Atlas of Igneous Rock

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Atlas of Igneous Rocksand Their Textures


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The English Language Book Society is funded bythe Overseas Development Administration of theBritish Government. It makes available low-priced, una bridged editions of British publishers'textbooks to students in developing countries.Below is a list of some other books on earthsciences published under the ELBS imprint.

Adams, MacKenzie and GuilfordAtlas of Sedimentary Rocks under the MicroscopeLongmanBlyth and de FreitasA Geology for EngineersEdward ArnoldDeer, Howie and ZussmanAn Introduction to the Rock-Forming MineralsLongmanEvansAn Introduction to Ore GeologyBlackwell ScientificHallIgneous PetrologyLongmanHolmes; Holmes (reviser)Holmes: Principles of Physical GeologyVan Nostrand Reinhold (UK)Kearey and BrooksAn Introduction to Geophysical ExplorationBlackwell Scientific

MacKenzie and. GuilfordAtlas of Rock-Forming Minerals in Thin SectionLongmanRead and WatsonIntroduction to Geology Vois 1 and 2MacmillanTuckerSedimentary PetrologyBlackwell ScientificWatsonGeology and ManA1Ien & Unwin


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Atlas of Igneous Rocksand Their TexturesW. S . MACKENZIEC. H. DONALDSONC. GUILFORD

I [ E L I B s ] 1English Language Book Society/Longman


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Longman Scientific & TechnicalLongman Group UK Ltd,L ongm an H ouse, B urnt M ill, H arlow ,Essex CM20 2JE, EnglandAssociated companies throughout the world Longman Group UK Ltd 1982

A ll rights reserved; no part of this publication m ay bere prod uced , sto red in a retrieval system, o r t ran smi tt edin a ny form or b y a ny m ea ns , e le ctr on ic , m ec ha nic al ,p hoto co pyin g, rec ord in g o r o th erw ise, w ith ou t th e p rio rwr it te n pe rm i ss ion of t he Publ is he rs ,Fit"sl p ub lish ed 1 982R eprin ted 1 98 4, 1 98 8E LB S ed itio n first p ub lishe d 1 98 8ISBN 058202641 5P rin te d in G re at Britain byW illiam C low es L td, B cccles and L ondon


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Contents

PrefaceA ckno w l edgemen ts

Part 1 The textures of igneous rocks{Numbers refer to photographs - not to pages)Introductiou,

Cryst.allinity

Granul,arityTerms referring to what the aided and unaided eye can or cannot se eTerms Indicating absolute ranges of grain sizeTerms indicating r el at ive s ize of crystals

Crystal shap~sTerms indicating quality of the development of faces,on crystalsTerms indicating three-dimensional crystal, shapeGeneral three-dimensional termsSpecific. three-dimensional termsSkeletal dendritic and embayed crystalsParallel-growth crystalsSieve-textured crystalsElongate. curved. branching crystalsPseudomorphs

Mutual relations o r crystals (and amorphous marerials)Equigranular texturesInequigranular texturesSedate texturePorphyritic. texture

1 - 9

[ot4IS-171 8 - 2 2

2 3 - - - 2 5

2 6 - 3 03 1 - . 3 : 23334-3738

,4 44 5 ,


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G lom eroporphyritic textureP oi ki li ti c t ex tu reO ph itic te xtu reI nt er st iti al t ex tu re s

O riented , a ligned and directed texturesTrachytic textureT ra chyto id textureP ara lle l-g ro wth te xtu reCom b texture (com b layering)O rbicular tex ture (orbicu la r la yering)

Intergrowih texturesConsertal textureM icrographic texture (or graphic, if v isib le w ith the naked eye)G ra noph yric tex tureMyrrnekitic textureln tra fa sc ic ula te te xtu reL am ella r a nd b le b-lik e in tergrow thsS ym ple ctite te xtu re

R ad ia te te xtu re sSp heru litic te xt IIreVariolitic textureRadiate intcrgrowth

O vergrow th te xturesSk eleta l or d end ritic ove rgrow thsC oron a te xtureC ry sta l z on in g

B and ed te xtures (b and in g)Comb layering, orb icular texture, and ocellar tex ture

C av ity te xtu re sV esic ula r te xtu reAmy gda lo id aJ textureM ia ro litie te xtu reLithophysa or (ston e-ba ll) texture

Part 2 Varieties of igneous rocks(Numbers refer to photographs - not to pages)

In t ro d u ct io n

Ultrabasic rocksDunitePeridotiteKimberliteG a rn et p er id ot it ePyroxeniteKomatiiteMeymechiteHornblendite

B asic ro ck sBasaltsvar, T ho le iitic b asa ltvar, A lka li olivine b asa ltvar. Lunar low -Ti basaltvar. Lunar high-Ti basaltva r. P ie ri tic ba sa ltv a l'. An k ar am i te

4 6 - - 4 74 8 - 5 15 2 - 5 75 8 - - 6 36 4 - 6 66 4 - - 6 667--f ,9

se e 3 1 - 3 27 0 - 7 1

se e 1 0 4

7 2 - 7 37 4 - - 7 67 6 - 7 77 87 98 0 - 8 38 4 - 8 5

8 6 - 8 88 99 0

919 2 - 9 49 5 - 1 0 21 0 3 - ] 0 41 0 5

\06-1071 0 81 0 9

110III11211 311 411511611 7

11 811912~1 2 11 2 21 2 3


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SpiliteGabbroTescheniteEssexiteDoleriteNoritcAnorthosite

12 412512612 712 8129130

Intermediate rocksAndesiteBonini teDioriteTonaliteKentallen ieMonzoniteDaciteGranodioriteTrachyteSyeniteShonkinite

1311 3 21331341 3 513613713 81 3 91401 4 1

Acid rocksRhyolitePan telleriteGraniteAlkali granite

1 4 21 4 314 4145

A lk alin e an d m iscella neo us ro ck sPhonoliteLeucite phonoliteNosean leucite phonolitePseudoleucite phonoliteBlairmoriteNepheline syeniteMaligniteSodalite syeniteNepheliniteljoliteUrtiteBasaniteTephriteOlivine melilititeLeucititeFergusiteMinetteAlnoiteMafuriteitzroyiteWyomingiteMadupiteCarbona liteChondrite (meteorite)Achondrite (meteorite)

1461 4 71 4 81 4 91 5 01 5 115 21 5 31 5 41 5 51 5 61 5 71 5 81 5 91 6 01 6 11 6 216316416 516 61 6 71 6 81 6 91 7 0

AppendixPreparation of a thin section 0.( rockReferencesIndex


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Preface

The commonest means of studying an igneous rock is to examine it in thin section,either with a petrographic microscope or a hand lens, which permits identificationof the minerals present and investigation of their textural relations. From such studythe skilled petrographer can interpret details of the history of the magma whichcrystallized to form the rock.To become skilled requires many hours of study and training. Much of thetraining is acquired by patient attention by the teacher to the student. The studentneeds his observations verified and this can result in the teacher being summonedevery minute or so; with a class of ten or more, the student is for long periodsunattended, becomes frustrated and loses interest. The remedy is for the student tobe able to verify his own observations by comparison with a photograph of a rockof the same type or showing the same feature(s).The main aim of this book is to provide such a la bora tory hand book to assist th estudent of geology (undergraduate and amateur) beginning to study igneous rocksin thin section. It is hoped that it may also be useful as a reference work for moreadvanced students and others interested iJ1 the natural history of rocks.The work is divided into two parts - Part I is devoted to descriptions and photo-graphs of textures found in igneous rocks and Part 2 consists of photographs ofcommon (and a few not so common) igneous rocks.We have selected those rocks and textural types which we believe may be en-countered in an undergraduate course in geology but have made no attempt toproduce a comprehensive coverage of all igneous rocks which have been givenindividual names because many of these names reflect only minor mineralogical ortextural differences.We have tried, as far as possible, to avoid any interpretation of the origin oftex lures and rocks, although the sim pie matter of arranging the rocks in some orderof presentation is based to some extent on presumed genetic relationships betweenthem.In a previously published Atlas ofRock-Forming Minerals we have illustrated theappearance of the common rock-forming minerals so that here we have not con-sidered it necessary to describe the optical properties in detail. To be able to give aname to the majority of igneous rocks it is only necessary to be familiar with theproperties of between twelve and fifteen minerals and we have assumed tha t the userof this book is already able to recognize these minerals.Thill sections can be observed under the simplest of microscopes fitted with twopieces of polaroid and a new field of interest is open to the amateur for only a modestfinancial outlay. Because some amateur geologists may be interested in preparingtheir own thin sections we have included a brief description of how this may be done.Many of the photographs show a combination of shapes and colours which have aspecial beauty of their own, reflecting the fact that while thin section study is ofpractical importance it can also be of aesthetic satisfaction. A few of the mostattractive pictures unashamedly represent the authors' self-indulgence.The usual criticism of photomicrographs is that it is very often difficult to deter-mine which feature they are intended to illustrate. For this reason many textbooksare illustrated by drawings in which the required feature may be exaggerated. Wehave tried to test the usefulness of OUf photographs by asking our colleagues toidentify the mineral assemblage or texture which we have illustrated. We are grateful


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to them for their help in this respect. Most of the photographs were made from thinsections of rocks in the teaching collections of the Geology Depa rtmen ts of Man-chester University and St Andrews University. Others were provided by friendsand colleagues who made available to us thin sections from their own researchcollections, and we are most grateful to them for their help in this matter. We areparticularly indebted to Dr John Wadsworth and Mr Ian MacKenzie who read andcriticized all the descriptions of the textures and rocks. However, any failings in thesedescriptions arc our responsibili ty alone.Finally, we caution those using the book not to regard the photographs as re-

presenting al l the known textures and varieties of igneous rock, or indeed all theirguises. These photographs are only an ai d to recognition of textures and rock typesand cun never substitute for looking at thin sections Linder (he microscope.

AcknowledgementsWe are much indebted to our colleagues and friends who ha ve generously given LISthin sections of" rocks from which to take photographs: they include the followinggentlemen: S. O. Agreli, BAtkinson, N. Binstead, K. Brooks, FM Broadhurst,LS.E. Carmichael, J. B. Dawsori .J . Esson, M. E. Fleet, F. G. F. Gibb, A. Hall, D. L.Hamilton, C. M. B. Henderson, A. M. Hopgood, E. lk i, R. Johnston, I. R.Mackenzie, R. Nesbitt, E. Sapountzis, J. Wadsworth, Rong-shu Zerig and J.Zussman, We have abo benefited grca tiy from having beem able to usc the collectionsa f the la te Pro f. H. I. D rever who was the teacher and friend of bo th WSM and CHDat St. And rews Un iversi ty, a Ithough 25 years in tcrvcned bet ween 0 II r u me as hisstudents: he and his colleague Mr R. Johnston were jointly responsible for arousingour interest in igneous rocks early in our careers.The staff of the publishers have been very patient and helpful and we especiallywish to thank them for their consideration and for that quality essential to allpublishers - a sense of humour,M iss Patricia Crook's help both in typing the manuscript and in preparing theindex is gratefully acknowledged. We are grateful to Dr Robert Hutchison of theBritish M useum (Natural History) for permission to photograph thin sections of thePrairie Dog meteorite and the Stannern meteorite. both of which are in the BritishMuseu r n co Ilcctions,


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Part 1The texturesof igneous rocks


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lotroductionTo English-speaking petrologists textures are the geometrical relationships amongthe component crystals of a rock and any amorphous materials (glass or gas incavities) that may be present. They comprise the following properties:I. Crystall inity (degree of crystallization) - i.e. the relative proportions of glass and

crystals.2. Granularity (grain size) - i.e. the absolute and the relative sizes of crystals.3. Crystal shapes.4. Mutual relations or arrangement of crystals and any amorphous materials

present.In this part of the book textures in each of these categories are described and illus-trated, some in plane-polarized light (PPL), some in cross-polarized light (XPL)and some in both. Some textures exhibit more than one of the above properties andwe have indicated where this is so.Petrography, of which textural relations are a part, is the descriptive and factualside of petrology, whereas petrogenesis is the interpretive side. Thus genetic terms,such as cumulate, cumulus crystal, cumulate texture, synneusis texture, exsolution

texture and fluxion texture should be avoided, as they combine factual descriptionwith interpretation; they rob any person reading a petrographic description ofunbiased observations and can cast doubt on the objectivity of the petrographer whowrote the description. For this reason, genetic textural terms are not included in thisbook, there being suitable non-genetic terms available fo r all of them.

Remarkably few igneous textures have been reproduced in the laboratory and theorigins of even fewer could be claimed to be adequately understood. For thesereasons, we have made no comment on the origin of most of the textures; readersshould consult the texts by Iddings (1909), Holmes (1921), Niggli (1954), Hatch.Wells and Wells (1972) or Cox, Bell and Pankhurst (1979), for discussion of the originof textures and their implications. However, it should be noted that many texturesare open to more than one interpretation and the newcomer to the subject i advisedto consider the possible origins and implications for himself before reading oneof these texts. He is then likely to interpret the crystallization of a rock more objec-tively and flexibly than if the 'standard interpretation' is adopted slavishly. Thiscomment is particularly relevant to the interpretation of 'order of crystallization'of minerals in a rock. We have found that both students and teacher can benefitfrom a two-hour discussion of the subject; the student who is unencumbered by pre-conceptions can be remarkably inventive and provide his teacher with copious newideas for considers tion.In studying rocks in thin section we must not forget that only a two-dimensionalview is present and hence the true three-dimensional texture has to be deduced from

examination of the dispositions of many crystals in the section. In rocks with a strongpreferred orientation of crystals, two or more sections of different attitude may berequired La reveal the texture adequately.

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Cry sta l l in iy

CrystallinityIgneous rocks range in crystallinity from entirely crystals to entirely glass. Adjectivesused to describe these states are shown on the following scale:10 0 % crystals 100% glassholocrystalline hypocrystalline Ior hypohyaline holohyal ine

The adjectives glassy, vitreous and hyaline all indicate that a rock is more or lesscompletely glass.IHypocrystalline rocks can be described more precisely by stating the relative proportions ofcrystals 10 gloss.

4

1 Ho)ocrystalline anorthositic gabbroElongate crystals of plagioclase feldspar, some wrapperound olivine crystals, form a framework in this rock,interstices of which are filled with smaller plagioclasolivine and augite crystals. The purplish-blue area attop right of this photograph is an augite crystal whiincludes a [J umber of small plagioclase and 0 Iivine crystaPe rp end ic ul ar F e ld sp a r gabb ro from M id dle B ord er G ro0/ Ie S ka er ga ar d in tr usio n, E a st G re en la n d ..m a gn ific a lx 7, XPL.

2 Holocrystalline gran,i.teCrystals of biotite, quartz, 'perthitic' potassium-rich fespar (large crystal bottom right) and zoned sodium-ricfeldspar makes up this granite. The speckled appearancin the cores of the plagioclase feldspars is caused by finclusions of mica.G ra nite fro m R oss 0/ Mu ll, S co tla nd ; m a gn ific atio n xXPL.


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3 Hypocrystalline pitchstone with perliticcracks

Crystals of plagioclase, biotite and magnetite in this rockare set in glass (black in XPL) which has spherical frac-tures known as pe rl it ic c rac ks : these appear as circles inthin section.Dacite from C hem nitz; East G erm any; m agnifica tionx 2 0, P PL an d X PL .

4 Hypocrystalline basaltSmall olivine phenocrysts (colourless in PPL) and colum-nar, skeletal ti tanaugi te crystals (pinkish-beige colour inPPL) are enclosed by murky brown glass. No plagioclasehas crystallized in this rock. The deeper pink colouraround the margin o f some of the titanaugites is a narrowmantle of Ti-rich amphibole.Basal tf rom Quarsut , Wesl Green land ; magn if ic a ti on x 35,PPL an d XPL .

Crystallinity

5


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Crystallinity

6

Hypocrystalline basalt (continued)

,5,GI;ilssy rock1The 'photograph shows abundant, very small cryst(probably quartz or feldspar) enclosed by glass, Notebanding caused by (a) differences in abundance or crystlites, (b) crystallites in the lighter bands baving a slipreferred alignment and (c) differences in colour ofglass, The small brown, isolated round objects are knowas 'spherulites' (see Spherul ut c t ext ure , p. 54). (See also 1Pitchstonefrom Arran, Scotland : magnification x 12, PP

6 Glassy basalt threads - Pele's hairThese f laments of basalt glass form when particles inmolten lava spray are caught by the wind and drawn oPele is a mythical lady, believed by native Hawaiiansreside within the volcano Kilauea. (Contrast 7,)Specimen from Erta Ale volcano, Ethiopia; magnificat iox8, PPL.


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7 Glassy particles of mare basalt in lunar soilPieces of glass, many of them spherical. are orange-brownor black in colour. Some of the darker ones are partiallycrystalline. These particles were formed by rapid coolingof droplets of basalt melt; it has been suggested that thedroplets formed either in a fire-fountaining lava eruption,or by meteorite impact Into a lava lake or into a moltenor solid lava flow. (Contrast 6.) The scarce, irregularlyshaped fragments are pyroxene (pale brown) and feldspar(colourless).Lunar basalt 74220 from Taurus Littrow Valley collectedby Apollo 17 astronauts: magnificationx ds , PPL.

7a Liquid ImmiscibilityGlobules of one glass in another are found in some rocksand these are attributed to immiscibility of the twoliquids. In this rock they can on Iy be seen at very highpower in thin films of glass between laths of plagioclase.Specimenfrom basalt lava, Lava beds National MonumentCalifornia, U.S.A.: magnification x 600, PPC

Crystallinity

Glass, or devitrified glass, is often an important constituent of the pyroclastic rocksknown as ash-fall tuffs and ash-flow tuffs (or ignimbrites). Such rocks typically havefragmental textures, i.e. they comprise mixtures of fragments of rocks, crystals andglass, predominantly less than a millimetre in size (8-9). In an ash-now deposit [heglass fragments may initially be plastic enough to be partly or wholly welded to-gether as the weight of overlying material causes compaction of the constituentfragments; such a rock is known as a welded It!ff'(8b). If sufficient heat is avai table,glassy fragments devitrify.

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Cryst al l i n it y

8

8a Glassy unwelded rhyolite tuffThe glassy fragments in this rock, some of which arebanded and slightly flattened, are not welded to oneanother. They and the crystals of quartz and feldspar areembedded in fine glassy particles (ash). .Recent ignimbrite, from Whakatane, North Is/and, NewZealand; magnificat ion x 46, P P L

8b Glassy welded crystal tuffThe glassy matrix in this rock has an apparent discon-tinuous lamination caused by extreme compaction andwelding of original pumice fragments. The regular align-ment of the flattened fragments is known as eutaxitictexture.Welded luff from Tibchi granite ring-complex, Nigeria;magnification x 36, PPL

9 TuffThis fragmental rock consists of crystals of quartz, alkalifeldspar and plagioclase of various sizes and shapes,pieces of glassy rhyolite (e.g. centre) and pieces offine-grained tuff, all enclosed ill a fine-grained bandedash matrix which originally may have been glassy. (Seealso 13.)Tuff from Lloneilwedd, Wales: magnification x /0, PPLand XPL.


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' Pe gm a ti ti c t ex tu re is" variety ofphanerocrystoltine in which I" ~[,I'J'/(I/.I are .,lri kiIlK(r i arg, h i}.rga than /-2 em, and in rarein sta nc es u p /0 IIWlIl' metres.'The term aphyric is sometimes used for aphanitic rocks whichlack phenocrysts (eg, 60, 63, 107).'F el si ti c t ex tu re is sometimes applied /(I siliceous rocks with ill"defined. almost crvptucrystulline. grey-polarizing areas composedof more 01' less equigranular aggregates 0/ quartz and alkalifetd-s pa r. T he /1an~~felsite i s o ft en applied to .HICit rocks. although 1111.1ismore conunonlv ofield termforfine-grained acid materiat of u,,-certain mode offoccurrence.

10 Phanerocrystalline granitesThe crystals in the two granites, illustrated here in handspecimen, are clearly visible to the naked eye. Althoughthe rocks contain the same minerals (alkali feldspar,plagioclase feldspar, quartz and biotite) the proportionsof the minerals are not the same, and this influences therock textures. Thus the Shap granite contains two distinctsizes of potassium feldspar crystals (pink), whereas theEagle Red Granite has only one ..Granite [rom Shop, England (opposite) and 'Eagle Red'gran ill', S ou th A fr ic a tnext page), both magnifications x I.A thin sec/ion view 0/ the Shap granite is shown in .144

Granularity

GranularityThis property embraces three different concepts: (I) what the aided and unaidedeye can or cannot see; (2 ) absolute crystal sizes (p, 12); and (3 ) relative crystal sizes(p.14).

Terms referring to what the aided and unaided eye can or cannot seePhanerocrystalline iphaneritic texture of American petrologists) - all crystals of theprincipal minerals can be distinguished by the naked eye (see 10). IAphanitic - all crystals, other than any phenocrysts present (see p. 14), cannot bedistinguished by the naked eye.! Two sub-types exist:(a) Microcrystalline - crystals can be identified in thin section with a petrographicmicroscope (11). Crystals only just large enough to show polarization COIOUI'S

(less than 0.0 I rnrn) are called micro/ill'S.(b) Cryptocrystalline' - crystals are too small to be identified even with the micro-scope (12 and 13). Globular, rod-like and hair-like crystals which are 100 small

to show polarization colours are known as crystallites.

9


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Granularity

10

Phanerocrystalline granites (continued)

11 Microcrystalline olivine basaltThis rock consists mainly of plagioclase feldspar, augiand olivine but, without the aid of the microscopeindividual crystals would not have been distinguishable.In parts of the photograph the randomly arrangedrectangular plagioclases are enclosed by areas showinuniform yellowish interference colours. these arc augicrystals,Olivine basah from North-west Skye. SCOIland , magnification x 1 1. PPL.

12 Cryptocrystalline rockComparison of these two photographs shows that thbrown material in the PPL view is birefringent but thathe individual crystals are of submicroscopic size. Thclear areas in the PPL view are slightly more coarselcrystalline, as can be seen in the XPL view.Rhyolite from Island of Pantellerta. ltaly ; magnifica/ionx 72. PPL and XPL.


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Cryptocrystalline rock (continued)

13 Cryptocrystalline matrix in a tuffCryptocrystalline texture is common in tuffs (i.e. con-solidated ash), as in the matrix of this rock. Here thematrix encloses fragments of shale and quartz crystals.(See also 8 and 9.)Tuff from unknown locality: magnification x 16, PPL andXPL.

Granularity

1


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Granularity

Terms indicating absolute ranges of grain sizeCoarse-grained - crysta I diameters > 5 mmMedium-grained - crystal diameters 1-5 mmFine-grained - crystal diameters < 1mm IThe next six photographs (15,16 and 17) were all taken at the same magnification(x 27) to indicate bow grain size relates to the number of crystals seen in a given

field of view (4.2 x 3.1 mm), and hence the extent of the texture visible at that magni-fication. While the overall texture is recognizable in the fine-grained rock, it is not soin the coarse one and a low-power objective lens would be necessary to examine itadequately. Petrographic microscopes rarely have a sufficiently low-power objectivelens for examining the textures of coarse-grained rocks; a hand lens should be usedfor these, with two sheets of polaroid, if available.

ISome petrologists include another range.


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Fin,'-grained gabbro (continued)

16 Medium-grained olivine gabbroThespaces be tween the t abu la r crystals of plagioclase inth is rock are occupied by augite and ilm enite. At the topright o f t he p ic tu re th e p la gi oc la se abuts onto an olivinecryst al . The augi te crystals contain lamellae of orthopy-r o x e n e .G ~bbrofro m Lo wer Z on e b o f the Ska erg aa rd in trusio n,EastG reenland; magnif ication x 27. PPL and XPL.

Granularity

13


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Granularity

Terms indicating relative size of crystalsEqui gr an ul ar - all crystals are of approximately the same size.lnequigranular - crystals differ substantially in size. A common variety, porphyritictexture, involves relatively large crystals (phenocrystsl) embedded in finer-grainedgroundmass. (N.B. The same mineral may be present as both phenocrysts andgroundmass.) In naming a rock with porphyritic texture the minerals present asphenocrysts should be listed and followed by the suff ix -phyric, e.g, 'hornblende-pigeonite-phyric andesite'. However, if the ground mass is glassy, the term 'vitro-phyre' is used, e.g. an 'olivine vitrophyre' has olivine phenocrysts set in glass; thetexture in thi case is referred as vitrophyric (3, 142). Seriate texture involves a con-tinuous range in sizes of crystals of the principal minerals; if the crystals show abroken series of sizes, the inequigranular texture is said to be hiatal. Caution isnecessary in the identification of seriate and hiatal textures, since the dimensions ofa crystal in a thin section depend on the atti tude of the intersection of the crystal inthree dimensions.I The prefix micro- may be added /0 phenocrysts which have diameters between 0.05 and0.5mm [e.g. 'olivinemicrophenocrysts),14

17 Coarse-grained olivine gabbroAt this magnification only parts of three large olivinand one plagioclase are visible, such that textural relationare not determinable in this single view.G abbro from Rhum , Sco tland , m agnification x 27 , PPa nd X PL .


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18 Equigranular peridotiteUniformly-sized olivine crystals, some of them in clots,form the bulk of this rock, with plagioclase filling theinterstices. The black material is microcrystalline haema-tite formed by oxidation of olivines and the green materialisa clay mineral.P erid otite from th e S ka erg aa rd in tru sio n, E as t G re en la nd ;magnification x 27 , PPL and XPL.Addtttona! views of equigranular rocks are shown in 43,113,117,125,130 (first photo) .134,140 {third photo], 168.

19 Porphyritic andesiteIn this rock the phenocrysts (some of them in clots) ofplagioclase. hornblende (khaki colour in PPL), augite(pale green in PPL) and magnetite, are surrounded byfine-grained groundrnass of plagioclase. magnetite andg l a s s .Andesitefrom Siebengebirge, Germany: magnification x 23,P PL a nd X PL .

Granularity

15


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Granularity

16

Porphyritic andesite (contin LIed)

20 leucite-phyric micro-ijoliteTwo, large, shapeless crystals of leucite (very dark anshowing multiple twinning in XPL photograph) are hesurrounded by an equigranular grcundmass consistingcrystals of elongate augite (bright interference coloursequant nepheline (grey in XPL) and interstitial biotitleucite and magnetite. The amorphous material in tPPL view is a clay mineral.Micro-ijolitefrom the Batsberg intrusion, East Greenlandmagnification: 11, PPL and XPL.Many other examples a/porphyritic rocks can be seenleafing through the book.


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21 Plagioclase-augite-magnetite vitrophyrePhenocrysts of the three minerals plagioclase, augite andmagnetite, some of them in clots, are set in glass whichcontains crystallites of plagioclase.Basalt from Arran. Scotland: magnification v Zt), PPL.See 132 for another example 0/ vitrophyre,

22 Seriate-textured olivine basaltThe crystals of olivine, augite and plagioclase in thisbasalt all show a wide range of grain size from as small as0.01mm up to 4mm. Note the abundance of groundmassinclusions in some of the crystals, giving them a sponge-like appearance,Olivine basalt [rom Arthur's Seal, Edinburgh, Scotland;magn!f tcationx 11. PPL and XPLSee 44 and 1.17jill" other examples ofthis texture.

Granularity

17


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25 Anhedral olivine phenocryst in basaltThe entire perimeter of the large olivine crystal, at extinc-tion in this picture, has an irregular outline and no planarfaces are present. (The narrow brown rim on the crystalis 'iddingsite' formed by hydration and oxidation of theolivine.)O livin e ba sa lt fro m M auritiu s, In dia n O ce an ; m ag nific a-tion x 32 , XPL.

Fig. A Examples ofequidimensional crystal shapesThe words grain and granule are often used for equi-dimensional crystals, and drop and bleb for particularlysmall examples.

Crystal shapes

Terms indicating three-dimensional crystal shapeIn hand specimens of coarse-grained rocks it is often possible to see the three-dimensional shape ora crystal on a broken surface. For finer-grained rocks, however,the crystals have to be examined in thin sections and the two-dimensional shapes ofseveral crystals of different orientations used to deduce the three-dimensional shapesof the crysta Is in general.General three-dimensional termsThe shape may either be an equidimens ional (syn. equant) or an inequidimens ionalone, as illustrated in figs. A and B where the names applied to the various shapes arcshown.

equant polyhedral

equant anhedral spherical

19


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Crystal shapes

tabular(svn. platy) lamellar acicular(svn, needle-like,

fibre, fibrous,hair-like)

bladed' columnar(svn.prismatic)

Specific three-dimensional termsSkeletal, dendriticand ernbayed crystalsSketetal crystals are those which have hollows and gaps, possibly regularly develop-ed, and usually with particular crystallographic orientations, In thin section' thesespaces appear as embayments ' and holes in the crystal, filled with groundmasscrystals or glass. Dendritic crystals consist of a regular array of fibres sharing acommon optical orientation (i.e. all part of a single crystal) and having a branchingpattern resembling that of a tree or the veins in a leaf or a feather. In practice, manycrystals can be described as either skeletal or dendritic because they have characteris-tics of both,

'A common misconception among petrologists is tha! the terms 'embayment' and 'embayed ,imply resorption ofa crystal by reaction with liquid, While this may be true of some crystals{e.g. 29), others [e.g, 26 and 27) have embayments which probablyformed duril1g grow/h.

20

Fig. B Exam ples of in eq uid im e ns io na l s ill/ pic 'SN.B. Although these are euhedral examples, they COLLIsubhedral or anhedral.

Bladed [eldspor crystals hy common IIsag/! are [requently dccribed as ' lath-shaped' or as 'Iaths of [eidspar' , ill allusion 10thslats (laths) in a Venetian blind.

26 Skeletal olivines in picritic basaltAll the large crystals in this rock are olivines and eacshows a different shape in section; some are compleskeletal crystals (e.g. elongate yellow crystal on the leftothers are relatively simple skeletons (e.g: equant orangcrystal, middle right) and yet others have only smaembayments,Picritic basal! [rom Ubekendt Eiland. West Greenlandmagnification x 40, X PL.


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27 Skeletal olivineWhile superficially resembling the euhedral outline of theolivine in 23, the crystal occupying the bulk of this picturehas a complex interior form and incomplete prism anddome faces.P icritic basa lt fro m U bekendt E jland, W es! G reenland:magnificat ion x 15, PPL.

28 Dendritic olivinesAll the delicate, dendritic crystals in this photograph areolivines which formed during exceedingly rapid solidifica-tion of the basalt melt, part of which became the yellowglass.Specim en of olivine basalt m elted and then cooled 1/ 1! 400~ jh r i n t he l ab o ra to ry ; magn if ic a ti on x 40, P PL.

29 Embayment in augite phe.nocrystThe large augite crystal in this photograph contains adeep embayment filled with the basaltic groundmass, Theirregular outline of this embayment distinguishes it fromthe embayments in the skeletal crystals in 27. Note alsothe distinct marginal zoning and the delicate 'patchy zon-ing' within the crystal.O livin e ba sa lt fro m A rth ur's Se al, E din bu rg h, Sc otla nd ,magnificat ion x 23 , XPL.

Cry sta l s h


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Crystal shapes

Parallel-growth crystalsThe term is applied to an aggregate of elongate crystals of the same mineral whosecrystallographic axes are mutually parallel, or almost so, Although in thin sectionthe individual parts of the aggregate may be isolated from one another, in the thirddimension they are probably connected, A parallel-growth crystal is therefore asingle, incomplete crystal formed by a particular style of skeletal growth,

22

30 Embayed quartzThe deeply embayed quartz crystal in this olivine bacontains brown glass and small, columnar, skepyroxenes, It is also surrounded by a film of the glassan aggregate of equant granular augite crystals wseparate it from the basaltic groundrnass.Olivine basal! from Lassen Park, USA: magnification xPPL.

31 Olivine parallel growthThe elongate olivines near the middle of the photograand showing blue interference colour all have the scrystallographic orientation, and hence represent a sinparallel-growth crystal. The crystal with yellowish-grinterference colour shows how the parallel-growth crymight appear, if sectioned at right angles.Picritic basalt from Ubekendt Ejtand, West Greenlamagnification x 23, X PL.


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32 ParaUel growth in a very coarse-grainedrock

Here the parallel growth is of a very large olivine crystal.The actual width of the field of view is 1.7 em and thisshows only a small part of the parallel growth, whosetotal width is 50cm and height is 150cm. The wholecomprises several hundred parallel units like the onesshown here. Plagioclase and augite occupy the 'channels'between the parallel growths. In the XPL picture thepolars have been rotated so that the olivine is not inextinction. The slight differences in birefringence of theolivine at the top and bottom of the picture are caused bythe section being thinner there. This rock has the specialtextural name harrisite.Feldspathic peridotite/rom Rhum, Scotland; magnificationx 7 , PPL and XPL

Crystal shapes

Sieve-textured crystalsThese contain abundant, small, interconnected, box-shaped glass inclusions, givingthe crystals a spongey, or porous, appearance.23


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Crystal shapes

E lo ng a te , c ur ve d, b ra n ch in g c ry st al sThese a re rarely gen u inely ben 1, rather the ell rvature is caused by develo pment 0[branches along the length of the crystal, each branch having a slightly differentcrystallographic orientation to its neighbours (e.g, 34--36).

24

33 Sieve-textured feldspa,rThe COIC 'of this xenocrysr consists of glass and alkfeldspar in a fine-mesh-like arrangement; the narrow ris an overgrowth of plagioclase.O liv in e b as alt fro m L as se n P ar le U SA .. m ag nifi ca ti on xPPL.

34 Cu.rvedbranching augiteThe highly coloured crystals in this photograph are coplex, branching crystals of augite in subparallel aligment. They form part of a pyroxene-rich band indifferentiated dyke. (See also 71.)D olerite FO In N orth Skye, Scotland ; m agnifica tio n xXPL.


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35 Branching augite in lamprophyre dykeThe acicular, aligned phenocrysts in this photograph areall of augite, forming composite, radiating, curved andbranching groups. Individual needles can be seen to con-sist of several straight portions offset slightly from oneanother, and having very slightly different orientations;this gives each ' needle' its curved appearance. The rna rgi [1o f the dyke lay to the left. (See also 70.)F ou rc hite [r om F is ka en es se t a re a. S ou th -w e st G re en la nd :maglJijicaiiol1 x 20, X PL.

36 Curved and branchi1ng pl;agioclase crysta,lsin doleriteThe large composite plagioclase crystals in this rock areelongate parallel to the c crystallographic axis and flat-tened parallel to (010). From the direction in which theybranch, and from [hat in which the crystal at the bottomwidens, it can be deduced that the crystals grew from rightto left The matrix consists of fine-grained plagioclase,olivine, pyroxene, amphibole, devitrified glass and clayminerals.Feld spathic do lerite. U bek end t E flan d, W est G reenla nd ;magnifica/ion x 16, P PL and XPL.

Crystal sbapes

25


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Crystalshapes

PseudomorphsIt may be found that crystals in a thin section, although having the characteristicshape of a particular mineral, prove to be of another mineral, or an aggregate ofcrystals of another mineral. The name pseudomorph is used for such a crystal. If thepseudomorph has the same composition as the original crystal (e.g. 'quartz' inplace of tridymite) it is known as a paramorph.26

37 Composite branching augite crystalThese photographs illustrate a particularly intriguingshape of branching augite crystal: it consists of groups oslightly diverging needles, subparallel to the length of thcrystal, which apparently have grown from curved branch-ing needles oriented approximately at right angles to thcrystal length. Despite the uniform interference colourof many oft he needles, a sweeping style ofextinction occurswhen the microscope stage is rotated under crossed polarsindicating that the needles are not aIIof the same crystallo-graphic orientation.Peridotitic komauite from Munro Township, OntarioCanada; magnification x 52, PPL and XPL.


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38 Carbonate pseudomorphs after olivineTh e phenocrysts in this altered basalt show typical sec-tions of skeletal olivine, with inclusions of groundmass inth e embayments. However the photograph shows thephenocrysts to be occupied by finely crystallized carbon-ale, indicati ng tha Ireplacement of oli vine has occurredA ltered basa l! from C astleto n, D erbyshire , Eng land:magnif ication x 27 , XPL.Another example 0/ pseudomorphs is sh ow n in 1 49 .

Crystal shapes

Mutual relations of crystals (and amorphous materials)The various patterns of crystal arrangement which can exist are convenientlyintroduced under the following headings: equigranular textures: iuequigranulartextures; oriented textures; intergrowth textures, radiate textures; overgrowthtextures; banded textures; and cavity textures. Particular textures may belong tomore than one of these categories and some also belong to the categories of crystal-linity, granula.rity and crystal shape. Thus certain of the textures introduced in thissection have already been mentioned and reference is made to photographs of themin previous sections.Equigranular texturesDepending on the general shape of the crystals, three textures can be distinguishedin which crystals of the principal minerals in a rock are of roughly uniform grainsize:

allotriomorphic granular(granitic and granitoidtextures apply to si liceousrocks only)

deflnitionbulk of the crystals areeuhed ra1and of uniform sizebulk of the crystals aresubhedral and of U niform sizebulk of the crystals areanhedral and of uniform size

name synonymspanidiornorphic granularu hedral gran ular

su b h ed ra 1 granular hypidiom orphic granular(anhedral) I granular

Boundaries between these categories are not sharply defined and consequentlythe terms are applied very SUbjectively. Furthermore a rock may not fit neatly into asingle category, thus one in which ~ 50 % of the crystals are euhedral and ~ 50 %anhedral might best be described as having a mixed euhedral and anhedral granulartexture.In addition to the examples of these textures in 39-43, others may be found in18, 111, 113, 117, 125, 130, 134, 140 and 168.

L This adjective is commonly omitted from this textural name.27


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Mutual relations of crystals: equigranular textures

28

39 Euhedral granular hoenblendlteRocks possessing truly euhedral granular textures avery rare. The one in this figure is a good example ofmore common si t uation in which on l y some of the crystaof the principal mineral, hornblende, are euhedral asome strictly are subhedral. In contrast to 40, there arehigher proportion of crystals with faces and the ter'euhedral granular' is therefore suggested as most apropriate. It should be appreciated, however, that anothepetrologist might prefer 'subhsdral gran ular'.Homblendite from Ardsheal Hill, Scotland; magnificatiox 7. XPL.

40 Subhedral granular gabbroThe stout prismatic plagioclase feldspar crystals whidominate this rock are mostly subhedral, The anhedrainterstitial crystals are of orthopyroxene, augite amagnetite.Gabbro from Middle Zone of the Skaergaard intrusioEast Greenland; magnification x 20, XPL.

41 (Anhedral) granular troctoliteOnly a few of the plagioclases in this equigranular rocpossess a face and none of the olivines do. The crystaare therefore predominantly anhedral and the 'mosaictexture is granular.Troctolite from Garbh Bheinn intrusion, Skye, Scotlandmagnification x 17, XPL.


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42 Granular graniteExcepting the scarce biotite crystals, the quartz, micro-cline and albite crystals which make up the bulk of therock are anhedral and have slightly interdigitating bound-aries (i.e: consertal texture - see p. 45).Granitefrom Madagascar; magnification 13, XPL

43 Granular lherzoliteThe crystals of olivine (colourless in PPL), and pyroxenes(pale brown in PPL) which make up 95% of this rock,lack any crystal faces.Lhrrzolite xenolith [rom the Matsoku kimberlite pipe,Lesotho: magnification x 16, PP L and XPL.

Mutual relations of crystals; equlgranular textures

29


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Mutual relations of crystals: inequlgranular textures

luequigranular texturesThis category includes seven kinds of texture: (a) seriate; (b) porphyritic; (c)glomeroporphyritic; (d) poikilitic; (e) ophitic, (f) subophitic; and (g) interstitial(intersertal and intergranular).lt is not uncommon for a single thin section to displaymore than one of these textures.Seriate textureCrystals of the principal minerals show a continuous range of sizes. (See also p. 14.)

30

44 Seriate-textured basaltThis basalt, consisting of just plagioclase, augite andsmall proportion of magnetite, shows a range in sizes oplagioclase and augite crystals from


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45 Augite -01i,vine-Ieucite-phvric meUI ititeAugite (greyish-green and green in PPL) is present Inthree generations in this sample - large euhedral pheno-crysts. subhedral microphenocrysts and minute ground-mass crystals. The leucite occurs as colourless, equanteuhedral microphenocrysts, most easily identified bytheir very low birefringence in the XPL picture, and theolivine as faint-grey, euhedral, columnar mieropheno-crysts, Note the complicated zoning pattern in one of thea u g i t e phenocrysts, the prominent marginal zoning andthe line of small inclusions of groundmass crystals inanother. Melilite is confined to the fine-grained gran u l argroundmass and cannot easily be seen in these photo-graphs.Me lilit ite from M a law a, C ele be s .. m a gn ifi ca ! io n x 11, P P Land XPL.M any m ore exam ples of porphyritic texture m ay be foundb y le afin g th ro ug n th e b oo k.

Mutual relations of crystals: inequigranular textures

Porphyritic textureRelatively large crystals (phenocrysts) are surrounded by finer-grained crystals ofthe groundmass. (See also p, 14.)

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Mutual relations of crysta Is: inequigranular textures

Glomeroporphyritic textureA variety of porphyritic texture in wh ich the phenocrysts arc bunched, or clustered,in aggregates or clots called glomerocrysts. (A minority of petrologists maintain thatthe term applies only to monornineralic clots and for polymineralic clots they usethe term cumulophyric texture.y Glomerophyric is usually used synonymously withglomeroporphyritic, though the lormer term strictly should be reserved for clusters ofequant crystals (Joha nnsen, 1931). iSynncusis texture a l so describes crystal clots butincludes the genetic irnplica tion that the crystals 'swam together' and is thereforebest avoided.)

32

46 Glomeroporphyritic tholeiitic basaltThe photograph shows crystal clots of different sizecomposed of plagioclase, augite and olivine crystalsenclosed by fine-grained intergranular- and intersertaltextured groundrnass.B as al t from u nk now n lo ca lity : m a gn ifi ca tio n x / l , X PL .

47 Glomeroporphyritic hawaiiteDiscrete phenocrysts of plagioclase and olivine, anclots consisting of a lew crystals of the same minerals, arset in a fine-grained groundrnass, in places showing slighalignment of plagioclase needles. Some plagioclases iindividual clots are aligned - this arrangement is commonin plagioclase glomerocrystsHawaii te [rom pla tea u la va s 0/ NOr/ I I S ky e, S CO I/a ndmagnification x 11, XP L.Additional views of g lom eroporphyritic texture m ay bse en in 122,127, 154 and IS8,


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48 Poikilitic enclosure of olivine crystals byaugite

In this photograph approximately 100 crystals of olivineof fairly uniform size are enclosed by a single augitecrystal (at extinction),Peridotite [rom Quarsut , West Greenland: magnificationx22, XPL.

49 Plagioclase chadacrysts enclosed byaugite

Part of a single augite crystal (yellow colour), exceeding30mm in size, is shown here enclosing plagioclase crystals,some of which form clots. The orange crystal at upperright is olivine and the crystal almost at extinction isanother augite crystal.Gabbro from North Skye, Scotland: magnification x 7,XPL.

Mutual relations of crystals: ineqnigrannlar textures

Poikilitic textureRelatively large crystals of one mineral enclose numerous smaller crystals of one,Or more, other minerals which are randomly oriented and generally, but not neces-sarily, uniformly distributed, The host crystal is known as an oikocryst (or enclosingcrystaii and the enclosed crystals as chadacrysts. Although chadacrysts are generallyequant, or nearly so, they need not be uniform in size; sometimes they display pro-gressive change in size from the interior to the margin of an oikocryst, indicatingdifferences in extent of chadacryst growth at the time of enclosure.ll isnot customaryto apply poikilitic texture to the arrangement in which scarce minute crystals ofaccessory minerals are embedded in a crystal, nor to that in which the enclosingmineral is approximately the same size as that included.

33


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Mutual relations of crystals: lnequlgranular textures

Ophitic textureThis is a variant ofpoikililic texture in which the randomly arranged chadacrysts arcelongate and are wholly, or partly, enclosed by the oikocryst, The commonestoccurrence is of bladed crystals of plagioclase surrounded by subequant augitecrystals in dolerite (sometimes referred to as doleritic texture): however the textureis not confined to dolerites, nor to plagioclase and augite as the participatingminerals.Some petrologists distinguish the arrangement in which the elongate chadacrystsare completely enclosed (poiktlophitic textures from that in which they are partiallyenclosed and therefore penetrate the oikocrysts (subophiuc textures. Poikilophitic

texture could also be used when oikocrysts surround elongate chadacrysts of onemineral and equant chadacrysts of another.Fine- and medium-grained rocks made up of many small oikocrysts have a patchyappearance, sometimes described as ophimottled.

34

50 Olivine gabbro containing poikiliticdomains

Large plagioclases, enclosing or partially enclosing, rouolivines at their margins provide a framework torock. the interstices of which are occupied by large augalso enclosing round olivines and small stubby crystalsplagioclase.Olivine gabbro from Middle Border Group of the Skagaard intrusion, East Greenland, magn i fi ca t i on x 12. XP

51 Olivines enclosed by plagioclase oikocrySubhedral, equant olivine crystals here are enclosedsingle large plagioclase crystal,Feldspar peridotite from Rhum, Scotland; magnificatix 21, XPL.Additional views of poikilitic texture may be foundJlI, 114 and 167.


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36

54 Subophitic alkali olivine doleriteIn this view ' plagioclase laths a re embedded in olivrather tha n pyroxene. O ne olivine crysta l is a t ex ti nc tin the X PL photograph and another shows orange inference colour. T he other m afic m inera l in the picturea ug ite s howin g a p ur pl e in te rfe re nc e c ol ou r.Olivine dolerite from Shiant Isles silt, Scotland, magnifilion x 26, PPL and XPL.Se e 121, 126, 128 and 164.for additional exam pless ub op hi ti c le x lure " 121 is particularly i nt er es ti ng b e cahere the p yro xe ne is s ub op hiiic atly e nc lo se d by plagioclaand in 164 pyroxene is s ub op hi uc al ty e nc lo se d b l' k at sii

55 Poikilophitic texture in olivine gabbroF or the texture shown here the term poikilophiticpreferable to ophitic beca use (a) the large augi te enclosome equant olivines in addition to plagioclases, a(b) many of th e p la gio cl as es a re not markedly elongate.Olivine gabbro [rom Lower Zone (I of the Skaergaai nt ru si on , E a st Greenland; magnification lO , XPL.


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56 Ophimottled texture in olivine basaltApproximately fifty augite crystals are shown here en-closing bladed plagioclases and giving the rock a mottledor speckled appearance.O liv in e b a sa lt fro m I sle o j" M u ll . S co tl an d: m a gn ifi ca ti onx 14, XPL .

57 Feldspar-olivine-phyric ophimottledbasaltPhenocrysts of plagioclase and olivine, some in clots, areet il l fine-grained ophimonled ground mass.Olivine basaltfrom Skye, Scotland; magntf ication x 12,XPL .

Mutual relations of crystals: lnequlgranalar textures

Interstitial texturesTwo varieties are recognized on the ba i of the material occupying the angularspaces between feldspar laths:I. In terserta l texture - glass or hypocrystalline materia) wholly, or partly, occupiesthe wedge-shaped interstices between plagioclase laths. The glass may be fresh orhave been altered to palagonite, chlorite. analcite or clay minerals, or it may havedeviuified, Ifa patch of glass is ufficiently large and continuous to enclose anumber of plagioclases, some petrologists would describe the texture as hyal-ophitic, (See also hvalopilitic texture, p. 41,)2. ln te rg ra nu la r te xture - the spaces between plagioclase laths are occupied by one.or more, grains of pyroxene (olivine and opaque minerals). Unlike ophitictexture, adjacent interstices are not ill optical continuity lind hence are discretesmall crystals. Tile feldspars may be in diverse, subradial or subparallel arrange-ment (see also pilotaxitic andfdlY textures, p. 41).

I\S shown by some or the photographs illustrating these textures, a single thinsection may contain both types or interstitial texture In separate, but contiguous,textural domains.

37


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MutuaJ relations of crystals .inequigranular textures

38

58 Intor5ert&1 (hyalophitic) texture intholeiitic basalt

Certain parts of this photograph show lath-shaplagioclasesenciosed in pools of devitrified, deep-broglass. Other plagioclases are surrounded by augitesubophitic manner.Oceanic tholeiite from Leg 34 of the Deep Sea DrilProject; magnification )(65, PPL.

59 Int.ersertal texture in alkali doleriteThe intersertal texture in !is dolerite COLl sists of placlase crystals embedded in analcite (colourless in Pand isotropic in XPL). Other plagioclases are partiaenclosed by pyroxene in a subophitic manner. A cryof o li vin e c an be seen a t the right-hand ed ge o f the viewPPL.Alkali dolerite from Howford Bridge sift, Ayrshire, Seland; magnification )(23, PPL and XPL.See also 126 and 127.


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60 Intergranular doleriteAnhedra l equant crystals of augite and pigeonite OCCllPYthe paces between plagioclase crystals in this sample,Dole rit efr om n ea r t he l ow er m a rg in 0/ Pa li s ade s s il l, Nell'J e rs e y, USA ; magn if ic a ti o n x 60, PPL and XPL.

61 Intergranular olivine gabbroln this example of intergranular texture the rock is coarse-gra ined and the plagioclases have a subpara llel a rrange-ment. Note that the interstitial augites ar e anhedralagainst the e uh ed ra l p la gio cl as es ,Ol iv in e gabbro [rom Lower ZOl1e b o f t he S ka er ga a rdlntrusion. Ea st G re en la n d: ma g ni fi ca ti on xIS, XPL.


39


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40

62 Tholeiitic basalt with two types ofinterstitial texture

In this photograph. patches between some of the placlases lire occupied by brown glass (partly devitrifleand betw een others b y clots of sm all augite crysta ls wout any glass present, i.e. domains of both interserta lin tergran ular tel' lure are prescn l.Tholeiitic basedI[rom Ubek.endt Ejland, West Greenlamagnification x 2 7 . PPL and XPL:

63 Intersertal, intergranular and subophititextures in dolerite

AU th re e o r th ese te xtu re s c o-e xist in [h is r oc k.Doleritefrom Whin sill. Northumberland, England: mafication x 26, PPL and XPL.


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Intersertal. in te rg ra n u lar and su boph itic textures indolerite (continued).

Mutual relations of crystals: lnequlgranular textures

Oriented, aligned and directed texturesSeveral classes of this textural type exist: (a) trachytic texture; (b) trachytoidtexture, (c) parallel-growth texture: (d) comb texture: and (e) orbicular texture.Trachytic textureA subparallel arrangement of microcrystalline lath-shaped feldspars in the ground-mass of a holocrysta l l ine or hypocrystalline rock.N,B, the term is not restricted in use lO rocks of trachyte composition (e.g. seegroundrnass of 47).Some petrologists subdivide trachytic texture with microlite-sized feldspars intopiloiaxitic /(!XI!iJ'e and hyalopiliiic leX/UN!, depending 011 whether the materialbet ween the feldspars is crystalli ne or glassy.' Strictly, however, the 11 1 icrolites inthese textures may be more or less aligned. (For a pilotaxitic texture in which themicrolites are essentially randomly arranged [he term/elly texture exists.)Trachytoid textureA subparallel arrangement of tabular, bladed O~prismatic crystals which are visible[0 the naked eye (Holmes, 1921). While the term is usually applied to crystals offeldspar, Johannsen (.1931) states that it may equally well be used for orientedcrystals of any other mineral.The termsjlow and fiuxion texture are sometimes used as synonyms fo r trachyticand trachytoid textures, however they should be avoided on account of their geneticimplications.

4'1


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Mutual relations of crystals: oriented, aligned and directed textures

42

64 Trachytic texture in a trachyteThis rock illustrates trachytic texture with no gla stween the small, aligned alkali feldspars (i.e. pilotaxitvariety). Note that, rather than there being a single uversal alignment direction there are several domainsthe photograph, each having its own preferred directiof feldspar alignment.Trachyte [rom unknown Czechoslovak ian tocattty : magficattan x 16. XPL.

65 Trachytic texture in trachyteThe somewhat tumpy ground mass alkali feldspars inrock display a subparallel alignment which is part icularnoticeable where they follow the outline of the phencrysts.Trachyte [rom unknown German l oca li ty : magn tf ic a lix 15 , XPL.

66 Hyalopilitic texture in rhyoliticpitchstone

The feldspar rnicrolites in this glassy rock have a preferelongation direction from lower left to upper right; nthe feldspar phenocrysts and opaque crystals the oriention of the ruicrolites follows th e outline of these crystaNote the tendency for the microlites to be arrangedbands.P itch sto ne fro m Isc hia , B ay 0.( Nap le s ,' magn if ic a ti on xPPL.


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67 Trachytoid dioriteThis medium-grained rock contains aligned columnarplagioclases, The cloudy appearance to the plagioclasesresults from very small inclusions of iron ore and mica.Dioritefrom Comri , Scotland; magnifica/ion x 16, PPL.

68 Trachytoid gabbroThis trachyioid texture consists or platy plagioclases,here seen edge on, stacked upon one another. Note thatwhen this rock is sectioned parallel to the plane or theflattening, the crystal alignment would not be evident.Gabbro f rom Lower Zone b of the Skaergaard intrusion,Ea st G re en la n d; magni fi ca : i on X J 2. X PL.

69 Qlivines in trachytoid arrangement inolivine dolerite

In this view, large columnar phenocrysts of olivine, someof skeletal type, are aligned. and embedded in inter-granular-textured plagioclase and augite.Olivine dolerite from Isle ofSkye, Scotland: magnificationu, XPL.

Mutual relations of crystals: oriented, aligned and directed textures

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M utual relations of crystals: oriented, aligned and directed textures

Parallel-growth textureA single elongate skeleta l crystal w hich in thin section appears to consist. of a clot ofcrysta ls havi ng the sa m e elO I1ga tion d irecti on and the same optical orien tation(For illustra tions see 31 and 32.) r n rocks with trachytoid texture it is not uncommonto r neighbouring parallel-grow th crystals to be aligned ( see 31 ).Comb texture (comb layering)Elongate, possibly curved, branching crysta ls sharing th e sa me direction of elonga-lion ..Th e c ry st al s typically form a band, layer, pr fringe with the elongation directionof the crysta ls inclined at 60-90" to the plane ot the layering. (Synonyms me Willow-Lake layering and crescumulate layering, though the latter is a genetic term and.hence, should be avoided.)

44

70 Pyroxene comb layer in a thinlamprophyre (fourchite) dy.keLon g b ra nc hin g augite crysta ls arc aligned at right ato the boundary between the comb-layered rock (beand pyroxene-phyric rock (above). The V of the branchwidens in th e d ire ctio n of growth, which is away fromdyke w all. (See a lso 35 .)Lamprophyre dyke from Fiskaenesset area. South-Greenland; magnificationx S , XPL

71 Comb layers in dolerite dykeTwo types of comb-textured layer are present inphotographs: the first and third bands from theconsist of elongate branching olivine (now lars e rpen Iin ized) and pi agiocl a se crysia Is , Ihe secondfourth bands are pyroxenite dominated by com pelongate, branching augite crystals w ith scarce plagioclcrysta l s in betw een. Til e 111[1rg i 11 0 r the d yk e Iies to the(See also 34 ..)Dolerite. from No rth-west Skye. Scotland: magnificax8. PPLand XPL


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Mutual rei atiuns of crystals: oriented, aligned and direeled texturesomb layers in dolerite dyke (contin lied)

Orbicular texture torbicular layering )See p. 69 lor definiuon an d i llustruuon. In connecuon with th e groupof texturesbe ing considered here. no le 1ha t in some 0 rbicules t he concen 1ric shells have elongatecrystals aligned radially about the centre ofthe orbieule

I nterg rowth tex tu re sI n ti l in sect ion the junci ion be l ween twocrysruls m ay appear .[> .1 strarghi hne, II,iillpl'e I;Urve, 0 r a com plex curve: in t h e Ih iI' d t;


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Mutual relations of crystals: intergrowth textures

.,' /. "I I ',,,'

Micrographic texture (or graphic, if visible wit" the naked eye)A regular intergrowth of two minerals producing the appearance of cuneiform,semitic or runic writing. The best-known instance is of quartz and alkali feldspar,the quartz appearing as isolated wedges and rods in the feldspar. (A micrographicintergrowth of quartz and alkali feldspar is also known as micropegmatit ic tex lurE,)A graphic intergrowth of pyroxene and nepheline is shown in 100,Granophyric textureA variety of micrographic intergrowth of quartz and alkali feldspar which is eithercrudely radiate or is less regular than micrographic texture.46

72 Consertal texture in granodioriteThis photograph of a quartz-rich portion of the rshows several quartz crystals with intergrown boundar(See also 42.)Granodiorite from unknown source; magnification )XPL.

73 Consertal intergrowth texture in gabbroThis picture illustrates an extreme example of intergrboundaries between crystals; the participating cryare all augites (purple, pale yellow, grey and orange),Gabbro from Lower Zone a of the Skaergaard intruEast Green/and; magnification x 25 , XPL


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Graphic granitehotograph of a polished hand specimen of graphic-extured granite in which the dark material 1S smokyuartz and the light material is alkali feldspar.a ph ic g ra nit e fro m u nk no w n l oc ali ty ; m a gn ific a/i on x 3.

5 Micrographic texture in apliteo of the crystals in this view show an intimate micro-raphic intergrowth of quartz and alkali feldspar. In onemiddle right of XPL photograph), the alkali feldspara t extinction, and in the other (middle left) the quartzat extinction. (The PPL photograph is deliberately

to show the Becke line in the higher-reliefineral (quartz) when the objective lens is 'raised")c ro -g ra nite fro m W orc este r, Massachusetts, USA.-

x 60, PPL and XPL.


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76 Micrographic and granophyric texturesmicrogranite

The photographs show several units of in tergro wn q uand alkali feldspar; most are o f m icro gra ph ic typ esome have a radiate arrangement (granophyric texa t th eir m a rg in s, In th e S co ttish H eb rid ea n ig ne ou s pince, rocks like this on e were formerly known as g rphyres in allusion to their distinctive textures.Microgranite [rom Eastern Red Hills of Skye. Scll//Omagnif ication x 20. PPL and XPL.

77 Granophyric textureIn this rock, radiate intergrowths of quartz and afeldspar ar e arranged about euhedral, equant plagioccrystals.M i cr og ra n it e FOn ! S ka e rg aar d iIU fusion, East G I'l'ell/magnif ication x 37. PPL and XPL.


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Granophyric texture (contin ued)

78 Myrmekitic texture ingraniteMuch of the lower part of this photograph is occupied byan intergrowth of quartz and plagioclase: this formsembayment in the microcline crystal which OCCUplCSmost o f the upper part of the field of view.Granite Fain Rubislaw quarry, Aberdeen, Scar/and:magnification x 30. XPL.


Myrmekitic texturePatches of plagioclase inter grown with vermicular quartz. The intergrowth is oftenwart-like in shape and is commonly to be found at the margin ofa plagioclase crystal,where it penetrates an alkali feldspar crystal. The texture could be regarded as avariety of symplectite texture (see p, 53).

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Intrafascicukue textureHollow, columnar plagioclase crystals filled with pyroxene.

Lamelfar and bleb-like intergrowthsParallellamellae, or trains of blebs ,o r one mineral, and all of the same optical orienta-tion, are enclosed in a single 'host' crystal of another mineral. Well-known examplesinvolve lamellae or blebs of sodium-rich feldspar in a host of potassium-rich feldspariperthitic texturey; the converse (antiperthitic textures; and lamel.1ae or blebs of onepyroxene in a host of another (e.g. augite in orthopyroxene or vice versa, and pigeon-ite in augite or vice versa). Other examples include: ilmenite lamellae in (ulvospinel-magnetite) solid-solution crystals; metallic iron rods, and blebs in lunar plagicclases:plagioclase lamellae in pyroxene; amphibole lamellae in pyroxene; and chrome-magnetite lamellae in olivine. Careful examination may reveal lamellae of more thanone orientation and scale and sometimes even fine lamellae within coarse lamellae,i .e, multiple generations of lamellae.Lamellar and bleb-like intergrowths are often attributed to exsolution of thelamellae and blebs from- the host crystal (i.e, solid-stale reaction) and the geneticterm exsolution texture is often therefore applied to tbem. However, laboratoryexperiments in which antiperthite formed from a melt as a result of co-crystallizationof two feldspars, and others in which ilmenite lamellae formed in pyroxene duringco-crystallization of the two phases from the melt, highlight the danger of uncriticaluse of the term exsolut ion texture.50

79 Intrafascicu late texture in doleriteThis medium-grained rock has an iruergrowth textuwhich the gaps in the columnar plagioclase crystaloccupied by augite.Dolerite [rom Garbh Bheinn intrusion, Isle of Skye,land; magnification x 72 , XPL.


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0 Microperthitic texturesThree examples ofperthites are represented here:The first photograph shows fairly broad sinuous lamellaeof a lb it e traversing the tartan twinning of a microclinecrystal.Specimen from pegmatite. Topsham. Maine" USA: magni-

16. XPL.he second photograph shows narrow albite lamellaeform ing a braided pattern in an orthoclase host (upperc imen [rom granite. Ratagan, Scotland: magnification

x3 i1 . XPL.e th ird photograph shows two l arg e a re as of th e fieldview with different orientations of crystals consistingf a n intimate intergrowth of a potassium-rich feldsparnd a sodium-rich feldspar In each case the darker greyre pr ese nts th e potassium-rich feldspar. The pro-o r th e two materials ar e approximately equal so

hat neither is dearly the host - in this case the feldsparis k riown as a mesoperthite.of nepheline syenite from Langesund [jord,y ; magnif ica tion x 32. XPL.


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Mutual relations of ervstals: inrergrowth textures

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81 Antiperthitic texture in tonalitic gneissThe poorly aligned, bleb-like inclusions in the piagclases in this rock are potassium-rich feldspar o r i n tmediate structural state ( i. e. o r thocl a se ) . It i s l ik el y tthe te xt ure fo rm ed in this rock during prolonged higrade regional metamorphism rather than during crysliza tion of m agm a.Tonalitic gneiss from Scourie, North-west Scm/ant i; I 1nification x 2 0, X PL.

82 Lamellar intergrowths of two pyroxenein gabbroThe host crysta l to the lamellae is an orthopyroxene ( ct o ex ti nct ion ); it contains two kinds of lamellae - rtively broad and continuous ones of augite, an d narrodiscontinuous ones of a ugi te , i nc li ne d to th e b ro ad v a r i eG ab bro fro m B ushve ld in trusio n, So uth A fric a; / r I a g n i j ilion x 9, XPL.

83 Bleb-like intergrowth of augite inorthopyroxene in olivine gabbro

In this sample blebs of augite a re embe dd ed in an ortpyroxene host, form ing an 'em ulsion-like' texture. Thothe blebs are irregular in shape they have a com melongation direction and the s am e o pti ca l o rie nta tio n.Olivine gabbro from Lower Zone b of the Skaergaai ntr us io n , E as t G re en la nd : ma gn if ic ati on x 27, XPL.


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84 Symplectite of iron ore andorthopyroxeneIron ore (probably ilmenite) and a small crystal of ortho-pyroxene are intim ately intergrown in a verm icular fashionin the spaces between plagioclase, augite and ilmenitecrystals.Olivine gabbro [rom Lower Zone b of the Skaergaardmuu si on , E as t G re en la nd : m a gn ifi ca ! io n x 72 . P P L.

85 Fayalite-quartz symplectiteBetween the opaque m inera l (ilmenite) and the silicate

in this rock, there exists a com plex boundaryn si st in g o f a narrow rim of fayalite im mediately adjacentthe opaque m inera l, which in places abuts onto aymplectite intergrowth of fayalite and quartz . Theaya lite in the intergrowth and that which rim s the ilm enitev e the sam e op tical o rien tatio n.

Fum Upper Zone II 0 ./ S ka e rg a ar d i nt ru si on .Green land: magni f ica t ion x 32. PPL and XPL.


Symptectite textureAn intimate intergrow th of two minerals U1 which one m ineral has a vermicular(w orm lik e) ha bit.

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Radiale texturesRadiate textures are those in which elongate crysta ls diverge from a commonnucleus. They are m ost frequently found in fine-grained rocks, bu t no t exclusively;f or e xampl e, 34, 35, 36, 70 and 71 show large branching pyroxene, plagioclase andolivine crysta ls in Ian-shaped r ad ia te a rr an gemen ts . A remarkably large number ofterms exists to describe the various patterns, i I1c1uding: fan, plume, spray. bow -tie,pherical, sheaf-like, radiate, radia l, axiolitic, spherulitic and variolitic. A ll exceptth e last three (w hich are defined and i ll us tr a ted here), a rc or se lf -ev iden t meaning.SpheruUtic textureSpherulites are approximately spheroidal bodies in a rod: they ure composedor an aggregate or fibrouscrystH ls or one or more minerals radiating from a nucleus,with glass or crysta ls in between. The acicular crysta ls may be either single, simplefibres or each may have branches along its length; any branches mayor may no tshare the sam e optical orientations as their parents. The most com mon occurrenceof spherulitic texture i a radiate aggregate or acicular a lkali feldspar w ith glassbetw een them , though quartz or other minerals may be present resulting in an inter-grow th texture. Should the spherulite have-a hollow cen tre it is known as a hollowspherulite, and i r i t compri se s a series of concentric, pa rtia lly hollow shells, th e termlithophysa is u se d.A xiolites differ from spherulites in that radiating fibres extend from either end of a

linear nucleus (i.e, from a small acicular crysta l) ra ther than a point. They could beregarded as a variety of overgrowth texture (p. 58), as indeed could those spheru-liles w hich grow about visible crysta ls rather than 011 subm icroscopic nuclei (e,g. 88).54

Fayal i le-quartz sym plectite (contin L ied)


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86 Plagioclase spherulite in doleriteThis pherulite com prises approxim ately tw enty elongatec ry sta ls o f plagioclase, eacb having a different opticalorientation. It is an 'open ' spherulite, in the sense thatthere is much space between individual plagioclase crys-t a l s ; t h e spaces are occupied by coarse augite, columnarplagioclases no t r ela t ed to the spherul ite, and sm allerspherulites.Dolerite from Garbh Bheinn intrusion, Skye, Scotland:lIIagl1ift~'(/lio!lx32, PPL and XPL.See 126/01 ' a similar example.

87Spherulite in rhyolitehe spherulite at the centre of this photograph consists ofdense mass of very fine intcrgrown needles of bothuartz and alkali feldspar radiating from a commonAbove and below, th e spherulite abuts onto

whereas to left and right there is glass.from Hlinik, Hungary: magnification x 27, XPL.

Mutual relations of crystals: radiate textures

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Variolitic texture

A fan-like a rrangem ent of divergent. often bra nching. fibres: usually the fibres areplagioclase and the space between is occupied by glass or granules of pyroxene,olivine or iron ore. T his texture d iffers from sp her ulitic in tha t 110 d is cre te s ph eric a Ibodies are identifiable; ill fact, each fan as seen in thin section is a slice through ac on ica l b un dle o r a cic ula r c rysta ls.56

88 Compound spherulites in rhyoliteBoth single and com pound, or clum ped, spherulitesurrounded by glass in this photograph. The spheren clo se m icro pb en oc rysts o f p la gio cla se a nd b io titecolour variation in the spherulites is ca used by v a r i ain d en sity o f fib re s.R hyolite from G lashuue. H ungary .. m agnifica tionPPL and XPL.


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89 Variolitic olivine doleriteTheolivine phenocrysts in this sample are set in a ground-mass consisting or many fans of diverging plagioclasen e e d l e s with augite crystals in the interstices. Note howall tbe fans diverge from lower right to upper left, indicat-ing progressive solidification in this direction. Note alsothe branching character of some of the plagioclase fibres,Olivine dolerite from Skye. Scotland: magnification x 27.PPLandXPL.

Radiate intergrowth of plagioclase andaugite in dolerite

his unusual radiate texture occupying the centre of theiew consists of two, mutually perpendicular, columnar,lagioclase crystals, the elongate gaps in which have aadiate distribution; these gaps are occupied by a singleugite crystal. rather than by many crystals. This kind of

texture differs from a spherulite ; it is more akin toeletal growth (p. 20).leritefrom lngia intrusion. West Greenland: magnifica-x 27, PPL and XPL.


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Overgrowth texturesThis term applies to textures in which a single crysta l has been overgrown either bymaterial or the same composition, or by ma te ri al of the same m ineral species butdifferent solid-solution composition, or by an unrelated mineral. There are threetypes: (a) skeletal and dendritic overgrowths; (b) corona texture: and (c) crysta lzonmg.Skeletal or dendritic overgrowthsPorphyritic rocks w ith a glassy or very fine-grained groundmass may show delicatefibres or pia tes extending from the corners or edges 01' ( be phenocrysts. The over-growth and the phenocryst. need not be the same mineral.

.'11

, j'if, - II~' - I : " ( I t

" ! " " ' 1 I, i\ .-\. ''lV I- . . . . - A. "58

Radiate intergrowth of plagioclase and augite in do(continued)

91 Overgrowth textures in rhyoliticpttchatoneThe faces of the phenocrysts of alkali feldspar andnetite ill this glassy rock have acted as locations focleation of dendritic overgrowths of (?) alkali feldDendritic crysta llites arc also present in thegroundmass .Pitchstone [rom A /'/'ClI1. Scotland: magnifica/ioll x it ,


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92 Corona textureIn the centre of the photographs a twinned and zonedaugite crystal is mantled by green hornblende of non-uniform width.Quam dioritefrom Mull ofGalloway, Scotland: magnifica-tion x 43. PP L G ild X PL .

Mutual relations of crystals: overgrowth textures

Corona textureA crystal of one mineral is surrounded by a rim. or 'mantle', ofone Of more crystals o ranother mineral, e.g, olivine surrounded by orthopyroxene, or biotite urroundinghorn blen d e, Such rela lion ships are often presumed to resul r from incorn pletereaction of the inner mineral with melt or fluid to produce th e outer one and for thisreason [he equivalent genetic terms reaction rtm and reaction corona are frequentlyused. The special term Rapakivi lex lure is used to describe an overgrowth by sadieplagioclase on large, usually round, potassium-feldspar crystals, and kelyphilictexture is used for a microcrystalline overgrowth o r 1 1 b J " 0 u S pyroxene or horn blondeon olivine or garnet.

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60

93 Corona textureBetween olivine and plagioclase crystals in this rock tis a 0.02-0.06 mm wide corona which consists o f eone or two zones (I) radially oriented, fibrous, b rhornblende; or (2) colourle s pyroxene (see middlphotograph) surrounded by radially oriented, fibrobrown horn blende. Ana lysis of the pyroxene suggestsit is a submicroscopic intergrowth of augite and orpyroxene.Olivine gabbro [rom Thessaloniki. North Greece .. m a[icauon x 100. PPL and XPL.

94 Rapikivi textureThe texture is of large, round potassic feldspars, somw hich a re mantled by sodic plagioclase rims, o th ersno plagioclase rims. In the first photograph, which i spolished hand specimen, the plagioclase rims havgreenish colour contrasting with the pink potassicspar. The second photograph is of a thin .ection ofsame rock.Granite [rom Eastern Finland; magnification x 2 (photo); x 3, XPL (second photo).


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Rapakivi texture (continued)

95 Zonal arrangement of melt inclusions inplagioclase

Several stages in the growth of this plagioclase crystal canb e picked out by the bauds of minute melt inclusions. (Seealso 45.)Feldspar-phyric dolerite from Isle of Skye, Scotland:magnification x 9, PPL.


Crystal zoningOne or more concentric bands in a single crystal are picked out by lines of inclusions(95) or by gradual or abrupt changes in solid-solution composition of the crystal.As regards the latter type of zoning, a large number of patterns ar e possible, th ecommoner ones being illustrated graphically and named below, using plagioclase asall example.Normal versus reverse zoningT he se te rm s specify the general trend of solid-solution composition from core to rim., orrnal' indicates high-temperature compcnent-s low-Iemperature component(e.g. An-rich plagioclase .....A b-ri ch plagioclase, see ig. C) and 'reverse' indicatesthe opposite.Continuous' versus discontinuous I zoningThese terms indicate respectively a gradual or an abrupt change ill composition.Figure C shows examples of continuous normal zoning and Fig. D an example ordiscontinuous normal zoning. ontinuous and discontinuous zoning may alternate(Fig. E).

IThese terms are nOI the same as continuous reaction and discontinuous reaction oj crystalswith melt,

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Mutual relations of crystals; overgrowth texturesFig. C Three examples ofcontinuous norma! zoningrepresented on a sketch graph

Fig. D Discontinuous normal zoning

F ig . E Combined continuous and discontinuous normalzoning

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co.;::'~,o0.EouOJ


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96 Zoned plagioclaseThe central plagioclase phenocryst in this photograph isdiscontinuously zoned, having a homogeneous corernantled by a more sadie rim; the rim has continuousnormal zoning resulting in variation of the extinctionposition on rotation of the microscope stage. The crystali s th us partly discontinuously and partly continuouslyz o n e d .D olerite f ro m Isle o f Sk ye, SCOI/a nd ; m ag nifica tio n x 43 ,XPL.

F ig . F M ultiple. even zo nin g


Multip le zo nin gThis term is used lor crystals having repeated discontinuous zones, J r the zonesshow a rhythmic repetition of width. the pattern is known as as ci i/ a t o ry z o tung .The overall compositional trend of the multiple zoning may be normal or reverse orellen (in which there is no general trend from core to rim). Individual zones may beofuniform or variable composition, such that the zoning pattern on a composition-distance graph is square wave, step-like. saw-tooth, curved saw-tooth, or somecombination o f these (see Figs. H-1). However, these are details which only lIerycareful and lengthy optical examination or electron-probe microanalysis wouldreveal.The reader should appreciate that the sketches in figs. C-J are all idealized andthat in real crystals the oscillat ions will be less uniform: furthermore multiple orosci Ilatory zoning may only occupy pan of , I crystal, the remainder perhaps beinghomogeneous or continuously zoned.

'-- -AnOL- ___

Core Rim63


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Mutulll relations of crystals: overgrowth texturesAn100

AnaCore

Fig. G Oscillatory, even zoning

- - - L...........J '---l

RimFig. H Oscillatory, normal zoning: step-like

AnoL_--------------------------------------Core Rim

Fig. I Oscillatory, normal zoning: saw-tooth

AnoL_ _Core

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Rim


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Fig.J Oscillatory, normal zoning: curved saw-tooth

Zoned plagioclaseshis photograph illustrates several styles of zoning in thewo plagioclases comprising the glomerocryst. Combina-ions of discontinuous, oscillatory and convolute zoningre present, together with zoning picked out by a band ofelt inclusions near the margins of both crystals.orphyritic andesite from Hakone volcano, Japan; mag-

x 24, XPL

Mutual r el at ion s o f c ry s ta ls : overgrowth t ex tur esAn100

An O L- _Core Rim

Convol ut e z on in gThis is a va rie ty of m ultiple zoning in which some of th e z on es are erra tic a nd h av en on -u nifo rm th ic kn es s ( se e 9 7) .

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Sec lor (or h0111'glass) zoningAs seen in thin section, this ideally lakes the form 01' four triangular segments(sectors) with a common apex (Fig, K(b)). Opposite sectors arc chemically identical.whereas adjacent ones differ in composition (though possibly only slightly) an dhence in optical properties, Each sector may be homogeneous 01' show continuous ordiscontinuous or oscillatory. normal or reverse or even zoning. In three dimensionsthe sectors are pyramid shaped (Fig. K{a, and, depending on the orientation of ihec ry st al w ith respect to the plane of ,1 thin section, a variety of patterns may be seenin thin section (Fig. K(b) (rn. If the sector boundaries are curved, the pattern canresemble that of an hourglass (Fig. Keg), Sector zoning is a com mon feature ofpyroxenes in alkali-rich basic and ulrrabasic rocks, It has also b een seen in p la gio -ciases in a few quickly cooled basalts.

b d e 966

98 Zoned olivinesZ on in g is n ot con fin ed to feldspar crystals, Here, eachofthe three olivine phenocrysts in the cluster has a home-geneous core surrounded by a continuously normal-zonedmantle, as indicated by the variation in interferencecolours,Ankaramite from Mauna Kea, Hawaii: mognif icauon x 4 3 ,XPL.

Fig, K Schematic representations a/sec/or zoning


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99 Sector-zoned augiteTIle picture shows a simple sector-zoned augite pheno-cryst containing elongate melt inclusions: the crystalspartially enclosed by two of tbe sectors are olivines,B ss ex ue from C raw fo rd jo hn , S co tla n d: m a gn ific ati on x 40.XPL.

100 Sector-zoned pyroxenesTwo sector-zoned titanaugite crystals are illustrated inthese ph tographs ; that on the left is complicated byFonning at one end a graphic intergrowth with nephelineand leucite: the other crystal has an intriguing figure-B.shaped core, with a discontinuous, sector-zoned mantle.M elanocra I i c nephe line 1 1 1 i cr os ye n it efr om Vog el sb er g, WeslGermany ; magn i fi ca t ion x 7, PPL and XPL.

M u tu al r ela tio ns o r c ry sta ls ; o ver gr ow th tex tu re s

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Banded textures (band ing)Textures or this type involve two, or more, narrow up to a few centimetres), sub-parallel bands in a rock which are distinguishable by differences in texture, and/orcolour and/or mineral proportions. The (em) layering is also used by petrologists;while it includes banded texture, it is also used for larger scale stratification, Anexample of banded texture due to textural differences is illustrated En5, and 10 3and 104 show examples resulting from extreme differences in mineral proportions.68

101 Oscillatory- and sector-zoned,bearing pyroxene

The augite phenocryst occu pying most of thisissector-zoned and each sector displays oscillatOrInclusions of nepheline augite and magnetiteranged in trains parallel to the oscillatory zones.Tephrite FOn! Monte Vulturi, Ma/fi, Italy;x27. XPL.

102 Osci Ilatory- and secto r-zoned pyroUnlike the pyroxenes in 99 an d 100, this S!Xpyroxene has. some sectors bounded by moreI~lce, e.g, the sector on the right is terminatedfaces, and that on the left by three faces. Thecloses plagioclase laths, an olivine (blue coloupyroxene crystal (orange colour).Essexite from Crawfordjohn, Scotland: magnifica/lmXPL. .


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103 Olivine and chrome-spinel banding (orlayering)

The photograph shows two bands, one rich in olivine,with scarce disseminated chrome-spinel crystals, and theother rich in equant chrome-spinel crystals with scarceinterstitial olivine.Banded dunite-chromitite from Skye, Scotland; magnifica-tion x 11, PPL.

104 Anorthosite-chromitite banding (orlayering)

This hand-specimen photograph shows alternating bandsof anorthosite (white) and chromitite (black). Theyellowish-brown crystals in the anorthosite are enstatiteand the black particles are single crystals and glomero-crysts of chrornite.Banded anorthosite-chromitite from Critical Zone of theBushveld intrusion, South Africa; magnification x 2.

Mutualrelations of crystals: banded textures

Comb layering, orbicular texture, ocellar texture andeutaxitic textureComb layering (see p. 44, 70 and 71) and orbicular texture (105) are particularlyexotic kinds of banding. In the latter, 'orbs' consist of concentric sbells of rhythmi-cally alternating mineral consti union. Within the shells the texture may either begranular or elonga te crystals may be radially arranged. 'Orbs' may reach a fewtens of centimetres in diameter. A further variety of banded texture, eutaxitic,occurs in some tuffs a nd ignimbrites and consis

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