, 379. 
 
 v. RATH. Verb. d. niederrhein. Ges. f. Nat. u. Heilkde. Bonn. 8. Mz. 1875. 
 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1877. XXIX. 274. 
 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 323. 
 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1883. II. 97. 
 
 KLOOS. N. Jahrb. f. Min. u. Geol. 1884. III. Beil.-Bd. 19. 
 
 Diopside (Omphacite and Sahlite). 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 21. 
 
 v. DRASCHE. Tsch. Min. Mitth. 1871. 58. 
 
 v. KALKOWSKY. Tsch. Min. Mitth. 1875. II. 
 
 DATHE. N. Jahrb. f. Min u. Geol. 1876. 225, 337. 
 
 RIESS. Tsch. Min. u. petr. Mitth. 1878. I. 168. 
 
 BECKER. Zeitschr. d. deutsch. Geol. Ges. 1881. XXXIII. 31. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 297. 
 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 321. 
 
 Disthene (Cyanite). 
 
 V. KOBELL. Pogg. Ann. f. Phys. u. Chem. 1869. CXXXVI. 156. 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1872. 835. 
 RIESS. Tsch. Min. u. petr. Mitth. 1878. I. 165, 195. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 225, 231. 
 
 Dolomite. 
 
 INOSTRANZEFF. Tsch. Min. Mitth. 1872. 48. 
 LEMBERG. Zeitschr. d. deutsch. geol. Ges. 1876. 519. 
 O. MEYER. Zeitschr. d. deutsch. geol. Ges. 1879. 445. 
 
 RENARD. Bull. Acad. royal Belg. XLVII. 5. Mai 1879. Comp. Ref. N. Jahrb. 
 f. Min. u. Geol. 1880. II. 146. 
 
 Elajolite. 
 
 SCHEERER. Pogg. Ann. f. Phys. u. Chem. 1863. CXIX. 145. 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1870. 810. 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1880. II. 141. 
 KOCH. N. Jahrb. f. Min. u. Geol. 1880. I. Beil.-Bd. 140. 
 
BIBLIOGRAPHY TO PART II. 203 
 
 Enstatite. 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 17. 
 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 273. 
 
 TRIPPKE. N. Jahrb. f. Min. u. Geol. 1878. 673. 
 
 TELLER u. JOHN. Jahrb. d. kk. geol. R.-Anst. Wien, 1882. XXXII. 589. 
 
 Epidote. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1869. XIX. 121. 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1872. 837. 
 BECKE. Tsch. Min. u. petr. Mitth. 1879. II. 25, 34. 
 - Tsch. Min. u. petr. Mitth. 1882. IV. 264. 
 v. KALKOWSKY. Tsch. Min. u. petr. Mitth. 1876. II. 87. 
 REUSCH. N. Jahrb. f. Min. u. Geol. 1883. II. 179. 
 TORNEBOHM. Geol. Foren. i Stockholm Forh. VI 185. Comp. Ref. N. Jahrb. 
 
 f. Min. u. Geol. 1883. I. 245. 
 
 BACHINGER. Tsch. Min. u. petr. Mitth. 1884. VI. 44. 
 KUCH. Tsch. Min. u. petr. Mitth. 1884. VI. 119. 
 
 Fluorite. 
 
 LASPEYRES. Zeitschr. d. deutsch. geol. Ges. 1864. XVI. 449. 
 
 Glaucophane. 
 
 HAUSMANN. Gottinger gel. Anz. 1845. 195. 
 
 BODEWIG. Pogg. Ann. f. Phys. u. Chem. 1876. CXLVIII. 224. 
 
 LUEDECKE. Zeitschr. d. deutsch. geol. Ges. 1876 XXVIII. 248. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1879. U. 49, 71. 
 
 WILLIAMS. N. Jahrb. f. Min. u. Geol. 1882. II. 201. 
 
 STELZNER. N. Jahrb. f. Min. u. Geol. 1883. I. 208. 
 
 BARROIS. Ann. Soc. geol. du Nord. Lille, 1883. XI. 18. Comp. Ref. N. Jahrb. 
 
 f. Min. u. Geol. 1884. II. 68. 
 v. LASAULX. Sitzungsber. d. niederrhein. Ges. F. Nat. u. Heilkunde. Bonn, 
 
 1884. 3. XII. 
 
 Graphite. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. 68. 
 Pogg. Ann. f. Phys. u. Chem. CXLIV. 1871. 319. 
 
 RENARD. Bull, du Musee royal d'hist. nat. Bruxelles, 1882. I. 47. Comp. Ref. 
 N. Jahrb. f. Min. u. Geol. 1883. II. 68. 
 
2O4 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Gypsum (and Anhydrite). 
 HAMMERSCHMIDT. Tsch. Min. u. petr. Mitth. 1883. V. 245. 
 
 Hauyn (comp. Nosean). 
 
 ZIRKEL. Basal tgesteine. 1870. 79. 
 
 N. Jahrb. f. Min. u. Geol. 1870. 818. 
 
 VOGELSANG. Mededeel. d. k. Akad. v. Wetenschapp. Amsterdam, 1872 (2). 
 SAUER. Zeitschr. f. d. gesammt. Naturwiss. Halle, 1876. XIV. 
 DOELTER. Tsch. Min. u. petr. Mitth. 1882. IV. 461. 
 
 Hematite. 
 
 G. ROSE. Zeitschr. d. deutsch. geol. Ges. 1859. XI. 298, 306. 
 KOSMANN. Zeitschr. d. deutsch. geol. Ges. 1864. XVI. 665. 
 ZIRKEL. Basaltgesteine. 1870. 71. 
 
 Hercynite. 
 v. KALKOWSKY. Zeitschr. d. deutsch. geol. Ges. 1881. XXXIII. 533. 
 
 Hornblende (ordinary and basaltic). 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. 99. 119. 
 
 - Zeitschr. d. deutsch. geof. Ges. 1871. 43. 
 
 - Basaltgesteine. 1870. 74. 
 
 TSCHERMAK. Sitzungsber. d. Wien. Akad. d. Wiss. 1869. LIX. i. I. 
 
 Tsch. Min. Mitth. 1871. 38. 
 
 RIESS. Tsch. Mm. u. petr. Mitth. 1878. 165. 
 
 SOMMERLAD. N. Jahrb. f. Min. u. Geol. 1882. II. 139. 
 
 BECKER. N. Jahrb. f. Min, u. Geol. 1883. II. i. 
 
 STRENG. XXII. Bericht d. oberhess. Ges. f. Natur- u. Heilkunde. Giessen, 1883. 
 
 KLOOS. N. Jahrh. f. Min. u. Geol. 1884. III. Beil.-Bd. 24. 
 
 Hypersthene. 
 
 KOSMANN. Sitzungsber. d. niederrhein Ges. f. Natur- u, Heilkunde. Bonn, 
 3. Febr. 1869. 
 
 N. Jahrb. f. Min. u. Geol. 1869. 374 and 1871. 501. 
 HAGGE. N. Jahrb. f. Min. u. Geol. 1871. 946. 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 17. 
 
BIBLIOGRAPHY TO PART II. 2O5 
 
 NIEDZWIEDZKI. Tsch. Min. Mitth. 1872. 253. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1878. I. 244. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1883. V. 527. 
 
 FOUQUE. Santorin. Paris, 1879. 
 
 BLAAS. Tsch. Min. u petr. Mitth. 1881. III. 479. 
 
 TELLER u. JOHN. Jahrb. d. kk. geol. R.-Anst. Wien. 1882. XXXII. 589. 
 
 ROSENBUSCH. Gesteine v. Ekersund. N. Magaz. f. Naturvidenskaberne. 
 
 XXVII. 4. Heft. 
 HAGUE u. IDDINGS. Amer. Journ. of Science. 1883. XXVI. 222. Ref. N. 
 
 Jahrb. f. Min. u. Geol. 1884. I. 225. 
 CH. WHITMAN CROSS. The same. XXV. 1883. 139. Ref. N. Jatfrb. f. Min. u. 
 
 Geol. 1884. I. 228. 
 KRENNER. Gr. Zeitschr. f. Kryst. 1884. IX. 255. 
 
 Ilmenite. 
 
 LASPEYRES. N. Jahrb. f. Min. u. Geol. 1869. 513. 
 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 70. 
 
 SANDBERGER. N. Jahrb. f. Min. u. Geol. 1870. 206. 
 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 385. 
 
 GtJMBEL. D. palaolith. Eruptivgest. d. Fichtelgebirges. Miinchen, 1874. 35. 
 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1874. XXVI. i. 
 
 COHEN. Reisen in Siidafrika. Hamburg, IS73 1 . 2. Friedrichsen'sche Jahresber. 
 
 der geograph. Ges. Comp. Ref. N. Jahrb. f. Min. u. Geol. 1876. 213. 
 v, LASAULX Verh. d. naturw. Ver. d. preuss. Rheinlande u. Westphal. 1878. 
 
 XXXV. 
 
 SAUER. N. Jahrb. f. Min. u. Geol. 1879. 575- 
 CH. WHITMAN CROSS. Tsch. Min. u. petr. Mitth. 1881. III. 401. 
 CATHREIN. Gr. Zeitschr. f. Kryst. 1882. 244. 
 
 Labradorite. 
 
 VOGELSANG. Archiv. Neerland. 1868. III. 
 
 SCHRAUF. Sitzungsber. d. Wien. Akad. d. Wiss. Dec. 1869. LX. Bd. 
 
 STELZNER. Berg- und Huttenmann. Zeig. XXIX. 150. 
 
 HAGGE. N. Jahrb. f. Min. u. Geol. 1871. 946. 
 
 SCHUSTER. Tsch. Min. u. petr. Mitth. iSSi. III. 183. 
 
 Leucite. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1868. 97. 
 
 Basaltgesteine. Bonn, 1870. 
 
 V. RATH. Monatsber. d. Berlin. Akad. d. Wiss. Aug. 1872. 
 
206 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 KREUTZ. Tsch. Min. u. petr. Mitth. 1884. VI. 135. 
 
 v. CHRUSTSCHOFF. Tsch. Min. u. petr. Mitth. 1884. VI. 161. 
 
 loiebenerite. 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1868. 719. 
 
 Magnesite. 
 
 ROSENBUSCH. N. Jahrb. f. Min. u. Geol. 1884. I. 196. 
 
 Magnetite. 
 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 67. 
 
 VELAIN. Descript. geol. d'Aden, Reunion, des iles St. Paul et Amsterdam. 
 Paris, 1877. 
 
 Meionite. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1866. XVIII. 608, 626. 633. 
 v. KALKOWSKY. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. 663. 
 
 . Melanite. 
 
 FOUQUE. Compt. rend. 15 mars 1875. 
 
 WICHMANN. Pogg. Ann. f. Phys. u. Chem. 1876. CLVII. 282. 
 
 KNOP. Gr. Zeitschr. f. Krystall. 1877. 58. 
 
 Melilith. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1866. XVIII. 527. 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1868. XX. 118. 
 
 Basaltgesteine. Bonn, 1870. 77. 
 
 HUSSAK. Sitzungsber. d. Wien. Akad. d. Wiss. April 1878. 
 STELZNER. N. Jahrb. f. Min. u. Geol. 1882. II. Beil.-Bd. 369. 
 
 Meroxene (Biotite). 
 
 TSCHERMAK. Sitzungsber. der Wien. Akad. der Wiss. 1869. May. LIX. 
 
 Gr. Zeitschr. f. Kryst. 1878. II. 18. 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 76. 
 
 - Ber. d. kgl. sachs. Ges. d. Wiss. July 21, 1875. 
 v. KALKOWSKY. Die Gneissformation d. Eulengebirges. Leipzig, 1878. 28. 
 
 N. Jahrb. f. Min. u. Geol. 1880. I. 33. 
 
BIBLIOGRAPHY TO PART II. 2O/ 
 
 KISPATIC. Tsch. Min. u. petr. Mitth. 1882. IV. 127. 
 WILLIAMS. N. Jahrb. f. Min. u. Geol. 1882. II. 616. 
 BECKER. N. Jahrb. f. Min. u. Geol. 1883. II. i. 
 
 Microcline (Microperthite). 
 
 DES CLOIZEAUX. Ann. de chim. et phys. 1876. 9. 433. 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1877. XXI. 274. 
 
 - Zeitschr. d. deutsch. geol. Ges. 1882. XXXIV. 12. 
 M. LEVY. Bull. d. la societ. miner. No. 5. 1879. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 196. 
 KOLLER. Tsch. Min. u. petr. Mitth. 1883. V. 218. 
 KLOOS. N. Jahrb. f. Min. u. Geol. 1884. II. 87. 
 
 Muscovite (Sericite). 
 
 LOSSEN. Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 509. 
 
 WICHMANN. Verh. des naturf. Ver. f. d. Rheinlande. XXXIV. 5. F. 4. Bd. 
 
 TSCHERMAK. Gr. Zeitschr. f. Kryst. 1878. 40. 
 
 v. LASAULX, N. Jahrb. f. min. u. Geol. 1872. 851. 
 
 v. GRODDECK. Jahrb. d. kk. geol. R.-Anst. Wien, 1883. 397. 
 
 Nepheline. 
 
 ZIRKEL. Pogg. Ann. f. Phys. u. Chem. 1867. 298. 
 
 - N. Jahrb. f. Min. u. Geol. 1868. 697. 
 Basaltgesteine. Bonn, 1870. 
 
 ROSENBUSCH. Nephelinit v. Katzenbuckel. Freiburg, 1869. Ref. N. Jahrb. f. 
 
 Min. u. Geol. 1869. 485. 
 BORICKY. Archiv. d. naturw. Landesdurchforsch. Bohmens. Prag, 1874. Die 
 
 Phonolithe. 8. 
 
 Nosean. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1862. XIV. 663. 
 ZIRKEL. Pogg. Ann. f. Phys. u. Chem. 1867. CXXXI. 312. 
 ROSENBUSCH. Nephel. v. Katzenbuckel. 1869. 35. 
 
 BORICKY. Archiv. d. naturw. Landesdurchforsch. Bohmens. Prag, 1873. Die 
 Basaltgesteine. 27. 
 
 - The same. 1874. Die Phonolithe. 10. 
 
 Oligoclase. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 100. 
 M. SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 164. 
 MUGGE. N. Jahrb. f. Min. u. Geol. 1881. II. 107. 
 
2O8 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 Oligoclasalbite. 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 159. 
 
 Olivine. 
 
 TSCHERMAK. Sitzungsber. d. Wien. Akad. d. Wiss. 1866. LIII. 260. 
 - Sitzungsber. d. Wien. Akad. d. Wiss. 1867. July. LVI. 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 55. 
 
 Zeitschr. d. deutsch. geol. Ges. 1871. 59. 
 
 HAGGE. Ueber Gabbro. Kiel, 1871. N. Jahrb. f. Min. u. Geol. 1871. 946. 
 
 ROSENBUSCH. N. Jahrb. f. Min. u. Geol. 1872. 59. 
 
 DATHE. N. Jahrb. f. Min. u. Geol. 1876. 225, 337. 
 
 PENCK. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. 97. 
 
 BROGGER. N. Jahrb. f. Min. u. Geol. 1880. II. 187. 
 
 COHEN. N. Jahrb. f. Min. u. Geol. 1880. II. 31, 52. 
 
 v. FOULLON. Tsch. Min. u. petr. Mitth. 1880. II. 181. 
 
 BECKER. Zeitschr. d. deutsch. geol. Ges. i8Si. XXXIII.' 31. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 322, 355, 450. 
 
 Tsch. Min. u. petr. Mitth. 1882. V. 163. 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 321. 
 KREUTZ. Tsch. Min. u. petr. Mitth. 1884. VI. 142. 
 
 Opal. 
 
 M. SCIIULTZE. Verh. d. naturf. Ver. d. preussischen Rheinlande u. West- 
 
 phalens. 1861. 69. 
 
 G. ROSE. Monatsber. d. Berlin. Akad. d. Wiss. 1869. 449. 
 BEHRENS. Sitzungsber. d. Wien. Akad. d. Wiss. 1871. LXIV. I. Abth. 
 VELAIN. Descript. geolog. d'Aden, Reunion . . . Paris, 1877. 32, 322. 
 KISPATIC. Tsch. Min. u. petr. Mitth. 1882. IV. 122. 
 
 Orthoclase (Sanidine). 
 
 REUSCH. Pogg. Ann. f. Phys. u. Chem. 1862. CXVI. 392, and 1863. CXVIII. 
 
 256. 
 ZIRKEL. Pogg. Ann. f. Phys. u. Chem. 1867. CXXXI. 300. 
 
 Sitzungsber. d. Wien. Akad. d. Wiss. 1863. XLVII. 237, 246. 
 
 N. Jahrb. f. Min. u. Geol. 1866. 775. 
 
 Zeitschr. d. deutsch. geol. Ges. 1867. XIX. 87. 
 LASPEYRES. Zeitschr. d. deutsch. geol. Ges. 1864. XVI. 392. 
 
 S. WEISS. Beitr. z. Kenntn. d. Feldspathbildung. Haarlem, 1866. 
 ROSENBUSCH. Verh. d. Naturf. Ver. Freiburg. VI. i, 95, 98, 103. 
 STRKNG. N. Jahrb. f. Min. u. Geol. 1871. 598. 
 
BIBLIOGRAPHY TO PART If. 2OQ 
 
 Ottrelite. 
 
 v. LASAULX. N. Jabrb. f. Min. u. Geol. 1872. 849. 
 TSCHERMAK u. SiPOcz. Gr. Zeitschr. f. Kryst. 1879. 509. 
 BECKE. Tsch. Min. u. petr. Mitth. 1878. I. 270. 
 
 RENARD et VALLEE POUSSIN. Ann. de la Soc. geol. Belgique. VI. Mem. 51. 
 N. Jahrb. f. Min. u. Geol. 1880. II. 149. 
 
 Perowskite. 
 
 BORICKY. Sitzungsber. der math.-naturw. Classe d. k. bohm. Ges. d. Wiss. 
 
 1876. Comp. Ref. N. Jahrb. f. Min. u. Geol. 1877. 539. 
 
 HUSSAK. Sitzungsber. d. Wien. Akad. d. Wiss. math.-nat. Classe. April 1878. 
 STELZNER. N. Jahrb. f. Min. u. Geol. 1882. II. Beil.-Bd. 390. 
 
 Phlogopite. 
 
 TSCHERMAK. Gr. Zeitschr. f. Kryst. 1878. 33. 
 
 Picotite. 
 
 ZIRKEL. Basaltgesteine. Bonn, 1870. 97. 
 
 STELZNER. N. Jahrb. f. Min. u. Geol. 1882. II. Beil.-Bd. 393. 
 
 Finite (and other decomposition-products of Cordierite). 
 WICHMANN. Zeitschr. d. deutsch. geol. Ges. 1874. XXVI. 675. 
 
 Plagioclase. 
 
 TSCHERMAK. Sitzungsber. d. Wien. Akad. d. Wiss. L. Dec. 1864. 
 WEISS. Beitr. z. Kenntn. d. Feldspathbildung. Haarlem, 1866. 
 ROSE. Zeitschr. d. deutsch. geol. Ges. XIX. 1867. 289. 
 STELZNER. N. Jahrb. f. Min. u. Geol. 1870. 784. 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1871. XXIII. 43, 59, 94. 
 - Basaltgesteine. Bonn, 1870. 28. 
 HAGGE. Ueber Gabbro. Kiel, 1871. 
 STRENG. N. Jahrb. f. Min. u. Geol. 1871. 598, 715. 
 COHEN. N. Jahrb. f. Min. u. Geol. 1874. 460. 
 v. RATH. Monatsber. d. Berlin. Akad. d. Wiss. 24. Feb. 1876. 
 ROSENBUSCH. Verh. d. naturforsch. Ges. Freiburg i. Br. VI. I, 77. 
 PENCK. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. 97. 
 PFAFF. Sitzungsber. d. phys.-med. Societ. z. Erlangen. 1878. 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 117. 
 Tsch. Min. u. petr. Mitth. 1882. V. 189. 
 
210 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 HOEPFNER. N. Jahrb. f. Min. u. Geol. 1881. II. 164. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 253. 
 KLOCKMANN. Zeitschr. d. deutsch. geol. Ges. 1882. 373. 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1883. II. 97. 
 KREUTZ. Tsch. Min. u. petr. Mitth. 1884. VI. 137. 
 
 Pleonaste. 
 TELLER u. JOHN. Jahrb. d. kk. geol. R.-Anst. Wien, 1882. XXXII. 589. 
 
 Protobastite (Diaclasite). 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871. i. Heft. 20. 
 STRENG. N. Jahrb. f. Min. u. Geol. 1872. 273. Anm. 2. 
 
 Pyrope. 
 
 DOELTER. Tsch. Min. Mitth. 1873. 13. 
 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 321 and 1884. II. 21. 
 
 Quartz. 
 
 H. CLIFTON SORBY. Quart. Journ. geol. Soc. Nov. 1858. XIV. 453. 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1868. 711. 
 
 Pogg. Ann. f. Phys. u. Chem. 1871. CXXXXIV. 324. 
 ROSENBUSCH. Reise n. Siidbrasilien. Freiburg i. Br., 1870. 
 BEHRENS. N. Jahrb. f. Min. u. Geol. 1871. 460. 
 
 LEHMANN. Verh. d. niederrhein. Ges f. Nat. u. Heilkunde. Bonn, 1874. XXXI. 
 
 Verh. d. naturhist. Ver. d. preuss. Rheinlande u. Westphalens. 1874. XXXIV. 
 v. CHRUSTSCHOFF. Tsch. Min. u. petr. Mitth. 1882. IV. 473. 
 
 Archiv d. naturw. Landesdurchf. Bohmens. 1882. IV. No. 4. 12. 
 
 Ripidolite (Chlorite, Helminth). 
 O. MEYER. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. i, 24. 
 
 Rubellan. 
 HOLLRUNG. Tsch. Min. u. petr. Mitth. 1883. V. 304. 
 
 Ruffle. 
 
 SAUER. N. Jahrb. f. Min. u. Geol. 1879. 5 6 9- 
 
 N. Jahrb. f. Min. u. Geol. 1880. I. 227, 279. 
 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1880. II. 281. 
 
BIBLIOGRAPHY TO PART II. 211 
 
 CATHREIN. N. Jahrb. f. Min. u. Geol. 1881. I. 169. 
 
 - Gr. Zeitschr. F. Kryst. 1883. VIII. 321. 
 
 H. GYLLING. N. Jahrb. f. Min. u. Geol. 1882. I. 163. 
 
 PICHLER u. BLAAS. Tsch. Min. u. petr. Mitth. 1882. IV. 513. 
 
 SANDBERGER. N. Jahrb. f. Min. u. Geol. 1882. II. 192. 
 
 v. LASAULX, Gr. Zeitschr. f. Kryst. 1883. VIII. 54. 
 
 Serpentine (comp. Olivine). 
 
 WEBSKY. Zeitschr. d. deutsch. geol. Ges. 1858. 277. 
 WEISS. Pogg. Ann. f. Phys. u. Chem. 1863. CXIX. 458. 
 TSCHERMAK. Sitzungsber. d. Wien. Akad. d. Wiss. LVI. July 1867. 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1870. 829. 
 J. ROTH. Abhandl. d. Berlin. Akademie d. Wiss. 1869. 
 DRASCHE. Tsch. Min. Mitth. 1871. i. 
 WEIGAND. Tsch. Min. Mitth. 1875. 183. 
 DATHE. N. Jahrb. f. Min. u. Geol. 1876. 225, 337. 
 LEMBERG. Zeitschr. d. deutsch. geol. Ges. 1877. XXX. 457. 
 BECKE. Tsch. Min. u. petr. Mitth. 1878. I. 459, 470. 
 - Tsch. Minn. u. petr. Mitth. 1882. IV. 322. 
 HARE. Serpentin von Reichenstein. In.-Diss. Breslau, 1879. 
 HUSSAK. Tsch. Min. u. petr. Mitth. 1882. V. 61. 
 SCHRAUF. Gr. Zeitschr. f. Kryst. 1882. 321. 
 SCHULZE. Zeitschr. d. deutsch. geol. Ges. 1883. XXXV. 433. 
 
 Sillimanite. 
 
 v. KALKOWSKY. Die Gneissform. d. Eulengebirges. Leipzig, 1878. 
 SCHUMACHER. Zeitschr. d. deutsch. geol. Ges. 1878. 427. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 189. 
 
 Scapolite. 
 
 MICHEL Llvv. Bull. Soc. miner. France. 1878. No. 3 and 5. 
 
 BECKE. Tscherm. Min. u. petr. Mitth. 1882. IV. 369. 
 
 TORNEBOHM. Geol. FQren. i Stockholm Forhandl. VI. 185. Comp. Ref. N. 
 
 Jahrb. f. Min. u. Geol. 1883. I. 245. 
 CATHREIN. G. Zeitschr. f. Kryst. 1884. IX. 378. 
 
 Sodalite. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1866. 620. 
 
 Verh. d. niederrhein. Ges. f. Nat. u. Heilkunde. 1876. 82. 
 
 VRBA. Sitzungsber. d. Wien. Akad. d. Wiss. LXIX. Feb. 1874. 
 
212 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 v. KALKOWSKY. Zeitschr. d. deutsch. geol. Ges. 1878. 663. 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1880. II. 141. 
 KOCH. N. Jahrb. f. Min. u. Geol. 1881. I. Beil.-Bd. 149. 
 
 Staurolite. 
 
 PETERS u. MALY. Sitzungsber. d. Wien. Akad. d. Wiss. LVII. 1868. 15. 
 
 v. LASAULX. Tsch. Min. u. petr. Mitth. 1872. III. 173, and N. Jahrb. f. Min. 
 
 u. Geol. 1872. 838. 
 O. MEYER. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. i. 
 
 Talc. 
 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1872. 823. 
 TSCHERMAK. Tsch. Min. Mitth. 1876. I. 65. 
 
 Titanite. 
 
 ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1859. XI. 522, 526. 
 
 Pogg. Ann. f. Phys. u. Chem. 1867. CXXXI. 325. 
 
 v. RATH. Zeitschr. d. deutsch. geol. Ges. 1862. XIV. 665. 
 
 Zeitschr. d. deutsch. Geol. Ges. 1864. XVI. 256. 
 GROTH. N. Jahrb. F. Min. u. Geol. 1866. 46. 
 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1872. 362. 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1880. II. 159. 
 MANN. N. Jahrb. f. Min. u. Geol. 1882. II. 200. 
 DILLER. N. Jahrb. f. Min. u. Geol. 1883. I. 187. 
 
 Titaniferous Magnetite. 
 
 v. WERVEKE. N. Jahrb. f. Min. u. Geol. 1880. II. 141. 
 CATHREIN. Gr. Zeitschr. f. Kryst. 1883. VIII. 321. 
 
 Tremolite (Grammatite). 
 
 TSCHERMAK. Tsch. Min. Mitth. 1871. 37. and 1876. 65. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. IV. 338. 
 
 Tridymite. 
 
 ZIRKEL. Pogg. Ann. f. Phys. u. Chem. 1870. CXL. 492. 
 v. LASAULX. N. Jahrb. f. Min. u. Geol. 1869. 66. 
 
 Gr. Zeitschr. f. Kryst. 1878. II. 254. 
 STRENG. Tsch. Min. Mitth. 1871. 47. 
 
 N. Jahrb. f. Min. u. Geol. 1872. 266. 
 
BIBLIOGRAPHY TO PART II. 21$ 
 
 ROSENBUSCH. Verhandl. d. naturf. Ges. Freiburg i. Br. 1873. VI. I. Hft. 96. 
 SCHUSTER. Tsch. Min. u. petr. Mitth. 1878. 71. 
 
 Tourmaline. 
 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1875. 628. 
 
 TORNEBOHM. Geol. Foren. i. Stockholm Forhandl. 1876. III. 218. 
 
 MEYER. Zeitschr. d. deutsch. geol. Ges. 1878. XXX. i, 24. 
 
 WICHMANN. N. Jahrb. f. Min. u. Geol. 1880. II. 294. 
 
 DATHE. Zeitschr. d. deutsch. geol. Ges. 1882. XXXIV. 12. 
 
 PICHLER u. BLAAS. Tsch. Min. u. petr. Mitth. 1882. IV. 512. 
 
 Uralite. 
 
 G. ROSE. Reise nach dem Ural. II. 371. 
 BECKE. Tsch. Min. u. petr. Mitth. 1882. V. 157. 
 
 Viridite (Delessite, Chlorophaeite). 
 
 VOGELSANG. Zeitschr. d. deutsch. geol. Ges. 1872. XXIV. 529. 
 
 KOSMANN. Verh. d. naturw. Ver. d. preuss. Rheinlande u. Westph. XXV. 239. 
 
 and 289. 
 
 TSCHERMAK. Die Porphyrgesteine Oesterreichs. Wien, 1869. 42, 66, 134. 
 Tsch. Min. Mitth. 1872. 112. 
 
 Wollastonite. 
 
 FOUQUE. Compt. rend. 15 Mar. 1875. 
 
 LAGdRio. Andesite d. Kaukasus. Dorpat, 1878. Ref. N. Jahrb. f. Min. u. 
 
 Geol. 1880. I. 209. 
 
 CH. WHITMAN CROSS. Tsch. Min. u. petr. Mitth. 1881. III. 373. 
 T8RNEBOHM. Geol. Foren. i. Stockholm Forh. 1883. VI. No. 12. 542. Comp. 
 
 Ref. N. Jahrb. f. Min. u. Geol. 1884. I. 230. 
 
 Zeolite (Analcime). 
 
 ROSENBUSCH. Nephelinit v. Katzenbuckel. Freiburg i. Br., 1869. 
 KLOOS. N. Jahrb. f. Min. u. Geol. 1884. III. Beil.-Bd. 37. 
 
 Zircon. 
 
 SANDBERGER. Wurzburger nat. Zeitschr. 1866/67. VI. 128 and 1883. 
 
 Zeitschr. d. deutsch. geol. Ges. 1883. XXXV. 193. 
 
 N. Jahrb. f. Min. u. Geol. 1881. I. 258. 
 
214 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 ZIRKEL. N. Jahrb. f. Min. u. Geol. 1875. 628. 
 
 N. Jahrb. f. Min. u. Geol. 1880. I. 89. 
 
 TORNEBOHM. Geol. Fohren. i Stockholm Forhandling. 1876. III. No. 34. and 
 
 N. Jahrb. f. Min. u. Geol. 1877. 97. 
 
 MICHEL LEVY. Bull. Soc. mineral. France. 1877. No. 5. 77. 
 ROSENBUSCH. Sulla presenza dello zircone nelle roccie. Atti d. R. Accadern. d. 
 
 Science. Torino 1881. Vol. XVI. 
 BECKE. Tsch. Min. u. petr. Mitih. 1882. IV. 204. 
 FLETCHER. Gr. Zeitschr. f. Kryst. 1882. 80. 
 NESSIG. Zeitschr. d. deutsch. geol. Ges. 1883. XXXV. 118. 
 v. CHRUSTSCHOFF. Tsch. Min. u. petr. Mitth. 1884. VI. 172. 
 
 Zoisite. 
 
 RIESS. Tsch. Min. u. petr. Mitth. 1878. I. 188. 
 
 BECKE. Tsch. Min. u. petr. Mitth. 1878. I. 249. and 1882. IV. 312. 
 
EXPLANATIONS OF CUTS ACCOMPANYING 
 PART II. 
 
 FIG. PAGE 
 
 51 ILMENITE. Grain, partly decomposed into leucoxene, with undecom- 
 
 posed earthy filaments interlaminated in 
 
 52 OPAL. As filling of a cavity, in concentric layers, inclosing small 
 
 groups of tridymite tablets. (After Fouque.) 112 
 
 53 HAUYN. Cross section with opacitic border and vitreous inclosures; 
 
 penetrated by a network of black lines crossing each other at right 
 angles 115 
 
 54 a. MELANITE cross section, zonally developed 117 
 
 b. ALMANDINE GRAIN, with inclosures of quartz-grains ; traversed 
 
 by irregular cleavage fissures 117 
 
 55 PYROPE GRAIN (P) with border of so-called kelyphite (K). From the 
 
 serpentine (S) from Kremse, Bohemian forest. On the serpentine 
 portion (S) showing the "mesh-structure" is a thin layer of fresh 
 olivine grains, followed by the fibrous metamorphosed zone (A") of 
 pyrope; this has been called kelyphite by Schrauf, and is a " pyro- 
 gene" product, although regarded by others as an " hydatogene" pro- 
 duct, and has been regarded as allied to an augitic mineral 117 
 
 56 PEROWSKITE GRAINS in the so-called "hacked" figures. (After Stelz- 
 
 ner.) 120 
 
 57 LEUCITE cross-section in polarized light, showing the polysynthetic 
 
 striation. (After Zirkel.) 122 
 
 58 Cross sections of small LEUCITE crystals and grains (constituents 
 
 II. order), with vitreous inclosures regularly distributed 123 
 
 59 RUTILE CRYSTAL. Knee-, heart-shaped, and polysynthetic twins. 
 
 (After Reusch.) 122 
 
 60 ZIRCON CRYSTALS. (After Fouque.) 124 
 
 61 SCAPOLITE cross-section, at right angles to the chief axis, with rectan- 
 
 gular cleavage 124 
 
2l6 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 FIG PAGE 
 
 62 MELILITE. a. Cross-sections parallel to the chief axis. The upper 
 
 shows a separation into fine fibres and cleavage-fissures parallel #/*; 
 the under, the so-called " pflock-structure," pear-shaped and spin- 
 dle-shaped canals originating from the face oP, which appear as a 
 small circle in sections (parallel) oP (Fig. 62, b}. Fig. 62, c, shows a 
 larger cross-section of an irregular grain, wherein small leucite 
 grains are developed, (a and b after Stelzner.) 127 
 
 63 QUARTZ, a d are cross-sections of the conchoidal crystal skele- 
 
 ton, which occur interpenetrated with orthoclase " micropegmatitic." 
 a. Section parallel to the chief axis. b. Section parallel to the 
 base. c. Section at right angles to the prismatic edges, d. Section 
 inclined to the same. (After Fouque.) e. Cross-section of an ortho- 
 clase wherein quartz is developed micropegmatitic 129 
 
 64 TRIDYMITE. Crystal groups of thin hexagonal tablets overlapping 
 
 each other like roof-tiles. (After Fouque.) 131 
 
 65 CALCITE GRAIN, with rhombohedral cleavage and twinning striations. 
 
 After \R 132 
 
 66 NEPHELINE. a. Transverse section, b. Longitudinal section, with 
 
 augitic inclosures zonally distributed 134 
 
 67 APATITE, a. Transverse section, b. Longitudinal section, with 
 
 cleavage-fissures and acicular inclosures parallel to the base 137 
 
 68 TOURMALINE, a. Longitudinal section, b. Transverse section zo- 
 
 nally developed 138 
 
 69 TOURMALINE CRYSTAL. (After Reusch.) 138 
 
 70 OLIVINE cross-section in different degrees of decomposition, a. With 
 
 undecomposed centre, b. " Serpentinized " only on the edges and 
 cleavage-fissures 141 
 
 71 OLIVINE cross-section, a. Cross section parallel oP. b. Cross-sec- 
 
 tion parallel oo P oo. (After Fouque.) 140 
 
 72 SILLIMANITE. a. Transverse section, b. Long, broken needle, with 
 
 transverse fissures 142 
 
 73 STAUROLITE. Twin with inclosures of quartz granules ; the -f- sign 
 
 annexed indicates the position of the directions of vibration in the 
 individual which is hatched 142 
 
 74 ENSTATITE and BRONZITE transverse sections, a. Optical orienta- 
 
 tion according to Tschermak's position, b. According to G. v. 
 Rath's position 144 
 
 75 ENSTATITE longitudinal section, with the cleavage fissures parallel to 
 
 the vertical axis partially decomposed into bastite 145 
 
EXPLANATIONS OF CUTS ACCOMPANYING PART 77. 
 
 FIG. PAGE 
 
 76 ANDALUSITE cross-sections, a. Transverse section with rectangular 
 
 cleavage-cracks, and opaque granules distributed centrally and in a 
 cross-shape. (Similar to chiastolite.) 153 
 
 77 CORDIERITE GRAIN, with a fibrous decomposition on the cleavage- 
 
 cracks, with inclosures of sillimanite needles 155 
 
 78 Transverse section of a twinned cordierite crystal. The apparently 
 
 hexagonal crystal, composed of three individuals, divides into six 
 fields in polarized light, two of which lying opposite extinguish to- 
 gether ; the position of the directions of vibration is designated by 
 a mark 154 
 
 79 ZOISITE cross-sections, a. Transverse section, b. Longitudinal sec- 
 
 tion, showing cleavage- fissures and fluid inclosures arranged in a 
 series 155 
 
 80 BIOTITE leaflet, parallel oP; the outer portions are decomposed into 
 
 chlorite and contain earthy granules and epidote needles ; the irregu- 
 larly defined kernel is fresh 156 
 
 81 BIOTITE longitudinal section, showing cleavage-cracks parallel oP 
 
 and inclosures of calcite lenses 156 
 
 82 OTTRELITE. Section at right angles to oP, twinned polysynthetically 
 
 after oP. The annexed -f- indicates the position of the directions of 
 vibration 164 
 
 83 SANIDINE cross-sections, a = parallel oP or <x> P co. b = Carlsbad 
 
 twin, c = Baveno twin, d = parallel oo^co with a combination 
 
 of oP . COP co . 2P oo. e = parallel coP oo. (After Rosenbusch.). . . 166 
 
 84 AfJGiTE cross-section, a. At right angles to the vertical axis. b. 
 
 Parallel to the orthopinacoid. c. Parallel to the clinopinacoid. 
 (After Fouque.) 168 
 
 85 URAI.ITE cross-section. The seconary hornblende is partially de- 
 
 veloped over the augite, with a twin lamella after co/'co. (After 
 Becke.) 175 
 
 86 HORNBLENDE cross section. a. Transverse section. b. Parallel 
 
 GO^PCO. c. Parallel co P oo. (After Fouque.) 172 
 
 87 EPIDOTE. Optical orientation. (After Klein and v. Lasaulx.) Opt. 
 
 A. = optic axes (for red and green), I. a first negative middle line, 
 II. c = second middle line, b = b optic normals, a. Clinodiagonal 
 and one direction of cleavage, c. Vertical axis 176 
 
 88 EPIDOTE twin after <x>P co. (After Reusch.) 176 
 
 89 EPIDOTE CRYSTAL. (After Reusch.) 176 
 
218 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 FIG. PAGE 
 
 90 TITANITE. Cross-section of crystals and grains ; simple individuals 
 
 and twins after oP 176 
 
 91 MICROCLINE. Section parallel oP shows the latticed twinning stria- 
 
 tions and lenticular albite developed within, with polysynthetic 
 striations also 180 
 
 92 MICROCLINE from Lampersdorf, Silesia. (After A. Beutell.) Section 
 
 parallel oP. The microcline is in part homogeneous, in part shows 
 the latticed structure ; the larger albite bands run parallel to the 
 edge oP : P co and show fine twinnings striation parallel oP : co p oo . . 180 
 
 93 MICROPERTHITE. a = section parallel to the separation-plane cor- 
 
 responding OOP co, shows a peculiar network composed of filaments 
 refracting light powerfully, crossing each other at right angles. 
 b = section parallel oP, c parallel co/ co, both with entered poly- 
 synthetically twinned albite lamellae. (After Becke.) 180 
 
 94 Plagioclase crystal showing the position of the obtuse edge P/M, 
 
 and the bearing of the directions of extinction toward them. (After 
 
 Schuster.). 182 
 
 195 PLAGIOCLASE. Cross-section parallel M(<nP<x). Right longitudinal 
 plane (co P oo) of a crystal correctly oriented. (Compare Fig. 94.) 
 
 The obtuse edge P : M lies above 182 
 
 96 PLAGIOCLASE. Cross-section parallel P(oP). Upper terminal plane 
 
 (oP) of an oriented crystal; the obtuse edge P : M lies to the right. 182 
 97-101^. Interference-figures of PLAGIOCLASE on cleavage-leaflets par- 
 allel M and P. They have reference to the upper oP- and right 
 oo P co-planes of an oriented crystal (Fig. 94), and are all in the same 
 position as Figs. 95 and 96. 
 
 Fig. 97 Albite, parallel M( co P oo) 182 
 
 98 Oligoclase, parallel M( oo P oo) 184 
 
 gga Labrador, parallel M(<x> P oo) 186 
 
 99< Labrador, parallel P(op) 186 
 
 loofl Bytownite, parallel M(ao P co) 186 
 
 ioo<5 Bytownite, parallel P(oP) 186 
 
 Anorthite, parallel M(<a P co) 188 
 
 Anorthite, parallel P(oP) ; . . 188 
 
 (Figs. 95-101 after Schuster.) 
 
 102 ANDESINE. Cross-section parallel oP. Zonally developed. (After 
 
 Becke.) 185 
 
 103 AGGREGATES of acicular zeolite crystals and concentric-conchoidal car- 
 
 bonates as cavity-deposits , 195 
 
CUTS ACCOMPANYING PART II. 
 
 FIG. 51. 
 
 FIG. 52. 
 
 FIG. 54. 
 
220 DETERMINATION OF RQCK-FORMING MINERALS. 
 
 CUTS ACCOMPANYING PART \\.-Continutd. 
 
 <> 
 
 FIG. 56. 
 
 FIG. 57. 
 
 FIG. 58. 
 
 FIG. 61. 
 
CUTS ACCOMPANYING PART II. 
 
 221 
 
 CUTS ACCOMPANYING PART II. Continued. 
 
 
 
 FIG. 63. 
 
 FIG. 64. 
 
 FIG. 65. 
 
 a 6 
 
 FIG. 66. 
 
 FIG. 67. 
 
 FIG. 68. 
 
 FIG. 69. 
 
 FIG. 70. 
 
222 DETERMINATION OF ROCK-FORMING MINERALS. 
 CUTS ACCOMPANYING PART II. Continued. 
 
 w 
 
 ^< A> 
 
 /p 
 
 a 
 
 FIG. 71. 
 
 FIG. 72. 
 
 df 
 
 ,P 
 
 
 
 ? 
 
 *v 
 
 sta 
 
 FIG. 74. 
 
 FIG. 73. 
 
 FIG. 75. 
 
 FIG. 76. 
 
 FIG. 77- 
 
CUTS ACCOMPANYING PART II. 
 CUTS ACCOMPANYING PART \\.-Continued. 
 
 223 
 
 FIG. 84. 
 
224 DETERMINATION OF ROCK-FORMING MINERALS. 
 CUTS ACCOMPANYING PART \\.-Continued. 
 
 1 
 
 FIG. 85. 
 
 FIG. 86. 
 
 FIG. 87. 
 
 FIG. 88. 
 
 FIG. 
 
CUTS ACCOMPANYING PART II. 
 CUTS ACCOMPANYING PART II. Continued. 
 
 225 
 
 FIG. 91. 
 
 FIG. 93. 
 
 FIG. 94. 
 
226 DETERMINATION OF ROCK-FORMING MINERALS. 
 
 CUTS ACCOMPANYING PART II. Continued. 
 
 FlG. 95. 
 
 FIG. 96. 
 
 FIG. 97. 
 
 FIG. 98. 
 
 FIG. gga. 
 
 FIG. 99 b. 
 
CUTS ACCOMPANYING PART II. 
 
 227 
 
 CUTS ACCOMPANYING PART \\.-Conelu4etL 
 
 FIG. ioo a. 
 
 FIG. ioo b. 
 
 FIG. ioi a. 
 
 FIG. ioi b. 
 
 FIG. 102. 
 
 FIG. 103. 
 
INDEX. 
 
 Aegerine *97 
 
 Aggregate 188 
 
 Acmite i? 2 . *97 
 
 Actinolite i?4. J 97 
 
 Albite 182, 198 
 
 Almandine 116, 198 
 
 Ammonium-magnesium Phosphate 62 
 
 Amorphous Minerals 112 
 
 f Behavior of, in polarized light.iy, 30, 
 
 106 
 
 Analcime 118, 198 
 
 Analyzer 8 
 
 Anatase 124 
 
 Andalusite 152, 198 
 
 Andesine 184, 198 
 
 Anisotrope 106 
 
 Anomite 158, *99 
 
 Anorthite 188, 199 
 
 Anthophyllite 22, 146, 199 
 
 Apatite 53,136, 199 
 
 Arragonite *94 
 
 Arfvedsonite 174, *99 
 
 Augite, Ordinary and basaltic 168, 199 
 
 optical orientation of 24, 28 
 
 shell-formed structure of 91, 92 
 
 cleavage of 84 
 
 interpenetrations of 93 
 
 twins 4 1 
 
 Axes of elasticity, Determination of the 
 
 position of 26 
 
 Axial plane, Determination of the posi- 
 tion of the optic 33 
 
 Axial colors, Determination of the 45 
 
 B 
 
 Backinger 2O 3 
 
 Barrois 201 
 
 Barium-mercury solution 75 
 
 Bastite 22, 150, 192, 200 
 
 PAGE 
 
 Becke 92, 164,197-211,213, 214 
 
 Becker 202, 204, 207, 208 
 
 Behrens 51,59,200,208,210 
 
 Belonite 87 
 
 Bertrand 7 
 
 Biotite 3 6 > 89, 156 
 
 Bitumen ; " 
 
 Blaas.. 205, 211, 213 
 
 Bode-wig 203 
 
 Bohm 198 
 
 Boricky 5*. 55, 207-210 
 
 Bourgeois 5 x 
 
 Brogger 208 
 
 Bronzite 22, 146, 200 
 
 Biickrng 200 
 
 Biitschly *99 
 
 Bytownite 186, 200 
 
 Cadmium-boro-tungstate solution 73 
 
 Caesium alum 63 
 
 Calcite 38, 132, 200 
 
 Calcite plate 12 
 
 Calderon 7 
 
 Double-plate 12 
 
 Calderon y Arana 201 
 
 Carinthine *7 2 
 
 Cancrinite 136, 200 
 
 Cathrein 76, 210-212 
 
 Centring adjustment on microscope 13 
 
 Chabazite *94 
 
 Chalcedony 192, 200 
 
 Chemical investigation. Methods of 50 
 
 Chiastolite 152, 201 
 
 Chlorite 162 
 
 Chloritoid 162,201 
 
 Chlorophaeite 192. 213 
 
 Chromite 110.118. 201 
 
 Chrustschoff. 99, 205, 210, 213 
 
 Chnochlore l6a 
 
230 
 
 INDEX. 
 
 PAGE 
 
 Cohen i, 66, 81, 85, 197, 205, 208, 209 
 
 Condenser 10 
 
 Cordierite 40,47,154,201 
 
 Corrosion of the rock-forming minerals. . 88 
 
 Corundum 138 
 
 Couseranite 126, 201 
 
 Cross. ..' 198, 201, 204, 205, 213 
 
 Crystal formation, Disturbances in the.. 87 
 
 Crystallites 86 
 
 Crystallization, Determination of the sys- 
 tems of 106 
 
 Crystalloids 86 
 
 Cyanite 178, 203 
 
 Dathe 198-202, 207, 208, 211-213 
 
 Decomposition of the rock-constituents. . 101 
 
 Delessite 192, 213 
 
 Dus Cloizeaux 207 
 
 Determination of the system of crystal- 
 lization of the rock-forming minerals. . . 106 
 
 Diaclasite 15. 2I 
 
 Diallage 170, 201 
 
 Dichroite 154 
 
 Diller 212 
 
 Diopside 170, 202 
 
 Dipyr 126, 201 
 
 Dispersion of the optic axes 36 
 
 Disthene 29, 178, 202 
 
 Doelter 79, 204, 210 
 
 Dolomite 132, 202 
 
 Double-refracting minerals in parallel 
 
 polarized light 17 
 
 Double-refraction, Determination of the 
 
 character of 31, 34 
 
 v. Drasche 198-200, 202, 211 
 
 Eisen-kies 108 
 
 Elaeolite 134, 203 
 
 Enstatite 21, 144, 203 
 
 Epidote 24, 42, 176, 203 
 
 Extinction, Oblique 25 
 
 , Parallel 19 
 
 Feldspar, Shell-formed structure of 91 
 
 , Decomposition of 102 
 
 Fisher . . . 197 
 
 Fletcher 213 
 
 Fluid inclosures 94, 95 
 
 PAGE 
 
 Fluor-spar (Fluorite) 120, 203 
 
 Form of occurrence of the rock-compo- 
 nents 81 
 
 v . Foullon 208 
 
 Fouque i, 66, 76, 79, 81, 197, 205, 206, 213 
 
 Fourth-undulation mica 13 
 
 Gas-pores 94 
 
 Gastaldite 194 
 
 Gisevius 67 
 
 Glasmasse 112 
 
 Glaucophane 174, 203 
 
 Globulite 86 
 
 Goldschmidt 66, 201 
 
 Garnet 116 
 
 Graphite no, 204 
 
 v. Groddeck ' 207 
 
 Groth 1 6, 45, 212 
 
 Green earth 192 
 
 Giiutbel 205 
 
 Gylling 211 
 
 Gypsum 178, 204 
 
 H 
 
 Hagga 199, 202, 204, 205, 208, 
 
 Hague 
 
 Hammerschntidt 
 
 Haradu's apparatus 
 
 Hare... 
 
 Hausmann ' 
 
 Hauyn 54, 114, 
 
 Heating apparatus 
 
 Helminth 162, 
 
 Hematite no, 138, 
 
 Hercynite 1 18, 
 
 Hexagonal minerals 106, 
 
 , Behavior of, in polarized light. 18 
 
 Hopfner 
 
 Hollrung 
 
 Hornblende, Cleavage of 
 
 , Ordinary and basaltic 
 
 , Opacitic border of 
 
 , Optical orientation of ... 24, 
 
 Humboldilith 
 
 Hussak .' 201, 206, 209, 
 
 Hydrofluosilicic acid 
 
 Hypersthene 148, 
 
 , Inclosures in 
 
 , Optical orientation of 
 
 , Pleochroism of 
 
 209 
 
 205 
 
 204 
 
 70 
 
 211 
 
 20 3 
 204 
 
 15 
 210 
 204 
 204 
 128 
 
 2 !O 
 2 TO 
 
 I 7 2 
 8 9 
 
 126 
 
INDEX. 
 
 231 
 
 hidings 205 
 
 Ilmenite no 
 
 Index of refraction, Determination of. 14, 44 
 Inclosures of the rock-forming minerals.. 93 
 
 of gases 94 
 
 of fluids . v 95 
 
 of vitreous particles 97 
 
 of foreign minerals 99 
 
 Inostranzeff 200, 202 
 
 Interference-figures 32 
 
 Interpenetration of the rock-constituents 93 
 
 Investigation, Optical methods of 16 
 
 , Chemical methods of 50 
 
 Isotrope 106 
 
 J 
 
 Jeremejeff. 198 
 
 v. John 198, 203, 205, 210 
 
 K 
 
 v. Kalko-wsky 202-204, 206, 211, 212 
 
 Kaemmente 162 
 
 Kispatlc 208 
 
 C. Klein 7 
 
 D. Klein 66, 73 
 
 Klein s solution 73 
 
 KlockinanM 210 
 
 Kloos 202, 204, 207, 213 
 
 Knop 54, 206 
 
 v. Kobell 202 
 
 Koah ..197,200,203,212 
 
 Koenig 199 
 
 Koller 207 
 
 Kosjnann 204, 213 
 
 Krenner 205 
 
 Kreutz 199, 205, 208, 210 
 
 Kiich 203 
 
 L 
 
 Labradorite 186, 205 
 
 Lagorio 199, 200, 213 
 
 v. Lasaulx.. .7, 198, 201-204, 207, 209, 211-213 
 
 Lasfieyres 7, 45, 49, 203, 205, 208 
 
 L.ehmann 210 
 
 Lemberg 210, 201, 211 
 
 Leucite 120, 122, 205 
 
 Liebenerite 136, 206 
 
 L, icbisch 7 
 
 Longulite 86 
 
 Lessen 198,207 
 
 L uedecke 203 
 
 M 
 
 Magnesite 132, 206 
 
 Magnetite 108, 206 
 
 Magnet-kies no 
 
 Maly 212 
 
 Mann 197, 212 
 
 Margarite 86 
 
 Measurement of angles 83 
 
 Mechanical separation of the rock-forming 
 
 minerals 66 
 
 by means of solutions of high 
 
 specific gravity 67 
 
 by the solution of the iodides of 
 
 barium and mercury 75 
 
 by the solution of the iodides of 
 
 potassium and mercury 6,7 
 
 by the solution of cadmium 
 
 boro-tungstate 73 
 
 by means of the electro-magnet. 79 
 
 by means of acids 76 
 
 , Apparatus for 70 
 
 Meionite 124, 206 
 
 Melanite 116, 206 
 
 Mellilith 126, 206 
 
 Meroxene 156,206 
 
 Meyer 200, 202, 210-213 
 
 Michel Le"vy ...i, 28, 44, 51, 66, 76, 81, 197, 207, 
 
 211, 214 
 
 Microchemical Methods 51 
 
 of Boricky 55 
 
 of Behrens 59 
 
 Microchemical reactions with Aluminium 62 
 
 Barium 63 
 
 Boron 65 
 
 Calcium 57, 60 
 
 Chlorine 64 
 
 Fluorine 64 
 
 Iron 57, 63 
 
 Lithium 57, 63 
 
 Magnesium 57, 62 
 
 Manganese 58, 63 
 
 Phosphorus 64 
 
 Potassium 56, 60 
 
 Silicon 65 
 
 Sodium 56, 61 
 
 Strontium 58, 63 
 
 Sulphur 64 
 
 Water ... .66 
 
 Microdine 180, 207 
 
 Microlites 85,86,207 
 
 Micrometer 14 
 
232 
 
 INDEX. 
 
 PAGE 
 
 Micropegmatite 93 
 
 Microperthite 180 
 
 Microscope 7 
 
 Monochnic minerals 107 
 
 , Behavior of, in pol. light.. 2?, 36, 40 
 
 Morphological properties of^ the rock- 
 forming minerals 81 
 
 Miiggt i97, 207 
 
 Miiller 198,201 
 
 Muscovite 36,160,207 
 
 N 
 
 Natrolite 194 
 
 Nepheline 53, 134, 207 
 
 Nessig 314 
 
 Pfiedzwiedzky 205 
 
 Nigrine 122 
 
 Non-pellucid minerals 108 
 
 Nosean 114, 207 
 
 Ocular micrometer 14 
 
 ( ^ebbeke 66, 76 
 
 Oligoclase 184, 207 
 
 Albite 182,208 
 
 Olivine 45, 89, 140, 208 
 
 , Decomposition of 101 
 
 Omphacite 170, 202 
 
 Opacitic border 89 
 
 Opal 112, 208 
 
 Optically-uniaxial minerals 18, 30, 46, 106 
 
 Optically-biaxial minerals 20, 32, 47. 107 
 
 Orthoclase 25, 42, 164, 208 
 
 Oschatz 200 
 
 Ottrelite 164, 209 
 
 P 
 
 Penck 208, 209 
 
 Penninite 162 
 
 Perowskite 120, 209 
 
 Peters 212 
 
 Pfaff 209 
 
 Phlogopite 158. 209 
 
 Pichler 211, 213 
 
 Picotite 118, 209 
 
 Finite 154, 209 
 
 Plagioclase 209 
 
 , Shell-formed structure of 91 
 
 , Twins of 43, 44 
 
 Pleochroism 45 
 
 Pleonaste no, 118, 210 
 
 Pohlig 198, 201 
 
 Polarization-microscope 7, 8 
 
 I'AGE 
 
 Polarizer 7 
 
 Potassium fluo-borate 61 
 
 Mercury solutipri 67 
 
 Platinum chloride 61 
 
 Preparation of microscopical sections 3 
 
 Prism, Nicol's 7 
 
 Protobastite 150, 210 
 
 Pseudo-crystals 89 
 
 Pyrite 108 
 
 Pyrope n6, 210 
 
 Pyrrhotine "o 
 
 Quartz. 
 
 iredge 
 
 plate, Biot-Klein's n 
 
 R 
 
 v. Rath 198, 202, 205-207, 209, 211 
 
 Regular minerals 106, 114 
 
 , Behavior of, in pol. light 17, 30 
 
 Renard 163, 198, 200, 202. 204. 209 
 
 Reusch 200,203,208 
 
 Rhombic minerals 107, 140 
 
 , Behavior of, in pol. light 21. 35 
 
 Riess 198, 202, 204, 214 
 
 Ripidolite 162, 210 
 
 Rohrbach 67,75 
 
 Rose 201, 204. 208, 209, 213 
 
 Rosenbusch. i, 7, 16, 45, 51, 76, 81, 92, 197-200, 
 205, 207-210, 213 
 
 Roth ioi. 211 
 
 Rubellan 158, 210 
 
 Rutile 38, 122, 210 
 
 S 
 
 Sagenite 122 
 
 Salite 17. 202 
 
 Sandberger 205,211, 213 
 
 Sanidine 166, 208 
 
 Sttuer 204, 205, 210 
 
 Scapolite 124, 211 
 
 Scolecite *94 
 
 Scheerer 203 
 
 Schonn 5 1 
 
 Sch8rl 138 
 
 Schrauf 198, 200, 202, 205, 208, 210, 211 
 
 Schultze J3> 2 8 
 
 Schulze 211 
 
 Schumacher 211 
 
 Schuster 198, 200, 205, 207, 209, 213 
 
 Sericite 160 
 
 Serpentine 19, 2" 
 
INDEX. 
 
 233 
 
 PAGE 
 
 Shell-formed structure of crystals 90 
 
 Siderite 132 
 
 Silico-fluorides 56, 57, 58 
 
 Sillimanite 142, 211 
 
 Single-refracting minerals 17 
 
 Sipocz 201 , 209 
 
 Sismondme 162, 201 
 
 Sjogren 199 
 
 Smaragdite 172, 174 
 
 Sodalite 114, 211 
 
 Sommerlad 204 
 
 Sorby 44,210 
 
 Specific gravity, Determination of 68 
 
 Spinel 118 
 
 Stage, Heating 15 
 
 of the polarization-microscope 7 
 
 scale 14 
 
 Staurolite 39,142,212 
 
 Stauroscopic apparatus 7, 13 
 
 Stelzner 200, 203, 205, 206, 209 
 
 Stilbite 194 
 
 Streng 51, 200, 202, 203, 205, 208, 210,212 
 
 Structure of the rock-forming minerals... 87 
 Szabo 51, 198, 201 
 
 Talc 160, 212 
 
 Teller 198, 203, 205, 210 
 
 Tetragonal minerals 106, 122 
 
 , Behavior of, in pol. light 18, 30 
 
 Thoulet 6, 44, 66, 81, 201 
 
 Titanite 25, 42, 176, 212 
 
 Titaneisen 102,110,205 
 
 Titan ,magneteisen 108, 212 
 
 PACK 
 
 Tdrnebohm 197, 200, 203, 211, 213, 214 
 
 Tourmaline 46, 138, 213 
 
 Tremolite 174, 212 
 
 Trichite 87 
 
 Triclinic minerals 107, 178 
 
 , Behavior of, in pol. light. . . . 28, 37 
 
 Tridymite 130, 212 
 
 Trippke 203 
 
 Tschertttak 45, 197-204, 206-213 
 
 Twins, Behavior of, in pol. light 37 
 
 U 
 Uralite 174,213 
 
 V 
 
 Valee-Poussin 209 
 
 Velain 206. 208 
 
 Viridite 192, 213 
 
 Vitreous inclosures 97 
 
 Vogelsang 15, 85, 204, 205, 213 
 
 Vrba 199.211 
 
 w 
 
 Websky 211 
 
 Wedding 199 
 
 Weigand 211 
 
 Weiss 208-211 
 
 v. Wer-veke 66, 199, 202, 203, 210-212 
 
 Wichman 6, 198, 201, 206, 209, 213 
 
 Williams 203, 207 
 
 Wollastonite 25, 172, 213 
 
 Z 
 
 Zeolites 194, 213 
 
 Zircon 124. 213 
 
 Zirkel i, 51, 76, 197-214 
 
 Zoisite 154, 214 
 
{283 
 
 UNIVERSITY OF CALIFORNIA LIBRARY