INTROD U CTION TO Opti cal Mineralogy a n d P e t r o g r a p h y The P racti cal M etho ds o f Id e n tify i n g M i n e ral s i n T h i n S e c t i o n W i t h t h e Mi cro sc o p e a n d The P rincipl es Invo lved in The Classificatio n o f Ro cks By D M . M . G . ED WAR S A . , In s truc to r i n Ge o l o gy and M in e ral o gy Ca se Scho o l o f App lied Sc i en ce . CLE E V LAN D, O H IO , 1916 . 1 9 1 6 C opyright , , by D M . G . E W A RD S Th G r ne r P ri n i n o e a d t g C . PREFACE. I N THE preparation of this volume the writer has at tempted to gather together and systematize in a manner accessible f or ready reference those facts which are essen tial to a field geologist or to a mining engineer in an understanding of the fundamental principles involved in the class ification and identifica tion of rocks . In the field , a prelimina ry classification is usually made by macro scopic means . However, it is often necessary to make a more careful classificati on by a microscopic examination of a thin section of the minerals comprising the rock mass . To do this successfully requ ires a knowledge of the application of light to crystalline substances . Thi s volume differs from most of the reference and text books relating to this subj ect in that it incorporates in one volume the elements of o ptical mineralogy and the elements of petrography . In Part One , eight general operations f or the determination of unknown minerals in thin section are described , prefaced by a short summary of the principles of optics which apply to the trans mission of t polarized light through minerals . D escriptions of fif y eight of the most common of the rock-making minerals are given , special attention being given to the criteria for the determination of these minerals in thin section . o n bi Their f rm , cleavage , twinni g , color, refringence , refringence , extinction angles , pleochroism , absorption , optical character, inclusions , alterations , occu rrences , ff a uses , and di erenti tion from similar minerals , are all discussed whenever applicable . An elementary knowl edge of crystallography and descriptive mineralogy is assumed . 4 OPTICAL M INERALOGY AND PETROGRAPHY of a In Pa rt Two , the principles petrogr phy are dis a cussed briefly . Attention is given to the cl ssification and description of the more important i gneous rock types and o a Following Iddings , Winchell , ther Americ n and petrographers , the symbols X , Y , Z , are here em ployed in referring to the axes of ether elasticity , instead of the German a , b , and c , used in many text and refer o ence books . This is done to av id confusion , especially in conversation or discussion , with the crystallographic axes . o o The writer is indebted to Pr fess r Frank R . Van Horn for suggestions . Among the reference and text books most frequently consulted the writer wishes t o ’ " acknowledge Winchell s Elements of Optical Miner " ’ " " o Johannsen s Ma a of o a al gy , nu l Petr gr phic Methods , ’ " " ’ " Lu uer s M r q inerals in Rock Sections , Rog e s s Study of ’ " ’ " M a a o iner ls , Findl y s Igne u s Rocks , Kemp s Hand f " ’ ’ " book o Rocks , Ries and Watson s Engineering Geol " ’ " " ogy , and Farrell s Practical Field Geology . M . G. EDWARDS . Cleveland Ohi o Februar 1 9 1 6 , , y, . T A B L E O F C O N T EN T S N I TRO D UCTION . PAGE " PAR T O NE . O P TICA L M IN ERA L O GY . — A TH CHAPTER 1 . THE E LE MENTS OF OPTICS ND E APPLICATION OF POLARIZED LIGHT TO CRYSTALLINE S UBSTANCES The Nature of Light — Isotropic and Aniso tropic Media — Uniaxial and Biaxial Crystals Index of Refraction D ouble Refraction Interference Polarization . R Z CHAPTER 2 . THE POLA I ING MICROSCOPE AND ITS PARTS Microscope Nicol Prisms Condensing Lens Cross Hairs Stage Mirror Obj ective Bertrand Lens Ocular Micrometer Ad j ustment Screws . _ — E CHAPTER 3 . GENERAL METHODS OF MINERAL D TERM INATION 1 . By the General Physical Properties ; 2 . By the Relative Refractive Index Method of D ue de Chaulnes Immersion M e t h o d B ecke im Method Scale of Refringence ; 3 . By the R f ring ence Interference C o l o r s Axes of Ether Vibration Optic Plane Scale of Ri refringence . — CHAPTER 4 . GENERAL METHODS OF MINERAL DE TERM INATION ( Continued ) — 4 . By Axial Interference Figures Uniaxial 6 OPTICAL M INERALOGY AND PETROGRAPHY D 6 . and B iaxial ; 5 . By ispersion ; By Optical Character Quartz-Sensitive Tint Quarter Undulation Mica Plate Quartz Wedge ; 7 . By the Extinction Ang le Parallel an d Oblique Extinction ; 8 . By Pleochroism . — OF R CHAPTER 5 . D ESCRIPTION I MPORTANT OCK MAKING MINERALS Isometric Minerals . — OF M Con CHAPTER 6 . DESCRIPTION INERALS ( tinued) Tetragonal Hexagonal . — OF M Con CHAPTER 7 . DESCRIPTION INERALS ( tinued) Orthorhombic Monoclinic Triclinic . R R H . PART TWO . PET O G AP Y — U CHAPTER 8 . GENERAL D ISCU SSION OF IGNEO S ROCKS Classification — Essenti al and Accessory Min erals — Primary and Secondary Minerals Texture — Rosenbusch ’ s Law — Volcanic and Plutonic Rocks Petrogeny Magmas D if f erenti ati on Magmatic Stoping Crystal — lization Influence of Gases on a Magma Relation between Composition of Igneo us Rocks — and Mag mas Aids in the D etermination of R Igneous ocks in Hand Specimen . C 9 — — HAPTER . IGNEOUS ROCK TYPES PLUTONIC ROCK S Granite Syenite Nephelite and Leucite Syenite D iorite Gabbro and Norite Es TABLE OF CONTENTS i li ni e sex te Theralite , Shonkinite , Ma g t , Ij ol M o ite , issourite P e r i d t i t e Pyroxen ite , Hornblendite . — — CHAPTER 1 0 . IGNEOUS ROC K TYPES VOLCANIC Rhyol ite Trachyte Phonol ite Andesite D acite Basalt Trachydolerite Tephrite , B asanite Leucitite , Nephelinite Limburg ite Augitite . Pyroclastic rocks . - A CHAPTER 1 1 . SEDI MENTARY ND META M ORPH IC ROCKS Sedimentary Rocks Classificati on—Cong lom erate B reccia Sandstone Shale Loess Sand D unes Limestone Gypsum An hydrite Halite Flint Iron Ores Phos phate Rock Carbonaceous Rock . Metamorphic Rocks Composition , Chemical and Mineralogical Agents of Metamorphism Gneiss S c h i s t Quartzite Slate and Phyllite Marble Serpentine Ophicalcite Soapstone . AP PEND IX . S U GGESTIONS FOR GEOLOGICAL WORK Observation f or Geological Mapping . Criteria f o o Relative Age . Table for the Examination f o r R cks in the Laborato y . I NDEX N ROD C IO I T U T N . THE TERM Petrology is derived from the two Greek s words p etros ( rock) and log o ( discourse) , from which the modern definition , the science or treatise of rocks , has been evolved . The term has a wide scope , and embraces not onl y the study of the origin and transfo rmation of rocks but a consideration of thei r mineral composition , classification , description and identification based upon r either megascopic o microscopic characteristics . Petrology may be subdivided into the following spe cial studies : Petrogeny , which is concerned with the origin of rocks , and Petrography , which is concerned with the systematic classificati on and description of rocks megascopically and microscopically . It is the latter phas e of the subj ect which i s dealt with chiefly in the following notes . Petrography may be divided f or the sake of con venience into megascopic petrography and microscopic petrography , depending upon whether or not the student i s basing his identification , classification and description upon a study of the rock in hand specimen or in thin sec tion with the aid of the polarizing microscope . The use of the polarizing microscope necessarily em tails a brief review of the elements of optics and a con sideration of the application of polarizing light to crys ll ta ine substances . This is a specia l study in itself, and is called Optical Mineralogy . Assuming that the student has had little or no previous experience with the study of the optical properties of minerals , a short review of the optical characters of the more important rock-making 1 0 OPTICAL M INERALOGY AND PETROGRAPHY a to minerals is given . Speci l attention is given the criteria f or the determ inati on of the minera l in thin sec tion and diagn ostics f or the differentiati on of the mineral from similar minerals . — a a Hi s to ry o f Pe tr o gr aphy . Gre t adv nces in the knowl edge of mineralogy marked the latter half of the eight a a at eenth century . Incident lly there followed sever l 1 787 tempts to classify rocks , which resulted in in the publication of two cla ssifications by Karl Haidinger ’ G. (Vienna ) and A . Werner ( Dresden ) Werner s classi o ficati on was stratigraphic rather than petr graphic , but he described rocks in terms of mineralogical compo sition f a be and physical characteristics , and he dif erenti ted o tween essential and access ry minerals . " r o In 1 80 1 , Abb R .