Petrology for students : an introduction to the study of rocks under the microscope

Front matter

Cambridge Geological Secs

i Ke:

Day

Contents

CHAPTER PAGE

I. INTRODUCTION . : : : : : A ]

meee HONIG ROCKS . -:. 4 : : : 5 23 II. GRANITES . : : : ; 28

Introduction

In this chapter will be included some notes on the optical properties of minerals, which may be of use to a novice; but there will be no attempt to supersede the use of books dealing systematically with that subject.

Microscope. We shall assume the use of a microscope specially adapted for petrological work, and therefore fitted with polarizing and analysing prisms, rotating stage with graduated circle and index, and ‘cross-wires’ of spider’s web properly adjusted in the focus of the eye-piece. The sub-stage mirrors attached to such instruments usually have a flat and a concave face. With day-light the flat face should be used; with artificial ight things should be so arranged that the mirror, used with the concave face, gives as nearly parallel rays as possible.

A double nose-piece, to carry two objectives, is very useful, although it usually gives very imperfect centring for high powers. The most useful objectives are a 1 inch or 14 inch and a 4 inch, but for many purposes a + inch is also very desirable. For minute objects, such as the ‘crystallites’ in glassy rocks and the fluid-pores in crystals, a high power is indispensable, and for very fine-textured sedimentary rocks an immersionlens offers great advantages.

A selenite-plate, a quartz-wedge, and other special pieces of apparatus will be of use for various purposes. Tho methods involving their use may be found in the mineralogical textbooks; where too the student will find guidance 1s to the examination of crystal-slices by convergent light.

Form of section of a crystal and cleavage-traces. A well-formed crystal gives in a thin slice a polygonal section, the nature of which depends not only upon the forms present on the crystal, but also on the direction of the section and on

HP 1

2 CLEAVAGE-TRACES its position in the crystal, as, e.g., whether it cuts through the centre or only truncates an edge or corner. Again, the same shape of section may be obtained from very different crystals. Nevertheless, if several crystals of one mineral are present in a rock-slice, we can by comparison of the several polygonal sections obtain a good idea of the kind of crystal which they represent. Further, if by optical or other means we can determine approximately the crystallographic direction in which a particular crystal is cut, we can usually ascertain what faces are represented by the several sides of the polygon.

For this purpose we may require to measure the angle at which two sides meet, and this is easily done with a microscope provided with a rotating stage and graduated circle. Bring the angle to the intersection of the cross-wires, adjust one of the two sides to coincide with one of the cross-wires, and read the figure at the index of the circle. Then rotate until the other side is brought to coincide with the same cross-wire, and read the new figure. The angle turned through is the angle between the two sides of the section. This angle is the same as that between the corresponding faces of the crystal, only provided the plane of section cuts these two faces perpendicularly. For a section nearly perpendicular to the two faces, however, the error will not be great.

In consequence of the mechanical forces which affect rock-masses, and also as a result of the process of grinding rock-slices, the minerals often become more or less fractured or even shattered. In a strictly homogeneous substance the resulting cracks are irregular; but if there be directions of minimum cohesion in crystals (cleavage), the cracks will tend to follow such directions, and will appear in a thin slice as fine parallel lines representing the traces of the cleavage-planes on the plane of section. The regularity and continuity of the cracks give an indication of the degree of perfection of the cleavage-structure; but it must also be borne in mind that a cleavage making only a small angle with the plane of section will, as a rule, not be shown in a slice.

In the case of a mineral like augite or hornblende, with two directions of perfect cleavage, the angle which the two

COLOURS OF MINERALS 3 sets of planes make with one another is, of course, a specific character of the mineral, or at least characteristic of a group ‘of minerals, such as the pyroxenes or the amphiboles. In a slice perpendicular to both the cleavages the traces will show the true angle. For any other direction of section the angle between the cleavage-traces will be different; but it will not vary greatly for slices nearly perpendicular to both the cleavages, and will often suffice for discrimination, as for instance between the 87° of the pyroxenes and the 554° of the amphiboles. In a slice parallel to the intersection of the two cleavages the two sets of cléavage-traces reduce to one, and a slice of a mineral such as augite or hornblende which exhibits but one set of cleavage-traces may be assumed to be nearly parallel to the intersection of the cleavages.

A mineral not possessing any good cleavage often shows irregular cracks in rock-slices (e.g. quartz and garnet). This is especially the case in brittle minerals.

Transparency, colours, and refractive indices of minerals. Only a few rock-forming minerals remain opaque even in the thinnest slices: such as graphite, magnetite, pyrites, and pyrrhotite; usually hematite, ilmenite, limonite, and kaolin; sometimes chromite or picotite. These should always be examined in reflected light: the lustre and colour, combined with the forms of the sections and sometimes the evidence of cleavage, will usually suffice to identify any of these minerals. The great majority of rock-forming minerals become transparent in thin slices. Those which seen in hand-specimens of rocks appear opaque, are often strongly coloured in slices, while those which in hand-specimens show colours are frequently colourless in thin slices. In the case of many minerals these “absorption-tints’ are thoroughly characteristic; but still more so are the differences of colour (pleochroism) in one and the same crystal according to the direction of the slice and the direction of vibration of a polarized beam traversing it, as noticed below.

The colours ascribed to minerals in the following pages and the epithet ‘colourless’ apply to thin slices of the minerals.

Apart from colour, the aspect. of a mineral as seen in thin 1—2

4 REFRACTIVE INDEX slices by natural light varies greatly according to its refractive index}, and it is of great importance for the student to learn to appreciate at a glance the effects due to a high or a low refractive index.

If a thin slice of a single crystal be mounted by itself in some medium of the same colour and refractive index as the crystal, its boundaries and surface-characters will be invisible, while its internal structure may be studied to the best advantage. Quartz mounted in Canada balsam (both colourless and of very nearly the same refractive index) is almost invisible. If olivine, a colourless mineral of much higher refractive index, be mounted in balsam, its boundaries and the slight roughness of its polished surface will be very apparent. In ordinary rock-slices, mounted in balsam, a roughened or “shagreéned’ appearance may be taken as the mark of a mineral having a refractive index considerably higher than that of the medium used.

Again, a highly refringent mineral surrounded in the shes: by others less highly refringent is seen to be more strongly illuminated than these, and its brightness is made more conspicuous by a dark boundary which is deeper in proportion to the difference in refractive index between the mineral in question and its surroundings. For these reasons a highly refringent crystal seems to stand out in relief against the rest of the slice.

Such considerations must be borne in mind in examining the minute inclusions in which many crystals abound. These inclusions may be of gas, of liquid (usually with a gaseous bubble), of glass, or a crystal of some other mineral; and these may be distinguished by observing that the depth of the dark border depends upon the difference in refractive index between the enclosing and the enclosed substance. The most strongly marked border is seen when a gaseous is enclosed by a solid

1 By this must be understood its mean refractive index. A crystal of any system other than the regular has in any section two refractive indices, the magnitudes of which depend further upon the direction of the section; but these differences in any one mineral are usually small as compared with the differences between the mean indices in different minerals.

REFRACTIVE INDEX 5 substance. A liquid-inclusion in a crystal has a less marked boundary, but a bubble of vapour in the liquid is strongly accentuated. A glass-inclusion is still less strongly marked off from its enclosing crystal, while a gas-bubble contained in it shows a very deep black border.

When two minerals (or a mineral and Canada balsam) are in contact with one another in a thin slice in such a position that their surface of junction is cut approximately at right angles by the plane of section, it is easy to determine which of the two has the higher refractive index. For this purpose the illumination should be limited by a diaphragm placed below the stage, and a high-power objective focussed upon the line of junction at the upper surface of the slice. This line is then seen to be bordered by a narrow bright band on the side of the more highly refringent mineral and a narrow dark band on the other side. If the objective be depressed until the lower surface of the slice is in focus, these appearances are reversed.

The refractive indices of the several rock-forming minerals may be found in the tables or books of reference, but the student will find it useful to carry in his mind such a list as that given below.

Refractive indices of the common rock-forming minerals. Very low (1:43-1:51): tridymite, sodalite, analcime and most other zeolites, (volcanic glasses), leucite. Low (1:52-1:63): felspars, nepheline, quartz, (Canada balsam), micas, calcite, dolomite, wollastonite, actinolite, melilite. Moderate (1:63-1-645): apatite, tourmaline, andalusite, hornblende.

High (1-68-1-8): olivine, silimanite, pyroxenes, zoisite, idocrase, epidote, garnets.

Very high (1:9-1-95): sphene, zircon.

Extremely high (2-0-2-7): chromite, rutile. Extinction between crossed nicols. When the polarizing and analysing Nicol’s prisms are used together, with their planes of vibration at right angles to one another

6 AXES OF EXTINCTION

(‘crossed nicols’)!, if no object be interposed, there is total darkness (‘extinction’); and the same is the case when a slice of any vitreous substance, such as obsidian, is placed on the stage. If, however, a slice of a crystal of any system other than the regular be interposed, there is in general more or less illumination transmitted, often with bright colours. On rotating the stage” carrying the object, it is found that extinction takes place for four positions during a complete rotation, these being at intervals of a right angle. In other words, there are two axes of extinction at right angles to one another, and the slice remains dark only while these axes are parallel to the planes of vibration of the nicols, which are indicated by the cross-wires in the eye-piece. If we rotate the slice into a position of extinction and then remove the nicols, the cross-wires will mark the axes of extinction in the crystal-slice.

Without attempting to deal fully with this branch of physical optics, we may remark that all the optical properties of a crystal are related to three straight lines conceived as drawn within the crystal at right angles to one another (the axes of optic elasticity) and to a certain ellipsoid having these three straight lines for axes (the ellipsoid of optic elasticity). The positions of the three axes may vary in different minerals, but they must always conform with the symmetry proper to the system, and the same is true of the relative lengths of the axes of the ellipsoid. The plane of section of any slice cuts the ellipsoid in an ellipse, the form and position of which depend upon the direction of the section (ellipse of optic elasticity), and the axes of extinction are the axes of this ellipse.

In certain cases the ellipse of optic elasticity may be a circle. For this any diameter is an axis, and accordingly we find that such a slice gives extinction throughout the complete

1 Tn using the two Nicol’s prisms, it should always be ascertained that they are crossed. For this purpose the rotating prisms are usually provided with catches in the proper positions, but the true test is total darkness when no object is interposed.

2 In some microscopes, such as that devised by Mr A. Dick, the stage is fixed, and the two nicols rotate, retaining their relative position, an arrangement with several advantages. We shall assume for distinctness that the stage is made to rotate, as in the most usual models.

_ STRAIGHT AND OBLIQUE’EXTINCTION u rotation. In crystals of the triclinic, monoclinic, and rhombic systems there are two directions of section which give this result. They are perpendicular respectively to two straight lines in the crystal (the optic axes), which lie in the plane of two of the axes of optic elasticity, and are symmetrically disposed towards them. In crystals of the tetragonal and rhombohedral systems the two optic axes coincide with one another and with the unique crystallographic axis, and only slices perpendicular to this give total darkness. In the regular system, the ellipsoid being a sphere, the ellipse is always a circle, and all slices give total darkness between crossed nicols.

Crystals of the regular system are spoken of as singly refracting or optically isotropic, and their optical properties* are similar to those of a glassy or colloid substance. Crystals of the other systems are doubly refracting or birefringent, and they are divided into uniaxial and biaxial according as they have one or two optic axes.

It is evident that the chance of a slice cut at random from a birefringent crystal being perpendicular to an optic axis is very small. If more than one crystal of a given mineral be present in a rock-slice, and all remain perfectly dark between crossed nicols throughout a rotation, it is a safe conclusion that the mineral is a singly refracting one.

Straight and oblique extinction. By bearing in mind that the ellipsoid of optic elasticity, and consequently all the optical properties of a crystal, must conform with the laws of symmetry proper to the crystal-system of the mineral, we can foresee all the important points as regards the position of the axes of extinction in crystals of the different systems cut in various directions. For instance, a longitudinal section of a prism of apatite (a hexagonal mineral) will extinguish when its length is parallel to either of the cross-wires: this is straight eatinction. A longitudinal section of a prism of albite (a triclinic mineral) will, on the other hand, have axes of extinction inclined at some angle to its length: this is oblique extinction. It is to be noticed that these terms have no meaning

1 That is, such of them as we are here concerned with.

8 EXTINCTION-ANGLE unless it is stated or clearly understood from what direction in the crystal the obliquity is reckoned. In these examples we reckoned with reference to one of the crystallographic axes, defined by the traces of known crystal-faces. Another character often utilized is the cleavage. Thus in a monoclinic mineral -with prismatic cleavages, such as hornblende, we select a crystal so cut that the two cleavages give only one set of parallel traces. These traces are then parallel to one of the crystallographic axes (the vertical axis), and we examine the position of extinction with reference to this. First we bring the cleavage-traces parallel to one of the cross-wires, removing if necessary for this purpose one or both of the nicols, and note the figure indicated on the graduated circle. Then, with crossed nicols, we rotate until the crystal becomes dark, and again note the figure. The angle through which we have turned is the extinction-angle. Observe that if a rotation through, say, 15° in one direction gives extinction, a rotation through 75° in the opposite direction would have given the same. For most purposes we do not need to distinguish between the two directions of rotation, but take merely the smaller of the two angles.

To obtain a measurement of use in identifying a mineral we require more than the above. Slices of a crystal of hornblende cut in various directions along the vertical axis will give different extinction-angles, from zero (straight extinction) in a— section parallel to the orthopinacoid to a maximum value in a certain other section. This maximum extinction-angle is a specific character, being nearly the angle between the vertical crystallographic axis and the nearest axis of optic elasticity. We may determine it with sufficient accuracy for most purposes by noting the extinction-angles in two or three vertical sections of the same mineral in a rock-slice and taking the largest value obtained.

By attention to the following points it is in most cases possible to refer to its crystal-system an unknown mineral of which several sections are presented in a rock-slice:

Regular system: singly refracting; all slices extinguish completely between crossed nicols, as in glassy substances.

CRYSTALLOGRAPHIC SYSTEMS DISCRIMINATED 9

Tetragonal and Rhombohedral (including Hexagonal): birefringent and uniaxial; straight extinction for longitudinal sections of crystals with prismatic habit and for any cross-sections of crystals with tabular habit. The two systems cannot be distinguished from one another by optical tests, but in cross-sections of prisms the crystal outline or cleavages will usually suffice to discriminate.

Rhombic (this and the remaining systems birefringent and biaxial): straight extinction for longitudinal sections of crystals with prismatic habit; sections perpendicular to the vertical axis have axes of extinction parallel to pinacoidal faces or cleavages and bisecting the angles between the traces of prism-faces cr prismatic cleavages. A section nearly parallel to the vertical axis will give nearly straight extinction, except in minerals (e.g. hypersthene) which have a wide angle between the optic axes.

Monoclinic: two important types may be noticed according as the intersection of the chief cleavages (and direction of elongation of the crystals) lies in or perpendicular to the plane of symmetry. In the former case longitudinal sections may give any extinction-angle from zero up to a maximum. value characteristic of the species or variety: in the latter (e.g. epidote and wollastonite) longitudinal sections give straight extinction. The former case is the more frequent.

Triclinic: no sections give systematically straight extinction.

Twinning. The existence of twinning in a slice of a crystal is, in general, instantly revealed by an examination of the slice between crossed nicols, since the two individuals of the twin show different interference-tints, and extinguish in different positions! (fig. 3, B). When twin-plane and face of association coincide—the most common case—a slice perpendicular to the twin-plane will give in the two individuals of the twin extinction-angles which, reckoned from the line of junction, are equal but in opposite directions. Conversely, a crystal

1 The only exceptions (apart from opaque crystals) are in minerals, like thé spinels, optically isotropic, and in cases in which the law of twinning is such that the directions of the axes of optical elasticity are ' not altered (e.g. quartz).

10 EXTINCTION-ANGLES IN FELSPARS which gives equal but opposite extinction-angles may be assumed to be cut very nearly perpendicularly to the twin-plane. If the plane of section cut the twin-plane of a crystal at a very small angle, the two individuals of the twin will overlap for a sensible width, and we shall see between the two a narrow band which does not behave optically with either.

When repeated twinning occurs, as in felspars with albite lamellation, the lamelle divide, as regards optical behaviour, into two sets arranged alternately. |

Extinction-angles in felspars. The discrimination of the several felspars by means of their extinction-angles measured on cleavage-flakes, as perfected by Schuster, is a method of great precision, but is not applicable to crystals in rock-slices. For these the method advocated by Michel Lévy and others will often be found useful. There are two cases in which it is readily applied.

(i) For crystals with albite-lamellation:—Select sections cut approximately perpendicular to the lamelle. These are known by the extinction-angles in the two alternating sets of lamelle, reckoned -from the twin-line, being in opposite directions and nearly equal; also by the illumination of the two sets of lamelle being not very different when the twin-line is parallel to a cross-wire. Measure the angles in question in three or four crystals so selected, and take the greatest value found. This will be very nearly the maximum angle for all such sections, which is a specific constant for each kind of felspar, as indicated for certain types in the annexed diagram (fig. 1). The values for types not given in the diagram may be judged with sufficient accuracy by interpolation, since the maximum extinction-angle changes steadily from one end of the series to the other. It will be noticed, however, that in certain cases different kinds of felspar (viz. those placed on the same vertical lines in the diagram) give equal angles, and in this connection two remarks are to be made.

(a) A slice of a crystal has two directions of extinction, at right angles to one another. Hitherto we have taken the angle to the nearest direction of extinction, but the diagram shows that for angles of 37° or more this introduces an ambiguity. —

EXTINCTION-ANGLES IN FELSPARS ll

It is then necessary to distinguish between the two directions (by means of the quartz-wedge or some other contrivance) and to select that one which corresponds with the least axis of the ellipse of elasticity (indicated by an arrow-head in the diagram). In this-way anorthite and bytownite are discriminated from the medium labradorites. Other criteria may sometimes be used, e.g. the stronger birefringence of anorthite, as pointed out below!.

(b) The signs + and — denote angles measured in opposite directions crystallographically. Unless other means of discrimination can be made use of, we have usually no way of distinguishing the two directions, and there is consequently an ambiguity between albite and the more basic oligoclases (with oligoclase-andesine). Since the latter have about the same refractive index as quartz and Canada balsam, while the index for albite is distinctly lower, a discrimination may sometimes be made by rough observations of comparative refringence.

Summarily, we have the following characteristic angles for different felspars :— 0° to 5°, oligoclase, the more acid types. 6° to 16°, albite and the more basic oligoclases (with oligoclase-andesine). 16° to 22°, andesines. 27° to 45°, labradorites. 45° to 50°, bytownites. 50° and above, anorthites.

(iu) For microlites, assumed to have their length parallel to the intersection of the two principal cleavages:—Here we measure extinction-angles from the long axis of the microlites, and select the highest angle obtained by measurements on several microlites. The characteristic maxima for certain varieties of plagioclase are given in the annexed diagram (fig. 2), and the values for intermediate varieties can be interpolated. As before, there are two points to be noted.

1 Another point worthy of notice is the frequency with which certain angles (less than the maximum) occur in a number of sections perpendicular to the albite-lamelle. For anorthite the favourite angles are 32° and 41°, for medium labradorite 21° and 36°.

1 EXTINCTION-ANGLES IN FELSPARS noe)

AL8- OLIc.

0 OLIG. 5 27 ACID 16 20 LAB. ~ 40 OLIG. aa AES MED. ay ye | -45|45 LAB. 2 <40\50 XY st¢ Ana BY T. |

BAS.

Fie. 1. MAXIMUM EXTINCTION-ANGLES OF PLAGIOCLASE FELSPARS IN SECTIONS AT RIGHT ANGLES TO THE ALBITE-LAMELLA.

OLIG.- AND. eae LAB

Fia. 2. MaxIMUM EXTINCTION-ANGLES OF PLAGIOCLASE FELSPARS IN LONGITUDINAL SECTIONS OF MICROLITES.

EXTINCTION-ANGLES IN FELSPARS 13

(a) If the angle of extinction as measured is 26° or more, we must discriminate by the quartz-wedge or otherwise between the two directions of extinction.

(b) If the angle is 20° or less, an ambiguity occurs which cannot be removed by this method; wz. between albite and andesine or andesine-labradorite and between acid oligoclase and oligoclase-andesine. There is thus more unavoidable ambiguity in this case than in that of albite-lamelle, as appears from the following values for different felspars.

0° to 7°, oligoclase with oligoclase-andesine. 8° to 10°, albite-oligoclase and andesine. 10° to 20°, albite and andesine-labradorite with acid labradorite. 30° to 42°, labradorite, medium to basic. 49° to 56°, bytownites. 58° to 64°, anorthites.

Becker! has suggested another test applicable to microlites, which may very conveniently be used to supplement the above; since, although it is of little use for the more basic varieties, it affords a useful criterion for distinguishing the oligoclases, andesine-, etc. Instead of longitudinal sections, perpendicular cross-sections of the microlites are selected. These are small, - nearly square, and sharply defined. The extinction-angles vary from — 18° for pure albite to 425° for anorthite, and from Becker’s figures we may deduce the following approximate values :—

0° to 4°, oligoclase, acid. 4° to 7°, oligoclase, medium, and albite-oligoclase. 7° to 13°, oligoclase, basic, and albite.

18° to 22°, andesine.

264° to 38°, labradorite, acid to medium.

38° to 424°, medium labradorite to anorthite.

If the sections selected for measurement be as much as 10° from the true perpendicular cross-section, the resulting error is

1 18th Ann. Rep. U.S. Geol. Sur. Part 1 (1898), 32-34, and A. J. S: (1898) v, 349-354, pl. mi. For a more general account of the modern optical methods of discriminating the felspars see Winchell, Amer. Geol. (1898) xxi, 12-48, pl. u-vun, and Iddings, Rock M inerals (2nd ed. 1911).

14 ZONARY BANDING IN FELSPARS only about 1$° to 25° in the more acid half of the plagioclase series, and therefore does not vitiate the conclusion.

Zonary banding in felspars. In many rocks the felspars show between crossed nicols concentric zones roughly parallel to the boundary of the crystal, the successive zones extinguishing in different positions (fig. 3, A). (If there be albite-lamellation, we confine our attention to one of the two sets of lamelle.) This difference in optical behaviour among the successive layers which build up the crystal may arise in two ways: firstly, from the successive zones being of different kinds of

Fic. 3. PLAGIOCLASE CRYSTALS; x 14, CROSSED NICOLS.

A, Albite-lamellation combined with twinning on the Carlsbad law (c) and with strong zonary banding.

Bb. Pericline-twinning (p) in addition to albite and Carlsbad.

felspar-substance; or, secondly, from ultra-microscopic twinning affecting in various degrees the different layers of a crystal chemically homogeneous. This has been pointed out by Michel Lévy, and he gives a test which will resolve all except certain rare cases. It will be found, on rotating the slice between crossed nicols, that there are certain positions in which the albite-lamellee disappear. If simultaneously with this the zonary banding disappears also, so that the whole crystal’ is

1 Or if there be Carlsbad-twinning also, the whole of one individual of the Carlsbad-twin.

INTERFERENCE-TINTS 15 uniformly illuminated, the appearances can be explained by ultra-microscopic twinning alone: if this is not the case, the zonary banding may be ascribed to the successive layers of felspar-substance in each crystal differing in chemical composition. When this occurs, the rule generally holds that the layers or zones become progressively more acid from the centre to the margin.

Interference-tints. We have remarked that a thin slice of a doubly refracting crystal, examined between crossed nicols, is in general not dark except when placed in certain definite positions. In any other position it does not completely extinguish the light, but its effect, in conjunction with the nicols, is partially to suppress the several components of the white light in different degrees, so that in the emergent beam these components are no longer in the proportions to give white light. In this way arise polarization-tints or interference-tints. These belong to a definite scale, known as Newton’s scale, on which the several tints (though graduating into one another), are distinguished by names and divided into several ‘orders.’ The student should learn the succession of these tints, in the first place from the coloured plates accompanying some mineralogical works!, but ultimately from the minerals themselves. The precise position in the scale of a given tint observed between crossed nicols can be fixed by means of a quartz-wedge? or other contrivance for ‘compensating’ or neutralizing the birefringence of the slice; but for ordinary purposes, at least with colourless or nearly colourless minerals, the interference-tint can be judged by eye with sufficient accuracy. The most brilliant colours are those of the second order and at the top of the first; the lowest colours of the first order are dull greys;

1 Michel Lévy and Lacroix, Les Minéraux des Roches (1888); Rosenbusch (transl. Iddings), Microscopical Physiography of the Rock-making Minerals; Iddings, Rock Minerals (2nd ed. 1911).

2 The edge of the quartz-wedge is usually not thin enough to give the lowest tints of the scale. This can be remedied by means of a film of mica, properly oriented: see Mahony, Nature, 2 August, 1906, xxiv, 317-318. On some special applications of the quartz-wedge see Evans, Min. Mag. (1905) xiv, 87-92. On the mica-wedge, which for many purposes affords an efficient substitute, see Dick, Notes on a New Form of Polarizing Microscope (1890).

16 INTERFERENCE-TINTS while in the third and fourth orders the tints become brighter but paler, ultimately approximating to white.

The interference-tints given by a crystal-section depend (i) on the birefringence of the mineral, which is a specific character; (11) on the direction of the section relatively to the ellipsoid of optic elasticity, the tint being highest for a section parallel to the greatest and least axes of the ellipsoid; (ii) on the thickness of the slice. These last two are disturbing factors, which must be eliminated before we can use the interference-tints as an index of the birefringence of the crystals, and so as a useful criterion in identifying the mineral.