OPTICAL MINERALOGY THIRD EDITION Previous Editions McGRAW-HILL BOOK COMPANY 1959 t PDF

Preview

Summary

OPTICAL MINERALOGY Paul F. Kerr, Ph.D. PROFESSOR OF MINERALOGY, COLUMllIA UNIVERSITY o !I THIRD EDITION Previous Editions by Austin F. Rogers and Palll F. Kerr McGRAW-HILL BOOK COMPANY III Ncw York Toronto London 1959 t To the Memory of LEA McILVAINE LUQUER 1864-1930 \' OPTICAL MINERALOGY Copyri...

Description

OPTICAL MINERALOGY Paul F. Kerr, Ph.D. PROFESSOR OF MINERALOGY, COLUMllIA UNIVERSITY

o

!I

THIRD EDITION Previous Editions by Austin F. Rogers and Palll F. Kerr

McGRAW-HILL BOOK COMPANY III

Ncw York

Toronto

1959

London t

To

the Memory of LEA McILVAINE LUQUER

1864-1930

\'

OPTICAL MINERALOGY Copyright

©

1959 by the McGraw-Hill Book Company, Inc.

Copyright, 1933, 1942, by the McGraw-Hill Book Company, Inc. Printed in thc United States of America. All rights reserved. This book, or parts thereof, may not be reproduced in any form without pemlission of the publishers. Library of Congress Catalog Card Number 58-13880

. 20 21 22 - MAMD - 9 8765

fs nN 07 -O:J~ 21O-r,

t

Preface

Austin F. Rogers, the senior author of the first two editions of this text and professor emeritus in mineralogy at Stanford University, passed away at Berkeley, California, in April, 1957. His wise counsel as a former professor and his judgment as a mineralogist have been greatly missed during this revision. On the other hand, many readers, particularly those most familiar with Professor Rogers and his work, will recognize the influence of his teaching and will remember portions of the text which remain unchanged in the third edition. Sixteen years have elapsed since the second edition of this text appeared. The fidelity of the readers who have maintained a steady demand over this period indicates that the general features of the second edition have been found useful and consequently they are retained. The first objective in this revision has been concern over the student who has found difficulty with the phraseology or explanations of previous editions. Within the limitations of space every effort has been made to prepare a text which could be used with a minimum of supervision and a maximum of self-instruction. Optical mineralogy is acquired by the student with greatest facility with a good set of illustrative material under competent classroom instruction. On the other hand, experience has shown that a considerable number, lacking classroom facilities and desirous of learning the techniques described, have made considerable progress with representative thin sections and the text alone. The format of mineral description has been retained. The length of the tcxt is essentially the same. However, each mineral description has been reviewed, many have been revised, a few have been added. Descriptions of opaque minerals have been reduced in order to make space for other material. Selected references have been added in an attempt to extend thc scope of the text without undue enlargement. Th e polarizing microscope has undergone considerable evolution in n'c('nt years. Ncw illustrations have bcen substituted to call attention to ill q)l'(lVCd cqu ipmcnt now available. Ph ase microscopy is illustrated. The (' lla plcr intend cd to g,lide th e stlld ent in thc selection of methods of '~ I'i lldill g thill sec liolls ha s hcc n rcviscd. A chapter is includ ed to serve as iO lI olillino ill a(;('pliri ll g a working knowl edge of the universal stage. The vli

/1

PIlE FACE

1,," 1111" .11 ion tables have been revised in an attempt to make th em more '1 " I,d III lhe solution of the problem of identifying unknown min crals. 'I'll " I, 'x t is intended primarily for thin-section study, but both thc "'/I ', llifhU IS and the tables will be found useful for work with min cral II', JiIl IIl I S. The feldspars have been the subject of considerable rcvision I I Ill' ligh t of recent studies. Other mineral groups have not b een so di lll "lv,· ly revised, although frequent revision will be noted throu ghout. '1d'i II pplie's to the pyroxenes, amphiboles, chlorite, serpentine, the clays, 1111 IIVII por i.tes. ' 1'1 11 1 W I ilcr is particularly indebted to colleagues and research assoIII I ( \~ II I Columbia University who have offered suggestions. Profcssors l in "oldervaart, Brian Mason, and Ralph J. Holmes; Miss P. K. HamilIII , It" s('arch Associate; Mr. Martin Molloy, Mr. William Bassett, and 1,'. ' )!lvis M. Lapham, Graduate Assistants, have all provided assistance l VII I h)lls ways. The manufacturers of optical equipment have co1>" 1'11", n" the mineral is negative . nw is constant in a given uniaxial mineral, whereas the index of the ext raordinary ray v aries from nw t o n •. n , and n. = the lesser a nd greater indices of refraction of the two rays in a ny crystal section a t ra ndom orientation. X = the ax is of greatest ease of vibration . Light vibrating parallel t o X travels with m ax imum velocity (also indicat ed by a ) . Z = th e ax is of least ease of vibration. Light vibrating parallel to Z travels with minimum velocity (also indicated by -y). Y = t he intermedi ate axis at right angles t o t he plane of X and Z (also indicated by (3). e = the ax is of vib ration of the extraordin ary ray . w = the ax is of vibration of t he ord ina ry ray (in a pla ne a t right angles to .). r - the d ispen;ioll for red . II ..

t he di ~ p o l'~ i() II [0 " v iolct. t ho a xial a ll ~ l o wit hill t ho m ill era l.

:l V -

d ll

•

v

ABBREVIATIONS

= the axial angle observed in air. r. = acute bisectrix. ro = obtuse bi~ectrix. ~. pI. = the plane of t he optic axes. = micron, tho usandth of a millimeter (0.001 mm.). ,.. = millimicron, millionth of a millimeter (0.000001 mm.). = angstrom unit , tenth of a millimicron (0.0000001 mm.). = retard ation in m,.. (millimicrons). = thickness of a t hin section. Usually given in hundredths of a millimeter (0 .01 mm.). b, and c = t he crystallographi c axes. c..1

'"',.

The quartz wedge mounted on a glass plate and in a metal frame. The arrow marks the slow-ray direction. Ordinarily a wedge covers four orders from the thin edge to the thickest portion. (Am erican Optical Co.)

FIG. 2-17.

Mica

Gypsum

red

0Nr

LN (0)

1/4

t

--

Fast

Slow

A

0L

N

(b)

IF'"

-

Slow

(c)

2-18. The gypsum plate (a), mica plate (b), and a centering pin (c). (E . Leitz, Inc.)

FIG.

directly against the bottom of the cover glass. In case the slide is poorly mounted and a space intervenes b etween the top of the slice and the bottom of the cover glass, the extra distance should be considered as so much additional thickness of cover glass. In order to obtain the best results with objectives, cover glasses of standard thickness should b e employed . Precautions to Be Observed in the Use of the Microscope. Even under the h es t conditiolls mi crosc(lpe work prodll('('s :I ('('rtain allloHnt of stra in

THE POLARIZING MICROSCOPE

29

upon the eyes. It is essential, therefore, to employ the best possible conditions of work in order to reduce such strain to a minimum. The student should assume an erect but not too rigid position. Such a position with the microscope tube inclined allows him to work with maximum comfort. Both eyes should b e kept open while looking through the instrument. If it is difficult to do this at IIrst, a shield should b e placed over the eye not in use. It is also a good plan to learn to observe equally well with either eye and not to de- FIG. 2-19. The Berek Compensator. (E. . Leitz Inc.) vclop the so-called 1nlC1'OSCOpe e y e . , . Care of the Instrument. A polarizing microscope is expensive. Properly Ilscd, it should last a lifetime. Otherwise, it may become useless with littlc real service. Most of the precautions to be observed in the use of the instrument are such as should be applied to any piece of fine apparatus. A few, however, are of special nature and should be specially mentioned. Fine-textured lens paper or, still better, a camel's-hair brush should be used for cleaning all optical parts. This applies to the ocular, the objectives, the substage system, the mirror, and the two nicols. Objectives should be brought into focus by moving the tube of the microscope upward rather than downward. Possibility of contact between the lower lens of the objective and the thin section is thus I".. !2 20. A cOlllpensator to mcaSUl e llilo ill dili'ercnces in retardation. Mica avoided . High-power or oil-immer1'"111 ', II willdow in the accessory plate. sion objectives should be cleaned II !l Illy 1)(' lilted liy lurning the drum. with lens paper and xylol or ben11,11 " ,t:t ld atioll is y", red. (E. Leitz, zinc (not alcohol). III' Chemicals should not be used on lilt' ~ I "I.'t 1111 I('ss sp('ci:11 preca utions are takcn to protect the objective. (lIl l"" ti v,'s 'lill y Ill' prol('('I ('d hy lit e usc of cover glasses fastened to the I"\\'~' r 1('ll s. OI'I':l sioll;tlly i lll old objective is reserved for chemical work d"" Il,

30

MINERAL OPTICS

Illuminators. At ordinary magnifications a good north light with a broad, clear sky forms an excellent source of illumination for the polarizing microscope. In case such illumination is not available, artificial daylight lights may be successfully employed. These consist of various types of electric bulbs mounted in cases with a speCial blue-glass light filter in the path of the illumination. Tlu'ee types are illustrated in Figure 2-21a,b,c. A low-voltage bulb with a condensing lens and diaphragm, as illustrated in Figure 2-21c, provides suitable illumination for a wide variety of magnifications. At high magnifications and for photomicrographic work a mechanical-feed arc lamp is sometimes used. The beam from the arc is very warm and should always be passed through a cooling cell of water in order to avoid injuring the cement in the prisms of the microscope (unless special prisms are employed). Phase Microscopy. The technique of phase microscopy has FIG. 2-21. Various types of artiRcial illufound considerable application in mination for the microscope: (a) small biological science where specimens substage Jamp. (Bausch and Lomb Oplacking in contrast may be illumitical Co.) . nated 111 such a way that structures become visible without using stains. In the examination of minerals the technique of late has received some attention. In case the refractive index of a mineral less than about 10", thick differs but slightly from the refractive index of the mounting medium, phase microscopy may offer a significant method of examination. The theory and application of phase microscopy have been reviewed at some length by Bennett et al. (1951). Phase differences between light waves passing through points in the mineral and in the surround are utilized to bring out contrast at the eye. An annulus at the level of the condenser diaphragm and a diffraction plate at the back focal plane of the objective are utilized to produce phase differences (Figure 2-22). Minerals with extremely low relief in balsam may be made to stand out more distinctly with this arrangement. The use of annular diaphragms develops a change in optical path, or phase relation, in light entering the objective directly from an object and light diffracted from an object. A phase-shifting element may be mounted at the rear focal plane of the objective. Such elem~nts may be made by tho doposition of films of predetermined thickness by high-vacuum ther-

31

THE POLARIZING MICROSCOPE

mal-evaporation processes. Patterns of annular shape which introduce a phase shift of one-quarter wavelength of green light have been found effective . An annular aperture diaphragm is placed at the front focal plane of the substage condenser. When illuminated it furnishes a light

,~·,~w~·~_"""",.

FIG.

2-21b. A strong lamp for general utility. (Am erican Optical Co.)

I",, :. 2-2 1c. A low-vollage lamp with V-slots for filters. (B ausch and Lomb Optical (II )

', 11111'('( : atinRnily with respect to the object plane. The two annuli when , \d( 'lly t'O Il Ct'lllric and. supcr imposed produ ce a phase difference of one' I" I"I('J' w;lv(' lcll gtll . Iloll I " ne wavelength is observed, light is singly colored, or monochromatic. White light may b e considered composed of seven different colors. These grade into each other, forming a continuous spectrum. The colors of the spectrum are frequently represented by arbitrarily chosen wavelengths representing mean values of the various colors, as follows: R ed Ora nge Y ellow Green Blue Indigo Violet

= = = = = = =

700 mM 620 mIL 560 mIL 515 mM 470 mIL 440 mIL 410 mM

The elech'omagnetic spectrum (Figure 3-3) extends fa r beyond the r ange of visible light. The mechanisms by which the different radiations Blue Indiqo

1\ A

~Gfileetf;n Yellow /,/,;Oronqe

VAt)

,

x- Ex- Ultro: roys tremf I violet,: I

U.

v. './

o

\~

A

Visible spectrum

3,900.00 390. 00 0.39 0.00039 F I G.

A{

Red , '. \

___ -'"

_----

_-/-Infro red

'w"'-7,600.00 10,000.00 760.00 1,000.00 0.7 6 1. 00 0.0 0076 0.001

Au MfL fL mm

3-3. The approximate range of visible spectmm.

are produced, however, must be much different because of the great difference in frequency. REFERENCES Coker, E. G ., and L. N. G . Filon: "A Treatise on Photo-elasticity," Cambridge Univcrsity Prcss, London , 1931. Crew, H.: "The Wavc Thcory of Light," Am crica n Rook Compa ny, New York, 1900 . F:dsn , E .: "Lighl ror SlllCkllls," Macmill a ll & Cn., 1,l d., 1,l)lI doll , 1930.

A SUMMARY OF THE PROPERTIES OF LIGHT

45

Hardy, A. C., and F. H. Perrin: "The Principles of Optics," McGraw-Hill Book Company, Inc., New York, 1932. Heyl, P. R. : The History and Present Status of th e Physicist's Concept of Light, ]. Opt. Soc. Am., vol. 18, pp. 183-192, 1929. "Huygens' Treatise on Light," trans. by Silvanus P. Thompson, Macmillan & Co., Ltd., London, 1912. Newton, Sir Isaac, "Opticks," repr., McGraw-Hill Book Company, Inc., New York, 1931. Pockels, F. : "Lehrbuch der Kristalloptik," B. G. Teubner, Leipzig, 1906. Saunders, F . A. : "Survey of Physics," H enry Holt and Company, Inc., New York, 1930. Webster, D . L. , E. R. Drew, and H . W. Farwell: "General Physics for Colleges," Appleton-Century-Crofts, Inc. , New York, 1926. Whittaker, E. 1'. : "History of the Theories of Aether and Electricity," Longmans, Green & Co., Ltd. , London, 1910.

47

REFRACTION

the trace of the plane normal to the incident beam I strikes the surface at II, the point 15 is still a considerable distance above the bounding plane. The positions 12 , l a, and I ., together with corresponding intermediate

CHAPTER

4

R efraction

Snell's Law. The Index of Refraction. When light p asses obliquely from one medium to another in which it travels with a different velocity, it undergoes an abrupt change in direction. This abrupt change in direction is known as refraction. The relationships of the incident and refracted

points, are also above the surface. Let the beam advance until the ray at 15 has reached R 5. During this advance the ray at II has penetrated the denser medium and has continued with diminished velocity until it has arrived at the circumference of a circle with a radius I 1R" which represents the distance traveled in the denser medium. Similarly, 12 has penetrated to the circumference R 2 , 1a to Ra, and 14 to R •. A tangent common to these circles represents the new wavefront, and the new beam is p erpendicular to the new wavefront. The spherical waves sent out from b and other points on the bounding plane destroy each other except along bc and corresponding directions. In the above construction, the distances 15R5 and 11Rl may be considered proportional to the relative velocities of light in the two media. It is apparent from the relationship of the lines of the diagram that ao sin i = bo

p

bo =~ sin i

or also

be sin r = bo be bo = sin r

or Air

a

I,

Rs l\

b\ \

Since bo is common, the equations may be combined, and ao

- .-

Wafer \

\

\

\ \

\

or

\ \

\

c

R;\

"t \

\

\.

rA

\

\ \

,

be

. = - .-

sm '!,

sm r

ao sin i be = sin r

The index of refraction is determined by the distance light will travel in a given time interval through a transparent substance as compared with air. In Figure 4-1 light h'avels the distance ao in air, while it travels the distance bc in water. It follows , therefore, that the index of refraction

\ FIG.

ao n = be

4-1. Light b eing refracted on p assing from a rare into a denser medium.

light m ay be illustrated by the adaptation of the construction of Huygens shown in Figure 4-l. L et us supposc, for example, that a rare medium- ail- is in contact wilh a denscr med iu m - walcr. An incid cnt b ca m I strikcs the surfacc 01' Lho wa leI' ohli (I', ely, mak in g all a ll gle i w ith a pe rpc ndi c ula r P. Whc n 1\(1

0 1'

n

sm '!,

- .smr

It appears from the foregoing equation that for any angle of incidence the ratio of the sine of the angle of incidence to the sine of the angle of

MINERAL OPTICS

48

refraction is a constant. It is also true that the respective velocities of light in the two media bear the same ratio. The relationship between the sines of the two angles and the velocities is known as Snell's law. It was discovered by Snell in 1621 but was not made known until after his death. Let n be the index of refraction of a transparent material referred to air.l Then V = the velocity in air, and v = the velocity in the transparent material; also

v

n = v

If n and n 2 are the indices of refraction of two different materials, then l nl V2 n2 Vl Thus the indices of refraction of two transparent substances are inversely proportional to the velocities of light in the two media. The angles i and r may be measured experimentally for many substances, thus determining n. The index of refraction depends both upon the substance and upon the kind of light. The indices of isotropic substances or general values are designated by the letter n. The extreme values for hexagonal or tetragonal minerals are deSignated by ne and nw. OrthorhombiC, monoclinic, and triclinic crystals h ave their extreme values designated by n)' (greatest) , n O! (least), and n j3, the value in a direction at right angles to the two others.2 The following t able gives examples of values for the indices of refraction of several well-known minerals that occur throughout the normal range: Minerals Fluorite .... .. . . ... . . . ... . .. . Quartz * . . .. .. ... . . . ... . . ' Calcite * ......... . . . . .. . ... . Apatite * . . ... . ..... . . . . . . . Aragonite * . . .. . Garnet (grossular ite) .. Sphalerite .. ...... .

(N aD)

Indices of refraction

n = 1. 4338 n, = 1.5533; nw = 1 5442 nw = 1 .6585 ;n, = 1 .4863 nw = 1.6461; n, = 1.6417 na = 1.5301;n~ = 1.6816;ny = 1 6859 (Yellow) n = 1.7714 (Yellow) n = 2.3692

* Quartz,

calcite, and apatite are a nisotropic with a r an ge of values for refractive indices between n, and n w , th...