Chapter II: 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