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Optical Mineralogy (A) Optical Mineralogy (a) Q Frontispiece Photomicrographs, taken using (a) transmitted light and (b) reflected light, of the same area of a polished thin section of quartzite containing pyrite (P), sphalerite (S), muscovite (M), apatite (A) and abundant quartz (Q). The features illustrated in transmitted light are: (i) opacity - pyrite is the only opaque phase, sphalerite is semi-opaque, and the others are trans­ parent; (ii) relief - very high (sphalerite, /1 = 2.4), moderate (apatite, 11 = 1.65), moderate (muscovite, n = 1.60), and low (quartz, 11 = 1.55); (iii) cleavage - perfect in muscovite (/1 is the refractive index of the mineral). The following values of reflectance (the percentage of incident light reflected by the mineral) are illustrated in reflected light: pyrite, 54% (white - true colour slightly yellowish white); sphalerite, 17% (grey); apatite, 6% (dark grey); and muscovite and quartz, both 5% (dark grey). Note that opaque grains, grain boundaries and cleavage traces appear black in transmitted light, whereas pits (holes), grain boundaries and cleavage traces appear black in reflected light. Optical Mineralogy Principles and Practice C. D. Gribble A. J. Hall Department oJGeology & Applied Geology, University oj Glasgow I IChapman & Hall Dn New York © c. D. Gribble & A. J. Hall 1992 Softcover reprint of the hard cover 1st edition 1992 First published in 1992 by UCL Press. UCL Press Limited University College London Gower Street London WCIE 6BT The name of University College London (UCL) is a registered trade mark used by UCL Press with the consent of the owner. First published in North America in 1993 by Chapman & Hall, Inc. 29 West 35th Street New York, NY 10001 ISBN-13 : 978-1-4615-9694-3 e-ISBN-13: 978-1-4615-9692-9 DOl: 10.1007/ 978-1-4615-9692-9 Library of Congress Cataloging-in-Publication Data Gribble, C. D. Optical mineralogy: principles and practice / C. D. Gribble, A. J. Hall p. cm. Includes bibliographical references and index. 1. Optical mineralogy. I. Hall, A. J. (Allan J.) II. Title. QE869.06074 1993 549' . 125-dc20 92-36265 CIP Contents Preface page ix Acknowledgements Xl List of tables Xli List of plates X111 Symbols and abbreviations used in the text XIV 1 Introduction to the microscopic study of minerals 1.1 Introduction 1.2 The transmitted-light microscope 1.3 The appearance of thin sections using transmitted-light microscopy 5 1.4 Systematic description of minerals in thin section using transmitted light 6 1.4.1 Properties in plane polarized light 6 1.4.2 Properties under crossed polars 10 1.5 The reflected-light microscope 16 1.6 The appearance of polished sections under the reflected-light microscope 19 1.7 Systematic description of minerals in polished section using reflected light 21 1.7.1 Properties observed using plane polarized light (PPL) 22 1.7.2 Properties observed using crossed polars 24 1.7.3 The external nature of grains 24 1.7.4 Internal properties of grains 25 1.7.5 Vickers hardness number (VHN) 25 1.7.6 Distinguishing features 25 1.8 Observations using oil immersion in reflected-light studies 26 1.9 Polishing hardness 27 1.10 Microhardness (VHN) 29 1.11 Practical points on the use of the microscope (transmitted and reflected light) 31 1.12 Preparation of thin and polished sections 32 V CONTENTS 2 Silicate minerals 2.1 Crystal chemistry of silicate minerals 35 2.2 Mineral descriptions 40 AlSiOs polymorphs 41; Amphibole group 47; Beryl 63; Chlorite group 63; Chloritoid 65; Clay minerals 66; Cordierite 69; Epidote group 71; Feldspar group 78; Feldspathoid family 97; Garnet group 101; Humite group 103; Melilite group 106; Mica group 107; Olivine group 112; Pumpellyite 118; Pyroxene group 119; Scapolite 138; Serpentine 139; Silica group 140; Sphene 145; Staurolite 147; Talc 149; Topaz 150; Tourmaline group 151; Vesuvianite 152; Zeolite group 152; Zircon 154 3 The non-silicates 3.1 Introduction 155 3.2 Arsenides 155 3.3 Carbonates 156 3.4 Halides 163 3.5 Hydroxides 165 3.6 Native elements 169 3.7 Oxides 172 3.8 Phosphates 186 3.9 Sulphates 190 3.10 Sulphides 196 3.11 Tungstates 213 4 Transmitted-light crystallography 4.1 Introduction 215 4.2 Refraction 217 4.3 The refractive index 217 4.4 The Becke effect and the Becke line 219 4.5 Birefringence and Newton's Scale of Interference Colours 219 4.5.1 Anomalous interference colours 222 4.6 Isotropic and anisotropic minerals 222 4.7 The isotropic indicatrix 222 4.8 The uniaxial indicatrix 223 4.8.1 Uniaxial crystals under crossed polars 225 4.8.2 Compensation and the determination of interference colour using the quartz wedge 227 4.8.3 The sensitive tint plate 228 VI CONTENTS 4.8.4 Interference figures in uniaxial minerals and their sign 228 4.8.5 Determination of the mineral sign 230 4.9 The biaxial indicatrix 231 4.9.1 Interference figures in biaxial minerals 233 4.9.2 Determination of the sign in biaxial minerals 235 4.9.3 Determination of the extinction angle in biaxial minerals 239 4.10 Pleochroism 240 5 Reflected-light theory 5.1 Introduction 243 5.1.1 Reflectance 245 5.1.2 Indicating surfaces of reflectance 252 5.1.3 Observing the effects of crystallographic orientation on reflectance 252 5.1.4 Identification of minerals using reflectance measurements 256 5.2 Colour of minerals in PPL 258 5.2.1 CIE (1931) colour diagram 260 5.2.2 Exercise on quantitative colour values 261 5.3 Isotropic and anisotropic sections 261 5.3.1 Isotropic sections 261 5.3.2 Anisotropic sections 263 5.3.3 Polarization colours 263 5.3.4 Exercise on rotation on reflection 266 5.3.5 Detailed observation of anisotropy 267 Appendix A Refractive indices 269 A.I Biaxial minerals 270 A.2 Uniaxial positive minerals 272 A.3 Uniaxial negative minerals 273 A.4 Isotropic minerals 273 Appendix B 2 V size and sign of biaxial minerals 274 Appendix C Properties of ore minerals 278 Appendix D Mineral identification chart 289 Appendix E Gangue minerals 291 Bibliography 293 Index 297 Vll Preface This book is the successor to A practical introduction to optical mineralogy, which was written in the early 1980s, and published by George Allen & Unwin in 1985. Our intention, once again, is to introduce the student of geology to the microscopic examination of minerals, by both transmitted and reflected light. These techniques should be mastered by students early in their careers, and this text has been proposed in the full awareness that it will be used as a laboratory handbook, serving as a quick reference to the properties of minerals. However, care has been taken to present a systematic explanation of the use of the microscope, as well as to include an extended explanation of the theoretical aspects of optical crystallography in transmitted light. The book is therefore intended as a serious text that introduces the study of minerals under the microscope to the intending honours student of geology, as well as providing information for the novice or interested layman. Both transmitted-light and reflected-light microscopy are con­ sidered, the former involving examination of transparent minerals in thin section and the latter involving examination of opaque minerals in polished section. Reflected-light microscopy is of increasing importance in undergraduate courses dealing with ore mineralization, but the main reason for combining both aspects of microscopy in this text is that they are both important in petro­ graphic studies of rocks, especially where opaque minerals consti­ tute a significant part of a rock's volume. Dual-purpose microscopes are readily available and are ideal for the study of polished thin sections, which is a necessary prerequisite of electron microprobe analysis, in which a narrow electron beam, about I ,urn in diameter, is used to analyze a point on the polished surface of a mineral. The sections on reflected-light microscopy have also been written to assist any experienced transmitted-light microscopists who may wish to gain a better understanding of these techniques. Chapter I provides an introduction to the study of minerals under the microscope, using both transmitted- and reflected-light tech­ niques. Chapter 2 deals with detailed descriptions of the silicate minerals in transmitted light, and includes all of the minerals likely IX PREFACE to be encountered in thin sections of most normal rocks. Chapter 3 includes occasional descriptions of some non-silicates in transmitted light, but mostly deals with reflected-light descriptions of the opaque non-silicate minerals. In both Chapters 2 and 3 the minerals are presented in broadly alphabetical order, but closely related minerals are grouped together: the best way to locate a particular mineral is to make use of the index at the end of the book. Chapter 4 gives a detailed account of transmitted-light optical crystallography, since it is felt - by ourselves and many others - that students should have access to detailed explanations of the various optical prop­ erties. In a similar manner, Chapter 5 gives an account of reflected­ light theory. Both of these theoretical chapters have deliberately been placed after the main mineral descriptions of Chapters 2 and 3. The appendices include systematic lists of the optical properties of minerals, for use in identification. As this book is intended as an aid to the identification of minerals in rock thin sections, by either transmitted-light or reflected-light techniques, the description or interpretation of mineral relation­ ships is not discussed, although the mineral descriptions in Chapters 2 and 3 contain brief notes on occurrence. We hope that our text fulfils its purpose, and that all students of geology find it helpful and enjoyable to use.
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