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 . J . H auy ( Pa is) , a mineral gi st , ma a published the first syste tic cl ssification , and his " Trait"de mineralogie with subsequent revisions re mained a classic for a long period . He distingu ished five on e classes of rocks : st y and saline , combustible nonm ll ta ic, metallic , rocks of an igneous or aqueous origin , o and v lcanic rocks . John Pinkerton ( England ) in 1 8 1 1 published a P e trolo a f 1 2 f g y, Treatise on Rocks , o 00 pages . In view o the fact that natural history was divided into three king o — a — d ms the anim l , vegetable , and mineral h e believed it the most n atural classificati on to subdivide the mineral o kingdom into pr vinces and domains . Accordingly he o o : introduced the f llowing three pr vinces Petrology , the no o or a k wledge of r cks stones in l rge masses ; Lithology , and a ll the knowledge of gems sm ll stones , and Meta ogy , o ’ the kn wledge of metals . Pinkerton s volume was lightly a reg rded even by his contemporaries . Cordier ( France) in 1 8 1 5 classified rocks as feld a or sp thic pyroxenic , and made subdivisi ons according to texture . INTRODUCTION 1 1

Karl von Leonh ard ( Heidelberg) In 1 823 and Alex andre Brong niart in 1 827 propo sed systems which mark the real origin of systematic petrography . Mineral com position was the chief fa cto r i n the classification . The former established fou r divisions : heterogeneous rocks , homogeneous rocks , fragmental rocks , and loose rocks . The latter made only two classes : homogeneous rocks and heterogeneous rocks . Hermann Abich in 1 84 1 made a classificati on of the eruptive rocks according to the content of the variou s feldspars . The term petrography was perhaps first used by Carl " Fried rich Naumann , who in 1 850 published his Lehr " l buch der Geognosie , in which he divided a l rock classes int o crystalline rocks , clastic rocks , and rocks which are neither crystalline nor clastic . In a later revis ion he

: a recognized only two classes the origin l , an d the de rived . Several cla ssificati ons were presented in the next few decades by von Cotta Senft B lum ( 1 8 6 0 ) Roth ( 1 86 1 ) Scheerer Ferdinand Z irkel

an d F . von Richthofen ( 1 8 68 ) based upon min eral constitution , chemical composition , structure , an d textu re , wi th an increasing tendency to emphasize the importance of mineral composition . A new era in the development of petrography dawned f with the introduction o the pola rizing microscope . With the greater knowledge of mineral composition and texture thus revealed , the old schem es were discarded or radically revised , new terms introduced , and the nomenclature became rapidly more complex . Although Henry Clifton Sorbey ( England ) perhaps first u sed the microscope in o the determination of rock sections , it was n t until the decade between 1 870 and 1 880 that microscopic methods began to exert a controlling influ ence in the devel opment 1 2 OPTICAL M INERALOGY AND PETROGRAPHY

1 8 73 D i e ' o of the science . Zirkel in produced mikr skop " s ische Beschaffenheit der Mineralien und Ge teine , which shows a remarkable and significant a dvance in the p rog ress of petrography in the eight years following the pub " " licati on of his Lehrbuch . He dealt chiefly with feld spathic , mass ive , composite , an d nonclastic rocks .

In France in 1 879 the Mineralogie micrographique ,

M . R a by F . Fouque and A . ichel Levy , appeared ock cl ssi

fication was bas ed upon the mode of formation , the geo i logical age , and the s pecific mineral properties , wh ch includes the nature of the mineral and the structure of the rock . Subsequ ent editions of the original works of Rosen busch and Zirkel , and a number of new noteworthy con tributi ons by Roth Teall Loewi nson Lessing ( 1 890 and Johannes Wa lther ( 1 8 97 ) appeared , chief attention being given to the classification of on igneous rocks the basis of origin , age , and char

acters . Within the las t twenty years a number of Ameri can petrographers have made noteworthy contributions to the science of rock clas sification , and with the coopera ti on of the field geologist who has gradually become more and more p ainstaking in the matter of collecting and c f labeling rock spe imens or future study , they hope to evolve from the present classification which is marred of a by a complexity nomencl ture , a logical and compre hensive system of classification which will approach in construction as closely as possible a truly natural arrange ment . Among the earlier American petrographers who made valuable contributions toward the development of the

. . . D science are J F Kemp , J S . iller, Whitman Cross , J . P .

Iddings , and F . D . Adams . THE ELEMENTS OF OPTICS

N ERA . PAR T ON E . O PTICA L M I LO GY

CHAPTER 1 .

Th E e n Of O cs an d the A c a i o n o f o ariz e d e l m e ts p ti , pp li t P l

Light to Cry s tallin e Sub s tance s . — The N a ture o f Light Light is a form of energy which in a homogeneo us medium as the ether is trans mitted in a rapid wave motion in straight lines with no change in the direction of propagation . This wave mo tion is considered to be a resultant of simple harmonic motion and a unifo rm motion at right angles to this . In other words , wave motion is a vibration which takes place at right angles to the direction of propagation of the light . A ray of light is a line which designates the direction of transmission of the wave . The intensity of light de

- pends upon the rate or wave length of the vibrations . Color sensation is determined by the number of waves of light which reach the eye in a given time . The wave leng th f or red light is 76 0 millionths of a millimeter , and the wave-length f or violet light is 3 97 milli onths of a millimeter . White light i s the sum of light of all these

- variou s wave lengths . The velocity of light of all colors o in vacuo i s the same , and is about km per sec nd . — I so tro p ic M e di a Light is transmitted with equal velocity i n all directions in certa in media , as air , water , and glass . Light which is transmitted through such a medium if it finds its source in that medium will be p ropa i - gated as spherical wav es , n which the wave front or 1 4 OPTICAL M INERALOGY AND PETROGRAPHY

- all wave surface forms a continuous surface , and points o on that surface are equidista nt from the s urce . A ray - n of light is perpendicular to its wave fro t . - a ma In an isotropic substance , this wave su rf ce y be e a s represented by the su rface of a spher . Any pl ne pas ing through this imaginary sphere in any p osition will a o have a circular outline . Gases , liquids , m rphous sub f stances as v olcanic glass , and crystals o the isometric lo system , are isotropic substances . The ve city of trans mission of light through these substances is independent o of the directi on of vibrati n . — o An i so tro p ic M e di a . In anis otr pic media ( as op m a c posed to isotropic edi ) , the velo ity with which l ight o is pr pagated varies with the directi on . All substances which are not amorphous or which do not belong to the om are is etric system optically an isotropic . Anisotropic crystals are divided into uniaxial and a bi xial crystals . — U n a a C r s a s . In a a n i x i l y t l uni xi l crysta ls , only o e direction exists in which there is no double refraction of light . This is in the direction of the vertical crystallo a a a gr phic xis , which is c lled the optic axis . It li es in the of a direction either the gre test or least ease of vibration . The wave-front which represents the optical structure of uniaxial crystals is an ima ginary spheroid of revolu o ti n in which the optic axis is the axis of revolution . A plane passing through the spheroid in any direction at

a e o h s right ngl s to the ptic axis a a circular outline . Any o an a ther section has elliptic l outline . Tetragonal and a a hex gon l crystals are uniaxial .

a a C r s — Bi xi l y tal s . In biaxial crystals there are two directi ons corresponding in character to the one optic a a a a xis of uni xi l cryst ls , which gives ri se to the term a a - bi xi l . The wave front which represents the optica l THE ELEMENTS OF OPTICS 1 5

x a with three unequal rectangula r a es . A plane p ssing through this ellipsoid in any direction at right angles to a either of the optic axes h s a circular outline . Any other a section has an elliptic l outline . O rthorhombic , mono clinic and triclinic crystals are biaxial .

' n — ha I n de x o f R e frac tio . The previous discussion s been concerned with light which has . passed through homogeneous media . If a system of light waves of the same wave-length passes obliquely from one medium into a another , there will be a change in the direction of tr ns mission depending upon the relative ease or difficulty with which the light may penetrate the new medium . If a the second medium , such as glass , is optic lly more dense i r f than the first medium , such as a , that portion o the wave-front which first strikes the glass will experience a greater difficulty in transmi ssion , and its vel oc ity will - be reduced , while the remainder of the wave front is still traveling with the same velocity in the air . When this

- portion of the wave fro nt finally reaches the glass , it has ga ined up on the first portion , with a result that the wave will have suffered a defl ection from its origina l course . From this position the v a ri ous portions of the wave-front continue through the glass with equal velocities .

This phenomenon is called refraction . It is a change of direction at the bounding su rface . Refracti on is toward the perpendicular (to the bounding su rface) when the passage of a light ray is from the rarer to r the denser medium , and away from the pe pendicular in the opposite case . 1 D In Fig . , C is the bounding surface between two a o medi , f which the lower is optically denser than the

. G a o upper H is perpendicular to the b unding su rface . Angle i is the angle of incidence and ang le 1) is the angle o f refraction . A constant relation exists between the S ines of these angles regardless of the direction of trans 1 6 OPTICAL M INERALOGY AND PETROGRAPHY missi n which ma be expressed as follows : the sine o , y a of the angle of incidence bears a constant r tio to the a sine of the angle of refraction . Th is r tio may be sin i n n i expressed by the equ ation , in which s

o a t o the index of refraction and is inversely propo rti n l

i i nd i n e re r i n . F g. 1 . Re flect on a s gl f act o e ( Wi nch ll . )

the wave velocity . In this formula there are two lim

z :: 0 iting relations to be considered . If i o, r , in

which case the angle of refracti on b ecomes zero . Thus , by perpendicular incidence , the ray proceeds in the

second medium without any change in direction . If

1 I. 1 90 z n or s m r Thl S value of r is known S l n r n

as or f a the critical angle , angle o total refl ection , and m y

1 8 OPTICAL M INERALOGY AND PETROGRAPHY

An adamantine luster is characteristic of minerals with an index of refracti on above

a o o Ordinarily , the index of refr cti n f a substance is determined by passing the incident ray into the sub stance from air, but other media than ai r might be used . The index of refracti on of the substance in air is the product of its index in the medium by the index of that medium in air . The index of refraction of air when referred to a vacuum is

a a Elementary phenomena in refr ction , such as the p parent bending of a stick of wood when partially sub i merged n water , were no doubt observed in early times . The constant ratio between sin 13 and sin r was first a established by Desc rtes in 1 637 , but it was not until Newton succeeded in producing a colored spectrum by a prismatic decompo sition of white light that the full o a w imp rt nce of n as realized . — D o ubl e R e fr ac tio n D ouble refraction Is the prop ‘. erty posses sed by all an isotropic crys tals of resolving

' a light ray into two rays pola rized at rig ht ang les to a e ch other and traveling in different directions . This is due to the fa ct that upon entering the anisotropic me dium the vibration s of light are made to confo rm to a the molecul r structure of the medium . In other words , light travels with different velocities in different crys tallog ra hi c o p directi ns in the same substance .

The ray advances with the greatest velocity when it is vibrating parallel to the direction of the greatest a o e se of vibrati n , and with least velocity when vib rat a e e ing par llel to the dir ction of l ast ease of vibration . a These r ys obviously have different indices of ref rac tion . The ray which follows the usual la ws of single refracti on is called the ordinary ra expressed by O y , . ra The other y is called the extraordinary ray ( E ) be THE ELEMENTS OF OPTICS 1 9 cause it does not follow the usual la ws of sing le re fraction . When a ray of light enters an anisotropic medium as perpendicular to its su rface , the ordinary ray p ses through without suffering refraction , provi ded sur

2 . P n w n i n e r en r Fig . la e ave adva c g p p dicula th e ver i xi s s wi n e e r to t cal a , ho g th vib rati o n a nd re ta rdatio n of

the O a nd E rays . i n e (W ch ll . ) face through which it emerges is parallel to the sur i face through which t enters . The extraordina ry ray is diverted . To this rule the following exceptions must be noted . If the original ray enters a substance per pendicular to the surface and at the same time par 20 OPTICAL M INERALOGY AND PETROGRAPHY

a allel to an optic axis , there is no refr ction nor polar a a i zation. If perpendicul r to the su rf ce and to an optic a w xis , there is no refraction but there is a division into t o rays traveling with different velocities and p ol arized at

right angles to each other .

. 3 Fig . Obli q u e i nci de n ce on a s u rfac e r e t h p e O i xi s . Wi n e a all l to p t c a ( ch ll . )

When the two rays emerge from the substance , resume parallelism , but the waves of one are slightly of in advance the waves of the other . Such waves are a to e a o s id int rfere with e ch ther, producing light of different colors . Upon this phenomenon is based much THE ELE MENTS or OPTICS 21 of the work done in examining minerals in thin section under the microscope in parallel polarized light . — In te rfe r e n ce . Two waves of like length and ampli tude , if propagated i n the same direction and meeting r f am in the same phase , un ite to fo m a wave o double i pl tude. If these waves differ in phase by half a wave length or an odd multiple of this , they interfere in such a way as to extinguish each other . For other relations of phase falling between these extreme cases they also interfere with each other, forming a new resultant wave , ' differing in amplitude f rom each of the component waves . We are assuming here the use of monochromatic light waves , or light waves of l ike l ength . If ordinary white light is employed , the waves in case of interference will be indicated by the appearance of the colors of the spectrum . — P o l a riz a tio n . Ordinary light is propagated by trans verse vibrations of the ether which take place in all directions a bout the line of p ropagation . Plane p olar i zed light is propagated by ether vibrations which take place in one plane only . This phenomenon is called polar i za i t on. It may be described as a change in the char f acter o reflected or transmitted l ight , which diminishes of its power being fu rther reflected or transmitted .

Light is p olarized by reflection , by single refraction , and by double refraction . The plane of polarization of light polarized by reflection is defined as the plane con taining the incident and the reflected rays , the vibra s a tion t king place at right angles to it . The plane of po larization of the refracted ray is the plane at right e to angl s the vibration direction , consequently at right a to ngles the plane of the inci dent and the reflected rays . That light is p olarized when refl ected may be shown e o exp rimentally by the use f two reflecting su rfaces . — N co r sm . N i l P i The icol prism , so named after its 22 OPTICAL M INERALOGY AND PETROGRAPHY

o a f or . inventor, Nic l , is device producing polarized light f k w It consists of a clea r transp arent crystal o calcite no n o as Iceland spar, as it is obtained alm st exclusively t a from caves in cert ain basa lts in Iceland . The ver ic l cleav faces are natura l cleavage faces , in which the end ges inclined 7 1 degrees to the obtuse edges of the a , prism are ground down and po lished so as to make , an angle of 68 deg rees with the obtuse vertical edges . It is then cut diagonally in two parts perpendicular to

i i i 5 . End vi ew F g. 4. S de v ew of of

the Ni ris m . the Ni co l p m m . col p

the short diagonal of the end face . The two parts are cemented together in their original position by Canada m of balsa , a resin obtained from a species fir . It has an index of refraction of Since calcite i s a doubly of refracting substance , the Nicol prism refracts a ray — light into two rays the ordinary ray , having an index of and the extraordinary ray , having an index of

The angle at which the two new planes are po lished ,

as well as the angle at which the crystal is cut, are so calculated that the ordinary ray will strike the bal THE ELEMENTS OF OPTICS

sam a t a n angle greater than the critica l angle . Couse ab quently , the ordinary ray is totally reflected an d is sorbed in the blackened walls of the cork mountings . The extraordinary ray passes through the balsam as a completely pol arized ray wh ich i s vibrating in a known direction , namely , p arallel to the short diagonal of the calcite rhomb . When two Nicol prisms have their short di agonals parallel , light passes through without being n f changed except for a decrease in intensity . If o e o the nicols is revolved , the light gradually diminishes until the nicols are at 9 0 degrees to each other , when darkness results . OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTER 2 .

n The Po l a ri z in g M ic r o scop e a d Its P arts . — The Po l a riz in g M ic r o scop e . In order to as certain the peculiarities of minerals of each of the crystallo . graphic systems as they are manifested in polarized i ru s . nst light, the polarizing micro cope is used This ment is applicable to the study of the form , optical prop f erties , and mutual relations o the minerals as they are of a found in thin sections rocks , making it a v luable a id to geological research . It is likewise u sed to great r advantage in the study of small isolated crystals , o fragments of crystals . A dete rmination of the follow ing characteristics of the unknown mineral is of pa rticular value in its identification :crystal fo rm as shown in outline , direction of cleavage lines , ref rac tive index , light absorption in different directions , the or isotropic anisotropic character , position of the axia l of a plane and the nature the axi l interference figures , the strengt h and character (positive or negative) of a o the double refraction , presence and n ture f inclusions , type of twinning . In addition to the parts essenti al to the ordinary microscope the polarizing microscope contains the fol : lowing parts two Nicol prisms , a lens f or convergent polarized light , a rotating stage , and an ocular with cross

not usually be distinguished except by a combination of NIC l O . two prisms The upper nicol is not revolvable ,

26 OPTICAL M INERALOGY AND PETROGRAPHY a nd is pl aced in a supp ort between the ocular a nd the

n out of . obj ective . It ca be pushed in or the tube at will

It is called the analyzer . a be The lower nicol , which is revolvable , is pl ced o a neath the stage . For rdin ry work , its principal sec ti on its shorter diameter) is placed at right angles with the principal section of the upper nicol . It may be raised or lowered without disturbing the centering . This a nicol is called the pol rizer . f The principal section o the analyzer is left and right , and that of the polarizer is front and rear . In this posi " o ti n the field is dark and the nicols are crossed . When a thin s ection is examin ed over the lower or nicol between two nicols without the convergent light , to o it is said be d ne in parallel polarized light . When

o o a the l wer nic l is used alone , its vibr tion plane must o n be kn w . A simple test is to place a cleavage frag m of w ent calcite ithin the field of view . It has a high , relief when its long diagonal is p arallel to the plane of f vibration o the n icol . A secti on of bi otite cut at right angles to its cleav ag e has its greatest absorption when its cleavage direc tion is parallel to the plane of vibration of the polar

izer. C onsequently , it is darkest in this position . Tour m on a aline, the other h nd , extinguishes vibrations at a o i right ngles to the ptic axes , . e. , it absorbs the ordi nary ray , and only the li ght rays vibrating parallel to a the crystallogr phic axis 0 emerge . N By removing the icol prism from the tube , the a sep rating plane of the balsam along which the two a fr gments of calcite were cut may be seen upon looking through the prism at an angle . The vibration direc tion of the ray which passes through the prism (the ex traordinary ray ) is normal to the layer of balsam . The polarizer may be removed from the microscope THE POLARIZING M ICROSCOPE AND ITS PARTS 27

o as and light reflected from a h rizontal surfa ce , such

o o . a plate of glass or a table t p , examined thr ugh it Since light is polarized in a plane parallel to the reflecting

surface , the polarizing plane of the nicol lies at right angles to the reflecting surface when the latter appears

dark . — L e n s fo r Co nve rge n t Light When the operation de

a o o mands convergent light , p werful c nvergent lens can be thrown into the tub e ‘ of the microscope over the

polarizer by means of a lever beneath the stage . This lens may be raised with the lower nicol until the surface of the lens i s practically in contact with the glass slide holding the thin section . — The R o ta tin g Sta ge The stage is a circular table

upon which the thin secti on is placed f or examination . The edge has a graduated scale an d vern ier reading to f minutes . The center o the stage must coincide with

the optical center of the tube . Centering is done by f lo means o two centering screws , 9 0 degrees apart ,

cated on the lower end of the tube . The thin section is i held n place by two spring obj ect clips . Recent forms of microscopes are equipped w ith mechanical stages which have freedom of movement in a left-and-right - - direction and in a front and rear d irection , thus allowing f a rapid inspection o every p art of the section . — C o ss H a . C r i r . ross hairs are placed in the ocular at right angles to each other , one running left and right o and the other fr nt and rear , in agreement wi th the principal sections of the nicols when crossed .

To measure a plane angle in thin section , or the in terf acial of a angle a sm ll , flat crystal , the stage is cen tered with the intersection of the two edges at the cen s ter of the cro s hairs . A reading is made when one edge of the crystal is parallel to the left-and -right cross hair , then the stage is revolved until the other edge is 28 OPTICAL M INERALOGY AND PETROGRAPHY parallel to the same cross hair but on the Opposite side

of the center . Another reading is taken . The difference between the two readings is the external angle . Other parts of the microscope which deserve expla n ation and suggestions as to proper use are taken up in order . — a The M irr o r. The mirror which is att ched to the substage reflects light from the source to the obj ect . A plane mirror forms one side and a concave mirror o the other . The former is used for low magnificati n , f o where a weak light is sufficient . The latter is used r higher magnifications . This mirror concentrates the light by converging the rays included within an angular aperture of ab out 40 degrees . For still higher mag nifications and f or all phenomena observed in convergent light the condensing lens is used . A proper use of the mirror is essential to the most

fi f . ef cient use o the microscope When parallel rays , such r as ordinary daylight , a e used , they are reflected from the mirror with a slight loss of intensity . They are re fl cted o e fr m the concave mirror with increased intensity , the rays coming together at the focal point. If the of s ource light is close to the instrument , the focal length

is . To larger meet this adj u stment , the mirror is at h d tac e to a sliding vertical bar . Since the condensing lens d has its focus some istance above its upper surface , the a pl ne mirror is used in connection with it . — The Obj e c tive Obj ectives are classified according to their magnification . An obj ective of low power has a focal leng th above 1 3 mm and a magn ification less th an 1 5 diameters ; it is of medium power when its focal length is between 1 2 and 5 mm and its m agn ification is 4 0 diameters ; of high power when its focal length i s less than mm and its mag nification exceeds 4 0 diameters . The obj ectives most commonly used are THE POLARIZI NG M ICROSCOPE AND ITS PARTS 29

f or Out numbers 3 and 7 , the former searching an obj ect and for making the preliminary examination , and the latter f or convergent light and high po wer . A thin section may be considered as made up of a series of planes supe rimposed one above the other , only one of which may be seen f or one adj ustment of focus . With low-power obj ectives one can see obj ects lying in

- slightly different planes , but with high power lenses this o is impo ssible , as the depth f focus diminishes inversely as the numerica l ape rture . The brightnes s of the image increases as the square of the numerical aperture . — R e so lv in g P ow e r . The resolving power of an Oh j ective is that property by vi rtue of which one is able to see the finer details of an obj ect . This resolving p ower increases with the number and obliqu ity of the rays coming from the obj ect, consequently an immersion flu id by increasing the number of rays brought to the obj ect increases the resolving power . In petrographic work no very great magnifying powers are required , and im mersion lenses are not much u sed except for p a rticular kinds of work . When two points are removed from the eye times the distance separating them they will appear as a single po int . The eye is able to distingu ish only 2 l about 50 lines to an inch . Thus pleu rosig ma ang u atum with about lines to the inch can be resolved by a one-half inch obj ective so as to be clearly seen with a three-quarters inch ocular but not with one-and-one -half inch . A much smaller line may be seen than the inter val between two lines . f i — Co s t o Obj ec t ve s . Obj ectives With a focal length of 25 mm and over cost about $4 each ; between 25 and 1 0 1 0 1 0 ’ t mm , to $ ; to 3 mm , $7 to $ 1 5 ; 3 o 2 mm , about $20 . Students are u rged to treat them with care . 30 OPTICAL M INERM J OGY AND PETROGRAPHY — B e rtr an d L e n s . In t he center of the microscope tube r m a above the an alyzer is the Be trand lens , which y be thrown in or out of the tube by means of a sliding car

a m a l o rier . It acts as s l microsc pe which is used with the ocular to magnify interference figures . — a The O cu l a r . The Huygens ocul r which is most gen erally used in petrographic microscopes consists of two simple plane-convex lenses placed with their plane sur faces toward the eye . The u pper lens i s known as the n o a or eye lens a d the l wer s the collective field lens . The focal length of the eye lens is one third of the field and a lens , they are separ ted a distance equal to the sum of their focal lengths . The rays of light emerging from the eye lens are parallel a nd thus cause the eye less fatigue .

The cross h a irs which are placed in the eyepiece are of h made spider web , t e dark threa d from the inside of the nest being the best . M — ic r o m e te r . It is desirable at times to measure a as f sm ll distances such the dimensions o small crysta ls . A special eyepiece called the micrometer has been devised f or o . o a a this purp se It c ntains sc le etched on glass . On the stage of the microscope a scale reading to hundredths of a mm is placed . It is then n ecessa ry to find to how many hundredths of a mm each division of the eyepiece is equivalent .

ma f or It y be well the student, in order to become a a f mili r with the use of the micrometer , to construct a table showi ng the v alue of the ocula r micrometer for a e ch obj ective . The stage micrometer is used for this

d u s m e n c e — A j t t S r w s . The tube ca rrying the eyepiece and o has a a bj ective fine dj ustment screw , the edge of a a which is gr du ted . It mOVes aga inst a fixed index at tached to the tube , by which means the distance through THE POLARIZING M ICROSCOPE AND ITS PARTS 3 1 which the tube is raised or lowered can be measured to . 00 1 mm . The student is advised a s a laboratory illustration to determine the amount which one revolution of the

fine a dj ustment screw raises the obj ective . To do this , measu re the thickness of a glass plate or cover glass by focus ing carefully on the lower su rface of the glass and then upon the upper su rface . This distance is meas ured by the micrometer by setting the glass plate on edge , slightly embedded in paraffi n or wax, or supported other

W i se . — U se o f the M ic ro scop e . The best light f or micro sc0 pic work is that coming from the north ; the next best from the east . D irect sunlight should never be used .

The table should be firm , and of a height to su it the convenience of the individual . The instrument should be placed directly in front of the observer , so that both hands can be used for manipulation . The eye which is not used for obse rvation should als o be kept open . Although it may seem difficult at first to concentrate the gaze on the thin section , it will be found to be far less fatigu ing . When using high powers , the eye must be kept very close to the ocular , with low powers slightly farther removed . When both ni cols are being used , more light i s advantageous than when only one is in use . As much of the examination of a thin sec tion as possible should be done with the low powers in order to save a strain on the eyes . The student is p articularly cautioned whenever f o cusing with high powers to focus upward and never

a . downw rd If this rule is followed , no thin sections will be broken . Place the eye on a level with the sta ge , and l ower the tube slowly until the obj ective i s almost

a o in cont ct with the thin section . Then looking thr ugh a o the tube , r ise the bj ective slowly until the portion of 32 OPTICAL M INERALOGY AND PETROGRAPHY

the section desired f or examination is in focus . If col orless minerals such as quartz are being examined , it is well to reduce the amount of the illumination and r look f or bubbles o other inclusions . Proper care of the nicols and lenses prolongs their life and increases their efficiency . They should not be exposed to severe sunlight nor to the heat from a steam radiator, lest the cement soften . The lenses should be kept free from dust . The obj ective Should never be to allowed come In contact with the cover glass .

34 OPTICAL M INERALOGY AND PETROGRAPHY

a f polarized light, which gener lly causes no great dif erence in the appearance of the mineral . The intensity of the

a of unpolarized light , h owever, is much greater than th t d the polarize . Minerals examined by ordinary light are of two classes : transparent and opaque . The former class is l v examined by transmitted light f or crystal form , c ea

f or ag e, and color ; the latter class by incident light crys

o tc tal fo rm , c lor, luster, e .

CRYSTAL FORM . The determination of crystal form is not of great imp ortance in the study of rock sections f or the reason th at individu al crysta ls have not had the

o or o opp rtunity f undisturbed devel pment , but have been hampered in their growth by interference with neigh boring crystals . In certa in porphyries a stu dy of the

of a form the phenoc rysts often le ds to their identificati on .

o CLEAVAGE . Pr nounced cleavage lines are developed in certain minerals in ch aracteristic directions during

of the process grinding to thin secti on . The direction and o of perfecti n the cleavage cracks is indicative . A mineral possessing no cleavage will have irregular cracks , as quartz .

Perfect cleavage is a cleavage in which the lines a re

a o sh rp and extend for c nsiderable distances . Examples mica , fluorite .

Good or distinct cleavage is cleav age in which the

a are o cr cks interrupted with ffsets , etc . Examples a o ugite , h rnblende orthoclase . i Poor or indist nct cleavage is very irregula r, with a uneven cr cks , though they follow roughly ce rta in di rec o ti ns .

a o a a a as Pin c id l cle v ge , shown in mica , is well devel o one ped in direction only . Prismatic cleavage , as METHODS OF M INERAL DETERM I NATION 35

shown by augite and hornblende , usually develops in of two planes . In certain minerals the isometric and

hexagonal systems , such as galena and calcite , three good cleavages develop .

Cleavage angles , of cou rse , depend upon the orien tati on of the random section shown in the thin section . Where the section is cut at right angles to the cleavage

e . planes , the angles are charact ristic Hence , if one i s S using cleavage fragments , he can orient it at will , ince the flat faces will bear definite relations to the crystal log raphi c axes

PARTING . Part ing is a fracture often developed par allel to a certain cleavage direction occurring along planes of weakness as may result from shearing or glid

ing planes .

TWIN NING . Twinning is impo rtant i n certain m in r l a e a s and will be discussed in Chapters 5 , 6 , nd 7 .

COLOR . All colored minerals may be divi ded into a two classes :idiochromatic and allochrom tic . Idiochro matic minerals are those in which the color is due to of a property the mineral itself, namely , its ability to

- absorb light of certain wave lengths , although the prop erty of absorption may not be the same in eve ry di rec

tion . Allochromatic minerals are those in which the

color is due to inclusions , which may or may not be dis tin uish ed i r g under the m c ois cop e. The pig ment may

be either organic or inorganic . Carbon , nitrogen and r n hyd oge have been found in zircon , smoky quartz , ame th yst, fluorite , apatite , calcite , microcl ine , barite , r halite and topaz . Free fluorine has been found in fluo

ite . Traces of iron are found in brown zircon . The pigment may be thickly and evenly distributed or i rreg ularly and so sparingly distributed that a thin section

appears colorless . 36 OPTICAL M INERALOGY AND PETROGRAP HY

2 D e e m ina o n o f the R e l Gen e r al O p e r a tio n N o . : t r ti x a ti ve R e frac tive Inde . Refraction is the change which light undergoes in direction in passing between two media which differ in f density . The index o refraction described under optics may often be j udged by the appea rance of the m ineral C a in the liquid in which it is mounted , usually anad bal a sam . This makes a convenient st ndard with which to compare the index of refraction of the unknown mineral . Although its index varies slightly during the process of mounting and with age , only those few minerals whose indices fall within the limits of variation of the a con balsam are aff ected . B alsam may ret in its sticky sistency and low index f or forty years if protected by a cover glass . If the mineral under examinati on an d the balsam have practically the same index of refraction , the min eral will appear smooth and will be visible with difficulty . " " It is then sai d to have low relief . If the two have a o quite different indices , the su rf ce f the mineral will appear rough and the borders da rk . Such a surface , f because o its resembl ance to shagreen , is called a sha " green surface . The mineral is said to have high re " li lief. This apparently rough surface is due to inequa of ties the surface , each elevation and depression re flectin g and refracting the light at a different angle . This irregular illumination causes the minera l to appear darker in some spots and lighter in others . A mineral embed ded in Canada balsam will have high relief whether or a its index is lower higher th n the liquid . In a rock section where a number of different m in erals having different indices of refraction lie in con a t ct , certain minerals appea r to stand out above the oth

ers in relief . Minerals with high indices seem to be of elevated from the plane the section . This is because M ETHODS OF M INERAL DETERM INATION 37

the rays of light from the lower surface of different minerals appear to come from the points of intersection

of the refracted rays . Since the index of refraction of a mineral is one of

its most important optical properties , many methods have been devised for its identification .

THE METHOD OF D UC DE C HAULNES . By this method one may determine the index of the mineral directly by focusing a medium or high-power obj ective accu r l r ate y upon an obj ect , and then inse ting between it and the obj ective a transparent plate with parallel si des . The

image becomes blurred . The tube of the microscope is

raised until the image is again in focus . The amount of change necessary is dependent upon the index of

refraction of the plate and upon its thickness . As a laboratory illustration the student is advised to determine the index of refraction of a plate glass or

cover glass by this method . If the student uses a glass whose true thickness has already been determined , the index may be obtained by measuring the apparent thick ness and dividing the true thickness by this latter

amount . If the thickness of the glass is not known , it is possible to determine the approximate true thickness by focusing on a point or Scratch on another plate of glass which is to be u sed for a support . Then place upon it the glass plate whose thickness is to be deter

mined , an d focu s on its upper su rface . This distance is the thickness of the glass plate plus the thickness

of the air film separating the plate from the support , and should be used only in case the thickness of the plate is so great that the thicknes s of the air film be

comes negligible . The apparent thickness can now be

measu red and the index calculated as mentioned above . A correcti on f or the air film can be made very easily

and should always be done if the glass plate is thin . 38 OPTICAL M INERALOGY AND PETROGRAPHY

Focus on the upper surface of the glass plate and then on the lower surface . This distance through which the obj ective moves is the apparent thickness of the plate .

Now focus on the surface of the support . This gives t the true thickness of the plate and the air film . Nex focus on the lower surface of the plate and on the upper O surface of the support . This gives the true thickness f the air film , which can readily be subtracted from the thickness of the glass plate and air film . The difference is the true thickness of the glass plate . The index can now be determined by dividing the true thickn ess by the apparent thickness of the plate .

IM MERSION METHOD . If a drop of a liqu id with an index equal to that of the mineral is placed upon a thin secti on of the mineral without a cover glass , the appear ance of roughness which characterizes the mineral in i a r disappears , since there is neither reflecti on nor re fraction at the contact , and the light passes through r without deflection . If the mineral is colorless , it p ac tically disappears from view . By the use of a series of immersion liquids whose indices of refraction are o kn wn , it is possible to experiment with the unknown mineral until a liquid is found whose index of refraction by the above test corresponds with the index of ref rac of tion the mineral . M . BECKE ETHOD By the B ecke method , which i n volves the use of total reflection in connection with re one fraction , may determine the relation which the re f ractive index of the unknown mineral has to that of one which is known and which i s in contact with it . Bring the focus directly upon the line of separation of

- the two minerals , using a high power obj ective in con vergent light . If the condenser is lowered and the an alyz in o g nicol is rem ved , it i s ob se rved that the field c be omes slightly darker , and a fine line of white light MET HODS OF M INERAL DETERM I NATION 39

sharply marks the contact of the two minerals . Upon raising the obj ective very slightly , this thin line of white light will be seen to Shift from the line of contact of the two minerals toward the mineral having the higher index . This phenomenon is explained as follows : The rays of light which enter the minerals perpendicular to their su rfaces undergo no refraction but pass directly through . Those rays which enter the mineral having the lower index of refraction reach the plane of contact of the two minerals and are all bent toward a normal to this . plane passing through the mineral having the higher index , because they are passing from a rarer to a denser medium . The rays which enter the mineral having the higher index must pass from a denser medium to a rarer . In such a case it is remembered that all of those rays which strike the mineral of lower index at an angle greater than the critical angle of the denser mineral , are totally reflected and emerge from the upper surface of the denser mineral . Only those rays pass into the mineral of lower index which strike the plane of contact of the two minerals at an angle less than the critical a of ngle the mineral of higher index .

Up on lowering the obj ective slightly , the white line shifts toward the minera l with the lower index . It is advisable to use the Becke test on contacts which are nearly or qu ite vertical . One can easily determine a vertical contact by shifting the focu s and seeing that the boundary remains sharp at all foci , and in the same f position . The ve rticality o the contact makes no differ ence with the result , provided the medi um having the lower index lies above . If it l ies below and the i ncli nation is great enough , the bright line may appear to move the wrong way . o to Acc rding this method , differences of . 00 1 be 4 0 OPTICAL M INERALOGY AND PETROGRAPHY

a tween indices are noticeable . It is especi lly useful in a o a determining minerals with low indices , s s d lite , leu

o a . cite , or in distinguishing between rthocl se and quartz Since the mean refractive index of qu a rtz is ab out the a a and same as andesine , is higher th n orthoclase , lbite o oligoclase , and less than labradorite , bytownite and an r ma thite, certa in definite inferences y be drawn regard ing these minerals in contact . The following scale of refringence ( after Winchell ) will aid the student to estimate the value of the mean index of refraction of minerals in thin section by means " " of reliefs .

OF S CALE R E F RINGE NC E .

w e n n e l o e ce . m fl V ry r fri g Ex a ple, uo rite,

Low e n ence . am a r fri g Ex pl e, qu rt z ,

o e a e e n ence . n n e 11 : M d r t r fri g Ex ample, hor bl e d , e n nc n : 1 1 5 High r fri ge e. Ex am pl e, augite , . 7 .

n 2 1 9 52 e n nc . am c 71 . V ery high r fri ge e Ex pl e, zir o ,

The negative relief seen in fluorite is caused by the total reflection of light striking the l ower su rface of the mineral . The indices of an unknown mineral compared with those of any known mineral by the Becke method will a lways give at least one limit , which in connection with am o f the visible unt o relief may be sufficient .

Ge n e a e a r l O p r tio n N o . 3 : D e te rm in a tio n o f the R e l a ve oub e R e f ti D l r ac tio n o r Bir e frin ge n ce . C — ISOTROPIC RYSTALLINE SUBSTANCES . ISOMETRIC M INERALS . Between crossed nicols , isometric minerals rema in dark in thin section during the entire revolution a of the st ge . Such m inerals allow the rays to vibrate with equal ease in all directions regardless of the dirce

o tion in which the secti n is cut . They have no inter

42 OPTICAL M INERALOGY AND PETROGRAPHY

a of taking the oblique direction . These r ys are , course , vibrating perpendicul ar to each other , and perpendicular ao to their direction of propagation . The extr rdinary ray vibrates in a plane containing the incident ray and

rinci al o ti c s ecti on . optic axis . This plane is called the p p p

o a On emerging from the thin secti n , the extraordin ry a ray is more or less a dvanced than the ordin ry ray .

Upon reaching the analyzer , each of these rays is again — resolved into two rays an extraordina ry and an ordi — nary the two extraordinary rays vibrating in one o a a plane , and the two rdina ry r ys in plane at right

a . a of a ngles Upon reaching the l yer Can da balsam , the a ordin ry rays are totally reflected and absorbed . The two extraordinary rays emerge from the an alyzer in a

o o a unif rm directi n , but not equally adv nced , conse quently in different phases . This interf erence produces o c lor . If the rays have a difference of phase of one half of - a wave length , or any uneven multiple thereof , dark ness will result .

COLOR . The kind of color produced depends upon : 1 . The mineral .

2 . The thickness of the section .

3 . e o The dir cti n in which the section is cut . The amount of color depends upon the angle between the principal optic section and the principa l section of either nicol . The color i s least when the angle is 0 de

, and a 4 5 grees greatest when the ngle is degrees , each f o these conditi ons occurring four times in one revolu tion of the section . Upon this phenomenon is based the determination of the angle of extinction ( General Op erati o n No . OF AXES ETHER VIBRATION . The direction in which ether vibrates in anisotropic minerals with the greatest a of I e se is called the greatest axis ether vibration . t M ETHODS OF M INERAL DETERM INATION 4 3

. C n Fig 7. ha ges of light i n pas si ng th rough e r r i mi r e a p t og a p h cal c os co p . 44 OPTICAL M INERALOGY AND PETROGRAPHY

of of is denoted by the letter X . The index refraction

n . light vibrating in this directi on is expressed by , The direction in which ether vibrates in anisotropic minerals with the least ease is called the least axis of i . n ether vibration . It is denoted by the letter Z The dex of refraction of light vibrating in this direction is expressed by

In uniaxial minerals , one of these axes always coin cides with the vertical crystallographic ax is 0 . The other axis is in all directi ons at right angles to this . Either the greater or the lesser axis of ether vibration may coincide with the vertical crystallographic axis . The value of the maximum double refraction or bire f rin ence is the difference between n and ng . Thus , g p — a n n n n . for c lcite , g is and , is g p which indicates a very strong birefringence . For quartz , n n — n ng is and p is g p which indi one cates weak birefringence , as the retardation of ray over the other in emerging is ve ry slight . It will be observed that in certain minerals the in dex of refraction of the extraordinary ray is greater t a a han th t of the ordinary ray , and in other miner ls the reverse is true . A further discussi on of this fact a will be t ken up under General Operation No. 5 . ’ NEWTON S COLOR SCALE . Thin sections of anisotropic minerals cut not perpendicular to an optic axis show a pol rization colors between crossed nicols . A careful study of these colors is most important for a successful

determination of unknown minerals . The color scale of Newton has been adopted as a a st ndard . It consists of a succession of interference

tints shading into each other . These sa me tints are o pr duced by anisotropic minerals in thin section . About forty distingu ishable tints in natural light have been METHODS OF M INERAL DETERM I NATION 4 5

named . The colors are best exh ibited by thin sections which have a thickness ranging between and ’ Newton s color scale as applied to the principal rock making minerals in sections mm th ick is given here with .

’ NEWT ON S C O LOR SCALE .

I nterfe re n ce of Colo rs Rock-fo rmi ng Be wee n X Ni i s . M ne r s t col al .

Bl ack n I ro gr ay . Leucit e .

Laven v n . der gray . Vesu i a ite av n a a L e d er gr y . A p tite. a Gr yis h blue. B eryl .

a s c e . Gr yi h bl ue. N eph elite, ri eb e kit a s e Lighter gr y . Stilbite , z oi it . e O o as m c c ne Light r gray . rth cl e, i ro li , n k aol i . 234 een s O ase a e a a Gr i h white . ligocl , lbit , l br

dorit e.

25 n m 0 White . Co ru du .

26 7 e s s m ens a e . Y ll owi h white . Gyp u , t tit

28 1 a a s a e . Str w y ell ow . Q u rtz , pphir 3 06 e o on e st au ro Light y ll ow . T paz , rhod it ,

lite .

2 n a e 33 B right y ell ow . Cli ochl o re, b rit . a s And l u ite.

n n e e s ene. Ora ge y ell ow . A orthit , hy p r th

an a s n e. Or ge y ellow . Woll to it

n r an e. O ra ge ed . Cy it Vi olet ( S ens itive ti nt — N . 1 e n Y e en e e. o ) Gr e , el H d b rgit m a n n e . I digo. T our li

e ne e. Blue. W r rit

u e. Sky blue. A git

o n en e . Sky blue . H r bl d

n a a e. G ree ish bl ue. Di ll g

c no e. u e. Green . A ti lit A git em e a ve son e Light green . Tr olit , rf d it .

n s e canc n e. Y ellowi sh gree . Diop id , ri it 46 OPTICAL M INERALOGY AND PETROGRAPHY

R - rm n I nte rfe re n ce of Colo rs ockfo i g

M i ne r s . Between X N ICO lS . al

v ne a u e . D ark orange viol et . Oli i , l z rit V iol et ( S ens itive tint

No.

o e . I ndigo . Ep id t

e uscov e . Greeni sh blu . M it

n hl ogo te anhydr te . Sea gree . P pi , i

mon te . Greenish y ell ow . Li i

a c. R ose red . T l Vi ol et gra y ( S ensi

tive ti nt No . a Greeni sh gr y .

C olor s very fa i nt .

n a e Not dis ti gui sh bl .

With sh a des of red

n u e. and gree . R til

The lowest colors of the above scale are the colors of o to of the first order , which includes all the col rs up the first vi olet , which marks the limit of the order . The colors of the second and third orders are successively a higher . In the fourth order , the colors begin to p a pro ch white light , due to an overlapping of the inter

ference . The highest color which the mineral i s capable of producing is usu ally taken for comparison with the

o o a . a a re c l rs of the t ble In uniaxi l minerals , such colors o given by secti ns cut parallel to the Optic axis .

D e te rm in a ti o n o f th e O rd e r o f Co l o r Pr o d uc e d by In te rfe ren — ce . The rank of an interference color may be METHODS OF M INERAL DETERM INATION 47

a determined by means of a qu rtz wedge . Thi s is a quartz plate of varying thickness , which gives the colors of the Newton scale from the grayish blue of the first order up . The quartz wedge is mounted on a plate of gypsum in such a position that the faster ray in the gypsum is the slower ray in the quartz . The gypsum plate i s ma de of such thickness that its effect is com pletely compensated by that of the wedge at the mi ddle of o re the latter . Between cr ssed nicols , darkness will sult at this p oint . Upon moving the wedge in either or direction , the colors rise successively from this zero compensating point to colors of the third order . When the wedge is superimposed over the thin section of a mineral , the col ors rise in the scale if moved in one di c re tion and fall if moved in the other di rection . Assume that a mineral is placed upon the stage of a microscope between crossed n icols and in its position of maximum illumination . Insert a quartz wedge in the proper slit in the microscope tube above the thin sec an tion , d note the change of colors as the wedge is moved over the field , the thin edge being inserted first . If the colors rise in the scale from yellow to red to violet to a blue to green an d again to yellow , it is an indication th t the greater axis of ether vibration of the thin section and that of the quartz wedge are parallel . Therefore , tu rn the stage 9 0 degrees to its former position and in of sert the wedge again . The order of the change colors n w o will o be reversed . The c lors will fall , indicating that the lesser axis of ether vibrati on of the thin section is p arallel to the greater axis of the wedge . Move the wedge over the mineral until the plate becomes dark or " " gray . This is the compensation point, where the accel cration of one of the rays of the plate corresponds ex e actly to the retardation of the same in the wedge . R o m ve the mineral from the stage . The interference color 48 OPTICAL M INERALOGY AND PETROGRAPHY

that the wedge displays is now the same as that orig

inally shown by the mineral . Slowly remove the wedge , o observing carefully the sequence of col rs . The number of times that any color recurs until the wedge is re moved gives the order of the original interference color of the mineral . From a birefringence chart it is possible to deter mine not only the order of birefringence of a mineral o a but the thickness of the section , provided s me miner l o a c ontained in the slide is kn wn . Let u s t ke , as an ex

i a . ample , granite n which quartz is e sily recognized It is fairly safe to assume that , if there are many frag ments of quartz in the fiel d of view, the frag ment with the highest interference col or is cut parallel to the optic a xis , and its birefringence has a maximum v alue , This value on the color chart is marked by a di agonal line , which should be followed toward the lower left

a o h nd corner to the intersecti n with the vertical , giv

ing the interference col or shown in the slide . The ordinate at the p oint of intersection represents the thick o ness of the secti n . Its v alue i s determined by follow ing the horizontal line through the intersection to the scale on the left . This reading gives the thickness of

the section in millimeters . Having thus determined the thickness of the sec o a ti n , find gain the highest interference color in a frag m of a ent the miner l which is to be determined . Take the intersecti on of the horizontal line of thickness in a o o the ch rt with this c l r . The diagonal line passing through this point of intersection indicates the bi ref rin gence of the mineral in question . D o ub e R e ac o n o f a a M n — l fr ti Bi xi l i e r al s . Minerals of the o o o , o o and m rth rh mbic m n clinic triclinic syste s . M i a of t a tw ner ls these three sys ems h ve o optic axes ,

OPTICAL M INERALOGY AND PETROGRAPHY

a have no fixed relation to the crystallographic xes . The discussi on of the determination of birefringence t of uniaxial minerals is applicable o biaxial minerals . The interference colors in sections of biaxi al minerals normal to the optic elements grade downward in the following order from highest to lowest :

1 . Optic normal .

2 . Obtuse bisectrix .

3 . Acute bisectrix .

4 . Optic axis . The following scale of birefringence ( after Win chell ) is useful for comparison in the estimation of the birefringence of an unknown mineral .

F SC A LE O BI RE F RI NG ENCE .

e ea e n ence r am e euc e V ry w k bir fri g o less . Ex pl , l it .

ea n n e am as W k birefri ge c Ex pl e, o rthocl e. o e ate e n nc am e en M d r bir fri ge e Ex pl , hypers th e . R ath er strong bi ref ri n n m e ce a a e . g Ex pl e, ugit — on e n ence 0 0275 0 0 355 . am s Str g bir fri g Ex pl e, diop id e . on e n ence am e m sc v e Str g bir fri g Ex pl , u o it . e s on e n ence 00 445—00 56 5 m V ry tr g bir fri g . Ex a pl e , a egirite . eme e n ence m n Extr bir fri g Ex a ple, tita ite . METHODS OF M INERAL DETERM INATION

4 CHAPTE R .

Gene ral M e tho d s o f M in e r a l D e te rm i na tio n (Con tin u ed ) .

n f h Ax a Gen e ra l O p e r a ti o n N o . 4 : D e te rm i a tio n o t e i l f I nte r e r en ce Figu r e s . Interference figures are obtained by the use of crossed

- nicols in convergent light . A high power obj ective must be u sed . When the eyepi ece is removed , a small image f o the interference figure can be seen . By sliding the

Bertrand lens into the tube of the microscope , a mag i n fied image of the figure is obta ined , in which case the a ocular is reta ined . Strong illumin tion is necessary , with the condensing lens close under the thin section .

Results are best with monochromatic light , but the ef f ects are the same with white light except that the rings will be variously colored instea d of l ight and dark . This operation aids the observer in distinguishing r i between i sot Op c, uniaxial an d b iaxial substances , and aids in the determ in ation of the rel ative double ref rac tion of minerals .

Isotropic minerals show no interference figures . UNIAXIAL INTERFERENCE FIGURES

or a . Secti ons cut perpendicular to the optic axis vertical crystallographic axis show a dark cross with or without colored rings . The arms of the cross are parallel to the vibration planes of the nicols , and the figure does a i not move with the rot tion of the sect on .

b . Sections cut oblique to this position show figures f which move about the center o the field . The center of the figu re may even be outside of the field , but upon 52 OPTICAL M INERALOGY AND PETROGRAP HY rotation its dark bars may be seen to move across the

field . Thes e dark bars remain straight and parallel to themselves .

i ni xi fi re . F g . 8. U a al gu

o o a a If the obliquity of the secti n is t o gre t , the b rs will show a curvature upo n entering the field and up on leaving, but they are straight upon crossing the center of the field . The cu rvature shifts upon crossing the cen one ter from side to the other, thereby differing from the biaxial figures , in which the b ars remain curved in the same direction .

c . If the section becomes so oblique to the optic axis

a to a o as to pproach parallelism it, the bl ck cr ss appears to break up into hyperbolas which a re symmetrically o placed with respect to the optic axes , and then unite t form a dark cross again upon completing the rotation of the section . Sections which are thick and have a strong double re fraction show the cross and rings clearly and sh a rply

outlined , many rings being crowded closely together . Thin sections with weak double refracti on Show broa d no crosses and rings . The observer m ay thus deduce inferences both in regard to the thickness of the section o and the strength of the d uble refraction .

BIAXIAL INTERFERENCE FIGURES

' a . Sections cut n ormal to an optic axis Show a series f o concentric colored cu rves crossed by a single dark bar. The bar changes into a hyperbola and back into a METHODS OF M INERAL DETERM I NATION 53

a r bar. Sometimes the cu rves are not observ ble . The ba when stra ight shows the direction of the optic plane with which it is parallel .

b . S ections cut normal to the acute bisectrix in which the angle between the optic axes is not too great will show both optic axes in the interference figure , the bisee f trix being in the center o the field between them . In 54 OPTICAL M INERALOGY AND PETROGRAP HY

one case a dark bar appe ars in the center of the field , its a a o arms varying in size . Th t line which p sses thr ugh the optic axes is narrower than the one passing between

Fi . 1 0 i x g . Op t c a is n r e n i te f re ce figu re.

out of . them . Its extremities widen on the edge the field

The intersection of the two bars marks the bisectrix . The trace of the Optic plane is the line p assing through n the loci of the optic axes a d the b isectrix . On rotating a a the section , the dark b rs sep rate into two hyperbolas , the summits receding from each other toward the edge n of the field , a d beyond it if the optic axes are not in view . They bend through the colored curves surround ing the optic axes , and unite again as a stra ight bar when

i . 1 1 . Bi se ri x i n e r eren e i i i F g ct t f c fi g . 1 2. B s ect r x nte rferenc e fi re gu . fi gu re at the plane of the Optic axes coincides with the vibration o plane of the nic l . The most distant positions of the e hyp rbolic summits are , therefore, after a revolution of 4 5 degrees . MET HODS OF M INERAL DETERM INATION 5 5

An excellent illustration of the biaxial interference figure may be obtained ve ry s imply by placing between crossed nicols in convergent light a thin sheet of musco

V ite mica . Since the optic angle is small , the loci of both optic axes will be seen in the field . Since the center of the small ellipses an d the black hyperbolas mark the loci 56 OPTICAL M INERALOGY AND PETROGRAPHY

a a o a o of the optic axes , they indic te ppr xim tely the ptic angle . The uniaxi al or biaxi al cha racter of a minera l sec ti on which shows only an indistinct bar may be deter mined as follows (La Croix) A bar of a uniaxial interference figure moves in the a same direction as the rotating st ge , and always remains straight , while the biaxial bar moves in the Opposite direction to that of the stage , and becomes cu rved .

D e e rm n a on o f the D i s Gene ra l O p e r a tio n N o . 5 : t i ti p e rs io n o f the Op tic A xe s . The colors of the interference figures in convergent light are caused by the difference of phase of different rays brought together by the analyzer so as to inter o fere . The phenomenon of relative position f the red and violet rays is caused by the dispers ion of the optic axes . When the red ray has the greater Optic angle it is expressed by R V ; when the violet ray has the greater optic angle , it is expressed by R V .

‘ lo When white light is used , the co rs on the convex side of the hyperbola (which is the side toward the acute bisectrix) are edged with red if the dispersion of red is a a f gre ter than th t o violet, and edged with violet if the reverse is true .

Labradorite , muscovite , orthoclase , and anorthite o have a dispersion f rmula R V . Albite and oligoclase a a r h ve dispersion fo mula V R .

Gene a e a on r l O p r ti N o . 6 : D e te rm in a tio n o f the Op tic al Cha r ac te r o r the Cha r ac te r o f the D o ubl e R e frac tion (after Winchell ) C OPTICAL HARACTER OF UNIAXIAL MINERALS . For light traveling perpendicular to the optic or vertical crys tallo ra hic g p axis , the vibrations of the ordinary ray are transverse to that axis and those of the extraordinary

58 OPTICAL M INERALOGY AND PETROGRAPHY mineral are the s am e the double refraction is in creased i n proporti on to the resultant thickness of the two o of plates . The color of the section rises thr ugh two the vertical divisions of the color cha rt . The color falls cor resp onding ly if the axes are not the same .

- a . B . With the qu rtz sensitive tint in polarized light The quartz -sensitive tint is a plate of qu artz cut parallel to its vertical or lesser axis of ether vibration , and is of such a thickness as to give the first violet color ’ o . of Newton s scale . It i s m unted between glass plates a The direction of the vertical crystallographic xis , as well

In p a rall el pola ri z ed l ight.

Colo r falls : Negative . - 1 . r z ens ve n . Fig . 7 U s e of the q ua t s i ti ti t as o of the optic axis , corresponds with the directi n Z , and is usu ally indicated by an arrow on the glass . The position of the axes of ether vibration in the un o kn wn mineral is first determined . This is done by deter on o a mining r t tion between crossed nicols , the positions t a at which ex inction takes pl ce . The section i s placed 45 degrees to this position . The brightest interference

- color is thus produced . Place the quartz sensitive tint over the section in such a p osition that the direction of Z 45 wi is degrees th the principal sections of the n icols . If by this superposition a color is produced which is higher in the scale than the sensitive tint of the quartz METHODS OF M INERAL DETERM INATION 59

a t plate , the axis Z of the quartz plate is par llel o the axis

Z of the thin section . If the resultant color i s lower , con e the axis of ether vibration of the mineral is X , in s n f que ce o a lessening of the reta rdation . Uniaxial minerals are positive when Z coincides with the optic axis , and negative when X does . Since the optic axis coincides with the vertical crystallographic axis , it is necessary when using this method to be able by the crystal outline to determine the direction of the vertical axis . This method is , therefore , practicable only when the section is approximately parallel with the ver tical axis and the crystal outline is distinct .

C. With the quartz wedge in parallel polarized light .

This method is the same as method B .

18 D s rb n e f h n Fig . . i tu a c o t e i terference fi gu re of a u nia xial r s a b t he rter- n i n m i c y t l y q ua u dulat o ca p late.

a - D . With the qu rter undulation mica plate in conver gent light . By inserting the plate with its Z axis 4 5 degrees with the cross hairs , the dark cross of the interference figure is destroyed and two dark spots are brought prominently into view . If rings are seen , they will a ppear disj ointed at the lines dividing the quadrants , and they will appear

expanded in those quadrants occupied by the dark spots . The mineral is optically positive if a line j oining the two dark spots is perpendicular to the axis of the mica plate . The mineral i s negative if the line uniting the 60 OPTICAL M INERALOGY AND PETROGRAP HY dark sp ots is pa rallel with the directi on of the a rrow on the mica plate . The positive and negative character of the mineral becomes a simple Operation if it is borne in mind that the line j oining the dark spots makes a positive sign and a negative sign respectively with the axis of the mica

a . plate , thereby indic ting directly the sign of the mineral

- . E . With the qu a rtz sensitive tint in convergent light

Upon inserting the sensitive tint plate , two opposite quadrants will appea r yellow and the other set will ap of pear blue . In determining the sign the mineral , the yell ow qu adrants may be considered equ ivalent to the dark spots .

When a s ection is cut p arallel with the optic axis , the interference figure i s not a black cross but may re semble a biaxial interference figure . The observer wishes of o to determine the direction the ptic axis . He may determine this by observing in which qu a drants the hyperbolas always leave the field . These will be the quad a a r nts cont ining the optic axis . Moreover , the inter ference colors in these quadrants are lower than f or o a corresp nding points in the other qu drants . After o f nce determining the direction o the optic a xis , Z can be determined by any one of the first three methods for parallel polarized light .

F OPTICAL CHARACTER O BIAXIAL MINERALS . If the greatest axis of ether vibration bisects the acute bisee trix , the mineral is negative . If Z bisects the acute bisec o trix , the mineral is positive . Theref re , a dete rmination of the optic sign of a bi axial mineral demands a di stinc tion of the acute from the obtuse bisectrix and a distinc of tion X and Z .

D i s tincti on between the Acute and btus s e ices O e Bi ctr .

The thin section is cut perpendicular to the acute bi sec METHODS OF M INERAL DETERM INATION 6 1 trix if the optic angle is so small that the loci of the optic axes or of one optic axis and the bis ectrix remain r in the field during a rotation of the stage . Othe wise it is necess ary to find sections cut perpendicular to both

X and Z and compa re them . o 1 . The section perpendicular t the acute bisectrix shows a lower interference color than the section cut perpendicular to the obtuse bis ectrix . o o o f 2 . The angle of r tati on between the p siti n o the black c ross and the position when the summits of the hyperbolas are tangent to the edge of the field can be er endicu measured . This angle is larger in a section p p lar to an acute bisectrix than in one perpendicular to the obtuse bisectrix . If the angle is more than 30 or 35 r n degre es , it is safe to assume that the section is p e pe i l r 1 5 d cu a to an acute bis ectrix . If the angle is less than or 20 degrees , it is perpendicular to an obtuse bisectrix . a Z D is tincti on between X nd . This involves a com parison of the velocities of the light ray in the direction of the axis to be determ ined with that in the direction of , a known velocity in another mineral . Rel ative retarda tion is indicated by the relative positions of the colors on ’ Newton s scale . The following methods are ava ilable :

- A . With the quartz sensitive tint in parallel polar

’ i z ed light . The section examined must be parallel to the Optic a pl ne , that is , it must conta in the axes Z and X . Z of the t a quar z pl te lies in the direction of the arrow , and X at right angles to this direction . Superpose the quartz plate over the mineral . If the resultant color is higher than the sensitive tint of the quartz plate , the Z axes of the of quartz plate and the mineral are coincident . If the resultant color is lower , the Z axis of the quartz plate i s a coincident with the X axis of the miner l . This enables 62 OPTICAL M INERALOGY AND PETROGRAPHY

the observer to determine the position on the two axes . It is now necessary to view the interference figu re in convergent light in order to determine in which qu a d rants the optic plane lies . If Z is foun d to lie in the acute

a a optic angle , it bisects the cute bisectrix , and the miner l is positive . r a r If only two hyperbo la s are obse ved , they e in the quadrants containing the acute bisectrix . If the optic a angle is large , hyperbolas may be visible in all four qu d o rants , but the hyperb las leave the field more slowly in the quadrants conta ining the acute bisectrix . The quarter-undulation mica plate may be us ed in the same manner as with uniaxi al m inerals to determine the directions of X and Z since the section i s cut parallel a to the plane cont ining X and Z .

B . With the qu artz wedge in convergent light . Obtain an interference figure from a section as nearly normal to the acute b isectrix as possible , and rotate the stage until the optic plane makes a 4 5-degree angle with a o the vibr ti n planes of the nicols . a Insert the quartz wedge , with the thin edge adv nced , in such a position that the Z axis coincides in directi on with a line passing through the optic axes of the figure . The optical character of the mineral is positive when the ellipses surrounding the loci of the optic axes appear o t to widen u , and move from the l oci of the optic axes toward the center of the interference figure and finally open into the outer col ored m argins surrounding the whole figure . The optical character of the mineral is negative if the movement of the colors is reversed from of the center the fig u re toward the axial spots . With certain interference figures the following simple : rule will apply If the dark spots approach each other , a the miner l is negative . If they appear to retreat from a e ch other , the mineral is positive . METHODS OF M INERAL DETERM INATION 6 3

- C. With the qua rter undulation mica plate in con vergent light . This plate is perpendicular to the negative bisectrix

X , and contains Z and Y . The direction of Z coincides

with the trace of the plane of the optic axes , since the axial plane always conta ins X and Z .

For sections perpendicular to an acute bisectrix .

When the mica plate is superposed in the usual way , there is an appa rent lengthening of the figure in the direction of the Z axis of the mica plate and an apparent short i en ng in this direction f or positive minerals . Winchell suggests that this observation be m ade with the optic n w plane parallel with one nicol . The dark spots will o appear in the qu adrants through which the arrow passes , the line connecting them forming an angle less than 4 5 degrees with the arrow , or an approximate minus sign a indicating a negative minera l . The reverse takes pl ce with a positive mineral . There is a shortening in the direction of the arrow , and the line connecting the dark spots forms an angle greater than 4 5 degrees with the o a rr w , making an approximate plus sign . Iddings suggests the following method f or sections perpendicular to an optic axis :Place the section with its a optic plane 4 5 degrees with the n icols . The hyperbol is

convex towa rd the acute b isectrix . Insert the mica plate with the Z axis parallel with the optic plane of the min

eral . The hyperbol a moves towa rd the obtuse bisect rix a o bisec when the mineral is neg tive , and t ward the acute o trix when the mineral is p sitive . For minerals of weak

a . birefringence , as the feldsp rs , this method is excellent For minerals of strong birefringence the following rule may be applied : The mineral is positive when the bla ck dot appears on the convex side of the hyperbola upon insertion of the mica plate with its Z axis parallel

with the optic plane of the mineral . 6 4 OPTICAL M INERALOGY AND PETROGRAPHY

SU M MARY OF THE OPTICAL SIGN FOR UNIAXIAL INER LS When the E ray is less refracted tha n the O M A . ray and advances with greater velocity , the mineral is a i . , negative , as in calcite In this c se X coincides w th the The ac optic or vertical axis . index of refr tion for the E

one n . ray vibrating in this direction is the lesser , p When the O ray is less refracted than the E ray and advances with the greater velocity , the mineral is posi

a . a tive , as in qu rtz In this c se , Z coincides with the verti of a of ra cal or optic axis . The index refr ction the E y

ne n . which is vibrating in this direction i s the greater o , g

bis ec FOR BIAXIAL MINERALS . When X is the acute

. trix , the mineral is negative , as in muscovite

When Z is the acute bisectrix , the mineral i s posi tive , as in augite .

7 e e m n a o n o f the Ex Ge n e r a l O p e r a tio n N o . :D t r i ti

he R e a o n o f the C r s a o r a c tinc tio n An gl e , o r t l ti y t ll g phi

Ax e s to the Axe s o f Ethe r V ib r a tion . This operati on is perfo rmed between crossed nicols with parallel polarized light . It will be remembered that the intensity of color de pends upo n the angle between the principal optic sec tion of the mineral and the principal section of either a nicol , the color being greatest at 4 5 degrees and le st

at 0 degrees . Thus when the section is in such a posi tion that its directi ons of elasticity are parallel to the a a of vibr tion pl nes the nicols , no light can p a ss through and the analyzer, the section is dark . This phenomenon a is c lled ext inction . Extinction is the m ost common phenomenon for dis tin uishin g g isotropic minerals from anisotropic . In the isometric system all minerals are completely da rk a of during a rot tion the stage . It is likewise of great

6 6 OPTICAL M INERALOGY AND PETROGRAPHY

i i n wave or shadow is called undulatory extinct on . It di cates that the minera l has been subj ected to mechanical of a forces , producing a change in the position the xes of elasticity in different parts of the mineral . It is difficult f or the eye to distingu ish small varia of tions in the intensity of light . By the use the quartz a sensitive tint, extinction is determined qu ite accur tely by a distinction of difference of color to which the eye a a is more susceptible . A thin qu rtz pl te is cut parallel to the axis of elasticity , having such a thickness that ’ it shows the violet col or of Newton s scale . Insert this plate in such a position th at its axis is 4 5 degrees to the cross hairs . The field of the microscope is violet . By placing the unknown mineral on the stage so as not to occupy the enti re field , it will be seen that the color of the minera l is not the same as the violet c olor of a the unoccupied field . Rot te the stage until the color of a the miner l is the violet color of the quartz plate . a at o The miner l is now extincti n . This phenomenon is due to the fact that the axes of ela sticity of the nicols and of the mineral are in the same p osition and producing no interference .

Gen e a e a o n r l O p r ti N o . 8 :D e te rm in a ti o n o f th e Pr e s e nce o r b s en A ce o f Pl eo chr o i s m .

Pleochroism is a property poss essed by all aniso tropic minerals of absorbing certain colored rays in cer a tain cryst llographic directions , thereby showing differ

ent colors in different directions by transmitte d light . It o or a is observed by p larized p rallel transmitted light . The axes of absorption coincide generally with the a of a xes el sticity, therefore with the crysta llographic

axes in the tetragonal , hexagonal , orthorhombic , and the b axis of the monoclinic systems . o Secti ns perpendicul ar to the optic axis can not Show METHODS OF M INERAL DETERM I NATION 67 differences in color since in this direction the ab sorp tion must be equal in all directions .

Uniaxial minerals are said to be dichroic , showing two different colors , produced by the rays which vibrate parallel to the direction of the vertical axis and parallel s to the plane of the basal axe . r Biaxial minerals a e said to be trichroic , as there are theoretically three differences in color , corresponding to the di rections of the three axes of elasticity . Ple

' l i n ochroi sm exi sts practica lly on y colored minerals . Pleochroism may be tested as follows : If a mineral is pleochroic , a change i n color will be observed upon rotating the stage . This may appear as an actual change a in color or as a change in shade of the s me color . In to case it is almost indistinguishable , it is best make the test with the condensing lens in position immediately under the section . An absorption formula is an expression of these dif f er n e t amounts of absorption in any mineral . Thus a b indicates that absorption is greater when the ether vibration of the polarized ray is parallel to the crystal l r i og aph c axis a than when parallel to b . A pleochroic formula expresses the colors that a min eral presents in polarized light vibrating parallel to each of its axes of ether vibration . For magnesium tourmaline the pleochroic formula is : Z D ark yellowish brown .

X Pale yellow .

In general , amphiboles show pleochroism , and pyrox enes do not . 68 OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTER 5 .

- D e sc rip ti on o f Im p o r tan t R oc k m akin g M ine ra l s .

N INTROD UCTIO .

The present-day classification of minerals is pri marily a chemical one, as minerals are arranged accord a a ing to the acid r dic l . In any chemical division , how m a ever, minerals of simila r che ic l composition , i f related as crystallographically , are placed in the same group , f or instance the six members of the calcite group .

About a thousand kinds of minerals are known , of which most are rare or found only in a few localities . has m o o Rogers co piled the f llowing informati n , which is of interest in a discussion of the derivation of min eral names : The following m inerals were named in honor of prom inent : scientists biotite ( B iot , French physicist) , brucite

(Bruce , an early American mineralogist) dolomite ( D ol omi eu , French geologist) , goethite (Goethe , the German ) M poet , millerite ( iller , English crystallographer) , scheelite ( Scheele , Swedish chemist) , smithsonite

( , a Smithson founder of the Smithsoni n Institution ) , wol lastonite o (Wollast n , English chemist) . The following minerals were n amed from prominent a a : a geogr phical loc lities ndalusite (Andalusia , a prov

in ) , o France ilmenite (Ilmen m untains , in the Urals ) , DESCRIPTION OF ROCK -MAKING M INERALS 6 9

labradorite ( Labrador) , muscovite ( Moscow , in Russia) , strontianite ( Strontian , in Scotland ) . The following minerals were derived from the Latin and Greek names for colors : albite (white) , azurite sk - ( blue) , cyanite (blue) , celestite ( y blue) , chlorite

( green ) , erythrite ( red ) , hematite (blood ) , leucite - (white) , rhodonite (rose red ) , rutile ( reddish) . The following minerals were n amed directly from their chemical composition argentite , arsenopyrite , barite , calcite , chromite , cobaltite , cuprite , magnesite , molybdenite , sodalite , stannite , zincite . At one time there existed a binomial nomenclature for minerals , as exist at the present time for animals r l and plants . Thus , barite was known as Ba a us ponder osus Bar l ri , an d celestite was known as a us p smaticus . The following minerals are discu ssed according to the

crystallographic system in which they occu r, isotropic minerals being considered first .

ISOTROPIC MINE RALS .

A MO RPHOU S .

OPAL .

m i t o nH Co p os i n : Si0 2 . 2 0 . Cri teri a for d etermina ti on i n thin s ecti on : No r Form crystal fo m , but sometimes concretion r ary , banded o with spherulitic structure . Optical P rOperti es :n z Relief so l ow that the mineral may be mistaken f or a hole in the section filled

with balsam . Feeble negative double refraction at times . r Colorless patches o veins . Fragments are dark and

irregular between crossed nicols . Occurrence :As a secondary mineral in cavities and seams in igneous rocks ; as sinter around hot springs and geysers (Yellowstone Park) ; as a constituent of 70 OPTICAL M I NERALOGY AND PETROGRAP HY

o a ia diatomaceous ea rth . D i at ms and r diolar secrete casts of opal silica . New Uses : As gems . Precious Opals are found in o South Wales and in Hungary . Fire Opal is f und in

o Mexic .

Comp osi ti on : Fesz . Cri teri a f or determina ti on i n thin s ection

’ r Form : Cubes , pentagonal dodecahedrons o combi nations of these . Sometimes in irregular grains .

Optical Properties : Opaque . In reflected light , pale

- brass yellow color with strong metallic luster . Alteration : Alters readily to limonite by Oxi dation and hydration .

Occurrence . As a vein mineral associated with other sulphides . As an original and secondary mineral in a igneous and sediment ry rocks . : Uses Used in the manufacture of sulphuric acid . In association with chalcopyrite as a low grade COppel‘ - ore . It is often gold bea ring .

PYRRH OTI TE .

Com osi tion :Fe S p G , to Fe n s" . Cri teria for d etermi na ti on in thin s ecti on : Form : Practically always in irregular masses and not in crystals . Cleavage usually not visible mi croscop i ll ca y.

Optical Properties . Opaque . C olor between bronze

Distincti ons : D istinguished from pyrite by its usual associati on in irregular masses and by its bronz e yellow

color in incident light . Occurrence : In basic igneous rocks ; as a vein min eral ; i n crystalline limestones . DESCRIPTION OF ROCK -MAKI NG M INERALS 7 1

r Uses : The nickeliferous pyrrhotite of Sudbu y , o r Ontario , i s an imp rtant o e of nickel .

M AGNE TI TE .

s Fe Comp o i ti on : , O, . Cri teria for d eterminati on i n thin s ecti on

Form : Octahedrons and dodecahedrons . Also gran ular . Cleavage indistinct . Twinning common after 0 . i Optical P rOpert es : Opaque . B luish black by reflected light , with a strong metallic luster . Index Of refraction high .

: e to . Alteration Alt rs hematite , limonite and si derite Occurrence : A common and wi dely distributed accessory mineral of igneous rocks ; magmatic segrega r tion in o e deposits , as in Scandinavia ; as a contact min eral between limestones an d igneous rocks ; in lenses , in g neisses and schists .

Uses : Important ore of iron , especially in New

York , New Jersey and Pennsylvania .

SP INE L .

m i ti n :M A1 Co p os o g 2 0 4 . Cri teri a for determinati on i n thi n s ecti on

Form : In grains or octahedral crystals , never decomposed in rocks .

Optical Properties : Strictly i sotropic . Index of refraction high . Color usually the lighter sha des of red ,

- blue green , yellow , brown . The most common colors are

- - n green ( in pl eonaste , iron bearing) and coffee brown ( i

- picotite , chrome bearing) D ifferentiation : D i stinguished from garnets by its octahedral form , by the more common green color, by its undecomposed condition and by its slightly lower rel ief .

Occurrence : As a contact mineral , in crystalline 72 OPTICAL M INERALOGY AND PETROGRAPHY

an a a limestones and schists . As ccessory miner l , in igne

- a of C . ous rocks . In the gem bearing gr vels eylon

- Uses : Ruby spine"is used as a gem .

GA RNET .

R 0 R Ca M Comp osi ti on : R 3 ( Si 4 ) 3 is , g , Fe or ’" or . Mn. R is Al , Fe Cri teri a for d eterminati on i n thin secti ons Form : Irreg ular grains or in simple crystals as

NO . dodecahedrons . Zonal structure frequent . cleavage

Irregular fracture .

Optical Properties : Normally isotropic , sometimes showing anomalous double refraction , due possibly to internal strain .

r . Colorless or nearly so, to yellowish o reddish Index of refraction : n to

Relief high and surface rough . r a to Alteration :Usually f esh . May be found ltered chlorite .

" a : Differenti tion From spinel , see under the latter, : e Occurrence In schists and gneisses , granites , p g matites - , peridotites , nepheline and leucite bearing a m in cryst lline li estones developed at the contact , sands . : Uses As an abrasive , p articularly for finishing oo a w dwork and le ther . Also as a semiprecious gem .

LEUCI TE .

Com osi ti on : Al i p K S Q Om Cri teria for d eterminati on i n thin s ecti on r : G a r Fo m r ins , o well defined , embedded crystals very near the trapezohedron or tetragonal trisoctah e

- o . C e o dr n ross s ctions r und or eight sided . Vary greatly

. a in size Fine stri tions due to twinning common . No

cleavage , though fracture may be noticed .

74 OPTICAL M INERALOGY AND PETROGRAPHY

o o a character . From other is tr pic miner ls , by a very low refractive index .

FLU O RITE . F Comp osi tion : Ca 2 . Cri teria for determinati on in thin s ecti on dodecahe Form : Crystals cubical , octahedral and

dral . Cleavage , perfect octahedral , appearing Often in section as triangular cracks . i o Optical P rOp ert es : Is tropic . n On account of the low index of ref rac

tion the negative relief is marked .

Abnormal birefringence may Show , due to internal

tension .

Color is due to inclusions of hydrocarbons . Some crystals appear green by transmitted light and blue

by reflected light . Color not uniformly distributed . Occurrence : As a very common vein mineral o t gether with calcite , barite , sphalerite , and galena . In limestones . Kentucky and Illinois are chief sources . : - Uses As a flux in iron smelting and foundry work . Also f or the manufacture of hydrofluoric acid and enamels . DESCRIPTION OF ROCK -M AK ING M INERALS 75

CHAPTE R 6 .

D e sc rip tio n o f M inera l s (Co n tinue d ) .

ANISOTROPIC MINERALS . UNIAXIAL .

TETRAGONAL .

RUTI LE . i Composi ti on : T OZ. Cri teria for determinati on i n thi n s ecti on

Form : Embedded grains , acicular inclusions , mas sive or i n crystals , which are sharp , elongated and pris - matic , or in net shaped groups . Twinning lamellae com mon in basal sections . Prismatic cleavage not Observed .

E longation parallel to 0 .

Optical Properties : Uniaxial and positive .

Refringence very high . Relief high and surface

rough . n and

B irefringence extreme . Interference colors very

high , hence may not be noticed when mineral i s strongly colored

Extinction parallel to prisms . : ed Color R , brownish red to black .

Pleochroism usually not noticeable . X is yellow Z ish , is brownish yellow to yellowish green . : Ma Alteration Qu ite stable . y alter to ilmenite . Occurrence : More widely distributed as a micro i f scOp c mineral than as one o megascopic size . Occurs in igneous an d metamorphic rocks and in veins . As a sec

ond r . a y mineral in clays Virginia is source . OPTICAL M INERALOGY AND PETROGRAPHY

Uses : As a source of ferro-titanium and as a col oring matter for porcelain .

ZI RC O N . i Zr . Comp osi ti on : S O4 Criteri a for determination i n thin secti on °

Form : Small , short prismatic crystals usually elon gated parallel to 0 . Always crystallized . r i s Optical P Op ert e : Uniaxial and positive .

Refringence very high and su rface rough . n

and B irefringence very strong Interference

colors of fourth order, minute crystals showing bril

liant colors .

C : a olor Colorless to p le gray or brown . o a Ple chroism usu lly not noticeable , but when a a observed little bsorption t kes place parallel to c.

Extinction : Parallel to c. Interference figure in basal section shows several

rings in addition to dark cross . : Alterations Rare . D f : if erentiations From apatite , by much higher and s a relief tronger double refr ction . From cassiterite , by much weaker double refraction , and by mode of occur rence .

Occurrence : As an accessory mineral of ig neous

rocks , especially the more acid varieties . In sands and gravels .

WERNERI TE ( S C A P O LI TE GROUP ) .

Com osi ti on : mCa A1 p 4 6 0 2 5 . Cri teria f or determinati on in thin s ecti on : : C l Form rystals rough , coarse and large , in c eav a , and ble columnar massive forms . Cleavage distinct , a . o par llel to square prism E l ngati on parallel to a . P : i Optical roperties Uniaxial and negat ve . DESCRIPTION OF ROC K -MAKING M INERALS 77

Refringence considerable . n and

Relief not marked and about the same as quartz . Birefringence rather strong to Inter ference colors of the second order more brilliant than o those f most of the colored minerals .

Interference figu res distinctly uniaxial .

Extinction parallel in longitudinal sections .

Colorless . : Alteration Alters to ka olinite , muscovite , etc . D f of if erentiation : From feldspars , by absence twinning .

From quartz , by cleavage , higher order of inter

ference colors and Optical character . Quartz is

positive . l From apatite , by lower index of refraction , c eav

age and higher order interference colors .

o Occurrence : F und in gneisses , crystalline sch ists , and limestones . E G H X A ONA L .

H E MATI TE .

Comp osi ti on : Fe. O Criteri a f or d etermi nati on i n thi n s ecti on

a . Form : Irregular scales , minute gr ins and earthy

No cleavage .

Optical Properties : Uniaxial an d negative .

Refringence very high . n and Birefringence very strong

Opaque . By reflected light , black with tinge of red

and a metallic luster , or red without luster .

Pleochroism absent , or S light . X is yellowish red an Z d is brownish red . : Alteration Common by hydration to limonite .

Occurrence : Very widely disseminated . As micro SCOpic inclusions and as a common alteration product in 78 OPTICAL M INERALOGY AND PETROGRAPHY

ore all rocks . As a commercial iron from the Lake

Superior district .

I LMENI TE .

FeTi O . Comp osi ti on : g Cri teri a for determinati on in thin s ecti on a Form : Irregular masses , without cryst llographic outline , or rhombohedral crystals . Optica l Properties :

Opaque . Rarely translucent , and dark brown in

o very thin sections . S metimes brownish in reflected

light , with metallic luster.

Alteration : To leucoxene , which is believed to be a variety of titanite . This alteration often develops along definite rhombohedral directions .

D ifferentiation : From magnetite , by occurring in irregular masses and by a whitish strongly refracting decompositi on product . Occurrence : A common though sparsely distributed accessory mineral in igneous rocks and as a mag matic segregation in igneou s rocks .

C ORUNDUM .

Com osi ti on 1 : A . p 2 0 3 Cri teri a for determi nati on i n thin s ecti on : Form Prisms , grains or plates . Rhombohedral a cle vage may Show .

a Optical Properties : Uni xial and negative . Co lorless or with patches or zones of blue . R efringence very high and surface rough . n and Birefringence weak like quartz Inter o of ference col rs middle the first order , yellow to blue . Interference figure of basal secti on shows indis

tinct cross . DESCRIPTION OF ROCK -MAKING M INERALS 79

Pleochroism marked when color is deep . Z is blue ;

X is green .

X axis coincides with crystallographic a .

Occurrence : In crystalline metamorphic rocks , such erido as marble , gneisses , mica and chlorite schists , in p s tite , in sands and gravels .

Uses :Ruby , the red transparent variety , is valuable a as a gem . S pphire , which is likewise valued as a gem , is the blue transparent variety . Bu rma furn ished the best rubies and Ceylon the best sapphires . It is also used as an abrasive .

QUARTZ . Comp osi ti on : Sio Cri teria f or determinati on i n thin s ecti on

Form : Crystals usually prismatic , terminated by rhombohedrons . Allotriomorphic in granitoid rocks , rounded grains in clastic rocks . Rarely in distinct crys tals in any rocks . May be mutually interpenetrated by a or an acid feldspar . Cleavage nearly always bsent difficult .

Optical Properties : Uniaxial and positive .

Colorless . By reflected light it may appear cloudy

if it contains many inclusions . lo S Refringence w. NO relief and mooth surface . n and Birefringence weak with interference colors of white or yellow in the middle of the first order

Pleochroism absent .

Extinction takes place , but is not distinctive , due to a r the bsence of cleavage o crystallographic outline . Interference figure of a basal section shows a d ark

cross without any rings .

: D so a Alteration oes not alter , th t the fresh appear a a nce of the miner l is an important aid i n i dentification . 80 OPTICAL M INERALOGY AND PETROGRAPHY

Inclusions : Minute fluid or gas inclusions common

o o a in granitoid rocks . N t s abund nt in porphyritic rocks . Occurrence : One of the most abundant minerals r found in nature . It occurs in sedimenta y , acid igneous , metamorphic rocks and veins .

Differentiation : From sanadine, by uniaxial and positive character . f a From nephelite, by absence o hexagon l outline ,

stronger double refraction , and fresh undecomposed

appearance . m f or instru Uses : For orna ental purposes , optical - or ments , for glass making , f pottery and porcelain , and as an abrasive .

CALCI TE .

i i . Comp os t on : CaCOS Cri teri a for determinati on i n thin s ecti on : r Form : Grains and aggregates . May be fibrous o oolitic . Never in crystals in rocks . Polysynthetic twin

o of ning common , pr bably due in part to the grinding the section . Shows in crossed nicols as a series of light and a dark b nds . Cleavage parallel to R appearing as a m ny cracks .

o Optical Pr perties : Uniaxial and negative . C o ol rless when pure , but may appear colored by o transmitted light , due to rganic pigments . R efringence low . Relief not m arked and surface oo sm th . n and

Birefringence very strong , with pale , iridescent interference colors of the fourth order Extinction parallel to cleavage cracks when they

appear . o m . NO of o Ple chrois change c lor Observed , but a o o o bs rpti n can be noted if secti n is not too thin .

OPTICAL M INERALOGY AND PETROGRAPHY

From dol omite and magnesite , by common poly

synthetic twinning . It is the only mineral of the Calcite group with b oth indices of refraction higher than that of balsam except the rarer smithsonite and i rhodocros te.

- Occurrence : In limestone , clay iron stone , clay slate , gneiss . Also in veins with metallic ores .

APATI TE . I m i n Ca . F . Co p osi t o : a ( C ) ( PO) 3 i Criteri a for determi na ti on i n thi n sect on. : M Form inute , slender, hexagonal prisms , with reg a ular hexagon l boundaries . Grains . Clusters Of crys tals . Elongation parallel to a . Cleavage seldom observed .

Optical Properties : Un iaxial and negative .

Colorless usually in thin section . Sometimes gray , or blue brown , the color being irregularly distributed , a to microscO i c due perh ps p inclusions .

Refringence moderate . Relief more ma rked than of : the associated colorless minerals . n and

en : B irefring ce Weak , with interference colo rs or grayish blue white , of the lower first order a Extinction p rallel to c axis .

Interference figure shows a cross without rings . o a f or Pleochr ism bsent white crysta ls . Colored a v rieties weakly pleochroic .

: M a Alterations ineral alw ys appears fresh . D : ifferentiation From nephelite , by occurring in smaller and longer crystals , and invariably fresher in appearance .

From zircon , see under the latter mineral .

From feldspars , when granular , by higher relief and the uniaxial interference figu re . DESCRIPTION OF ROCK -MAKING M INERALS 83

From quartz , in having a higher reli ef, weaker

birefringence , and a negative S ig n .

Occurrence : Widely distributed as an accessory con it n st ue t of igneous rocks and in crystalline schists . With metamorph ic limestones . As a vein mineral in gabbro and in pegmatites .

of Uses : As a source phosphates for fertilizers .

E NE PH LI TE .

m s ti l i o o i on : NaA . C p S 0 4 Cri teria for d etermina ti on i n thin s ecti on :

S - Form : Crystals thick , ix sided prisms with base prominent . Massive and in embedded g rains . Cleavage a imperfect , p ar llel to the prism Of the first order and the base , better in partially altered sections .

Optical Properties : Uniaxial and negative .

Colorless in thin section . : Refringence low . Relief absent . n and

Birefringence very weak Interference

colors grayish white of the lower first order , a little

lower than the feldspar colors . Extinction parallel to cleavage lines when they

appear .

Pleochroism absent .

Interference figure is a broad cross without rings .

: M o f Inclusions icrosc pic needles o augite , also fluid and gas generally in zones .

Alteration : Readily to fibrous zeolites with stronger birefringence .

D : r ifferentiation From quartz , by weaker bi ef rin

gence , better hexagonal outline , and negative sign . OPTICAL M INERALOGY AND PETROGRAP HY

From feldspars , by uniaxial character and absence

of twinning . s Occurrence : In nephelite syenite , phonolites , and - o . o rare soda rich r cks It is never ass ciated with quartz , but often with orthoclase .

T OURM ALI NE .

i ? Ca Mn Comp osi ti on : RGS Os R chiefly Al , K , Fe , , ,

Mg , Li . Cri teria for determina ti on i n thin s ecti on : a r Form : Column r crystals , bunched o in radiating . aggregates . Irregular cracks may appear , but no cleavage is seen . Cross section shows trigonal outline parallel to base .

Optical Properties . Uniaxial and negative . C : olor Varies , with grayish blue , brown , and

black most common . Zonal structure may be shown f by dif erences in color .

R m o efringence mediu . C nspicuous against the

colorless rock constituents . Surface rough , n

Birefringence quite strong with bright interference colors Of the upper first or lower second

. order Often masked by strong absorption . Interference figure Shows a sharp cross with a

few rings .

o a Extincti n p rallel to the c axis .

X axis is parallel to the c axis .

Pleochroism distinct even in light-colored v ari i et es, increasing with the depth of the color . The greatest absorption takes place normal to the di rec

of a tion of elongation the miner l . Formula for Mg

tourmaline Z is pale yellow . X is colorles s .

Absorption very marked . Alteration does not take place commonly . DESCRIPTION OF ROCK -MAKING M INERALS

f D ifferentiation : From hornblende , by absence o cleavage , and by the fact that the greatest absorption takes place at right angles to the longitudinal axis . Occurrence :Widely distributed in crystalline schists and gneisses , in crystalline limestones ( New Jersey) , in granite pegmatites and veins with copper minerals . It is a common product of contact metamorphism .

Uses : Colored tourmaline is used as a gem . 8 6 OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTER 7 .

n n D e sc rip ti on o f M ine r al s (Co ti ue d ) .

- ANISOTROPIC BIAXIAL MINE RALS . O M C O RTHO RH BI .

ANDALU SI TE . l i Comp ositi on :A , S O Cri teria for determi na ti on i n thin s ecti on Form : Prismatic crystals always more or less elon gated parallel to the vertical axis , in rough or embedded crystals . Cleavage may show parallel to almost square prism . : Optical Properties B iaxial and negative . : Color Colorl ess to reddish .

R z efringence medium . n and

Birefringence weak Interference colors , of middle the first order , white or yellow .

Interference figure shows large optic angle .

Extinction parallel to c.

Pleochroism marked only in colored varieties , c of being reddish parallel to , which is the direction

elongation or cleavage . Pleochroic halos Often su r

round inclusions .

: C a Inclusions arbon ceous matter common , distrib uted through the crysta l in some geometrical form con

forming to the symmetry . a : R Alter tion eadily to colorless mica . D f : i ferentiation From diopside , by weaker bire f ring ence of and absence extinction angles . DESCRIPTION OF ROCK -MAKING M INERALS 87

Occurrence : In granitic eruptive rocks and in meta m r h e a o p os d sediment ry limestones .

T OPAZ . 1 i s i A F . Comp o it on : 2 Q S O4 Cri teria f or determina ti on in thi n s ecti on

Form : Colorless crystals of short prismatic habit .

Cleavage perfect parallel to the base . a Optical Properties : B iaxi l and positive .

Refringence medium , about the same as calcite .

n 2 and Birefringence weak about the same as that

of quartz with interference colors , middle Of the first

order white and yellow .

Interference figure shows large optic angle .

E xtinction parallel to cleavage .

Z axis parallel to c. Alteration : To kaolin or muscovite by loss of F and addition of water and alkalies . i D ifferentiation : From quartz , by cleavage and b ax ial character .

From andalusite , by its cleavage and its smaller

optic angle .

From orthoclase , by its higher relief, absence or rarity of twinning an d extinction parallel with the

cleavage . Occurrence : In contact metamorphic zones and in e rnatite p g , associated with cassiterite , flu orite , tou rma i line , beryl , etc . In cavities n rhyolite . : Uses Occasionally as a gem . O STAUR LI TE .

Com osi ti on : FeAl i p 5 ( OH ) ( S OG) Cri teri a for d eterminati on in thin s ecti on r : s 9 Fo m Short prism twinned at 0 or 60 degrees .

Cleavage variable , both prismatic and pinacoidal . 8 8 OPTICAL M INERALOGY AND PETROGRAPHY

Optical Properties : Biaxial and positive .

Color : Yellowish to brown .

Refringence rather high and surface rough . n and B irefringence weak with interference col

ors middle of first order white to yellow , about like

quartz . a Optic angle large . Pl ne of optic axis is parallel

to 1 00 .

Pleochroism distinct but not strong , showing c of the darker color parallel to , the direction elon Y g ati on (Z is golden yellow , is pale yellow , X is col orless )

Extinction parallel to cleavage or crystal outline .

of z Inclusions rutile , tourmaline , garnet and quart occur , the l atter abundantly .

Alteration :To a green mica and chlorite.

D ifferentiation : From titan ite , by the fact that in convergent light the optic plane i s Shown to be in the longer diagonal of the cross section . Occurrence : In mica schists and phyllites associated with garnet , cyanite and andalusite .

SERPENTI NE .

Com o i ti on : H M i s 0 . p 4 g 3 S 2 9 Cri teria for d etermi nati on i n thi n secti on o : Not F rm known in crystal form . Fibrous or a sc ly masses with elongation parall el to c. Prismatic a of 1 30 cle vage degrees seldom visible . a : Optic l Properties B iaxial and positive . C r olor in thin section : Pale green , yellow , o

colorless . R efringence low , about the same as Canada bal

. No o z sam relief , and sm oth surface . n

B irefringence rather weak, with interference

9 0 OPTICAL M INERALOGY AND PETROGRAPHY

o or in place of s ome of the magnesium , is col rless

o m nearly so , and sh ws strong pleochrois with X a Y and pale yellow, a brownish yellow , Z a bright

green . o Refringence high and surface r ugh , about the n same as in the monoclinic pyroxenes . and

Birefringence weak much weaker than r the monoclinic pyroxenes . Inte ference colors low

of first order. Interference figures not marked onaccount of the

weak double refraction .

Extinction parallel to cleavages , both pinacoidal of and longitudinal prismatic , and bisecting angles

intersecting prismatic cleavages .

Axial plane parallel to brachypinacoid , that is ,

parallel to the best cleavage . Axial angles large . r Pleochroism weak o absent .

Alteration : To serpentine by ordina ry weathering . f Also to uralite ( a variety o hornblende) , but much less o a m comm nly th n the onoclinic pyroxenes do.

a : Differenti tion From hypersthene , by the optic S and a o ign bsence of distinct col r and pleochroism .

From the monoclinic pyroxenes , by parallel exti nc

tion on vertical sections , and lower interference

colors . Occurrence : A common constituent of basic igneous

rocks as well as of serpentine derived from them . Also

found in crystalline schists and in many meteorites . Bronzite contains about 1 0 per cent FeO and has a char acteri stic bronzy luster due to inclusions .

HYPE RSTHENE .

Com osi ti on : M i p ( g , Fe) S o3 . Cri teri a for determination i n thin secti on DESCRIPTION OF ROCK -MAKING M INERALS 9 1

Form : S imilar to enstatite . More often massive in ll lame ze . E longated parallel to 0 .

Optical Properties : B iaxi al and negative .

Color :Brownish to greenish .

Refringence slightly higher than enstatite , due to

increase in percentage Of iron .

B irefringence S lightly stronger than enstatite , due

to increase of iron . Weaker than monoclinic

pyroxenes .

Extinction same as enstatite . e Axial plane parallel to brachypinacoid , i . . , parallel

to the best cleavage . Optic angle about X becomes

smaller with increase in i ron content .

Pleochroism distinct , increasing with increase in Y iron . Z is bright green , is yellowish brown , X is

clear red .

Inclusi ons : Gaseous , liqu id , glassy . Also a reddish brown material regularly a rranged , which gives it a - peculiar submetallic bronze like luster . They are believed to be inclusions of ilmenite , either primary or , produced at depth under pressure by circulating waters r acting along a cleavage o parting plane . Alteration : TO a variety of serpentine called bas to tite , less commonly to uralite , occasionally talc . Occurrence : Important constituent with plagioclase in basic igneous rocks , as norites and gabbros . Abundant in andesites . Found in meteorites .

BASTI TE .

Bastite is a variety of serpentine to which the ortho rhombic pyroxenes poor in iron alter frequently through the ordinary processes of weathering . It is geometrically o riented on the altered pyroxene , replacing crystal f or crystal . It is composed of fibers Often traversed by irreg ular cracks . Cleavage traces of the two minerals coin 92 OPTICAL M INERALOGY AND PETROGRAPHY

o a . cide , but the ptic l properties differ The pyroxene has

a cleavage parallel to the trace of the optic plane . In to of bastite , the cleavage is perpendicular the trace the t Optic plane, and to the negative acu e bisectrix . This

is the surest distinction between them .

Bastite is light yellowish or greenish . Refringence is less than that of the orthorhombic pyroxenes and about

the same as Canada balsam . B irefringence is weak .

Extinction is parallel to the fibres . Pleochroism is weak

and seen only in thick S ections .

O LIVI NE ( CHRYS OLI TE )

e i Comp osi ti on : (Mg . F ) 2 S o, . Cri teri a for d eterminati on i n thin s ecti on re Form : Idiomorphic , or in grains or granular ag g r gates . Also massive . Longitudinal sections more o

- less lath shaped with pointed ends . Outlines of crystals or often corroded rounded . Interpenetration twins

occur . Cleavage distinct , parallel to brachypinacoid ,

less distinct parallel to macropinacoid , Often made more e visible by d composition . An irregular fracturing is

often conspicuous , especially where alterati on to ser o pentine has c mmenced . Elongation usually parallel to 0 . : an Optical Properties B iaxial d positive . C : olor Nearly colorless , becoming reddish with

high iron content . R efringence high . Relief marked and su rface

rough . n and

B irefringence very strong, with i nterference colors or of the second third order , higher than the colors of augite

Extinction always parallel to cleavage cracks .

Axial plane parallel to the base , that is , at right

angles to the general direction of elongation . Axial

angle very large . DESCRIPTION OF ROCK -MAKING M INERALS

Pleochroism absent except in reddish varieties .

:M . Inclusions agnetite , spinel , apatite , common Also liquid or gas .

Alteration :Alters readily . Altered forms are more frequently observed than the fresh . Serpentine i s the commonest alteration product , with frequently a separa tion of magnetite or hematite . The first alteration goes on along the cleavage and fracture cracks . It is easily altered by atmospheric weathering to car bonates with limonite and opal or quartz . Calcite may a usually be distinguished in this case . In cont ct with a feldspar it may alter to an amphibole by regi onal meta morphism . The amphibole appears as a zone of pale green or colorless needles between the Olivine and the feldspar . D ifferentiation :From light colored monoclinic pyrox enes by parallel extinction , by poorer and unequal cleav ages and stronger birefringence . Olivine should be easily n recogn ized by its high refringence , a shagreen surface , o color, strong birefringence , and a large optic angle . : Occurrence E specially in basic igneous rocks , asso ciated with augite , hypersthene , plagioclase , magn etite .

An essential constituent of many meteorites , constituting the stony portion of the mass . Uses :The transparent variety is sometimes used as a gem under the name peridot .

TALC H M i Comp osi ti on : 2 g 3 S o3 ) 4 . Criteria f or d etermina ti on in thin s ecti on

Form : Colorless plates , elongated like rods . More rarely with round to hexagonal outline . May be arranged r in rosettes . Often more o less compact foliated mass es .

Cleavage perfect parallel to the base like mica . E longa tion parallel to c. OPTICAL M INERALOGY AND PETROGRAPHY

a Optical Properties : B iaxial and neg tive .

Colorless in thin section .

Refringence moderate . n and of Birefringence strong, with interference colors o to the third order , like musc vite

Extinction parallel to basal cleavage lines .

Plane of Optic axes parallel to 1 00 . Optic angles

small . o D ifferenti ati on : From musc vite , by its small optic angle . From sericite , by lower refringence .

Occurrence : Most abundantly in crystalline schists , o Often forming r ck masses , as soapstone . As a secondary

a a o miner l in b sic ign e us rocks , altering from Olivine , en i stat te, tremolite . : oa a Uses For s p , t lcum powder , French chalk, and in the manufacture of p aper .

NA TRO LI TE .

Comp os iti on : Cri teri a for determination in thin s ection : a Of o Form Aggreg tes col rless , fibrous crystals , Often r in interlacing groups o divergent . Prismatic angle 9 0 . a o nearly degrees E long ti n parallel to c. Microscopic twinning on 1 1 0 . a : Optic l Properties B iaxial and positive .

R no efringence very low , with relief. n and

a B irefringence we k , th ough slightly stronger than

quartz .

Interference colors , middle of the first order yel

low , a little higher than quartz . f Inter erence figu re shows dark cross . Optic angle a l rge .

a a a t 0 1 0 Pl ne of optic xis par llel o .

Axis Z parallel to c. DESCRIPTION OF ROCK -MAKING M INERALS 9 5

a E xtinction par llel to fibers . : N Occurrence ever found as a primary mineral . As a secondary mineral in basic igneous rocks filling amyg l i l da o da cavities . Common alteration product of sodalite , nephelite , and acid plagioclase .

P YR OX E NE G RO UP .

Comp osi ti on : The pyroxenes are metasilicates Of cal cium or , magnesium , iron , more complex silicates , often two or o o tri va containing m re bases , b th bivalent an d r stallo lent . They are closely related to ea ch other in c y graphic and phys ica l properties . They crystallize in the orthorhombic , monoclinic and triclinic systems . Cri teri a for d etermi na ti on i n thi n s ecti on : Form :Fundamental form is a Short prism with inter cleav facial angles of about 87 and 9 3 degrees . D istinct age occurs parallel to both prism faces . Twinning if l present is para lel to 1 00 . E longation usually parallel

to 0 .

Optical Properties :B iaxial . Most species positive .

C olor : In thin section usually pale to colorless . o Soda pyroxen es have a distinct green col r .

Refringence high . Relief distinct and surface

z : rough . n to

Birefringence strong , being stronger in the pale

or colorless pyroxenes with interference colors , bright tints of the second order to o Extinction :Maximum a ngle fr m 0 to 9 5 degrees , with common species varying between 30 and 54 de

grees . In sections showing parallel cleavage lines , a a p rallel extinction in orthopinacoid l sections , and in

all other sections an extinction angle is observed . The maximum extinction angle is large and is obta ined only when the section of the crystal is parallel to the

clinopinacoid . OPTICAL M INERALOGY AND PETROGRAP HY

1 Optic plane is parallel to 0 0 . Pleochroism weak or absent except in the soda

pyroxene .

Alteration :Alters readily to amphiboles . D es cribed under each species . Differentiation : Pyroxenes may be distingu ished from amphiboles by the following cri teria

P yroxenes . Amphi boles . Cleavage angle about 93 de Cleavage angle about 1 24

grees . degrees .

Crystals short prismatic . Crystals long prismatic: o o a C l r usually we k . Color marked . o Ple chroism weak . Pleochroism marked . Extinction angles 0—9 5 de Extinction angles 0—25 de

grees . grees . Mo c st spe ies positive . Most S p ecies negative .

Alter to amphiboles . Alter to chlorite , biotite ,

etc .

M N O OCLINIC MINE RALS .

M on oclin ic Py ro x en e s .

D iopside .

D iallage .

Augite .

Aegirite . O D I P SI D E . Com os ition : Ca M p ( g , Fe) ( Si0 3 ) 2 . Cri teria for determination in thi n s ection : Form Long, columnar crystals and grains . o . G c arsely lamellar ranular masses . Cleavage always distinct parallel to 1 1 0 in two directions nearly at right a . ngles to each other Parting parallel to the base , yield a ing fine twinning lamell e in this direction . E longation a a p r llel to c.

OPTICAL M INERALOGY AND PETROGRAP HY

Y or as follows : Z is greenish , is brownish reddish a brown , X is greenish . It cont ins inclusions like those - . of hypersthene , which gives it a bronze like luster

AUGI TE .

n M Al Fe Comp os iti on :mCaMg ( g , Fe) ( , )

Cri teria for d etermi nati on i n thi n s ection

Form : C rystals short thick prisms coarsely lamellar , i parallel to 00 1 or 1 0 0 . Granular . Tw nning common , giv a a ing polysynthetic lamellae parallel to 1 0 0 . Cle v ge a 1 1 0 imperfect, but distinct in two directions par llel to

0 . nearly at right angles . Elongation parallel to

Optical Properties : B iaxial and positive . R Color green , greenish black , brown . arely

yellow .

Refringence high . High relief and rough surface . n and

B irefringence rather strong , being stronger in the

p ale or colorless pyroxenes . Interference colors are bri ght tints of the second order Interference figures distinct on acc ount of the

strong birefringence . Axi al angles l arge . Optic plane parallel to the clinopinacoi d ( 0 1 0 ) Extinction angle : Maximum from 3 8 to 5 1 e d grees , which is Obtained when the section of the crystal is parallel to the clinopinacoid ( 0 1 0 ) varying from these angles to 0 degrees when the section is parallel to the orthopinacoi d Pleochroism usu ally absent or weak unless rich in o a Y ir n , in which c se Z is greenish , is brown ish to red o dish br wn , and X is green . o : Ga Inclusi ns seous , liquid or glassy , sometimes a a rr nged in zones . DESCRIPTION OF ROCK -MAKING M INERALS 9 9

Alteration : Mos t commonly to uralite , a variety of a or to hornblende , either cryst l for crystal , a fibrous aggregate of uralite . The alteration begins around the periphery of the crystal or along cleavage cracks . It may alter to biotite and then to chlorite , or directly to chlorite , sometimes forming calcite , quartz or epidote simultaneously . D : . ifferentiation From diopside , see under the latter From aegirite and spodumene in the extinction

angle in the vertical zone .

From amphiboles , see under Pyroxene group .

a o From epidote , by the fact that the pl ne f the Optic axis is parallel to the longitudinal axi s and cleav

age cracks . r i Occu r ence : Abundant n igneous rocks , but found also in metamorphic rocks . Occu rs in some stony meteorites . I ZE GIR TE ( A CM I TE ) . Comp osi ti on :Na Fe Cri teria for d eterminati on in thin s ecti on

Form : In crystal form , S imilar to augite , although or C a to 1 1 0 often longer acicular . leavage p rallel , more

: distinct than in au gite , almost at right angles . Part ing parallel to 1 00 . E longation parallel to 0 .

Optical Properties : B iaxial and negative .

Color in thin section greenish or brownish .

Refringence h igh , with high relief and rough su r

face . n and

Birefringence quite strong , with bright tints of

the second order, although it is stronger in the pale or colorless varieties of pyroxene Interference figures distinct on account of strong

birefringence . Optic angle is large . Optic plane parallel to the clinopinacoid ( 01 0 ) OPTICAL M INERALOGY AND PETROGRAPHY

Extinction angle : About 5 degrees . o Y Pleochroism marked . Z is yell wish green , is

olive green , X is dark grass green .

Alteration :To analcite and to the iron oxides .

' D ifl erentiation : From the amphiboles , see under pyroxene group . o From other monoclinic pyr xenes , by the very

small extinction angle , the negative S ign , the stronger

birefringence and m arked pleochroism . a- Occurrence :In pegmatite veins , in sod rich igneous a rocks , as nephelite , syenites , phonolites and sod granites . M A PHIB OLE GROUP .

Comp osi ti on : The minerals of the amphibole group are orthorhombic , monoclinic , and triclinic silicates Of a m gnesium , calcium iron or sodium , with aluminum or ferric iron in some cases . Cri teri a f or d etermination in thi n s ecti on :C Form rystals usually prismatic , elongated parallel c to , possessing very marked and regular prismatic cleav ages , varying little from 1 24 degrees between the cleav

a e . a g faces Twinning common p rallel to 1 00 . a : a M Optic l Properties B iaxi l . ost species negative . C o ol r in thin section are green , brown , blue , yel Or low colorless .

R a efringence aver ges less than the pyroxenes , a of incre sing with increase iron . Relief distinct . n to

Birefringence quite strong , but a little weaker o than in the pyr xenes . Interference colors are bright

tints of the second order . May be masked by strong absorption to

: M a o Extinction aximum ngle fr m 0 to 25 degrees .

OPTICAL M INERALOGY AND PETROGRAPHY

M o no c lin ic Am phib o l e s

Tremolite .

Actinolite .

Hornblende .

Riebeckite .

TREM OLI TE . i o . Comp osi ti on : Ca Mg s ( S , ) 4 Cri teri a for d etermina ti on i n thi n s ecti on - r Form :Crystals long bladed o short p rismatic . Often

fibrous or acicular . Perfect prismatic cleavage at an angle of about 1 24 degrees . Cleavage sometimes distinct , 1 00 parallel to 0 1 0 and . Transverse fracture frequent .

Cleavage more perfect than in pyroxenes . : Optical properties Biaxial and negative .

Colorless in thin section .

Refringence high , with distinct relief, but not as

marked as in the pyroxenes . n and

B irefringence quite strong, but a little weaker than in pyroxenes

Optic plane parallel to 0 1 0 . Maximum extinction angle is 1 8 to 1 6 degrees in

vertical zone . D ispersion weak . Inclusions of carbonaceous matter and biotite in of a tremolite met morphic rocks .

: o Alteration To talc , beginning al ng cleavage lines .

Also to calcite . D f : i ferentiation From hornblende , by light color . It has the lowest index of refraction found in monoclinic a mphiboles .

From pyroxenes , see under Amphibole group . r : Occur ence In schists , contact rocks , and veins . : Uses Fibrous varieties sometimes used as asbestos .

As j ade , sometimes used f or ornamental pu rposes . DESCRIPTION OF ROCK ’ MAKING M INERALS

ACTI NO LI TE .

Comp osi ti on : Ca (Mg , Fe) Cri teri a f or d eterminati on i n thin secti on

Form :Similar to tremolite .

Optical Properties : B iaxial and negative .

Color in thin section pale to dark green , depending

upon the percentage of iron . Refringence and birefringence S imilar to tremo lite Maximum extinction angle in vertical zone is 1 5

degrees .

D ispersion weak . o Pleochr ism pronounced , and absorp tion marked , being greatest in the general direction of the cleavage

lines in longitudinal sections . Z is pale to dark green , Y is greenish yellow , X is very pale yellow .

Inclusions :Similar to tremolite . : Alteration To chlorite , epidote , talc , etc . : as Occurrence Same tremolite , with which it is often o ass ciated . Uralite is the name given to the amphibole to which the pyroxenes frequently alter . It u sually cor responds to actinolite .

H ORNB LEND E .

a Fe n l Comp osi ti on : mC (Mg , ) o (A , Fe)

( F , OH ) Si0 3 . Cri teri a for d eterminati on i n thin s ection

Form : Prismatic elongated , parallel to the vertical axis , sometimes fibrous . Prismatic cleavage perfect , 24 making the characteristic angle of 1 degrees . Parting an d polysynthetic twinning are sometimes present, par allel to 1 00 or 00 1 . Cross sections may be acutely -S rhombic , with acute angles truncated , hence six ided , - whereas the pyroxen es are usually eight sided . Longi - tudinal sections lath shaped . Zonal structure occurs f re OPTICAL M INERALOGY AND PETROGRAP HY

quently in the brown h ornblende . Twinning frequently a parallel to the orthopin coid .

Optical Properties : Biaxial and negative . a Colorless , gr y, green , greenish blue , brown or

black . n Refringence high , with distinct relief .

and be B irefringence quite strong, ing strongest in

the basaltic hornblende . In common hornblende , n : a a n : In b s ltic hornblende , o Interference col rs , bright tints of the second order ,

Often masked by strong absorption . Maximum extinction angle in common hornblende o n 20 degrees , in basaltic h r blende from 1 to 2 degrees .

Optic plane parallel to 0 1 0 , in which face Z makes a variable angle with the axis 0 in the Obtuse angle

Beta . D ispersion distinct . a Y w a Pleochroism :Z p le green , pale bro n , X cle r f or brown , common hornblende .

Inclusions abundant but not characteristic . Rutile c ommon . Alteration :By ordinary weathering to chlorite often a a t ccomp nied by epidote , calcite and quar z . Sometimes

to . biotite By heat to augite .

a : a Differenti tion From other mphiboles , by stronger o c lor and pleochroism , higher interference colors . o From pyr xenes , see under Amphib ole group . : Occurrence Widespread in igneous , regional meta m a orphic and cont ct rocks . Hornblende schists .

I RI EB E CK TE .

Com osi ti on : nNaFe i p S 2 0 6 . FeSiO Criteri a for determi na tion i n thi n s ecti on

o : a F rm Simil r to hornblende .

OPTICAL M INERALOGY AND PETROGRAPHY

o ored micas , v arying fr m to Interference

colors of the third order , which may be very brilliant a in thin sections of colorless mica , often appe ring

iri descent . Occasionally masked by strong absorption .

Extinction about parallel to the cleavage lines .

Very small extinction angles may be noticed in biotite .

Absorption strong in colored micas . Optic angle large in white micas and small in the

- ferro magnesian varieties , appearing almost uniaxial . i D ifferentiation : Character zed by distinct relief,

strong birefringence (chlorite has weak) , one perfect a a cleav ge m rked by parallel , fine lines , practically par allel extinction , mottled appearance between crossed nicols , maximum extinction in col ored varieties parallel

to the vibration plane Of the polarizer .

O MU SC VITE .

Na Comp osi ti on : H ( K , ) Al Criteri a for determination i n thin s ecti on

See also under Mica group .

Colorless in thin section . : Not Inclusions as common as in biotite . Z ircon , apatite , spinel , garnet , quartz , and magnetite . : a Alteration By ordinary we thering to sericite , ser

a . pentine , t lc : M Occurrence ost common of the micas . Normal of c constituent igneous ro ks , especially g ranites . Abun dant in gneisses and schists . Present in veins . Occurs as Of an alteration product the feldspars , nephelite , etc . D f : if erentiation From talc , by large optic angle .

From kaolinite , by strong birefringence . o Fr m other micas , by being colorless in thin

section .

From chlorite , by strong birefringence and lack of color . DESCRIPTION OF ROCK - MAKI NG M INERALS 1 07

S E RI CI TE . f t Sericite is a fine , scaly or fibrous variety o muscovi e ,

with a greater degree of hydration . It is nearly uniaxial

in character , with a small optic angle .

BI OTI TE .

P s e u d o he x ago n al .

m si i M Co p o t on : ( K , H ) , ( g , Fe) (Al , Fe) , Criteri a f or d eterminati on i n thin s ecti on

See also under Mica group . Optical Properties :

Color : Black , green , brown , red , yellow . Angle of Optic axes almost 0 degrees in most bio

tite of ig neous rocks . B irefringence increases with increase i n iron

content .

Absorption marked .

Pleochroism distinct. Z is dark to opaqu e brown ,

Y is the same , X is pure yellow .

Inclusions : Apatite and zircon common . Pleochroic halos abundant about inclusions . eail Alteration : R y to chlorite , often accompanied of by the formation calcite , epidote and quartz . D ifferentiation : From alkaline micas by small optic angle color and distinct pleochroism .

From chlorite , by strong birefringence and color .

From hornblende , by extinction parallel to cleav age and almost uniaxial interference figu res in con

vergent light . Occurrence : Important constituent of many igneous rocks , gneisses and schists . D eveloped by regional and contact metamorphism .

LE PI D OLI TE .

om osi ti on : AI F l i C p ( Li , K ) ( , OH ) , A ( S O Cri teri a for determinati on i n thin s ecti on OPTICAL M INERALOGY AND PETROGRAPHY

See also under Mica group .

Colorless in thin section . o Pleochroism distinct . Z and Y pink ; X , c lorless . Differentiation : Microscopically indistinguishable from muscovite . Pink color is believed to be due to traces of manganese . n a Occurrence :In veins a d dikes in granite , associ ted with cas siterite , tourmaline , etc . : Uses As a source of lithium salts .

P ITE PH LO GO . i M M Comp osi t on : ( K , H ) ( g , F ) g Cri teri a for determina tion in thin s ection

See also under Mica group .

Color : Brown , brown ish red , green , yellow . o Y r Pleochr ism , Z and a e brownish yellow , X is col orles s

r Inclusions : Hematite , rutile and tou maline are m co mon . o Differentiati n : From minerals of other groups , a s me as biotite . e From muscovit , paragonite and lepidolite by color . o Fr m biotite , by mode of occurrence . : a l Occurrence Only in cryst l ine limeston es , dolo and r mites , se pentines , associated with spinel , graphite ,

. o etc Absent in ig ne us rocks . : Uses As an insulator in electrical apparatus .

C R HLO ITE GROUP .

Penninite . C linochlore .

C a a : m a to l h r cter Si il r the micas , with perfect c eav ag e a 00 1 p rallel to , the basal pinacoid . This cleavage ma not o o y be n ticed in fibr us or secondary chlorite . a of , Aggreg tes small flat scales of irregular outline , usu

OPTICAL M INERALOGY AND PETROGRAPHY

CLI NO CH LORE .

Comp osi tion : Same as penninite . D D ifferentiation from penninite : istinctly biaxial ,

S positive ign , higher birefringence , occasionally Oblique extinction , common polysynthetic twinning .

EPID OTE GROUP . Al i Comp osi ti on : Ca, ( , OH ) ( S O Cri teria for d etermina ti on in thin s ecti on : o a a Form C lumn r crystals , nearly always elong ted , p arallel to the b axis . Fibrous , massive , or in irregular grains as aggregates . Twinning common parallel to 1 00 . a Cleavage parallel to the basal pin coid , imperfect parallel o a to the orth pinacoid . B sal cleavage cracks not very m nu erous , and appear parallel to the general direction of elongation . a : Optic l Properties B iaxial and negative . Co o to o l rless range yellow in thin section .

R a efringence high with rough surf ce . n and

o B irefringence variable , ften strong, with high e int rference colors . Va riable in a single crystal

Extinction p arallel to cleavage in elongated sec

. tions In other sections , extinction angle varies from 0 to 28 degrees . Interference figure of cleavage flakes show an axial bar o with c ncentric rings . Axial plane at right a to o ngles the elongati n of the crystal . Axial angles

are large . : Pleochroism Z is colorless , yellowish green , ; Y a to pink is p le blue greenish yellow ; X is colorless , o lem n yellow , pale green .

: e a Alteration Epidot is very resist nt to weathering . D ifferenti ation : From light colored m onoclinic DESCRIPTION OF ROCK -MAK ING M INERALS 1 1 1

pyroxenes , by having optic plane at right angles to the principal cleavage cracks , which are parallel to the direc ti on of elongation . Epidote is characterized by form and color, high refringence , parallel extinction in l ongitudinal sections , strong bi refringence variable in a single c rystal . : Occurrence Very common , especially in schists and in zones produced by contact metamorphism between granites an d limestones . Also as an alteration product of the ferro-magnesian minerals and feldspars in igneous rocks . O Z I SI TE .

( Orthorhombic member of the Epidote group . )

m s i Co p o i t on : Same as epidote without the iron . Cri teri a for d etermin ati on i n thin s ecti on :

Form : Prismatic crystals or granular aggregates . r a l v Lamellar, fibrous o in comp ct masses . Perfect c ea t ag e, parallel to 0 1 0 ; difficult , parallel o 1 0 0 . Longer indivi duals Show transverse parting . Microscopic twin ning in polysynthetic bands occur .

Optical Properties : B iaxial and positive .

Colorless to yell ow tints . Usually lacks color .

Refringence high , with rough surface . n an d a B irefringence weak , with gr yish or whitish inter ference colors

Extinction always parallel . f : Di ferentiation From epidote , by its lack of color and its weaker birefringence . It is ch aracterized by parallel extinction , high relief, very weak birefringence , strong dispersion . : Occu rrence In crystalline schists , associated with amphibole , particularly hornblende . In igneous rocks , a of a as an alter tion the feldsp rs . In veins in altered basic igneous rocks with quartz . OPTICAL M INERALOGY A ND PETROGRAP HY

KAOLI NI TE . H A1 Si O Comp osi ti on : , , , Cri teria for d eterminati on i n thin s ection

or . Form : Pseudohexagonal , in thin plates scales

- Usually in clay like masses . a Optica l Properties . Neg tive .

Col orless in thin section . Aggregates are cloudy .

Refringence low with no relief. n Birefringence weak Differenti ation : From muscovite an d talc by weak birefringence . Occurrence : Kaolinite is the most common second ary mineral . It is derived from the feldspars by ordi nary weathering . Occurs in large sedimenta ry clay masses as a result Of the decompos ition of aluminous l si icates .

Uses : It is used in the manufacture of porcelain , pottery , and china .

TI TAN ITE ( S PH ENE ) .

Comp osi ti on : CaTi SiO Cri teria f or d etermina ti on in thin section Form : In deta ched crystals an d in disseminated

- . m grains Often wedge shaped when pri ary , and irreg a ular gr ins when second a ry . Flatten ed parallel to the

. a a base Elong ted p rallel to a or c. Cleavage imperfect, to a parallel the pri sm , ppearing as a few rough cracks . C a e leav ge rarely observ d in secondary forms . Twinning s seen only between cro sed nicols , the twinning boundaries

c n the bise ti g acute angles of the rhombs . Optical Properties : C o ol rless , brown ish or yellowi sh . R efringence high , with rough su rface . n and

1 1 4 OPTICAL M INERALOGY AND PETROGRAP HY

a to 0 1 0 . matic , somewhat fl ttened , parallel Narrow or bands of albite , intergrown with orthoclase microcline , " " forming perthite common . Plagioclase feldspars are

9 0 . triclinic , but angle alpha varies little from degrees Cleavage : Perfect parallel to the basal p inacoid and almost as perfect parallel to the clinopinacoid . Cleavage cracks usually noticed only in very thin sections . The two cleavages intersect at 9 0 degrees in orthoclase and a at 93 or 94 degrees in the plagioclase feldsp rs . The cleavage is not as distinct as the cleavage of mica or hornblende . Twinning : Twinning common in the feldspars f ol

o a a M l wing the C rlsb d , annebach , Baven o, Albite and

Pericline laws . CARLSBAD TWINNING is the simplest type of feldspar twinning, and it occurs in both monoclinic and triclinic varieties , in the latter case causing confusion with other of types twinning . The twinning plane is the orthopina coid and the composition face is the clinopinacoid . Carls bad a twins alw ys consist Of two individuals , a fact which may be used to differentiate between the plagioclase feld spars and orthoclase . MANNEBACH TWINNING :The b asal pinacoid is the composition face and twinning plane . This type of twin ning is not common . BAVENO TWINNING : The twinning plane is the clino m do e to which the twinning axis is no rmal . Sections cutting such a twin Show squ are or rhombohedral out

lines , the cleavages being parallel to the sides . ALBITE AND PERICLINE TWINNING are especially com mon on the plagioclase feldspars and are used as a means of o identificati n . They are usually visible to the n aked eye . In thin sections they appea r as polysynthetic stria tions in narrow alternating light and dark bands , which DESCRIPTION OF ROCK -MAK ING M INERALS

extingu ish a lternately upon being rotated . In the albite linO ina type of twin , the twinning axis is normal to the c p

coid. Hence the lamellae are parallel to the clinopinacoid and the striations are visible only on the basal pinacoid

and the orthopinacoid .

In the pericline twinning , the twinning axis is parallel

to b . on Therefore , the pericline striations are visible all

faces of the crystal . It is obvious that if twinning occurs

on the clinopinacoid , it must be of the pericline type . In

thin section , the pericline twinning is visible in any sec

tion except a section cut parallel to the composition face .

i . 1 9 . Tri i ni e r rm wi n h s i i ns the F g cl c f lds p a fo , s ho g t e po t o of r ns cha ract eri s tic al bite an d peri cli ne s t iatio .

D ff n i i i ere t a tion o f the Fe l d sp a r s by Twinn ng .

Orthoclase occurs in simple twins , after the Carlsbad ,

Baveno and Mannebach laws , but never in polysynthetic

twins . Microcline is always polysynthetically twinned in two

directions , a combination of albite and pericline twinning producing a rectangular crossh atching between crossed

nicols . OPTICAL M INERALOGY AND PETROGRAPHY

Plagioclase feldspars practically always sh ow a poly law . synthetic twinning , after the albite Albite shows twinning lines that are fine and far

e . apart , irregular and interrupt d Oligoclase shows twinning lines that are clear and of regular widths . Labradorite shows twinning lamellae which are clear one and definite , but the wi dth Often varies from lamella to another. Anorthite shows twinning lamellae which are broad law o of and regular, after the albite , while th se the peri cline law are distributed only in certa in of the albite bands .

— a O p tical Pr op e r tie s . The optic pl ne containing the

Optic axes and the bisectrices is the chief optic element . Its position in each of the feldspars has definite relations a to the cleavage , extern l faces , axes , and the positions

of . an the albite twinning In orthoclase d microcline , for example , the optic plane is almost parallel to the basal pinacoid , hence agrees with the direction of the most perfect cleavage . Z is perpendicular to the clino pinacoid . X lies in the plane of the clinopinacoid alm ost parallel to the base , varying about 5 degrees in I microcline .

R low a efringence is , simil r to that Of quartz . The

Becke test is advised .

a Birefringence is we k , similar to that of quartz .

OF OF D TA BLE REFRINGENCE TH E FE L S PA RS .

TL n 72 m

1 1 8 OPTICAL M INERALOGY AND PETROGRAPHY

a o To di stig ui sh one plagi oclase feldsp r from an ther ,

several practical methods have been devised .

ON AND 1 . EXTINCTION ANGLES BASE BRACHY

PINACOID .

Schuster established relations existing between the

extinction angles on the base and the brachypinacoid . The prevalence of favorable cleavages aids in this deter

mination . As these minerals are all triclinic , extinction takes place in all sections unsymmetrically with respect

r s . onse to crystallographic , twinning o cleavage line C r quently , extinction angles will always be obse ved . When the extinction angles on both the basal pinacoid and

the brachyp inacoid are large , anorthite is in all prob

ability the mineral observed . When the angles are both

small , the feldspar is Oligoclase . Albite and labradorite

show intermediate extinction angles . Orthoclas e ha s n extinction o the bas al pinacoid of from 5 to 9 degrees . The extinction angles given in the following table or are marked plus minus . The angles on the base and brachypinacoid are marked plus when the direction of e extinction has apparently mov d , as the hands of a watch , with reference to the upper right-hand edge of the crys

tal , between the base and pinacoid . The angles are

marked minus when the reverse is true .

QEXTINCTIO A L N NG ES . Section parallel to base Section parallel to brach measured from trace of ypinacoid measured from a a pinacoid l cleav ge . trace of basal cleavage . Albite 4 Albite Oligoclase 2 Oligoclase Labradorite Labradorite Anorthite 37 Anorthite DESCRIPTION OF ROCK -MAKING M INERALS

2 . STATISTICAL METHOD . The method proposed by Michel Levy is practical in all sections , Showing the albite twinning . This method consists in finding the maximum equal extinc tions on opposite sides of an albite twinning line . The position Of the plane which gives maximum extinctions in the zone normal to the brachypinacoi d is different for di different feldspars . Th is method , though te ous , is reliable , in that the various species have characteristic maxima . Sections norma l to the brachypinacoid may be recogn ized by the fact that the twinned parts Show equal illumination eight times upon a complete rotation of the stage , once every 4 5 degrees , in which position , the two parts seem to belong to one individual . The faintly discernible twin line must be parallel to the plane Of f vibration o either Of the nicols at equal illumination . Maximum extinction angles in sections perpendicular to albite twinning : Albite Oligoclase Labradorite

o n c n M o li ic Fe ld s p ar . O RTH OCLAS E .

m si ti on : Al i Co p o K S , O, . Cri teri a for determi nati on i n thin s ecti on

See also under Feldspar group .

Twinning after Carlsbad law common , after Baveno and Mannebach less common . : Optical Properties B iaxial and negative .

Colorless in thin section .

Refringence low . Relief absent and surface

smooth . OPTICAL M INERALOGY AND PETROGRAPHY

B irefringence very weak , with interference colors

a . of the lower first order, bluish gr y , white , etc , not

quite as bright as the colors of quartz and plagioclase . Alteration to kaolinite so prevalent that surface usually appears cloudy .

D ifferentiation : From other feldspars , see under

Feldspar group .

From quartz , by cloudy appearance , and negative a charact .

Occurrence : Abundant in acid plutonic rocks , pres ent in interm ediate and certain basic i gneous rocks , in

a . schists , gneisses , and in cont ct zones As perthite , with bands of albite .

Uses : In the manufacture Of porcelain and china . A variety of o rthoclase called moonstone is used as a gem .

SANAD INE .

ana i le r l ss S d ne is a c a , g a y variety of orthoclase , . occurring in rhyolite , trachyte , Obsidian , etc . It decom poses less readily than orthoclase , has a smaller axial angle , and usually contains more inclusions .

n T ric li ic Fe ld sp ars .

MI CROCLI NE .

Com ositi on l i p :KA S 3 0 8 . G a t a eneral ch rac ers s me as orthoclase . D ifferentiation : From orthoclase ; simple crysta ls not Showing the cros sed twi nning have extinction angles of about 1 5 degrees on the base with reference to the brachypinacoidal cleavage .

From other feldspars by the characteristic crossed , a rect ngular , grating structure . : to Occurrence Similar orthoclase , but more abun dant in pegmatites .

OPTICAL M INERALOGY AND PETROGRAPHY

ANORTH I T E .

Comp os i tio n : Differentiation : Anorthite has the strongest bire f ring ence and the highest refringence of all of the feld

Spars . See under Feldspar group . Occurrence :As an essential constituent of basic igne ous rocks . D eveloped by contact and regional metamorphism . DESCRIPTION OF ROCK -MAK ING M INERALS

— RA H . PART TWO . PET R O G P Y

CHAPTE R 8 .

Gen era l D i scu s s io n o f Igneou s R o c k s .

Petrography is that division of Petrolog y which is concerned with the systematic classification and descrip

tion of rocks megascopically and microscopically . The broad classification of rocks according to origin M a is :1 , Igneous ; 2 , Sedimentary , and 3 , et morphic . Igneous rocks are those which have solidified by cool

ing from a molten condition . Sedimentary rocks are those which have b een dep os

ited under water or on land by mechanical , chemical or

organic processes . Metamorph ic rocks are those which are derived from previously existing igneous or sedimenta ry rocks by heat

alone or by pressure and resultant heat .

Igneo u s R o cks . — Clas sificatio n . Severa1 methods for classifying igne

ous rocks have been devised , two of which seem to be r more o less satisfactory for practical purposes . These methods are the qualitative and the quantitative classi i n f ficat ons . By a examination o the minerals comprising the rock many inferences may be derived as to the mode o f origin , the conditions of crystallization , the general

chemical composition , whether aci d or basic , etc . A macroscopic Observation alone gives the Observer some basis for a rough classification . In the field , the mining OPTICAL M INERALOGY AND PETROGRAPHY engineer or geologist may find a rock which is essentially of quartz and feldspar, with a small percentage ferro magnesian minerals . He may call it a granite . His clas sificati on i s correct if the rock contains 25 or 30 per

cent of quartz . But if it contains only a few per cent , he will hesitate as to whether the rock is a granite or a syenite . As long as such a doubt exists as to the proper clas

i ion of of s ficat a rock , it is Obvious that the system a classification is at fa ult . It is of course cle r that all possible gradations in mineral percentages exist between f a the various igneous rock types . In so r as this is true , a qualitative classification based upon mineral per centages is defective . On the other hand , this method is exceedingly rapid in that one who is skilled in the manipulation of the microscope and in the interpretation of the phenomena observed in thin section can infer much from a glance about the nature of the rock .

For more complete descriptions of a rock , the quanti tative classification is more satisfactory in that the chem ical composition of the rock is used as a basis f or clas sific ti on a . But such an analysis usually takes two or three a d ys of careful work by a skilled chemist . Obviously , a qualitative classification with the aid of the microscope meets the requirements of the great maj ority of cases generally met with .

Es sen a an d cce s so n — ti l A ry M i e r a l s . Of the thousand minerals which are known , only about ninety occur in - igneous rocks . Twenty five of these are of prime impor

tance in determining the classification of a rock . These " " are the essential minerals , f or their presence is essen tial to the classification an d definition of the rock type in

. which they appear The remaining minerals , which com " a " prise the m j ority , are the accessory minerals , whose

OPTICAL M INERALOGY AND PETROGRAP HY about the molten mag ma at the time of thes olidification o of of the rock m ass . M st impo rtant these are

1 . The rate of cooling ;

of 2 . The chemical composition the magma ;

3 . Pressure ;

4 . Temperature ; H l as t C . 5 . Acti on Of mineralizers s eam , , Fl , B

The rock solidifying at great depths cools very slowly , allowing the minerals time to crystallize into well-formed e individuals . Many minerals crystallize simultan ously , and these minerals interfere with each other as they grow . The interpenetration or i rregular boundary line between any two crystals is a mutual adj ustment of a simultaneous formation . Molten m gmas which are suddenly subj ected to rapi d cooling, such as would aecom a on z p ny an extrusion or near the surface , crystall i e rel ativel y rapidly , with the result that a portion of the rock mass crystallizes as a glass . Microscopic crystals usu a a ally h ve time to m ke thei r appearance . It is also found that crystals of some minerals grow more rapidly than of crystals other minerals . When a rock shows a glassy appearance , with minute crystals embedded in the glass , " a the glass is reg rded as a groundmass . ma The common textures y be reduced to four , as follows 1 . Glassy ; 2 . Felsitic , or stony ;

3 . Porphyriti c 4 . Granitoid .

Glassy texture is characterized by absence of crystallization .

Felsitic or stony texture Shows some crystallization of m inute crystals enmeshed in a glassy or dense ground GENERAL DISCUSSION OF IGNEOUS ROCKS 1 27

a m ss , giving the rock a stony or n oncrystalline

appearance . Porphyritic texture results from conditions within the magma which allow the crystallization of certain min erals o t take place before any other appears . These well " " defin ed crystals , which are called phenocrysts , are b em edded in a finer ground mass , which may be wholly r glassy , partly crystalline , o very finely crystalline

throughout .

Granitoid texture is applied to those rocks which con tain no groundmass and which are composed of crystals of the same general time of growth or which separated o ut in order of their basicity . In this case the earlier

- minerals Show well defined boundaries and crystal planes , whereas the later minerals fill the interstices and assume S r an irregular hape , dete mined by the position Of the

earlier minerals .

Extrusive flows and intrusive lavas and dikes are

usually characterized by the presence of a groundmass . The deep -seated rocks are characterized by a granitoid

texture . All gradations in texture between rocks con s isting entirely of glass and of wholly c rystalline mate

rial exist .

To illustrate the use of m ineral composition and tex

ture in classifying rocks , the following examples are given

O . R CK . MI NE RA L S . TE XT U RE

an a s a a nd a an Gr ite Alk li feld p r qu rtz Gr itoid .

n ; same n m s n Sye ite Grou d ass pre e t .

e c e s a s an o Diorit A id f ld p r G r it id .

n s s am e n m s n A de ite Grou d a ss pre e t .

a o me e s a s an G bbr Li f ld p r Gr itoid . a ame n n B as lt s Grou dma ss prese t . D a a se s am e but nte me a e i b with i r di t e textur . OPTICAL M INERALOGY AND PETROGRAPHY — T e x tu r a l T e rm s . Convenient terms which are applied to igneous rocks to describe the amount of crystallized : matter present , are

r 1 . Glassy , in which no c ystals are present ;

2 . C ryptocrystalline , in which crystals are present but visible neither to the eye nor to the microscope ;

3 . Microcrystalline , in which crystals are present but visible only under the microscope ;

4 . Hypocrysta lline , in which the rock consists partly of glass and partly of crysta llized matter ;

5 . Holocrystalline , in which the rock is completely crystallized and no glass exists .

A classification Of terms which describe the form and shape of the crystals

1 . Idiomorphic crystals are those which have their r own peculiar geometric form . The first minerals to c ys

lliz ta e from any solution are idiomorphic , as they were allowed to grow without interference . 2 . Hypidiomorphic crystals are th ose having part Of their planes present and part absent . This may be brought about by an overl ap in the time of crystallization a of series of minerals . One mineral is not g iven time to crystallize completely before an adj acent m ineral interferes .

3 . Allotriomorphic crystals are those in which no a crystallogr phic planes are present . This is true of the to last minerals crystallize . Thei r S hape is determined

by previously existing minerals . Simulta neous crysta l liz ation sometimes results in the development of allotrio morphic crystals .

’ R o se nbu s c s — h L aw . There is a norm al order of crys t ll a i zati on in igneous rocks which in general is a law of

OPTICAL M INERALOGY AND A PETROGRAP HY

— o Geo l o gical O ccu rre nc e . The f llowing table lists the commonly Observed original structures of igneous rocks .

K V O LCANIC RO C S .

t us ve Ex r i .

or Pyroclastic or fragmental deposits , as ash

Volcanic necks .

o . Lava flows or Sheets . Overfl w from fissures

V O L CAN IC O R PLU T ONI C R O CKS .

e I ntrusiv .

r Intrusive sheets o sills .

s li h By ma t s .

a L ccoliths .

D ikes . N C O CK PLU TO I R S .

si v I ntru e.

8 . Bosses or stocks .

9 . Batholiths .

P e tr o gen y . — M agm a A magma is a fused rock mass in mutual f o solution . The essential feature o a soluti n is its ten

deney to become homogeneous . This tendency is pro d uced by diffusion , convection currents , differences in m o te perature , sinking f frag ments of superincumbent o r cks , etc .

Magmas do not origin ate in the places where they a re

now observed . They m ove

1 . In the zone of flow :

. a wa a By rising gradu lly , like a bubble of air in o ter , with a fl wag e of the rocks ab ove SO as to allow passage ;

b . By overhea d stoping and absorpti on ;

. a c By ssimilation . GENERAL DISC USSION OF IGNEOUS ROCKS

f r 2 . In the zone of ractu e

a . By following the course of least resistance

through whatever openings exist .

b . By overhead stoping . — D iff e r e n ti a tion The possible causes of differentia tion in a still fluid ma gma are gravity and differences in temperature , of which gravity is by far the more impor tant . This accounts for the accumulation and concem trati on of magnetite along the lower border of a magma .

It is the first mineral to crystallize .

— o o M agm a tic Stop in g . Marginal a ssimilation is ne f the methods Of magma advance through overlying rock

fo rmati ons . This method is effective chiefly in the early ’ part of the magma s history and takes place at the main n contacts a d along a relatively limited surface .

According to the theory of m agmatic stoping, each batholithic magma in its gradual advance upward through the overlying rocks engulfs large blocks of rock

from the roof and walls . This process is facilitated by the shattering which it is believed accompanies an intru

sion , du e to unequal heating of the country rock along

the contacts . These blocks are thus dissolved at depths

forming a compound m agma by assimilation . The aver a age crust rock is more soluble in basic rocks th n in acid .

z — two Cry s ta lli a tio n . A eutectic is that proportion of or more substances that has the lowest freezing point

for those substances . Eutectic aggregates represent later

products Of crystallization because the first mineral to . crystallize i s that which is in excess as compared with

certain standard proportions . Thus , an intimate inter growth of quartz and feldspar is a proof of simultaneous

crystallization . If a third substance were added to a eutectic propor

o tion , it would l wer the temperature so as to approach OPTICAL M INERALOGY AND PETROGRAPHY

a tern ary eutectic , unless the third substance were pres ent to an amount less than one per cent , in which case it may be c onsidered negligible . A mineral which crys talliz es late has an appreciable effect on a eutectic p ro

portion . One which crystallizes early , as apatite , has no of an es effect . Thus the importance acc sory mineral

a depends upon its solubility in eutectic , although it is usually present in such a small amount that it may be disregarded . - On the other hand , the gases will be present throughout the crystallization of the magma . They tend to lower the temperature of crystalliz ation to a greater extent than do the access ory minerals , and they aid the magma to solidify to a c rystalline mass instead of to a glass .

o In perfect isom rphism , A and B form mixed crystals an i o in y proportion , so that there is a complete ser es f

o a p ssible v rieties between end members . Such a series is Obtained between minerals which agree very nearly

a m and in molecul r volu e crystalline elements . The a lbite a a northite series is an ex mple . In imperfect isomorphism only certain mixtures are

as r possible , A with some B , o B with some A . The ortho - clase albite series is an example . Orthoclase mav con a a t in some lbite , but no continuou s series of mixtures o c nnects pure orthoclase and pure albite .

Influ en c e O f Gas e s o n a M a gm a —Gases present in of magmas are in a condition unstable equilibrium , par ticularl at y slight depths , liberating heat by reaction with a e ch other. With decrease in pressure the reaction a a between g ses incre ses . Thus it was observed in Hawaii that the temperature of a lava lake changed a few hundred degrees in temperature as the amount of

gases passing through it increased or decreased . The

1 34 OPTICAL M INERALOGY AND PETROGRAPHY

The texture of the rock should be examined first . Hav ing noted the presence or absence of a groundmass , the feldspars should be examined . Pink feldspar i s usually orthoclase or microcline . If Carlsbad twinning can be or Observed , the mineral is probably orthoclase microcline .

r Plagi oclase feldspars are usually white , gray o bluish

a . gray , sometimes with flashing blue surface Albite twinning is Often Observed as a polysynthetic striation . Labradorite is of a d arker blue or gray than the other a feldspars . The feldspars are less transparent and gl ssy than quartz .

a Quartz is recognized by its vitreous , fresh appear nce and transparent qu ality . A rock containing quartz will contain neither leucite nor nephelite . If leucite or nephe a lite can be determined , qu rtz is absent . This fact is o f inherent in the chemical compositi n o the magma . A rock containing leucite or nephelite is too low in S ilica f or any to be present as quartz in excess . D ark minerals which appear in an orthoclase -micro cline rock are bi otite , hornblende , or augite . Biotite is determined by the flashing black surface due to the per feet cleavage plane . A knife blade may be us ed to test diffi the softness and the ease of cleavage . It is more cult to distinguish hornblende from augite in th at they n a are both hard a d not readily cleav ble . In good crystals , a - ugite shows an eight sided cross section , whereas horn a - blende has six sided cross section . The black minerals of a pl agioclase rock are biotite and hornblende rather than augite . If the rock is mainly

a o a b sic , has a dark color , and a dull , st ny ppearance , the a m o bl ck ineral is pr bably augite . Olivine occurs in basic a - l vas in clear , glassy greenish yellow grains . GENERAL DI SCUSSION OF IGNEOUS ROCKS

A dense , volcanic rock which shows a groundmass and visible quartz is either rhyolite or dacite . Without fur ther examination the Observer would be j ustified to call the rock rhyolite , as it is far more abundant than dacite .

If the volcanic rock is black and felsitic or stony in appearance , it is a basalt . If the rock answers neither of these descriptions but is evidently v olcanic , it may be or a trachyte , a phonolite , an andesite . Of the three , andesite i s the most probable , as it is the most common .

It usu ally appears medium dark , midway between the acid and basic members of the series . If Carlsbad twin a ning is seen on the feldspar , the rock may be tr chyte instead of rhyolite . If leucite is distinguished , it is a phonolite , otherwise there would be no j ustificati on for naming it thus . A rock possessing a granitoid texture and quartz in some abundance may be called a granite rather than the rarer quartz diorite . If it contains orthoclase and no quartz , the observer would doubtless classify the rock r either as a syenite o as a nephelite syenite , although the o former would be the more pr bable . D iorites are d arker than the syenites and may be inferred from this fact alone if the character of the feldspars cannot be deter mined . If plagioclase can be distinguished the classifi cation is simplified . The latter may also show the dark a or gray blue color of l bradorite .

D iabase may be determined by a peculiar texture , commonly called diabasic texture , on account of its char i acteri st c appearance . White plagi oclase is intimately intergrown with augite crystals , the plagioclase having - developed first , hence taking a lath shaped texture . The interstices between the feldspars were later filled by the augite . It is dark and Of medium grain . OPTICAL M INERALOGY AND PETROGRAPHY

The more basic rocks are determ ined by the total n absence of quartz a d feldspar, and the n ature of the ferro-magnesian mineral comprising the greater part of the rock .

1 38 OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTER 9 .

IGNE O U S R O CK TYPES .

Plu to n ic R oc ks . M THE GRA N ITE FA I LY .

— a al M i ne r a lo gic a l Co m p o s iti o n . E ssenti l minerals : kali feldspar and quartz . Common minerals : biotite , o muscovite , amphiboles , pyr xenes . Accessory minerals

a a ta . magnetite , p tite , zircon , ti nite , garnet , tou rmaline — T e x tur e . Granitoid . — Cha r ac te r . Granites a re generally light in color , in shades of white , gray and pink , occasionally darker , due to an a a a or rox incre sing mount of biotite , mphiboles py a o r enes , in which c se the r cks a e liable to grade into the o syenite or di rite families . Microcline is more common in granites than in any of other kind a rock . The quart z may conta in minute e or rutile n edles tiny c avities filled with gas bubbles . B io o tite is the c mmonest dark silicate .

a e e s o f an — V ri ti Gr ite s . There are two va ri eties of granites . The most common type consists of the alkali

a - lime v riety , and the rarer type is called the alkali granite variety . The essential difference between the two types - is that the alkali lime variety grades toward and into the

o a - di rite f mily, and the alkali granite variety gra des toward and into the alkali syenites and finally into the nephelite syenites . The pyroxen es and amphiboles of the - alkali lime variety are more basic , and contain consider IGNEOUS ROCK TYPES — PLUTONIC ROCKS a o ble am unts of magnesium and calcium . These min erals not do appear in the alkal i granites , but are sub sti tute d o by alkali pyr xenes and amphiboles , such as acgi rite and riebeckite . Classification

G N E M R A IT FA ILY . - — Alkali Lim e Gr an ite s . Containing alkali feldspar e ( orthoclase , microcline , albite , perthit ) and quartz .

o a . Granitite , with addition of bi tite .

O b . Amph ibole granitite , with addition f biotite and amphibole . b c . Pyroxene granitite , with addition of iotite and pyroxene .

d . Granite , with addition of muscovite and biotite .

of e . Amphibole granite , with addition muscovite , bio tite and pyroxene .

o f . Pyroxene granite , with addition f muscovite , bio tite and pyroxene .

g . Tourmaline granite , with addition of tourmaline . — Alk a li G ran ite s . Containing alkali feldspar and quartz .

o a . Alkali granitite , with addition of bi tite .

. Of a b Aegirite gran ite , with addition egirite .

. R c iebeckite granite , with addition of riebeckite .

d . Aplite no subordinate mineral except possibly a

i little mu scov te .

BORDER P HASES OF GRANITES . — P e gm a tite s . A peg matite is a border phase of a gran ite often observed on the edges of bosses and batholiths . a - They are usually very co rsely crystalline vein granites . of a consisting quartz , feldsp r , muscovite , tou rmaline , r be yl , spodumene and others . D ue to the immense size 140 OPTICAL MINERALOGY AND PETROGRAPHY

e which the crystals attain , pegmatit s are sometimes " called giant granites . The largest crystal Of spod ‘ f umene on record was f ound in the Etta tin mines o the

a . Black Hills . This cryst l measured thirty feet in height

Beryl crystal s weighing over a ton have been recorded . Muscovite mica in sheets three feet in diameter are qu ite

common . In pegmatites , the essential minerals Of gran and ites are not always present . Quartz and beryl , quartz

a , tourmaline , mica and qu rtz , feldspar and tou rmaline are all possible combinations .

Pegmatites are usu ally regarded as a late phase of the eruption which produced the granite . The common

‘ occurrence of such minerals as tou rmaline , topaz and apatite in pegmatite leads to the suggestion that the influ ence of the rare elements flu orine and boron may have had some influence in effecting the coarse crystallization which S O frequently exists . — Gr ap hic Gr an ite Graphic granite is a variety of a peg matite which consists of a curious form of inter growth of qua rtz and the feldspars in such a manner that the cross fracture of the vein rock exposes a cuneiform or wedge-shaped texture resembling the writing charac of ter the ancient Chaldeans and Assyrians . The most om c mon intergrowth is qua rtz incl osed in orthoclase , c micro line , or perthite . Since neither of the minerals comprising graphic granite possesses any definite crystal shape , it is evident that they crystallized from solution at the same time .

e n — Gr i se . Greisen is another border phase of a gran a ite m ss which , although not occurring abundantly , is Of economic value the world over as the mother rock for s ca siterite , the tin ore . It is a granitoid rock composed a or of qu rtz and muscovite , some related white mica , as o lepid lite or zinnwaldite .

142 OPTICAL M INERALOGY AND PETROGRAPHY

as limes tone to escape , it will be retained in the marble o f or calcite or dol omite . If there is an Opp rtunity its release through fissures or j oint cracks , the resulting a Of a a m ass may consist essenti lly second ry silic tes , some of which develop by a recrystallization Of the origin al constituents of the limestones and others by the addition of materi al from the magma .

The contact metamorphic effect of an intrusive magm a n a a on shale is pronounced a d ch r cteristic . Immediately " at the contact , the shale is converted into a hornfels a rock , which is dense , very finely cryst lline , extremely o o and of hard , has a c nch idal fracture consists chiefly n quartz , feldspar a d biotite . From hornfels to the unal tered shale the foll owing stages are often Observed : o highly metamorph osed m ica schist , sp tted mica slate , sp otted sl ate lacking the conspicuous development of the a a mic s , un ltered sh ale . This change may be almost im n perceptible , a d may extend for miles from the actu al a o o contact . The chemic l c mp sition of hornfels and shale ’ aFEoften a very S imil r , although at times it Shows an addi a tion of materi l in the hornfels .

con o m c se s f — E i U o Gr an ite s . Granite is more exten s ively used f or structural purposes than any other igne a o ous rock , lth ugh any crysta lline rock is often loosely a a a c lled gr nite in the qu rry if it consists of silicates . G a m r nite is the strongest of the co mon bu ilding stones , the crushing resistance ranging from to er - pounds p square inch tested on two inch cubes .

The following resistance tests Show the average in range :

G a . C o M r nite from St l ud , inn . , to

G a o M R C o r nite fr m ystic iver , onn . , t G a o Ca r nite fr m pe Ann , Mass , G a om M r nite fr Vinal Haven , aine , IGNEOU S ROCK TYPE S — PLU TONIC ROCK S

Upon the following points are based the desirability of granites f or structura l purposes

1 . Homogeneity of texture .

2 . Adaptability to tool treatment . a 3 . Good rectangul r j ointing in the quarry .

4 . Pleasing color .

5 . Transportation facilities . d 6 . D urability a s affected by gra in an mineral content .

A light color is generally more desirable than a dark a a one, and a medium grain is more f vorable f or dur bility a a i than a coarse gra in . The R p k wi granite of southern r o Finland is used freely in Petrograd f o columns . It c n tains large red orthoclase crystals , which give the rock a prevailing red color , greenish plagioclase , sm oky quartz and biotite . The disintegrati on i s found to be rapid , as the j ointing or fracture occasioned by the cleavage planes of one mineral tends to continue into the others .

n — ns i R e l a ti o s hip . Granite approaches syenite by ins e o a ble gradati ns with decrease in quartz . It appro ches di orite with increa se in hornblende or bi otite and plagio clase . Intermediate v arieties are called granodiorites .

With increase in augite and plagioclase , granite ap roach e p s gabbro . — e " G o gr aphic a l D i s trib u ti o n . Granite occurs abun dantly along the Atlantic Coast from Virginia into Can ada . It is extensively quarried . The Quincy granite M - from Quincy , ass , is a well known bu ilding granite . M a In innesot , Wisconsin , and northern Michigan , and - a northward in Ontario , much of the Pre Cambri n crys talline area known geologically as the Laurentian High

a . a l nd is granite It is found widespre d in the West , a existing in the Bl ck H ills , in the Wasatch , the Rocky a and the Sierr Mountains . OPTICAL M INERALOGY AND PETROGRAPHY v An aly se s o f Gran ite s

Total

Sp . Gr .

1 n e om a s a o em a us a . . Gra it fr C rl b d , B h i , A tri

N an m av n a a e a . Gr ite fro B e o, L k e M ggi or , I t ly O O m n an te m a s ace e a . Gr i fro B rr , Lo wer Al , G r y Q m an a h n o e e om B rt o a e . A phib l gr itit fr g , Swed

I

U o ne a n om Lavellm e os es oun a ns ance . Pyr xe gr ite fr , V g M t i , F r

'J

O a a n m e as sac s e s . Alk li gr it e fro Chest r , M hu tt

Q u a an m K k a i a nn a A gite sod gr ite fro e equ b c L ke, Mi esot .

Di scus s ion o f An a ly s e s

1 . Granite contains more silica than any other plu tonic rock .

2 . a Alumin content is not as high as in the syenites .

It is present chiefly in feldspars and biotite .

3 . Iron content is generally low . It is present chiefly am in biotite , phiboles , pyroxenes , and possibly magnetite . 4 . Ma gnesia content is low, indicating an absence of a o- m ny ferr magnesian minerals . 5 . Lime content is low . It is present chiefly in a a a few acid pl gioclases , in mphiboles and pyroxenes . 6 . Potash predominates over soda , occurring in a and lkali feldspar biotite .

7 . a a Soda rarely predomin tes over pot sh . When it o ( a d es see An lysis albite is the chief feldspar . It a a h m rks a gradation tow rd t e diorites .

1 46 OPTICAL M INERALOGY AND PETROGRAPHY — a a re . Alkali Sy e n ite . The alk li syenites rare The Same three types occur in this group as occur in the alkali lime group , except that the dark minerals are alkali pyroxenes and amphiboles . Belonging to the alkali-lime group is a rock called a " i as monzonite , which grades over into the dior tes , it o contains both alkali feldspar and plagioclase . A r ck r o associated with the copper o e deposits of Butte , M n

a a tana , is a more cid rock of this type , called a qu rtz a monzonite . It covers an rea seventy miles by forty , and is known as the Bowlder Batholith .

The corundum syenites north Of Kingston , Ontario , of are alkali syenites composed pink orthoclase , and a greenish corundum which is used as an abrasive . of The colors the syenites are light, although usually a darker th n the granites . The crushing strength is greater .

An aly se s o f Sy en ite s 4 5

Tot al S p . Gr .

1 . ca n Mi sye ite ( alkali type ) from Tonsenoos nea r Chri s tiani a , , o N rway .

2 . ca n M s ye te ( alkal l me ty e fr m Gan enbach l ack i i i i p ) o g , B o es e man F r t , G r y . IGNEOUS ROCK TYPES — PLU TONIC ROCKS

m n a 3 . n en s en e a . H or bl de y ite from Bi ll , Pied o t , I t ly

4 . n e om B sk ven nea au v o a . Aug ite sy e it fr y o , r L r ik , N rw y m a o 5 . en e s e n o . Sy it fro Cu t r C ou ty , C lor d

n n n h n n a es en . 6 . H or bl e de sy e ite from Pl auensc en Gru d , e r Dr d

D i scu ss i o n o f An a ly se s

a to 1 . Silica content is lower than in gr n ites , due de crease Of quartz .

2 . Alumina content is higher than in granites , due to relative increase in feldspars .

3 . Iron , magnesia and lime contents are higher , due

- to increase in ferro magnesian minerals , chiefly horn blende .

4 to of . Alkali content is higher , due increase felds spars .

5 . The high water content is due to hydration of the secondary minerals .

6 . The specific gravity is higher in that of granites ,

- due to the increase in ferro magnesian minerals .

- 7 . The alkali lime syenites contain more lime and magnesia than the alkal i syenites .

D E C E NEPHELITE AN L U IT SYENITES . — M ine ral o gic al Co m p o s iti on . Alkali feldspar and nephelite or leucite .

e u re — T x t Granitoid , sometimes porphyritic .

ar e n D — e Ch act r a d i s tribu tio n . Nephelite and leucit syenites are white to smoky gray in color, and contain very few accessory minerals . When present , they usu ally a are biotite , egirite , an d an alkali amphibole called bar

kevikite.

These types are comparatively rare , occurring espe i l c a ly as dikes . They are known in North America at 1 48 OPTICAL M INERALOGY AND PETROGRAP HY

i l Montreal and D ung ammon ( Ontari o) , L tchfie d Red New M (Maine) , Hill ( Hampshire ) , Salem ( assa r ville New chusetts ) , Beeme s ( Jersey) , and near Little a - o Rock (Arkans s ) , in well known exp sures , though they have a widespread occurrence . Of economic importance is the occurrence of rare min a eral containing zircon ium , tant lum , titan ium , yttri um ,

a and cerium , l nthanum , terbium , other rare elements . In the nephelite-syenite pegmatites of southern Norway a 00 of bout 8 these rare minerals have been recorded . A corundif erous nephelite syenite is found in commercial quantities in Canada .

An aly se s o f N ephe lite and L e uc ite Sy en ite s 1

e e e s en e om o ans van a N ph lit y it fr Ditr , Tr yl i a , Hung ry .

e e e s en e om Beemersville usse oun New N ph lit y it fr , S x C ty , J ersey

e e e s en e om Litchfiel d N ph lit y it fr , M a i ne .

euc e s en e om a ne v n L it y it fr M g t C o e , A rka sas .

D i scu s s ion O f An aly s e s

1 . e o a Silica cont nt is l wer th n in the syenites , as neph has a elite silic , and the minerals which it replaces

have several per cent more .

OPTICAL M INERALOGY A ND PETROGRAPHY

An aly se s Of D io rite s

ne m o o e. ec c ea e o Pyrox e a phibole bi tite di rit El tri P k , Y ll w n s to e P a rk . a ca s ene m P f undersb r Qu rtz mi hyper th diorite fro e g , Tyrol . a s n e m Cam omai or a Mic hyper th e e diorit fro p , Portug l . m o e o e om Neunseestein a sace A phib l di rit fr B rr , Al .

o m c m n nnes A ugite di rite fro Ri h o d , Mi ota .

D i scu ss io n o f A n aly se s These analyses when compared with the analyses of granite Show that

1 . Iron , lime , magnesia and alumina contents are higher . 2 . Alkali content is lower.

3 . Soda predominates over potash .

4 . to f Silica is lower, due change o feldspar .

5 . The specific gravity is higher .

G O AND NO ABB R RITE FAMILY .

M n e a lo ca Co m o s n — i r gi l p iti o . Basic plagioclase and a usu lly a pyroxene .

e u e —G x t r . N T ranitoid ever porphyritic .

R e a on s — l ti hip . Gabbros grade by decrease in plagio clase to the more basic pyroxenites and peridoti tes . IGNEOUS ROCK TYPES — PLUTONIC ROCKS — V a ri e ti e s . E ssential to all , basic plagi oclase .

of 1 . Gabbro , with addition diallage .

of . 2 . Hornblende gabbro , with addition hornblende o 3 . Olivine gabbro , with addition f olivine and diallage .

4 . Norite , with addition of hypersthene , bronzite or enstatite .

5 . Olivine norite , with addition of hypersthene , bron zite or enstatite and olivine .

6 . Anorthosite , composed chiefly of labradorite . It metamor may contain a few dark minerals which , when o ph sed , cause the development of almandite garnets in considerable quantities . — f Co n c en tra tio n o f M a gne tite . The concentration o magnetite in many gabbroi d magmas took place during the process of solidification along the lower border of the magma . This concentration was effected by the early i crystallization of the magnetite from s olution , ts high specific gravity , convection currents , etc . Magnetite of this occurrence has been found in commercial quantities in the Adirondacks and in Lake and Cook counties of M . to northern innesota It is u sually titaniferous , due an intimate association with the mineral ilmenite . — Ni cke life r o u s Pyrrho tite . In the Sudbu ry district of

of Ontario , great quantities nickeliferous pyrrhotite and workable amounts of chalcopyrite are found in the norite .

They occur as magmatic segregations . The pyrrhotite is an important source of nickel .

In Lancaster County , Pennsylvania , nickeliferous pyrrhotite is Observed along the contact of a metamor h p osed basic i gneous rock called amphibolite . Platinum is believed to occur minutely disseminated of in rocks this type , the weathering of which has sup plied the placer deposits . OPTICAL M INERALOGY AND PETROGRAPHY

An aly s e s o f Gabb ro s a nd N o rite s

Tot al

Sp . Gr .

om on ac s New o . l . A northosite fr A dir d k , Y rk

om oun o e a mo e a an . 2 . Gabbro fr M t H p , B lti r , M ryl d

n a om an en o s o e us a . 3 . Olivi e g bbro fr L g l i , L w r A tri

n n e a o om u u nneso a . 4 . H or bl e d g bbr fr D l th , Mi t

m Monsino nea I orea e mon a . 5 . Norite fro , r , Pi d t , I t ly — Di scu ss io n o f A naly se s . The analyses compared with analyses of diorites show

1 . A lower silica content due to the absence of quartz and the decreasing basicity of the feldspars .

2 a . Higher alumina , lime , iron and m gnesia content . 4 High magnesia , as in Analysis , suggests olivine .

3 . Lower alkali content . 4 . Higher specific gravity .

S ES EXITE FA MILY .

M ne a o ca — Co m o s o n . i r l gi l p iti Basic plagioclase , with a of v rying amounts subordinate orthoclase . Nephelite or sodalite may be present . The dark minerals are au gite , biotite , and a brown amphibole called barkevikite .

Olivine and apatite sometimes occur . The plagioclase Iss usuall y labradorite , rarely andesine .

1 54 OPTICAL M INERALOGY AND PETROGRAPHY

' o o Malig nite was named by Laws n , fr m Puba Lake , ae t o Ontario . It contains chiefly girite , augite , bioti e , orth

a . clase , nephelite and tit nite

Di scu s s io n o f Analy s e s

. 1 . Chemical resemblance to essexite

S . 2 . Low ilica

e . 3 . Equal lime and mag nesia cont nt

M . 4 . agnesia higher than in essexite

A na ly s e s

s e e m a m c n E s xit fro S l e R o k , nea r B osto .

e a e om a ns a e a n a ns n ana Th r lit fr M rti d l , Cr zy M ou t i , M o t .

on n e om ua u e oo n a n Mon Sh ki it fr Sq re B tt , Highw d M ou t i s , n t a a .

Mal i nite om u a a e a n a n g fr P b L k , R i y L ke Di s tri ct , O ta rio .

o e om Iwaa ra n a n Ij lit fr , Fi l d .

ssou e om on n ee n a ns Mon Mi rit fr Sh ki Cr k , Highwoo d M ou t i , n ta a .

JO AND M I LITE ISSOU RITE .

M ne a o c a Co m o — i r l gi l p s itio n . Ij olite contains wg i rite a o a and ugite and nephelite , ften with patite , titanite , as andradite accessories . It is nonfeldspathic . IGNEOUS ROCK TYPES — PLU TONIC ROCKS

M o o a o and iss urite c ntains ugite , leucite , livine , biotite with accessories . It is nonfeldspathic . — R e l a ti o n ship . These rocks are end products of the series beginning with essexite . They are closely related to the theralite-shonkinite rocks and are distinguished from them by the fact that they contain no feldspars . Ij olite was first found on Mount Iwaara in northern

Finland . D PE RI O TITE FA MI LY .

M n e — a o c a C m s o n . i r l gi l o p o iti Olivine with pyroxenes ,

or amphiboles , biotite . No feldspars are present .

R e a n h — l ti o s ip . The peridotites gra de from Olivine gabbros by the elimination of the feldspars . They are o f und on the edges of gabbro and norite bosses . They are regarded as ultra basic . — Cla ss ific a tio n . Olivine is essential in all varieties .

1 . Sherz olite of , by addition diopside and enstatite . 2 H r r i . a bu g te, by addition Of enstatite .

3 . Wehrlite , by addition of diallage and hornblende .

4 or landi . C t te by addition of hornblende .

5 . D unite chiefly Olivine . It may contain chromite or " chrome spine . 6 . Kimberlite , which was named from its occurrence o in Kimberley , South Africa , is a peridotite f und in the truncated cones of extinct volcanoes . In its type local " ity it weathers to a soft serpentine rock called blue " ground . It is the mother rock of the diamond . A s im a ilar rock has been found in southern Ark nsas , where diamonds are likewise found in commercial quantities . Small diamonds have been found in a peridotite rock in

C . E lliot ounty , Kentucky

Garn ierite , the chief ore of nickel , is a s econdary min eral associated with serpentinized peridotite , probably as 1 56 OPTICAL M INERALOGY AND PETROGRAP HY

- an alteration of a nickel bearing olivine . The French locality of New Caledoni a is the only important locality .

An aly se s

Total S p . Gr .

1 e o om S eb rei z heim a en e m an . . Amphibol perid tite fr , B d , G r y man 2 a o m a e a a u e . . Mi c perid otite fr K lt s Th l , H rzb rg , G r y n m a m a an 3 . Pyrox e ite fro B lti ore, M ryl d .

n a o n ana 4 . Pyrox e ite from M e dowbro k , M o t .

Di scus s io n o f An aly se s

1 . Lower silica and alumina content than in gabbro , o due t the absence of feldspars . 2 . Iron content varies , depending upon the dark min eral present .

a 3 . M gnesia content higher than in any other normal plutonic rock .

4 . Lime content varies .

5 . Alkali content less than that of any other igneous

' rock . In a mica peridotite , potash predominates over

- soda , an unusual case among alkali lime rocks .

6 . Specific gravity highest of the normal plutonic rocks .

1 58 OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTE R 1 0 .

IGN E O U S R O CK T YPES .

Vo l can ic R oc ks .

THE RHYO LITE FA M ILY .

— o a o M in e r al o gic a l C o m p o s itio n . O rth cl se , olig clase , t a re o . quartz . Biotite , h rnblende The rhyoli es chem a i cally the equivalents of the granites , particul rly the

- alkali lime ty pe .

— a T e x tu r e . Well devel oped groundm ss , often largely glassy . Frequently porphyritic . — R e l ati o n ship . Rhyolite grades imperceptibly into o trachyte , granite , and dacite . Unless quartz is rec g n to ized , a microscopic examination is n ecessary dis in i h t t g u s rhyolite . It may easily be confused with daci e unless the polysynthetic twinning of the feldspar ch ar acteri s i t c of dacite can be seen . — Cha r ac te r . The term rhyolite comes from the " G rhei n flow reek verb , to , because of the fl ow structure frequently Observed . Lip arite is a synonymous term a used l rgely in Europe . It was named from the Lipari a Isl nds , in Sicily . Quartz porphyry is a term Often applied to the rhyolites which h ave crystallized as e a intruded she ts , l ccoliths , dikes , and sills . The glassy portion is ch aracterized by its behavi or between crossed o a nic ls , rem ining dark during a c omplete revolution of the stage . IGNEOU S ROCK TYPES VOLCANIC ROCKS

The processes of weathering Of the rhyolites are the same as take place in granites , ordinary decomposition by atmospheric agencies giving rise to the formation of the hydrou s aluminum silicates . Metamorphic processes develop schistose textures leading in extreme cases to the development of sericite schists .

Early in the study of volcan ic rocks it was customary to distinguish two types those which had erupted pre vio s to r u Te tiary times , and those which had erupted after a o Terti ry times . The f rmer were called Pale ovolcanic , N and the latter were called eovolcanic . Fortunately , this classi fication did not survive . — Cla s s ifica tio n . Rhyolites are regarded by some writ ers as porphyritic rocks with phenocrysts of quartz an d alkali feldspars in a groundmass which is wholly glassy or a very finely crystalline aggregate of quartz and feld " " v l spar . They classify in the glasses all varieties of o canic rocks in which chilling has prevented crys

l za i n tal i t o .

The classification here adopted combines the glasses with the rhyolites .

a Volcanic glasses are obs idi n , pumice , pitchstone , and perlite .

OBSIDIAN is a dense , homogeneous glass with a low percentage of water . PU M ICE is a cellular glass formed by the expansion of the cooling magma by the escaping steam bubbles . It - is light , very porous , and may resemble blast furnace slag .

PITCH STONE is essentially the same as Obsidian , with a higher percentage of water . It i s more resinou s in

or . appearance , giving it a greasy pitchy luster OPTICAL M INERALOGY AND PETROGRAPHY

PERLITE is a pitchstone which has a spheroidal

to arrangement of the particles , giving rise a rounded fracture .

— o P an te lle rite . Pantellerite is a v lcanic rock corre

c sponding to the alkali granites . It is rare , and o cu rs n ell r S O far as known only on the island of P a t e ea , in the

a Mediterranean Sea . It conta ins a r re feldspar called a of northoclase , which is an isomorphous mixture albite and orthoclase . — D i s tr ibu tio n Rhyolites are widespread throughout

the Western States . Obsidian Cliff in Yellowstone Park , C e N w M Silver liff in Utah , extinct volcan o s in e exico ,

a M a and a M a Ut h , ontan C lifornia ( ono L ke) , are well

a known examples . In Leadville , Color do , they are asso ciate ore d with the deposits .

Along the Eastern Coast , remnants of rhyolite lavas from ancient Pre-Cambrian volcanoes have b een found New M M P nn l in B runswick , aine , assachusetts , and e sy vania .

An aly se s

1 62 OPTICAL M INERALOGY AND P ETROGRAPHY

R and the Auvergne , along the hine , in the Azores , in the

C M a . Black Hills , in Custer County, olorado , and in ontan

An aly se s o f T r achy te s (See un de r Phono lite s ) . — D iscus s io n o f An aly se s . Compared with rhyolites , the trachytes show :

i . 1 . Lower S lica , due to decrease in quartz

a a 2 . Higher alumina , m gnesi , lime and iron , due to - increase in ferro magnesian minerals .

a o o . 3 . Higher alk lies , p tash usually pred minating

M THE PH ONO LITE FA ILY .

— na a M ine r al o gi ca l Co m p o s iti o n . Sa dine nd nephelite

a of or leucite . Aegirite . Occasion lly members the soda lite group . Garnet as an accessory .

— a o e T e x tu r e . The groundm ss is crystalline , s metim s porphyritic , rarely glassy . — R e l a ti on ship . Phonolite grades into trachyte with

a a a decre se in sod . The two types are closely a ssoci ted .

- f Charac te r . Phonolite is a translation into Greek o " " a G o Kl n s tein or erman w rd i g , cluck stone , so named because certain phonolites with a pron ounced horizont al

o j ointing when hit give forth a metallic sound . The r ck has a a greasy appe rance , due to the presence of nephe lite . Nephelite if identified serves at once to distinguish o ph nolite from other volcanic rocks .

r f re Leucite phonolites a e rare . Leucite may and o quently d es occur with nephelite in the typical phonolite . Concentrically arranged inclusions of m agnetite specks occur in the leucite .

The pyroxenes are more common th an in any other

o a o - v lc nic r ck . They are usually aegirite augite or aegirite - a In long tufted , r gged , bright green prisms . The acces IGNEOUS ROCK TYPES VOLCANIC ROCKS

sory minerals sodalite , brown garnet , and titanite are in themselves characteristic .

Phonolites are relatively not common . They occu r in ’ dikes , sheets and isolated buttes ( D evil s Tower) in the Black H ills and in the Cripple Creek mining districts of o Colorado , where they are ass ciated with purple fluorite r and calaverite in the o e bodies . The phonolite magmas being rich i n alkalies may have had a solvent effect upon o the g ld , thus accounting for the present association .

In Germany , phonolites occur in great masses as vol canic r necks o plugs in southern Baden , near the Swiss border . Many old castles have been erected on the summits .

n Kilimanj aro , o e of the v olcanoes which has recently been active , is said to have given forth phonolite lavas .

A n al y s e s o f T r achy te s a n d Pho no lite s

TRA CHYTE S . PH ONOLI TE S . 1 2 3 1 2 3

. 70

. 40 . 22 . 6 3

30 . 08 . 30 .39 .

. 96

. 20 . 85

TRACHYTE S .

a om ve ne ance . 1 . Tr chy te fr A u rg , F r

s ene ac e om uscan a . 2 . B iotite hyper th tr hyt fr T y , I t ly m ame e us e oun o a 3 . Trachyte fro G Ridg , C t r C ty , C ol r do. OPTICAL M INERALOGY AND PETROGRAP HY

PH ONO LI TE S .

m a n om c ss e e o e . 1 . Pho olite fr S hlo b rg, T plitz , B h i

n om Mi aune ance. 2 . Pho olite fr , F r " " ’ n om ev s o e ac s W om . 3 . Ph onolite fr D il T w r , Bl k Hill , y i g — Di scus s io n o f A n aly se s . Compared with trachytes , the phonolites Show :

of 1 . Lower silica due to the substitution nephelite

f or sanadine.

a . 2 . Higher alumina and lkalies

a to of 3 . Lower iron , m gnesia and lime , due absence iri e dark minerals . In case a g t is present , soda and iron are increased . t 4 . Traces of chlorine and sulphu r are due o the pres o o ence of members of the s dalite gr up .

A D D M THE D A CIT E N AN ESITE FA ILY . — M ine r al o gical Co m p o s iti o n . Acid plagioclase ; biotite ,

o a hornblende, augite , di pside ; magnetite , ap tite , zircon as common accessories . Quartz is present in dacite and absent in andesite . T — e xtu r e . Groundmass present as gl ass or as an inti mate mixture Of minute indistinguishable feldspars , " which may be described as a pepper and salt texture . Plagioclase feldspa r shows irregular outlines with zonal a of arr ngement inclusions frequent .

Cha ac e — r t r . This group is the volcanic equ ivalent of o the quartz diorite and di rite group . The dacites are not common . They were named from an old Roman province of D a acia , now part of Hungary . Andesites derived their name from the abundance Of lava of this type in M the Andes ountains .

D ifferentiation from other volcanic rocks may be a " b sed upon the pecul iar pepper and salt texture . D a

1 6 6 OPTICAL M INERALOGY AND PETROGRAPHY portion of the rock consists of prominent augite and Oliv o ine crystals with good outline , and with plagi clase poorly of developed in small crystals . The pale buff color the augite phenocrysts is characteristic . The texture of diabase is intermediate between that of gabbro and of basalt . It is essentially an intru sive basalt, entirely crystalline and granitoi d . The plagio clase crystals are idiomorphic , occurring in long, lath shaped crystals which lie in all positions . The interstices are filled with allotriomorphic crysta ls of augi te and mag " " netite. This texture is called ophitic . It is an impor tant microscopic criterion for the identification of diabase .

D iabase is variously classified , sometimes as a pluton ic rock and sometimes as a volcanic rock . Since it grades into porphyritic forms at the contacts and since it is really volcanic in its nature , occu rring in sheets or d ikes of limited thickness close to the surface , it is considered here with the basalts .

Basalts are volcanic equivalents Of the gabbros . They to l are difficult c assify in the field , as they are all heavy , black gray or brown rocks f or which a common and u se " " f ul e field term dolerit has been applied . The term dia " G e di abai ne n base originated from the re k verb i , to pen " etrate . Trap is a common field synonym appli ed to s of rock diabasic texture .

Clas s ifica i n - S t o . A imple classification of the basalt family which meets all field requirements is 1 . B asalt . 2 - . Olivine basalt .

3 . D iabase . - 4 . Olivine diabase .

D s bu n — i tri tio . Basalts are abundant particularly along the Atlantic seacoast where diabase has intru ded Triassic IGNEOU S ROC K TYPES VOLCANIC ROCKS 1 6 7

shales . It has formed prominent landmarks , such as the w Palisades of the Hudson , E ast and West Rock near Ne N of Haven , and D eep River , orth Carolina . Thousands feet of basalt of Pre -Cambrian age are found on Kewee naw Point . Native copper , secondarily precipitated in the amygdaloidal cavities of these flows , is an important ore . The Columbia Plateau and the D eccan Plateau fur nish the two greatest examples of basaltic extrus ion .

The lavas from many volcanoes are chiefly basaltic . Among these are Kilauea and Mauna Loa in the Hawaiian sands I , Mount Etna , and various volcanoes in Iceland .

n a s e s o f D ac e An A ly it , d e s ite , B a s a lt an d 1 2 3

. 90

. 40 40

’ e assen a o n a D acit from L s P eak , C lif r i .

e ene an es e om oun as a a o n a Hyp rs th d it fr M t Sh t , C lif r i . n s e m m o a o A ugite a d e it fro Chi b r zo, M exi c . a n nn Di ab ase from New H ve , C o ecticut . va om h orsa ce an B a salt l a fr T j , I l d . a om if ak sc s an n n I ron b as lt fr N , D i o I l d , Gree la d . — D i scu ss io n o f B asa lt A naly se s . When compared with andesites , the analyses show :

1 . Lower S ilica .

2 . Higher alumina . OPTICAL M INERALOGY AND PETROGRAPHY

a 3 . Lower alkalies , all due to the increasing b sicity of the feldspar .

a 4 . Higher iron , m gnesia and lime , due to the addi tion of dark minerals .

. 5 . Sod a predominates over potash

TRACHY D O LERITES .

— a M ine r al o gic al Co m p o s itio n . B sic plagioclase and o alkali feldspar ; pyroxene ; members of the s dal ite group , olivine , and h ornblende . — T e x tu r e Often porphyritic , with phenocrysts Of

a basic plagiocl se .

— rach oleri R e l ati on ship . T yd tes are the volcanic equiv r alents of essexites . They a e intermediate between alkali trachytes and phonolites on one side and tephrites on the other .

An — aly s e s (Se e n e x t tabl e ) . The analyses Show low

S ilica , high alumina , iron , mag nesi a , lime , and the alkalies . A D TEPH RITES N B ASANITES .

M nera c — i l o gi al Co m p o s itio n . Basic plagioclase and

either nephelite or leucite . Augite is common . B asanite o a c nt ins olivine and tephrite does not .

R e a o n — l ti ship . These rocks are the volcanic eq uiva sho lents of the theralites and the nkinites .

Clas s ifi a i — c t o n . Basic plagioclase is common to all a v rieties .

1 . Leucite tephrite , with addition of leucite and au gite .

2 . a Leucite basanite , with ddition of leucite , augite and olivine .

3 . N ephelite tephrite , with addition of nephelite and a ugite .

1 70 OPTICAL M INERALOGY AND PETROGRAPHY regarded as the volcanic equivalents of peridotite and hornblendite . — An aly se s . Chemically they show a decrease in the

. alkalies , due to the absence of the feldspathoids Lim a burg ite shows an increase in magnesi , due to the pres ence of olivine .

a a rra an f ne . E Tr chyd ol erite from Ch j o , I sl d o Te riffe rup n tio of 1 798 .

2 . euc e e e om Atre de Cava o es v us . E ru L it t phrit fr o l ll , V u i p

n M a 5 . tio of y, 1 85

3 e e e e m av s am on a n n a a e . N ph lite t phrit fro l a tre S A t o, C p

a n Verde I sl d s .

euc e om a o de ova V i a a om L itit fr C p B , Appi , R e. e e n e om San nta o a n s N ph li it fr A , C p e Verde I sl a d . m u e om m u a s n a n a en Li b rgit fr Li b rg, K i ser tuhl M ou t i s , B d , n G erma y .

7 . u e om u e nea s n n A gitit fr H tb rg, r T et che , Germa y .

O C O K PY R LASTI C R C S .

Pyroclastic rocks are those made up of fragmental a volcanic deposits , usu lly more or less stratified by trans ortation p through the air or under water .

Cla ifi — ss c atio n . 1 . Volcanic agglomerate consists of the large sized volcanic products of a fragmental nature

which have been deposited near the crater . IGNEOUS ROC K TYPES VOLCANIC ROCKS 1 7 1

2 . Volcanic brecci a consists of angular fragmental products which have been firmly cemented together .

3 . Tuff is a deposit of volcanic ash or dust consoli o dated by cementation . In the historical e ruption f 9 AD Vesuvius , in 7 , Pompeii was buried in ash and Her n culaneum was buried in tuff . Excavation has gone o rapidly in Pompeii , as the loose ash offers less resistance to removal than does the tuff which covered Herculaneum . OPTICAL M INERALOGY AND PETROGRAPHY

CHAPTE R 1 1 . E M R H SEDIM EN TAR Y AN D M T A O P IC R O CKS .

Sedimentary rocks are of secondary origin in that they are formed from previ ously existing rocks which m a or metamor y have been either igneous , sedimenta ry , h c r ic. o p They may be me hanically chemically deposited , r either on land o under water . The agents of deposi r tion a e water , wind , and ice . By the weight of overlying strata and through the

of r agency siliceous , calcareous o ferruginous cement , they become consolidated from a l oose aggregate to a

Solid mass . C — N 1 . LASSIFICATIO Sediments of mech anical origin .

A . Water deposits .

a . Conglomerate .

b . Breccia .

c . Sandstone .

d . Shales .

B . Wind deposits .

a . Loess .

b . Sand dunes . 2 . Sediments of chemical origin formed from solution .

. C A oncentration .

a . Sulphates . G ypsum .

Anhydrite .

1 74 OPTICAL M INERALOGY AND PETROGRAP HY

an of 3 . Volcanic breccia is formed by accumulation angular fragments ej ected by volcanic action and later solidified . — San d s to n e Sandstone is composed of sand grains which have been rounded by water action and separated by the classifying action of moving water to deposits Of o uniform texture . Quartz is the essential c nstituent , m although i purities are always present , such as feldspar , a mica , garnet , m gnetite , etc . — CLASSIFICATION . According to the character of the cement :

1 . Silice ous s andstone . 2 . Ferruginous sandstone .

3 . Calcareous sandstone . 4 . Argillaceous sandstone .

According to mineral content

1 . Arkose , conta ining much feldspar: 2 a a o - . Gr yw cke , c nta ining ferro magnesian min r l e a s . M 3 . a icaceous s ndstone , etc . — Shal e Sh ale is a rock consisting of the finer mate a a ri l , usually cl yey , deposited beyond the zone of depo iti s on . a of the sandstone It cont ins compacted clays , and muds silts , which possess a finely stratified structure a c lled bedding . C — . o LASSIFICATION Acc rding to composition .

1 . o Argillace us Shales .

2 . Arenaceous Shales .

3 . o Ferrugin us Shales .

4 . C arbonaceous Shales .

Shales form about 87 per cent of the sedimentary o , a o 8 r cks s ndst nes about per cent , and limestones about 5 per cent . SEDIMENTARY AND METAMORP HIC ROCKS 1 75

— - o e s s . L Loess is a fine , homogeneous , clay like sub l f stance , largely siliceous , which lacks a l semblance o f stratification , and when eroded forms precipitous clif s .

It contains angular quartz , mica flakes , clayey material , with ofte n high percentages of calcium carbonate .

- o a Loess i s believed to be a wind blown deposit , pr b bly assisted in certain localities by aqueou s deposition . It is used in the West for brick manufacture .

Adobe clay is a form of loess abundant in the arid o southwestern porti n of the United States . It is used in - the manufacture of sun dried brick f or adobe houses .

— - S an d D un e s . Sand dunes are formed by wind blown sand , which always exhibits a characteristic shape with S on its long , gentle lope the windward side , up which the sand grains a re blown , and with a steeper slope on the leeward side , which i s the angle of repose for s and grains . n Sand dunes show stratification a d ripple marks .

The m i gration of san d dunes has been known to create " " r havoc in certain parts of the cout y . They are fixed by transplanting with beach grass and sand hedges . One of the ra ilroads temporarily checked the progress of some advancing sand dunes by spraying them with crude petroleum .

Se dim en ts o f Che m ic al O rigin . — Gyp sum a nd Anhy drite . Gypsum and anhydrite , the of hydrous and the anhydrous sulphates lime , occur inter i er r ti e a bedded or in irregular masses , nt st a fi d with cl ys ,

S hales , sandstones , and limestones , or with hal ite .

They originate from concentration of oceanic waters by evaporati on , and in inland lakes in which the evapora tion equals or exceeds the amount of inflow .

Anhydrite changes to gypsum by normal hydration , due to exposure . A tunnel in E urope which was driven 1 76 OPTICAL M INERALOGY AND PETROGRAPHY through anhydrite was thrown out of alignment by the volume increase produced by this change .

— a Halite Halite occurs in massive , granul r form ,

a o . interstratified with clays , marl and s ndst ne It is espe o cially associated with gypsum , anhydrite and dol mite .

f tra sf ord G . The deposits o S s , ermany , are feet thick It is here associated with the chlorides and sulph ates of potassium and magnesium .

Flin t — Flint is a cryptocrystalline variety of silica

a occurring as a hard , grayish to bl ckish rock , its color as being due to carbonaceous matter . It occurs nodules or lenses in limestones . It is used for road material , and in tube and ball mills .

Se dim e n ts o f O rgan i c O rigin . — Lim e s to n e Limestone is a widely distributed cal cium carbonate rock containing impurities of magnesia ,

S a a o a . ilic , cl y , iron , and rganic m tter It is quite soluble , and a llows the formation Of sink holes , caves , solution f o cavities , etc . Buildings erected o limest ne thi rty or forty years ago Often show the effect of weathering by

pitted and etched surfaces . Limestone forms by chemical precipitation and o of thr ugh the agency animals and plants . C — LASSIFICATION . The classification of limestone is o based up n composition , texture , and uses . It has a wide a Of o r nge ccurrence .

1 . Ca lcic limestone is chiefly calcium carbonate .

2 . Dolomite refers usually to any magn esian rich limestone .

- 3 . C a o fine a com h lk is a soft , p rous , gr ined variety of posed minute shells of foramin ifera . 4 . Hydraulic limestone is a clayey limestone used in

cement manufacture .

OPTICAL M INERALOGY AND PETROGRAP HY to the composition of the rocks from which they are

derived . In so far as this is true , chemical analysis is an important criterion f or discriminating between meta morphosed sedimentary and metamorphosed igneous rocks . Frequently there is addition or subtraction of con stituents accompanying metamorphism which renders more difficult the interpretation of the origin of the meta morphic rock .

Mineral composition m ay or may not be the same in re the metamorphic rock as it was in the original rock . F quently metamorphism is accomplished by a granulation and rotation of the original particles . In the greater number of cases , there is a development of platy minerals which are best adapted to withstand conditions of higher pressures and temperatures . In these minerals the mutual parallelism of the greatest , mean and least dimensional r axes causes a more o less perfect cleavage in one plane , i which is called schi stoc ty . The average ratio of the a f gre test to the mean dimensions of mica is 1 0 1 , o o 4 1 n h rnblende , a d of quartz and feldspar 1 .

A metamorphic rock contains a higher percentage of the minerals mica and hornblende than the original rock . r For example , shale may contain no mica . By metamo

hism a o p , mic schist is devel ped , containing over 50 per of cent mica . NO change in chemical composition has a a t ken pl ce . Obviously the mica was developed by a recrystallization of the constituents originally contained roc m in the k ass .

Minerals which are characteristic of metamorphic o are o r cks staur lite , cyanite , sillimanite , zoisite , chlorite ,

a . a t lc , etc Quartz , feldsp r , mica , pyroxene , and amphi o o to o nd b le are comm n b th igneous a metamorphic rocks . SEDI MENTARY AND METAMORP H IC ROCKS 1 79

C riter i a fo r the D i scrim in a tio n o f M e tamo rpho se d Ign e ou s and M e tam o rp ho s e d Se dim e n tar y R o cks :

1 . Mineralogical composition . The minerals which are strongly indicative of a sedi menta ry origin of the metamorphic rocks in which they occur are staurolite , andalusite , sillimanite , cyanite . They all conta in higher percentages of alumina th an those

a found in igneous rocks , and as alumina is lmost insoluble there is practically no possibility of an addition of alu mina from other sources .

2 . Original textures and structures . n If ot too severely metamorphosed , sedimentary rocks S - or may how remnants of bedding , fossils , cross bedding, other features . Igneous rocks may show amygdaloidal cavities , flow structu re , etc .

3 . Field relationships . Areal distribution and association of the metamor ho d p se rock with surrounding rocks may give some clue . By tracing the metamorphosed rock laterally along the t f strike , one may come o a less metamorphosed portion o the rock which still shows original sedimentary textures or structures .

4 . Chemical composition .

a . D ominance of magnesia over l ime is indicative of sedimentary origin .

b . D ominance of potash over sod a is suggestive of sedimentary origin .

c . The presence of several per cent of alumina over the 1 1 ratio necessary to satisfy the lime and alkalies of is suggestive sedimentary origin .

d . A high silica content is suggestive Of sedimentary origin if supported by other criteria . OPTICAL M INERALOGY AND PETROGRAPHY

CLASSIFICATION —The classification of metamorphic rocks is based upon composition , texture , and structure

1 . Gneisses .

2 . Schists .

3 . Quartzites .

4 . Slates and phyllites .

5 . Marbles .

6 . Ophicalcite , serp entine , and soapstone .

T ABLE OF S EDI MENTARY R OCK S AND TH EI R O M E TA M RPH I C E QUI VA LE NT S . Cons oli da ted M etamorp hic Loos e S ediments Rock Rock

G . C ravel onglomerate . Gneiss , sch ist .

a . S nd Sandstone . Quartzite .

. a Silt and clay Sh le . Slate , phyllite .

. M Lime deposits Limestone . arble .

TA BLE O F I GNE O U S RO CK S AND TH EI R

METAM O RPH I C EQUI VALENT S .

I ne g ous Rocks . e M tamorp hi c Rocks . Coarse-grained feldspathic

rocks , as granite , syenite ,

Gneiss . etc . F i n e-grained feldspathic G f neiss , schist . rocks , as tuf , etc . o Basic igneous r cks , as ' Sch 1St diorite , basalt.

Gne - s s . A a i gneiss is banded metamorphi c rock , of i either gneous or sedimentary origin , in which the a b nds are mineralogically unlike , consisting chiefly of quartz and feldspar, with or without the parallel dimen sioual arrangement necessary for rock cleavage . G o a a o neisses are devel ped by gr nulation , r tation and a i f recryst ll zation o the original minerals rather than by

OPTICAL M INERALOGY AND PETROGRAP HY

are found along the foliation planes . These planes prob ably represent original bedding planes i n the sandstone .

Quartzite is used to advantage as a building stone , although its extreme hardness is found to be a handicap both in quarrying and in dressing . — Sl ate an d Phyllite Slate is a dense , thinly cleavable , homogeneous rock , whose cleavage pieces are mineral so og ically unlike , and whose mineral grains are small in size as not to be distingu ished by the eye . This cleavage is not to be confused with original bedding planes . It is a secondary structure produced in the development of the secondary minerals . Slates are composed of the finest particles of mineral matter which are carried in suspension and deposited considerable distances from shore . Volcanic ash and tuff more rarely give rise to slate deposits . Phyllite is the next step in the metamorphism of a slate , intermediate between slate and mica or sericite schist . Quartz and mica are the essential minerals .

M a rbl e - Marble is the metamorphic equivalent of a limestone or a dolomite . It is completely recrystallized , and when pure shows the development of large rhombic calcite crystals or fine sparkling surfaces .

r Few original limestones are pure . The metam o

hism o p of an impure limest ne containing silica , clayey material , iron oxides and carbonaceous matter is charac teriz ed not only by the recrystallization of calcium car bonate but by the development of variou s secondary sili

a o a o cates , p rticularly biotite , w ll st nite , diopside , tremo lite , actinolite , grossularite , and hornblende . At least seventy secondary minerals have been found to exist in metamorphosed limestones .

When pure , marble is massive , and shows no indica SEDIM ENTARY AND METAMORP H IC ROCKS 1 83

ti on of a schistose structure . All traces of fossils and original structures are obliterated . — O phic alc ite Ophicalcite is a variety of marble asso ciated with streaks and spots of serpentine . Verde antique is a name more popularly used . It results from the metamorphism of an ori ginally impure limestone to a calcite -S ilicate rock in which the S ilicates were later altered by hydration to serpentine . Ophicalcite is valu r able f o decorative purposes , as it takes an easy polish .

It occurs in quantities in Quebec , in the Green Mountains , and in the Adirondacks .

- S e rp e n tin e . Serpentine rock consists essentially of S the mineral serpentine , a hydrous magnesium ilicate , in l association with o ivine , pyroxene , hornblende , mag netite , chromite and the carbonates . Garnets and micas are common accessories .

Serpentine is derived by metamorphism of igneous or other metamorphic rocks which are essentially com of posed magnesium silicates , as olivine , pyroxene , or hornblende . Such rocks are basic igneous rock an d horn blende schist .

Serpentine occurs in the crystalline area of eastern i United States , n eastern Canada , and in a few of the western coast States , but seldom in large masses . It is r used as an o namental stone and as a sou rce of asbestos . — So ap s to ne Soapstone is essentially the mineral talc . l It becomes a ta e schist by taking on a foliated structure .

Impu rities are mica , chlorite , tremolite , enstatite , mag i net te, quartz , and pyrite . Soapstone has a similar origin to serpentine as a sec ondary product from the magnesium silicates . It is found in association with talcose and chloritic rocks in crystal line areas . OPTICAL M INERALOGY AND PETROGRAP HY

Soapstone is mined extensively in Virgini a . The rock

o o has many uses . It g es int the manufacture of tubs , r - switchboards , insulators , sinks , stoves , fi e brick and lubricants .

OPTICAL M INERALOGY AND PETROGRAPHY

? a or o b . Are cont cts conformable unc nformable

c . In case of intrusive bodies , are there large dikes

or small masses and few in number, or are they small and widely distributed through the intruded formations ? ? d . Are the rocks much altered

o a ? e . Is metam rphism prominent feature

3 . Collect specimens of the different formations , giv ing locations as closely as possible . 4 . Note roads , tra ils , water , and possible camping places . — B . GEOLOGICAL MAPPING GENERAL . 1 . Locate boundaries between formations .

a . Simple boundaries . i Take d p and strike . D oes boundary indicate conformable or uncon formable contact ? Are there evidences of faulting ?

b . Obscure boundaries .

f or Look fragmental traces of the formations . Work up hill and locate the highest points at which fragments of the lower formation

appear . Note whether scarps or change of slope are e connect d with the boundary .

. C c omplicated boundaries .

Intrusive boundaries . Ma p carefully dikes and arms . Note alteration and metamorphism in the

neighborhood of the boundary . Note variations in texture of the igneous

rock in approaching the boundary . SUGGESTIONS FOR GEOLOGICAL WORK 1 87

B oundaries Sh owing contact metamorphism . Map the general relations Of the m etamor i ph c patches . Note the metamorphic minerals and their

succession . Note presence and ass ociation of ore min

erals .

2 . Work within the boundaries of a formation . Trav

ersing .

a . In areas of sedimentary rocks .

Strike and dip of beds .

Color, thickness and general character of beds .

Minerals composing the rocks ; nature Of the grains or fragments (angu lar or rounded )

cementing material . In conglomerates l ook f or recog nizable fragments of earlier

formations .

Presence Of fossils .

Areas of alteration . of Areas metamorphism . Systems of folds— minor folding directi on and — pitches of axes of folds relations of fold

ing to faulting .

b . In areas of igneous rocks . R ock texture and variations in texture . o Variations in comp sition .

Segregations .

Inclusions of other rocks . D ip and strike of schistosity or gneissoid

structure .

Flow structure .

c . In areas of metamorphic rocks . Is rock of sedimentary origin ? D oes it Show traces of bedding ? 1 88 OPTICAL M INERALOGY AND PETROGRAPHY

a su es Are gneissoid lamin e continuous , g g tive of sheared beds ? Are the grains rounded or angular in out line ? What are their relative sizes ? Are minerals such as to suggest erosion or metamorphic processes ? Is the rock of igneous origin ? Are the minerals typical of igneous rocks ?

Are the gneissoid laminae noncontinu ous , suggestive of sheared minerals ?

Are the changes suggestive of dynamic action , chemical action or both ? Is the rock texture suggestive of folding and Shearing ? D oes it suggest an impregnation and meta

morphism by replacement process , due to action of solutions ? Is the rock widely different in structure and composition from the original typ e ? — C. M GEOLOGICAL APPING DETAILED WORK .

1 . Faults .

a . Strike and dip .

- b . Evidences of movement slickensides , striae

(their direction and dip) , gouge , drag , etc .

c . Cross fracturing.

2 r . Veins and other o e bodies .

a . Strike and dip .

b . General character of mineralization .

Strong or weak . e Oxidiz d or unoxidized vein mate rial .

. M c inerals and groups of minerals .

(1. R o elative age f minerals .

1 90 OPTICAL M INERALOGY AND PETROGRAPHY

b . Included minerals are Older than the ones which

include them .

o c . Minerals abutting with ut crystal boundaries

are of the same age approximately .

re of . d . Intergrown minerals a the same age

e . Minerals cutting others are younger than those

they cut .

Alteration and metamorphism .

a . D egree of change , extent to which original min

r l e a s are changed .

a b . Ch racter of the change , secondary minerals

due to alteration , metamorphic minerals .

B . SEDIMENTARY ROCKS .

1 a . Rel tive sizes and shapes of the component par i le t c s .

. t la a Unassor ed material , rge and small frag ments

together, imply that the source of the materi al is near at hand or that the transporting agent

is very powerful .

. b Angular grains , fresh in appearance , indicate

disintegration without decomposition , and o little m vement from the source .

0 . R a ounded gr ins , fresh , imply disintegration of and transportation the material .

. d Sorted material , where the grains are S imilar a in mineralogical char cter, indicates that the deposits were made some distance from the o or s urce , the original rock disintegrated and a we thered also , differentiating the more resist

ing minerals . SUGGESTIONS FOR GEOLOGICAL WORK 1 9 1

2 . Look for fragments which give some clue as to the source from which the sedimenta ry material is

derived .

of 3 . D etermine the character and probable origin

secondary cementing material .

C. WITH META MORP H IC ROCK TRY To D ETERM INE

1 . The nature of the original constituents and the

original rock . Look f or traces of original min r l e a s in form or cleavage .

2 . The nature of the alteration . D n — a . y amic folding, shearing, etc . , distortion of r c ystals or fragments . — b . Chemical change older minerals partially dis

solved by later ones .

1 94 OPTICAL M INERALOGY AND PETROGRAPHY

89 1 07 E ns ta tite 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Biotite 1 1 0 40 44 48 50 6 0 E dote 0 0 0 0 0 0 0 0 Birefri ngence , , , , pi 49 ssen a m ne a s 1 24 Bi sectrix E ti l i r l 1 73 E ssex te 1 52 Breccia i 4 1 1 1 1 9 n . 42 6 8 E xtinction a gl e , , , 1 22 42 Extraord na ry ra . 8 44 8 0 i y C al cite , us ve fl ow 0 0 0 0 0 0 0 0 0 0 0 1 27 36 42 Extr i C a nad a b al s am , 4 C arl sb a d 1 1

F a rrell 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 C ementing m a te ri al 1 74

a 0 0 0 0 0 0 0 0 0 0 F ault recc 0 0 C ente ring screw s 27 b i

e s a O O O O O O O O O O O O O O O O Chalk 1 76 F ld p r

e s c e u e 0 0 0 0 0 0 0 0 0 0 Chl ori te 1 08 F l iti t xt r

n O O O O O O O O O O O O O O O O O Cl eavage 34 Fli t O O O

uo e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Clinochl ore 1 1 0 Fl rit

Goal 1 77 Compensation point 47 5 1 43 42 44 4 O O O O O o n O O O O O O O O O O O O O C ol r , , , G a r et C ol or s ca l e 45 G a rnieri te

ncen a on 1 51 s s nfluence of Co tr ti Ga e , i 1 73 0 n 0 0 C ongl omer ate Geol ogi cal m app i g 0 0 0 Contact metamorphi sm 1 41 G eol ogi cal ob serv a ti ons

27 0 0 n n n 0 0 0 0 0 ve s 0 0 0 0 0 Co rge t l e Gl a ss 0 0 0 0 0 0 0 n 78 Co ru dum Gl a ssy texture 0 0 0 0 0 0 0 0

1 6 0 0 ca an e n 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Criti l gl G ei ss 0 2 0 3 0 a 7 0 0 0 0 0 oss s n 0 0 0 0 0 0 0 0 Cr h ir , Gra ite 0 0 0 n 1 28 oc s a e 0 0 0 C rypt ry t lli Granito id textur e 0 0 0 0 0 0 4 m 3 0 0 s a o n 0 0 0 Cry t l f r Graphi c gra ite 0 0 0 0 s a a on 1 28 1 31 Cry t lliz ti , G ra yw ack e 0 0 0 0 0 0 0 0 0 0 0 0 0 0

G rei sen 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 5 1 6 4 Gypsum 3 , 1 35 1 5 , 6 9 7 H alite f e en a o ma 0 0 0 0 0 0 0 0 0 0 0 0 0 0 D i f r ti ti n 13 1 H e tite 0 0 Diop side 9 6 H ex agonal mi ne ral s Di orite 1 49 H ol ocry stalli ne Di spersi on 56 H ornblen de

o e e n n 0 0 0 0 0 0 0 0 0 0 D l rit 1 6 6 H or bl e dite 0 0 o om te 1 1 o n D l i 8 , 76 H r fel s

ou e e ac on . 1 8 21 40 48 e ene D bl r fr ti , , , Hyp r sth Duc de au nes me o 7 Ch l , th d of 3 Hyp idiomorphic un e 1 55 0 0 0 0 0 0 0 0 0 D it Hypocry s talli ne 0 INDEX

I cel and spa r 22 M aun a Loa 1 6 7 1 2 1 2 1 77 I ddi ngs 63 M eta morphic rock s . 3 7 I diomo rphic 1 28 M etam orphi sm 1 77 1 23 1 3 7 1 38 ca 1 05 I gneous rocks . , , Mi 1 54 ca a e 5 59 63 Ij olite Mi pl t 7 , , Ilmenite 78 Microcline 1 20 n 38 n m c oc a 0 0 0 0 0 1 28 I mmers io eth od Mi r rys t lli e 0 0 0 0 0 n f ac n 1 5 36 m 0 I dex of re r tio , Mi cro eter 3 n n 21 42 44 45 48 50 I te rfere ce , , , Microscope 24 I nterference fi gures 5 1 -56 Mineral d ete rmi nation n v 1 27 - I trus i e . 33 6 7 1 33 0 6 9 70 - s me m ne a s . 1 4 4 n n 1 22 I o tric i r l , , , Mi era l d escriptio 6 8

I rO i m a 1 3 n a sot c e e e s 0 0 0 0 0 0 0 0 0 2 p di Mi r liz r 0 0 0 0 1 6 Mirro r 28

sso e 1 4 Mi urit 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 M onocli nic mi neral s 48 9 6 -1 1 9

c v o o o o o o 1 06 us o e o o o o o o o o o o o o M t o o o o o o o o o o i

La Croix Ne ga ti ve ch a racter L a bra do rite Nep h elite L epid olite N eph elite b asalt L eucite N eph elite syenite ’ n L eucite b asalt Newto s col or scal e c m Leucite ph onolite Ni ol p ri s Leucite sy enite No rite L eucitite na u e of Light , t r Obj ective Limburgite Oblique extinction Limes tone Ob s idi an

a e use sec Lip rit Obt bi trix 0 0 0 0 0 0 0 0 0 0

Lith og r aphi c limestone . Ocul a r

oess O a s L ligocl e 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Olivine

M a gma Olivi ne b as alt M a gma tic s topi ng Olivi ne di ab a se Ma gnetite O olitic limestone Malig nite O pal M anneb ach Op aque mi neral s M appi ng Ophi cal cite M a rbl e Ophitic texture OPTICAL M INERALOGY AND PETROGRAPHY

49 oc a s c oc s O ptic axi al pl ane Pyr l ti r k 26 4 1 5 1 yroxene O ptic axi s , , P 49 Optic normal 49 o e Optic pl ane Pyrrh tit 42 Optic section 4 ua a ve c a ss fica on 1 23 O ptical ch a racter 56 6 Q lit ti l i ti a fi n 1 23 O ptical mi ner al ogy 9 Q ua ntitative cl ss i ca tio . n Orbi cul a r gr anite 1 41 Q ua rter undul atio mi ca 4 7 a e 57 59 63 Order of color pl t , , 1 8 22 42 ua 44 79 1 34 Ordinary ray , , Q rtz , , Orthocl ase 1 1 9 Q ua rtzite 1 8 1 8 6 a -sens v n a . . 48 u e Orthorhombi c mi ner l s , Q rtz iti ti t 58 6 0 6 1 6 6 , , ua e e 47 59 6 2 P antell erite 1 6 0 Q rtz w dg , , Pa rall el exti ncti on 6 5 ec s a a on P egma tite 1 39 R ry t lliz ti e ac on Penninite 1 09 R fr ti e e Pericline twi nni ng 1 1 4 R li f o e P eridotite 1 55 Rhy lit Perlite 1 6 0 9 1 30 R ock cl as sifica ti on Petrogeny , o a 9 1 0 1 23 R gers Petrogr phy , , ’ osen usc s l aw Petrol ogy 9 R b h u e Phenocry s ts 1 27 R til Phl ogopite 1 08 1 6 2 S ana in Phonolite 35 , 1 d e Phosph ate 1 77 S and dunes Phyllite 1 82 S and s to ne Pitchs tone 1 59 Scal e of birefri ngence c 6 6 6 7 ca f n n Pl eo hroi sm , S l e of re ri ge ce u on c oc s 1 29 1 38 ca Pl t i r k , S polite Pola riz a tion 21 Schi st Pol a rizer 26 4 1 S econd a ry mineral s Pol a rizi ng microscope 24 S edimenta ry rocks P orphyritic texture 1 27 S ericite Pos itive ch a r acte r 57-64 S erpenti ne Primary mineral s 1 25 S erpentine rock n c a O c sec on . . 26 42 Pri ip l pti ti , Sh al e

os ec n 1 85 l n Pr p ti g Sho kinite 0 0 0 0 0 0 0 0 0 0 0 0 0 0 um ce 1 59 P i Siderite 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Pyrite 70 Sl a te