MINERALS IN ROCK SECTIONS
THE PRACTICAL METHODS OF IDENTIFYING MINERALS IN ROCK SECTIONS WITH THE
MICROSCOPE
ESPECIALLY ARRANGED FOR STUDENTS IN TECHNICAL AND SCIENTIFIC SCHOOLS
BY
LEA McILVAINE LUQUER, C.E., PH.D.
_Late Assistant Professor of Mineralogy,
Columbia University, New York City_
_FOURTH REVISED EDITION_
[Illustration: [Logo]]
NEW YORK
D. VAN NOSTRAND COMPANY, INC.
250 FOURTH AVENUE
Copyright, 1913
BY LEA MCILVAINE LUQUER
LANCASTER PRESS, INC
LANCASTER, PA.
PREFACE TO FOURTH REVISED EDITION, 1913.
This edition is practically that of 1908, with necessary minor revisions
and corrections, especially the data referring to indices of refraction
and strength of double refraction. The values of the refractive indices
α and γ are recorded as well as the mean values. References are also
given to “The Methods of Petrographic-Microscopic Research,” by F. E.
Wright, 1912, for exact (quantitative) methods of determining crystal
constants.
LEA MCI. LUQUER.
DEPARTMENT OF MINERALOGY,
COLUMBIA UNIVERSITY, NEW YORK, May, 1913.
PREFACE TO THIRD REVISED EDITION, 1908.
The 1908 edition is essentially the same as that of 1905; the only
changes being in the nature of corrections, and the addition of a brief
description of the Schrœder van der Kolk method of determining
refractive indices and also an abbreviated diagram for determining
triclinic feldspars in sections showing both Carlsbad and Albite
twinning. The Naumann symbols for crystal forms have also been omitted.
LEA MCI. LUQUER.
DEPARTMENT OF MINERALOGY,
COLUMBIA UNIVERSITY, NEW YORK, April, 1908.
PREFACE TO REVISED EDITION, 1905.
In preparing the revised edition, Chapters I. and IV. have been
rewritten and enlarged and the part relating to the determination of the
plagioclases has been greatly amplified. Many additions have also been
made to Chapter III. and the Becke method, for the determination of the
relative indices of refraction of minerals, has been given in detail.
Some new and useful tables have been introduced; as tables of refractive
indices (mean) and double refraction (maximum). A diagram has also been
added, showing the relation existing between strength of double
refraction, interference colors and thickness of section.
Professor E. Weinschenk’s admirable text-book, “Die Gesteinsbildenden
Mineralien”, Freiburg, 1901, has been specially referred to, and the
tables of refractive indices and double refraction have been compiled
from Weinschenk’s new Tables.
Many new cuts have been added, among them being semi-ideal drawings,
showing typical outlines of crystal sections, cleavage, optical
orientation, etc. In describing the “Usual Appearance in Sections” of a
mineral, it is of course only possible to mention the usual crystal form
in which the mineral occurs in a rock. The crystal may be cut in any way
by the plane of the section; but a general knowledge of the crystal
forms will furnish an idea as to the outline, etc., that the mineral may
show in the section.
LEA MCI. LUQUER.
DEPARTMENT OF MINERALOGY,
COLUMBIA UNIVERSITY, NEW YORK, July, 1905.
PREFACE TO FIRST EDITION, 1898.
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 descriptions 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 footnotes 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 otherwise 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 Rosenbusch
(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 taken mainly from Les
Minéraux des Roches, by Lévy and Lacroix.
The terms axes and directions of elasticity, used throughout this book,
are very commonly employed in petrographical literature 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.
LEA MCI. LUQUER.
DEPARTMENT OF MINERALOGY,
COLUMBIA UNIVERSITY, N. Y. CITY, October, 1898.
TABLE OF CONTENTS
PAGE.
CONVENTIONS AND ABBREVIATIONS ix
CHAPTER I.
INTRODUCTORY OPTICS FOR OPTICAL MINERALOGY.
Ordinary Light.—Plane Polarized Light.—Effects Produced by
Crystal Sections on Transmitted Light.—Amorphous
Bodies.—Isotropic Crystals.—Anisotropic Crystals.—Double
Refraction.—Uniaxial Crystals.—Biaxial Crystals.—Principal
Vibration Directions.—Axial Angle.—Dispersion. 1
CHAPTER II.
PETROGRAPHICAL MICROSCOPE.
Reflector.—Polarizer.—Nicol Prism.—Condensing Lens.—Rotating
Stage.—Objectives.—Analyzer.—Eye-pieces. 7
CHAPTER III.
INVESTIGATION OF MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS.
Opaque Minerals.
Transparent Minerals:
With transmitted light: _Form_, _Color_, _Index of Refraction and
Relief_, _Becke Method_, _van der Kolk Method_, _Cleavage_,
_Fracture_, _Inclusions_.
With polarized transmitted light: _Pleochroism_.
With crossed nicols: _Isotropic Character_, _Anisotropic
Character_, _Interference Colors_, _Extinction_ and _Extinction
Angles_, _Vibration Directions of Faster and Slower Rays_,
_Order of Interference Color_, _Strength of Double Refraction_,
_Determination of Minerals and Thickness of Section_,
_Structure_.
With convergent light: _Uniaxial Interference Figures_ and
_Optical Character_, _Biaxial Interference Figures_ and
_Optical Character_, _Determination of Axial Angle_,
_Distinctions between Orthorhombic, Monoclinic and Triclinic
Sections_.
Resumé of Uses of Parallel and Convergent Polarized Light. 13
CHAPTER IV.
MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS.
Amorphous Minerals.—Isometric Minerals.—Tetragonal
Minerals.—Hexagonal Minerals.—Orthorhombic Minerals.—Monoclinic
Minerals.—Triclinic Minerals.—Mineral Aggregates. 51
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. 117
CHAPTER VI.
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.[1]
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;[2] 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.
2_E_ = the apparent axial angle measured in air, 2_V_ 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.
The object of this introduction is merely to give a practical discussion
of elementary optics, as applied to optical mineralogy,[3] 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_[4] 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[5] 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.[6] In these crystals any section, however
cut, will transmit all the rays of light, incident to the surface at
right angles, with no change.[7] 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.
[Illustration:
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_,[8] 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.[9] 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.[10] 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_.[11]
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_[12] 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.[13]
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_.”[14] 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.[15] 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.
In the _Triclinic_ system, the principal vibration directions are not
parallel to the crystallographic axes, and there is no definite relation
between these directions and the crystallographic axes; hence in all
possible sections there will be extinction angles.
In all biaxial crystals the two _optic axes_ are inclined to each other,
making what is called the _axial angle, 2V_, the apparent angle measured
in air being _2E_. The optic axes lie in the plane, called the _axial
plane_, which contains the principal vibration directions a and c. The
axial angles are bisected by these principal vibration directions, the
direction bisecting the acute angle being called the _acute bisectrix,
Bx_{a}_, and that bisecting the obtuse angle the _obtuse bisectrix,
Bx_{o}_. An approximate idea of the value of the axial angle can be
obtained by the use of the petrographical microscope, as described
later, p. 47. The axial angle is often a convenient distinction between
such minerals as muscovite and biotite.
The axial angle will vary with the temperature and with light of
different wave-length or color, and this variation is called
_dispersion_ of the optic axes. Dispersion of the principal vibration
directions also takes place in monoclinic and triclinic crystals, but
can be usually disregarded.
In closing it is very important to remember that any section of an
anisotropic crystal (not at right angles to an optic axis) will always
transmit two rays of light with different velocities and with vibration
directions in planes at right angles to each other. Isometric crystals,
of course, produce no double refraction of light.