Contents

Part 1 The tex tures of igneous rocks (Numbers refer 10 pholOgr:lphs nOl 10 pages)

lntrOtluclion

Crp' lallini l r

Granularit.I'

TernlS refefflng 10 "hat the aidt'd and unaided eye can Of Cltnnot see

Terms indlc;lling ab~olulC Tange:. of grain Size

Terms indicating rc!a!lvc sil'" of crystals

Crystal Shllpes

Terms indicating q ualil~ of Ih(' development of faces on cryst;t )s

Terms mdicaling three-dimcnsioMI crystal shape General three-dimensional terms Sp..--cific three-dimensional terms

Skeleta l. dendritic and embaycd crysluls Parallel-growth crystals Sieve-textured c rystals Elongate, curved. branching crystals Pseudomorphs

Mutual relations or crystals (and arnorphous materials)

Eq uigranular (("lUTeS

lnequigranular tc.~ \urcs Seriale IC,lll.lrc Porphyritic texture

1- 9

10 14

15 17

18- 22

23 2S

26 30 31- 32 33 34-37 311

39-43

44 45

GJomeroporphyntic tcxture 1'00kllillC texture OphillC te.~ture Inter~llllal te"ture~

Oncnted. aligncd and dIrected te .~1Urcs T mchytic to:turc T rachytoid texture Par.tlld.growl.h te.~turc Comb texture (comb layering) OrbIcular texture (orbIcular byermg)

Jnlcrgrov.lh textures Con>ertallcxture M1crographic texture (or graphic. If VISible with the naked C) e) Granophync texture Myrml.'kllil: texture Illlrllfasciculate texture Lamellar lind bleb·hk;:: intergrov. th~ Sympil'C'lItl" te.-nure

RadIate textures Spherulitlc texture \':HlohIlC texture RadIate lfltergrowlh

O\'ergro\\ th texturC$ Skektal or dcndTll){' o\crgrowlh ~

Corona texture Cry~tal zonmg

B.mdc-d 1C: .~tures (banding) Comb layering. orbIcular texture. Ilnd occllar IC.~turc

CaVH) textures Vc~icular Inturc Am)gdalOldal texture Mia rohllc texture LLlhophys;t or (slonc-·ball) texturc

Part 2 Va rieti es or igneous rocks (Numbers refer to phOiographs not 10 pages)

Introducllun

lJltr:lba~ i c rocks

Dunae Pcridotlte Kimbcrhlc Garnet peTldo!ite Pyroxenllc Komatinc i\kymtochnc Homblendltc

Basic rocks

Basalts vaL Tholeiitic bas.11t vaT. AlkalI oltvlIle baSlllt \'ar. Lunar low-T i basalt var. Lunar hlgh-Ti basalt vaT. PICTitIC basalt ""r Ank aramite

~,

~

46 47 485 1 52 57 58 63

.-166

M" 67 69 31 32 70 71 104

72- 7.l 74 76 76 77

" " 80 to "'115

MH8 8. ... 91 92 94 95 102

103 104 105

1116 lOS 109

110 II I 112 11 3

"' 115 11 6 11 7

11 8 119 12() 121 122 123

107

SpltilC Gabbro Teschenite EssexilC Doler ite Norite Anorthosite

intcrn)('dillte rocks

Andesite Boninit!.' Diorite Tona.lite Kentul!enitc Monzonite Dacite Granod iorite Trachyte Syenite Shonkinite

Acid rocks

Rhyoli te Pantellcrite Granite Albli granite

Alkaline and miscellanl"Ous rocks

Phonoli te Lcucite phonolite Nosean iellcile phonolite Pscudoleucite phonolite Blairmorile Nepheline syenite Malignite Soda lile syenite Nepnelinite ljalite Urtil~

Basanite Tephrite Olivine melilitite Lellcitile Fergusite Minette Alnoite Mafuri tc Fi!zroyi!e Wyomingite Madupi!e Caroonalile Chondri te (meteori te) Achondrite (meteorite)

Pr~p(JrmiOIl oj (I lhill seC/ion oj rock

Refi,'rf!nccs

Imll'X

124 125 126 127 I2S 129 130

131 I3Z 133 134 135 136 137 138 139 140 141

142 143 144 145

I" 147 1411 I" 150 151 152 153 154 155 156 157 158 159 160 161 161 163 164 165 166 167 16Jj

169 170

Crystallinity

Igneous rocks mnge in crystallinity from entirely crysta ls to entirely glass. Adjcctives usc<! to describe t!lese stales are shown on the followin& scale:

f(XJO ", cry:;wls

holocT)'STUllme hypocrY:;Tallinl'L or h),pohyaline

IOO "'o glu~

holohyulllll'

The adjectives g/(lS5),. )'iTreoll.l' and hyaline all indicate that a rock i.~ more or less completely glass.

L flJ'f/OCrplaflml' r(){"ks ran IN! d .. ,raibt'11 mOrl' prI'CIJI"), by Sllllll lg 1M r,"IIlI/'·,. prO/N)f/WIIJ of tTl'SIOIs /0 gllJSS,

4

1 Holocrystalline anorthositic gabbro

Elongate crystals of plagioclase feldspar. some wrapped round olivine crystals. fonn a framework in this rock. the interstices of which are filled with smaller plagioclasc. olivine and augite crystals. The purplish-blue area at the lOp nght of this photograph is an augite crystal \\ hich incl udes a number of small plagioclase and olivine crystals.

PerpendiCli/ar Fe/dspflr gabbro f rom M idille Border Group of Ihe Skaergaard i/llrusiOIl. EasT Greelt/l/ltd .. magnifica/iOIl xl, XP./.. .

2 Holocrystalline gra nite

Crystals of biOlile. quartz, 'pcrthitic' pot:lssnml·rich feld· spar (large crystal bottom right) and zone<! sodium-rich feldspar makes up this granite. The speckled appearance in the cores of the plagioclase feldspars is caused by fine Inclusions of mica ,

Gralliu.' from Ross of Mu ff. SCQf/mlll : maglfijirmiQI/ x 14 . NPe

3 Hypocrystalline pitchstone with perlitic cracks

Crystals of plagiocla~. biotit~ and magneti te in this rock are set in glass (black in XPL) which has spherical frac­tures known as pt>rhtic cracks : these appear as circles in thin section.

Dacite /rum Chenmil:, Easl Germany; magnifico /ioll )( ZOo PPLand XPL.

4 Hypocrystslline basalt

Small olivine phenocrysts (colourless in PPL) and colum­nar, skeletal titanaugite crystals (pinkish-beige colour in PPL) are enclosed by murky brown glass. No plagioclase has crystallized in this rock. The deeper pink colour around the margin of some of the ti tannugitcs is a narrow mantle ofTi-rich amphibole.

BII.wll from QUtlrsl/t . West Grt'cn/and: magnifica/ion )( 35. PPL (Inti XPL.

Crystallinity

5

CrystaliinilY

"ypocr}~lalhlle ba~lh (continued)

5 Glassv rock

The photograph s.hows abundanl. vcry small crystals (probubl) quartl or feldspar) cnc1os;,.-d b} glass, Note the banding caused by (al differences in abundam.:e of crystal­lites, (b) crystalhtes in the lighter bands having a shght prcfcrrl-d alignment and (c) differences III colour of the glass, T he small brO\.\n, isolated round Objl"Cts arc known a~ 'spherulth .. os' (~ec Splwruill/('/t'_\'/lirt'. p. 54). ($cc liiso 14.J

l'ill"ll,I /{Jfl'· (rom Amm. SC{JlfOfld .- flitly,wjiclIIioll )( 12, PI'L.

6 Glassy basalt threads - Pele's hair

These filaments of bils:Jh glass fonn when partkles III a molten lava spray are Cilught by the v.md and drawn OUI Pek IS a mythical lady. believed by native I-Iawumms 10 reSide wllhm the volcano Kilauea. (Colllrasl 7.)

Sf/t'/'IJIII'" from frl(l AI,: I"()/C(/IIQ. ElhioplU , IIItlKlllilca/irm x 8, "I'L.

7 Glassy particles of mare basalt in lunar soil

Pieces of glass. many of them spherkal. are orange·bro\\ n or black in colour. Some of the darker ones are partially crystalline. These particles were fonned by rapid cooling of droplets of basalt melt: it has been suggested that thl: droplets formed ei ther in a fire-fountaining lalla erupllon. or by meteorite impact lnlO a lall,l lake or into a molten or solid lalla flow. (Contrast 6,) The scarce. irregularly shaped fragments are pyroxene (pale bro .... n) and feldspar (colourless).

Lunar ba.wlt 74110 from Tallrus Li//fOIl Voller colle,,/!,tl b .• ' Apollo 17 Q.f/rOIffIll/.f ; magll!/ic(l/iOlI x 43, P PL.

7a liquid Immiscibility Globules of one glass in another are found in soml: rocks and these arc altributed to immiscibility of the two liqUIds. In this rock they can only be seen lit IIc ry high po"cr in thin films of glass between la ths ofplagioclasc.

SIX'dmen/rom hasafll(ll'(l. lA I'(I beds NO/iOlw/ M OllumelJ/ Cali/Ofilia. U.S.A.: IIItl.l:llijiC(I/ioll x 600. PPl~ .

C!')'s tall initr

Glass. orde\'itrified glass. is oftcn an important constituent of the pyrod:lstic rocks known as a.rir:tulf /1ifJ.( :111d ash-flow mffs (or Igll imilfllI'S). Such rocks tYP,c;III}' h;r\'c frogmcn/a/ /e:nllres, i.e. they comprise mixtures of fragments of rocks. crystals and glass. pn:domiml11tly less than a mil limetre In size (89). In an ash·O(m deposit the glass fragmcnts may ini tially be plastic enough to be partly or wholly wcld!..'(1 to­gcther as the weight of overlying materi,,! causes compaction of the constituent fmgmen ts: such a rod is known as a ,.·"Mrtlll!ff(8b), If sunk!cn' hl::11 is :l\ailahle. glossy fragments dcvitrify.

7

Cr,-Slallinil)

8a Glassy unwelded rhyolite tuff

The glassy fragments in this rock. some of which arc banded and slightly flattened. arc nOI welded to one another. They and the crys tals of quartz .tnd feldspar lIfC

embedded in fine glassy particles (ash).

Recel1l ig"imbritt>, f rom Whokaullle. NOrlh Island, New ZeolUfld: mag"ifiemioll x 46. PPL

8b Glassy welded crystal tuff

The glassy nlalr!;\: in Ihls rock has an apparent discon­tinuous lamination caused by extreme compilction and v.eldmg of original pumice fragmcnts_ Tho.: regular align­ment of the fl anencd fragments tS known :1S ellUI.Htll.' textllre.

Welded 1/41 f rom Tibchi gronile rll/g-,·amplex. Nigerill: magnificO/ioll x 36. PPL

9 Tuff

This fragmental rock consists of crystals of quartz, alkali fel<b par and plagioclase of various sizes and shapes_ PICCt'S of glassy rhyolite (e.g. centre} and pIeces of fine-grained tuff, ,Ill enclosed III a fine-grained banded ash matrix which originally may have been glassy. (Sec also 13.)

Tuff from L/{/nellll"('dd, Wales; magflijicllIioll x 10. PI'L anll XPL

1 PcgOlaUL1C I{,'IUI'<: 1.1' (1 l(lril'H!lf phall"'()(n',-/u/lm" on II /u, h th .. <"'.lW/.\ "I"(' Xlrildll.!flr ll"~". hi~Kt'r Ihllll I 11"m. Imil 11/ '(1'" III.</aIlCI'< lip If) IIIUIIl 111""('.(. :Tht" rerm l.lpll~nc u wm('lm/('S I •• <ed fo, uphuIII"(, 't"K'h "'/II('h Itl('~ pit,.IIIKrr .• /< (~I!. 6CJ. 63. 1117) ' l'd~IIIC tC.'tllre " «(HI,../mw. upp/ied {tI li/in',,".\ ,ock. \\ Ilh ill­,/<:/mtd. "lmOlI rr.\IU"Ul Hal/lIt(', ~'t.r-p'''arl:'''1!. Im'IIS N)lup,u"d (>/ """ " (lr '''.'.1 ('qmg'mlll/i" UX/:'I'KUIi'X Oll/ullrI: mit! ,,1~1I1i IHtI­'pur. TI,,, Hun" fclsne ,:, IJ(I('H IIppb<,t/ '" '1,,1. f/H-k •. ullhQI,gh Ihn il IIWrI' ."OmlUmrlr // (wid 1"'111 [<I' jrnr-~rllm"rI unrl mUlr,,,,1 of ,"'­<"N'r/III/IIIOt/f' ,..0 m-rlOF't'lIC'I',

10 Phanerocrystalline granit es

The crysla ls In the tWO granites. illustrated here in hand specimen. arc clearly visible 10 the naked eye. Although the rocks conUlin the same mineral~ (alkali feldspar. plagioclase feldspar, quartz and biolite) the proportions of the minerals are not the same. and this influences Ihc rock lexlure. ... Thus the Shap grani te conwins two distinci ~i les of potassium feldspar crystals (pink), whereas the Eagle Red Granite has only one.

GUlllifl' from SImp, EII!!/mlll (OPPQSif") 1II1I1·f.oj;{e Red' grllllill', SI)IIIII Africa (IIC"I pi/g.,): bolll l1lugll !fica/iO/u x I.

A Ihlll S('('IIOII 1'/('11 of Iltt' Slwp grallilf' i.f silo ll / / ill {44

Grnnuhtrit:

Gmnula ri ty

This property embraces three different concepls: (I) \\hal Ihe aided and unaIded eye can or cunnQl.sec: (2) abi;Olute cryst:l! SiLes (p. 12); and (3) relative crystal S l/.c~

(p.14).

Terms referri ng 10 ",'hal Ihe aidl'd and unaidl'il eye COli or cannot Sl'C

Pluulf'mr ,yswllinl' (plulIIuilic textu,e or American petrologists) atl crystals of thl' prinCIpal minerals cnn be distingUIshed by the naked eye (sec 10).1 Aplulllllic alt crystals, other than any phenOCr)')L~ prl$(lIt (see p, 14). (,1111101 be distmgUlsheo by Ihe na ked eye.' Two sub-types exist:

(a) Mit:rocr),sta/lim' crys tals can be idenlified ill thm $('("Iion with a pctrogmphic microscope ( I I). Crystals only just lll rge enough to show polarization colours (less than O.Olmm) are called micm/m'$.

(bl Cryplocry,wal/ille J - crYSlals 11rc tOO small 10 be identified even wilh the mIcro­scope (12 and 13). Globular. rod- like and hair-like crystals which are 100 small 10 sho" polarization colours ate known a..~ cr)"sfal/lfl'f.

•

Granula rit~'

10

Phanerocryslulli l1c granites (continued)

11 Microcrystalline olivine basalt

This rock consists mainly of plagioclase feldspar. aughc and olivine bUI. wi thoul the aid of Ihe microscope. individual cryslals would not have b<."'Cn distinguishable. In parts of the photograph the randomly armnged rccHlOgu!ar plagioclascs arc enclosed by areas showing unifonn yellowish interference colours. these are a ugite crystals.

Olivinl! bll$ali [rom North-west Skye. Seo/Ill/III ; !/lagllijil'll­tion x II , PPL.

12 Cryptocrystalline rock

Comparison of these two photogr<1phs shows Ihal Ihe brown material in the PPL view is birefringent but that Ihe individual crystals are of submicroscopic size. The clear areas in the PPL view arc slightly more coursely crystalline. as can be seen in the XPL vicw.

Rhyolile Irmf/ Island of Palliefieria. III/Iy: magnification )( 72. PilL olld XPL.

Cryptocrystalline rock (continued)

13 Cryptocrystalline matrix in a tuff

Cryptocrystalli~ texture is common in tufTs (i.e. con­solidated ash), as In the malrix or this rock. Here the matrh!:. encloses rr.lgmc:nlS or shale and qu:mz cl)'5lals (See also H and 9.)

Tuff from I/nkllowli localll)' ; magllijicalloll x 16. PPL (111(1 XPL.

Grunutarily

11

GranularilY

TtmlS indicating absolute ranges of grain s izt'

C(}(,,~e'f!,raill('d - crystal diameters > 5 mm Ml'llillm-gmineJ - crystal diameters I 5 nlEn rille.grained - crystal diameters < I mm I

The next six photographs ( 15. 16 and 17) were aU taken at the same magnificatIon ( )( 27) to indIcate how grain size rei:ltes to the number o f erysulls seen in it shell tlc1d of view (4.1 x 3.1 mm), <lnd hence the eXlent or the ICXl'llre visible al lhal magni. fication . While the ovcrall texlure is r.:cogniza ble in the fine-grained rock. it is not so in the coafS(" o ne and a low-power objective lens would be neces~lry to examine il 'Idequalely. Pelrogmphic microscopes rardy have a sufficiently low.power obJcctive l.::n5 for examimng Ihe textures of coarse-grained rocks; a hand lens should be uscd for these. wilh two sheets of polaroid, if available.

' Some' ptlrQlogLSIS mrlutk (lnm/!" r(lngC' . <O,OJmlJl. whu'II IIII')' rail \Cr) fine· grained.

14 Pitchs ton e containing crystallites of two s izes

Radiate clusters of crystlilliles arc SCI here in glu.ss. The bulk of the glass contains even smaller crystallites. causing the grey colour. whereas adjacent 10 Ihe larger crystallites the smaller o nes are absent. This is a higher magnification view oflhe rock illustrated in 5.

Pi/chSIQIlt' from Arr(lII. Scol/and: mllgmjh:(lIiOlr )( 52. PI'L.

1 5 Fine~9rain9d gabb ro

T his rock contains phlgioclase, orthopyroxene. augile and magnetite ; some of the onhopyrollene crysta ls (low birefringent mafic mineral) contain narrow J:unclJae o f augi te. Although the rock is fine grained. it is called a 'gabbro' because it is from a large intrusion : the fin!.' grain size results from quick cooling al the mtrusion margin. Another lenn thaI could be used for this rock is microgabbro (St.~ p. 78).

Gabbro from cllilll'd margin of Ille Skaergaard illlrllsiOll, Eas/ Green/and: mag"ffica/ioll x 27. PPL (Iml XI'/"

Fine-grained gabbro (continued)

16 Medium -grained olivine gabbro

The spaces between the Ulbular crystals of plagioclase in this rock arc occupied by augi te and ilmenite . Al lhe top right of the picture the plagioclase abuts onto an olivine crystal. The augite crystals contain lamelhlc of orthopy­roxene.

Gabbro f rom Lower ZOIlr b of the Ska(!rgaard illlfllSUJII ,

£fISt Gref'llftl/ld .. /I/(lgllijic(IIiOIl x 27, PPL (111(/ X PL.

Granularity

Granul:lril ~'

. '~ -' \

, , ~ " / .j •

TernlS indicating rl'l3 tiH~ sill' of crystals

f:.(f/lIgrtlnlllllr all crystals arc of appro:-.:unately the same Sll.c. lllrl/mgrtmu/ar - crystals difTt:r substantial I) in size. A common variety. /Wrphyruir texture, m ... ol..-cs rc1ati\'cly large crystals (phenocrYSlsl) embl.-dded in liner-gmincd groundmass. (N. 8. The Silme mineral may be present as both phenocrysts and groundmass.) In naming II rod. with porphyritic te.uure the minemls present as phenocrysts should be listed and followed by the suffix ·phyrie. c.g. ·hornblende· pigeomlc-phyric andesite' . Ho\\e\'cr. if th\: groundmass is glassy. the term 'Vltro­

ph~re' is used, e.g. an 'olmnc vllrophyre' has oli \'ine phenocr)sts sct in glass: the texture 111 thiS 1:1ISC IS referred as I'"ropilyril' (3. 142). Scrimt' tC~turc Involvc~ a con­tinuous r;Ulgc in sizes of crystals of the principal minerals; if the crystals show II broken series of SII.CS. the mC4U1gnlnuhlf texture is said to be Multll. ClUtion is neeeSSltr) in the identification of seriate and hiatal textures. since the dimenSions of II erystal,n ;1 thin section depend on the- :Ittltudc of the intcrsection of the cry~tal in three dmlcnsions.

I TnI' prrfi:r; mICro- nw.)' be added,o phenocrYSll> IOhich hal'!' lila"'''''''' bt-II'~.·" 0.1)$ ullIl /Umm ( t&. 'afmnt! minophf'nMT)'SU '),

17 Coarse -grained olivine gabbro

At this magnification only pans of three large olivin<.'s and one plagioclase! are viSible. such that lextural relations are not determmable in this single V1ew.

Gabbro frQm RJIllIn . SCOllond; magnifictl/iO// x 27, PPL alld XPL

18 Equigranular peridotite -

Uniformly-siLoo olivine crystals, some of them in clots, form the bulk of this rock, with plagioclase filling the interstices, Thc black malerial is microcrystalline haemtl­tite formed by Ollidntion ofoljvlnes nnd the green material is a clay mineraL

Peridotite from thf! Skaf!rgaard intrusioll. EMt Greenlal/d; magllijictllion)( 27. PPL (/lid XPL. Atldltiol/II/ "'('1I'S of eqlligramllar rocks tlf" shoWI! J/J IJ . 1/3. II 7. /25, 130 (first pll{lw). HI, 140 (thirtl piloto). 168.

19 Porphyritic andesite

In this rock the phenocrysts (some of them in clots) of plngioclase. hornblende (khaki colour m Pill), augite (pale green in PPL) and magnetite. are surrounded by fine-grained groundmass of plagioclase, magnetite and glass.

Alllies/I(> from Siebel/febirge. Germal/Y . lIIagl1ijietlllon )( 11. PPLundXfL.

Granularity

Gr!lnulari t~,

Porphyritic andesite (continued)

20 Leucite- phyric micro-ijolite

Two. large. shapeless crystals of leudte (llery dark and showing mUltiple twinning in XPL photograph) are here surrounded by an cquigranular groundmass consisting of crystals of elongate augite (bright interference colours). equant nepheline (grey in XPL) and interstitial biolite. leucile and magnetite. The amorphous materia! in Ihc PPL view is a clay mineT(l1.

Micro-ijolite f rom the Bmsberg ill/fIISi(lll. East Green/and ; IIIUglli!ico/ioIlX II . PPL OIlD XPL. }'hln)' Olher I':({/mples oj'porpli.l'ritic rocks call be $('O'n by leafing through Ihl.' book.

21 Plagioclase-augite-magnetlte vitrophyre

Phenocrysts of the three minerals plagioclasc. auglle and magneti te. some of them in clots. are set in glass which contains crystallites of plagioclase.

&1S1I1I from Arrall. Seollond; magnificO/iOIl)( 20. PPL. Set! 131 jor (If/Ulha f'xumpl(' of 1'llmp/1l1i'.

22 Seriate-textured olivine basalt

The crystals of olivjne. augite and plagioclase in thiS basalt all show a wide mngc of grain si/.e from as small as 0,0 I mm up to 4mm. Note the abundan~ of groundmass inclusions in some of the crystals. giving them a spongc­like appearance.

Olil';lw basalt j rom Arthur's St!O/, £dlllburgh. Scotland; III«Fnijicutio/l x II, PPL (lnd X PL. S"" 4" mul 117 for olher e.mmpl"SIl! IIIIS /1'Xlllf("

Granularit )'

Crysta l shapes

Two kmds (lf tenn arc used to describe crystal shape : (I) those rel:uing to the quality of the development of faces on c rystals and (2) those specIfYing the thn .. -c-dimensional shapes of indIVIdual crystal:, (po 19).

Terms indk alini; the quality of the den lopment of faces 011 crysta ls

Regrettably. three setS of words are in usc 10 describe the same ideas, the mo)[ commonly used sct being Iha\ in the first column of the followi ng table.

Prdt rred lomns Synon)'mous lerms 5)-000) mous terms Meaning

Euhcdral Idiomt.rphlc Automorphic Cryslal completely boundl:<! by us characteristic faces.

Subhl:<!l11l Hypldiomorphlc Hypautomorphlc Crystal bounded by only ~me of lL~ charactcristic facl'S.

Anlledl1ll Allotrtomorphic Xenomorphic ('rySta! lacks any of lIS characteristic facc:s.

23 Euhe dral olivine in olivine bas alt

The photograph shows the charnClerist1c six-sided eu­hedral sh:lJ>e of olivine in sections through the prism and dome faces. Note the slight enclosure of matrix material by one of the prism faces.

Olil'ine basall from Ulwkendt £j1and. West Grrrlliand: magnificatioll x 40. XPL.

24 Subhedra l olivine in pi critic basalt

Some of the faces on this equidimensional olivine cr}'Slal are Oat. planar ones. whereas othC-fS are curved and embaycd.

Pirritic lx,s"ll/rom Ubckrnd; /:.'j/llIrd. We.~1 Grreu/mui : magnification x 72, XPL.

25 Anhedral olivine phenocryst in basalt

The enllre perimeter of the largc olivinc crystal. :It extinc­tion in this picture, has an irregular outline and no planar faces are present. (The narrol!.' brown rim on the crystal is 'iddingsitc' fonned by hydmtion and oxidation of Ihe olivine,)

Oli~ille l)(Isall fram Mauritius, Indial1 OC(!(III ; maglJijiclI ­II(m x 12, XPL.

Fig , A Examples (if equidiml'lI.fiol1(l/ cr)'swl .\'hnp(!~·

The words grtlin and gr(llJl//t· lire often used for equi­dimensional crystals, and drop and hll'b for particularly small examples.

Terms indicating thre1!'-dimensional crystal sha pe.

In hand specimens of coarse-grained rocks it is often possible to SL"C the three· dimensional shape ora crystal on a broken sUrf'lce. For finer-grained rocks. however. the crystals have to be examined in thin sections a.nd the two-dimensional shapes of several crystals or different orientations used 10 deduce the thrcc-dimcnsion~ll shapes of the crystals in general.

Generu/ l hree-dimf'nsionu/ terms

The shape may either be an I!qllidillll'Il.I'ioIlUI (syn. equlIllt) or an illl!l{lddimellsiollul one. as illustrated in figs. A and B where the names applied to Ihe various shllpes arc shown.

equant polyhedral

equant anhedral spherical

Cry!>1:al shapt.'s

tabular fSV n. platy)

!ameUar

Spl'Cijic tlrrct-llinretfsiotful (t'rms

Skeli'wl, (/(ondriti, and mrooYf.'(1 cry-,tal.f

bladed · columnar (syn. prlsmatlCl

acicular (syn needle · like. hbre,llbrous, hem-like)

Skrlcwl crysf(lfs are tho;;e which have hollows and yaps. possibly regularly develop­ed. and usually with particular crystallographic oricntution$. In Ihin section thcse spaces appear as cmb.1ymcnts l :md holes in Ihc crYSlal. filled with groundmass crystals or glass. Del1dritic cry-,tab' consist of a regular array of fibres sharing :1 common oplical oriental ion (i.e. all part or a single crystal) und having a bnmching pancrn resembling Ihal ofa tree or Ihe veins in a \eafor a feather. In practice. many crystals can be described as either skeletal or dendritic bec:lUse Ihcy have ch"factens­lics of both.

I A rommnn misconrf'plrOll amo"g p('fro/ugl.flJ is Ihut the II'ma.r '/'I.rooymrlfl· (mr/ ','mbuy('d' impl)' reJ'QrpliOIl of" ("yslIIl b.1' rt'at:lirJ(! .. /lit liql/ld. IIIhllt this may ~ 1fI'" oj .<onr.' crysral.f (r.g. 29) . olhl'rs (r .g. 16 Ulrd 17) Il(l\'t rmborml'lIu .. hlch p,r.hobl)' jarmtd dJ/rmg gf OKl/I

N ,B. A lthough these are euhedral c~ampl~. they could oc subhcdral or anhecir'.tl.

-Bludl'illl'ldsJlUI urHII/.' /I, I'ommlm "Sllgl' an In'qlwl/lh d".!_ c-r;bt~1 us '/OIh-,hapl"d' Of /I.) '/UIM 01 Jrld<po'-. III ulluJron It> rlrt> sims f /(11/u) in 0 Vel/nil'''' M,rd

26 Skeletal olivines in picritic basalt

All the large crystals in this rock arc ohincs and c;lch shows a different shape in sectron : somc are complex skeletal crystals (e.g. elongate yellow crystal on the left). olhers arc relallvcly simple skeletons (e.g. equant orange crystal. middle righl) Bnd yel others have only small l"Tllbayments.

Picrill~ hosall from Ubekemlt Ej/(md. Wl'_~/ Grccl/hllld: magnifica/ioll x 40. XPL.

27 Skeletal olivine

While superficially resembling the cuhedral outline of the olivine in 23, the crystal oa:upying the bulk of this picture has a complex interior fonn and incomplete prism and dome faccs.

Picrilic INlS(l/1 from Ubekel1dl £j1alld. Wes/ Grcel,/aml : mngllijicalioll x 15. PPi...

28 Dendritic olivines

All the delicate, dendritic crystals in this photograph arc olivines which formed during exceedingly rapid solidifica­tion of the basalt melt. part of which became the yellow £11Iss.

Specimen of Qlivine hasafl meltcd alld Ihen coo/eel 01 /4(XF/h, ill the laboratory: magnification x 40. " PL.

29 Embayment in a ugite phenocryst

The large augi te crystal in this photograph contains a deep embayment filled with the basaltic groundmass. The irregular outline of this embayment distinguishes it from thc cmbaymenLS in the skeletal crystals in 27. Note also the distinct marginal zoning and the delic.1te 'patchy zon­ing' within the crystal.

Olil·in!' basalt from Arth/lrs Seat. Edinb/lrgh. Stolltllld: magflijiclIIioli x 13, X PL.

Crystal shapes

Cry SI~1 shapes

Pamfld-grawlh crysU/1s The lenn is applied to an aggregate of elongate crystals of the same mineral whose crystallographic axes are mutun)]y pnrallel. or almost so. Although in thin S<.'Ction the indh';dual parts of the aggregate rnJ.Y be isolated from one another. In the third dimension they are prob:lbly connCCted. A parallcl-grQy,th crystal is therefore II singlc. incomplclc crystal fonned by a particular style of skeletal growth.

30 Embaved quartz

The deeply em bayed quartz crystal in this oli\ine b:tsalt contains brown glass and sm:tll. (.oolumn:lr. skelel:tl pyroxenes. II IS 1I1so surrounded by a film of the glass and an aggrC!ate of equant granular augite cry:.tals y,hich separntc it from the basaltic ground mass.

Olin"e/xlsufl/rOIll lAssen Park . USA , magnification x 42. PPL.

31 Olivine parallel growth

T he elongate olh';nl'S ne:ar the middle of the photograph and showing blue interference colour all have the same crystallographic orientation, and hence repreKnt a single. parallel-growth crystal. Thc crystal with yello\\'ish-green interference colour shows how the p3raUcJ-growth crysta I mighl appear. if sectioned at right angles.

Pierilit- bas(lll/rOIll Ubekcml! Ejfuml. WeSI Green/llml: magnifica/ion x 13. XPL.

32 Para lle l growth in a very coarse-grained rock

llere the p;!rallel gro\\th is ofa very large olivine crystlil. The actual width of the field or vic .... is I.7cm and this sho .... -s only a J.mall part of the par-I llc! growth . .whose lotal width is 50~~m and height is 150cm. The whole comprises several hundred parallel uni ts like the ones shown here. Plagioclase and augite occupy the 'channels.' between the parallcl growths. In the XPL picture the polars have been rotated so that lhe olivine is not tn extinction. The slight differences in birefringence of the olivine ,1\ the top and holtom of the picture are caused by the section being thinner there. This rock has the special tex tural name harrisil#'.

Fddspa/hic peridolilt' from RJIIUII . Scoll'l/Id: mllgllijicllIlUlI x 7, PPL IIml ,rpL.

Crystal shapes

Si#, I'e-lexlUred cry.flo/s These contain abundant. small, interconnected. box-shaped g1ass inclusions. giving the crvstals a SOOlll!ev. or OOTOtlS ll" ......... ,.,.n .. "

Crystal shapes

Efongwl!, CllfI'ed, bralldl/IIg Cf),swfs These are rarely genuinely ben!, ralher the curvature is caused by development of branches along the length of the crystal. each branch having a slig.htly dilfercnt crystallographic orientation to its neighbours (e.g. 34 36),

33 Sieve-textured feldspar

The core of thiS ltcnocryst consists of gL.lSS and alkHh feldspar in a fine-mesh-like arrangement: the narro ..... nm IS an o~ergrowlh of plagioclase,

Ofil'ilU.' bmwl! from ul~'J"j'n Pork , USA; lI/(lgnijicllliOll )( 62, Pi'L.

34 Curved branching augite

The highly coloured crystals in this photograph arc com­plex. branching crystHls or augite III subparallel align. ment, They (onn part or a pyroxene-rich band III a differenliated dyke. (See also 71.)

Oulerj!t' (rom Nur!h Sk)'/', Scm/ami : mllgllijicllliU1l)( JI XPL.

35 Branching augite in lamprophyre dyke

The aCicular. aligned phenocrysts in this photograph are all of augIte. forming composite. radiBling, curved and branchmg groups. Individual needles can be seen to con­si,t of several straight pordons offset slightly from one another, and having vcry slightly different orientations: this gives cach 'needle' its curved appearance. The margin of the dyke lay to the len. (See also 70.)

Fourl'lrlfe from Fiska(,f1(!ss('I area, South'M'/,SI Grct.>l1irmd; mOf{lIifictlliOIl )( 20, X PI. .

36 Curved and branching plagioclase crystals in dolerite

The Inrge composite phlgioclase crystals rn this rock ar.:: elongate parallel to the c crystallographic axis and flat­tened parJlIe1 to (010). From the directIon in which the) branch, and from that in which the crystal at the bottom widens. It can be deduced thatlhecrystals grew from right to left. The matrix consists of finc-grdincd plagioclase. oJivme. pyroxene. amphibole, dcvitrificd glass and cia) mintmls.

FddsplIlhi(' dolt'r;I/!. U/x.'i-:/'ndr Ejland. W(!JI Greenlaml: nwgnifiralloll >< 16. PPL ond ,\' flL.

Crystal shllpn

Crysltll shaptS

PS('lI(lomorplu II may be fou nd Ihal cryslais In a thm section. aJlhough having the charucteristic shape of tI particular minerul. prove 10 ~ of another minerul. or an aggregate of crystals of .mother mineral. The name PSf'lIf.wlllorph is used for such a crystal. If the pscudomorph has the same composition as the original crystal (e.g. 'quartz' in _ , _~~ _~ •• : ... . _: . AI :. ;. 1.."" ... , .. ~ .. • " ... m .. ",.,, /,

37 Compos ite bra nching augite crys tal

These photographs ill uslrule a particularly intngumg shape of branchmg aUgite crystal : It consIsts or groups or slighlly diverging needles. subparallel to the length of tho: crystal. which apparently have grown from curved branch­ing needles oriented approximalc1y tit right angles to the cry>tai length. Despite the unifonn interference colour of mtl n y of the needles. a sv. ccpmg sl )·ie of eXII nCllOn occurs '" hen the microscope .)Iage is rotated under crossed polaN. indicating that the needles are not all of the same crystallo­graphic orienl;)lion .

P('mIOl/lic koma!ii!/' from MUl/fO T(lwns/,ip. Omario. Canaria .. magnifica/ioll x 51. I'I'L ami XPL.

38 Carbonate pseudomorphs after olivine

The phenocryMs in thi~ altered basalt sho\\ typical sec· lions of skelclal olivine. with mclusions of groundmass in the cmbayments. However the photogmph W10 ..... S the phenocrysts to be OCCUPied by linely crystallized carbo.:m· ale. mdicutLng that replaeemenl of olivme has occurred.

<IIwl'd bmalt trom GIS/INrll/ . D.'rby~hir(' . EII~lflnd . mugn;(irano/l )( 27. XPL..

Allo/Ill', ,·.Wlmp(e of pS/:,IIl/omorphs IS shOlnl III J.l9 .

Cr~'Sta l sha

Mutual relations of crystals (and amorphous materi als)

The various pallems of crystal arrangement which can exis\ are convcmcnll introduced under tll..: ro!!O\>,ing headings : cqulgranular Icxlure~: inequig.rnnuli textures : oriented t;~XIUres : intergrO\\lh lexturl~: radmte textures : o~cr@rowt textures: banded lexlures: and cavity tcxlurl.'S. Particullir texlurc~ mn} belong t

more than onc of Ihe!.C categories nnd some also helong to the l'lltc-gOri~ of crySl1I linit) . ~r:JnullUi ty and cr)sta l sha pe. Thus cerlmn of the textures mtroduced In th ~t'Ction h,ne already been mentioned ami reference is made to photogr.tph~ of thet in prevIOus st"Ctlons.

Equigranulli r lextures

Depending on the gener:!1 shape of thc erystllk three tcxtures cun be dlstmgUlshe. in Yo hich crystals of the principal minerals 10 a rock arc of roughly uniform gf'.!i l

size:

name

euhedral granular

subbedral granular

(anhedral)' gnmular

synOllYms

panid,omorphic granular

hypidiomorphic gr'Jnular

al101TJomorphic grurlular (gram\lc and gramlold Iclttures apply 10 siliccou) rcx:ks only)

definition

bulk of the crystals are cuhcdrnl and of uniform size

bulk of the cl)'Stab an: subhedral and of uniform size

bulk of the cryslIlls arc anhedral and of uniform 51/.<'

Boundaries between these ciltegories are not sharply defined and conSC<]uent!) the tenm aTC applied very subJectivciy. FurthemlOrc a rock may nOI fit neally into a single category. thus onc in v.hich - 50 ~" of lhe crystuls arc cuhedral and - 50 \ anhedral might best be described as having a mixed euhedral and anhedml granu1:u texture.

In addition to the examples of these le.~tures in 39-43. others may be found in 18. 111. II3. 117. 12S. IJO. Il4. 140 and 168.

' nilS udjffliW! /s eomn!tlfll" omll/rdjrom 1/1/5 ItXWflJl miff"'.

l\ lutunl r('lalions of crysta ls: e1luigranular texlures

39 Euhedral granular hornblendite

Rocks possessing truly euhooral granular textures are \'ery rare. The one In this figure is a good example of a more common situation in which only some of the cryst.lls of the principal mineral. hornblende. are euhcdral and some strictly are subhooral. In contrast to 40. there arc Ii higher proportion of cT)'stals with faces and the term 'euhedral granular' is therefore suggested as most ap­propriate. It should be appreciated. ho\\ever, th:1I another petrologist might prefer 'subhedrul granular ',

lIornblemlite Jrom A rdslll'ul Hill, Scol/lIftd: IIJagnijiclllioll x 7, XI'L.

40 Subhedral granular gabbro

The Stout prismati(' plagioclase feldspar crystnls Ilhieh dommate thiS rock are mostly subhcdral. The anhedral interstitilll crystals are of orthop}roxene, augite and magnetite.

Gabbro / ro/ll Mid,l/e Zone of Ihe Sktwrguurd IIIlnmun. £fJSl Gret'nfotl(l, mugmjiclilioll x 10, XPL.

41 (Anhedral) granular troctolite

Only a few of the plagioclases in Ihis equigrJ.nular rock possess a face and none of the olivlnes do, The crystals arc therefore predommantly anhedral und the 'moSillc' texlure is granular.

Troctolitl' from Garb/t BIt~1I1n intrusion, Sk),I', S{'QI/wul , lIIuKnijicUlion x /7, XI'L.

42 Granular granite

Ex~pting the scart"C biotite crystals. the qu'lrt7., micro­cILne and albite crystals which make up the bulk of th ... rock arc anhedral and have slightly interdigitat ing bound­aric~ (i.e. fOI/St'rW/ {('Xlllf(' see p. 45).

Grallill' from Mlulagascar .. magllijic{f/iQII x JJ. XPL.

43 Granular lherzolite

The crystals of olivine (colourless in PI'L), and pyro.~ell "" (pale brown ill PP L) whieh make up 95 Q

v of this rock . lack IIny crystal faces.

/)h'r:v/ile .I((!1I0/i11/ Jm'" III(> MOI$oklf kimhulile pipe, LI'.wlho : /Jl(/gnijiC"lIIiOIl X 16 . PI'L find XPL.

)\lu/u31 rela/ioos of crrst;lls : cquigranulllr Icxlures

;\ IUlual relalions or cryslal'l: iocquigranuhlr textures

Incquigranular textures

This category includes seven kinds of texture : (a) seriate: (b) porphyritic ; (e) glomeroporphyritie: (d) poiki!itic: (e) ophitic : (n subophitic: and (g) intcrstitial (intcrscrtal lind intergranu!ar). It is not uncommon for a single thin section 10 display more than onc of tht.'Se textures.

Seriate tt:cturt

Cr)'Slais of the principal minerals sho\\ a continuous mogc of sizes. (Sec also p. 14.)

44 Seriate-textured basalt

This basalt. consisting of just plagioclase. augltc and II small proportion or magnetite, shows II mnge in sires of plagioclase and augi te crystals from < O.OI- O.Smm.

Basall/rmll Isloml of Muurilills" maglli/imlioll x 43. PPI. and XPL.

Set' 11 IIIIIIIJ 7 for O/llt'r sertale-texlurctl rocks.

4S Augite-olivine-Ieucite-phyric melilitite

Augite (greyish-green and grt..-cn in PI'L) h prC'SCnt 111 three gcncmtions in this s,ample large euhetiml pheno. crysts. subhedr.ll microphcnOl:ryslS and minute ground­mass I:r~tals, The leucite occurs lb colourless. equant cuhcdral microphcnocrysts. most eas,ly identified b) theIr \'cry low bm!fringence III the XPL pIcture. and 111('

olivine as faint·grey. euhcdml. columnar mlcropheno· err-its, Note the compiicau.:d zoning p.,Ulcm III one or the augite phenocrysts. lhe prominent marginal loning and the line or small IIlcJusions or grollndmass cl)'Swis in another. Melilile )~ confined 10 the fine-grained granular groulldmuss and C"nnOI easily be $Ccn in these photo· grJphs_

Itelil/me/rom MU/IIIW, CeleMs, magniftcutio".", /I , PPL allll Xf'L

MOllY mor£> examples of porphyritic /ex/Url' mfly be fOil/it/ by /t'ofing /hrouglJ the hook.

POl'pllyritk IUIUn'

Rclumely largl.' crystals (phenocrysts) lire surrounded by liner.grained eryswb or the grollndm,lss. (Scc .!lso p. 14.)

Glomuoporph)'ri(ic U ,r:(Ure

A \'l1nety of porphyritic texture in which the phenocrysts ore bunched, or clusteroo, in Ilggregatcs or clots called g/omerocrysls, (A minority of petrologists maintain that the tenn applies only to monomineralic clots and for polymincmlic clots they use the tenn ('lImllloph)',ic (extllre,) G/olllf'ropizyric is usually used synonymously .... ith glolUl.'mporphyntlc, though the fonner term strictly should be reserved forclusters of equant crystals (Jolwnnsc!l. 1931), (SYl llll!llSIS texilire a lso describes crystal clots but includes the genetic Implic.llion that the crystals 'swam together' and is therefore best ;I\'Old(.'(I ,)

46 Glomeroporphyritic tholeiitic basalt

The photograph .ho\.\'S erystul clots of different sizc~

composed of plagioclase, augi te and olivine crystals, enclosed by fine-gr.lllled IIIlcrgmnular' and intcrscrlal textured groundmass,

iJtlSll1t from WIk!W\f1l /oc(lIII),: 1I/(lJ!lli{iauio/l x I I . X P L.

47 Glomeroporphyritic hawaiite

Discrete phenocrysts of plag;oda~ and oit\ine, ond clots consisting of a fc\\ crystals of the smile minerals, a rc SCt III a line·gnlillcd groundml\ss, in places shO\\-mg )1;ght alignment of plagioclaS(' needlcs, Some plagioclascs in individ uul clots are aligned - !.his arrangement is common in plagioclase gloffi\:rocrysts,

H(lll'aiit .. from pfateau ftJl'OS of No"h Skye. St'QtIUlW, magwficatioll x II, XPL. AdditiQnal V;t'H-S 0/ gfomeroporpliyrllic (('XII/'" 1IU1,' bf' Jet"! /II 112, 127, 15-1 t/lld 158,

48 Poikilitic e nclosure of olivine crys tals by augite

In Ihls phologmph appro1(mlUtc1y 100 crystals of oiivine of f.ml} uniform size arc cnc1oSl.'<I by :l single augite cry~lal (ut extinction).

I" 'fir/mill' Jrom QuafslII. W('SI Crt'lm/ullcI, /II(1~'lfJlr(/lioll xll. NPL

49 Plagioclase chadacr,sts enclosed by augite

Pari of a single auglle crystal (yellOl~ t:olour), c,ceedmg ].()mm In SIZC. is shown herc enclosing plagioclas.: cry~tak ,ome of V.IHCh rorm clots. The or-mgc crystal at upper nght IS olIVine and the cry~t<ll '1lmoSI at extinction is another augIte crystal.

Gubbro JfOIll Norih Skl'e. SCOIland , II/agnifica/ion x 7. rpL.

Poikili,ic tUtur~

Relatively large crystals or one mmeral enclose numerous smaller crystuls of one. or more. other minerals which a~ mndomly oriented and gencmlly. but not noces­S.'lril). ullIformly distributed. The host cryst.'lils known as un tIIkocryst (or (>/lc/osmg ((),5/(1/) and the enclosed crystals as c/tO(/lIcryslS. Although char/flcrysl.r are generally equant. or nearly so. they need not be uniform in size : sometimes they display pro­gressIve change in size from the interior 10 the margin of an oikocrysi. indlcalmg differences in e:l: lent of t:htldacryst growth :ttlhe lime of enclosure. II is not customary 10 upply poikilitic fe.\"fUre to the arrangement in which scarce minute crystals of accessory mmer-lls an: embedded in ::t crystal. nor to that in which the enclosing mmeral is appro;"imately the same si .. .e as that included.

Mutua l r{'ia,jons or ct)qals: inequigranuJar I{'xtures

Ophitic lutu".

Tim is a \'<lrianl or rwiJ" IIIlI r I(',tUm' in .... h,ch thc randoml) arr<lngcd chadl\cry~ls [lrc clongalc and arc .... holly, or partly, cndo~ by the olkOGrySl The comrnone.st occurrcnce is of bladed crystal-. or plagioclase surrounded by subequanl augite cry:;tals in dolerite (sometimes referred to as Jo/eraic 1('xllIrt'): howc\oer the texture is nOI confined 10 dolerites. nor to plagloc la~ and augIte as the partlclpatmg mincl"dls.

Some petrologists dlstmguish the arr:mgcment in .... hich the elongate chadacrysts are completely enclosed (poikilopillfir l('xlI/rel from Ihal In .... hlch thc~ are partiall~ enclosed and therefore pcnetnlle the Olkocrysls (~'lIhop"l/ic li'xtllre). PoiJ"i/ophill(' /I'x/urt' could (llso be used when oikocrYSls ~urround dongatc chadacrysis of one mineral and equant chadacry:;ls of anOlher

Fine- and medium-groined rocks made up of many small oikocrys[s have a p;ltch~ ~ " ...... " .... ,.." ... cnmPlimf'~ ril'''Crihed as on/rrmO/l/ed_

50 Olivine gabbro containing poikilitic domains

Lnrge plagioclases. enc1o~ing or parllal!) enclosmg. round oli\incs al their margins pro\ldc tl framework to [hI) rock. Inc interslices of .... h,ch are occuplt:d by IJrge llUgite:; abo enc1o~mg round olivmes ,l1ld ~m;jll ~tubby t:rYSlal~ or p!agloclase.

Oll\"ill~' gahbro lmm ,\Iuldlt· Border GrOllp ,,/1/1(' Sk{/('r ­fI,(mrd mlfllsiml. Easl Green/wId, IIIUgllijiC(I{/O/i " 12_ XPL

51 Olivinea enclosed by plagioclase oikocryst

Subhedral. equant ollVine crystal;; here arc cnclosed in a Single I.uge plagIoclase crystal.

Fdll.fpllr p('f;linli{{' /mlll RJIIIII/, SCiII/mu/; lIla};mjlcalll)ll xli , XPL.

Allditimlill 1''-('11'$ of poikdilic 1('Xt/ire IIItt)' be fOlllld ill

1/1. /14 (11/(1 167_

52 Ophitic-textured alkali olivine dolerite

Two large anhedral cry)tuls of augite enclose numerous, random ly arr.mged lath·shapl-d plagioclases, Note lhat mILn), of thc plagioclases are :Issociatt-d in groups. The larger nugue crystul h;ls II v;lriable colour due to a chemica I I.omng (St'C p, (1)

Ollllllt' t/fI/t'rw'.Irom SII/(/II/ hie)' .\'/1/1, Sf(){/jmd, IIlf1gmflm. /iollxJI,XPL.

53 Subophitic telCture in olivine dolerite

The photographs shoy. plagIoclas.: lath, embedded In

severnl aUgite cr)stals: whereas some of the plagiocla:.cs are "hall) embedded. othen; pcnetr.tte beyond the augite crystals, The other malic mlncral prescnt is oli\ine "hieh is pamally altered to a green clay.like minel1li and is distinguished from the augite by its colour in the PPL \'11:""',

O/mne doleTl//! from IIl/knOll1/ Brl/ish source , magnifica­/iQII x 17, PPL ami XI'L

\1utulil rehltions or crystal): inequi~ranula r U',\ tures

i\ 1ulual re lations of crystals: inl'quigrllnular textures

54 Subophitic alkali olivine dolerite

In thl~ \"ic\\ plagioclase laths are eml:,.:dded In olivine r.llher than pyro"~enc. One olivine eryslal is lit t'Xlinction In the XPL pholog.rnph and another sho .... 'S orange mler­ferenee: colour. The Olher mafic minerdl in the pictut't-s I) augIte showmg a purple interference colour.

Of '" hI(' dofl'rlll' from Shwm Isles sill. St"(lflami: 11/(/1:11(//("(1.

1;/)// )( 26. PPL allli XPL.

See Ill . 116. 118 and 164 jor adllilioll(ll t'x(lmpit's QJ ,l/bophilir It'Xlllr/! ; i21 is pllrticlilarly imerl's ring ht'eIliISt' hl'rf' tht' flJ'ro.~('ne I:~ slihophilic(l/ly enclosed br pk'gioda.H'. IIml in 1M p),ro.'(ene is sllhophilit'ally el1do.~/'(J by ka/siJif('.

55 Poikilophitic texture in olivine gabbro

For the texture shown here the lenn poikilophilic I~

preferable 10 ophitic because (a) the large augite encloses some equant olivines in 'lddition 10 plagioclascs. lind (bl man) of the plagioclases lire not mllrkedly elongate

Olil' ine gabhro jrom J..()H'er ZOne fI Of the Skflcrgflart/ imruslon . East Greenland: magmjira/IQII x 10. XPL.

56 Ophimottled texture in olivine basalt

Appro"'lmalely fifty augLte crystals are shown here en­closing bladed plagioclases and giving the rock a mottled or speckled appearance_

OblInl' basal/ /rolll I$f(' of MI/ff. Sct)ff/l/u/: /11(IKI1!ficilliOlI

x (4. XPL

57 Feldspar-olivine-phyric ophimottled basalt

Phenocrysts of plagIoclase and olivine. some in dots. are set in fine-grained ophlmottled grollndmass.

Olil'inl' haS(/1t from Skye. SeQ/fuml. /lUlgllijictlli(lllx fJ. XPL.

i\'lutual rclations of crystals: inequigran:.dar texlUr('S

JIlIi'rMiriu( t1'x/Urt'~

T\\o vaneties arc rt'Cognizcd on the basis of the matcnal occupying the angular spaces between fcldsp:lr hlths : I . ImerSUlIIlll'Xflirl! glass or hypocryslalline material II holly. or partly. occupies

the: wc<lge-sh:l ped interstices bct\\ccn plagioclase laths. The glass may be fl\."Sh or hale been altered to palagonlte. chlorite. analcite or cJny mlOernls. or it may have UCI'ilrificd. If 3 patch of glass is sulliciently large and continuous to enclose II

number of pla£.loclase~. some pelrologl~\S \.\ol1ld de~:ribc the le ~ture as hwf· ophifl(:' (Sec a l~o hraitlpilillr te:<ture. p. 41 .)

2. fil/ergratfl/ilif II!,r l(lr(! - the sp;~ccs bctwetn plagioclase lalhs arc OCCUPied b) one. or more. grains of pyrOllene (±olivllle and opaque mineralsl. Unlike ophaic I.:-xture. adj[Lt'cnt IOterstices ;Lrc nOI in oplical cOntinult) and hcnl't 3rt' di.)l.Tcte "m31l1."r)~\als. The feldspa r.; may be 10 d!~cr~e. ~l1brlldial or subp:lrallel drrJng<" men! (sec also pifo{(Jxilir andld/r textures. p 41,.

As shown by ~ome of the photographs illustrating these textures. a smgle thin section may contain both typt.-s of inlersti\lal t('.~turc m SCIXITate. but contiguous. texlUrnl dom;tins.

Mutual relations of crys tals: illcquigranular texture>

58 Intersertal (hyalophitic) texture in th o leiitic basalt

Certain paris of this photograph show lath-shaped plagioclascs enclosed in pools of dl.'vitrilicd, deep. brown glass. Other plagioclases arc surrounded by augite in a subophitic manner.

Oceal/ic tiloldile [rO/ll Leg 34 of the Dl.'l.'p Se(1 Drilling Project ,' magnific(ltjoll)( 65. PPL.

59 Inters ertal texture in alkali dolerite

The inlcrsertaltcxturc in this dolerite consists of p!i"lgio· clast' crystals embedded in analcite ((;olourless in PPL and isotropic in XPL). Dthtr plagioclascs are partially enclosed by pyroxene in a subophilic manner. A crystul of olivine can be ~cn atlhe righl.hand edge orlhe vicw in PPL.

Alkali duler;/(' [rolll flulliord IJridgl.' sill. Ayrshire. SCQI­{{lilt!: magnification x 13. PPL (lml XPL. Sf'e also 126 OI/d J 27.

&0 Intergranular dolerite

Anhctlral cqullnt cry~tals or augllc and plgconllc occupy the spaces bct\\CCIl plagtocla~ cryst;lls in this sample.

Odlrnll? {rom Ileur IIII' {()'H'r margll1 oj Pf/h~Q(Jt..t '·i!l_ NI'II

kney. CSA . lIIugmjlCQI I(/n )( 60, PPL and XPL

'1 Intergranular olivine gabbro

In thiS example of inu:rgranular tcxture the rock is wursc­~incd und thc plagloclascs have a subparallel arr.mge­mCn! Note that the interstitial ;"iugi\e-. arc unhedral Ug:ltnSI the euhc.:drnl plagioctlis~.

Olllin" gabbro i rom Lower 20111' h oj Ihl' Skaergaard in/rwlO/!. &m Green/aI/d. /lUI,f{/lijicaJion)( 15. XPL

Mutual relations or crYsl9ls; inl'quigr.lAular lexture<;

\ Iutua l r('la lions or crystals : int'q uigrJnular l('xrurt'S

62 Tholeiitic basalt with two types of interstitial texture

In this photograph patches bctwttn some of the plagio· c1ascs arc occupied by brown glass (panl)' devitrificd) lmd betwecn others by clots of small augi te crystal!. with­out IIny glass present, i.c, domains of both interscrtal and intcrgranu!ar texturc arc present.

ThQll'illic bllsall from Ulwkendt Ejfumi. Wi'J't Grel'lIlalld: IIItJgnijicalioll x 27, PPL /III(} XI>L.

63 Intersertal, intergranu lar and s ubophitic textures in dolerite

All three of thcse textures co-exist 10 this rock .

I)Qtl'fIIl' from Whin sill, Norllwmberlmul. Ellg/lmd .. magni­ficO/iOi I x 26. PPL (1111/ XPL

O ri rmted. aligned :md directed tcxlurK

Several clas!;eS of this Il'xlural type exist: (a) lrachytic ICXl'ure: (b) trachYlol, texture; (c) parallel-growth texture: (d) eomb texlure; :Ind (e) orbicular texture.

Trachyt;(' texture

A subparallel arrangement of mierocrys\allin.: lath-shaped feldspars in the grounc mass ofa holocrystatiine or hypocryslallinc rock.

N.B. the term is nOl restricled in use to rocks of trachyte composition (c.,g. se groundmass of 47).

Some petrologists subdivide lrachytic texture with mierolite-sizcd feldspars int, pifotaxilic 1(>;>;/1"<' and hyolvpifili, ICxmre. depending on wh.:ther the materil between the felds pars is crystalline or glassy. I Strictly. however, the mierolit~"S i thc~ tl'xturcs may be more or less aligned. (For a pilOiaxitic texture in which th microlites arc essentially randomly arranged the teml!dly /eX/Uri' c.\ists.)

TrQchyroid teXture

A subparallel arrangement of tabular. bladed or prismatic crystals which are visib to the naked eye (Holmes. 11)21). While Ihe term is usually applied to ef)'stals c fcldsp..'1r. Johannsen (l931) SIMes Ihal il may equally weI! be used for oriente cryst:lls of any other mineral.

The lemlsJiow ,Indjfll:don reXllm? arc sometimes used as synonyms for trachyti and Irachytoid textures, ho\\'e\'cr they should be aV01ded on account of their geneti implications.

Mutual reilltions of crystals: orienll'd. aligned and direcll'd textures

64 Trachytic texture in a trachyte

TIlis rock illustrates tntChYlic texture with no glass be­t'-"cen Lhe small. aligned alkali feldspars (i.e. pilotaxitic variety). Note that, rather lhan there being a single uni­versal alignment direction. there arc sever-al domains in the photograph, each having its own preferred direction of feldspar alignment.

Trachy//:' frQm Ill/k/lOlt"/1 C;I!c/wsIOl"akian /o£"Olily: maglll­jic(IIiQII x 16, Xf'L.

65 Trachytic texture in trachyte

T he somewhat stumpy groundmass alkalj feld spars. III this rock display a subparallel alignment which is pmticularly noticeable where they follow the outline of the pheno­crySts.

TrachYI(' from IlIIkllf1l'lI Germa/l locl/lil)'; mag'lijil"mi()'J x 15 , XPL.

66 Hyalopilitic texture in rhyolitic pitchs tone

Tht' feldspar microlites in this gla~y r(X:k ha~e a preferred elongation direction from lower left to upper right: near the feldspar phcnocr)"sls and opaque crystals the orienta­tion of the microlites follows the outline of these crystals. Note the tendency for the microlites to be armnged 10

bands.

Pilch.~IQItI! from Ischia. Bay oj Nllpl .. ~·: nw[:niJil"lllWn x 10. PPL.

'7 Trachytoid diorite

n ilS m~-dlUm.gramcd rock contams aligned columnar pl:lgioclases. The cloud~ appeamnce to the pragiocla~s resuh\ t'rom v~ry small inclusions of Iron ore and mica.

D"",/I'/,om Com',/,. Seolland. m(/gm/icullon >( 16. PPL

68 Trachytoid gabbro

This !rachytoid Ic~turc consists of phHY piagioclascs. here secn edge on. slacked upon one another. Note that whcn thiS rock lS sectioned parallel 10 thc plnne of the nattening. the crystal ajil!:nmcnl would nOI be cvident

GuM,o from [.oWl" ZOlll' h of ,lte Skoerg(w,d mlnl.lilJII .

&ISI Grren/rmd; ,/wgn!ficfllwII >( I Z. XPL

69 Olivines in trachytoid arrangement in olivine dolerite

In this \ole". large columnar phenocrysts of OhVlOC. some of skc1ctnl type. are aligned. and embedded in mlcr-1!fanular-lcxtured plagIoclase and auglte.

DIM" .. dol(',;II;' from Isle IJ( Sk,I'i'. SCQllmlll . "wgnijicmioll >( 21. XPL.

43

M utua l re-Ia tions of crys tals: oriented. alignCil and direcICil Ie-xlurc-s

Parallel-gro IN" tl!xtUJ'e

A single elongate skeletal crystal which in thin section appears 10 consist of a dot of cryslltls h:lving the same elongation direetion a nd the same optical orienta tion. (For lllUsI~l1ions see 31 and 32.) In rocks wilh Irachytoid texture il lS nOI uncommon for n.-ighbouring parallel-groll th cryslllls to be' aligned (se<! 31).

Comb texture (comh layering)

Elongate. possibly curved. bmnching nystnls sharing the same direction of elonga­tion. The crystals typically foml a band. layer. or fringe with the elong:llion di re<: lion ofthl' crystals inclined at 60-9{) to the plane of the layering. (Synonyms urI.' Willow­[..lIke layering lmd Crl'sl'wmdme layering. though the lutter is a genetic teon and. hence. should be avoido:d.)

70 Pyroxene comb layer in a thin lamprophyre (fourchite) dyke

Long branching augite erystal~ arc aligned at right angles 10 thc boundary between the comb-Iays:rcd rock (below) Md pyroxene.phyrie rock (above). The V of the bntJlehing widens in the di rection of growth. which is away from the dyke wall. (Sec also 35.)

LI.IIllprophyre dyke from Fiskoem'SSI!I (1r/:lI. SO!llh-wl!.H GreenllJnd: lIulgfli/iClIliOIJ )( 8. X PL.

71 Comb layers in dolerite dyke

T wo types of comb-textured layer arc present in IheiiC photographs : the first and third bands from the right consist of elongate branching olivine (now largely serpentinized) and plagioclase crystals: Ihe SL"Cond and founh bund~ arc pyroxenite dominated by complex. elongate. br:tnching uUl"rite c:rystitls with scarce plagioclase crys\,tlS in between. T he margin of the dyke lies to the left . (See also 34.)

Dolerite from Nortlr-wen Skye. Scotland: magl1ification )(8, PPLandXPl~.

Comb l;tycrs in clokflte dyke [contmued I

;\ lutUll1 rl!lations of crystals : oriented. aligned and dirl!etl!d Il!Xlures

Orbicular leXlllre (orhil"lIfur IUyt'fin,: )

StX' ['! 69 l'or ddi!111 10 !l ,Ind Illu'tr;llI'Hl In CtUlIl<.''\:l1tHl \\ )lh thc !,!TclU['! ilf tc~turc~ bemg considcred here, nOle that In some orblcules the concentric shell. have elongate crystals olif!IIt'd r;ldLllIy ;IOOut Ihe centre or lh..: orblcuk

lntcrwo\\ th te"lures

In 111m ",,-'('ILI)Il th(· JUlK'tLon hcl\\ccn L'\l1 ~'I),t;,b rn:t~ appear lI'.1 'tr;Llllht Imc. il

~Lmplc curvc. or U cI)Lnple]; cur\c, m thc thm.! .;:to...: lhe ny~t ,d~ mler(hgll ,uc or interlOl.'k, l}(hSibly $0 Illllmmely Ihat the~ appe<Jr' ICo he emhcdck"l.lm one ;Inother. ThC"'C IIlterpcnelr.Ulvc palLern~ ;Ire all c];:tmpJe~ of 1II/i'r!{f(l'I/iI li'nUf{'I, U~uall~ the crys tab concerned are anhedml but one or both may Ix skeletaL dendrnll' or mdl:tlc. Sc\en vanew:s are d l ~lIn.!!,U1sh<.'d here ' la) consertal leJ(lure , (h) micro, gntphlc tCJ(ture . (cll:lr:tnophync texture: {dl myrmekll ic texture, Ie) mlrafa-CKu1ute tell\ LIre _ {f) lamellar and b!cbb~ In ter!'ro\\ Ihs , Clod j g) ~)mrlcctlte tCll ture

Cflnse'·ruf u:,mre

Th~ bound;Jr~ bet\\een twO {Ty~t"ls lO\oIH'~ IOterd l glt;Jtion~ 1ll1d henL .... '([,!PC:IP. 10 be notched or serrall'd III ,erllon tlddlll~, 1909, Niggll. 19:;4),

' Til" appf'urum:., Of I'" ml('f(ilgll(Jllng D(mndllrt n.,/II',"w I .. (1 "'NIIII,>, "I </IIU 11, ,/,'p.,,,,£> un fill!

"Wf"l of mll'f{JI'nelrollOti (lnd Ihl' dln'rlll'" m "'''1('11 Ib" hOlIllJlln f,r 5.'{'(w,,,,d ,,,,,,.. II1ft'r­srf'/I"'U: "'<II <1\11" Ill{' f'n',f/Il/s 11Wt'lmg ;'111 (ompl ... r t'urw' _ o/h,,(\ flWI "hOll ,.f\ I/ul II "IU:f",\j',/ 1/1 B. /)/h..,.slIIuy J'lIoh I" .. /'IlIIn'rs .. ami )'tl I}llwf,f mill dill" t'IIrh "nda.,Itl!l III<' ",liN

74 Graphic granite

Photogr.lph of a polished hand specimen of gmphlc­te.uurcd granite In which thc dark m:ilcnal is smoky quartz and the light material is alkali feld~p:lr_

Graphic gf(lllile from III/know" foculity. IIwgniji((IIio/l x 3.

75 Micrographic texture in aplite

Two of the crystals in this \iew show an mlmlalC micro­gr'dph)c IntcrgroWlh of quartz and alkah feldspar. In one (middle right of XPL photograph), the alhl; feldspar IS al CXllnction. and In Ihe othcr (mIddle left) the quar\! i~ al extinction. (T he PPL phologfilph is deJil>t!ratel) lkrocussed to show the Becke Hne in thc higher-relief mineral (quartz) when the object;\e lens b 'r:tiscd'_)

\f1C7'o-grtlnill* [rom WorCts/t'r. Hussaclmsells. (;SA. IIIl1gm/icu/lfm x 60. PPL ami XPL.

Mutual relations or crystals : inte'l;TOl'lth Ie.Xlurl'S

t

74 Graphic granite

Photogruph of a polished hand 'pecUllt'll of gruphic­textured granite in whieh the dark material is ~moky quartz and the: light material is alkali feldspar

Graphic grollll(' lrom u"km)w/I locall,)', magniflcalioll x 3.

75 Micrographic texture in aplite

T\\o of the crystals In thIS \ICW sho\\ an inlim~lIc micro­gr.lphic Inlcrgrowth of qUllr!/. and alkali feldspar. In one (middle nghl of XPL pholograph). the alkali feldspar IS al extinction. and in the othcr (mIddle Icftjlhc quarl? b at extinction. (Thc PPL photograph is dclibcnuel) defocu~ to show the Becke line in the hlgher.rellef mmeral (quartz) \\hen the Objective lens is 'r:aiscd' I

Mlcro-grtll1i11! jm", iVor('('SII'r. \la.ulldwsN/S. US.4 mlll:,m/iclIllon x 60. PPL and X PL

Mutual Tchilions of crysl9.ls : intergrowth textures

•

t ~.

Mulual relations or c'1'SlaiS: intergrol'ilh textures

76 Micrographic a nd granophyric textures in microgranite

The photographs show several uni ts or inlcrgrown quartz and alkali feldspar; most arc of micrographic type but some have a radiate arrangemcnt (granophyric texture) altheir margins. In the Sconish Hcbridcun igneous prov­ince, rocks like this onc were formcrly known as grllllo­phyres in allusion 10 their distinctive textures.

Mirro~((/Ilj/(J from E(L\'II'rII Red 1Iif/$ (If SkYI'. SWIIIIIIII: magnijic(l/ioll x 20. PPL alld XPL.

77 Granophyric texture

In this rock, radiate intergrowths of quartz and alkali feldspa r arc arrang~'(\ about ("uhcdra1. equant plagioclase crystals. Microgronirl:' from Skaergaard imru.\'iOIl , £(lSI Green/lIml : lIIagnijicmioll x 37. PPL (mt/ XPL

Granophyric texture (continued)

78 Myrmekitic texture in granite

M lith orthe lower pari of this photograph is occupicd by an intergrowlh of qmtrlz ,tnt! plagioclnsc: this forms embaymcnls in the microcline crystal which occupies most of tht upper run of the field of \·iew.

Grallill' from Rllbi.~/1I1I qrUlrry . Aberdeell. Srolland: magflijirmio/l X lQ. KPI..

Mutu~1 rela tions or cr}'stals: intergrowlh textures

1\lyrmeki,ic texture

Patches of plagioclase intcrgrown with vcrmicular quartz. The intcrgro\\ Ih is orten wan-like in shape and is commonlYlo be round at Ihe margin or a plagioclase crystal. Ilhefe il penetrates an alkali feldspar crystal. The texture could be reg,lf(lcd as a vancly of symplcctilc texture (see p. 53).

Mutual relat ions or crystals : intergrowlh textures

fntra!rncicuiIJte tl.'xture

fl ollo\>', columnar plagioclase crystab filled wIth pyroxene,

Lamellar and hld-fikl' inli'rgroM'th~

Pamllellrllnellae,ortram~ofblcb),or onc mmcral. and all of the same OpIIC;'] OrlenLJ­Lion_ arc enclosed in a single 'host' crystal of another mineral. Well-known examplc~ m~olvt' lamellae or blebs of sodlum·rich feldsp<tr in a host of potll!>Sium-rich feldspar (pl'rllll//(' /exlI/rej: the converse (UlJllperthi/lc /l'xlI/rl'): and lamtllae or blebs of one p)- roxene in 11 hOSI of another (e,g, augltc m orthopyro"ene or Hrl' Iwsa,lInd plgcon­Ite III augtte or \'iu I'l'fSaJ, Other eXllmples indude : ilmenite Ill00eliBe 111 (uhospmel­magnelllc) solid-solution cryslals: metallic iron rods, find blebs 111 lunar plagloclases, plagioclase lamellae In pyroxene : amphibole lamellae in pyroxene : and chrome­m~lgnetile lamellae In olivine, Careful examination may re\'eallamell:le of more than one orientation and scale and sometImes e\en fine lamelli!e withm coarse lamellae. I.e. multiple generations of lamellae.

Lamellar and bleb-like intergro .... ths are often atlributed 10 c,so\ullon of the lamellae and blebs from the host crystlll (I .e. solid·state rcactionJ lind the genctic term 1!.'(SOilllioll /t'.\'/IIr1' is oftt:n therefore applied to them. However. laboratory experiments in which antiperthite formed from a melt liS a result of co-crystallization of two feldspa rs. and others to whIch ilmemte lamellae formed III pyroxcne during co-cry~tall ization of the t\\O phases from the melt. highlighlthc danger or unCfltlc,l1 u-.e of the tenn t:'xs(J/lllioll It'Xlllrr.

79 Intrafasciculate texture in dolerite

ThIS medIUm-grained rock has an mltrgro .... th tcxture in which the gaps in the columnar plagioclase cryst:lls afC (X:CUPIOO by :lugill:.

Dolem/:' Jrom GClrhh Bheinn intrusiOll, Islr or Sk,I'f', St·OI· IQ/ld : magnific(l/ion)( 72. XPL.

80 Microperthitic textures

Three examples of pcrthites arc represented here, The first photograph shows fairly broad sinuous lumeJla.: of albite traversing the tartan Iwmnmg of a mkrocline crystal.

SPl'cilllffl/rom pegmlllill', Topsham, Mallie, USA: maglli­firalioll x 16, XPL.

The second photograph sho .... s narro .... albite lamellae fomli ng tl braided pattern in an orthoclase host (upper centre),

SfWCIIllt!II from grmri/I', Rmagmr, Sfollrllld : maRlllfiratroll x 34, XPL.

The thIrd photograph shows t ..... o large areas of {he field of "ie .... y,ilh different orientations of crystals consisting of an mumate imergrowth of:l potassium-rich feldspar and a wdium-rich feldspar. In t-ach C'.dse the darker grc) colour represents the poUtssium-rtch feldspar, The pro­ponions of the IWO materials afC approximately equal ~o that neither is clearly the host - m this case the feldspar mtergro .... th IS knoy,n as:l mesopt'rliri/t'.

Spf!('iml'lr of lIepirelllli! S)'f!nI/f! from umgrslI/ll! fjord , ,'urII'm'; mClglli/irflliO// x 32, X PL.

MUiUlI] relations of crystals: intergrowth textures

Mutual re lations of crystals: inlugrOlolth textures

,

81 Antiperthitic texture in tonalitic gneiss

T hc poorly a ligned. bleb-like inclusions in the plugio­duSt.'S in this rock are potassium-rich fc ldsp;lr of inter­mediate structural state (i.e. Qfthoc]nse). It is likely that the texture forme<! in this rock during prolonged high­gradc region<ll metamorphism rather than during crystal­lization of rnagm(l.

Tmw/ilic gl/ri.t)· from S(:(lrme , North·west SCOI/tlm/; mlrg· IlijicatlOl1 x 20, XPL.

82 Lamellar intergrowths of two pyroxenes in gabbro

The hOSI crystal to the lamellae is an orthopyroxene- (clost' to extinction) ; it contains 1',1.'0 kinds of !:uncllae - rcla­th'e!y broad and continuous ones ofaugitc, and narrower discontinuous ones o f a ugite. inclined to the broad \~\ricty,

GlIbbrQ from Bus/II'cft! imrusicm, Smull A/rim : lI!(JgIJijica­lion x 9, XPL.

83 Bleb-like intergrowth of augite in orthopyroxene in olivine gabbro

In Ihis sample blebs of augite are embedded in lin ortho­pyroxene host. forming an 'emulsion-like' texture. Though the blebs are irregular in shape they have a common elongation dircclion and the sonne optical orientation.

OUI'ilJC' gabbro from Wn'('f 2m1£' b of the Sklwrg(l{lr(/ ilttrusi(ll1. Etw Gr('f.'It/mJti; m(Jgl1ijictlliC)IJ x 17. XPL.

84 Symplectite of iron ore and orthopyroxene

Iron ore (probably ilmenite) and Il small crystal of ortho­pyroxeneare intimately intergrown in a vermicular rashion in the spaces between plagioclase, augi te and ilmenite r.:Tyslals.

a/iI'im.' gabbro from l..ol>'('r Zone b of Ihe Sk(/j'rg(wrd i/l ln~~i()l/, £IISI GrecIIll/l/{l ; magnificatlQIl )( 72, /'PL.

85 Fayalite-quartz sympl ectite

Between the Opaque mineral (ilmcnite) lind the silicate minerals in Ihis rock, there exists a complex bound:iT) consisting of a narrow rim or fayaHte immediately adjacent to the opaque mineral. which in places abms onto a sympkctite intergrowth of rayalite and quartz. The fnyulitc in the intcrgrowth and that which rims the ilmenite have the s,:unc optical orientation.

Ft'rmgohhro [mill Upper ZOlle h oJ Sk(U'rgoard ill/rUslOlI.

Ea,\'! Green/wll/ .- I/Iagllijkmiol/)(, 12. PI'L /II!(I XPL.

Symplulire fe.fture

An intimate intergro\\lh (womllikc) habit.

1\"IUlual relations or cr)'slals : inlergrQI'!1h tex tures

of two mineruls in Ilhich one mineral has a vermicular

/'IIulual r('laliollS of cryslals: inll'r~ro"'lh testures

Radiale leslures

Radiate textures art' tho~ In "hich dongate crystals divers<' from a common nucleus. They are moSt frequently found In fine-grained rocks. bUI not e_~clu.si\ el) : for example. 34. 35. 36. 70 and 71 \>ho\\ large branching pyroxene. pt:lgloclase and ollvlOe cryStals to fan-shapl.:d radiate arr.mgemenls. A remarkabl) lar~ number of terms eXis ts 10 dt-"cfl!x: the l.trlOU~ p:lltcm~. lOcludmg : fan. plume. spnly. bo\\·tl~. ~pheflelli. sh<':<lf-hke. radllltc. radiul. ilXIOlitic. ~pheruillic and \o<lriollilC. All e.lcept the la~1 three ,"hlen an: dehncd :.nr.llllu~lnl\ed here). arc of,elf-~\lir.lcnl meanmg.

Sphrrulitic texture

SphcruJlh:" aTe arrrn~lm:Ilc1) 'phcrold;11 hlldlc, m it f()l' ~ Ihc~ .In: COmpd:-l."'d 01 • .Ill 'Iggregate ollibrow. cr),t:ll~ 01 one or moll.' mmcral~ rudt,lung Imm..l nur.:ktb. II Ilh glass or cr)'~ta ls m bct"ccn. The :1r.:Kular r.: ryslals m:l) be either smgle. ~lmplc fibres or each rna) have branches along us length. any branches 01:1)' or m:l)' not ~h:lre the samc optICal orientations as their parents. The mOSI common occurrem .. "C

of ~pherulitic te:<tull.' is a radiate ag!!regate of acicular alkuh rcld~pars with gla~s be\\\C'en them. Ihough quaru or other mmerals may be presenl. rcsuitmg In:tn inlcr· gro\\th texture. Should the sphcruhle have a hotlo" ,:entre II IS kno\\n:lS II hU/J(JM .fpll('rulllt!. lind if It comprises a serIes of conccllIric. pan ially hollow shells. the term lil/l()phy~/J is used.

A:<lohtcs dlffcr from splleruhtcs In Ih"'t radiating fibres c:<lcnd rrom eI ther end of a !incur nucleus (i.t. from a small ar.:icular crystal) rather than a point. They could be reg(lrded liS a \'ariet) of ovcrgrowlh texture (p. 58). as indeed could those $pheru­lites" hich grow about \'islble crystab ralher than on suhmlcroscopic nuclei (e.g. SH).

J-'aY'.lltle-qu:tnz s),mplcctite (contlnucd)

86 Plagioclase spherulite in dolerite

This spherulite comprises approximately tI~ertt)' elol1g:lIe CT}stals of plagioclase. each having a different optical orientation. It is an 'open' spherulite. III the sense th.1t there is much space between mdlvidual plagioclase crys­tals; the spaces life occupied by coarse augite. columnar plagiodascs 1101 n:latcd to the sphcrulite. and smaller spherulitcs.

f){}/rrill' from Gar"" Bhl'UIII In/rl/sion. Skn.'. Scm/(iI/d. magl'ifteal/o,,)( J], PPL (Iml XI't.

See 116 /or 1/ .mmlClr I!.wlfllpll!.

87 Spherulite in rhyolite

The sphcrulite at the centre of this photogmph consist~ of a dense mass of very fine intergrown net:dk"S of both quartz and alk:llt feldspar mdililing from II cammon nucleus. Above lind below, the spherulile abuts onto others, .... hereas to left and right there is gluss.

R/lJ'lI/i li.' f rom I/Ili/lk , HI/ngary: lI/aglli/iL(I/IOII )( 27. XPL.

Mutual f(' lations of crYSluLs: rad iate tc)o.tur('S

Mutual rehllions of ('T~"$t a ls: radiale lex tur~

-•• . ' ti>

VariolitiC' ttxturt

..

• •

• J.

A fan-like arrangemcnt of divergelll. often hranchmg, tibres; usual1~ the fibres arc plagioclase and thc spat't' bet\\e1!n is occupied by gll1~~ or granu!e~ of pyrox!!nc. olivine or iron or;:. This I!!Xlllre diffcr) from spherulitic in that no dlscretc sphcnclli bodies arc Identifiable, in fact, each fan as secn in Ihm ~cclion is a )1icc through :1 ,-~}n iC'.I1 bundle of acicular erul:lls.

88 Compound s phe rulites in rhyolite

Doth single and compound. or clumped. spherulites arc surrounded by glass in this photograph. The spherulites enclose micro phenocrysts of plagioclase and blollte. The colour variation in the spherulites is caused by variations in density of fibres .

RIlY(llitl! IrQIII Glas/wt/t'. Hlll/g(It)': mllgl/f/icmiv/I >( J 2 . PPL and XPL.

89 Variolitic olil/ine dolerite

The olivine phenocryMs in this sample are set In a ground. mo'>S consist mil. of many fans of d iverging plagiocl3se needles \~ith lIugite crystals in the interstices. Note ho\\ all the fans diverge from lower nght to upper left. indicat· 109 progressiH~ solidification in this direction. Note also Ihe branching ch~l r.lc!cr of some of the plagioclase fibres.

O/mllt! do/crill' trom Sk.H.'. Srolltmd. magl/ilil'(l//oll )( 27. PPLllIIdXPL.

90 Radiate intergrowth of plagioclase and augite in dolerite

This unusual radiate texture occupying the centre of the .. iew consists of t\\'o, mutually perpendicular, columnar. plagioclase crysta ls, the elongate gaps 10 which have ;l

mdililc distribution ; these gaps arc occupil'<l by a sillgll' augi te crystal. r.lther than by many ~·ryst aJs. This kind of r:ldiate texture dlfTefli from a sphcrultu:; it IS more akin to skeletal growth (p. 20),

Dolerite from IIIgKt intrusioll, West Grf1'l1/allt/; magllifica. IIQnX 27, ffL (flu/ XfL.

Mutual relutions of crystals : radill!t' !('xlnrC:!.

Mutual rdations of er)'sfals: radiate textures

QI'erJ:Towth textures

This teml applies 10 teXlures in \\hich a single crystal has been olergrown either by material of the same composition. or by material of the same mineral species but different solid-solution composition. or by lin unrelated mineral. There are three types: (a) skeletal and dendritic overgrowths; (b) corona texture: and (c) crystal zoning.

Sktdetillor dendritic OI'('rgrooNh:,'

Porphyritic rocks with a glassy or very fine-grailled groundmas~ may show delieatc fibrcs or plllt~ extending from the cornerS or edges of the phenocrysts. The over­growth and the phenocryst need nOI be the same mineral.

',.

J'

"

.-

· , .' , .. 1 (\'

• • ,,~

" ' .. • •

I, . , ~"I '

•

• ,

".11 , y~

" •

'\ ,,'

R:ldiate intergrow th of plagioclase and augite in dolerite (continued)

91 Overgrowth textures in rhyolitic pitchstone

The faces of the phenocrysts of alkali feldspar and mag­netite in this glassy rock have acted as locations for nu­cleation of dendritic overgrowths of (7) alkali feldspar. Dendrit ic crystallitcs arc also prescnt in the glassy groundmass.

Pitcns/olle from Arra/l. Sco/falld. magnification x 31. PPL.

92 Corona texture

In the cen(re of the phOWgrdphs a (winncU nnd zoned augile crystal is mantled by green hornblemle of non­lm i form \\ uJlh.

Quorl: diorite jrom ,11'111 oj Gal/oltay. SrolflllUI : magmflCl/-11011 x 43. PPL IIIItI XPL.

Curona tex ture

A cryswl of onc minewl is surrounded by u r1111. or 'mantle', of one or rnorecryslals of ano ther mineral. c.g. olivine ~urrOunded by orthop;.-Toxene. or biotite surro unding hornblende. Such relationships are often presumed 10 result from incomplete reaction or lhe inner mineral with mclt or fluid to produce the oUler one ;mcl lor (his reason the equivalent genetic ICrnlS {('(Iniull rim <lnd r('IIClioll CUfOlll/ arc fn .. -qucntly us.:--d. The spl'cialterm RllPllkil'I 1('."(/lIrt' is used to dcscribe an overgrowth by sodil' pl:tgioclase on large. usually round. potassium-fc!d~par crystals. and kelyphili(' /n/Ilf1' is used for a microcrystalline overgrowth of tibrous. pyroxene or hornblende on olivine or garnet.

59

i\ 1ulual rl'l:ll ions of crystals: ovcrgroll-Ih textures

••

93 Corona texture

IJetween ohvme :md pktgiodast c')stals 11\ this rock ther~ is (I O.02-O.06mm wide corona ,"hich consists of either Qne or IwO zones : ( I) radially Oriented. fibrous. brown hornblende: or (2) colourless pyroxene (sec middle of photograph) surrounded by radially oriented, fibrous. brown hornblende. Analysis of the pyroxene suggestS that .t is a submicroscopic inlergrowth or au~ite and ortho­pyroxene.

Olil'lIIi' gahhru from Tht'SJalomkl. \ '0"" Grl'N·t'. nragl1l' ilemloll >( 100. PI'I~ (lml XPt.

94 Rapikivi texture

The texture is or large. round potassIc reid spars. some or which are munllcd b) sadie rlaglocla~ rim~. others ha\e no plagiocla~ rims. I n the first photograph. '" hieh is or a polished h:md specimen. the plagioclase rims have II

grccmsh colour contrasting with the pink potassic rcld­Sp.lr. T he second ph()tOgrnph is or (I thin S<.'Ction or the same rock.

Granire from £(HII'rII Fill/ullri; IIl1lgnijirtllifJ,,){:2 (jir.fl (lhfJfo): x J. XPL (s('(:lJlld (llIuw) .

Rap:lkivl texture (conu!luedJ

95 Zonal arrangement of melt inclusions in plagioclase

Several stages in the gro"'th of this plagioclase crystal can be picked QUI by lhe bands of minule melt indusions, (Sec also 45.)

F~/d,spnr-phyrl(' dolerite from Isle oj Skyl' , SCOII(lIId: mllg/lijic(lII~m x 9. Pi'L,

Crystal :IUlinR

One or more concenlric bands in a single crystal arc pick«i om by lines of inclusions (95) or by gradual or abrupt changes m solid-solution composillon of the crystal. As regards the laller type of zoning. a large. number of pallcms IIrc possible. the commoner ones beIng iIlusmned graphically and named belo",', u~mg plagioclase as an enmple.

Norm(}/I·I"~II.~ rel'Ust! ::(J/lwg Th~"SC terms specIfy the gencml tfcnd of solld-solullon composilion from core 10 rim. 'Nonnal' mdicatcs high-temperature componcm-lo'W·lcmpcmlurc component (l'.g. An·rich pJagllx:JaS<.'-Ab-rich plagmclasc. sec Fig. C) [me! 'fevcrse' indic:lle~ the opposIte.

CO/llll1llOu,s 1 I'cr,\l(f l/is('omil1l/Qus J ::l)lIlIIg These terms ind ica te respectively a gradual or an abrupt change in composition , Figure C shows examples of comiwlQJIJ IIOTIIW/ :oIJ;ng and Fig, 0 lin e.~;mlplc of (/i,fCOntmIlOUS f/fJmwl :onirrg. Continuous :lnd dl!>Conlinuous zoning may alternate lI- ig, E).

J Th!,5~ temu art' nO! fM 50"'" /U continuous reaC'l1on and disconl1nuous reaCuon oj " ,"Slall "lIlt 111;-1,.

"

i\ luluul r('IOlion.<; of crysluls: on'rgrowlh Il'XllIn'S

FIg. C Tl1rl'o' ( ,.\·alllp/('s oj C()IIII11I1/J10 lIormal : (mI/lX " 'prtlst'lI/l'Il 0/1 a s/.:t ' lfll ~mph

FIg. D D iSCQlllimlfllU lIorlllal : rmlllJf

Fig, E C/Jillbilll'd {·omil/llOus (//ill rlis('(JI/IIIII/OII.1 /101"111(11 : olliflX

A',L-____________________________ ~--Core Rim

Distance from cenlle of crystal

A', L _____________ _

Core

A', L-____________________________ ___ Core Rim

96 Zoned plagioclase

The central plagioclase phenocryst in this photograph I~ discontinuously zoned. having ~I homogeneous COTe

m:mtlcd by a more sorlic rim: the nm has continuous nonnal loning resulting In variation of Ihe e~linetion

position on rotation of the micros..:::opc )I:lb>e. The crystal IS thus panly discontinuously and partly eontmuousl~ toned.

Dolmll' from 'sll' of Skye. Srolltmd: mogllifir(lluJII x 4),

,\'PL.

Fig r MlIllip/e. erel! :OIlIng

Mutual relations of (.'1)-5Ials: O\Cfgrowlh lexlUres

M,lllipl .. :OIlIlIg

This term IS used for crystals h:l\ mg TCPC:Il00 dlsconllnuous mnes If the lone~ sho .... a rh~thmic r ... petition of "Idth. Ih ... p,lI\cm is Known as asciI/ilion :0111111( .

The overall composilional lrcnd of the mUltiple "zonmg ms> be nQrmal or rt' I"('f$(' or ('I.",' (m .... hieh there is 110 gcncr,iltrcnd from core to rim). Individualzoncs mil) be of umfonn or vlIriable compoSition. such Iha[ Ih!.' loning pattern on :1 compo)ition. d istance graph is square "'live. step· like. saw-tooth. rUf\OO s;lw·tOlllh, or som ... rombination of thc..;c (see Figs. H J). lI o\\e\'er. the:.t.' (Ire dctaib which only I'en e;ITcrul and Jcnl!thy orlleal examlnallon or electron.prolx mirroanJlysls \\ould rc\e;11.

The rc:tder should apprcculle that the skctehC!t 10 figs. C J are all idcahl .. :d :lT1d that in real crystals the oscilhttions \\111 be less unifonn: furthermore multiple or oscillatory zo ning rna) only occupy part of:1 cf))tal. the remainder pt'"rhapi:< being homogcm."Ous or continuousl} zoned.

An.,L-______________________________ ~--COle Rim

63

i\'lulual r('lal iOllS of cl)"stals: OH·rJ:rO .. "lh leXlurcs

An lllO

- ~ - ~

Ao.

Core

Ao • Core

~

Rim

Aim

A~~ __________________ ~~_

Core Rim

Fig . G Oscillowr.v" (· ."en :Qmllg

Fig . If Olcilfutof),. lIorm(l/ ;QlliIlR : I/ep-likf'

Fig" I OsciflmorJ'. lIormoi : ()//illg : sml"' IO(Ilh

Fig. J Oscilfot{Jrr. normal :onlllg ' cUrlwl ~{/\I'-I()(Jlh

97 Zoned plagioclases

This phOlograph lllusiraies several sirles ofzomng In the 1\\0 plagioclases comprising Ihc glomcrocrysi. Combina­Ilons of dlsconllnuous. oscillatory and convolute zoning arc presem. togelher with loning picked oul by a b..'lnd of mel! inclusions ne'lr Ihe margins of both crystals,

P(Jrphyrillc wulesift' jrom Hokum' l'o/mllo, japall: IIIlIg­lIijirfllioll x 14. 'yPL.

Ao,L-____________________________ ___

Corc

Com'ohm' : O/llllg

This IS a \,anety of multiple lOning m \\hlch :.ome of the lones are erratIC and hale nOll-uniform thi('kncss (sec 97).

"

l\'lutultl rela tions of eryshtb : ol"ergroMth textu res

S"Clor fur IUlllrg/W;"f J :fJll/lIg As 'lCen In Ihm section. tIllS ideally lakes the form of four tri;i!1!!u!;tr scgmenb (sector.)) \\ IIh a common ajX"x (Fig. K(b)). OpP()~;tc :.«tors :m: chemically idcntlc:d. v.hereas adjacent ones differ In compo<;lllon (Ihough pos~Ibly only slIghtly) and hence in optical propenlcs. Eaeh :Sector may be homogeneous or sho\\ contmuous or d'llContmuous or oscillatory. nonnal or rC\o,'rse or e\en 'lonmg. In thl\-'C dimensions the 5(."'(;10111 aTe pyramid shaped (Fig. K(a)). and. depending on the orientation of the crystal \\;Ih rcsp..'Cllo the plane of a thin !>Celion. a variety ofp:Hterns may be SI."'Cn In Ihm secllon (Fig. K(b) (f)) If the sector boundaries 3rt' curv ... -d. the pallcrn can resemble [h:1I or an hourglass (Fig. K(g)). $e(tor zoning is 11 common f('atuTe of p)roxen.-s In alkall-nch txl~1C and uhrnb:lSlc rocks. 11 has also been secn HI phtgio. ciascs III u rew quickl) coolc<l basalts.

, • 9 ..

98 Zoned olivines

Zoning is nOI confin .. -d 10 feldspar crystals. H ... re. each of the three olivine phenocrysts in the duster has a homo­geneous core surrounded by a continuously nonnal-zoned mantle, as indicated by the variation In interference colours.

Al1k(lr(lltliTejrom Mmj/J(I Keu . Hml'(1ii ; m(l1!.l1Ijicoliol1 x 43. XPL.

Fit; . K SeMl/llllie repreSl'IIf(JliOlU oj ~l.'CfOr :/II/ll1g

99 Sector-zoned augite

The picture shows a simple seclor-zoned augite pheno­cryst contai ning elongall:! meh inclusions: the crystab partially enclosed by two of the sectors are olivines.

Essexile f rom Crall/OrlljoJIII. Scotitllld; IIUI/:llljim/ioll '" 40. ,rPL.

, 00 Sector-zoned pyroxenes

Two sector-zoned litanauglle cl)::otals arc iIIu;.traled In

thes(! photographs; that on the left is complicated b} forming at one end a graphic intcrgrowth wi th nepheline and lcuclle: the other crystal has an Intriguing figure-S­shaped core. with II discontinuous. sector-zoned mantIc.

Mdllllo('ru/ic nepheJim:' IIIICfOSrem/I'/mm V(Jge/sMrg. IVf!.!'!

G.'rlllfmy: flUlgllificmioll x 7. PPL fllld XPL.

67

Mutual rci:ltiOIiS of crystals: uH'rgrowth textures

Banded texture!; (banding)

T",xtures of this type involve two. or more. narrow (up to a few centimetres). sub· parallel bands in a rock which ~ I n: distinguishable by differences in texture, and/or (,;olour. a:ld/or min..:ra] proporLion~. ~h'" term laymllg is 11150 used by...pqtr~logisl s; whtle It mcludes banded texture. It IS also used for larger scale strnmrcallon. An example of banded texture due to textural differences is illustr,ned in 5. and 103 and IIH show ",.~amples rcsulling from extreme differences in mineral proportions.

68

101 Oscillatory- and sector· zoned, inclusion­bearing pyroxene

The augite phenocryst occupying most of this photogr<lph is sector-zoned and each s<x:tor displays oscillatory zoning. Inclusions of nepheline, augite and magnetite are ar­ranged in trains parallel to the oscillatory zones.

Tephrill' /roll! MOille Vulturi. M!llft . Italy ; m(lgllifiCllliQn x 27. XPL.

102 Oscillatory· and sector· zoned pyroxene

Unlike the pyro.~cni:s in 99 and 100. this scctor-wned pyroxene has some sectors bounded bv marc than one facc. c.g. the sector on lhc right is terminated by two faces. and that on the left by three faces. The cry~\al cn­cl()sc~ plagioclase laths. an olivine (blue colour) and a pyroxene crystal (orange colour).

Ess/,xile/m/ll Cmlljordjohn. Scot/and; mtlgllijil'fllir'm x 15. XPL.

103 Olivine and chrome-spinel banding (or layering)

The photograph shows tWIl bands, one nch in olivme, "Ith &;Cllree disseminated chrome,spinel crystlllS, and the other nch in equant chrome-spinel crystals with scarce interstitial olivine.

BllIl(led dunitt- chromi,"e from Skye , SCQ/fond, magnifico­lion x II , PPL.

104 Anorthosite-chromitite banding (or layering)

ThiS hand-specimen photograph shows alternating bands of anorthosite (white) and chromiti te (black), The yelloVo ish·brown crystals in the anorthosite are enstatite and the black particles are smgle crystals and glomera. crysts of chromite.

Bamh'lI IIllOrthruitt ·cirromilile j rom Crilical Zone of the 8I1shw'/d intrusion, South A/rica . mllg"ijirlllitlll x 2.

. -_. __ .. _._ ... - -- "~ -'--. -_ .. _-- ........ -

Comb IUYI'ring, orbiculur leXlure, ocellur (!'.'ftur#! und I!utu.l'itir teXfU N.!

Comh {flJ" 'ring (5(.-": p. 44. 10 tmd 11) :Hld orbicular Int!l re (1 05) lirc p:t rt iculnrly exotic kinds of ba nding. I n the latter. 'orbs' consiSI of concentne shells of rhythmi­cally altern:lling mineral constItution . Within the shells the texture 11m> either Ix­granul:lf or elonga te crystals may be radially arranged. 'Orbs' 013 ) reach a few (cns of centimetres in diameter. A rurther variety of banded texture, t'III/IXI/lC.

occurs In some wffs and ignimbrites and consists of a regular alignment of flat­tened glassy fragments (8b),

"

Mutual reili tions or crystals: b:lndcd lexlures

105 Orbicular m om :o di or ite

The fir~t photograph shov.'s the texture in a hand specimen . The arrangement orlhe concentric darker bands aboUllhe lighter coloured. homogeneous nuclei is well displayed . The ~econd photogr.\ph shows the core and a few inner b~mds or one orbieule in Ihln seclion. The bands can be secn IQ differ from one another in their contents of biotite and alkali feldspar. and in their grail! site.

MOII:;:ori/orilt, from tilt.' Islrmd IIf Sltur; UIIII~w{/ri, Rlloko­!trhli. S.E. fln/al/d; IIw1(lIijiclllillll x I Uir,f/ pIiQ/o) x 3. (/ll(/ X PI. (J-l'(WIi/ p!/()/o).

106 Vesicular feldspar-phyric basalt

Large subspherical gas cavities are randomly distributcd In this volcanic rock. Note the tIl 0 vesicles;1\ the lOp left which have coalesced.

Basalt from MOUIII fllji. )tlplm : nltlgllijicQ/iollx 7. PPL Wid XPL.

Ca,it ~ IC"llLrt"S

These :Ire a collection of textures which feature either holes III the rock or likely former holes II hich IIrc now p;lUI} or completel} filled with crystals.

VeS;C/Ilar teXllIrr

Round. ovoid. or clong;llc Irregular holcs (vcslcles) rormed b} cxp.msk,n of gas. in a magma .

A mY.KduluiJullt'XlI/fIt

Formcr vesiclcs arc here occupied. or p:miall) occupied. by lute-$tl1ge nmgmlltic and or post-magmatic mlller:lb. such as carbonatc. 7.eolites. quartz.. chalcedony. analcite. chlorite. and or. [;ITdy. glass or fine groundm:lSS. The filled holes are known li S nm~gdllies or amygdu[es.

OcrJlllr feXlI/re

Certrlln spherical or ellipsoidallcueocr.lIic p:llehes enclosed In a more mafic host arc known as ocelli (singular ocellus). Unlike ;ull),gdalc.s. the miner:lls Iilling un ocellus can nOflllHl1y all IX! round in the host rock; the}' may include any of: nephe­line. analcllc. zcohtcs. calcite. 1cucite. potassiulll feldspar. sodium feld)par. quartl. chlorite. biotite. hornblende and pyroxene. or elen glass. and the m11lerals arc commonly distributed in it 7.0nall1 rrJngement ( 109"). Often. plllly und acicular eryst~ls III the host bordermg an ocellus lin: tangentially ILrr;ll\gcd (as in l09b) but somcllmcs proje~'1 into the 1X.-clJu~. Ocelh arc nomlHlly less than 5 mm 10 diameter but rna) reaeh 1 em. TheIr origlO has been ascrilxd on the one hand to separation of droplets of immi"Cib!c liquid from m:lgma. and on the o ther hand 10 seepagc of residual liquid or nUld into vesicles.

j\1iarofjt;c tuturt

These arc irreguhlrly shapt.'d e,(lilies (druses) in plutonIC and hypaby,~al rock~ Into II hich euhcdral crystals of the rock prOJCCt.

Litlwphysa (or stone-ball)

This 1$ the !Con gwcn IC) u sphere cOnSiSl11l1,!, of cOI1l'cntrtc sh(.'115 II ilh hollol\ lmer­space).

"

Mutual relut ions of crystals : clil-ilY lextures

"

107 Ves icul a r trachyte

Irregularly shaped, elongate vesicles afe streaked out through this trachyte; the columnar reldspars show 1I weak alignmenl in the same di rection ,

Trachyte from the AIII'ergll/', frIllICI': lIlagllification x J 2,

PPL.

'08 Amygdaloidal basalt

The onginal vesicles in this volcanic rock are now filled with an aggregatc of small calcite crystals: calci te is also prcst:nl as pseudomorph~ a fter olivine in the groundrnass. Pyroxene and glass in the rock a rc :tltcrcd to cla}' minerals_

Bosult from Matlock, Derbyshire. Ellg/alld: magn!ficulion x II , PPL l/lld XPL. J 24 ~-hu lI'j ollother llmygda/Qid(ll rock ,

109 Ocellar textu re Thl: upper photograph shows three ocelli in an olivine dolerite sill. Each ocellus is outlined by a more or less complele \encer of lin), magnetilcerystals. At the b..'lscof the 1\.0 largest Ql'dli the groundmass outsIde the ocelli c;C:lends across Ihc Illugnetitc \enccr. except that o livine: is absent inside thc <x:elli . The remainder of each ocellus eompn~s clear .l.eolilc, turbId. very fine-graincd leolilc ;lnd scartt nlllgl1c\lle. The len·hand ocellus Illso l:ol1laillS thr~'C elungate: p)'ro.~enl·s on the lefl side.

\ ~III-flllrphyrilic f(/ci~' .. or'lII (JI/I'me iloft-rill' sill. Igil/(Jrs­'·lIi1. l. '/"If!Knllll Ej/lIIu/. Il'e.I"t Greellltmd: /II11,~llificll/i0I1 x J 1. I'PL.

The :;..'Cond picture ~hows ' ..... 0 ocelli. occupied by calcite. :tlkall feldspar. chlorite and fine·grained patches or clay (possibly altered glass). Lllhs of biotite an.- arranged tan­gcntiall) about each ocellus.

l/ulelle fwm W('~lm(Jr/llIItJ. /:."lIg/mul. /I/ujtllijicatioll x 16. PPL

\ lutu31 re la tions of trystals : cao'ilY leXlul

73

Introduction

In this Part are defined and illustrated many of the more common igneous rock type). For mo~t types plane.polarized light and erossed.polarized light views are shown . In:t fe\\ cases more than one example of the rock type is illustrntcd and in somc \I e ha\e used more than onc magnification \0 show:t particular f;:.ature of II rock . ! Il IIddition. reference is mllde to Part I on textures \\ here other examples of a sJX."Cific rock type an.' illustrated. Thus, although gabbro may be represented b) on ly two photographs in Part 2. we have noted where other photographs of gabbros appear in Part I .

No two igneous rocks are identical in e\'ery respect but many are sufficiently alike that they eM be illustrated by II few I)pical sIX"'Cimens. Thu), an olivme gabbro from one localit)' m:ly be vcry ~Imilar to a 11Irge number of olivine gabbro) from different parts of the world. We have therefore selected thin S<."'Clions \I hich lire fuirl)" typical of lhe rock type being ill u stra t ~-d .

Fig. L 1\ (mwnc/(I/Ilre of Ih/! COI/1I1!()IIf" t}fIU'()II.f rock.r b(f~,,'d (m Ilr/,Ir stlinl (111(/ (J/~(J/I ("()lIlelll~.

The choi~"e of which rocks to include has not been easy and undoubtedl} \\e hll\ e omitted somebody's fa\o uri te. In Johan nsen's f)(' .~crlpli\·e PNrography of the /g lll'OIIS Rm-ks more than 540 differcnt names for igneous rocks are listed in the index, nO\ counting those names which have a prefix indicating the presence of a particular mincmi or textur~, thus we have counted diurill' as one name rather than the eightccn varieties of dIorite listed by Johannsen. Holmes listcd about 340 differcm Igncous rocks in his Nomellr/a/llre of Pl'lrolQg." but protxlbly less than 15Q of these arc now in l'ommon u'>t , We have selected about siXlyoftht.'SI.' names as rocks .... hich thc student lIIay expect 10 sec in an undergraduate course in geology. Certain rock types C;InnOI be distinguished by 1I cursory cxanllnation of a Ihin section. much Icss from Olle or two photographs. Thus. for example, because IIwge(lrilf'I and IWlI"aiill's cannot rcadily bcdislinguished from (llhlli Qasaltswilhout a dctermination of tile plagioclase composition. photographs of these rocks in thin slX"tion have not been indlldoo.

N(lfne~ offine grained rocks lIre shown in smalllcllcrs ,lI\d those of eoar~-gra ined rocks in capllul letters (modified from Cox, Bell and Punk hurst , 1979) For each rock illustrated we have given what we consider to be the definition of

16

10

8

6

,

2

/-, , ' / "

1/ ................

/~ .... , / , .... ,

/ ' phonolites .,.. .... / "') r.t£PH£Llr.tESV£r.tlns ;/ "

/ '" ~ , /', / .~1I.1i' / / ....

/ *<::- .,"" / ' / " I :s;:1I.i!-'" /' / '

// <;:-II.Q§!:"",.;J // e"" / trachytes ~ ..... , / ,to -i!-4; / :<,\~o, >- __ -<. SVENITIS /' / if" / . ~ ..... eQ*./', /',

/ :s;:o., / )..~ ..... ~ 1<-# ././ benmoreites " / , / / Q!;:-", / "S:'o(;<~",v"f- ./ MONZOOlOllrtES SYENITES ~ / I "

4. .f! / Q ",,,,'f' ""./ 0< $VENQDIOfIlTES .... ..... .... '..... / '\

1/ "..... /A" "".----~'I;~~;;7..... 'i rhvolites \ / ,/., .... / t;'> <bqj .. g; / / ' GIIA~ITES I

/ I J!- Imugeantes R.b'f'(;j ./ Ifachyandeslles' I / t' I ... I'S I o~o:.? / orlati\es I, I

/ "Xi!" ., I if" ~ / ~#."'q.t.,;/ / MONzor.tnES / \ \ I / II.~'"" l.$i ~----- tt ( .J,./

/ ~ &' I b .... "" -, \ "" ( <;:-11.'::; I ",t:.' II hawaiites I ............. 1 dacites \ -"./ I I$' I I \' "' Il /L _ _____ -..+ __ ...,_ I GIlANOOlOIlITES ,.>""

, 1.,.0;., / 'l1 " I I I / , -OJ!- -l'(;< .' III ./ ,1'/~'o, ... I!!I 1"-'I i' I :;;., c~~" basalts I .~ III I andesites I ././

'("' it I"'~ '0" GAB.8ROS 0\ l "tl W I OIORtf£S I"" .... , I ./ . ~~~)..,(;<'b I ; i I ..) ....

v . 01o..'b ~'6 ........ o, ..,0 .....

'''.' ""'6.'1 '':; 0 1 ~ ..... -.. .... ....., . ~ ." -. .

, """' ,' v I III I .... , i'~ " <,: '0 '" .... , .... 6aSalts l.l:l I _ .......... _____ __ __ L _ _ ...L-

77

the name and this is followed by a brief description of what is visible in the- field of view of the photomicrographs. In addition to defining the rock illustrated, we have also dcfUlcd. though not necessarily illustrated. the names of other.; which arc subtypes and whose names are still in usc (e.g. gralloph),re as a variety of mkro­gr(lIIilt!). Agreement among petrologists on the characteristics of individual rock types is improving but will always be open to some differences of opinion. The names used and defined here are as near to consensus opinion as we can sense it. using. the text of Holmes (920). Johannsen (1931 ). Halch &/ (II. (1972). and Nockolds /'1 oJ. (1978). the paper of Wilkinson (1968). and our own experience. In most of the definitions we have refrained from stating ranges for Ihe amounts of the essentia l minerals. since agreement amongst petrologists on this is generally poor. On the olher hand, the photogmphs give the reader an indk~ltion of whether 1I particular mineral is abundant or scarce in the rock .

The photograph dcscriptions arc deliberately short because t.hey arc only of thosc featllres which can be St!en in the photomicrographs. The complete pctrogmphic d~scription of 3 rock requires a careful examination of the whole slide at different magnifications and the student is likely to see much mor(- than can be illustra ted in one view al onc m'lgnific<ltion .

We have not set out a system for the classification of igneous rocks becauS(" this is beyond the scope of this book. The sequence in which the rocks are arranged is broadly ultrabasic and basic rocks first. followed by intcrmr<l1ntc and then acid rocks. leaving the alkali- rich rocks to the cnd. Among the alkali-rich rocks arc includl-d a number of rare rod types. simply because they arc ran: and beca use the photographs are visually allraetive. 1n any treatment of petrography those rocks traditionally groulX>d together under the name lamprophyres pose a problem be­cause of their diverse characters. Although we have defined some of them. we have ill ust r(LIed on I y three //Iilll!1 /('. allllJile and .Iourchilt·.

Wh ile we have avoided a fo rmal classification scheme of rocks. it is nonetheless helpful 10 have in Ihe mind's eye a series of pigeon holes in which to locate rotk names with respect to one another: otherwise the brain tends to succumb to the \\'eight of names and refuses to attept morc than a few or them. Figure L (p. 77), modified from Cox. Bell and Pankhurst (1979). isu chemical diagram on which many rock composi tions may be plotted. The outennost line encloses most known volcanie rocks and the bounded arC<\ has been subdividoo and the mimCS of fine-grained and eO~lrs.c-grai nC'd varieties of rocks indicated. Thc e~act positions of the dividing linc..~ and the na!n!.·S in cadI area lire open to debate hut. in gtncraL most petrologists would actept this classification . While a great many of the rock types illustrated hcre arc shown on Fig. L, a small proportion arc not - e.g. the names on the ligun.:: apply 10 the chemical eoridilion in which Na is less than K. Other nam(:S;trc used for rocks with the much less common condition of K grealer than Na (e.g, 11!1If/l ill.· in stead of I/t'phefillill').

In the photograph descriptions ~l 1l\lmbcr of terms :m: used which ure worthy of definition here : Esse/ltial milti'mls: those which arc nccesSilry to the naming of the rock. They need not be major constituents. e.g. a ainall i l l! contains only a small pcrcent3ge of essential 'lI1alcite. Accessory minerals: those which arc prescnt in sueh small ,lmounts in ,i rock that they arc disregarded in its definition. e.g. a small percentage of qua rtz in a gabbro. However. it may be uscful in the name to note the pre-scnce oLl pllrticuiar accessory mineml in a rock and this can be done by adding the mineral name as a prefix. e.g. quart: gllbbro. A/ f'/(mIJCrIIl lc. l/IesoCT(l/ic lind leUC(}CrlIIIC (synonymous with dar~-. met/il/Ill-. and 1if.: 'U-co{fJllft·tf) : terms \C'I indiclLtc t.he colour inde~ of a rock il!ld hence the rcla ti\'e proportions of dark-to light-coloured minccil ls. The bolmdarics arc at 66 6 ." and 33 (, 6

dark minerals re~fX..'Ct i \'dy. M(ific and felsit' ma y be applied to r<x:ks whieh arc com­posed pn'tlommaJltly of m;lfic minerals (olivine. pyroxenes. amphiboles, biotite, opaq ue mincral~) oroffebic miner:! Is (q ua r17. feldspa r;md f cJdspa t hoid). respect ively. They are thus less precise than the colour index terms. The term liI/raml/jie is used for rocks with Irivi,11 ,lnlOunts of. or no. felsic minerals. The rarely-used colour index term hypl'rme/onic (90- 1 00 0 ~ dark minerals) is more or leils equivalent to ultramafic. Ullrabasic, basic. illlerllledi(ll(! (/tU/ acid: chemical terms to designutc rocks with less than 45 ; ~ . 45--52°, . 52- 66 % and more than 66 0 ~ by weighl ofSiO: respectively. Since a large SiO! contenl is reftectt.'tI in a Jarge amount oflighl-coloured minerals, these terms correspond approximately to the colour index ones, Micro liS a prefix : most igneous rocks have fine- . medium-. and coarse-grained varieties. The fine- lind coarse-grained varieties always have different names (e.g. basalt and gabbro). Medium-grllinoo varieties may also have a distinel name (e,g. dolerite), or more often these days. the name for the coarse-grained rock is used and prefixed by mi,'flJ (e.g. microgranite, microsyeni{e or even microgabbro).

78

11.

Dunite

This is the name used for an ultramafic rock which consists almost entirely of olivine. onen accompanied by accessory chrome spinel.

The granular. textured sample we ha\'c illustr.Hed oon­sists of only \1\10 minerals. olivine and a chromium-rich .pinel. The spinel appears opaque in the PPL view but. with II more intense light than can be used for photo­graphy. II can be secn to have a deep brown colour. A banded structure is visible In the large crystal showing a blue interference colour \0 the right and slightly up from the centre of the photograph. and in 1'01.'0 of the crystals showing brown interference colour.;: to the right of the crystal showing blue. Above the blue crystal u crystal shows irregular extinction. These features indicate Ihat the olh'ines an: strained.

Ormilt'.!rom \1/111111 DIIII . .... I:'II Z.'olllnd: mll!;l1ijIC'UtiOI1>< 16. PPL (Jnd XPL. Another (limile is illllstrllll'd ill 103.

79

Ullrabasic rocks

80

111

Peridotite

This tenn is used for coarse-grained o livine-rich rock~ in which olivine is thl." dominant mineral but is 11.".% than 9O Q,~ of the rock. Textbooks usu:dly state that the acces­sory minerals arc llil fcrromagnesians ,md that oli\'inc­rich rocks containing plagioclase and pyroxene should be called picrilcs (01' Irl)cf()lile. if olivine and plugiodasc only). \41 , 51), I--I owever. pierile is nOl much used no\\ , and modem usage allows for plagioclase to be present in peridotite. as eun bo: indicated br the teml$ plagjoc/I/St· or !elll~pillhk perit/o/llt' as In 51. Peridoti tes eonlal11mg both orthopyroxenl." and clinopyroxene (113) <lTe o ften Called Jllf'r:(lIiIi.'J. I f dinoPYToxcne is present and onhopyroxenc in a minor amoum or absent. wehr/rtt' is used. and hllr:­bllrgilr for the conveTse.

We ha\'e chosen to illustrate this rock by two different samples.

The upper and middle photogmph" show a poikilitic textured peridotite in which. in the lower left p~trt of the field. a number of round cryStals of o livine are embedded in two clinupyroxene crystals. and elsewhere the olivines aTe enclosed by plagioclase crystals. In thl' centre of the fidd. o ne elongated olivmc crystal is surroundtoJ by plagioclase fddsp:lr. T he sm:ltl opaque c ry~ l als arc chromite. The differences in colour and relief octwccn the plagioclase and the pyroxene are alro obviou~ in the PPL vicw : stray polari7.3tion produces the pale grC1!nish and pink colours in this view.

The lower photograph shows an XPL view ofa perido­tite in which nlJmerous olivine crystals aTe poikililJcally enclosed in a b:t~ic plagioclase feldspar. Only a ~mall proportion of pyroxene is present in tIllS rock.

First and st'((J//d plwtogmphL' Pail/()Iilt' jrmll RJWIII , S,-o/((II1<I : IIIU[!,lIijl((IIIOII )C 11. I'PL amI ,\ PL. 11lird p/Jof(Jgraph : PI'rir/lJlill'{rNllf/w S/ulIIll I.~{/"~, $1'01· lal/d: magill/leal ion x 15. XI'L 0,/11" paidolires Ilrl' illllJlr{l/('d in 18 (lml 48.

112

Kimberlite

Kimberlite is a porphyritic pOla1>sium •. WlllCT-. :md carbon dioxide-rich peridotite which foons dykes. sills lind I)ipcs. It consists of phenocrysts of olivlI1C. ph1ogopitc. ilmenite and pyropc garnet in a groundmass which commonly contains olivine. phlogopllC. serpentine. calcite. t:hloruc. magnetite, apatite and perovskile.

All of the phenocrysts in the photographs arc olivine. as are many of the smaller crystals: some of the smilll cryst,lls are pyroxene. The olivine crystals have found oul hm."S und <Ire surrounded by rims of microcrystalline serpentine. In the gToundmass are patches of calcite and ,I high density of small crystals which appear black in lhe pholOgJ"'Jphs : these afC oxide miner:Lls. including pcrovs­ki te (CaTiO) which is a common constituent of kimbcr­lites. The large olivine :1\ the 101' left contains ncoblUSI:S. i,e. new, smaller erysllils which are believed \0 h:we grown from highly strained parIS of the big crystal.

KimbnUfe from Kimberley, Sowh Africa: magnificaflOl/ lC 7, PPL (/11(/ XPL.

81

82

11;;:

Garnet peridotite

A plutonic rock consisting of mor!: than 40 0" olivine

with two kinds of pyrox<:nes and a small amount of garnct.

T he granular-textured specimen illu~(rlltl"(l has only one gOlfnC\ fairly clcnrly shown in thc field of \'ie\.\'; il is al the left of the lower edge. About the centre ofthl' lower edge is a crystal of strained phlOgopitic mica. The rcs\ of the field is occupied by olivine (grey). onhopyroxcnc (pinkish · brown). and chrome diopside(paJe i'.rc.."t:n e.g. middle of lop edge), the olivincs and chromcdiopside showing mOd em!c inl(:fferenc!; colours. onhopyroxcnc showing low grey colours in the XPL view.

The veins which penet!"'.! lc most o f the cryslals are of serpentine bUI around thc garnet crys tals arc thin veins which aTC occupied by a pale brown mica : in Ihc X PL view Ihe inlerferem.;c colours of the micas arc moderate $Ceond order.

Gflrl!C'l peridOlile/rom Kim/xrll')'. Sowl! A/ri('(l : IIIOgl1ijiW' /hm x 7. " PL mul XPL.

"' Pyroxenite

An ultnnnalic rock consistmg mlun])' of pyroxenc(s): possible accessory mmerals Include olivine. spineL gamel. hornblende. blollle. feldspar. and m.:phe1inc. A rock con­sisting ofbolh orthopyroxene and clinopyroxene is known as a ,\"~b$lf'ril(!. Ifacger;nc-:lugJte or nepheline is present thc term alkali p),rO;t:l'mll' is u.'>Cd.

The sample illustrated here IS. strictly speaking, a websterite and shows round orthopyroxene crystals. recognized by their low first-order colours and a lamellar structure. poikiliticallY L"fIcloscd by a large zoned din!r­pyro.~ene crystal showing bluc and rcd interference col­ours. A few areas of low relief seen in the vicw in PPL arc i).1sic plagioclase. At lhc bollom left is II hole in Ihe slide. A slight difference In colours of the orthopyroxene crystals can be secn in the PI' L vicw a nd tbis is chiefly due to stray polaril.ation in the photographic equipment.

Webslulle fmll! Ihe Sa/bra/(', camplex. M OT/lalla , USA : IIIIIKlllji("(l/ilJll)( 7. PI'L ami XPL.

8J

Uhrabasic rocks

84

11.

Komat i ite

This has been dclined as an uitrnmaf.c \'oicanic rock with more than 18 0 ~ MgO. While the rock lacks plagioclase, it is rich in magnesian olivine and clinopyroxene crystals SCI in devilrified gln~s. though theo]jv;neisusuullyall but com­pletely serpentinized. The tenn 'spinifex textures' has been used to describe parnllcl and radiale arrnngements of elongate olivines and pyroxenes common in komatiites. Thcse tcxtures imply rapid crystallization from an ullm­mafic liquid.

The photomIcrographs weTC made from a large thin section (l Scm by 6cm) in which pseudomorphs after olivine crystals are up to lOon in lengths. In the PPl \'icw the long colourlt:ss shapes were originally olivines but are now completely replaced by serpcntllle. In thin sectIon the olivine crystals appear 10 be acicular in habit bUI in faCI IU'C cross sections of thinly tabular crySlals stacked p:lrallelto one another. The brown m:tterial (PPL view) was m311l1y chnopyroxene and glass. though the clinopyroxene has been replaced by chlorite and tremolile or laic, and the glass replaced by chlorite.

}.omutiilP from HUllm TOlI'lIsbip . Ol/Illrio : IIwgnifi("(l//01I )(3. PPL(lIld .\l'L

". Meymechite

This is the name gIven by the Russiaus to a rock dis­covcroo in Siberia which is a porphyritic ul tramafic e~trusive rock consisting mainly of olivine phenocrysts in II b'l'oundmass of clinopyroxene. mica and chlorite.

The illustrated sample comes from the type locality und shows p:Lrts O[ IWO large phenocrysts of partially serpentin­il.oo olivine in a grouTldmassconsisting mainly of brownish pyroxene and some iron ore. The fOJC-brown mineral. of Ilhieh there llrc only small rragments. is mica : SOffie chlorite is present. The groundmass is highly serpentinized.

Mf!ymechi/I' frol/l /hl.' MI?J'mecila ,h'er, NOr/hem Sim:rill.' IIIlIgllijirolJo/l x 10, PPL ami XPL.

as

86

117

Hornblendite

An ultramafic igneous rock consisting mainly of horn­blende. Thcnameamphiboliteis reserved fora metamorphic rock consisting essen I ialiyofa n amphibole and plagiochlsc_

The first and second photographs afC of a homblcllditc in which almost the whole of the field of view is occupied by amphibole crystals . Many of the crystals are zoned, 1I few arc twinned a nd a few arc cuI in the correct orientation \0 show the two cleavages intersecting al J 20 _ Notice lhe lack of preferred oricnt.1tion of Ihe crystals and their interlocking relations.

/ll)mblendi/!' jr;;m OQllegul. Ifl'''lIId. magnificatioll x fl , I'P/~ Imd KPL. AI/otizer IlQmb/endile is ilflls/rawN in .19.

Basalts In the mOSt generalized definition these are fine-grained mafic rocks with ess.::ntial augite. labradorite-sodie bytownite and opaque minerals (titanomllgnetite ± ilmenite). They may be subdivided. ir so wished, into tholeiitic ba.mlts (tholtiilt's or subolkaline basalts) and alkali olMne basalts (fig. L) on the basis or the presence or absence of accessory olivine. quartz and 10w·Ca pyroxenes (pigeonite or ortho­pyroxene). Phenocrysts or mierophenocrys ts of aU the essential and accessory minerals (except quartz) may be presenl. Tholeiitic basalts (118. 46. 58. 62) contain both augite and lo ..... -Ca pyroxene (pigeon. ite. hypersthene or both). Olivine is either absent or present only in small amounts (less than 5 % by volume) as phenocrysts only, never in the Sroundmass. The ground· mass commonly contains varying amounts or interstit ial bro ..... n glass. or devitnlicd glass (intersertal texture); in more slo ..... ly cooled rocks the place of the glass is taken by granophyric intergrowths or quartz and alkali feldspar. The remainder of the groundmass usually has an intergmnular or subophi tic texture. Alkali o/Nine basalts (119 and II ) contain no lo ..... ·Ca pyroxene but plentirul olivine, both as phenocrysts (ir present) and in the ground mass. The augite is often some­what purplish.grey in colour due to high Ti content. Less than IO }o of the feldspar is of alkali type. The groundmllss texture is usually intergrnnular or subophilic and glass is very rare, though accessory interstitial nepheline or analcite may be present. Iralkali reldspar is present. illS in the interstices and as rims on plagioclase.

The terms o/il'inl' tlroleiite and olil'inl' bas(llt (22, 23. 44, 56, 51) have been used for rocks which ha\'e certain chal"dcleristics or both tholeiites and alkali olivine b"salts. They lack 10 ..... ·Ca pyroxene. olivine is CSSt!nt ial. exceeds 5% and may be present as both phenocrysts and in the ground mass. The augite is nOI T i-rich and it is not as Ca·neh as thaI in 1I1kah olivine basalts. Interstitial glass may be present : nepheline and analcite nrc absent.

Chemical d"13 greatly assist in making these distinctions: normath'e hypersthene is the hallmark or a tholeiite (true tholeiite and olivine tholeii te) and absence of nonnative hypersthene characterizes alkali olivine basalt; nonnative olivine and hypersthene characterize olivine tholeiite. Without such data the pctrogmpher must rely on the mineralogical ehamcteristiC$ mentioned above. which or course may not be distinguishable if the rock is very fine grained .

Part icularly olivine·rich varieties of both alkal i olivine basalt and ohvine Iholeiite exist (up to 50% olivine) and these may be referred to liS alkalic ptcritl's and tlro/l'iitk pirritl's (or tholeiitic piuitt' basalts) or generJ lly pirritic bastlll.r (122. 26, 27, 31). Pyroxene·rich basalt is called ankaramite (98. 123).

The term trllcll}'lxlSlllt l is sometimes used ror rocks slightly richer in alkalis and silica than alkali olivine basalt and hence having a more socii..: plagioclase and more alkali feldspar (10-40% or total reldspar) than alkali olivine basalt. Strictly. thc term should be used for those rocks which on an alkall·silica plot (fig. L) lie between alkali olivine basalt and trachyte, namely hawoiite (andesine. anorthoclase, olivine, ,lUgite and biotitc. Sl'C 47). lI1uge(lritt (same but oligoclase ror andesine) and bell­lIIoreile (same but anorlhocla$C for oligoclase) and hence show reat ures gnulational bctll.·een trachyte and basalt. Syenogabbru is the equivalent coarse·grained name.

A very uncommon group of basalts are both alkali rich and have K greater than Na. in contnlst to common bas.."Ilts. These cont .. in essenti .. 1 K·reldspar in the groundmuss in addition to augite. plagioclase (labrudorite) and opaq ues. Olivine and biotite are common accessories. The tcrms absarokite and shosltonite arc: used ror these, the rormer being more mafic than thc latter.

Lunar basalts. two or which li re illustrated here (120, 121). arc cl:lssified d ifferently but being poor in sodium and potassium arc more akin to terrcs trialtholeiitcs than to alkali olivine basalts.

'Om.H·'U·U,i (m 1111' IIInmit,g lifllris I l'r", is poqr I I 'HIS orfgmufly lI.fl'djor ",'hItlIS IIUII' II'rm~Jo bujun ill' ( 157) and jOlll1' (ll'Ir%giJts/rm'" usnl ilfarha.wlll' III ... ·lrn·h Iho: II rOlIlNIt Url'f'dllhul

u/Nu.

87

Basic rocks

88

11.

Basalt var. Tholeiitic basalt

T he firs! and second photographs are ofll!holeine which is somewhat coarser in grain Si7.e than is usual. Clino­pyroxenes subophlt ically cndos(' laths of plaglOcI~IS(:. The brown Inter~ti t ial regions arc of much finer grain sile lind consi~ 1 of p];\gioclase. clinopyroxene. opaque miner;.!1 and deYitrified glass, A few skeletal oxide crystals tlre visible.

The third photograph shows an XPL view oran olivlOe tholeiite. The few rounded cryslals showing bright inter­ference colours arc ffilCrophcnocrysts or olivine III II

groundmass of clinopyroxene. plagioclase and interstitial glass,

First and second photographs : Tholdi(t' from Del'p Sea Drilling {'rojl'C( f,olll ug 34 , Nllc;" {,Iml' S.E. Pacific mawufiCfllio/l x II, 1'1'1. ami XI'L. nlird pil%grtlpil : Olivill(' tholeiite from Columbia Rh'er , USA ; IIl(lgn ijlcllI ill /l X. 20, XPL.

11. Basalt var. Alkali olivine basalt

The photographs show a rock conslstmg of brownish augite crystals subophilically enclosing dear laths of f.:1dspar. One microphenocrysl of plHgiociase feldspar is clearly visible 31 the lOp edge of the field of vicw. Two rnicfophcnocrYSI$ of olivine afC easily identified by their bnght interference colours - one blue crystal adjacent 10 the fcldsp;lf microphcnocrySI al Ine lOp edge orl he photo­graph and one pink crystal to thc right of Ihe ttntre of the fidd. Smaller crystals of olivine can be identified by C0111-

paring me \1'10 pholographs - in thc PPL view thc olivine crystals haw much puler colour than the clinopyroxcncs. which in this rock arc quite strongly coloured.

A/hili o/iI'ine baSlIl1 from lffJWl/h : IIIflgnijic(J(iflll x /5, PPL (lml Xl'/....

8.

B:I5ic rocks

90

120

Basalt Lunar low-Ti basalt

The photographs show phenocrysts of olivine and pyrox­ene set in a matrix of plagioclase and. pyroxene. The pyroxcnes can be distinguished from the o!ivincs by the fact that they have II reddish-brown colour. t.he intensity of which increases towards the rims of the crystals: the olivines arc nearly colourless in PPL and within them are ine.lus;ons of glass and also opaque mineral!;. mainly chromitc lind :m Fe-Ni alloy. There is rather a small proportion of plagioclase in the view shown here: it fonns elongated crystals intcrgrown in variolitic lashion with pyroxene. The opaque mineral in the groundmass is mainly ilmenite.

LoII'-lilmlilllll porphyritic basult QbwilletJ by tile Apoflo 11 misxi()II/mm the Rip},a('lIs Mountains. sOlllh o/Copernicus ( NASA sllIIlple IJIllllber 12002: 399) ; maglli{iC(1/iOIl x 18. PPL flild XPL.

121

Basalt Lunar high-Ti basalt

The thin section of this rock shows reddish-brown pyrox­ene. plagioclase and ilmenite. The completely unaltered stllte of the pyroxene and plagioclase is probably the first characteristic which stri\.:tl) the observer. Zoning ill the pyrollcnc is distinct in some crystals. Noticc the unusual texture in which plagioclase laths subophilic;jlly enclose pyroxene, contrary 10 nonna! suhophitic tC:l:lure. Acces­sory crislObalite is also prescnt. It can best be seen in the lOp right-hand eorner of the vicw 10 PPL. where ils 10\\ refractive index means lha! it stands out in relief againSi the calcic plagioclase - its low interference colours are secn in the I·iew under crossed polafs.

COllfse-grailled IlIgh-lilalrilllll hll.m/l ob/ailled by the Apollo 17mi.iSio/llrom Ihe 7imrri~-LiIl'oll' Vtllley, Ttmflls MOIIII­lohl', (NASA sample IIIlmlX'f 100/7. 216): ItwgnijicoliOir x 25, I'PL (md J.' PL.

"

Basic rocks

92

'22

Basalt va r. Picriti c basa lt

nll~ sample has abundant phenocrysts and glomero­cryslS or subhedml !lnd euhcdral ollvlnc In a groundmass or olivine, pyroxcne and small proportions or plagioclase and iron orc.

Pierilit" baSillt from Ube-kem/t b/olld. W. GrN!lIll1ml , magllil/em/on >(8. PPL (lml'yPL. See also 16, 27 GIIlI j I

123

Basalt var. Ankaramite

The specimen iUustrated here has zoned phenocrysts and mlcropnenocrysts \.\ hieh are mainly of pyroxene. although a few olivine crystals can be s{'''Cn. The oli\'ine crystals occur mostly as microphcnocrys\s and they are slightly paler in colour in the PPL photograph than thc pyroxenes. We C'.tn identify 1\\-0 of these olivines In thc field of view one shows a blue interference colour and is juSt to the right of centre at the top edge of thc photograph, and another is at the cent~ of the bonom edge of the pholo­gruph lind ~hows a pule yellow interICrencc colour. The groundmass contains minute laths of plagioclase. tOO small to be \'isiblc at Ihis magnification. embedded in "ryplocryslallinc material.

AIIJwrWlI/fl! /rolll /l'fJhilra. Madagascar : magni/iC'(lli')Ii x /5, PPL "nt! XPL

93

Basi!' rocks

94

124

Spilite

A basic rock, commonly amygdalolda!. in .... hich the original mincruls have been affected by some tylX' of altemtion. so thaI the feldspars arc of albi te composition and the pyroxenes have been rcphlcOO by other minerals. Although it has been suggesled many times thaI this name should be droppt.'d on the grounds thaI spilites are meta­morphosed basalts it is still in usc.

The sample illustrated shows an amygdaloid:tl rock in which the amygdales are filled with calcite ..... hne in both the X PL and PPL views and chlontc. green in the PPL vic\\ and showing anomalous blues and purples 10 the XPL vicw, In the groundmass of the rock are lalhs of feldspar of albite composition Sci in dark patches .... hich are mainly of finely crystallized (',hlonIC, calcite and haem .. !i\c,

Spflill~ from Chipley qlla,,)', DeI'O", EIIg'imri; IIl(1gllijirll­/1011)( 41, I'PI~ f1/1(J XPL.

12.

Gabbro

A coarse-grained. dark- \0 mcdium--colouroo rock con­sisting essentially of a ugi"lc and a plagioclase of labradorite. or morc calcic. composition. j,e. Ihc equivalent of ba!::llt and dolerite. Other minerals which may be present arc orthopyroxene. pigeonilc, olivine or quartz. Gabbros of tholeiitic affinity commonly show lamdbr Icxlure~ in the pyroxenes.

Gabbros containing feldspathoids (including analcite) andlackinglow-Capyroxenc have Ihe .group name alkali· gabbros or syl!"og(lbbro.~. A nwnber of names are used for these alkali gabbros: if Ihe gabbro contains abundant analcite and linle nepheline (eschenil e (126) is used: if nepheline amounts [0 more than IO o,~ Ihera/iIe is used, or if ohvine j$ present 1I is Icmloo an olil'/fle thera/ite; jf nephel· inc e.~ceeds 10 0

ft and between IO }o and 4O o.~ of the feldsl)ar is of alkali type. the name /:j.I'C'>:II(' ( 127) is used.

The rock illustrated is a granular olh'inc g."lbbro. A group of three olivine crystals a rc loc,all.:<i at the centre of the field towards the top and another crystal is allhe right t-dgl" of the tield. T he rCSt of the field is occupied by augite and u basic ph~gioclasc - the section is slightly too thick since some of Ihe plagioclase crystals show a very pale yellow inlcrfcrencccolour. The augite shows slight lOlling in some crystals and a lamellar texture is also visible: this is probably due 10 exsoJulion. Absence of orthopyroxene and presence of olivine make this rock the coarse· grained equivalent of olivine tholeiite.

Olil·jlll.' gahbro from New ClIfedonia; magllifica/ioll x f f. PI'Lmul XPL. Other g(lbbros or;> i!ll/str(llcd ill 15, 16. 17. 40. 49.50.61, 68, 93.

95

OlI~IC rOCKS

..

126

Teschenite

This is the name used for an alkali gabbro or dolcnt which con~iStS essentially of a plagioclase feldspar 0

labradorite o r more calcic composition. II clinopyroxen .... hieh is commonly a purphsh-brown colour. and amllcite If olivine is present. ofjl'ine leSCh(!lIill' is used . The tern crimmile is sometimes used to denote II mcdlUm-gruine< olivine-bearing rock of lhis Iype which contams onl~ trivial. but essential analcite.

The sample "'c have i!!ustr:ttOO contains olivine purplish clinopyroxene. plagioclase and analcite. Ohvincs ranging in size from O.2-2 mm. arc scattered throughoUi the rock and may be recognized by their gre) colour 111

PPL. The sUbophitie clinopyroxene shows Ihe colour typical of titaniferous pyroxenes and z(mmg can be clearly secn m the large crystal near the top of the ficld jusl to the left of centre.

In the XPL view, lhe analY-Ler has been rotated through a few degrees so thai the analcite can be distinguished from the opaque ores present in the rock mstcad of being completely black the analcite has a slightly brownish colour (e.g. left of centre): the triangular crystal or iron oxide near the top centre of the field is partially surrounded by analcite. This rock a lso cOnlains some nepheline bUI it cannot be easily secn III lhis photograph. NOle Ihe un­usual radiale arrangement of plagioclases at lower centre.

TI!$ch(,lIitl! frolll Dippul sift. ArNm. SCQt/and . magnijimlio" x 5. PPL ami XPL.

127

Essexite

This mlmc is used for a mooium- or c03l'S('-grained rock consisting essentially of labnldoritc or a more (".licle feldspar. clinopyroxene and olivine with small and vari­able minor amounts of alkali feldspar, and a fcldspalhoid which is nepheline wilh or without analcite (sec 125).

The upper photograph shows an XPL viC'w of a rock consisting of large lOlled clinopyroxene phenocrysts. small olivine crystals. laths of plagioclase. iron ore and analcite. An enlarged vicw of the central area of this photograph is produced as thc middle and lower photo­graphs. tn the PPL vicw. the brown colour of the clino­pyroxene crystals serves [0 distinguish them from the olivincs - there is one olivine crystal fairlY close to the right edge of the photograph and one: just to the right of thc renlre orthe field. panly surrounded by biotite and panly by Iron ore. M \leh orthe largeclc;lrare;\ in thcCenlre orlhe Pil L vicw IS analcite but within this area there ;Lrc alkali feldspar and nepheline crystals whose relief is such that they do 110\ stand out agllinsllhcana!citt asclcariy liS docs Ihe calcic plagioclase. The sm:lll hexagonal and needlc­shaped crystals of high relief are ofllpatile which is widely distribUlcd in this rock. One useful observation is that alkali Icldspar commonly rims phlgiociasccryslals. as can be seen in the long phlgiodase crystal lying sub-par-,dlel to lhe right edge of the enlarged XPL ~md PI)L photographs: at the lower extremity oflhis crystal there is II rim of alkali fcldsp;IT.

Essexile from Crmljordjohn . Scolltmd: /I/(/gllijicmion)( 7 ( lipper) . Xp~. x 16 (middle ( /lid lower) , PI'L (lIIl/ XI'L.

97

12.

Dolerite

This is the name used for medium-gruined b:lsic rocks consisting essentially of labradorite. augite lind ore minerals. i.e. equh'31enl of basalt and gabbro. In North America the term dillbase IS used III preference to dolerite 10 denote the §arne rock . Like b.asalls and gabbros. there arc tholeiitic and alkulic lIa rielit'S which can be identified from the presence or absence of low-Ca pyroxenes. nepheline. analcite, quartz and the absence or presence and amount of olivine. Coarse-grained names are orten prefixed by micro- to name alkalic varieties (c.il. micro­teschenitc).

The photographs show I'PL and X Pl views of a sub­ophilic tholeiitic dolerite this isconlirmt:d bylhe presence of both orthopyroxene and clinopyroxene. A smull ilnlOunt of olivine i~ prescnt in this rock and it h.a~ been p..1r11y rcpbce<l by serpentine this C'.Ul bc~t be secn in the rPL vie .... 1'0 here the olivl!-lIrt.'C1l patches arc of serpentine after olivine. Most of the pyroxene 31 the lo .... ;::r left of the lield ofvicw is orthopyroxene. It has a lamellar t~ture but this cannot be seen a t this magnification. The crystals :thowing red and blue interference colours to the right of the field of view are of clinopyroxene. The ophltic texture is rair1~ typical of \hls type of rock.

DQlerile from Palisades .tlll, New )'ork , USA , "WKlllfICtl·

lion x 21. I'P1- (/lid X PL. Addi/imlal doleriles art! WI/Slrll//'(l ill 51. 53, 59. 60. 6.1 , 89 and 90.

"9

Norite

ThiS;$ [he name ust.'d for" cOllnlC-grained rock consIsting mamly of a calCIC plagioclasc lind orlhopyroxene.

The sample illustrated shows plagioclasc. orthopyrox­ene and some clinopyroxene in a subhedrul granular texture. In lhe PPL view it IS difficult to distinguish the IWO pyroxenes but we can delCCl the presence of inter­growths in the IWO largest areas of brownish pyroxene. In the XPL vicw the areas wilh a brownish-yellow inter­ference colour arc of orthopyroxene with clinopyroxene lamellae showing higher interference colours. Smaller cl)~lal s showing blue and green interference colours arc of eli nopyroxcnc and these hn vc lamellae of o rthopyroxene. One such are.1 sho\\ ing a blue intcrrcrcncccolour is located aboullhe centre Just above the OOllom cdgeoflhe field .

Nome from IJlIsllrc/dt complex. SUlltll Afrku: magll!/ira­liOIl )( 11. PPt (11111 Xl'L.

.. ..." ,~ ' ......... "

••

Basic rocks

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... Q 1 • • " • , • •

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13 •

Anorthosite

An anorthosite IS a coarse-grained rock consisting of more than 90 ~;. plagioclase. usually labradonte or bytownite. There arc a variety or differcnl types of anor­thosite depending on the type of o(;currence and the com­position of the plagioclase ; many anorthOSlt~'S. although perhaps originally igcnous, are now metamorphic rocks.

The specimen illustrated in the upper and middle photographs is of an anorthosite Ja~'er from lln ignCQus complex. Most of the field of view is occupied by byto ..... n­lte: towards the left edge orthe field of ... iew some pyroxene can be recognized among the plagioclase crystals. The mineral .... hit h appears opaque is a very dark brown chrome spinel. There is a strong preferred orientation in the tabular plagioclase c!)'lilals in this rock giving it a lamLnaled texture. The thin section is slightly thick as is indicated by the pale yellow tinge in the plagioclase Lntcrference colours.

The lower photograph lS of a lunar anorthosite. It shows a strained and broken (Le. cataclastlc) texture whrch probably resulted from meteori te impact. The feldspar composition in IhLS sample is An 'J"J and is much more calcIUm-rich than most terrestrial anorthosi tes. Only Imy crystals of pyroxene ~XiSI among thc ~ma ll feldspar fmgments. The large feldspar crystal occupying the lower!cft part of the fidd of vie .... shows patchy extinc­tion as docs the crystal just above Ihc centre of the field . ThlS materIal is one of the oldest kno ..... TI rocks having an age between 4.000 and 4,500 million years.

First allli second photographs: Anortilosite Imm RIll/II! . Scol/aNtI: II1I1Kllijit:(lfioll x 9. PPL IIlId XPL. Third photograph ' AnortJ/o.filc I rom Cayley Forlllmioll. Desct/ru:s regiOIl 01 the LUllllr /Nell/awls : sUlIlp/e brollglll back by Apollo 16 mission ( NASA sllmple lIumlH.'r 60025, ljj ). magnificatlOlI x 12. XPL.

AIIQ/III'r allorlhosif(' If iffustr(Jlf!tlm 104.

131

Andesite

ThiS IS the name gLvcn (0 :l \'okamc rock with esscnl1al IIndcsme in the groundmass and on~ or more ferro­magnesian minerals. commonly pyroxenc(s) or hom­blcndc:tbiotltc. The rock type is commonly porphYritIC and the feldspar phenocrysts may be complexly zoned and ~mbayed \\ lth the composllLon of thc cores of the cr)'~t 'lls liS calcic Ill> b)'lownuc. The eqUIValent coarse-grained rock L~ (/IOrtl,..

The first pholOgrJph shows II PPL VICW ofa porph}ritic rock in .... hich thc phenocrysts arc of plagioclase. a brown 3mphilx>Ic and a fey. scattered microphenocrysts of pyr­oxene in a microcrystalline groundmass consisting mainl) of plagioclase. Some of the plagioclase phcnocryst ~ are qUIte diftcrcnl from OIher'S; at the right edge ofthc field. II large ~quarc phenocryst contains numerous mclusions and has u dark rim of inclusions,

" he SC'{'ond :tnd third photographs sho\\ an llndl"'lIe cQll1ammg clc-.tr !)hcnocryslS of plagioclase and bro\\ nlsh phcnocrysh of fI) roxene. The view has been selecte<l 10

mclude orthopyroxene and augite In the group of t'r)stals 10 the right of the centre of the licld, The IO\\I,!f crystal. li!;.httr 10 colour In the PPl vic\\!, IS orthopyroxene, Ihe darker crystal IS :lUglIC: The g.roundma:.~ COnlain~ r1agj(). d;Isc and p) roxcne hut thl-'> Il> tuo tinc gralnL'<i 10 determine the rHtture oflhe pyroxene

""SI pholOgrupli. Amitmr(' JTQm 80111';(1. mtlgllijic(llioli x 17. PPL

SI'f1md amI tlard plwlOgfllph.r. TlI'o-prrfJ.wlw emdt'Slll' tmlll Ilak,vII(' rule/JIIO, JafXm, mUL!,III/lCII/IC/fI x Ii. PPL ulld XPL.

Addlli()1I1I1 (1IIt/"Sllt' p!tOIOf(fllp!tS arc' ,hOLm 1/1 19 {l1I(1 97.

lnlermetliatl' rocks

101

Intermediate rocks

102

132

Boninite

This is the n:lme given to a voknnic rO\.'k Ihecomposition of \\hich IS Ihut of a high m::tgncsl:! [mdeslle. It is pre­domina n Ily glassy but wn tai ns microphenocrysts of ortho­pyro>;cnc lind sometimes olivine and clinopyro.\cnc also.

The specimen illustrated shows orthopyroxene cfyst:tls with first-order grcy illterferenee colours. clinopyroxene:. showing first- and second-order colours Ilnd somewhat lurger skeletal crystals of olivine. A Iterm ion of the olivines to serpentine IS seen in brownish patches in the thin sec­tion. No plagioclase is visible in any of these photographs.

The thi rd photogra ph is an cnlllrgcmcm of the central a rc'::! of the other photographs. This shows a Jarge onho­pyro:tene crystal with two clinopyroxene crystals parallel to the length of the orthopyroxene. AI the len extremity of this orthop)ro.~ene is :mothcr one (at extinction) 1ll,Inlled on three sides b~' clinopyroxene.

IJ()IJlllift' from Chichi lima. Bo/lill Islwlt/s. Japm • .- nwgl1!/i­canOlI x 11 (jlrsf and .w.'("ond pholOs ) . rPL (/lid XPL, x 71 ( third phIJto) . X/'L.

133

Diorite

A coarse-grained. mooium-colourcd rock con)!slmg of andesme pbgioclase and one or more of the mafic minerals elmo- and ortho-pyroxene. hornblende and bioI lie. A little quam and K-fcld~par rnllY be present . Olivine is a minor constituent of some diorites. This IS the coa~­grained equivalent of andesite. If quurtz is abundant (up to 2O ~J the name quort; dioriu! is used.

The photographs sho\\ a field OrllleW In which there are IhrC(' mafic miner-dis: biotite. pale to dark brown colour~ in lhe view under PPL: orthopyroxene; and clino­pyroxene. Some of the clinopyroxene t-rysla1s cun be e<mly recognized in the XPL vic .... bcclIuse they sho\\ second-order interference colours of blue and purple Tb" onhoPYTOxcncs show only first order (;O)OUh. T he plagioclase feldspar shows a very sllghl cloudmg which can be deteCioo in the rPL vic ..... A rock from the same intrusion showing pronounced cloudmg ofthc feldspar is shown m Pan I in 67. A small amount orquaru is present in thiS rock and is recognized by absence of alter.nion.

I)iorlt(> jrom Crllllrie. PerlhslJire, SOOI/tIllJ: /IIagllljicUliOiI )( I:!. PPI- ami XPL.

A/roIl,er '''orlll.' iJ- shon II ill 92.

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'03

InlermcdillU' rocks

104

13'

Tonalite

A coarse-gramed rock consisting of u pt.lgloclasc (appro>:. An Jo), hornblende. or blOlllC or both. and ncccsoor)' quartz. Thc name is synonymous wllh quem: diorite (In Nonh Amenea tonalite implies a quartz content of more than 2Qo o' bUI in Bntain smaller quortzcontents are included.)

The mincr.tlog) of the rock illu~trat~'(j is Mrnplc: lOned plagiodase and strained quart.: arc readily Identified, and the only ferronmgncsian mmcraJ present i~ biotitc. The cores or the plagioclase crystals conlnin a dense mass or\ery fine mica and day minerals. generally nscribed to aherntion. and the biolHC crystals h:l\C mciuSlons. man} of \\ hich produce pleochroiC haloes.

TOII{JlIle from 1'1r/~I.\· . Pit} dt' Dr)me. /-"//11("1.'; II1Uglllftcatl(JII

x 11, PPLand XPL

13'

Kentallenite

A (.'Qar>e-graincd rock consisting of essential olivine. llugite. biotite, plagioclase (more calcic than An~<l) and orthoclase. Tills i ... a coarse-grained I,:qulValent of utlchy­naSl"tlt (i.e. a ~ycno£:tbbro), iI lIlay be rcgard<.'(1 :I$:m oih'me· bearing monzonite.

In Ihc porphyritic ... pecmlcn iIIu:.tral~"(\ here. the ferro­magnesian minerals olivine. augite and biotite. arc fUlrly easily identified in the view m PPL. Most of the biotite crystals show II fox-brown absorptIOn colour. The crystals ct,)\'cred by networks of block cracks :,r.::: 01;"'mc5. tlnd the augite crystals can Ix Identified by their relief against the colourless feldspar: there arc only three augite crystals visible and \\\0 of these show yellow Interference colours lind the third.:'11 the lOp right Mthe field o(vil:w. shows a blue mterrerence colour. Some pale 1!rcen chlor;le elm be seen near the left edge orlhc phologmphs.

The mineml showmg grey. black ,l/1d white mterrerence CQlours is largely labradorite. It is dIfficult to $how Ihat or[hoda~ is present In this field of ... ;cl'. bUI t .... o unt'" mned crYSlltls II ith unIform grey mlerrerellcc colours. just below the oliv;ne crystal shol'. iog a purple Interference colour at the right I.:dge of the field. arc orthoclase.

Kplllallel1l//' from Kl'llwlfl'll . Scm/lind: mugllijim/ iOIl )( '1. I'PL ond XI'L.

105

Intermedhue rocks

'.0

'3. Monzonite

This is the name used for a coarse-gramed rock m v.hlch thc amount of alkali feldspa r is about the same as Ihnt of plagiodusc feldspar. The a mount of quartz pre.~ent should be 0 5 "y. and mMie minerals 10-25" 0' The rock is intermediate III character between s,I'f'/Iil;' and (/il}l'lli.'. Its fine-gruincd C<luivalenl is called ftllll~ or IWl"Ii)'lllllieslle.

The rock illustrated conSISts mainly of clinopyroxene. alkali feldspar and plagioclase feldspar. In thc I' PL vie" Lhe clinopyroxene can be readi ly identified. tiS can small amounts of bloille and opaque OXides. In this rock the distinction between plllgioclase a nd alkali feldspar IS easily made because the alkali feldspar shov.~ no multiple twmning. "herell~ almost alllhe plagioclase crystals In [he field ofvlCw arc multiple twinned. vcry fine IIllergrov.lhs lITe pre!>ent III the alkali feldspa r but they ea nnot easily be SI.'tn ,II Ihe magnification o f Ihese photogra phs. A vcry small !lmounl of quartz is present in th is rock.

M O/l=onile from M Ol/III Dmmf'llm)' CQmplt·x. Nell'SOIIIh Willes. AIISlrtlfi(l , lII(Jgnijicmioll x J J. PPL (lmf XPL .

'" Dacite

Th!~ IS (I volcanic rock which usulI11y conillins phcno· cry~l~ of plagioclase, less calcic than andes,ine. quartz and subordinate ferromagnesian minerul(s) in a fine-gramed groundmass ; 11 small proportion of alko li feldspar may be present. The coarse-grained cquivu1cn l 15 gml1odwr ire.

The specimen illustrated shows strongly-zoned pheno­crysts of plagioclase, and quartz lind microphenocrysis of a brownish-green amphibole In a fine-gramed groundmass composed of the same minerals and biotite. The phtoo­CI)"SIS of quarlz can be disllnguished from those of plagio­clase bll a number of criteria . the lntcrferem:e colour \hov,'fl by Ihe quanz crystals is JUSI slightly higher than thai of the feldspar : Ihe quart/. crystals show no zoning or [\\inning: and they have round outhnes. wbcre.1S the plagioclase crystals show lhe outl ines of crystal faces only ~1il!htly rounded lIt the corners. Some of the plagioclase phenocrysts have Steve texture (top left). The fddspars ~how;1 complete j.!r.tdation in sile of crystals from pheno­cry~ts through microphenocry~L\ to groundmass crystals (scnate texture). Some of the vartiHion 11\ size of the cr}"~lah I~ due to the faCI thai Ihe Sf."Ctlon does nOI cut through Ihe centres of all crystals.

Darll~ from IInknol<lllocullf.lll1 Argentum. magnifica/Ion xH. PPL and XPL.

, IIIQlit"r dudlt> is illl/sITu/I'I1 ill J .

'07

'.8

138

G ranod iorite

A gruno<horite is a medium- to tighto<:olourcd coarse­gruined rock containing essential quarll. plagioduSt! feldspar (oligoclase), and alkali feldspar. In amounts bcl\~een 10"0 and 35\ of the IOtal f<'ldsP.1r. and lesser amounts of mafic rnmera!:.. oornmonly hornblende or biotite. or bOlh. Whereas granodiOrites hale 10 ]0.

0

qu.tn1.. dionte has only accessory quartz. If :my. The specimen illustmtcd ,s a granular blotite·horn·

blende granodiori te. The hornblende IS pleochroic in shades of brown and green in PPL vic .... (e.g. Ihc [hr« green cryst:ll~ neM Ihc centre of the field) . Two or lhCl.c hornblende crystals sho .... twinning m thc X PL ViCII.

J USI below them fI rc two biotite crystals showing light a nd dark brown absorption colours. Quartz can be distin­guished from the feldspa r in thai it is rdatively dear in the PilL view nnd some cryst:lls show II slightly highcr inter­ference colo ur than the fe ldspars. It is more dil1kult to d istinguish thc alkali feldspar from the piligiodase. but In

this rock the fonner shows only simple twins: the crystals at the lOp right corner of the field of view are of alkali feldslXtr. whereas thc large f.:rystal ncar the centre of the ficld is plagioclase. The distinction is most e.1sily made by the difference in relief which cannot easily be shown in photographs, but the plagioclase in this rocl has:1 higher relief than quarlz whereas the alkali fdd:.pa r has a lo\\er relief.

Gmll(l(liQrll~ (mil! Criffe/-D(llhe(lIll~. S(,()llolUl. lIIogmfi((l-1/01/ l( J J. PPL ulld XPL

'3' Trachyte

A trachyte is a finc-gramoo volcanic rock consisting mainl} of alkali feldspar or sometimes of \\\0 feldspars. tI sodic plagioclase lind tl potassic feldspar. A small ,[mount of ferrornagncsian minerals is usually preso:nt. Quart1. or nepheline may be pre:.cnt as lIccessorics but tlTC confined to the groundmass. The cO<lrse-gruined equi­valent is :f),('I11/(.'.

The sampk illustrated is n porphyritic rock in which the rredominuntly cuhedral phenocrysts IIfC of both sod!!.: plagIoclase and sanidine. In the centre of the fidd there is a ~roup of plugioclasc cr}"ilals which in the PPL view can Ix seen to hal'c slightly higher relief than the large snnidincs whIch CK.'CuPY Ihc 00110111 len corner of the field In the XPL view the multIple twinning III the plagIO­clase crysilils is ob,· ioU5. The tWO large crystals at the right of the phOiograph are of sanidinc. o ne showin1! a simple !II III. AI the bonom left there iS:I hole In the slide­in \Ihich the large s;tnidine shows broken fragments. A rew microphenocrysb of pyroxene are I·istble. one lying JUst above the centre of the field. Unfortunately it does not shOll Up "cry \le1l in the PPL view bcc'.lUse the substage dmphragm was :.topJ>l-d <lo\ln to show the relief in the plagioclase feldspar. and in the XPL view this pyroxene is;n the e.'(tinction position.

Tmclnl/t' from IJdrl<l, lIaty: /Uuj;llificlI/iQII x /4. Pf'L lIl/I/ ,\PL.

Adl/ilio/!a/ trachyte plrolOgrapJI5 or(' ~lrow" III 64. 65 (/lid 107.

infemlcti iale rocl

'09

Inlcrrnediatl' rocks

"'

140

Syenite

A syenite is a Iight-coloureO, coarse-grained rock consist­Ing maml>' of alkali feldspar wIth less than 50

0 quar!l, or less Ihan 5~" of feldsp.llhoid . Clinopyroxene, hornblende. biotite and even olivine may 00 pmsent m small amounts. Plagioclase may be prescnt as an accessory. This rock is the plutonic equivalent of /radIY/I'_

The upper photograph is of.\ polish~-d slab of larn·ki/e. a syenite commonly used as a decorative building materi.).1 because of thc mOOnstone schiller shown b~' the alkali feldspars. A thin section cut from this rock is illustmted in XPL view in thc middle photograph. Mosl of Ihe field of VICW is occupied by cryptopcrthitic alkali feldspar. A small (lmount of nephcline is presenl in this sample but cannot be casily illustrated In one photograph. The crystal showmg a green interference colour at thc right edges of thc field is lin iron-rich olivllle: cli nopyroxene and biotite arc also prescnt.

The lowcr photograph is of a granular-textured syemte. A field of view has be~n chosen so as to show the fcrro­magnesian minerals at the edges only. The mineral on the top edge showtng a brown interference colour is of pyrox­enc: a purplish :Ibsorplion colour being added to the interference colour is the cause of the slightly anomalous colol1r. The crystals showing red and green colours at the right-hand edge are oli'lines with alkaline amphibole rims to the crystals.

First ami sec(JmJ pho10graphs: Synllle fmm 1.Lln·ik ill Norway .. maglli/ictllion )( J (first photo), x J I ( seCunt! photo ) . XPL. Third phu/ograph .· SYl!lIiu.' fr(l/ll lfima/I$saq, SOlllh-wesl Gre/'lIlund; lI1agllijic(lIiOI1 x 16, X PL

141

Shonkinite

This name is used for a melanocraticor mesocrallc potassic Hem/e.

Tbe photographs sho" crystals of blolllc and elmo­pyroxene, with one olivine crystal toward~ the bouom leO comer of the field its interference colour In the XPL view is dark green. Olivine is only an accessory cOIlSllIucnt of a ShQllkilll/f'. The remainder of the field of \ic .... is occupied by un alkali feldspar with a s),TIlplcctitc-hkc intcrgro .... th, best seen In the PPL view; somcsodic pl:.lglo­cJasc is also prescnt . The p..1rls of the symplcclitc .... hleh appear hrown ure fine grained alteration products of another mmeral. A sTIl:t1l amount of nephehne is also visible in Ihis \ lew bUI is not c:tsily dlStlngUlshed In the pholograph. With Uic rcasmg amount of nepheline Ihis rock would grade mlO a maliglll/f' (sec 152).

Sil(mkinll(' from Shonk;n Sflg, Hig/rll'oOl/ Marul/lIlI/s, MOII­tand. USA. /ullgll!fiClItlQII x 22, I'I'L (/1/(1 X PL.

111

112

142

Rhyolite

A rhyolite IS an acid \'ok'lOic rock generally containing phenocrysts of quurlz il!ld alkali-rich fcldspnT in a fine­J!.l1Iincd or glassy groundmass. Unfo rtunately, because the quartz phcnocryslS may be absent, it is sometimes im­possible to slate without a chemical analysis whether a given rock Isa rhyolite. The fcldsparmaybcsanidinc-. sodic plagl()Ci:lsc or bolh. Grul1Ill' IS the coarsc-gnllncd C'qulva­tent. Man)' rhyoliles lire wholly glassy and some have a high proportion of ghl ~s the tcmlsohli(/rUII :lOdp/U'hsIOIIl'

then apply respectively; these terms do nolo however. iml"l)' rhyolile composition. It m:ly be n01l'<l that some petrologists define obsidi:ln and pitchstone nO! on the basis ofcryswllinilY but on water content : in the former it is usually less than 1"0 and in the Jailer lip to 10"0'

We have chosen a rock containmg twO types offddspar phenocrysts in II microcrystalline to glassy groundmass sho\"' ing perlilic fracturing. Most of the phenocrysts are of alkali feldspar. some of which show simple Iwinnmg and comain glass inclusions: one phenocryst in the field is u pillgioclase. Th ... small crystals showmg higher interference colours are clinopyroxene. Some opaque Iron ore crystals arc also present.

Rhyolite from Elg~ . S('mlulltl : m{/gmjicllllI)ll x 7, PPL lind XPL.

A11dWon1l1 \'iI!II~~ (1/ rhyIJIiII!.f 1/IIt! \'iel\~ 0/ plldmol!l's lIrt' $11011'" ;113, 5 . 12, 14. 21 . 66, 87, 88. 91 (/1/(1 141.

143

Pantellerite

This IS the name given 10 a pcralkaline ' rh:yohle. [\ is usually distinguished from a nonna! rhyolite by the l)re~· cnce of phenocrysts of anorthoclase and Ihc sodium­titanium mineral aenigmatlte. Sometimes an alkali pyr­oxene IS present but II IS not ;1[\\.a)'5 green in colour (5(.'C

betow), Another name used for pcralkahnc rhyolites is romt'll(lirC' but this type is difficult 10 distinguish from a pantdlerilc by optical observations only. A comcndite tends to have less mafic minerals and is more likely to contain quartz phenocrysts.

We ha\c choscn two 5.1mplcs from the type locality \0 illustrate this rock . The l'irst and second photographs sho\\ phenocrysts of IInorthoclasc. easily idcnlilk"<l by the vcry tine cross-ha tched twinning. together with microphcno­crysts of pyroxene and a fayalitic olivine SCI in a fine­gf'dinl-d groundmass whIch IS mainly qUlIrtz and feldspar. Only one olivine cry~ta[ is visible in thi!; fic!d afvie\\'. it is to the [eft of ccntre oflhe field. sho\\ ing a blUe-green Inler· fcrcnl'ccolour. Thec~tal~ "hlch arc almost oraque arc of aenigmaute. In the third photograph \\'C hn~'e shown II

higher magnilicallon XPL view of another sample of pamellcnte in "hi<:h t .... o crystals of aemgmlltilc showUlg ~L dark brown colour are beside;\ .:rystnl of pyroxene. In Ih('....: rocks the pyroxene IS commonly It Mxhc he<knbcrg­Itt:. M) u~ absorption colour may be pale brown inste-Jd of grccn.

Two (h{fl!felll .tpl'/'mwllS 0/ pall/ellerill! lrolll P{II1I1'I/I'rill. 1I1IIy , rtwgnijictllwnx 11 (first (1I1l1 ,~l!Cf)lId photos ), 1'1'1.. (/lid X PI.. )C 17 (tlllrd piloto), XPL.

.\'a + K 'If rod m "hII'll mulllr AI > I (1.1'. '" u'hu'll/ltf'rf' l~ /J dl'ft-

rime" of I" ","ma "ill! re,~pl'l'l f(> al/..ali.t). Till,! f,'SI,/IJ 1/1 rfl'swll­i:aliOfl nj.umlf' ul~uli P.I·rO'U'1I' Of alkali amphlbol,. and, ill $(ilI1f'

C'(IJes,ormgm,'(lft.

• IT I . .. . , , . r

.' "

,

113

11 .

144

Granite

Granite is the name used for lcucocratic coarse-grained rocks containing mainly quartz and feldspar. alkali feld ­spar constituting bctwcl:n 90 " Q and 35 U D of the IOllIl feldspar. Accessory hornblende or biotite are the .:-orn­monesl malic minerals. Muscovite may be prcscnL Rhro­lile is the fine-grained equivalent. A microgranitc with micrographiC quartz-alkali feldspar intergrowlh is known as a graMphyre (77). A IcucQCrnlic micrograni!c occuTTIng as dykes or veins is known as apli/e. Granites in which more than 90 "" of the feldspar is an alkali feldspar arc known as atkafi gr(miles (sec 145). When alkal i feldspar is between 35 % and 65\, of the \olal feldspar the !ltlmes aliul1l1'llile and qllllfl: mon:onilc have been used for rocks wilh qtlarlz contcnt between 5\ lind 20"".

The first and second photographs urc of the Westerly gr'.mite. a rock which hus ix'en used as 11 sumdard granite for a variety of studies. From Ihc photographs it can be seen to consist mainly of a granular mixture of quartz and feldspar. with biotite and a fell crystals of muscovite. Sodi!' plagioclasc lind POlllSS1C rc!dSIXH arc prescnt: more than half of the feldspar in Ihis rock is potassic feldspar. Much of it does not show microcline-type twinning which ap­pears only in patches in somc of the crystals. e.g. JUSt above the cell! re of the field and slightly to the right is u crystal showing vague cross-hatched twinning. At the left edge of the photograph one potassium Icldspar crystal shows 11

s nnplc twin. The third photograph is an XI>L vicw of !.he gr'.ll1 i!~

from Shap. In this view the right lower part of the held is occupied by a group of fairly 1;lrge phenocrysts of alkali feldspar showing a microperthitic texture. Mosl of the plagioclase crystals show some alteration and CliO be recognized in the photograph by the presence of multiple twinning and zoning. The quartz crystals Ciln be recog­nized as free from alteration and the two areas which appear black in this photograph. to tile left of the centre and at the left bollom corner. are quartz crystals at ex­tinction. The on ly other mineral present in a significant amoulII in this field of I'iew is bioli te. (Sec also the h:md specimen photograph in 10.)

First und second pholograplls: Grimift' from Weslerl!". Rhode /.I·I/ll1d. USA: I/Ulgmficallon x 14. PPL {lilt! XPL. Third phOIOKNlP/t: Granile from Shop, Engiond: IlUlgllijinl­/;QI/x l, XPL.

144

Granite (continued)

The granular textured specimen illustrated here shows mainly quartz and two feldspa rs. The quartz is recognized in Ih" PPL view by the lack of alteration. (lnd in the XPL view. by its interference colours which are slightly higher than thaI of the feldspar. and by the non-unifonn extinc­tion shown by one crystal ill the top right of the vicw. Microclinc is clearly identified by the Iypical cross­hatched twinning and there arc slight signs of micro­perthitic texture also. In this field of view there lire o nly a few plagioclase crystals visible; one. near the lOp kfl corner of the field is a simple twinned crystal showing very dark grey interference colours: vagw' signs of a lbi te twin lamellae can just be seen in this crystal. Another crystal justlQ the right of the centre o f the field and show­ing. a low grey colour is also plagioclase. To the left of the biotite crystal at middle left is a smull patch of myrmekite. From the rela tive proponions of the two feldspars visible in this field of view this sample could be close to the boundary between a lkali gn.lOilc lind granite.

GrllI/;11! from Sow/! DukoUl. USA; magnific(I/lvl/ x /}, PPLolldXI'L.

Additional \'iews of granitl?~ are sholl'lI ill 2. /0 . 42 . 76, 94. 105 arul/09.

115

" ,

...

•

".

'4S

Alkali granite

A leucocratic coarse-grained plutOnic rock cQnsisting essentially of qu:Htz lind alkali feldspar. any plagioclase constituting less than 10 '-';, of the total feldspar content. The fcrrolUugnlosian minerals present are a lkali amphibole. or alkali pyroxene.

The PPL view of this rock shows lin area of colourless minerals and;} few fairly d;lrk-l'olour~-d crystu ls these arl' of an alkaline ,tmphibo!c. ricbeckilC. which shows strong pleochroism from a brown to mdlgo-blue. The interference colours shown in the XPL vicw urc masked by the absorption colours. The large areas of fairly uni­form interference colour :lrC quartz phenocrysts. and the rcst of the fi eld is made up mainly of albite laths ophitic311y and subophilically enclosed in subhedral patches of microclinc: cross-hatched twinning. characteristic of microdinc, is visible. The crystals of :L lbite tend to be smaller than those of microcline and show only albite twinning.

Alk(lli gr(lnilejrom Jos. Nigeria; m(lgnijiclilioll )( 16. 1'J>1. (lnt! XPL.

,.6 Phonolite

This is the n:lmc gl\'l!n to a fine-grained rock ,,:ons.isling chlcny of sallldinc or anonhoclase 3S groundnlass lmhs and frequently \lIsa as phenocrysts. nepheline as ground­mass crystals and often 3S phcnocl)sts as well. ..... ith some alkali pyroxene or alkali amphibole. II is fairly common 10 h,l\'c a mineral of the sodahtc group prescnt also. The coarse eqUivalent is nepheline syenite.

The first and second photographs show phenocrysts of nepheline and alkali rcldsp.1T In a fine·grained groundmaf-S. The greenish· brown microphcnocrysis arc mainly of alkali 3mphibo1e. although in lhe group o f crystals near [he bottom left corner of thc field there arc one or two small crystilis of biOlite. II is difficult 10 know which of the phenocrysts are sanidine and whIch are nepheline bill those showing simple twinning are invariably sanidine. Thus Ihe cryslllilo Ihe len of centre of the field. showing 1 .... 0 triangular arcus with different interference colours, is 31most certuinly ~1 ~I\'eno twin of sanidine. In this partIcular rock the nepheline shows a distinct cleavage and this can be seen in the PPL \iew in two crystals near to the 0011001 right-hand corner. This IS unusual bt."'CaLlS<!. ahhough feldsp.1r may show one or two cleavages. nepheline rarely does so. The groundmass of this rock is nlllde up of sanidine. nepheline and needles of a green pyroxene.

The third photograph is thaI of a phonolite with a small amount of nosean prescnt. ThL" photograph i~ included here mainly for historical interest in thliithe Ihin seclion used for Ihis pholoS"lph was preparL""Ci for J. R. Gregory in 1895 and has been in the Manchester Unher· slIy collection Since that lime. The \'iew shows. nepheline and feldspar phenocrysts and one phenocryst and a few Inlcrophenocrysts of nosean. \.\ hich appear almost black in PPL due to the high density of inclusions. The lath· shnp.:d cry~lals arc hkely 10 be s.midinc. whereas the nepheline crystals arc rectangular. In this rock the nephel. inc crystals show zoning by the margins of the cryslab haVing a higher refractive index than the interior. nnd they C,IO be disl1nguished from the feldspars by this reature. The groundmass of th is rock consists of nepheline.. sani· dine, .1 green pyroxene and nosean.

First and s('coII(1 photographs: PhonQlite from MarUl/glld:i. Zimbal;w(' : I/lagnf/iclllioll x 14. PI'I, fl/ld X PL. '111;,,/ photograph NoseD" phonolitt' ) rol1l Wolf. Rm:k. Gml\wll. I::"ghmd, IIJ(lg1l1jicalJQ11 x 14. PPL.

117

147

Leucite phonolite

This is the name given tO:l volcanic rock which l'Onsiqsof csscntl<tl leucilc. nepheline and K.fcldspar: 1111 "I"ali pyroxene flo usuaU~ prcl>Cnl. The term lerldwphrr(l was fonnerly used for varieties which contamed no ncpbchnc but this dlstmction is perhaps not necessary.

The photographs arc of a rock \\hrch contams !cUCltc

microphcnocrysts. each of which IS surrounded by 11 ring of small pyroxene crystals. These lie in a groundmas.s mainly composed of nepheline. leuclle. pyroxene and a small amount of alkali fetdspar. The nepheline IS easily Tl"COgnized b)' the rectangular and he.~agonal shape orlhe crystals and one hexagonal cross-section of zonooncphcl. mc is almost exactly In the centre of Ihe field of Ilell .

Wldl.' pll/J/lo/ueJrom O'I>nirl;, £ijel. G,·nm",.\'. nll/gll/fico­Iit'il/ x 17. PPL llIuJ XPL.

148

Nosean leucite" phonolite

Most usage of Ihc term phonolite regards nephelm.: and alkali feldspar as essential constituents so thaI a noscan Icutite phonolite contains nosean. leudlC. nepheline and feldspar, usually with an alkali pyro)(cne as accessory.

The first \\.\0 phologruphs sho .... a rod: containing phenocrysts of ]cueile. (clear In the PPL VIC\\). brownish n~an crystals wilh darker brolOo'Tl borders. due to thou­sands of small inclusions. and phenocrysts and micro­phenocryst5 of a green pyroxene. The multiple twinning in the Icudlc makes II fairly easy 10 identify. Around the noscan crystals In the cent re of the field the mineral which appears almost white in the XPL view is calcite and it can also be seen fllirly well disLIibutcd in the groundmass ncar o ther nosean crySllIls. The rest of the groundm:lss is composed of sanidine. nepheline. nose"n. lcucite and green pyroxene.

The th ird photOgraph shows;J very similar rock to the one shown above but the nosean crystals h:.we orange borden.. The phenocryst,; arc of lcucitc and noscan with micro phenocrysts of pale brown pyroxene having greenish flUb. and nllerophcnocr):)ts 01 lcuclte and noscan 111 a groundnlUss of samdllle. ncphcll1lc. n05ellll, kuclte and pyroxene. One elong.lled samdinc mlcrophcnocry:.1 ar­pears at Ihl.' bottom right of thl! fidd. A small amount of sphene and calcite can also be dcICCu..'<I1Il the groundmass bUI ("":1nnOI be discerned III the photograph .

Firs/ and second pllO/ograpll.l . Nos~all It'Ucl/e phollolll" j(/Jm Rellll'II , Eifel. GUIIIlIIIY. fIIl/glI/fim/lolI )( I I . PPL (IIul XPL. Third photograph Nosl"tm lell(ite phOilOlltt Imm UJacher See, Gtrman) : magnificatloll x y, PPL

119

120

14'

Pseudoleucite phonolite

This tl'nn b used to describe a rock which contains pheno­crysts having. the )hapc of leu cite crystab but comlXlsed of a pscudomorph aggregate of nepheline and K.feldspar. The groundma.\S conSIsts of nepheline. alkali feldspar and alkali pyroxene.

The specimen illustrated sho ..... s \\.\0 sizes of phenocrysts bolh haYing the outline of leucilc or analcite crystals. j,e. eight- and six·sid~ sections. The 111'0 larger phenocrysts are predominantly brown m colour and this is mainly altered sanidine: Ihe clear areas within them are mo~lly of nepheline and analcite. The clear area in the large pseudo­Icueilc allhe right edge of the field i ... analcite. The smaller phenocrysts differ in that they show very liule brown alter­ation and have a higher concentration of needles of pyrox­ene. It appears thatthcsc may represenllwo gcner.l.Iions of original icucilrs. The groundmass IS an aggregate of millute crystals or nepheline. feJdsp~1T :tnd pyro~ene.

Pseu(loleucile pho/lo/ilt? /rlJm Beurpuw Hmmtu;lls. M Ol/· /(J1Il1. USA; /t1agllijlCflluJil x II, PI'/' mul X PL

150

Blairmorite

A very rare \'olcanic rock, known only from tlm."C or [\lur Ioc-.ditic.s, it is chamctcrizcd by the presence of phcllo­CI):~lS of analci te and slinidinc in a groundm:.ss of 3nalcilc, alkali feldspar and pyroxene. Melan itc garnet is frequently present. Despite ils rdrity. 11 is inciudt'(l here because it is Inc only extrusive rock .... hieh has both analcite and game\ as phenocrysts. Its chemical composition is thai of II sodium-rich phonolite but the nepheline which is prescnt In a phonolite is here represented by Analcite. and il 111I li been suggestcO thai the analcite is U primary minerai and not a replacement orteuel!c.

The photogrnphs show three phenOCl)'sl!; of sanidinc (@rey interference colours). Two of Ihe phenocrysts arc p:millily surrounded by analcite .... hich has a slightly )dlowish colour in the PPL \'icw: the d,trk brown mlcro­phenCK'rysts are or mclanitc garnet: and the green crystals are of aegirine augite, The groundmass consists of laths or :i:lnidinc. equant analcite and pyroxcne.

BllIirll/orlll' from all/irll/ort' , Crnll"s N('XI POss, A/berra , Ctmadtl: /lllIgllifit'miQI/ x 11. ,'PL alld XPL .

Alkaline and miscellaneous rocks

'"

151

Nepheline syenite

This is the name used for a ooarsc'graillcd, felsic rock consisling essentially of alkali feldspar and nepheline with a small proponion of mafic miner.ils. usuall~' ,llk"li amphibole or pyroxene or both. [\ is the plutonic cqui­... alcnI of phonofi!!'. The medium-grained equivalent i~ Ix.-S\ called l1l'plwlill(, micros),ellile. though the special mime lillguailf' also exists for this.

T he nmncj!J),l.lile has been used for nepheline syeni tes which have a lrachyioid texture and the first two rocks illustrated fall in Ihis category. The Icrm tiilroiu.' is some­limes used for nepheline syenitcs in which Ihe texture is subhedml granular, and the third rock illustrated could be so described.

We have chosen 10 illuSlnllc Ihis rock type by three XPL photographs, since lhe PP L views lire dominated by colourless low relief minerals and henee arc less informa· tive. 111e fi rst photograph shows interpenetrating tabular crystals of feldspar in which the interference colours arc patcby. This is partly due to the fact that the crysUl.ls are microperthitic. and partly duc to the presence of mult iple twinning in both thl: sodium- and potassium-rich phases of the microperthite. A region of uniform grey interfer­encc colour can be secn just below the centre of the field. This is nepheline. as is a small triangular lIrea towards the top right-hand part of tbe field. The small black triangular area jusl above the centre of the fie ld and to the left is analcite. AI the top left of the field arc 1I few crystals of pyroxene showing a green interference colour. and an alkalinc amphib<l!c Showing a very dark brown colour can be Sl.-en (It the lower right of the centre of the field.

The second photograph shows tabular phenocrysts of mieropcrthitic-fcldspar and mierophenocrysts of nephel­ine showing homogeneous gJl:y interference colours - a large area just above and to the left of the centre of thc field of view is nepheline. Between the phenocrysts is a trachytic- textured groundmass consisting of nepheline. feldspar. pyroxene and a few biotite crystals showing bright sccond·order interference colours. The magnifica­tion used here is such that we c;tllnOI easily distinguish lhe ferro-magncsian minerals from the onc photograph. All thc regions in which the interfercnce colour is a uniform light or dark grey arc of nepheline-.

The third photograph is of a slightly coarser-grained rock than the olher two. bUI here again the alkali fe ldspar and nepheline can be fairly easily distinguished by the homogent.-ous interference colour sho .... 'T1 by the nepheline. in contrast to lhe mieroperthitic lexture of the feldspars. A few crystals of nepheline arc visible to the right of the (-entre of the field. The small coloured crystal to the right of lhe centre is biotite and in lhe oouom right comer a few crystals of sphene are visible.

Firsl pholOgraplr : Nepheline sYl'nile/wm PjfQr~~berg. SOIllIr A/rim; mo.gllijirallol! x f2. XPL St!colld photograph : Ncpirelillt! mkrosyellilf' /r,"" Barulltl. PUr/ugtll ; IIl11gnijictllioll x 7. XPL Third pirotograph : Nepheline s),l'l1ile from Lmrgesrllldfjord. Nonray ; mtlg/rijial/iOn x II, XPt.

Malignite

This IS thc name given (0 a rock consisting essentially of pyroxene, alkali feldspar nod nepheline in which the pyroxene is thc dominant con~liluent (about 50" v) lind the other IWO minemls are in approximately equal amouniS. It can be considered to be a mesocralic variet), of nepheline syenile.

The rock illustrated 5ho" 5 a large number of equant cuhcdral green pyroxene crystals. The dear crystals to Ihe left of Ihe centre, showing uniform interference colours are of nepheline, Ihc other parts are offddspar. The region al the right edge of the field is composOO of a nepheline­feldspar intcrgrowth. A crystal of biotite is vi~ib!e al Ihc top left corner of the field.

Mallglllujro/fl Shllll-xi. Chlllu : magl1ijiclllioll x 7. PPL mI(l XI'/"

123

/

124

,Y , "\ . , l-

'-~--.I'" -l ,.1' .. , ')

.J:.. -' \

, y

153

Sodalite syenite

A coarse-grained rock cons.isling es'-Cntially of socialite and alkali feldspar wilh generally somc nephehne llho. Alkali amphiboles and pyroxene arc Invariably prescnl.

The view illuslnned here. ShOW1l large cuhcdral sodllhlC phenocrysts, isotropic in the XPL vicw, amphibole and pyroxene in a fine r-gminL-d aggregate of alkal i feldspar lind nepheline. The alkali feldspar shows the same patchy extinction as seen in the nepheline syenites illuSInLloo. whcrells the nepheline has unifonn inlcrreren~ colours ­one nepheline crystal showing a pale grey interference colour can be secn adj3CC'nllo tWO sodalitc crySUtls, In the left pan ofthc \ lew. Anothcrclear nepheline cry~la[ can be seen to the left of the large sodalite at the boltom edge of the field. The two Illth-sh:lped isotropic cryst:lls :In:' :Ilso ofsodulite. ind icli ting a I:lbular habit of the cryst:11s In thIS rock .

Sadi.Jlilt! .~yel1llf' jmlll .I11/illll/wuh. Greell/lInd .. l1/(1gllijklll 10/11

x //.I'Pl. ,mdXPL

154

Nephelinite

This is a fine-gramoo rock consl)lmg essentIally of nephel­me and pyroxene \Vuhoul oli\'ine (when olivine is prc:;eru the name would be O/II'/lfI> IIl'pht'IIIJlIl'). J f morc (him 50 ~ ~ of mafi!: minerals IS present the tcnn mi!lallt'phdimtl' i~ used. Ijolll(> is the coarse equivalent of nephelinitc.

The rock illustrated 1/1 the fir.;IIWO photogruphs conSIsts mamly of pyroxene ffilcrophenocryslJi in II fincr-grained. seriate-textured aggregate of pyroxene. nepheline and Iron ore. The. pyroxenes show a pale greenish colour In the PPL photograph, whereas the nepheline crystals un: smaller and are transparcni. They show a fl'Clangula r or square shape. and one nepheline crystal is clearly \iSlblc near the top right comeroflhe field. Thesmal1 palchcsofu mmeTal showmg a fox-brown absorption colour arc of bIotite; Iron ore is widclydistributcd throughout the rock. The third photograph sho\\S a porphyntic nephelinite in which the phenocrysts are of nepheline and p~ ro;<enc in a ground mass II hich is extremc:ly fine grained. and made of the SlIme: mmcOils. The only other mineral present in this rock is iron ort!.

FirSI fwd second plUllogmplu: Nt.'phelimlt' .from Mm'ollf', Com(Jr(J I,Ialtt/s. 'ilt/fUll Ocean; f1!f1xmjlt'um", x51. PPL alld XPL. Third pho/OgrapJI ' Nephelinill' (rolll Marol/I'. Comuro l.s/wuk India" Quon. IIu1l.mfictlll(Jn x II . ,\ PL.

125

126

n.

Ijolite

An ijohte IS a mesocralic coarse-grained rock consisting essenllally of nepheline and clinopyroxene in upproxi­malcl), equal amounts. Frequently mclanite garnel is p~.

scnt. Thl~ is the coarse-gramed equivalent of l1l'p/!elume. Other names used for coarse-gr:l1nl'<l nepheline-pyroxene mixtures arc alkali P),Toxt'lrirr. meftdgiu.' and IIrllle (Sl.."e

156). these rl"spl.:ctively bting hypennelunic « fe ..... per cenl nephcline). mclanocrJlic and leucocralic. The pyro­.~cnc can rAnge in composition from sodie diopsidc 10 aeglnne augite to aegIrine \0 [ilannugll!!. If tilamlUgilc is the pyro'en!! in IlIl alkali pyroxenite. then the namcjntll­pifllllgill' is used.

In Ihe granular-Ic:uured specimen we have used \0 illustrate this rock type, the nephehne is recogni7.cd by ils 10\\ Telkf In the PPL vicw and by lhe first-order grey mterrerence colours in the XPL view, At the edges orthe nepheline cry~tals, and in eracks within them, the mineral which has first-order pule yellow and while colours is cancrinitc, The pyroxene in this rocl.: IS not strongly coloured but shows only a pale green llbsorplion colour.

Ijohte frum AItIQ, Swedf'l1 ; mugmjic(lliol1 x 16. I'I'L amI XPL.

A leucill'·bearillg \'Ori,,')' ojmil'roijolill! is sllo1l'/1 III 20,

15" Urtite

A coarse-grained. [cucocratic rock consisting mainly of nepheline, but generally an alkali pyroxene and/or an amphibole IS prescnt In small amounts. II IS more felsIc than ijolile. ( I SS)

The field of view is mainly occupied by nepheline. some· what altered and showing a brown colour in the PPL \ic\\ . The dark crystab in the field arc of an alkali pyroxene considerably altered in paris. Al the centre of the bouom edge of the field, an urea which was probllbly pyroxene is almosl entirely filled with fine-grained alteration products of the pyroxene. except for a group of clear crystals (PI'L vic ..... ) .... hieh life of apatite.

UrI;//? fro", Kh,bilUl. Ko/u fIenilisula. USSR. magnificol ;"" x II. PPL (111(/ XPL.

127

'"

157

Basanite

A fine.grd Ined mcsoc::ralic rock conta mingessclH ial olh me. augite. plHgioclase feldspar and a fcldspalhoid. g.enerally nepheline. with or " lthotll amllcitc or Icueite'. I he ellul\ a­lent olivine-free rock I~callcd lI'phm('(sec 158). Thc!,:oarse­gnuned eqUlv<llent I~ known a~ (1I1rlllt' (hemU/I',

Thl~ sample contains Illicrophcnocrysis of olt\ mc "hlCh stand o ut in the PPL VICW because of their \:u,:k of colour and high relief. The l1Ioundmtlss is made up of pall.' brownish pyroxenes. scarce laths of plagioclase (e.g. at top right) and abundant poiki]ilic are'.ts of nepheline (e.g. in the ccnlrc oftke field of view and allhe len side showing low grey interference colours). To Ihe bol1ol11 left of the phOlogmph the pyroxene is inlcrgrown with a colourl~s mineral of lower refraCl1\'C inde., lhan the nepheline and wi th almost Lero blrerrmgence: [hiS ha~ been identified as analcite by microprobe analysis. The presence of abundant nepheline and limited pl<lgiochlsc in this rock i~ wh:ll dis· tinguishes it fro m lin olivine basalt .

BlLWIIIII' jr(J1II Ja/l(,I I).f Sau/Ia. Libra , maglll/II·(J//011 x li. PPL //lJd ,\ IlL

' The name I.·unlt· },.,MUI"" ~h(,uld be u .... -d II 1<'u...I IC IS praclll ~H1ee )~I,fII""t' l~ ~cncrJJly Ul~cn to mean thai nephehne IS Ihe fddsp'Hhold.

158

Tephrite

A Icphme is a fine-gr.uned mCSQCralic rock con taining. in addition 10 plagioclase and pyroxene. nephehne o r another fcldspalhoid I. Olivine IS absent: If it is prcsem. (lIe rock IS c'1l1ed ;I hu~l/ItC' (l>l'C 157). c\cn Ihou£h 01/1'1/1("­

tl'piJrill' ""QuId IX' more JogicaL Thl'f(IIIIf' 1~ the l'O<Ir5f'­

gnuned equivalem of tcphrilc. The sample we ha~c used \0 illustrate Ihi'> rock type is.\

h:lUyne Icphnte. IL contlllns phenocryl>ls of clinopyro1(cne !lnd hauyne. The clinopyroxene cr)'5tal~ are cuhedral and have a dark ol1\'e.gr~n colour in the PPL VI~: both LOnlng and tWinning can be S\.'Cn in th ... XPL \iell , The hau)'ne phenocrysts are ellher blue or colourless and hUI·e rims of a darker blue or brown colour. The groundm;'Iss r.:onsisb ofl uth s.haped crystals of plagioclase. small round crystals of t('uclle. together with hauyne and citnopyro,"· ene.

1I(IlI,1'I1(' /('plJrut> from I/ome lulwrJ. lIet" MIIIII , /if/h '; IIwKmjicalioll )( 27. PPL (I/Jd NPL .

• Tcplrml' /lnpil/'s IlrOI "tPM/Illi' 1$ IIw {t'/dspt.lIhOld, if <1'10111<"

IcldspllllrOlJ;5 prt},·'.1 III plurt II/ ""phdmt'. " p' .. /Ixn ,Ir. III"''' ('.g lcucllt lcphruc

"9

159

Olivine melilitite

This rock is composed of essential olh~ne, melilile and pyroxene. Pcrovskilc is a frequent accessory constituent and nepheline may be present. The name meliliu basal/ has been used for this type of rock but il is not appropriate since plagioclase is absent. its place being taken by melili te.

The sample we have illustrated contains very lil1k pyroxene and so consists mainly of olivine and melilile crystals in a glassy groundmass. l n the PPL vicv.' the rock appears to consist of olivine and plagioclase microphcno­crysts. but it can be clearly seen from the X PL photograph mal what onc might take to be laths of plagioclase arc in fact mdilites. These are dearly identified by their anomal­OtiS interference colours. Closer inspection of the PPL view shows thai many of the mclilil!! laths have a line along the centre of crystals which is chamcterist;c of melilite crystals. In addi tion to olivine and melilite, pcrovskite is fairly abundant in this rock. The third photograph is a high.magnilic!.Ilion view of part of Ihe field of view shown in the first IWO photographs. and shows a number of multiple-twinned p<:rovsk ite crystals: in this photograph they arc dark green in colour. This section must be slightly thin because the olivine crystals do not show colours as high on Newton 's scale as expected from a mineral whose birefringence is betwee n 0.0]5 and 0.OS2.

Olil'inc melilirile from KOllmga. Uganda; magllijicllliOIl x 15 (firsl (lnd second plJoros) PPL (lnd XPL ,x 72 ( third phoro) , XPL.

'60

Leucitite

This is Ihe name given [0 liD cxtrusi .. c mesocralic rock consisting essentially of [eucile lind II clinopyroxene without olivine. If olivine is present the nnmc used is olMne fellcililf.'. and if Ihe olivine and pyroxene dominate il is known as IIgandit/!. The int rusive equivaknt of II lcuc!!;tc ;~ ~omctimt-S tOilk.-<I aferguslle (sec 161 ).

The sample we have illustrated shows, in the PPL vie ..... dear phenocrysts and glomerocrysls of lcucite and zoned olive-green crystals of pyroxene, sel in II very fine-grained ground mass composed mainly of these two minerals and glass. There is no evidence of any other minerals such as nepheline or feldspa r.

Ullt'IIlIe from Cf'lebe.~: lIlugnijicmioll x 12. PPL lIIld XPL.

131

134

163

Aln6ite

Thl' r,..:l hdllllg' hi lhealkah lampmph} TI' group 11 (.111-Ul1n~ 011\ IIlC. p) roxene and biotite. u~uaJI)' a~ ph~'no\: r)~I$. III it groundm;t~~ 1;ontalnlng mcilhll". II c,lroonatl' Illmcrnl I~ commonly I)Tc<-cnl :ll~o . Monti.:clhlc m:l) he pre,en! CI/IIlPWI1III' con'I~I~ or Cl>~n1!aJ plagulCla'\t.' and hftll-ln

,Hlll'hlhllk . llwmlulPtc mil) he rrc!>C1l1. P'l"lhl} In ""l'C'o..' (If Ihc amrhlhvlc . a Iluk anOJ!..:lh: 01,1) I!I.'UIP} the mler­"Ill',', 1/"111/111111111' ha' (,'"cnll<,1 "lnUll'. 1I1,tnauglic and hm .... n h.,rnhkmk In ,I m,llri'\ tlf ,In.lil'uo.: Jnd f'I"'lhl) 'lime hlllilil' fIJl/rlhl/, ' /ilf ",hl .. h an unu,uaJ !,',luT\"(1 lanelY is sho'M'fl In J55 and 70) is the ~me hUI \.\ IlhoUI oliline.

In thc specimen IlluSlrJlcJ here rno" of thc rh\'lh M;r~ .. I" m Ihc centre of Ihe field lIf vie\\ an: lonc.'d dmop} ru.\enc' A fe\\ 100~· rortwnul,:ry .. t .. of ollvrne ;.Ire rrc'oCnl At tlw n.lttUIll rlghl fifth .. , th.:ld 'If vie\\ 1~ ;1 lur~e ,lre;1 .... tlll·h r~ un aggrq;;rk 01 I:ry,1td, til' nUllllC rnterj!Tlm n \\ II h p~ rt\~,me In the groundmass tHe pOlklhl11: reglom shO\\U\i! a pale hWI"!l .:olour ,mu Ih .. ''oC ar.: .. hllll i hlullll: A p.lk hnll'.n .:Inngated lT~,t,Ll ,II mll'a ,.;an lx' !>C'I,'n t,'"ard .. the rlglH \'\.1111,' III the li.:ld

Much of the de-.H !foundmass mateT1al:.(!en m the PPL \lev. I' m.:llh1<.' hut hc .. :a uloC ur rb \er~ 1,,\\ ,Inurtlal"u, mlcrl .. 'r.::ncc "'"Iuur It I' mIl e;NI) rtlcnl1til'(l In the '\I'L 11<'\\ ,It till .. m"gnlfil'iltlun Thc dar~ hrn\\n lT~ .. t;ll, arc nl;tml~ f't.'nl"~Il<'

A/,,{iui' lrom Ok(J QUl'ht't·, "wgmjWIIIIIIII >C /5, PPL IIm/ .\ 1'1

168

Carbonatite

This name covers 11 considernble variety of different vol­canic and intrusive igneous rock tylX"S whose main can· stituent (greater than 50 ~~) is 11 carbonate mineral. The mOSi common carbonatite LS called SOI'iIt> which is a calcite carbonatite. Others contain dolomite. or siderite or alkali carbonate. The texture is generaUy granular but rarely may be trachytic or comb layered.

The specimen we have chosen to i11ustrate is u carbona­tite composed of more than 90~": calcite with minor amounts of apati te. pyroxene. mOnlicellite, mica and an opaque mineral which we have not identified.

Cllrbol1{f/i/e from Oka tompll.'x, Quebec. emU/da ; maglli. fiwllon x7. P/'Llll1dXPL.

;\lkalin(' and miscellaneous rocks

139

140

169

Chondrite (meteorite )

Meteoritcs are c.lassed as stones, stony-irons or irons depending on the proportion of silicates 10 nickel-iron alloy. The stones are mainly composed of olivine and or!hopyro.~ene and arc subdivided into chondrites and achondrites according to the presence or Ilbsent'C of chondrules. Chondrule:s <Ire spherical objects, with an average diameter of I mm, and are compose<! of olivine or pyroxene or bolh. We have illustrated IWO chondritcs and one achondri te (170). Most authori ties on meteorites prefer 10 consider chondrites as mCI3morphic rocks. though lhe chondrulcs may be of igneous origin.

The: first photograph is a PPL view of an olivine -hypersthene chondri!e which is the commoneSl type of chondritc. In the thin section on ly Ihc one chondrule (which we have iBustraledj is present. It consists of radiating crystals of orthopyroxene. The rest of the field is made up of an aggregate of onhopyroxenl: and olivine and some opaque regions. The opaque regions are mostly ofa metal phase with some sulphides but these cannOI be distinguished in transmitted light.

The second photograph is an X PL view of a chrondrite in which the chondrules are unusually well developed and can be seen to bave different minerillogy Hnd texture. One small chondrule, just above the centre of the field of view . showing a yellow birefringence colour with black lameUue is composed of olivi ne wilh li1mellae ofglilss.

The third photograph is a higher mllgnification XPL view of lho;' top kft corner of the second phologr<lph . The chondrule at the left or the field consists ofbladcd tll·inned eryslal~ of clinobronzite. Because of the twinning lind 10\1 birefringence it could be mistaken in a photograph, for plagioclase. This chondruk appe;lrs to have been broken at SQmc st:lge since in section it is not a complete circle. The other large chondrule is alSQ composed of pyroxene but is too fine grained for opticill identification. Th ree separate olivllle cryst;l!s art: vi$ibk at the bOIlQm of lhe field of view.

Firs/ phoIPgraph : C/u:m(/ri/t' frQm Rrudf'rheim . Alber/a. Cml{uili . II1l1gllij"·(JliQn X 28< PPL. Second (Jlld third pholOgraphs : Chondri/e from PrUJrir Dog Creek , KOlls(,~· . USA .. lI/agllificlJlioll)( 16 ( ue-ond plioto) . XPL. )( 4) ( third pholo ) . XPL.

17.

Achondrite m et eorite

AchondrllCS ,He siany meteorites (sec I). 140) in which chondrulcs arc lackmg and a nickel Iron phllst IS Cit her :Ibscnt or present In vel)' sm:11l amounlS. These are relatively ran.~ meteorites.

The phologmph shows coarse-gnuncd clasts or fmg­menls In a brecciated matrix. This specimen IS classed 85 3 eucri/e and it consists mainly ofbylOwmtC and a pigoonitic pyroxene together with an !lugill'. To the left of lhc centre of the field is a fragment consisling of an intcrgrowlh of plagioclase and clinopyroxene. The plagioclase contains trails of minute inclusions unevenly distributed within the crystah and it is these inclusions which cause the bro\~n colours III the crystnls in the PPL view. The clinopyroxene i~ relnth ely iron-rich and a fine lamellar structure can just be detected In the XPL photograph. The rest oflhe licld is occupied by the same minerals but (If mUl'h finer gram size in some paris. The pyroxene crystals a re almost black due to a very h1gh concentration of an Op.1que mineral. pro­bably magneti te. There is 110 olivine in this spc."'Cimen.

The n:IOll' eller/Ie' IS also used for a terrestrial gllbbroic rock consisting of II calcic plagioclase (An 11l~90) and a clinopyro.~enc_ Sollie of the achondrites have textures Slmii:tr to those of terrestrial gabbros.

l)yroxelU'-p/agiocflW! {lr/wlI{/rile .frfJ/II SUmlJt'fII, C:l.'d'(J­.I"/U)·/IJ.;"I , (ub.\·l'fI't'l1 filII. 1808): 1fI(lgllljlc(lliQlI)( 14. PI' L (/lid .'PL.

141

All references refer to page numbers. Page numbers in iralics refer to additiona l photographs of rocks in part I where detailed descriptions of them are included in part 2.

Absarokite 87

Accessory mineral 78

Achondrite 141

Acicular crysta l 20

Acid rock 78 , 112- 16

Adamellite 114

Alkali gabro 95

Alkali granite 114,116

Alkali o livi ne basalt 87 , 89

Allotriomorphic crys tal

Alnoite 134

Alkali picrite

Aligned texture;

Amphibo lite

87

41 - 5

86

Amygda loidal texture

Andesite 15, 65. 101

Anhedral crysta l 18

18

71, 72

Ankaramite 66 , 87 , 93

Anorthosite 69, 100

Antiperthitic texture

Aphanitic texture

Aphyric texture

Aplite 47, 114

Ash-fall tuff 7

Ash-flow tuff 7

9

Automorphic crystal

Axiolites 54

9

50, 52

18

Banded textures 68- 9

Basalt 5, 10, 17, 18,19, 21 , 22,30, 32,37, 38, 40,71,72, 87- 93

Basanite 128

146

Basic rock

Benmorei te

Bladed crysta l

Bla irm orite

78 , 87- 100

87

20

12 1

Bleb-like intergrowths 50, 52

Boninite 102

Branching crysta l 24--5

Camptonite 134

Ca rbona tite 139

Cavi ty textures 71

C hadacryst 33

Chondrite 140

Coa rse-grai ned rock

Co lumna r crystal

12

20

Comb layering 44

Comb texture 44

Comendite 11 3

Conserta l texture

Corona texture

45- 6

59

C rescumula te layering

Crinanite 96

C ryptocrystalline texture

Crysta llinity 4-8

C rysta llite 9

Crysta l shapes

Crysta l zoning continuo us convo lute

18- 27

6 1- 8 61

65 discontinuous 6 1 even 63 hourglass 66 mUltiple 63 no rmal 61

44

9

oscill ato ry 63 reverse 6 1 sector 66

C urved crysta l 24

Daci te 107

Dendritic crys ta l 20- 2 1

Dendri tic overgrowth 58

Diorite 43,59,103

Directed tex ture 41 5

Ditroite 122

Dolerite 24 , 35,36 , 38.39.40,43, 50,55, 57,63,98

34 Doleri tic texture

Druse. drusy cavity 71, 73

Dunite 69. 79

Embayed crysta l 20, 2 1

Eq uant crysta l 19

Eq uidimensiona l crystal 19

Equigranular. textures 14,27 9

Essential mineral 78

Essexi te 67,68,95,97

Euhed ra l crys tal 18

Eutaxi tic tex ture 8

Exsol ution texture 3. 50

Feldspa thic peridoti te 80

Felsic rock 78

Felsite 9

Felsi tic texture 9

Felty texture 41

Fergusite 131, 132

Fine-grained rock 12

Fitzroyite 136

Flow texture 41

Fluxion texture 41

Fourchite 25, 134

Foyaite 122

Fragmental texture 7

G a bbro 12, 13, 14,28,33,34,36 , 39, 43, 46 , 52, 53, 60,95

G a rnet peridotite 82

Glassy glo bules 7

Glassy tex ture

Glomerocryst

4, 5- 8

32

Glomero phyric texture 32

Glomeroporphyritic textu re

G rain size 12

32

Granite 4, 9, 10,29, 47, 49,51, 60 , 70,73, 114-16

Granodiorite 46, 108

Granophyre 114

Granophyric texture 46-8

Gra nula rity 9- 17

Granu le 19

Graphic texture 46-8

Har risi te 23

Ha rzburgite 80

Hawaiite 32, 87

Hiata l texture 14

Ho llow spheru lite 54

Ho locrysta lline texture 4

Ho lohyaline tex ture 4

Hornblendite 28, 86

Hyaline tex ture 4 , 5

Hyalophitic texture 37

Hyalopilitic texture 41

H ypauto mQrphic crysta l 18

Hypermelanic rock 78

Hypidiomorphic crystal 18

Hypocrystalline texture 4, 5

Hypohyaline texture 4

Idiomorphic crys tal 18

Ignimbrite 7

Ijo lite 16, 126

Ineq uidimensiona l crystal 19

Inequigra nula r texture 14, 30-41

Intergranular texture 37

I ntergrowth texture

Intermediate rock

I ntersertal texture

Interstitial texture

45- 54

78, 101 - 11

37

Intrafasiculate texture

Jacupira ngite

Katungite 135

Kel yphitic texture

126

Kentallenite 105

Kersantite 133

Kimberlite

Komatiite

8 1

26 , 84

37

50

59

La mellar & bleb-like intergrowths 50-2

Lamella r crystal 20

Lamproite 136

Lamprophyre 133, 134

Larvikite 110

Lath-sha ped feldspar 20

Layering 68

Leucite basanite

Leucite phonolite

Leucite tephrite

Leucitite 131

128

11 8

129

Leucito phyre 118

Leucocra tic 78

Lherzolite 29, 80

Light-coloured rock 78

Liquid immiscibility 7a

Lithophysa 54, 7 1

Lunar low-ti basalt

Luna r high-ti basalt

Madupite

Mafic rock

138

78

Mafurite 135

Malignite 123

Medium grained rock

90

9 1

Melanephelinite 125

Melanocratic rock 78

Melilitite 31 , 130

Melilite basalt 130

Melteigite 126

Mesocratic rock 78

Meteorite 140, 141

Meymechite 85

12

Miarolitic texture 7 1, 73

Microcrystall ine texture

Microgabbro

Microgranite

12

48, 78

9

Micrographic texture 46-8

Microlite 9

Micropegmatitic texture 46

Minette 133

Monchiquite 134

Monzo nite 106

Mugearite 87

Mutual relations of crystals

M yrmekitic texture 49

Nepheline syenite 51, 122

Nephelinite 125

Norite 99

27- 73

Nosea n leucite-phonolite 119

Obsidia n 11 2

Ocell a r tex ture

Oikocryst 33

Olivine basa lt

Olivine leucitite

7 1, 73

87

131

Olivine melilitite 130 /

Olivine nephelinite 125

Olivine tephrite 129

Olivine teschenite 96

Olivine thera lite 128

Olivine tholeiite 87

Ophimottled texture 34, 37

Ophitic texture 34-7

Orbicular gran ite 70

Orbicular texture 69, 70

Oriented textures

Overgrowth texture

Pantellerite 113

41 - 5

59- 68

Pa ra llel-growth texture 22, 23

Paramorph 26

Pele's ha ir 6

Peridotite 15, 23, 33, 34, 80

Perlitic cracks

Perthitic texture

5

50, 51

Phanerocrystalline texture

Phenocryst 14

Phlogopite leucitite 137

9

147

Index

Phonolite 117

Picrite 80

Picritic basalt 18,20, 21,22, 87 , 92

Pilotaxitic texture 41

Pitchstone 5, 6 , 12, 58, I 12

Platy crystal 20

33, 34 Poikilitic texture

Poikilophitic texture

Porphyritic texture

34, 36

14-16, 31

Pseudoleucite phonolite

Pseudomorph 26--7

Pyroclastic rock 7

Pyroxenite 83

Quartz diorite

Quartz gabbro

103, 104

78

Quartz monzonite 114

Radiate intergrowth 57

Radiate textures 54-7

Rapakivi texture 59, 60

Reaction corona 59

Reaction rim 59

120

Rhyolite 10, 42,55, 56, 112

Seriate texture 14, 30

Shonkinite III

Shoshonite 87

148

Sieve texture 23, 24

Skeletal crysta l 20, 21

Sodalite syenite 124

S6vite 139

Spessartite 133

Spherulitic texture 54-6

Spilite 94

Stone-ball 71

Subalkaline basalt 87

Subhedral crystal 18

Subophitic texture 34-6

Syenite 110, III

Syenogabbro 87 , 95

Symplectite texture 53

Synneusis texture 32

Tabular crystal 20

Tephrite 68, 128, 129

Teschenite 95 , 96

Texture (Definition) 3

Theralite 129

Tholeiite 87

Tholeiitic basalt 87

Tholeiitic picrite 87

Tholeiitic picrite basalt

Tinguaite 122

Tonalite 52, 104

Trachyandesite 106

87

Trachybasalt 87

Trachyte 42, 72, 109

Trachytic texture

Trachytoid texture

41,42

41,43

Troctolite 28, 80

Tuff 7,8

Ugandite 131

Ultrabasic rock

Ultramafic rock

Urtite 126

Variolitic texture

Vermicular texture

Vesicular texture

Vitreous texture

Vitrophyric texture

Vogesi te 133

Websterite 83

Welded tuff 7,8

Wehrlite 80

78 , 79- 86

78

56, 57

53

7 1,72

4

14

Willow-lake layering 44

Wyomingite 137

Xenomorphic crystal 18

Zoning see Crysta l zoning