Minerals in Rock Sections

CHAPTER VI.. Chemical and Mechanical Tests.

CHEMICAL AND MECHANICAL TESTS. Chemical Tests on Crystal in Section: Carbonates, Gelatinizing Silica.—Etched Figures.—Heating Section 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. 129

APPENDIX. Brief Scheme of Classification into Systems by Optical Determinations.—Tables of Double Refraction (maximum) and Indices of Refraction (mean).—Diagram, showing relation between strength of double refraction, interference colors and thickness of section.—Order of Consolidation of the Constituent Minerals in Plutonic Rocks.—Optical Scheme with Special Introduction. 141

INDEX. 149

CONVENTIONS AND ABBREVIATIONS.

a = The assumed direction of the ether vibrations of the fastest ray.

b = The assumed direction of the ether vibrations of the ray with intermediate velocity.

c = The assumed direction of the ether vibrations of the slowest ray.

In some American Text-Books (by Iddings, Winchell & Phillips) a = X, b = Y and c = Z.

a′ = The assumed direction of the ether vibrations of the faster ray in the given section.

c′ = The assumed direction of the ether vibrations of the slower ray in the given section.

(+) = Optical character positive.

(−) = Optical character negative.

|| = Parallel to.

(γ − α) = The difference between the indices of refraction of the slowest and fastest rays, respectively, transmitted by the crystal, and indicates in decimals the relative strength of the double refraction.

n′ = The mean index of refraction; hence

= (α + β + γ)/3 or (ε + 2ω)/3.

α = Index of refraction of the fastest ray.

γ = Index of refraction of the slowest ray.

a, b and ć relate to the crystallographic axes commonly represented by these letters.

2E = the apparent axial angle measured in air, 2V being the true angle.

Bx{a·}_ = The acute bisectrix.

Bx{c·}_ = The obtuse bisectrix.

Ax. pl. = The axial plane, i. e., the plane containing the two “optic axes.”

ELONGATION relates to the appreciable extension often shown by the crystal section. A crystal of long prismatic habit, cut about parallel to the ć axis, would show marked elongation; while a tabular crystal (like mica) would show elongation if cut at right angles to the tabular faces. 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 octagonal. Very often the relation of the cleavage is given to the elongation and also to the directions a′ and c′, which makes it possible to test for a′ and c′ even when no marked elongation can be observed.

MINERALS IN ROCK SECTIONS.

CHAPTER I.. Introductory Optics for Optical Mineralogy.

INTRODUCTORY OPTICS FOR OPTICAL MINERALOGY.

The object of this introduction is merely to give a practical discussion of elementary optics, as applied to optical mineralogy, and 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 entering into a theoretical discussion as to the cause of these phenomena.

Light may be regarded as transmitted in straight lines by vibrations of the “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 passing through the direction of transmission.

By experiment 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 of crystals in the six systems, and the manner in which these crystals affect transmitted light ascertained.

▄Isotropic Crystals▄: It will be found that all sections of Isometric crystals transmit light with equal velocity in all directions; that is, the crystals are optically equivalent in all directions and, hence, can produce no double refraction. In these crystals any section, however cut, will transmit all the rays of light, incident to the surface at right angles, with no change. The same is true of Amorphous bodies, glass, etc., unless they have been subjected to strains or peculiar conditions during cooling. A single image is seen through these isotropic sections.

FIG. 1. ]

▄Anisotropic Crystals▄: It will also be found that nearly all sections (the exceptions being given later) of crystals in the remaining five systems, produce quite a different effect on transmitted light. In these crystals the velocity of transmission of light varies with the vibration direction of the light rays. This property, called double refraction, seems to result from the power of resolving a ray of ordinary light, with ether vibrations in all directions, into two rays with ether vibrations in planes at right angles to each other; the two resulting rays traversing, usually, divergent paths in passing through the section.

The mineral calcite (Iceland spar) exhibits this property to a marked degree, and in certain sections will show a double image, Fig. 1. That the vibration directions of the two doubly refracted rays are in planes at right angles to each other, can be easily proved by using a nicol prism. 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 crystal section for double refraction will be given later, p. 27.

The crystals that show double refraction are further divided into two groups, uniaxial and biaxial:

(1) ▄Uniaxial▄, or those in which the optical characters are symmetrical to one direction, called an optic axis. This optic axis is the crystallographic vertical axis, ć; and parallel to this direction there is a single value only for the light velocity and no double refraction takes place. Hence any section parallel to the base (001) being at right angles to the optic axis, acts like a section of an isotropic crystal and transmits all the perpendicularly incident rays of light with no change. In any other section double refraction takes place and it can be proved by using a nicol prism that the two rays have ether vibrations, one in the plane passing through the incident ray and the ć axis of the crystal, and the other in a plane at right angles thereto, hence in the basal plane. This latter ray, which has a constant velocity, is called the ordinary ray O; and the other ray, with velocity varying with the inclination of the section to ć, is called the extraordinary ray E.

The vibration directions are either parallel or symmetrical to cleavage cracks and crystal outlines. In sections parallel to the optic axis, the two doubly refracted rays have the maximum difference in velocity of transmission, and hence their vibration directions are called principal vibration directions and the plane containing them an optical principal section. In these sections the refractive index of the ray vibrating parallel to ć (extraordinary ray) is denoted by ε, and that of the ray vibrating parallel to the basal plane (ordinary ray) by ω.

To this group belong all Tetragonal and Hexagonal crystals.

(2) ▄Biaxial▄, or those in which the optical characters are no longer symmetrical to an optic axis but to three planes at right angles to each other (for monochromatic light). These crystals have, however (for light of each wave-length and for each temperature), two directions parallel to which there is a single value only for the light velocity and hence no double refraction. These directions are called “optic axes.” An investigation of these biaxial crystals shows that of all the rays traversing these crystals there are three rays which advance with maximum, minimum and some intermediate velocity. The vibration directions of these three rays are called the principal vibration directions and are at right angles to each other (being the intersections of the three planes above referred to). The direction of ether vibration of the fastest ray is denoted by a, of the slowest ray by c, and of the ray advancing with intermediate velocity by b. Each of the three planes, containing two principal vibration directions, is called an optical principal section. The index of refraction of the a ray is denoted by α, of the b ray by β, and of the c ray by γ.

To this group belong all crystals in the Orthorhombic, Monoclinic and Triclinic systems.

In the Orthorhombic system, the principal vibration directions are parallel to the crystallographic axes; hence all pinacoidal sections contain two of these principal vibration directions. In all sections parallel to the three crystallographic axes ă, ƃ and ć, the vibration directions are parallel or symmetrical to cleavage cracks, crystal edges, etc.

In the Monoclinic system, one principal vibration direction is parallel to the ortho axis ƃ, the other principal vibration directions are in the plane of symmetry, at right angles to ƃ, but are not parallel with either the vertical axis ć or the clino axis á. In clino pinacoid (010) sections the principal vibration directions will make definite angles with crystallographic lines, such as cleavages or crystal outlines. These angles are called extinction angles. They will vary, in this system, with reference to the direction of the ć axis from a maximum on the clino pinacoid (010) to 0° on the ortho pinacoid (100), when the vibration directions of the two doubly refracted rays will be parallel and at right angles to the plane of symmetry. Hence the vibration directions are parallel or symmetrical to cleavages, edges, etc., only in sections parallel to the ortho axis ƃ; but in all other sections are unsymmetrical.

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