GIFT OF MICHAEL REESE 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, BELOIT, WISCONSIN. NEW YORK : JOHN WILEY & SONS. 1886. Copyright, 1885, by 3 ^3 03 JOHN WILEY & SONS. \ \ EARTH SCIENCES LIBRARY 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, " Minralogie Micrographique." EUGEN HUSSAK. GRAZ, November, 1884. TABLE OF CONTENTS. PART I. METHODS OF INVESTIGATION. PAGE Preparation of Microscopical Sections 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 5 1 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 ?6 V. Separation of the Rock-constituents by means of the Electro- magnet 79 VI 11 TABLE OF CONTENTS. 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. Structure of the Rock-forming Minerals 87 Shell-formed Structure of Crystals go 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 I. 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.. 229 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. FOUQUE ET MICHEL LEVY. Mineralogie micrographique. Paris, 1878. E. COHEN. Zusammenstellung petrographischer Untersuchungsmethoden. 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 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. VV C ' 3 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 with 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 fragile, 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- 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 knife, 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. 7 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.") TIL LIEBISCH. Bericht iiber die wissenschaftlichen Instrumente auf der Ber- liner Gevverbeausstellung. Berlin, 1879, P- 34 2 - H. ROSENBUSCH. N. Jahrb. f. Miner, u. Geol. 1876, p. 504. Ueber die Anwendung der Condensorlinse bei Untersuchungen im con- vergentpolarisirten Lichte: v. LASAULX. N. Jahrb. f. Miner, u. Geol. 1878, p. 377. E. BERTRAND. Societ6 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, ti) ; (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, mi, 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. 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, tt). In order 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 " teinte-sensible") of the circular polarizing quartz appears. The mineral to be examined is then placed beneath the objective. METHODS OF INVESTIGATION. FIG. 2. POLARIZATION-MICROSCOPE, BY R. FUESS. (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 ail 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 diagonals, grinding away a wedge-shaped por- tion from either half, and again cementing the polished surfaces. If the projecting and re- FlG ' 3 * entrant angle of the twin thus formed be ground CALDERON DOUBLE- . . , . i j j j 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, m, 11), or the tube acting within a socket can be moved by two screws (Fig. 2, ;;/;;/, mi). 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 fiom 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 ME THODS OF INVESTIGA TION. 1 5 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 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/ 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 isjitrope. 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 every direction. 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, analcine, 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. II. 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 o^tic 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 (= f) 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 oo > , and the mineral is negative ; if c = C and oo < e, 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 crystallographicaxes. Fig. 4 gives a clear idea of this parallel extinction in an optically-uniaxial mineral cross-section abed, c is the chief axis, and viv 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 induce 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 a, 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 refraction, a, fi, y, corresponding to these three axes of elasticity. Minerals crystallizing in the rhombic, monoclinic, and tri- clinic systems belong to the double-refracting 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 = t,6 = a f c If AP || oo ^oo , then c' ~ a, d c f -, . r o =. ; c' = c, d < = a 1 IiAP\\ oo P oo , then c' a, b = c ) . - > a = b. c 1 c, b a I 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 crystallographical 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. HYPERSTHENE. || 00 P 03. FIG. 7. ENSTATITE AND BRONZITE. || 00/00. FIG. 8. ANTHOPHYLLITE. II 00/00. 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 : co/^oo, so all longitudinal sections parallel to the vertical axis (c') from the zone ooPoo : oo/^oo 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 or at right angles to the plane of symmetry ooPoo. In the monoclinic minerals there are the following possibilities for optical orientation : If AP\\ co Poo, then I. M. = c ) i fc . or i.M. = a) and c and a are inclined to c' and a. If, on the contrary, AP oo jPoo, then I. M. = b = 0, 1. M. = b = c; or 2. M. = b a, 2. M. = b = c. In this case b and c, or b and a, are inclined to c' and a. 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. FIG. g. HORNBLENDE. II 00 P 00. (After Fouque".) FIG. io. AUGITE. HooPoo. (After Fouqu^.) FlG. II. WOLLASTONITE. II oo Poo. (After Fouqud.) FIG. 12. EPIDOTE. II oo Poo. (After Fouque".) METHODS OF INVESTIGATION . optic axes, d and C middle lines, and c the vertical 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 1 oo^oo. A^B 1 are the optic axes where AP\\ oo jPoo, A^B, FIG. 13 TITANITE. HcojPoo. (After Fouque.) FIG. 14. ORTHOCLASE. II oo f> oo. (After Fouque.) where AP is J_ ooPoo; in both cases a is at an angle of 5 to the edge oP : oo Poc. 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 coincide 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 extinguislies obliquely. Fig. 15 represents the oblique extinction of an optically- 26 DETERMINATION OF ROCK-FORMING MINERALS. 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 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 co P co. The angle of extinction can also be measured in relation to another known edge in the METHODS OF INVESTIGATION. section || oojPco; e.g., to the edge oP : oojPoo, 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 co 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 '.&>&> and oo P oo : co P oo always extinguisli at an angle. The angle of extinction finally reaches o when the section is parallel to oP or oo^co. Thus, according to Michel Levy, the value of the oblique extinction varies in augite and hornblende with the direction of the section iii the following manner : 28 DETERMINATION OF ROCK-FORMING MINERALS. Direction of the section in the zone. Augfite. For 2v = 58 59'. Hornblende. For -2V = 79 24'. oP : co Poo ; = :=f 2 :#l Maximum. In sections parallel to cof > J a : a = 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 f oo ; it then lessens and becomes o in sections parallel to oP. Maximum of 29 58' to 14 58' parallel to 00^*00, accord- ing to the species of horn- blende, then decreases and becomes o parallel to oP. CQ-^ co ; co f oo Maximum of extinction ob liquity on coPoo, c : c = 38 44'. According to the in- clination towards co/oo the angle decreases and becomes o parallel co^ oo. Minimum parallel QOJ? co, 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 oo f oo ; then decreases and equals o parallel oo P oo. oP : 00/5 All sections possess parallel extinction 'WJ5' Triclinic Minerals. In the triclinic minerals no one of the ^ CP three axes of elasticity coincides with the crystallographic axes. e Fig. 16 is an example of the opti- cal orientation of a triclinic rock- >JP 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 ooPrc; a is exactly per- pendicular tO OQ^CC. 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. DISTHENE. HOODOO. (After Fouqud.) METHODS OF INVESTIGATION 2$ 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 jPo>. Exact measurement of the extinction- obliquity must, however, be made on cleavage-lamellae parallel oo/^co 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 ocular is 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 cor*, 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-TTniaxial 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 V 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 thaj: the line joining them is perpendicular to the plane of the "bptitTaxes of the mica (generally indicated by a mark on the plate), then the mineral under examination is optically positive ; 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. FlG. 17. INTERFERENCE-FIGURES OF DOUBLE-REFRACTING MINERALS ON USING THE CONDENSER IN THE POLARIZATION-MICROSCOPE. (After Fouqu^.) $ 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,

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 = b. The reverse is true in case the second middle line is positive : First middle line a (negative) ; Second middle line = c; Optic normals fc. 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 c'}\ as on the one hand but few of the rock-forming minerals, e.g., oliv-ine, 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 > ^>, 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 ooPoo, the sections at right angles to the vertical axis and parallel oo Pco 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\ oo Poo or oo P oo : oo P oo) ; 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 oPicoPoo. 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. 1 oP. Mica I. Class. (After Fouque".) 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, ft, 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 oo^Poo 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 : ooPoo. 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 SPco. 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, = 114 26' (Fig. 20). C,6>re the chief axes of both individuals, and N is the twinning-seam. Whdn 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. 2 i. CALCITB TWIN. According to - /?. METHODS OF INVESTIGATION. 39 If the tvvinning-plane in calcite is the R-tace, although never occurring in the rock-forming individuals, the chief axes form nearly a right angle with each other, C : C^ 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 lt 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 f/'i; $Po. 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 ^-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 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. bro Augite, amphibole, epidote, and gypsum may be ward as examples of the rock-forming minerals with repeated twinning according to ooPoo. Sections perpendicular to the twinning-plane and parallel to Poo 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 : c, = 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 FIG. 24. AUGITE TWIN according to oo/'oo. || oGjP FlG. 25. POLYSYNTHETIC AUGITE TWIN according to oo^oo. 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 jP2, 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 120* 34 r~ FIG. 26. EPIDOTE TWIN according to oo^oo. Section || oojPoo. FIG. 27. TITANITE TWIN according to oP. Section || oojPoo. 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 Pen. 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 oo Pco, 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 i^oo, 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 oo /^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 FlG. 29. POLYSYNTHETI_C PLAGIOCLASE TWIN. FlG. 3 O. PLAGIOCLARE TWINNED || oo/'oo. according to the A! bite and Pericline laws. twins were not possible in the monoclinic system, as the plane in the monoclinic system ooPoo corresponding to the plane oo Poo 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 Pec, 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 oo/^co a double system of twinning-striations cutting each other at nearly right angles. Disthene occurs, though in rocks more rarely, as twins, according to the following laws : I. Twinning-plane OQ P OQ. II. " at right angles to the A.) (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 ooPco or oo/^co. 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 > f or c > a, i.e., the ordinary ray is more powerfully absorbed than the extraordinary. In cordierite C > b > a, 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 > c ; 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 > a, 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. loy, 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. naturw. Landesdurchforsch. Bohmens. III. Bd., V. Abthlg., Prag, 1877. SZABO. 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. BORICKY. N. Jahrb. f. 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. Zeitschr. fiir 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 -J 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 ^-i 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 (ooP.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, \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. CC -, ^^W J the point of a knife added. If now the en glowing for some minutes, whereby the sulplrl fills the crucible, and then, still covered, is allowed 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. Almos? all of the rock-fprming 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 ooP;/ . raPoo, 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 P2 ; 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. 77. SODIUM SILICO-FLUORIDE. 33 (After Boricky.) FIG. 34. CALCIUM SILICO-FLUORIDE. (After Boricky.) 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 a . 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. MAGNESIUM SILICO- 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 rilled, 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 Pco . P, generally lying on coPco or arranged in rosettes. Often larger crystals of the well- known swallow-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 r^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 (Fig. 37, a) are formed within a few minutes, and generally on METHODS OF INVESTIGATION. 6l 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 a. POTASSIUM-PLATINUM CHLORIDE. , , r i 1 r b. POTASSIUM FLUOBORATE. another drop of the solution from (After Be 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, a 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 hydrofluosilicic 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-MAGNESIUM PHOSPHATE. (After Behrens.) FIG. 9. CAESIUM ALUM. tfter 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 mg. 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 <9oo . O) 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 fourlings. 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. 6 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. Their 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 <9oo ; 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. Water. 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, H. p. 17 and 189. Fouqufi 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 : I. 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 oo . 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 the 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. kolben 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 kolben is weighed and then emptied back into the beaker-glass and the solution tested with the fragment of mine- ral ; the kolben 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. koiben gave : First weighing = 77.981 grams. Second " = 77.919 " Third " = 77-973 " Mean = 77.957 - Kolben = 11.682 " 66.275 " 66.275 -v- 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. Westphal 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- lution after the powder is added to it is well stirred with a glass rod and allowed to settle ; IN (Copy A from S ' K. Oebbeke.) 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 tJie 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 (9\VO 3 . B 2 O 3 . 2CdO, 2H a O + 16 aq.) for 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 a : 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 th^ 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 9WO 3 . B 2 O 3 . 2CdO, 2H 2 O + 16 aq. is formed and filtered from the insoluble BaSO 4 . Cadmium boro-tungstate dissolves in less than T ^ 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 INVESTIGATION'. f$ 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, which 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 3.5/5-3.588. 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 DETERMINA TION 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^i874, 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 2 O B ), Titaniferous Magnetite (difficultly soluble). II. Soluble with evolution of CO 2 : Calcite, Aragonite (Ca), Dolomite (CaMg), Magnesite (difficultly soluble), Siderite (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 Bytowniie (more difficultly soluble, Ca, Na), Anorthite (Ca). V. Soluble with separation of gelatinous SiO 2 : Sodalite (Cl), Hauyn and Nosean (SO,), Ncpheline (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 78 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*. t 7Q ^^/r ^ k -*- * 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, Chromite, Siderite, Almandine, Hedenbergite, Ankerite, Limonite, Augite (rich in iron), Pleonaste, Arfvedsonite, Hornblende, Augite (light-colored), Epidote, Pyrope, Tourmaline, Bronzite, Idocrase, Staurolite, Actinolite, Olivine, Pyrite, Chalcopyrite, Biotite, Chlorite, Rutile, Hatty n, 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. FOUQUE ET MICHEL LEVY. 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 co 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 Pco, 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- 8 4 DETERMINATION OF ROCK-FORMING MINERALS. 8 1 "s g 5- *o 1 >^-%^? < S c ?> v S > ??^-?S)vS 2 Z *g g t^OO O C 1 * "*> ON COCO trir^w-'ftN.fOw ONO 10 1OVO \O ^O VO tx 1^00 O O M N -^VC* t^oo m i .s % p o ^ .p 1 C '^3 N 4> t/J ! "o = = + o o Jo^Sj^ lo^^S) {^^vo \0 vO^vF K So? S 2" oT ?VO CO M 8 * ^ 8 ^ r 8 S! 7. 4 8 ft," . 'k O 5 g "jjj !J w cJ ^5- co in^ cT^- w s S X ? W( 5?) 2> * 10 1 1 o en 8* ^ ^ Of, O i ~ 4 i i l3S 1 5.?;^ 00 5- N ^?% vovo s- 8 ^ONONO OOt-t M N coro-^- tovo IN.CO *" rt ft, .5 ^ G 4J O 8 G "3 O O 8 a, 8 t. 2 ^ g C/J fti 'fl ^M O VO VO '"t CO 00 00 ^-WWOOOVOOO O VO ^00 \O O O W ^-lO COCO COfOlOM CO IO ^-(NCON^ cJ> D , o ' 8 ^ 2 1 ! f 8 | 8 % 1 | % 10 in 1000 Ofor^HiOHtx-^NM o OOO OOOOOOCQOOOOC^OOHMNrO^ m 10\O 00 0! O 2 2 KH : 1^ 2 o cj O j 5 C. METHODS 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 complicated, 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 determined 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, 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 DETERMINATION 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 are 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 MICROLITES. FIG. 42. MICRO-FLUCTUATION STRUCTURE. BELONITES AND TRICHITES. METHODS OF INVESTIGATION. 87 briefly as follows : The crystallites are the primitive form, the globulitcs being first in order ; the crystalloids mark a further progress in development; these form a transition to the microliteSj 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. FIG. 43 . CORRODED QUARTZ CRYSTAL. (After Fouqu^.) 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 which 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 already formed, if any change in the 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 OPACITIC- BORDERED 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 QO 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 ZONALLY-DEVKLOPED dist in gu ished from each other easily, es- AUGITE. Section ii oo POO. pecially if multicolored, as is so commonly the case with augite (Fig. 4$) or hornblende, where a green 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 and 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. Geol., 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, FIG. 46. AUGITE WITH " HOUR-GLASS STRUCTURE." Section II ooPoo. (After L. v. Werveke.) FIG. 47. SCHEMATIC 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 coPoo are similar. While sections perpendicular to the vertical axis show the METHODS OF INVESTIGATION. 93 ? ordinary zonal structure. At first a crystal-skeleton shaped li 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 ROCK-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. In ex- 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 INCLOSURES. 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- FlG. 48. 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 rapid. 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 iriclosure is generally an irregu- <)6 DETERMINATION OF ROCK-FORMING MINERALS, lar one ; 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 INVESTIGATION. 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 merely on the amount of iron present. FIG. 49. 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 broad 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 in hypersthene and bronzite (Fig. 50), or the opaque microlites and tablets parallel to the r'-axis in labradorite. The zonally arranged inclosures of small quartz granules in the garnet and staurolite INCLOSURKS o? 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. IOI 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 mo|t 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 undergoes 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 IO2 DETERMINATION OF 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 1 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 I. 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. _L = At right angles. n = Index of refraction. For optically- j GO = Index of refraction for the ordinary ray. uniaxial minerals. ( = Index of refraction for the extraordinary ray. For optically- I 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 i a = Clinodiagonal axis, minerals. ( V = Orthodiagonal axis. Under the heading "Structure:" I. O. = Components first in order of separation. II. O. = Components second in order of separation. io6 DETERMINATION OF ROCK-FORMING MINERALS. hO U CD 0) 4-> ho Q rt H o-l > HTI^ O K in the triclinic, gives sis alf B:Y*3' B 3?v,fl angles to one of the black cloud with or double-refraction. lar to the middle line, . On revolving the st If this cloud, i.e., hy the concave, then th > p. The angle of in ographic axes in sect parallel oP and P termining them. c sections polarize g to the or less pe t or within ntrary dir side and v\ if the re ity to the linic miner rable me Isot ver acco is m wit in a con is p e^ast mon an a ,o 7 Transverse sections show a fixed black axial cross, with or without colored gs, according to th power of double- refraction and the thickness of the mineral leaflet. Jis! si rt - < 8 8 * rt sijSuv pazuBpj SinjfcdAuoQ m uoi;Burarexg[ 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. (Magnet- Fe 3 4 (FeO + Fe 2 3 ). 4.9-5.2. According toO. Grains and octahedra. According toO. eisen.) Easily soluble in HC1. . Squares and equilateral ^ triangles. t \ 2. Titaniferous FeO 4.8-5.1. * Octahedra Magnetite. ** e *-' 1 KeTiOq and ( Titan- magneteisen.) Distinguished from magnetite only by chemical 6 K grains. analysis. (Reaction for titanium.) 3. Pyrite. FeS 3 . Easily soluble in HN0 3 , with separation of S. 4-9-5-2. According to oo O oo. 2 Regular hexagons and pentagons. Penetration- twins of 2 TABLES FOR DETERMINING MINERALS. of Opaque Minerals. 109 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. cruciform 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 a"H of P the crystallinjBr ^^jpeks. ^Tisaecomposi- tion-product of olivine, augite, cleavage- fissures. hornblende, and biotite. Ditto. iflQB 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 brass-yellow. 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. Sueciflc gravity. System of crystalliza- tion. Cleavage. Ordinary combinations and form of the cross-section. Twins. 4. Ilmenite. FeTi0 3 + 4.56-5.21. R and oR\ Tabular With parallel (Titan-eisen ) X (Fe 2 3 ,. Difficultly soluble in HC1. \ conchoidal separation (absonde- R . oR; also - y^R, - SA-, axial systems. Polysynthetic Ti-reaction rung). and grains twins with which are not af.er R. microcosmic spherical salt. but for the most part long rods. Cross-sections generally hexagonal, long. th readlike, jagged, or netted forms. rt 5. Graphite C. 1.9-2.3. rt M oP. Rarely in thin (and bitumen). Bituminous black rocks, .. Like rutile. Like rutile. 5. Meionite Group, a. Meionite. /3.Scapolite. Ca 6 (Al a )Si 9 36 R 3 Al. 2 Si 6 O 21 R = predomi- nating Ca, some Mg, Na 2 soluble 2.734-2.737 2.63-2.79. Perfect co/>oo. (See Fig. fa.) Crystals after ooP. oo/'oo . P Double- refraction; rather strongly Brilliant like quartz. in HC1, with and larger negative. separation of pulverulent grains \ or elongated SiO a . prismatic 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 ivine- yelloiv; very strong refraction ; notice- able. one of the first-formed rock con- stituents; quartz, orthoclase, plagioclase, biotite, inclosures, acicular cavities, and accessory constituent in garnet, syenite, characterized by crystalline form, polarization- colors, and therefore therefore hornblende, elongated quartz, powerful the con- common as augite. undeter- porphyry, 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 .0=1.92 accompany- e = 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. M = 1.594 - 1-597 < = 1.558- 1.561 With sanidine, sodalite, augite. Primary accessory constituent; very rare in trachytic Scapolite can be easily distinguished from orthoclase and calcite by Colorless. rocks. the optical properties and White. cleavage; w = 1.566 e = 1.545. [Scapolite appears often to With quartz, plagioclase, Poor; fluid in- closures; Opaque, fibrous ; de- composed Rare in crystalline sch ists, meionite is recognizable by the crystalline replace plagioclase and to be calcite, augite, garnet, rutile in scapolite. into calcite. with plagioclase secondary form. a decom- rutile, and accessory position- titanite. constituent. product from it. 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. Couse- ranite and Dipyr. Similar to scapolite; rich in the alkalies, 2.69-2.76 (2.613). 2.62-2.68. According to oo /'oo. According Long prisms 00/>.W/>00; with As in scapolite. Rather energetic. Rather brilliant. H 2 O. termina- Not attacked tions by acids (HC1), oral either rounded or least only fibrous. with difficulty. S. Melilite. (Humbold- ite.) (Ca,Mg,Na 2 ) 12 , (Al 2 Fe 2 ) 2 , Si 9 36 . Easily soluble in HClwith 2.90-2.95. Parallel to oP and oo P. Nearly always in crystals; thin tablets predomi- Rarely penetra- tion twins, with chief axes Double- refraction feebly negative. Not very brilliant ; if yellow and fibrous, separation of gelatinous SiO 2 . nating, oP . oo P . 00 POO. at right angles to each other. shows polariza- Irregular grains. Cross- tion- colors ; if colorless, sections bluish-gray for the polariza- most part tion- rectan- colors. gular, more rarely circular. TABLES FOR DETERMINING MINERALS. 127 Color and power of refracting Pleo- chroism. Structure. Association. Inclosures. Decomposi- tion. Occurrence. Remarks. light. Bluish, colorless in thin Crystals developed With calcite, actinolite, Very rich; particles of carbon, Fibrous decomposi- tion, with As contact- mineral in metamor- Can be distinguished from chiastolite section, limestone, and mica. quartz- formation phosed by the clear as often rich grains, of calcite limestone. structure ; ivaier, in and on the Very rare. from black inclosures. leaflets of crevices. andalusite from muscovite by the inclosures. distributed optical <> = 1.558. at random. properties. = 1-543- Generally lemon- yellow, ho ney - Longi- tudinal sec- tions ; Rectangu- lar longi- tudinal sections With nepheline, leucite, augite, Poor. The formation of fibres is a As primary constituent often Easily recognizable by the crystalline form. yellow, rect- show a and result of replacing color, and fibrous colorless to yellowish white. angles show a very striation and fibrous olivine. the decomposi- tion into nepheline in the nepholine tendency. If colorless, easily confounded feeble dichro- tendency parallel zeolitic substances. and leucite with nepheline, although the ism. to the (Compare basalts hexagonal short with and 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 " / flock- structure.'' 1 (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-j. Unattacked bv HC1 and "H 2 S0 4 : dissolved by HF1. 2.65 average (2.651). Imperfect according to/?. The sections uneven owing to the conchoidal Grains or crystals R . R or >R.R.-R. Generally in large ndividuals; regular hexagons, With parallel axial- rystems. As a rock- constituent never or rarely twinned. Double- refraction positive. Strong. Brilliant yet weak in very- thin sections; bluish-gray, like 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; clear as Often colored by Fe 3 3 With orthoclase Poor in mineral None. Changes [. As primary com- ponent : In grains often water; vitreous entering fissures ; (and sanidine), inclosures. Apatite resulting from the (a) In eruptive rocks as component similar to sanidine, lustre. cloudy from more rarely prisms. action of I. O. As macro- but can to = 1.548. e = 1.558. numberless inclosures, as plagioclase, biotite, In clastic schists and melted magma are scopic constituent in grains and be easily distin- n the granites and the clastic rocks. Grains lornblende, and augite. Never as granites very rich in fluid not un- common among the crystals with fluid inclosures in granites, quartz- guished optically from it. and crystals. Quartz from the eruptive primary component in augite- inclosures and long brown or quartz of the eruptive rocks, or porphyries, quartz- trachytes, and their glasses; in dacite Distin- guished from rocks give olivine black, 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 >eing 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 and Fig. 43.) (V) In nearly all refrac- with ortho- gas-pores. crystalline schists in tionj from clase, as irregular grains calcite by graphic- with fluid inclo- cleavage granite sures, as with the and = inicro- feldspars; especially twinning. egniatite. in gneiss, mica- imp. Fig. schist; in minute 63.) grains in clay In the schists. porphyritic II. As secondary eruptive product through rocks also 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 inclosures, joined together, reaching to the very edges. With Inter- IV. As simple rock, muscovite twined building quartzite and fluid and quartz biotite. inclosures. 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 IP. 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 P.) A.P. differing but little from the normals to oP. i.M. = c nearly Axial angle 65-70. A more exact crystallographical and optical determination of tridymite is generally impossible, owing to the minuteness of the crysta s occurring in the rocks. Microscopical tridy- mite behaves like an hexagonal rain- eral in p.p.l. TABLES FOR DETERMINING MINERALS. Color and power of refracting light. Pleo- chroism. Structure. Association. Inclosures. Decomposi- tion Occurrence. Remarks. Colorless, clear as water. , i.e. /3 = 1.4285. Generally in Aggregates of minute thin tablets; either With quartz, sanidine, plagioclase, augite, biotite, Fluid inclosures. Primary as accessory constituent, and secondary as decom- The tendency to form aggregates is very characteristic for microscopic tridymite; the hexagonal or and horn- position- product in optical properties and the irregular tablets, lapping blende. Secondary rhyolites, trachytes, hornblende- twinnings for the larger crystals and over with opal and augite- those suitable each ft her and chalce andesites. for optical like sh ingles. dony. Exceed- ingly investigations. Often in common the in the neighbor- younger hood of 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. CaC0 3 . Easily soluble in HCI, with rapid evolution of CO 2 . Easily 2.6-2.8 (2.72). ( Perfect R. (See Fig. 65.) Only in regular grains and crystalline aggregates. Poly- synthetic twinning- striation after -\R. (See 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. I \ 2. Dolomite. (Ca) (Mg) CO 3 . More difficultly soluble in acids than calcite. 2.85-2.95. ditto. Grain sand R. See " Re- marks." ditto. 3. Mag- ditto. nesite. MgC0 3 . 2.9-3.1. ditto. ditto. More difficultly soluble in HCI. 4. Siderite. FeC0 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. aggregates, bearing very poor building by ized by w = 1.6543 e = 1.4833 in cavities; in threads and fibres. augite, hornblende, biotite, in mineral inclosures. itself a simple rock, limestone; the rhombo- hedral cleavage In irregular and not yet and grains, plagioclase. assuredly twinning- single individuals. proved as such in striation. If occurring between eruptive in minute the other rocks. granules, con- Here as often stituents. deco m 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 ditto. See As decomposi- tinguished to brown. Calcite. tion-product 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 streujfth of double- refraction. Polarization- color. 5-Nepheline Group, a. Ekeolite. 2.65 (2.591). Imperfect; coarse fissures. Only in larger grains. Generally bluish gray, not very brilliant. (Na, K) 2 Al 2 Si 3 O 8 . Easily soluble inHCl with separation of gelatinous SiO 2 ; on evaporation cubes of NaCl. Double- refraction negative, not energetic. Similar to the feldspar in very thin sections. /3. Nephe- line. (2.58-2.65) 2.56. Imperfect fffand 00 P. Hexagons and rectangles; in -minute crystals aoP.oP, in short prisms, and in minute irregular granules. (See Fig. 66.) TABLES FOR DETERMINING MINERALS. 135 Color and power of refracting Pleo- chroism. Structure. Association. Inclosures. Decomposi- tion. Occurrence. Remarks. light. Gray, reddish brown, fatty lustre; colorless Irregular grains inter- penetrated with With sodalite, microline, hornblende, titanite. Poor; often colored green by hornblende. Fibrous, zeolitic meta- morphosis. As primary essential constituent in the older eruptive Well recognizable macroscopically. The solubility and Na-reaction are characteristic. in thin other con- rocks; in the section. stituents. elaeolite- syenites. Colorless, clear as In crystals or in With leucite, Inclosures of augite Generally fresh, in As primary essential Distinguished from: apatite by water, w = 1.539 aggregates of minute augite, olivine, very commonly phonolites more often constituent in the the imperfect cleavage, - 1.542 e = 1-534 irregular granules. or with sanidine and augite, or with hornblende arranged parallel to the faces. (See Fig. 66.) opaque and meta- morphosed into zeolites ; younger eruptive rocks: nephelinites, nepheline- microchemical reactions, and crystalline forms, as apatite commonly and then and leucite- shows /", titanite. polarizing basalts, besides P, oP, like an phonolites and occurs aggregate. and in long prisms; tephrites. quartz never occurs in such minute crystals as nepheline; feldspar-th reads are long and twinned; melilite has no hexagonal transverse sections; zeolites generally evidence their secondary character. The short rectangular longitudinal sections are wanting in tfidymite. If nepheline occurs in aggregates of minute granules, then it is similar to a colorless vitreous mass or quartz and orthoclase 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 strength of double- refraction. Polarization- colors. y. Cancri- nite. Composition like a 2.448-2.454. Imperfect Larger irregular Double- refraction Rather brilliant; nepheline, grains. negative. aggregate poor in polariza- potassium, tion. together with CaO, C0 2 , and H 2 O. Soluble in HC1 with effervescence, with separation of gelatinous Si0 2 . 5. Lieben- erite. Potassium- aluminium 2.799-2.814. Very imperfect Only larger crystals Double- refraction Remark- able silicate. CO/*. oo P . oP. negative (?) aggregate H 2 O, traces of Fe, Mg. Cannot be confirmed, polariza- tion^ -very Similar to as the brilliant. pinite, crystals incompletely decomposed are always completely by HC1. decom- posed. 6. Apatite. Ca B P 3 12 Cl. Ca 5 P 3 12 Fl. Soluble in acids. Reaction for phosphoric acid. 3.16-3.22. Crystals, more rarely grains. Imperfect cleavage parallel oP and oo P. P.P and more rarely oP. Generally long prisms. (See Fig. 67.) Double- refraction negative, feeble. Generally not very brilliant, as with nepheline. Remarkable "separa- tion" (abson- derung) II oP, the acicular crystals thereby separating into several members. CABLES 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 yelloiv, nearly inter- penetrated elaeolite. Leaflets of ferric oxide, decomposi- tion with elaeolite; rare. dis- tinguished colorless, with etc., like formation of 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 CaCOg. Oil-green; in sections colorless; Only as "spring- ling" b With flesh-red orthoclase It is probably a completely decomposed Rare. In orthoclase- Easily recog- nizable white. in macro- and mica. nepheline(?) or porphyry. by the scopic crystals. cordierite (?) consists crystalline form and principally of decom- minute position. m uscovite leaflets, which i.p.l. appear very clearly. Colorless, white; colored reddish, If colored, plainly di- In irregular grains, or inelongated often very Proven in nearly all rocks. Vitreous inclosures, gas-pores. Very characteristic Always fresh. As accessory constituent in nearly Dis- tinguished 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 (comp. clear like nepheline; separation. Inclosures ! the whole crystal; in this first eliminated nepheline), inclosures more (See respect, in the in very char- prominent transverse formation acteristic; among the Only sections of the hattyn in rock-con- accessory especially, rocks. the longi- stituents. constituent. showing a great tudinal n = 1.657. As similarity to sections in closure 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 apatite. 138 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 doublu- refractioii. Polarization- colors. 7. Corun- dum. A1 2 3 . Insolu'ile 3-9-4- R and oR. oo /> 2 . R. R and grains. In rocks seldom (?) Double- refraction Very brilliant. in acids. Hexagons strongly and negative. rectangles whose angles are truncated by/?. 8. Tourma- line. (Schorl.) Very complicated. R 6 A1 2 B 2 SuO* +R 3 (A1 2 ) 2 .B 2 Si 4 O 20 R = Na predominat- ing and R = Mg, Fe. 3-59- Imperfect R and /? 2 . Separation \\oR. Very perfect. Almost only in crystals. R /?.oo/> 2 . -. 2 Transverse sections three-, JZJT-, and nine-sided when observed parallel to the chief axis; often showing good hemi- morphism. Double- refraction negative, energetic. Rather brilliant; between brown and red. Contains boric acid. Not attacked by acids. oR below, R above. (Comp. Figs. 68 and 69.) 9. Haema- tite. (Eisen- glanz.) Fe 2 O 3 . Easily soluble in HC1. 5.19-5.28. R.oR. Not charac- teristic in microscopic individuals. Mostly tabular thin leaflets, <7/?.00/> 2 , 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 MINERfyS^ 139 . Color and power of refracting Pleo- chroism. Structure. Association. Inclosures. Decom- position. light. Colorless, azure-blue. If colored, Rough section- sitvface. With quartz, orthoclase, Very poor; fluid and Very rare. Contact- When in grains spotted. y crystalline brown. to = dark exceedingly muscovite In certain form and to = 1.64 e = T.02. >lue, brown to black, e = light minute crystals, rarely in in granite. With quartz, orthoclase. granites in grains. Accessory in pleochroism. Distinguished from : blue to light gray and brown. granules, as in certain metamorphic rocks. In mica, and other accessory minerals, as many crystal- line schists, especially clay-schists, hornblende by the optical properties; biotite by the granites in grains inter- penetrating quartz. The staurolite and garnet. also in clastic rocks. Finally, common and repeated formation of 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 form lustre; in thin leaves blood-red, also the fissure of minerals. Is always a secondary with decomposed biotite, hornblende, and brown pulver- ulent secondary mineral. of occurrence and 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. 140 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 o-. No INTERFERENCE-FIGURE VISIBLE 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. Tabular 1. Olivine. (Chryso- lite.) Mg 2 SiO 4 + FeSi0 4 Rather easily soluble in 3-2-3-4- Perfect II oo Poo, imperfect 00 ^00. Con- crystals. <*>P._Pv>. ooPoo. Hexagonal cross- Very rare according ft. i. M. -L OO/OO a = C. Z = a. Double- refraction very strongly positive. HC1, with separation , f - gelatinous Si0 2 . choidal fracture not so evident in sections 00/>=I 3 02'. Poo = 7 654' or large rounded also pene- tration- twins. Feeble dispersion of the axes. sections. Gene- grains. (See P < 7', large rally twisted Fig. 71.) angle. crevices. TABLES FOR DETERMINING MINERALS. 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- ohrolam. Stmcture. 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 from : stronger than in augite. rarely greenish ; rough section- surfaces. Relief surtace ; the manner of deco tit- posit ion into yellowish- plagio- clase, nephe- line, leucite. Also with the minute brown picotite octa- hedra, serpentine (compare aggregates), whereby the picotite inclosures in all basaltic rocks in olivine- felsand picrite, quartz in iso trope sections easily ; zoisite by the marked. red, brown horn- vitreous remain. melaphyr, crystalline |3 = 1.678. or greenish blende and Also into a olivine- form (never serpentine, beginning at the and biotite. Almost rarely fluid inclo- brown fibrous aggregate ; into picntes gabbro, olivine- norite, in long needles) and polari- crevices, is never sures, and pseudo- olivine- zation- charac- teristic ; with primary magne- tite, very morphs of calcite alter diabase. (In colors ; colorless also the inclosures quartz or ortho- rarely other olivine. By the de- crystalline schists ?) a ugite 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 hydroxide, porphyries. angles to sharp and magne- the c-axis ; crystals, tite in the sanidine partly in crevices. by the fragments of them, Totally decomposed rough surface and or in olivine, the ex- irregular grains. very rich in iron, always ceedingly brilliant Constituent I. 0. in in sharp tabular polariza- 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. fi. AXIAL PICTURE VISIBLE IN SECTIONS || oP. aa. Appearance of the I (-J-) NAME. Chemical composition and re ctions. Specific gravity. Cleavage. Ordinary combinations and form of the cross- section. Twins. Optical orientation. Character and st. ength of double- refraction. Direc- tion of extinc- tion. 1. Silli- manite. A1 a Si0 6 . Not 3.23-3.24. || 00^00 Separa- Prismatic individuals A. P. || oo A i.M. X oP. Double- refraction attacked tion of without c' = c. positive. by acids. the thin needles defined termina- Z = a. ^ t. - \\oP. tions. 00 P = III . (See d 0. Small axial angle Fig. 72.) p = 44. Strong dispersion. 2. Stau- rolite. H 2 R 3 (A1 2 ) 6 Si 6 O 34 . R = 3Fe + iMtr. Insoluble in acids. 3-34-3-77- Perfect CO/* 00, imperfect 00 P. Separa- tion \\oP. Rarely irregular grains. Crystals ooP. oofoo oP. oo/> = 128 42'. Penetra- tion- twins^ wherein the c-axes cut each other at right or nearly A. P. || oo .Poo i. M. -L oP. c' = C. = a. a = 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. Decom- position. Occurrence. Remarks. light. Very brilliant, some- what Colorless, often colored red by In exceedingly long, thin needles, With quartz, orthoclase, biotite, and Very poor. Fluid inclosures. As primary accessory constituent in Distin- guished from : zoisite like Fe 2 3 generally muscovite. crystalline by the musco- on in large schists; character vite. fractures. numbers, rare. of the often double- fituly refraction fibrous and polar- and ar- ization- ranged colors; parallel; andalusite developed by the' in quartz, character cordierite, of the and other double- minerals. refraction (See and Fig. 77.) cleavage. Very brilliant. Light or deep yellowish brown. Relief very marked. Easily recogniz- able, especially in the longitu- dinal In large and small crystals the num- berless in- closures are character- With quartz, orthoclase, mica, and garnet. Inclosures of minute quartz grains, bitumen, hematite, are As primary accessory constituent in crystalline sch ists, especially Well character- ized by the color and pleochro- ism. /3p == 1-7526 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. MgSi0 3 . Only with difficulty attacked by acids. (3-I53) 9 Perfect w5! oo/oc. Longi- tudinal Long prismatic 00 P . 00 /OO oo/oo .mPo=. Octagonal transverse N.B That position is selected where 00/>= 9 2 lies to the Double- refraction positive, rather strong. Weaker (?) sections inclining sections with two front : A. P. 1 00/00 than in monoclinic to fibrous. controlling Pairs ff i. M. -L P c' = C. augite. oo P about 92. faces, similar to b = b. Separa- tion \\oP. monoclinic augite. # = a. (See Fig 7.) Like bronzite. The positive axial angle increasing with iron present. Dispersion not strong. P > v % and clear. [Comp. Zirkel. Min. p. 597. Ac- cording to Tscher- mak's position the optical orientation in enstatite and bron/ite is the following: A. P. || 00 /CO c 1 = C. i. M. b =a. d = b.] (See Fig. 74-) TABLES FOR DETERMINING MINERALS. 145 Polari- zation- colors. Color and power of i ell-acting light. Pleo- eliroism. 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 constituent Distin- guished from: olivine marked. which monoclinic of talc. in basic by the show lira longi- tudinal augite. Into bast it e (compare). Decompo- sition porphy- ritic eruptive rocks. fibrous tendency IU-; 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.) More rarely in crystals, generally into serpentine, yet mostly crystalline outlines quartzose rocks as quartz- porphy- ntes; in refraction and polariza- tion-colors; si Hi ma - decom- obtained. porphy- ite by the posed. rites, form More often diabase- (never in interpene- porphy- so minute trated || c rites, needles) with melaphyrs ; and monoclinic augite. also in gabbro cleavage ; the follow- and nonte. 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 3 ) n (Fe~Si0 3 ). Not 3-3-5 (3.12- 3-25)- Perfect and separates Elongated acicular. 00/>. 00/00 00/>00 Not rarely the large bronzite Like enstatite. [According to Double- refraction positive, like attacked by acids. according to predomi- nating; cleavage- leaves Tschermak emtatite. In sections oo Pea. Qf* ^>/oo || 00/00 or at right very bent, c a 1 angles to similar wavy, Large axial the middle to the through angle, lines (i.e., tnonoclinic repeated 6a - 9 o. Il^-Pand augite. twinning after Inter- mediary product oo/oo) only an opening cross, with Rarely in between traces of por- enstatite the lemnis- phyritic eruptive and hyper- sthene. cates. is visible. rocks In sections knee- at right shaped angles to twins an optic after axis one or ;;/ /oo in no ring is asteroid visible. crystal- line groups. 5. Antho- phyllite. n (MgSiOg) 3-187- 3.225. || 00/00 oo/>, In leaf-like masses, A. P. = 00/00 Double- refraction Not 3 ' attacked by acids. 00/00 imper- feet very rarely in crystals. Transverse i.M. i_oP & = b strongly positive. [Accord- sections a = a. ing to Tscher- mak imperfect like those of monoclinic hornblende. Dispersion clear about C = -v > p; large axial || 00/00, angle. con- choidal (See Fig. 8.) separa- tion after oo/o,.] */ = about 125 or 55- TABLES FOR DETERMINING MINERALS. 147 Polariza- tion- colors. Color and power of refracting light. Pleo- ehroisin. Structure. Associa- tion. Inclosures. Decom- position. Occurrence. Remarks. Inter- /3 = 1.639 Very Partly in With Inclosures Similar Like Can be dis- ference- Dark slight. large olivine, of brown to enstatite, tinguished figures brown. irregular plagio- rectangular bastite, accessory from : /ess grains in clase, leaflets, or into a primary monoclinic brilliant by far coarsely- granular mono- clinic opaque needles green fibrous constituent. Often augite only by examin- than in mono- rocks; partly in augite, magnet- distributed || 00^00 aggre- gate, with replacing monoclinic ing the transverse clinic sharply- ite ; like (or, after elimina- augite as sections and augite. defined crystals in enstatite. Tschermak, ii /> \ II oo./ ooj. tion of Fe 3 O 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- ism. Bril- liant. Dark brown. Strong pleo- Inclosures similar to With olivine, Inclosures of minute Very rarely accessory Distin- guished ftp =1.636. Relief not marked. chroism. Greenish yellow parallel bronzite. The longi- tudinal sections plagio- clase, augite, and horn- brown and greenish mica-like leaves, as secondary constituent. Decom- from : biotite by cleavage, strength of to the striations generally fibrous as blende. often regularly position- product of pleo- chroism, ( II ^). reddish brown at right angles to a conse- quence of cleavage. arranged ; otherwise very poor in inclosures. olivine in gabbro and olivine- fels. and magnitude of axial angle; hornblende them. Magnetite. by the optical orientation ; bronzite and hyper- stkene 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 of the cross- section. Twins. Optical orientation. Character and strength of double- refraction. Direc- tion of extinc- tion. 1. Hyper- sthene. Like bronzite, yet far 3-3-34. (3-34-) oo P perfect. 00/00 Large irregular grains and Knee- A.P. = oo/co shaped 2.M. -L oP. twins i.M.-Lco/'co \P positive. || 00/00 richer in iron. con- choidal minute colu tn ns in the crystals c' = C negative. Feebler separa- tion. 00/00 imper- $L about 92. of 00/>. 00/00 00/00, also : /oo 2/00.3/3. as in bronz- ite. a = (See Fig. 5.) Large axial angle. Dispersion about a than in the monoclinic augites. (Compare bronzite.) P ^ ?\ feeble. [According to Tscher- mak the acute oo P- angle lies to the front. then A.P =00/00 c' = c. a = b.'] (See Fig. 74.) TABLES FOR DETERMINING MINERALS. Second Middle Line (-[-) on oP. 149 Polariza- tion- colors. Color and power of refract- ing light Pleo- chroism. ritnicture. Association. Inclosures. Hccoin- powttoa. Occurrence. Remarks. Rather bril- liant. Com- pare bronz- ite. Light :o dark arown. Black from ron in- closures Strong, especially in the ongitudi- nal sections and the In large irregular grains in the granu- lar older , and in small, With plagioclase, olivine, and monoclinic augite. Numberless inclosures of brown or violet rectangular leaflets often in the Hyper- stheneoften decomposes into a dirty- brown or In grains in gabbro, norites in younger eruptive rocks ; especially Distin- guished from : bronzite by the character of the 1.639. thicker sections. Axial augite-like crystals in the large grains with marked greenish fibrous aggregate in augite- andesites, and feld- double- refraction and power- f.ll .tlAXV colors: (l "= younger porphyritic il 00^0, the d>~b. distributed especially augite. transverse often occur on those sections. in the at right and by the crystals. angles to feebler Otherwise the t'-axis. double- poor in It is a refraction ; inclosures. " bast it e- biotite by Vitreous like" absence particles. decom- of the position. marked cleavage ; hornblende by optical orientation and prismatic angle. ISO 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- 3- 54- bastite. (Dia- clasite.) In sections \\oP Partly in positive; rather conse- sharply- de fined quently negative /3. Bastite. Like bronzite. Contains H a O. 2.6-2.8. 00/00. oo P. crystals, partly in grains of columnar form, and in irregular leafy in- dividuals. oo/>= 93 . Very rare. Penetra- tion- twins. A. P. = II oo -Poo. 2. M. -L p. I. M. -L 00/00. c' = C Z= a & = b Dispersion p > V about a. Rather strongly doubly- 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. (J- 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 || 00/00. 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 oo/oo. 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 brilliant. Light yellow- Proto- bastite, Into bastite. As primary Distinguish- able from 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 deconipcsi- augite- plagioclase tion into rocks. bastite^ 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 tion c > a Commonly interpene- trated wiih olivine, i.e., serpentine. A metallic clinic augite, magnet- ite. bronzite. Inclos- ures of picotite and chromite. Bastite itself is always a decom- position- product As secondary decom- position- product of rhombic Distinguished from : serpentine by the stria- lion parallel to the vertical lustre on of augite in axis ; COA. rhombic olivine-fels, chlorite by Finely augite. gabbro, less perfect striated norite, cleavage, and par al lei to the vertical andesites, rarely in not running II oP, by the axis. Often melaphyrs, feebler pleo- shows a and 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 MONOCLINIC. sections of monoclinic augite, 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 augite in very thin sections are generally yellowish white I. O., II 0/>and a>/~oo (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 r-axis. The common poly- synthetic twins of monoclinic augites Hoodoo are wanting in the rhombic (seen especially well on transverse sections). 152 DETERMINATION OF ROCK-FORMING MINERALS. y. AXIAL PICTURE VISIBLE IN SECTIONS || oP.; 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- lusite. Al a SiO B . Not acted on by acids. 3.10- 3.17. Prismatic Imperfect after 00 /^OO, Rarely grains; long columns OOP.0/ J .POO. . c /->; Quadratic and transverse Poo. sections, COP = 90 50'. Separa- tion longi- tudinal sections, elongated \oP. rectangles. A.P. || 00/00 i.M. -LoP. Double- c' = refraction 1 = b strongly j3. Variety of anda- lusite: a = C. Large axial angle. negative. Ohiastolite. ditto. 2.9-3.1. Perfect 00 P. Long columns COP = t>> a. Often in long closures, of cor- hornfels. ".oisite by the pleo- 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 n 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- in e ta- by the regular present a peculiar of mus- covite lusite; whole in or ph ic schists inclosures. structure. and pseudo- (contact on Quadratic biotite. morphsof granite). cores, quartz, nclosuresof a mus- bituminous covite, substance, are and arranged chloritic at the centre substance and on the after four corners; chiasto- 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- ite.) Mg 2 R 2 2.59-2.66. oo/oo, imperfect Pco. In large grains and small If in crystals so often A W /oo l! i. M. -L oP. Negative, not very energetic. A1 2 2 Fe 2 . Scarcely 119 io~'. crystals. 00 P. 00/00. pene- tration c' = a b = C attacked oP. twins a, = b by acids. Hexagonal transverse and four- Axial angle rather sections and rect- angular lings after 00 PI large. Dispersion feeble longitu- rarely p < v. dinal after sections. 00/3. (See Figs. 23 and 78.) Finite Large (Decom- crystals = position- 00 />. 00/00. product oo/oo . P. of cor- dierite.) bb. Appearance of 2 M. Zoizite. H 2 Ca 4 s't Attacked by acids only with difficulty; after ignition, however, 3.22-3.36. 00/00 very perfect. Separa- tion II oP. Elongated grains and long transverse- limbed columns. P . 00/00 (see Fig. 79). Hexagonal A. P. II 00/00 always = c = I = b c' = a. Or if A. P. |i oP, InoP appearance of 2. negative M. positive. Feeble double- refraction. soluble transverse 7, n with sections. V U c' b' separation of amorphous Si0 2 . P = 116 26'. L U, very powerful dispersion p < v. About a! (if A. P. 1! oP, then dispersion p > 7/.) TABLES FOR DETERMINING MINERALS. 155 Polari- zation- colors. Color and power of refracting light. Pleo- ehroism. Structure. Associa- tion. Inclos- ures. Decomposi- tion. Occurrence. Remarks. Rather bril- Violet- blue. marked. Always in larger With quartz. Fluid inclos- Very common, Rare, as accessory In thin sections and in grains liant, like In very thin In thicker sections individuals; never in ortho- clase, ures, silli- especially if occurring primary con- stituent in often very similar to quartz. sections, well microlites. and manite in grains or granite, quartz, yet colorless. /3= 1.54- recog- nizable. In rounded 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. grains, or in small crystals ; plagio- clase, quartz, naste crystals, zircon. crevices, or completely decomposed and in grains in gneiss. Rarely in phenomena of decomposition on the crevices b = pale the latter in sani- vitreous (pinite) into crystals in in c. p. 1. to prns- eruptive dine. inclos- a greenish tracliytes If in crystals, sian blue. rocks. augite, ures. fibrous (twins !), and recognized by c = dark Meta- pleo- aggregate, in the 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- zation. of minute threads and ortho- clase, scopically by the crystalline leaflets. and orm, and from biotite. the decom- position. (negative) on oP. Gene- 1 Colorless The With Fluid Often Common in Distinguished rally feebly bril- white. /3p = 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. 79.) by the pleochro- 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); etistatiteby the optical orienta- tion, the polari- zation-colors, cleavage, and the oo /"-angle. 156 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 double- refraction. Direction of extinction. 1. Mica Group. 1 a. Mer oxene (Biotite). m R 4 SiO 4 n 2.8-3.2. Highly eminent as P quite 120. Rare. Twin- A. P. || cc Poo (mica As hexagonal The trans- n R 2 SiO 4 \P. OOP. 00 POO . n ing- second as a conse- verse VI Separa- oP. Thin plane, class). quence sections v R 2 Si 3 12 tions corre- tablets or short ooP; both in- A. P. paral- lel to two of the apparently generally apparent- R = K, Na. sponding columns. dividuals, opposite constant ly H, to the Transverse however, sides(coPoo) parallel isotrope; n pressure- sections forced of a extinction the lon- R= Fe, surfaces ( H oP) over each hexagon and of the gitudinal Mg, or hexagonal other in a and small sections R 2 = A1 2 , Fe 2 . Slightly "sliding- planes" (gleit- tablets without cleavage- plane quite H oP\ also with coinciding with a u fracture- axial angle, or, as the i. M. differs with parallel extinc- attacked by flache) cracks. several line" but little tion, HC1, - P 3 and More often lamellae (schlag- from the therefore but \P&>. with inter- linie), normal cannot completely "sliding polated. i.M =a to 0P, be decom- posed by plane" (gleit- Only the latter vary but little from apparently rhombic. studied in c. p. 1. H 2 S0 4 fiache), recogniz- the normal Negative. Ap- with three line- able to oP parently separation systems i. p. 1. Axial hexago- ot a silica- crossing angle nal. skeleton. each other at an generally very small angle of - 5~*3 60, and but rectangular variable, longitudi- being nal sometimes sections very large. ( || to the Dispersion b > a. feebler pleochro- itic, and generally arranged in tufts. 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 mer- Large Large See meroxene. biotite (meroxene); mer- oxene. oxene. thin hexagonal axial angle. rich in tables. iron. c. Phlogo- pite. A magnesian mica, nearly free from iron. According to Tschermak, a mixture of K 6 (A1 2 ) 3 Si 6 O 24 , H 8 S, 10 24 . 2-75- 2.97. See mer- oxene. See meroxene. See mer- oxene; also twins after oo P with indi- viduals lying near one another. A.P. n oopoo a nearly J-oP. c-.Q. = -2\. Dispersion Axial angle about 15. Negative Hke meroxene. Like mer- oxene, alwa; s parallel extinc- tion. and Mg 12 Si 6 O a4 in the proportion of nearly 3:1:4- d. Anomite. According to Tschermak, See mer- oxene; also here See meroxene. A.P.j. 00 ^ 00 (mica I. class). Negative. ditto. a mixture the o nearly of " gliding- \oP. H 2 K 4 (A1 2 ) 3 planes" Axial angle Si 6 O 24 very com- about and monly 12-16 Mg 12 Si 6 24 in dis- cernible. and less. Dispersion proportion One of p> V. of i : i the or 2 : i. gliding- planes parallel 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, olivme plagio- Augitic needles, ferric hydrate, Depositing ferric hydroxide. In basalts and lavas. Dis- tinguished from biotite only primary clase, nephe- and microlites only by the color. constituent 1.0. Is line, or leucite. regularly arranged only an at 60 Altered as in (pyrogene?) biotite. meroxene. 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 strength of double- refraction. Direction of extinction. e. Musco vite (and sericite). H 4 K 2 (A1 2 ) 3 Si.O M . Not attacked by acids. 2.76-3.1. Very perfect II oP: '' sliding- planes" Rarely crystallized in rocks; hexagonal tables. See merox- ene. A.P.j_oojPoo (mica I. class), a differing but little Strongly negative. Like magnesia- mica with parallel (gleit- from c' extinc- flache) as in nearlyj.o/'. Dispersion tion ; ap- parently mer- p > V. rhombic. oxene. Axial angle generally large, 60-70. (See Fig. 18.) 2. Taic. HjMgs Su0 13 . Not attacked by acids. Al- reaction. 2.69-2.8. Eminent \oP (imper- fect oo/>). Never in crystals; in rocks mostly in minute irregular leaflets like mica. A. P. || oo Poo II to a fracture- line (schlag- linie). a nearly JL0P. (According to Tschermak axial angle about 17.) Feebly negative. Apr parently rhombic ? TABLES FOR DETERMINING MINERALS. 161 Color and tion- colors. power of refracting light. Pleo- chroism Structure. Association. Inclosures. Decom- position Occurrence. Remarks. Exceed- ingly brilliant, Colorless, light green, As primary constituent With quartz, orthoclase, Very poor; rarely rutile As primary constituent Easily recog- nizable by the highly irides- oil-green I. O. in biotite. needles, in granites, eminent cent (red to yellow colors). large leaves and tables, in tufted and tourmaline hematite tablets, or tourmaline columns. especially tourmaline granites, and in cleavage and brilliant po- larization-col- ors; yet diffi- stellate Zircon. crystalline cult to distin- aggregates. schists: guish with the As especially microscope secondary prominent from talc. * product in gneiss, Sericite is in mica-schist, 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 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- iebenerite, line schists. etc. See musco- vite. Colorless, white, light green. Mostly in 'rregularly disposed interlaced With quartz, orthoclase, mica, Very poor. Biotite, actinolite. Like As primary constituent in many Difference be- tween musco- vite and talc : Muscovite oc- or or with muscovite. crystalline curs general- rosette- augite and schists. y in large in- sh aped olivine. Not dividuals stellate common. remarkable aggregates As second- for the basal of minute ary product cleavage, or ,- leaflets. in the de- 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 and groups. 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 nnd form of the cross-section. Twins. Character oiSffiSL stl ;Si h e- of refraction. Direction of extinction. 3. Chlonte Group. . Ripido- lite. (Chlorite in restricted sense.) Mixture oj /<2H 2 0. 3MgO. 2Si0 2 ) + q (2H 2 . 2MgO . A1 2 O 3 . 2.78- 2-95- Very perfect \\oP. Leaflets and six- sided tablets ooP. oP like hexag- onal. If Apparently hexagonal (optically- uniaxial), often clearly optically- Feebly negative. Cleavage- leaflets like isotrope. Longi- tudinal sections Si0 2 ). monoclmic, biaxial, with p :q t : 2. then ooP. wit ft very parallel oo Poo . oP. small axial extinc- b. Hel- Decom- ditto. Long ver- Six-sided a ngle. tion. minth. posed by ftiicttlar leaflets a J-oP. H 2 SO 4 . curled with re- columns. entrant angles. c. Pennin- ite. See a. f.f = 3:2. Decom- posed by HC1. 2.61- 2.77. duto. Crystals like rhombo- hedra oP. R or 3/? Pene- tration tkree- 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. Kaem- mererite. Contains Cr 2 3 . 2.617- 2.76. ditto. Irregular leaflets apparently P . oP. positions differing by 120 in leaves Clearly optically- biaxial. leaflets some- times isotrope, II oP.) some- times double- refract- ing. e. Clino- chlore. /:*= 2 : 3 . More 2.65- 2.78. ditto. " Sliding Crystals of motioclinic Com- monly in A. P. || co Poo, often also Generally positive. C : c = 12-15. difficultly planes" habit twins and _L ooPoo; decom- (gleit- &P . ooPoo. three- C quite-i-^f. 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 pvramid axial angle. " M>. Dispersion P < "!'. /. Chlori- toid H 2 R(AI 2 ) Si0 7 ; 3.52- 3.56. II oP. Not so ditto. Tablets. Common- ly tablets A. P. || oo Poo. Negative. a : c 5-12. R = FeO perfect of thin i. M. differs and and some as in the eaves de- about 12 MgO. others. veloped from the Decom- twin-like. -LoP. g. Sismon- posed by which are Feeble. dine. concen- placed at trated 120 to H 2 S0 4 . each other. TABLES FOR DETERMINING MINERALS. Color and Polari- zation- colors. POWIT (if ivf nil-tin}, light. Pleo- chroism. Structure. Assucia tion. IncJos- mvs. Decora position Occi rrence. Remarks. Primary. More com- Feebly bril- Light to dark Very feeble. The chlorites As do not for the primary Very poor. monly in leaflets in liant. green. most part con- Hema- chloritic bluish, M = 1-575 occur as rock- stituent tite and schists, as de- blue to green. constituents in large lamel- with quartz, hydrated ferric comppsition- product of lary hexag- ortho- oxide, mica, augite, onal tablets clase, and hornblende. ike the micas, biotite, needles ol and garnet. Difficult to but like talc in aggregates of tm nut e ir- regular leaf- lets either and musco- vite. i utile and actinolite. As decomposi- tion-product aftermicaand 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 See ripido- Leek to bluish Feeble. Green ditto. ditto. Rare as rock- constituent d isti nguish from each lite. green. shades. as above, in Clinochlore leaves. alone (also ottrelite) is well charac- terized through the Peach to Often inter- With By Rarely in pronounced blood-red. penetrated olivine, decom- serpentines. pleochroism with clino- augite, position as well as the chlore. and is de- common chromite. color- twinning; zedand more easily resem- determined bles by optical talc. examination. More brilliant than in the Dark oil to bluish green. Often very strong. In sections In larger leaves, yet not so marked by With quartz, ortho- clase, and ditto. Primary. Common in crystalline schists, as Ottrelite is marked by the greater lardness, less other -LoP lamellae as mica. chloritic perfect chlo- rites. Indigo- yellow. yellow ; II c light green, yellowish green. mica. With augite, horn- blende, olivine, schist, and secondary in serpentine. cleavage, absence of laminations, and richness of inclosures ; -L c blue- or ser- also dis- green, dark- pentine. inguished by chemical green. quantitative See Dark See ditto. With Fluid in- Chloritoid in ana ysis. clino- green. clino- quartz, closures certain semi- chlore. chlore. ortho- very crystalline \\c clase, and common. schists. yellowish mica. Rutile Sismondine green. With needles. rarely in -Lr augiie, glaucophane- greenish rutile, eclogite. blue. titanite, glauco- phane. 164 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 douijie- ref faction. Direction of extinction. k. Ottrelite. H 6 R 3 (A1 2 ) 2 Si 6 24 . R = Fe,Mn. 4-4(?)- oP very perfect. Besides, Thin spherical tablets; See sismondine; poly Optically biaxial ; i.M. rather Very feebly negative. Com- monly parallel Attacked by H 2 SO 4 according rounded synthetic sharply to the only with difficulty. with an angle of 110-120 sections || oP rare. Elongated striations \\oP. (Fig. 82.) the perfect cleavage- planes; axis of the cross- section. (Becke). rectangles small axial if the angle. * sections are inclined to oP. 2. MONOCLINIC act. PLANE OF OPTIC AXES GENERALLY _LcPco; PERFECT 1. a. Ortho- clase. K 2 AI 2 2.50- Eminent In grains, Very A. P. Rather In Si 8 16 . Not 2-59 (2-57). { tf/** or partly columnar: common, especially generally -L ooPoo, feebly negative. sections or attacked by acids. Small Cleavage- angle 89 40'. oP. oo Poo. 00 P. 2 POO. SPoo.P, after the following three laws: equally inclined with 0Pand cleavage- leaflets || oof oo 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 law. 69 n'. from the crystals. Twinning- c = ^ edge oP : co Poo. oo P. plane oopoo a: a = 5. ooPoo = oP 2 Poo. combined True axial a: a about Poo. or inter- penetrated in the direc- angle = 69. (See Fig. 14.) Axial 5 18'. Sections tion of the dispersion parallel b, ^-axis. 2. = p > v. that is. Baveno law. In sections from the Twinmng- parallel zone oP: plane2Poo, especially 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 Manebach visible. law. A. P. is rare. TABLES FOR DETERMINING MINERALS. i6 5 Poiariza- tlOII- coiors. Color and power of reiracting light. Pleo- chroism. Structure. Associa- tion. Inclosures. Decom- position. Occurrence. Remarks. Not Greenish Rather Com pare with With Generally Rare, in Ottrelite is brilliant, black; in power- "Inclosures." quartz, exceed- semi- triclinic similar to ripidolite. sections light to grayish ful; 11 oP lav- ender- In large tablets of a black color mica (musco- vite), ingly rich in in- closures crystalline and metamorphic (Renard). Cleavage after fP\ also not green. blue. (rather hard). rutile of color- schists. after oo />, but bluish Cleavage || oP needles, less intwodirections green, never so garnet. quartz cutting each II * 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 oaxis. CRYSTALS. CLEAVAGE [| oP AND cojPoo, 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 very thin from decom- in eruptive rocks; always blende, rarely closures, apatite opaque, and rocks. Essen- tial primary minerals by the twinnings sections position^ in grains in augite, needles, non- constituent in in sections and in micro- gray; tinged crystalline schists. plagio- clase, zircon. trans- parent; granite, 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 || oo^Poo; musco- rocks; more- and sanidine, are dull, generally graphic- granite-like, vite or over, in the crystalline so often appearing in the blue- gray, as, e.g., nephe- with quartz (micro-peg- matite). (See Fig. 63, (?.) epidote. schists, especially the gneisses; here often ground-mass of rocks, have often a marked similarity to hne. Zonal struc- glassy, as nepheline. ture rare; also sanidine. inclo^ures zonally 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 extinctioa b. Sani- dine. As above; Sanidine in Twinning- Parallel crystals minute, plane = oP. ooPoo: fur- rowed. long, narrow (See Fig. 28.) Cross- b = * a : & ' threads, as micro! ites, sections of twins: equals 5. or large a. In the crystals, Carlsbad never in twins the grains. rectangular Form of sections at cross- right angles sections ooPco parallel oP divided into and oo Poo halves long and thread-like; parallel to the edges parallel oP/ooPoo coPoo and distorted hexagons whose sides 00 P/ 00 POO. b. In the Baveno correspond to twins the quadratic oP.ooP. sections COPCO. at right In columnar angles oopoo types of the crystals: are divided into halves rectangular sections if by the diagonals. at right angles to tfP.ooPoo, octagonal if besides these also 2Pco JS present. (See Fig. 83.) . TABLES FOR DETERMINING MINERALS. Polariza- tion- colors. Color and power of refracting light. Pleochro- ism. Structure. Associa- tion. Inclosures. 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 -water. /3p = I. O. and minute threads 1 1. 0. clase; besides with inclosures, especially vitreous rarely opaque. In ent of the trachytes, rhyolites, hexagonal transverse sections are i-^n- in eruptive rocks. The large crystals augite, nephe- line, and leucite, generally zonally disposed, andesites and trachytes a decom- phono- lites, and the glasses of orthoclase. Grains of orthoclase are often crumbled or never with mus- augite- microlites, position into opal. the ortho- clase in isotrope sections fused, and covite. apatite rocks; are with an . exceedingly beautiful needles. accessory in nearly all of the easily distinguish- ed from zonal structure, seen younger plagio- clase quartz by the condenser, particularly well. rocks, as in orthoclase 1. p. p. 1. cross- Inclosures sections are common, one of the arranged in zones. optic axes is visible. Orthoclase is distin- guished from plagioclase by the optical orientation and absence of the polysynthet- ic striation. 1 68 DETERMINATION OF ROCK-FORMING MINERALS. bb. PLANE OF OPTIC AXES NAME. Chemical composition and reactions. Specific gravity. Cleav- age. O dinary combinations and form of the cross- section. Twins. Optical orientation. Character and strength of double- refraction. Direction of extinction. 1. Mono- (- 17-3 4i) clinic Augite Group. a. Ordinary RSiO 3 3-34-3-S 8 - Emi- Rarely in Very A.PJooPoo; Positive, In and basaltic R = Mg, Ca, Fe, nent 00 P. grains; in crystals: common. T win- the i. M. = cis strong. sections parallel Augite. and Fe 2 O 3 and A1 2 O 3 00 P. 00^00, ooPoo./>, ning- plane wanting in the c : c = Mixture of CaMg and sometimes ^ r> also in obtuse angle /3, about 39. Varies Si^0^+ Ca P .of poly- b = b from 39 Al a Si0 6 . (Tscher- mak.) /^-rich and -P Sections at right angles to the c-axis are synthetic twins. (See Figs. 24 and 25.) More rarely (See positive axial angle, as in the to 54. a : to edge 0/yoojpoo about 22. Sections augites. Not attacked by octagonal, with evident penetra- tion- twins rhombic augites, decreases 1 b have parallel extinc- acids. prismatic cleavage. The longi- tudinal after: twinning- plane a face with the iron present, about 60. tion. In sections inclined to oojPoo sections distorted hexago- nally with the Pv>\ or after: twinning- plane a face f>2. Sections at right angles to the c-axis and t:c de- creases to parallel c-axis parallel to parallel show the with cleavage- fissures. Also rect- angular condenser one optic axis exactly in the parallel centre of OOjPoo; the field. often a rhomb. (See Fig. 84.) TABLES FOR DETERMINING MINERALS. :6 9 EMINENT CLEAVAGE AFTER o> p = 87' Polariza- tion- colors. Color and JlllWCf Of 1C light Pleo- ohrolam. Structure. Associa- tion. In- closures. Decomposi- tioii. Occurrence. Remarks. Verv In Gen- In large crystals Princi- Vitre- Augite As essential Easily dis- brilliant, especially in the light- colored, yellow to red. sections green to brown, often violet to brown in the erally very feeble, yet augite is strong- I. O. and col- umns in micro- litesII.O. The first very com- monly show a zonal structure, a green core, pally with pla- gioclase, nephe- line, leucite, with or ous in- closures are com- mon, as are also crystals are com- monly de- composed into a pro- duct of chloritic primary constituent in many younger prophyritic eruptive rocks: tinguished from other optically- biaxial minerals by the important basalts. ly pleo- e.g. with brown without gas- material, diabases, oblique The chroitic layers which in olivine pores calcite, melaphyrs, extinction same as in turn are often and and ferric augite- C " C crystal the again com posed 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 c leafage 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. cal differences, quartz. minerals Rare and in tions ; more Absorp- as in directions after augite larger 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 frequent. (See More eruptive clined to = b. Fig. 45.) As rarely the rocks, and the oaxis with orthoclase, metamor- in the the successive phosis into crystalline cleavage- layers in twins hornblende schists. angle ap- of augite (uralitizing) proaches run equally and unimpeded through both in- dividuals. Au- wherein the form of augite re- mains with that of hornblende. Easily dis- tinguished ?ites often show the so-called tiornblendic cleavage. from epi- dote by the " hour-glass " Finally the color, direc- formation rare meta- tion of ex- where sections morphosis tinction, re- II OO,POO divide into lief, and into four fields of which each serpentine, with forma- polariza- tion-colors. two lying oppo- site extinguish at the same in- tion of talc and chlorite. If augite is perfectly colorless, stant. (See the polari- Figs. 46 and 47.) Augite crystals zation- colors are are often fused, very bril- also commonly separated into liant and resemble large aggre- olivine. gates, the so- called " augite- eyes," or needles radially grouped. DETERMINATION OF ROCK-FORMING MINERALS. Chemical Ordinary Character Direc- NAME. composition and reactions. Specific gravity. Cleavage. combinations and form of the cross-section. Twins. Optical orientation. and strength of double- refraction. tion of extinc- tion. Z>. Diallage. See augite. 3-23- 3-34- oo/>(8 7 ) concen- trically arranged after Rarely in clearly-defined crystals, mostly in large tabular or granular [| co Poo poly- synthet- ic; not rarely S ee augite. oo fcx>. individuals, afters/ 1 . fibrous parallel to the without and 03 POO, terminal planes. Cross-sections resemble augite. TABLES FOR DETERMINING MINERALS. 171 Polariza- tion- colors. Color and power of refracting light. Pleo- chruism. Structure. Associa- tion. Inclosures. Decomposi- tion. Occurrence. Remarks. See augite. Greenish brown. Very feeble. Occurs only in large ir- regular With plagio- clase, As in bronzite. Inclosures Formation of uralite common, in Primary constituent. Common in Often resembles bronzite. grains. A great simi- larity in tructure to bronzite es- ordinary augite, olivine, horn- blende. of brown leaflets of gothite parallel that diallage changes at the ends into dark gabbro, norite, rare in porphyritic, eruptive Easily dis- tinguished from it on sections or cleavage- pecially as regards in- closures, Rarely with quartz. otherwise poor in inclosures. green, strongly pleo- rocks. In serpen- tine and leaves {"'ri separating chroitic, olivine-fels. appearance into fibres, hornblende Rare in of one optic and twin- fibres. crystalline axis. nings. Into schists. Often inter- viridite; penetrated nto serpen- with ordi- tine with laryaugite, formation U1 J lornblende. of chlorite or mica. and talc. Rare in crystals. ditto. Grass- See Only With Rare. Fluid In eclogites See augite. green. augite. known in resh grains poor in in- quartz, horn- blende, inclosures and needles of rutile. and amphi- bolites. They are distinguish ed from : closures; garnet, aitgite by often inter- zoisite, the paler penetrated disthene. color with rutile. (small lornblende. amount of Often en- Fe) and by veloped by surround- the crystalline ditto. ditto. ditto. ing grains. With olivine, Very rare. Vitreous As primary constituent form; diallage by the lack of chromite. inclosures. in olivine- the perfect diallage, fels (so- separation and the called chro- after cofco. rhombic mium diop- augites. side). Rarely sec- ondary as metamor- phic pro- duct of garnet. (Pyrope.) Very Pale With Rarely In brilliant. green to colorless. quartz, horn- changed to uralite. crystalline schists. Relief blende, marked as garnet, a conse- scapolite quence o: 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. sectioii. refraction. f. Acorne. Na a Fe a Si 4 12 . 3-53- 3-55- Eminent 00/>, 87: In grains or columns PCO, com- mon. S A.P. || oojPoo. Large axial se augite. Positive. C : c = very small imperfect ca Pco, angle. angle OOiPoo. elongated Sections = 2-7*. by pre- or leaves dominance || co Pco show of faces a distorted oo Poo. axial picture of a biaxial mineral. g. Woilas- CaSiO 3 2.78- Parallel oo/> = 87. ditto. A.P. || oojPco. Positive. c forms tonite. by HC1 perfectly decomposed 2.91. 00^00, P, and /'co. Only in prisms, irregular, Apparent axial angle = about 70. strongly. with oP towards the front, "with elongated, (Compare 32 12'. separation fibrous. Fig. ii.) a : c = 12. of following amorphous Si0 2 . the /'-ortho- axis. cc. PERFECT CLEAVAGE 2. Horn- blende Group. a. Ordinary m RSiO 3 + 3-3- Highly eminent osP. oojPoo, co^oo, and ditto. A.P. || ocjPoo. The Strongly negative, C : c =. about and * R 2 O 3 . 00 P. oP ' . P or jPoo i. M. = a yet some- Varies basaltic R=Ca, Mg, 124 ii'; almost falls in the what from 2-18. Horn- Fe. imperfect always in obtuse feebler a : c = 75. blende. R 2 = Al a , OSPCO crystals. angle /3, than Fe 2 . Only those and coPoo. rarely in grains. b = b (see Fig. 9). augite. 29 58'. C : c = rich in Fe Transverse True axial 13-15 partially attacked by acids. sections generally hexagonal, angle about 79, the posi- tive axial in green horn- blendes, also angle becom- and = octagonal, longitu- ing larger with in- 11-13 and less in dinal creased per- brown. sections, as centage of in augite. iron. (Fig. 86.) Parallel oP and co Pco side appear- ance of one optic axis on the circum- ference of field. Feeble dispersion P < v. b. Smarag- dite. See uralite. TABLES FOR DETERMINING MINERALS. 1/3 Color Polariza- tion- colors. and power of ivt'r.i cl- Ploo- chroism. Structure. Associa- tion. Inclos- urcs. Deeom- posit.on. Occurrence. Krmarks. ing light. See augite. Dark brown, Rather strong. In large crys- tals in the With elseolite, Earthy par- Not rare in elceolite- dark cdark syenites, often sodalite, ticles. syenites, green. brown; with fibrous micro- phonolites, /3 above a brown- terminations. cline, 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 bril- Color- less, In aggregates of fibrous With calcite, Fluid inclos- As decom- position- Resembles tremolite, but liant. yellow- ish white. individuals in tufts or radially disposed. green augite, granite. ures. product or contact-mine- ral rare in distinguish- able by the prismatic Relief granular angle, solu- marked chalks meta- bility in acids, morphosed from eruptive rocks. R;ire and gelatin- izing; difficult to distinguish in elaeolite- from zeolites syenites and as scolecite, phonolites. e-g- oo /> = 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 augite; yellow /3p = 1.62. a = yellow- green or rarely in small crystals and microlites II. O. plagio- clase, quartz, Fluid inclos- ures, and bleached through granular and porphyritic eruptive augite by the prismatic cleavage- to honey- The green biotite; glass, decom- rocks : angle, slight green- yellow; hornblendes are more gas- position syenite, dio- inclination of ish b = often fibrous; rarely pores, into rite (green c : c, and brown. yellow- the brown often with earthy epidote, hornblende), powerful brown; c = beautifully de- veloped in augite and tides, calcite, ferric hy- porphyrite, trachyte pleochroism; biotite on black or zones. The olivine. apatite droxide, (brown, more sections at greenish brown horn- needles. then rarely green, right angles brown. blende of the often sur- hornblende). 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 of in olivine-fels and powerful (see Fig. 44), or mag- (green H.). dichroism is pseudomorphs netite; Common in wanting in of augite and always as crystalline such sec- magnetite after augite. schists(green, tions ( || 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 with With chlorite. hornblendic augite. ompha- schists.certain cite, gneisses, ec- garnet, logite (so- zoisite, called smar- rutile. agdite). 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. c. Actinolite CaMg 3 Si 4 12 + Ca Fe 3 Si 4 12 3.026- 3.160. As above; separation at right Long prisms, generally without Rare. See Hor nblende. C : c generally JS - Al-frce angles to terminations />- poor the c-axis. oo P. oojPoo. (Tschermak). RSiO 3 and R = predomi- nating Mg, less Ca, and a little Fe. d, Tremo- lite. 3 MgSi0 3 f CaSi0 8 . MgO pre- 2 -93-3- 00 P like hornblende. ooP. oojPoo generally in long narrow Rare. Like horn- See hor iblende. C:<: = 15. dominating. Separation prisms. blende. Una.tta.cked at right by acids. angles to the c-axis. e. Arfved- sonite. Na 2 (Fe) 2 Si 4 O l2 . Insoluble in 3-33- 3-59- oo P like hornblende. In large grains. Se : hornblen de. acids. i f. Glauco- phane (Gastal- dite). Na 2 (Al) 2 Si 4 12 . Contains Ca, Mg, Fe. Nearly 3-i- Like hornblende. Separation at right angles to Elongated prisms, generally without terminal See hor iblende. c:r = 6i -7 unattacked the c-axis. planes. by acids. g-. Uralite (Smarag- ditempart). Like ordinary green hornblendes. 3-1-3-3- Like hornblende; often, however, See " Structure;" single fibres show oo P Se s hornblen de. showing = about 124. in addition Part in the the augite- form of cleavage aiigite 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- Light to dark cdark green, a Generally occurring in With quartz, Very poor. Often pertect Rather common in Dis- tinguished blende. green. yellowish Jreen, ebler than in long narroTV needles or grains, often fibrous at the mica, chlorite, rulile. pseudo- morphs of biotite, chlorite, certain non- feldspathic crystalline schists. from 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 Colorless, In long. With Very Into As contact- Compare brilliant. relief columns, the calcite; poor. calcite mineral in vvollaston- 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- green. strong. often fibrous thoclase, elaeolite tinguished blende. grains and long micro- rocks. 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 eclogitfs, blue, with green garnet, are amphibo- c = dark hornblende. zoisite, com- lites, mica blue. chlorite, mon. ami Absorp- ompha- chlorite tion cite, schists. c > b > a. rutile, titanite. Generally aggregate polariza- tion, as the Dark to light green. Partly strong, partly weak. Finely-fibrous decomposition- product of augite and diallage, often With plagio- clase, olivine, diallage, In gabbros and serpentines; in augitic porphyries. Compare with dial- lage and ordinary hornblende. separate of the form of augite. Anordinary horn- blende augite and with remnants of the green hornblende threads augite or dial- occurring have not the same lage yet fresh. The fibres show in eclogue was also optical the prismatic called orienta- angle of horn- smaragdite. tion. blende. (See Fig. 85.) 7 6 DETERMINATION OF ROCK-FORMING MINERALS. dd. CLEAVAGE || P AND NAME. Chemical composition and reactions. Specific gravity. Cleavage. Ordinaiy combinations and form of the cross-section. Twins. Optical orientation. Character and strength of double- refraction. Direc- tion of extinc- tion. Epidote. H 2 Ca 4 (R 2 )3 Si 6 26 3-32-3-S- Highly eminent Generally very small prisms, Rare micro- A. P. || ooPoo at right Strongly negative. a : c = 2 10' (R a )=(AI 3 ) iFe,). Slightly \oP, and per- fect >P<* elongated in the direction of ortho-diagonal scopi- cally. Twinning angles to the elonga- tion of the C : a = 27 47' = C : oP. attacked by forming axis, the com- plane crystal. HC1. an angle bination 00/>. 00^00. b = , i. M. of H524'. oP . f . co Poo predominating. (See Fig. 89.) (See Figs. 26 and 88.) = a nearly coinciding with c. The longitudi- nal sections Sections || parallel oofoo oo Poo show are hexagonal. a biaxial The transverse interfer- sections at right angles to c and ence-figure, as the 2. M. sections parallel is at nearly oP : oo Poo are right long and nar- 1 IgUI. angles. row, rectangu- lar or (See Fig. 8? ) hexagonal, /*/ with one pair of sides longer ; in grains. ee. CLEAVAGE IMPERFECT oo p OR Titanite. CaSiTiO 5 ; 3-4-36. oo P Mostly crystals: Rather A. P. || oojPoo Strongly o: c = contains *33 5 2 ' oo P. oP. &Poo; common; i. M. = cat positive. 39 '7' FeO. jPw \4tP or %Pi contact- nearly right tt:a = Decom- "3 3', prominent with or pene- angles to 21. posed by imperfect. oP . Poo . l^iPoo . tration- l^foo; H TK) 2 4 ; dissolved with forma- tion of gypsum. jPoo, or in acute "wedge-sh aped grains. Such are character- istic crystal cross-sections. twins; twinning- plane = oP. (See Fig. 27.) very strong dispersion of the axes. P > 7'. (See Fig. 13-) (See Fig. 90.) TABLES FOR DETERMINING MINERALS. 177 %& Polari- zation- colors. Color and power of refracting light. Pleo- chr^ism. Structure. Associa- tion. Inclos- ures. Decom- position. Occurrence. Remarks. Very bril- liant, yellow Lemon- yellow, yello-wish green. Rather powerful in the thicker Generally in long minute prisms, With quartz, ortho- clase, Very poor. Fluid inclos- ! Secondary min- eral. Common as decomposi- tion product of Similar to augite, distin- guished from it by the even to red. Relief very marked. /3 = 1.72- 1-75- prisms. a = very pale yellow, b= brown to yellowish green, lying in chloritic matter, or in pseudo- morphs, more rarely in grains. plagio- clase, horn- blende, biotite, augite with chlorite. ures. the feldspars, hornblende, biotite, more rarely of augite, in eruptive rocks and crys- talline schists bearing these parallel extinc- tion in sections parallel to the longest development (= ^-axis) and slight inclina- c = green to lemon- minerals, also often as tion of a. '. c. The yellow yellow. Absorp- primary con- stituent in the color, powerful refraction of tion b > c> a. latter. light, and brilliant polari- zation-colors are character- istic forepidote. \ poo; ACUTE WEDGE-SHAPED CROSS-SECTIONS. Feeble, K ra y- i.e. like original color; much Pale yellow, reddish brown to colorless. Rather strong in the darker colored varieties, tt = red- Rough sur- face of sec- tion is characteris- tic for titanite. With ortho- clase, plagio- clase, horn- Very poor. Rarely pseudo- morphs of calcite after As primary accessory constituent in eruptive rocks. Granite (rarely), Easily recognizable by the almost constant wedge- shaped cross- sections, weaker than augite and dish brown. C = green- ish Commonly associated and inter- penetrated blende, augite, biotite, chlorite, titanite. syenite, phonolite, leucitophyr, elaeolite powerful refrac- tion of light, and rough surface. horn- yellow. with augite quartz, syenite, blende. c> b> a. and horn- and other trachyte, mica- /3p = Weaker blende. accessory andhornblende- 1.905. than in One of the minerals. andesite, Relief the horn- minerals diorite and uery blendes. first formed in crystalline, marked. in the especially eruptive hornblendic rocks. schists. Secondary as decomposition- product of ilmenite and titaniferous magnetite. 1 7 8 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 strength of double- refraction. Direc- tion of extinc- tion. Gypsum. CaSC>4 + 2H 2 6. Difficultly soluble in 2.2-2.4. Highly eminent clino- diagonal, In granules or elongated prismatic individu- Very rare in micro- scopic individuals. A.P. || oo Poo i. M. = a. One optic axis Strongly negative. d: c 52 3'. C: c 37 3'- acids. perfect als, crystals nearly according osP.cofoo. j_oof oo. to - P -P. One forms 83 with <:, the other 22. II. b. 3. Minerals Crystallizing a. LONG COLUMNAR CRYSTALS, COLORLESS OR OF A BLUE COLOR, Disthene. (Cyanite.) Al 2 SiO B . Acids have no action. 3.48-3.68. Highly eminent II oo/oo, perfect 00/00, Grains, or elongated prisms, 00/00 predomi- Common; more rarely on micro- scopic individuals. A.P. forms with the edge oo PM : oP an angle of Rather strongly negative. In sections parallel C0/>00 and parallel oP. nating, oo/oo with an angle of Twinning plane either: 30; with w/oo:<7/> an angle 90*7 (Gleit- 106 15', I. 00/00 60 15', and fldche.') rarely with terminal repeated; z. At right like the i. M. = a planes. angles to is at right Transverse sections the oaxis; 3. At right angles to oo/oo. rectangular angles to (See Fig. or hexagonal if oo 'P or oo /"is added to the above combina- the 2-axis; 4. Parallel oP, caused by pressure, and re- peated. 16.) Large axial angle, about 80. Feeble dis- persion of the axes, v < p. tion. In sections parallel oo/oo a bi- axial inter- ference- figure with negative middle line is visible. TABLES FOR DETERMINING MINERALS. Polari- zation' colors. Color and power of refracting light. Fleo- chroism. Structure. Association. Inclosures. Decomposi- tion. Occurrence. Remarks. Very bril- liant. Colorlfss, secondary, often In minute granules, Rarely with clastic constitu- Fluid inclos- ures. As simple rock, granu- lar or Irides- cent. colored by iron compounds. and tangled ents as or parallel quartz gran- fibrous ules or compact. aggregates mica of needles. leaflets. Rarely in crystals. in the Triclinic System. OR GRAINS. CLEAVAGE OO^CD . oo/'oo 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- 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, es- pecially parallel 00^00. blu7. less fissures parallel or at right angles to the chief axis, often irregularly rarely with orthoclase. finely fibrous, felt-like decomposi- tion- product. schists, granuhte, eclogite, and especially in many sillimanite, with which it commonly occurs; only possi- ble by the a = or com- micaceous determina- white. pletely schists. tion of the colored position of blue. the axes of Rarely in elasticity. a g re g ates of thin needles or filaments; the needles cracked and broken at right angles to the chief axis. ISO 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. l.Potassium Feldspar. Microcline. (Microper- thite, so-called See ortho- clase. 2-54- 2-57 (2-56). Highly eminent \\oP. Eminent Very similar to orthoclase, oo/oo . oP. Rare. Countless thin lamel- l, predomi- nating. clase are developed parallel to oo/oo and section with forms with the obtuse parallel oo/oo; positive double- a cleav- age- leaflet parallel at right refrac- oP angles to oP. ~ C /oo tion. does not it, so that in sections 5-6 in the obtuse therefore extin- parallel oP a latticed angle dc. guish parallel inter- Cleavage- like penetration leaflets ortho- of two systems of parallel 00/00 clase^ but 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- microcline. what in- -{-15-16; 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 of both species of plagioclase fall in one plane. (See Figs. 9I-93-) TABLES FOR DETERMINING MINERALS. 181 COLORLESS; CLEAVAGE PARALLEL cP AND Polariza- tion- colors. Color and power of refracting liKht. Pleo- chroism. Structure. Association. Inclosures Decompo- sition. Occurrence. Remarks. Exceed- ingly bril- Colorless. Relief not so In rocks only in grains ; commonly a. With orthoclase, elaeolite, Generally very poor; Fibrous decom- position As primary essential Distinguished from: orthoclase liant. marked interpene- sodalite, of mine- with constituent by the as in ortho- trated with quartz, like augite, and rals; horn- opacity as in with orthoclase oblique extinction on clase. graphic- hornblende. blende, ortho- in : 0/>, and the granite. biotite, clase. a. Elaeolite- interpene- also with b. With zircon, syenite; tration sodalite and quartz, apatite. of twins; elaeolite. orthoclase, b. In the other Compare the twinning development. An ortho- biotite, hornblende, muscovite. different granites, especially graphic- triclinic feldspars by the latticed clase or c. With granite; structure feldspar these and and (interpene- correspond- garnet, tration of ing to microcline cyanite. c. In crystalline twins) parallel oP was called schists (as 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 related to and albite, which gneisses. ' can be especially well observed in sections parallel co/* oo, or cojpoo, as spindle- 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 extinc- tion. 2. Plagio- clase, Calcium- Sodium Feld- spars: a. Albite Ab. 3.6 l- Emi- oo/oo .oP. Almost always A. P. forms Rather On Na 2 Al 2 2.63 nent oo'P.ao f". twinned. with the strongly cleav- Si.0, e with traces of (2.62). parallel oo/oo; very similar to i. Albite law. Tw in n ing -plane oo/oo and generally oaxis an angle of 96 16', with positive. age pieces: parallel CaandK, 1-2%. imper- fect orthoclase. (See polysynthetic; there- :ore in sections from the normal ;o oo/oo an oPthe oblique- Not at- oo /" Fig. 94.) the zone oP\ co/oo angle of ness of tacked by and P,. i. p. p. 1. the single 16 17'. extinc- acids. Si0 2 --= 68 #. Right edges oP: individuals appear as fine la mellce with varied polariza- i.M. = C. Dispersion feeble tion to the edge (and 00/00 = tion-colors. Only p < z>; oP : oligo- clase Ab 6 An lt 93 36'. those sections Parallel oo /oo show large axial angle. + 354~' albite). no twinning-stria- Cleavage- to tions. Two such leaflets + 4 5i' polysynthetically- parallel (+4 twinned albite indi- oo/oo show 30'); viduals are often quite com- parallel again combined plete dis- 00/00 according to the Carlsbad orthoclase torted inter- fere nce- also = twinning-law. 2. Pericline law. figure (ap- pearance of to -\- 20 (+ 19)- Twins according to the law : axis of the positive middle line rotation the ~b-axis^ perpendicu- lar to composition pla ne the rhombic section, oc/oo); yet, i.e., the plane so cutting the rhom- boidal prism ao'P. the large axial angle, co/* that the plane angles which these position of 45 the planes form with oo/oo are equal to hyperbolas do not lie each other. The in the field twinning-edge hereby forms with c inclined to the the edge . oo/oo an angle of 13-22. sharp edge oP : oo/oo. Such twins are (See often again united after the Manebach 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 that of m icrocline. Compare Figs. 29 and 30. TABLES FOR DETERMINING MINERALS. 183 Polariza- tion- colors. Color and power of refracting light. Pleo- chro- ism. Structure. Associa- tion. Inclosures. Decom- position. Occurrence. Remarks. For the Colorless, In large With Very Rarely Common in The most part very brilliant. Not so powerful as in quartz; in very thin sections feeble, blue- clear as water. Relief feebly defined. &P= 1-537- grains, rarely in crystals; often inter- penetrated with orthoclase and quartz. Compare microcline. In calcite, quartz, mica, and ortho- clase ; chlorite, more rarely with horn- blende. poor; fluid in- closures. decom- posed. Fibrous, opaque decom- position. See oligo- clase. granular limestones. In crystalline schists, in many semi- crystalline gneisses, phyllites, sericite- schists. polysynthetic twinning after oo^oo is peculiar to all plagio- clase, and is exceedingly characteristic for them. The triclinic feldspars can be distinguished green. eruptive Rare in from each other rocks as eruptive accurately only thin fibres. rocks, in by chemical grains in analysis or by diorite, determination in fibres in many of the oblique- ness of extinc- andesites tion on oP and and oo^oo on cleav- porphyries . age-pieces from grains or larger crystals. It is therefore impossible to specify with accuracy the minute plagio- clase threads 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 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. 2.62- 2.65. (2.63.) Most perfect II oP, See albite. Always polysyn- thetic Very similar to albite. In See albite. Parallel oP to the edge oP : oo /oo But little K. = AbsAnj also after oo/oo twinning according to the cleavage- planes parallel = + i 10' (AbgAnj = -1- 1 4'); to Ab 3 An 1 . like albite. Albite laiv; also 00/00 the axial parallel 00/00 oP. oo/'oo according to the points lie still farther to the edge oP: oo/oo right = 93 28'. Pericline law. beyond the field than = 2-4, (Ab 3 An 1 = in albite. 4 36')- c is inclined to the obtuse edge oP: oo Poo. (See Fig. 98.) c. An- desine. Ab 3 An 1 to AbjAnj. 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/~ - i 57' to 2 19'; parallel 00/00 -*Ig to TABLES FOR DETERMINING MINERALS. I8 5 Polari- zation- colors. Color and power of refracting light. Pleo- chroism Structure. Association. Inclosures. Decomposi- tion. Occurri'iuv. Remarks. See albite. Colorless clear as In large grains or With orthoclase, Fluid inclosures Generally fresh in the As primary essential or See albite. water or clouded, white, grayish white. crystals I. O. and as minute, elongated, and narrow quartz, hornblende, biotite, augite, olivine. rare, and vitreous inclosures common in the younger eruptive rocks, in the older accessory constituent in eruptive rocks, granite, threads (cross- sections of younger eruptive rocks, fibrous and clouded. Metamor- diorite, diabase, gabbro, thin tablets). augite- and apatite- phosis into epidote trachyte, andesite, Zonal microlites. called also basalts, develop- saussurite; and in ment (see into mus- crystalline Fig. 102) covite schists, and zonally similar to e.g. disposed orthoclase; gneiss. inclosures. observed Almost also in always nearly all twinned plagioclase. 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, in tonalite, hornblende. (quartz- biotite, diorite), in quartz. andesites, especially dacites and augite- andesites, porphy- rites, syenites, also in crystalline schists. 186 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- Ab, An! to 2.68 See Mostly The albite In planes Like OnoP- dorite. AbjAn 3 . Si0 3 = 55-5 49 -2.70 (2.69). ortho- clase; often in large grains; rarely in and pericline laws || oo/oo (right) a side appear- ance of one ortho- clase. - 4 3' to - 6 54' per cent. De- change of crystals; as orthoclase. combined are optic axis and indication of composable byHCl. color on 00/00. common. The individuals the lemniscates; axial point invisible; 10'); on 00/00 = - 16 40' twinned parallel oP to || CO/00 side appearance 21 I2 X again of the other (AbiAn, twinned axis, the axial = - 16). after the point also Carlsbad invisible. law or Dispersion according p > v. to co Poo (See Fig. 99, a or oP. and 6.) See structure. e. By- townite. An.* Si0 2 = 49-45 per cent. More easily soluble in HC1 than d. 2.70 Jke labrado rite. Similar to labra- dorite. Cleav- age-leaflets || oP and oo/oo show the side appearance of one optic axis, the axial point not falling within the field. Dispersion p > 71. (See Fig. 100, a and d.) Like labrador- ite. Parallel -14.5 to - 20, (AbjAn, parallel 00/00 = 27 to 3 2 - AbjAn 3 = -29 38'. TABLES FOR DETERMINING MINERALS. I8 7 Polari- zation- colors. Color and power of refracting light. Pleo- chro- ism. Structure. Associa- tion. Inclosures. Decom- position. Occurrence. Remarks. Gene- rally very bril- liant. Like ortho- clase. In grains and large crystals I.O. and microlites II. O. Compare inclosures and decom- With diallage, hyper- sthene, olivine, also with quartz, augite, Hornblende, olivine, diallage, magnetite, ilmenite. Especially prominent are the countless Like ortho- clase. Com- monly into epidote and Primary essential constituent in norite, gabbro, dolerite, especially in dacite, Like albite. position. horn- inclosures of musco- basalts, If labradorite blende, long acicular vite. and is twinned biotite. opaque micro- diorites. according to the Albite lites disposed parallel to the and Pericline vertical axis or laws, a also to the edge latticed oP.aoPoo; 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 labradorite direction per- pendicular to the are clearly microlites. distinguish- or countless able. minute colorless to greenish granules, so that the labradorite appears opaque 4 Likel abradori te. With horn- Like labrac yet with no lorite, Primary essential blende, microlites and constituent augite, leaflets. in eruptive biotite, rocks, diallage, hyper- diorite, gabbro, sthene, andesites. etc. 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. f. Anor- thite. CaAl 2 Si 3 8 .An. 2.73- 2-75 Perfect Like albite. i. M. =c nearly Like albite. Parallel Si0 2 = 45-43 (2.75)- *>>. P :M perpen- dicular - 3 6to per cent. Easily soluble in right = 94 10'. Dispersion p > v. -37. Parallel HC1 Leaflets osfco without 1 */ and = -37 formation 00/*00 to 43. of amor- show a An = phous side ap- -36. Si0 2 . pearance of one or the other of the optic axes. Axial point on margin of the field. (Comp. Fig. 101, a and b.) 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 in cleavage-leaflets parallel 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 : oo A, the same as the section of the plane ,P,< with oo A. 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. 190 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 3 Si 2 8 -f aq. Completely decomposed by HC1. 2.5-2.7. Green, more rarelv yellow, brown, reddish- brown, black. ft = 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. perpendicular to the direction of perfect cleavage. TABLES FOR DETERMINING MINERALS. Structure. Association. Occurrence. Decomposition product. Itt'inark*. The mesh-structure With olivine, rhombic or Massive, as For the most part Difficult to The decomposition begins on the walls of the olivine fissures; generally yellowish- green threads shoot out at right angles; thus a sort of monoclinic augite, hornblende, garnet, magnetite, chromite, chlorite, magnesite. product of olivine-fels; in pseudomorphs after olivine, in olivine-bearing eruptive rocks, and schists. As decomposition- a ugite free from alumina and hornblende. distinguish from the bastite and chloritic decomposition- products of augite. net is formed product of embracing within its meshes particles of fresh olivine, olivine, Al-free augite, and hornblende. which are subject to the further decomposition. The interior of the meshes generally appears filled with tufted serpentine x 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 angles, 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. - I 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 chloritic, 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 jrownish-green f jrehensive term ? on account of t >s show aggregat re finely radial a med or more or hornblendes, als ibrous aggregate viridite. An ex he minuteness c e polarization, nd concentric o ess laminated a 3 garnet and biotite, often decom- s, or, as in green earth, granular, act specification with trie 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. Coinp rhombic pyroxen very like that of 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 illel 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. SiO a . Snail percentage of H 2 0. See quartz. Colorless, transparent, often colored by ferric oxide or hydroxide. See quartz. = I-547- TABLES FOR DETERMINING MINERALS. 193 Structure. Association. Occurrence. Decomposition- prouucts. Remarks. <*, , c occurring in su very commonly in ps to the chemical comp silicate, and from th bles the chlorites ; b ch crypto-crystallint eudomorphs after ai osition, a is a hydro e high percentage c and c are iron-ma : aggregates occur jgite. According us h'eMg alumina i alumina resem- Ejnesium silicates, For the most part from 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 r-axis. As a c< of these threads it i bastite by studying i under " Bastite" (pag jnsequence of the r s often possible to c 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- crystalline 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 sphaerulites are to be distinguished from the chalcedony always appearing as decomposition- product ; these are direct eliminations from In the last case, melaphyr, and the eruptive quartz individuals elongated according to the chief 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 ground-mass. of the limitations (Abgrenzung). brilliantly polarizing show the interference-cross 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. (Cotnpar : Regular Minerals.) a. Natrolite. Na a Al 2 Si 3 O 10 + 2H a O. 2.172.26. Colorless, clear as water. Relief not marked. Rhombic. b. Scolecite. CaAl 3 Si 3 O 10 -f3H a O. 2.2-2.39. ditto. Monoclinic or triclinic. c. Stilbite. H 4 CaAl a Si 6 18 + 3H a O. 2.1-2.2. ditto. Monoclinic. d. Desmine. CaAl 3 Si 6 O ie + 6H 2 O. 2.1-2.2. ditto. ditto. e. Chabasite. RK^O^O. 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 under the -grained or radial aggre- Aragonite. CaCO 3 . Easily soluble in HC1 with effervescence. 2.9-3. Colorless, transparent. Rhombic, Polarization- colors as in calcite, often iridescent. Cleavage parallel oo PCD and ooP. A.P. U ooPw; a = c. TABLES FOR DETERMINING MINERALS. 195 Structure. Association. Occurrence. Decomposition- product. Remarks. Besides anal cime the followi g often occur a decomposition- products : Almost always in aggre- gates of long acicular crystals, generally radially disposed ivitk brilliant polarization-colors. A.P. 11 oo/oo ; c = C. Compare occurrence. With augite, olivine, magnetite, hornblende, biotite, Secondary minerals, especially prominent in the bubble- cavities (see Fig. 103) of The zeolites occur generally as decomposition- products of the feldspars, of nepheline, The distinctions are best effected by the micro- chemical examination. b to d inclusive can be accurately distinguished only by determining the relation of the Ditto. Generally in needles radially disposed. feldspar, etc., i.e. their decomposition- product; with feldspar-, nepheline-, and leucite-basalts; the basanites. leucite, and hauyn. axes of elasticity to the crystallographic axes a : c = 11-12. calcite and tephrites, aragonite. phonolites. and also in trachytic and andesitic Tabular crystals in radial eruptive rocks. groups, i. M. = c = b. As above. A. P. || coPoo. 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, hedral carbonates very commonly as decomposition-product occur : Besides these rhombo- 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 and geodes. Decomposition- product of calcareous silicates. Characterized by solubility with evolution of CO 2 , and by 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 frangaises. 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, Schvveizer- bart'sche Verlagshandlung. 1877. F. ZIRKEL. Die mikroskopische Beschaffenheit der Mineralien und Gesteine. Leipzig, W. Engelmann. 1873. Microscopical Petrography. Washington, 1876. w. XII PI. Acmite and Aegirine. TSCHERMAK. Tschermak'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. Forh. geol. Foren. i Stockholm. 1883. VI. 383 and 542. Comp. Ref. N. Jahrb. f. Min. u. Geol. 1883. II. 370. MttGGE. 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, Karinthine). 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. 1.867. 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. LI II. 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. pe*r. 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. KOENIG. 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. Sitzungsber. 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. BOOKING. 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. Bytownitel 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 &D $1 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. 56 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. Oordierite. 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. CALDERON 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. iSSr. I. Beil.-Bd. 225. Diallage. 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. Elseolite. 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. KticH. 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. BODEVVIG. 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. Basaltgesteine. 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. Min. 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. Jahrb. 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. GUMBEL. D. palaolith. Eruptivgest. d. Fichtelgebirges. Miinchen, 1874. 35. DATHE. Zeitschr. d. deutsch. geol. Ges. 1874. XXVI. i. COHEN. Reisen in Sudafrika. Hamburg, 1875. 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 Hiittenmann. Zeig. XXIX. 150. HAGGE. N. Jahrb. f. Min. u. Geol. 1871. 946. SCHUSTER. Tsch. Min. u. petr. Mitth. 1881. III. 183. Leucite. ZIRKEL. Zeitschr. d. deutsch. geol. Ges. 1868. 97. Basaltgesteine. Bonn, 1870. V. RATH. Monatsber. d. Berlin. Akad. d. Wiss. Aug. 1872. 2O6 DETERMINATION OF ROCK-FORMING MINERALS. KREUTZ. Tsch. Min. u. petr. Mitth. 1884. VI. 135. v. CHRUSTSCHOFF. Tsch. Min. u. petr. Mitth. 1884. VI. 161. Liebenerite. 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 lies 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. XVLII. 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. ZIRKKL. 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. BoiucKY. 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. 1881. 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. SCHULTZE. Verh. d. naturf. Ver. d. preussischen Rheinlande u. Wcst- 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. Feldspathbilduner. Haarlem, 1866. ROSENBUSCH. Verh. d. Naturf. Ver. Freiburg. VI. i, 95, 98, 103. STRENG. N. Jahrb. f. Min. u. Geol. 1871. 598. BIBLIOGRAPHY TO PART II. 209 Ottrelite. V. LASAULX. N. Jabrb. f. Min. u. Geol. 1872. 849. TSCHERMAK u. Sii'dcz. Gr. Zeitschr. f. Kryst. 1879. 59- BKCKE. Tsch. Min. u. petr. Mitth. 1878. I. 270. RKXARD et VALLEE POUSSIN. Ann. de la Soc. geol. Belgique. VI. Mem. 51. N. Jahrb. f. Min. u. Geol. 1880. II. 149. Perowskite. BOKICKY. 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. Flagioclase. 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. Mm. u. petr. Mitth. iSSi. 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. J 3- 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. Sudbrasilien. 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. BoftlCKY. 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. Rutile. SAUER. N. Jahrb. f. Min. u. Geol. 1879. 569. 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. 6r. 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 LEVY. Bull. Soc. miner. France. 1878. No. 3 and 5. BECKE. Tscherm. Min. u. petr. Mitth. 1882. IV. 369. TORNEBOHM. Geol. Foren. 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. XL 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. LAGORIO. 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. T6RNEBOHM. 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. i. 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. Mitth. 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 m 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 IJ 5 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 (5) showing the " mesh -structure" is a thin layer of fresh olivine grains, followed by the fibrous metamorphosed zone (fC) 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 oP\ 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 inqlosures 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 an4 cleavage-fissures 141 71 OLIVINE cross-section, a. Cross section parallel oP. b. Cross-sec- tion parallel oo p o>. (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 II. 21? PIG. 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 oo P oo. b = Carlsbad twin, c = Baveno twin, d = parallel co^Pco with a combination of oP. co f oo . 2P co. e parallel ooP oo. (After Rosenbusch.). . . 166 84 AUGITE cross-section, a. At right angles to the vertical axis. b. Parallel to the orthopinacoid. c. Parallel tx> the clinopinacoid. (After Fouque.) 168 85 URALITE cross-section. The seconary hornblende is partially de- veloped over the augite, with a twin lamella after oo/'oo. (After Becke.) 175 86 HORNBLENDE cross section. a. Transverse section. b. Parallel coiPoo. 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 oo P o>. (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 0/*. The microcline is in part homogeneous, in part shows the latticed structure ; the larger albite bands run parallel to the edge oP : P o and show fine twinnings striation parallel oP : oo P . . 180 93 MICROPERTHITE. a = section parallel to the separation-plane cor- responding , 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(P). Right longitudinal plane (oo j* 5 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 : Miles to the right. 182 97-101*5. 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( oo P oo) 182 98 Oligoclase, parallel M( P oo) 184 99# Labrador, parallel M( P oo) 186 99^ Labrador, parallel P(oP) 186 Bytownite, parallel M( P oo) 186 Bytownite, parallel P(oF) 186 Anorthite, parallel M( P oo) 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. 220 DETERMINATION OF ROCK-FORMING MINERALS. CUTS ACCOMPANYING PART \\.-Continu.d. FIG. 56. FIG. 59- ~~~~~ FIG. FIG. 58. FIG. 61. CUTS ACCOMPANYING PART II. CUTS ACCOMPANYING PART \\.-Continued. 221 fPIP FIG. 63. FIG. 64. FIG. 65. a 6 FIG. 66. FIG. 67. a / FIG. 68. FIG. 69. FIG. 70. 222 DETERMINATION OF ROCK-FORMING MINERALS. CUTS ACCOMPANYING PART \\. Continued. oP A I of FIG. 71. FIG. 72. FIG. 74. FIG. 73. FIG. 75. FIG. 76. FIG. 77 CUTS ACCOMPANYING PART II. CUTS ACCOMPANYING PART II.- Continued. UNJ | 223 FIG. 84. 224 DETERMINATION OF ROCK-FORMING MINERALS. CUTS ACCOMPANYING PART \\.-Continued. FIG. 85. FIG. 86. FIG. 87. FIG. FIG. 89. CUTS ACCOMPANYING PART II. CUTS ACCOMPANYING PART II. Continued. 22$ FIG. 93. FIG. 94. 226 DETERMINATION OF ROCK-FORMING MINERALS. CUTS ACCOMPANYING PART II. Continued. FIG. 95. FIG. 96. FIG. 97. FIG. 98. FIG. 99 a. FIG. 99 b. CUTS ACCOMPANYING PART II, 22J CUTS ACCOMPANYING PART \\.-Concludcd. FIG. ioo a. FlG. 100 . FIG. ioi a. FIG. ioi 6. FIG. 102. FIG. 103. INDEX. PAGE Aegerine 197 Aggregate 188 Acinite .. 172, 197 Actinolite 174, 197 Albite 182,198 Almandine 116, 198 Ammonium-magnesium Phosphate 62 Amorphous Minerals 112 , Behavior of, in polarized hght.iy, 30, 106 Analcime 118, 198 Analyzer 8 Anatase 124 Andalusite 152, 198 Andesine 184, 198 Anisotrope 106 Anomite 158, 199 Anorthite 188, 199 Anthophyllite 22,146, 199 Apatite 53, i3& '99 Arragonite 194 Arfvedsonite 174, 199 Augite, Ordinary and basaltic 168, 199 optical orientation of 24, 28 shell-formed structure of 91, 92 cleavage of 84 interpenetrations of 93 twins 41 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 Bach inger 203 Barrois 201 Barium-mercury solution 75 Bastite 22, 150, 192, 200 Becke 92, 164,197-211,213, 214 Becker 202, 204, 207, 208 Behrens 51,59,200,208,210 Belonite 87 Bertrand 7 Biotite 36, 89, 156 Bitumen no Blaas.. 205, 2xi, 213 Bodewig 203 Boh m 198 Boricky 51, 55, 207-210 Bourgeois 51 Brbgger 208 Bronzite 22, 146, 200 Buckrng 200 Biitschly 199 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 A ratio. 201 Carinthine 172 Cancrinite 136, 200 Catkrein 76. 210-212 Centring adjustment on microscope 13 Chabazite 194 Chalcedony 192. 2 Chemical investigation. Methods of 50 Chiastolite 152* 201 Chlorite 162 Chloritoid 162, 201 Chlorophaeite 192, 213 Chromite no, 118, 201 Ch rustschoff 99, 205, aio, 213 Chnochlore 162 230 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 Des Cloizeaux 207 Determination of the system of crystal- lization of the rock-forming minerals. . . 106 Diaclasite 150, 210 Diallage 170, 201 Dichroite 154 Diller 212 Dio'pside 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 E Eisen-kies 108 Elseolite 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 Fitter 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 ............................ ... Gastaldite ............................... Gisevius ................................. Glasmasse ............................ Glaucophane .......................... 174, Globulite ............................... Goldschmidt ........................... 66, Garnet .................................. Graphite ............................. no, v. Groddeck ............................. Groth ............................. 16, 45, Green earth ............................ Gum bee ......... ........................ Gylling .................................. Gypsum .............................. 178, 67 n 2 203 86 201 u6 204 207 212 ig 2 205 211 204 Hagge 199, 202, 204, 205, 208, 209 Hague 205 Hammer schmidt 204 Harada's apparatus 70 Hare. . . .. an Hausmann 203 Hauyn 54 , u 4 , 204 Heating apparatus 15 Helminth 162, 210 Hematite no, 138, 204 Hercynite 118, 204 Hexagonal minerals 106, 128 , Behavior of, in polarized Iight.i8, 30 Hopfner 210 Hollrung 210 Hornblende, Cleavage of 84 , Ordinary and basaltic 172 , Opacitic border of 89 , Optical orientation of ... 24, 28 Humboldilith 126 Hussak 201, 206, 209, 2ii Hydrofluosilicic acid 55 Hypersthene 148, 204 , Inclosures in 100 , Optical orientation of 22 , Pleochroism of 48 INDEX. 231 PAGE I Iddings 205 Ilinenite no Index of refraction. Determination of. 14, 44 Inclosures of the rock-forming minerals.. 93 of gases 94 of fluids 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 ii. Kalkowsky 202-204, 206, 211, 212 Kaemmente 162 Kispatlc - 208 C. Klein 7 D.Klein 66, 73 Kleiiis solution 73 Klocktnann 210 Kloos 202, 204, 207, 213 Knop 54, 206 V. Kobell 202 Koch ..197,200,203,212 Koenig 199 Koller 207 Kosmann 204, 213 Kren ner 205 Kreutz 199, 205, 208, 210 Kiich 203 L Labradorite 186, 205 Lagorio 199, 200, 213 v. Lasaulx.. .7, 198, 201-204, 20 7 20 9i 211-213 Lnspeyres 7, 45, 49, 203, 205, 208 Lehmann 210 Lemberg 200, 201, 211 Leucite 120,122,205 Liebenerite 136, 206 Liebisch 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 67 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 Microcline 180, 207 ! Microlites 85,86,207 | Micrometer 14 232 INDEX. PAGE Micropegmatite 93 Microperthite 180 Microscope 7 Monoclinic minerals 107 , Behavior of, in pol. light. .23, 36, 40 Morphological properties of the rock- forming minerals 81 Miigge 197,207 Mutter 198,201 Muscovite 36,160,207 N Natrolite 194 Nepheline 53, 134, 207 ffessig 214 PJiedzwiedzky 205 Nigrine 122 Non-pellucid minerals 108 Nosean 114, 207 Ocular micrometer 14 Oebbtke 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 PACK Polarizer 7 Potassium fluo-borate 61 Mercury solution 67 Platinum chloride 61 Preparation of microscopical sections 3 Prism, Nicol's 7- Protobastite 150, 210 Pseudo-crystals ... 89 Pyrite 108 Pyrope 116, 210 Pyrrhotine no Quartz 88, 128, 210 wedge 13 plate, Biot-Klein's n R v. Rath 198, 202, 205-207, 209, 2ii Regular minerals 106, u 4 , Behavior of, in pol. light 17, 30 Renard 165, 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. ., loi. an Rubellan 158, 210 Rutile 38, 122, 210 Sagenite 122 Salite 170, 202 Sandberger 205,211, 213 Sanidine 166, 208 Snuer 204, 205, 210 Scapolite 124, 211 Scolecite 194 Scheerer 203 Schonn 51 Schorl 138 Schrauf 198, 200, 202, 205, 208, 210, 211 Schultze 13.208 Schulze 2ir Schumacher 211 Schuster 198, 200, 205, 207, 209, 213 Sericite 160 Serpentine - 190, 211 INDEX. 233 PAGE Shell-formed structure of crystals 90 Siderite 132 Silico-fluorides 56, 57, 58 Sillimanite. 142, 211 Single-refracting minerals 17 Sip'ocz 201, 209 Sismondme 162, 201 Sjogren 199 Smaragdite 172, 174 Sodahte 114, 211 Somuterlad 204 Sorby ...44,210 Specific gravity, Determination of 68 Spinel 118 Stage, Healing 15 of the polarization-microscope 7 scale 14 Staurolite 39,143,212 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 Tornebohm >97 200, 203, 211, 213, 214 Tourmaline 46, 138, 213 Tremolite 174,213 Trichite 87 Triclinic minerals 107, 178 , Behavior of, in pol. light 28, 37 Tridymite 130, 212 Trippke 203 Tschermak 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. 2I 3 Vitreous inclosures 97 Vogelsang 15, 85, 204, 205, 213 Vrba 199, 2ii W Websky 211 Wedding 199 Weigand 211 Weiss 208-21 1 v. Werveke 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 MW14 RETURN EARTH SCIENCES LIBRARY TO * 642-2997 LOAN PERIOD 1 1 MONTH 2 3 4 5 6 ALL BOOKS MAY BE RECALLED AFTER 7 DAYS Books needed for class reserve are subject to immediate recall DUE AS STAMPED BELOW FORM NO. DD8 UNIVERSITY OF CALIFORNIA, BERKELEY BERKELEY, CA 94720 U.C.BERKELEY LIBRARIES