Minerals in Rock Sections. The practical methods of identifying minerals in rock sections with the microscope, etc

Preface

. The identification of minerals in rock sections with the microscope, including as it does a knowledge of optical mineralogy, is often difficult for beginners. This may be due to the fact that most of the publications on this subject are quite elaborate in their nature and in either French or German. While detailed descrip tions are very necessary, and, in fact, indispensable for advanced investigation, they are apt to prove cumbersome and confusing at first. For these reasons this text-book has been prepared by the writer, with a view of putting before the student only those facts which are absolutely necessary for the proper recognition and identification of the common minerals in rock sections. The foot notes refer the student to standard publications, in which are given details of the methods and investigations outlined in the text.

An elementary knowledge of crystallography and mineralogy is almost indispensable and is here assumed. The microscopic and optical characters of the minerals are recorded in the usual order in which they would be observed with a petrographical microscope. Nearly all the rock-forming minerals become transparent in thin sections ; but when opaque, attention is called to the fact and the characters are recorded as seen with incident light. White light is assumed to be used, unless other wise stated. The interference colors recorded in all cases are those given by very thin sections of 0.03 mm. in thickness. The order followed for the minerals is essentially that of Rosen busch (based on the symmetry of the crystalline form), with a few exceptions made for convenience, such as placing pyrrhotite after pyrite and zoisite after epidote. The statements regarding the occurrences of minerals in the common rock-types have been

PREFACE.

. iv taken mainly from Les Mineraux des Roches, by Levy and Lacroix. The terms axes and directions of elasticity, used throughout this book, are very commonly employed in petrographical litera ture of the present time. These axes and directions should probably more correctly be called axes and directions of vibration or extinction. The reasons for or against the elastic condition of the " ether " are of more interest, however, to the physicist than to the petrographer. An optical scheme is appended, with the minerals grouped according to their common optical characters. The writer's thanks are due to Dr. A. J. Moses, Professor of Mineralogy, and to Mr. J. F. Kemp, Professor of Geology, for kind suggestions offered during the preparation of this book. L. Mel. L. Department of Mineralogy, Columbia University, N. Y. City, October, 1898.

Errata and Addenda. P. i, ist foot-note. Add E. S. Dana's Text- Book of Mineralogy, last edit., 1898. P. 8, 1st foot-note. Add 14th. ed P. 32, (a), 1st line. Read, slightly inclined for normal (when sections are normal the curves become nearly circular). P. 97, 28th line. Read, intensity for moment. Read, Iddings' Rosenbusch for Iddings' Translation, wherever found. For a more complete discussion of the production of interfer ence colors (p. 22) reference is made to A. J. Moses, Characters of Crystals, p. 106 et seq. TABLE OF CONTENTS. PAGE. Conventions and Abbreviations, vii

Chapter I

Introductory Optics for Optical Mineralogy. Ordinary Light. — Plane Polarized Light. — Effects Produced by Crystal Sections on Transmitted Light. — Amorphous Bodies. — Isotropic Crystals. — Double Refraction. — Anisotropic Crystals. — Uniaxial Crystals. — Biaxial Crystals. — Optic Axis. — Directions and Axes of Elasticity or Vibration. — Optical Principal Section. — Ordinary and Extraordinary Ray. — Axial Angle. — Dispersion, i

Chapter II

Petrographical Microscope. Reflector.— Polarizer. — Nicol Prism Condensing Lens. — Rotating Stage. — Objectives. — Analyser. — Eye-pieces, 7

Chapter III

Investigation of Microscopic and Optical Characters of Minerals. Opaque Minerals. Transparent Minerals : With transmitted light; Form, Color, Relief, Cleavage, Fracture, Inclusions. With polarized transmitted light ; Pleochroism, Absorption. With crossed nicols : Isotropic Character, Anisotropic Character, Double Refraction and Interference Colors, Ex tinction and Extinction Angles, Structure. With convergent light ; Uniaxial Interference Figures and Optical Character, Biaxial Interference Figures and Optical Character. Resume of Uses of Parallel and Convergent Light, ii v CONTENTS. VI

Chapter IV

Microscopic and Optical Characters of Minerals. Amorphous Minerals. — Isometric Minerals. — Tetragonal Minerals. — Hexagonal Minerals Orthorhombic Minerals. — Monoclinic Minerals Triclinic Minerals. — Mineral Aggregates, .... 35

Chapter V

Methods of Preparing Sections. Cutting and Grinding Machines. • — Saws. — Cutting. — Grinding Plates or Laps. — Cementing. — Grinding. — Mounting. — Cleaning and Finishing. — Convenient Apparatus for Work, 81

Chapter VI

Chemical and Mechanical Tests. Chemical Tests on Crystal in Section ; Carbonates, Gelatinizing Silica. — Etched Figures. — Heating Sections to Redness. — Methods of Isolating Crystals or Fragments for Testing ; Specific Gravity Separation, Electro-magnetic Separation, Chemical Separation.- — Micro-Chemical Reactions; Borichy's Method, Behren's Method, Special Tests, 92

Appendix

. Brief Scheme of Classification into Systems by Optical Determinations. — Measuring Strength of Double Refraction. — Order of Consolidation of the Constituent Minerals in Plutonic Rocks. — Newton's Color-Scale. — Optical Scheme with Special Introduction, 103 Index CONVENTIONS AND ABBREVIATIONS. VII CONVENTIONS AND ABBREVIATIONS. Elongation = The general direction of greatest extension that a crystal may exhibit in a rock section. In crystals of long pris matic habit marked elongation would show that the section was taken approximately parallel to the vertical or prism axis ; while in crystals of tabular habit (the micas) elongation would in dicate that the section was approximately at right angles to the tabular faces (the basal faces in micas). At times, of course, no elongation is appreciable, as in the case of granular or broken crystals or where the cross-section is essentially square or octag onal.

Very often the relation of the cleavage is given to the elongation and the directions a' and c', which makes it possible to test for a' and c' even when no marked elongation can be observed. a' = A direction of maximum elasticity,* which need not neces sarily be the axis of maximum elasticity. c' = A direction of minimum elasticity, which need not neces sarily be the axis of minimum elasticity. (+) = Optical character positive. (— ) = Optical character negative. || = Parallel to. (j — a) = The difference between the indices of refraction of the two rays with ether vibrations parallel to the axes of minimum and maximum elasticity respectively, and indicates in decimals the rela tive strength of the double refraction. ;.'= The mean index of refraction ; hence a + fi + r £ + 2co = - - or - 3 3 a, b and c relate to the crystallographic axes commonly repre sented by these letters.

Iddings' Rosenbusch = Iddings' translation of Mikroskopische Physiographic der petrographisch wichtigen Mineralien, von Rosenbusch. * Or the assumed direction of the ether vibrations of the faster ray. The assumed direction of the ether vibrations of the slower ray is cr. Minerals in Rock Sections.

Chapter I

Introductory Optics for Optical Mineralogy. As the object of this introduction is merely to give a practical, elementary knowledge of optics, as applied to optical mineralogy,* no elaborate discussion of this important subject will be attempted. The explanations will be made as simple as possible, and, in most cases, only the optical phenomena will be described without enter ing into a theoretical discussion as to the cause of these phenomena. Light is transmitted by vibrations, of the " luminiferous ether," taking place at right angles to the direction of transmission. Ordinary light is light with the ether vibrations in all possible directions, the path described by any particle of ether constantly changing. Plane polarized light is simply light with the ether vibrations all parallel to one plane.

By experiments it has been proved that there exists a very close relation between the optical properties of crystals and their other physical properties, such as form, color, transmission of heat, etc. Therefore it is often possible, by a careful optical investigation of a crystal section, to determine important crystallographic facts, even in the absence of any distinct outline. The Effects Produced by Crystals on Transmitted Light. Consider that a series of optical tests are made on all possible sections f of crystals in the six systems, and the manner in which these crystals affect transmitted light ascertained. * For a more complete discussion of optics, in connection with Optical Mineralogy, the student is referred to A. J. Moses' Characters of Crystals, p. 85, et seq.; Groth's Physikalische Krystallographie, 3d edit ; Rosenbusch's Mikroskopische Physiographie, 3d edit ; and Iddings' Translation of Rosenbusch, 2d edit. t These sections are supposed to have plane parallel faces, such being the case in ordinary practice, and to be examined with parallel incident light.

In some crystals a single image of a point is seen through any section, and it has been proved that in these the light advances with equal velocity in all directions; that is, the crystals are op tically equivalent in all directions. This is the case with Isometric crystals,* and in these crystals any section, however cut, will trans mit all the parallel incident rays of light with no changes The same is true of Amorphous bodies, glass, etc., unless they have been subjected to strains or peculiar conditions during cooling. All such crystals are called optically Isotropic. It has also been found that sections of some crystals have the property of showing a double image of a point. Calcite, especially the variety Iceland spar, exhibits this phenomenon very beautifully. This is called double refraction,^ and seems to be due to the power of resolving the ether vibrations of a ray of light into two sets of vibrations, taking place parallel to planes at right angles to each other, § the two resulting rays traversing divergent paths in the section.

The velocity of transmission of light varies in different directions in these crystals, proved by the different values of the indices of refraction obtained by measurement. Of course, it is only possible to make this test for double refraction, i. c., the seeing of a double image, with thick crystal sections which have very strong double refraction like calcite. It is conve nient, however, to think of double refraction as producing this effect. In most cases the separation of the two images is so slight as not to be perceived by the eye, and the practical method of testing a substance for double refraction will be given later, see p. 20. All crystals in which the velocity of transmission of light is de pendent on the direction and which produce double refraction, transmitting rays of light with ether vibrations in two planes at right angles to each other, are called optically Anisotropic.

To this class belong all crystals except those of the Isometric system. * It is interesting to remember in this connection that in the isometric system there is also the greatest possible symmetry of " form." ■|* A. J. Moses, Characters of Crystals, pp. 85-97. X For this branch of optical physics see A. J. Moses, Characters of Crystals, pp. 97-100. \ This can easily be demonstrated by using a nicol and a plate of calcite which shows a double image. If the nicol is held between the calcite plate and the observer's eye it can be so adjusted that only one image is seen. If now the nicol is revolved 900 the first image will disappear and the other image alone will be seen. The crystals which show double refraction are further divided into two groups : i°. Uniaxial, or those that have one direction parallel to which there is no double refraction.* To this group belong all Tetrag onal and Hexagonal crystals. 2°.

Biaxial, or those that have two directions parallel to which there is no double refraction. To this group belong all Ortho rhombic, Monoclinic and Triclinic crystals. The direction parallel to which there is no double refraction is called an Optic Axis.| The directions, at right angles to each other, parallel to which the ether vibrations take place of the rays of light passing through a section of a doubly-refracting substance may be called Directions of Elasticity^ or Vibration. The directions of vibration (found always to be at right angles to each other) of the rays which advance with maximum and minimum! velocity and a third direction at right angles to the plane of these directions (corresponding to some ray with an intermediate velocity) are called Axes of Elasticity or Principal Vibration Directions.

In Uniaxial crystals the optic axis coincides with the vertical axis c , which is also an axis of either maximum or minimum elasticity. The optical behavior is symmetrical to c f and hence is the same for all directions equally inclined to c. The other axes of elas ticity, being equally inclined (900) to c, are equal and their abso lute positions indifferent. It will readily be seen that any section parallel to the base, be- * In some cases a peculiar form of double refraction does take place parallel to this direction, as in the circular polarization of quartz and cinnabar ; but in very thin sections these results are not noticed and can be disregarded. t In biaxial crystals the directions called optic axes are not strictly speaking axes, because they change their position, to a greater or less extent, for light of dif ferent wave-length.

For most cases, however, in observations with white light the optic axes may be regarded as approximately fixed in position. X The term elasticity is merely used for convenience in order to bring this text-book into conformity with most of the books on petrography published at the present time, by Zirkel, Rosenbusch, Harker, Hatch, etc. Levy and Lacroix do not use the expression. Groth, in his Physikalische Krystallographie, III Ed., treats of the sub ject without discussing the question of elasticity. Liebisch does not consider the ether as elastic. Fletcher regards the theory of the ether being elastic as untenable from a mathematical standpoint. § Proved by determining the indices of refraction of the rays with ether vibrations parallel to these axes. ing at right angles to an optic axis, acts like a section of an iso tropic crystal, and transmits all the parallel incident rays of light with no change.

Any section parallel to the vertical axis c contains two axes of elasticity and is therefore called an optical principal section ; and of the two rays of light, that with ether vi brations parallel to the basal plane is called the ordinary ray 0, the other ray being called the extraordinary ray E* The index of refraction of 0 is denoted by m and that of E by e.f Any other section contains two directions of elasticity, always at right angles to each other ; one direction being the trace of the plane, passing through the incident ray and the vertical axis c of the crystal, on the plane of the section ; the other direction being at right angles to this. In Biaxial crystals the three axes of elasticity, at right angles to each other, are denoted by a = maximum, 6 = intermediate, and c = minimum.

These axes of elasticity are the intersections of the three planes, towards which the optical behavior is symmetrical. Each plane contains two axes of elasticity and, as in the uniaxial crystals, is called an optical principal section. The index of refraction of the ray with ether vibrations parallel to a is denoted by a, that of the ray with ether vibrations parallel to b by /?, and that of the ray with ether vibrations parallel to c by y.% In the Orthorhombic system the axes of elasticity coincide with the crystallographic axes. In the Monoclinic system one axis of elasticity coincides with the ortho axis b, the other two axes of elasticity are in the plane of symmetry§ normal to the axis £, but do not coincide with either the vertical axis c or the clino axis a. The relative positions of these axes will be best understood by the accompanying figures.

Fig. i . represents a section parallel to the clinopinacoid, ooPoo (oio). The axes of elasticity are denoted by dotted lines, and the crystals-See A. J. Moses, Characters of Crystals, pp. 98, 99. f Instead of <_ and _, for conveniences in tables, etc., o and y are used, denoting the indices of refraction of the rays traversing the crystal with greatest and least velocity respectively, without regard as to which is the 0 or E ray. A good reason for this convention is that the symbol (y — a) is used to express in decimals the relative strength of the double refraction of a crystal, whether uniaxial or biaxial, y is always greater than a. X The ray with ether vibrations parallel to a advances with greatest velocity, the c ray with least velocity and the b ray with some intermediate velocity. §A. I. Moses, Characters of Crystals, p. 132.

INTR OD UCTOR 1 ' OPTICS. 5 lographic axes by solid lines. The ortho axis b and the remaining axis of elasticity are normal to the pinacoid plane. The angles x o c and c oy are called extinction angles, and vary with different species, being very characteristic of some minerals such as amphibole and pyroxene. Fig. i. Fig. 2. Consider now an orthopinacoid section, oo Poo (ioo). In Fig. 2 o b is the ortho axis b , oc the vertical axisr, eb an axis of elasticity parallel to b and e c a direction of elasticity to parallel c . The direction e c is not an axis of elasticity, because the elasticity (or the optical condition affecting the transmission of light) in that di rection is intermediate in value between the two values of the axes of elasticity lying in the plane normal to b . There is no extinction angle in an orthopinacoid section.

The same is true of a basal section and of all sections in the ortho zone* OP : °o Poo (ooi : ioo). We see that for sections in the prism zone oo Pcb : oo Pob (oio: ioo), .'. e., parallel to the vertical axis, the extinction angles will vary from 0°, when the section is parallel to the orthopina coid, to a maximum value when the section is parallel to the clinopinacoid. In the Triclinic system no axis of elasticity is parallel to a crystallographic axis, and there is no definite relation between the axes of elasticity and the crystallographic axes ; hence in all possi ble sections there will be extinction angles. It must be remembered, however, that there is still symmetry in the optical conditions to the three planes already mentioned. * A preliminary knowledge of crystallography and zones is here assumed.

See Moses and Parsons, Mineralogy, Crystallography and B. P. Analysis, and E. S. Dana, Text Book of Mineralogy, New Ed., 1898, p. 26. In all biaxial crystals the two optic axes are inclined to each other, making what is called the axial angle* denoted by _? E- This angle is really the apparent angle measured in air, 2 V being the true axial argle.f The optic axes always lie in the plane of the axes of maximum and minimum elasticity, and the angles be tween them are always bisected by these axes of elasticity. The axis of elasticity bisecting the acute axial angle is called the acute bisectrix. An approximate idea of the value of the axial angle can be obtained by the use of the petrographical microscope, as described later, p. 33. The axial angle is often a convenient differentiation between such minerals as muscovite and biotite.

The axial angle will vary with light of different wave-length or color, and this variation is called dispersion of the optic axes. Dis persion of the axes of elasticity also takes place in monoclinic and triclinic crystals, but can be disregarded in ordinary investigations. The location of the optic axis or axes is necessary when the uniaxial or biaxial nature or the optical character is required, and the determination of the axes or directions of elasticity and their relative values is also generally essential. In closing it is very important to remember that any section of an anisotropic crystal (not normal to an optic axis) will always transmit rays of light with ether vibrations in two planes at right angles to each other, the two sets of rays, however, following di vergent paths in the section.

Hence all such sections have two directions of elasticity at right angles to each other. Isometric crystal sections, of course, produce no double refraction. * For methods of measuring the axial angle, see A. J. Moses, Characters of Crys tals, Chap. XL, pp. 148-153. ■f* For convenience in many cases only 2 E is recorded, as then an indication is given as to whether the axial angle is visible with an ordinary microscope (arranged for observation with convergent light for interference figures). If _> __ is very large the axial angle can only be observed by covering the section with some transparent, strongly refracting fluid. For the Seibert microscope with objective J' the limit for good results is about .£= 90°-loo°. Fig.

Chapter II

The Petrographical Microscope. * The petrographical microscope is essentially an ordinary microscope f with the following important additional equipment. It must be provided with: i° a polarizer (a piece of apparatus for giving polarized light) placed below the stage ; 2° an analyzer (a piece of apparatus for analyzing the rays of light after they have passed through the polarizer and transparent section) placed be tween the objective and the eye ; 3° a stage rotating about an axis which is the line of sight of the microscope. A very conve nient type of microscope is that made by Seibert of Wetzlar (No. 11 a). Fig. 3. The reflector a is usually fitted with a plane mirror on one side, and a parabolic mirror on the other. The plane mirror should be used with sunlight, and the parabolic mirror with artificial light, in order to make the rays of light as nearly parallel as possible.

The polarizer is in most cases a nicol prism, set in a suitable frame b, and made as follows : For the nicol prism "a cleavage rhombohedron of calcite (the variety Iceland spar is universally used in consequence of its transparency) is obtained, having four large and two small rhombohedral faces opposite each other. In place of the latter planes, two new surfaces are cut, making angles of 68° (instead of 710) with the obtuse vertical edges ; these then form the terminal faces * For a short historical sketch of the use of the microscope in connection with Petrology, see G. H. William's pamphlet, Modern Petrography {Monographs of Edu cation), Boston, 1886. \ For description of the ordinary microscope, eye-pieces, objectives, magnification, etc., see Manipulation of the Microscope, by Ed. Bausch.

PETROGRAPHICAL MICROSCOPE. of the prism. In addition to this, the prism is cut through in the direction HH', Fig. 4, the parts then polished and cemented together again with Canada balsam. A ray of light, ab, enter ing the prism, is divided into two rays polarized at right angles to each other. One of these, be, on meeting the layer of balsam (whose refrac tive index is less than that of the ray be), suffers total reflection, and is deflected against the black ened sides of the prism and extinguished. The other, bd, passes through and emerges at e, a com pletely polarized ray of light, that is, a ray with vibrations in one direction only, and that the di rection of the shorter diagonal of the prism." * The vertical plane through the shorter diagonal may be called the plane of vibratwn\ of the nicol.

The polarizer must be below the stage c, and is generally adjusted so as to have its plane of vibration parallel to the N. and S. cross-wire in the eye-piece 0 . It is important to know the di- Fig. 4. rection of the plane of vibration of the polarizer or lower nicol ; as we can then determine, when absorption of light occurs in a mineral, the direction in this mineral parallel to which the ab sorbed rays are vibrating. The polarizer slides in an outer shell or frame, and, by means of a lever d, can easily be raised or lowered. On the top of the nicol is placed the condensing lens for getting convergent light, and the adjustments are so arranged that when the nicol is up as far as it will go, the condensing lens + is brought almost in contact with the lower surface of the transparent section resting on the stage.

The rotating circular stage, e, is supported on a suitable frame c, and arranged so that its axis of rotation coincides with the line of sight of the microscope. The stage is graduated, and, by means of an index fixed to the frame, the angular rotation can always be * Text Book of Mineralogy, by E. S. Dana, p. 133. \ It is convenient to assume that the vibrations of the polarized light are taking place in this plane, called the " plane of vibration," but all the phenomena caused by polarized light could be also explained on the assumption that the vibrations were taking place at right angles to this plane. X This condensing lens must be removed when very low power objectives are used. PETROGRAPHICAL MICROSCOPE. 9 obtained. It is also provided with two adjusting screws* / by means of which the axis of rotation can be accurately centered.

The method of centering is as follows : Bring some prominent mark in the section exactly in coincidence with the intersection ot the cross-wires in the eye-piece. Rotate the stage 1800, and cor rect one-half the error by means of the centering screws, and the other half by moving the section on the stage. Check the result by rotating the stage i8o° again, and if necessary make the cor rections in the same way until the adjustment is satisfactory. The objectives g screws into the collar /., which ha% a slot k, in the upper portion, for the introduction of a sensitive color plate, a r^ undulation mica plate or a quartz wedge. J The slot k, is usually so arranged that, when the sensitive color plates are introduced, the axes of these plates will make an angle of 450 with the planes of vibration of the crossed nicols, and the interference color produced will thus be at its maximum intensity.

A revolving nose-piece is sometimes used which can be attached to the collar /., and which is arranged to carry two or three objec tives, which can thus be very quickly brought into position for use. This is convenient in passing rapidly from observations with parallel light to observations with convergent light, which must be made with a high power objective. The difficulty is that recenter ing is generally required. The analysers or upper nicol is contained in the frame /, which is arranged so as to slide in and out of the lube of the microscope. The plane of vibration of the analyser is generally fixed by the instrument maker so as to be parallel to the E. and W. cross-wire in the eye-piece. Consequently when the frame / is pushed into * Some microscopes are provided with adjusting screws bearing on the frame holding the objective, which can then be accurately centered to the axis of rotation of the stage. f In the Seibert Microscope use objective No. 00 for the first general study of a rock section, No. II for general use and No. V for observations with convergent light.

In the case of an English Microscope a I" to }i" objective is used for general purposes and a %" to i" for observations with convergent light. X See Iddings Translation, p. 63, for description of quartz plate; p. 65, for quartz wedge; and p. 81 for % undulation mica plate. § In some microscopes the analyser is in the form of a " cap " nicol, arranged to be fitted over the top of the eye-piece, and not introduced in the microscope tube as shown here. This form is not so convenient, as the " cap " nicol must be set by hand every time it is desired to make observations with crossed n.cols. But at the same time it avoids any possible refocusing which may be necessary when the other type of analyser is in troduced in the tube. PETROGRAPHICAL MICROSCOPE. 10 the tube, the analyser is introduced in the line of sight between the objective and the observer's eye, with its plane of vibration at right angles to the plane of vibration of the polarizer ; that is the nicols are crossed.

When the nicols are crossed, if they are prop erly adjusted, no light can pass through to the eye and the field of view should be dark. The eye-piece* o, fits into the top of the tube, and, by means of a little projecting piece fitting into a slot in the frame, can always be adjusted so as to have its cross-wires parallel to the planes of vibration of the two nicols. Some instruments are provided with an additional slot in the tube between the analyser and the eye-piece for the introduction of a Bertrand lens, which is used to magnify the interference figures produced by convergent light. The first approximate focusing is made by the screw m, and the fine adjustment by the screw n. In focusing always start with the objective very near the section, and move it away until the right focus is obtained.

Never move down towards the section in obtaining the focus, as there is danger then of striking the objective against the section. * In the Seibert microscope eye-piece No. 0, is used for most purposes. Other eye pieces, Nos. I and III, with cross-wires, are used for different degrees of magnification, and one eye-piece No. II, without cross-wires, is provided to be used in connection with an eye-piece micrometer. Eye-pieces with cross-wires and different degrees of magnification are also provided with the English microscopes.