TEXTURE AND STRUCTURE OF OPAL-CTAND OPAL.C IN
947Thz CanadianM inerab g istVol. 35,pp. 947-958(1,997)TEXTUREAND STRUCTUREOFOPAL-CTANDOPAL.CIN VOLCANICROCKSTOSHIRONAGASE 1ANDMIZUHIKO AKIZUKIInstitate of Mineralogy, Petrolagy and Economic Geology, Farulty of Science, Tohaht tJniversity, Aobq Sendai 980-77, JapanAssrRAcrTextues and structures of opal-CT and opal-C in volcanic rocks fiom the Hoseka aad Akase opal mines in Japan were studiedusing an optical miooscope and scanning and transmission electron microscopes. Textures ofthe opal-CT are optically classifiedinto anisotropic colnmnar and isoFopic m:ssive (opal-CTy) type,s. Botl tex0rres consist of rhin and platy crysrgls showing ffiowide { 101} faces of low crisobalite.Image,s obtained by high-resolution transmission electronmicroscopy show that the cristobalitestrucfilreis thefundamentalcomponentof volcanic-typeopalCl, andthat many sacking faults arepresentrandomly in thestructure.Columns arepoduced by parallel growth ofplaty crystals; opal-CTy and lepispheres consist ofcriss-crossing aggregafesofblades.Each blade is composed ofroughly parallel aggregates ofplaty crystals. The variation ofdror values with texoral and strucaualchangesof the opal-CT was measuredby an X-ray powderdiftaction methd. With decreaseof d161value, higbly ordered cristobalitedomains develop surrounding the domains ofdisordered cristobalite in the crystal. The considerable difference in degree oforderof stacking bet'ween the two types of domain implies changes in the growth process of the crystal.Keywords: opal-CT, opal-C, Iepisphere, cristobalite, transmission elecEon microscopn 5sanning electron microscopy, opalminss, Jspan.SoMnaensLes texqles et les structures de I'opale-CT et de I'opale-C dans les roches volcaniques des mines d'opale de Hosaka et d'Akase,au Japon, ont 6t6 6tudi6e.spar microscopies optique et 6lecronique (en transmission et par balayage). Les textues de I'opale-CTsont classifi6es optiquement en colonnes anisoFopes et en masses isoEopes (opale-CTrra). Dans las deux types de lexture, il s'agitde minces cristaur en plaquette montralt deux faces { 101 } plus larges, typiques de la cristobalite drdonn6e. l,es images obtenuesparmicroscopie 6lectronique en transmission ihaute r6solution montrentque Ia sfucure de Ia crislobalite est l'6l6mentfondamentalde lastructure del'opale-CTdetypevolcanique, etqueles d6fautsd'empilementseddveloppentengrand nonbredefagon al6atoirerlans la structure. Les colonnes r6sultent de la croissance parallBle de cristaux en plaquette; opale-CTM et l6pispheres sont faitsd'agr6gats de lames enchev8tr6es. Chaque lame est faite d'agrdgats plus ou moins paralldles de cristaux en plaquette. la variationde d1g1avec les aspects texturuux et strucfirraux de I'opaIe-CT a 6t6 mesur6e par difFraction X sur poudre. A mesure que diminuela valeur de d161,des domaines de cristobalite fortement ordonade apparaissent autour dqs domaines de cristobalite ddsordonndedans le cristal. La diff6rence importante d?ns le degr6 d'ordre de l'empilement entre les deux sortes de domaines implique qu'ily a eu un changement de processus de croissance cristalline.(Iraduit par la R6daction)Mots-cUs: opale-CT, opale-C, l pisphBre, cristobalite, microscopie dlectronique par uasmission,balaYage,minss d'6pals, Jspe1.INTRoDusnoNMicrocrystalline silica ssmposed of stackings ofboth cristobalite and tridymite, as inferred from anX-ray-diftaction analysis (Jones& Segnit 1971,Graetschetal.l994),isrefenedto as"orpal-CT'. Opal-CT is oftenfound in silicic sedimentary r@ks and innodules of silicaq,ithin volcanic rocks. Segnrtet aL (1970) were the firstto report, on the basis sf ssanning electron microscope(SEM) observations,that qptically massiveopal{T consistsof spherical aggregatesof blade-shapedcrystals (so-calledlepispheres;Wise & Kelts 1972,Weaver &Wise1972,I E-mail address: nagase@mail.cc.tohoku.acjpmicroscopie 6lectronique parFldrke et al. 1976).Fldrke er al. (199I) showed,bytransmission electron microscopy (TEM), thatopal-CTLS(opticallylength-slow)and opal-CTLF(optically length-fast) are composedof aggregatesoffibrous andplaty microcrystals,respectively.Itwas difficult to detemrinethefundamentalstructureof opal-CTfromX-ray- andelectron-diffractionpattenrbecauseof diffirsereflectionsocharacterizedby very lowintensity.Fliirke(1955)andJones& Segnit(197L,1975)suggeste4on the basisof an X-ray-diffraction QRD)analysis,thatthefundamentalstructureof opal-CTis thatofcrisobalite,whereasMirchell&T rfts (1973)andMlson
THE CANADTAN MINERAI'GISTet al. (t974)proposedinsteadthestructureoftridymite,on the basisofX-ray- andelectron-diffraction patterns.Recently,many detailedanalysesof ttre structureofopal-CT were carried out using several methods:high-resolutionTEM @ice& Elzea1993,Cady& Wenk1994,Rice et aI. 1995,Elzea& Rice 1996,Cadyet al.1996),XRD (Graetschet al.1987, l994,Fl6rkeet aI.1990),XRD panernsimulation(Graetsch& Fldrke 1991,CnaetschetaI. 1994,Gfihne et aI. 1995)andztSinuclearmagneticresonance(NMR) methods(deJongeraI. L987,Graetscheta\.1994).Mostresultsof thesestudiesagreewell with themodelproposedby Fliirke (1955),in whichopal-CT is composedof interstraffied cristobaliteandchangesto that of opal-C, composedof more orderedcristobalitecrystalscomparedto opal-CT,with a decrease' (CtKc)in thespacingof thestrongestrefler. lionat-21.7(Murata& Nakata194).asa resultof therecrystallizationto d1g1Thespacingofthe strongqstreflectioncorrespondsof low cristobali eand doorof low tidymite [we referthe structureof tridymite to the pseudo-orthorhombicstucture refined by Konnert & Appleman(1978)], andthed valueof thestrongestreflectionis typicallyreportedas"dr01".The volumeratio of interstratifiedcristobaliteandtridymite in the crystalis known to be a function ofthedrorvalue.A comparisonof calculatedXRD pattemswitl thosemeasuredfrom 75(A)dror-valueFIc. 1. Cross-polarized opticat photomicrograph of columnar texture of opal-CT from the Hosaka opal mine and correspondingdror values. The thin section was cut normal to the horizontal layer ofa silica- rich geode. Spherulites ofchalcedony me includedin the upper parts (white blorches). The dror values weremeasured using a Guinier-Hf,gg camera. The solid circles representthe average of the measured values shown by the open hehigh-resolutionTEM and opal-C contan 3U5OVo and ?-O-3OVoof Elzea& Rice(1996)andCadyet stacking, respectively (Graetsch et al. L994). However,GIRTEM)analysesal. (1996)providedirectimagesof thestackingsequence Guthrie et al. (1995) showed that the XRD patterns ofopal-CT are influenced not only by the proportion ofin opal-CTcrystals.Opal-CTis recognizedasa precursorofquartz at the cristobalite and fiidymit , but dso by severalother factors,transformationstagefrom biogenic amorphoussilica such as particle size and degree oforder ofthe stackings.(opal-A)toquatzduringthediagenesisof siliceoussedimenm Elzea & Rice (1996) analyzed the XRD patterns of(e.9.,Hesse1988).The)RD pattemof opal-CTgradually opal-CT in detail and integrated their findings with TEM
OPAI-CT AND OPAI{IN VOI.CANIC R@KS949FIc. 2. TEM images of opal-CT lepisphere from the Hosaka opal mine. The same samplewas prepared by (a) dispersion on a microgrid and (b) an is1-thinning method. Thecriss-cross blades of the lepisphere are composed of aggregates of polygonal and platycrystals, and the cross section of the lepisphere shows a fibrous texftre.observations.They showedtha,tthe degreeof the stackingdisorder reaches a maximum atthe drct value of about4.09 .4,.They also suggested that tridymite domain5 arscommon !n samplesof opal-CT with dror values largertnan 4.@ A. Cady et al. (1996) canidout detailedHRTEMobservafionsfor the structure and texture ofopal-CT fromsedimentaryrocks,andconcludedthatburial diagenesisis responsiblefor thee,piacticgrowthof orderedcristobaliteon disorderedopal-CTcrystalandfor a solid-statetransformation from opal-CTto quartz.Most opal-CT in volcanic rocks is consideredtoprecipitatedirectly from supersaturatedsolutions;thus
thetransformationof opal may not occurin the yelsanic(Fl6rkeetaI. owsomedifferencesandyet paredto thosein sedimentaryrocks.Thus,the objectivesof our studyareto describethe internal textureandcrystal structureof opal-CT and opal-C from silica nodule,sin volcanicrocks using TEM, SEM and optical microscopeobservafions,md to elucidaiethegrowthp'rocessof volcanic-typeopal-CTandopal-C.SnwmsIIF solution(48wtTo).andthenetchedwith aconce,ntratedwereobservedusing an SEMThe etchedcross-sections(JEOLJSMT-330A) afterdepositionof a gold co4ring.Thin foils for TEM observationswere supplied frompetrographicthin sections,and wereprepard by an Arisa-thinningmethod,with coolingusingliquid nitrogen.The sampleof opal-CT lepisphereswas also dispersedin aceoneandpipeuedontoholeyca onTEIvImicrogtids.TEM observationswere carried out using a copeaJanacceleratingvolage of 200kV. Electrondiftactionpatteflrswereobt2inedusinga selected-areaapernrrebywhich the areagiving the difftaction patternis limitedto 400 nm in diameter.After the photographsof thediftaction patternweretakenusing a low-doseelectronbeam,correspondingTEM imageswereobained"HRIEMi-ages were interpretedto provide information of thestrucurreof opaltT by comparisonwith simulatedimagescalculated using the MacHREM multislice program(Ishizuka 1980,Ishizuka & Ueda 1977).Varioustypesof stucture havebeenreportedfor naturalandsyntheticlow fridymite (e.9.,Dollase& Baur 1976,Ka,to& Nukui1976,Nukui & Nakazawa1980,Hoffrnannet al.1983);the crystallographicdata of the pseudo-orthorhombic(Po-type) structureznalyzedby Konnert & Appleman(198) wasusedforthe imagesimulationof thetridymite.Inage simulationin the caseof cristobalitewas carriedout usingthedatareportedby Scbmahlet al. (1992).Thedrorvaluesweremeasuredusinga Guinier-Hiiggcamerawith CuKa radiation(KitakazeL992).Theopalspecimensusedfor this studywerecollectedfrom mi & Sakamoto1973)andtheAkaseopal mine,IsbikawaPrefecture,Japan.Many spherulitesof fibrousanorthoclase,up to ten centimetersin diameter,occurinrhyoliteandperlitefrom theseopalmines.Thespherulitesareconsideredto bepoducedby deviEificationof tryoliteglass(Aki.anki1983).Thespherulitesof anorthoclaseincludea lens-like silica-rich geodeat the core.The silica-richgeodeconsissof opalandchalcedony,with someoriginallymillimefersin thicknrms.horizonhllayersfromoneto severalOn the basisof X-ray powder-diEfractionanalyses,the sampleswere groupedinto chalcedonyand threetypes of opal: opal-A (amorphouspattern),opal-CT(dror 4.07A) andopal{ (dror 4.07A). Thechalcedonyis colorlessandtransparentto translucent,whereasopalsamFlesare commonly whitish to pale bluish in scenceis rare in the sampleselaminsd, and is composedofRFSULTSopal-A. Most of theopalsamplesconsistof opal-CTandopal-C,andthe d1e1valuevariesfrom 4.05to 4.09 A. Tertureof opal-CTlzpisphcresfrorn thc Hosal nopalTWotypesof textuiecanberecopized in opal-CTusing rdandcolumnar(Fig. 1). Lepispheresof opal-CTarecommonOpal-CTlepispheresfrom the Hosakaopalmine arein samplesof opalfromsedimentayrocks,althoughsaryles composedof aggregatesof eundertheSEMarerareamong are severalmicrometersin diameter,as seenundertheftsss slamined. Texturesof opal-CT lepisphereswere SEM.ATEM imageof a dispersedsampleon a microgridobservedto consistofopal-CTy andcolumnaropal-CT. showsa polygonalandplaty morphologyof thecrystalsThefollowingthee re,presentativesampleswereexamined (Fig.2a),with prominent(101)facesshowingapseudoin detail (1) opal-CTlepispheresfrom th" llssaka mine; aggregatesoffine-grained lepispheresoccurrarely in a onthe[010]*-[102]*netoflow cristobalite.Thethincr'/stallayer befweenchalcedonylayersfrom fti5 mine; (!,) is muchsmallerin sizethanthebladeobservedundertheopal-CT showingcolumnartexturefrom the Hosaka SEM.The crosssectionof thelepisphereshowsafibrousmine;thetextureof this sampleis mostcommonin the texture(Fig. 2b). The bladecorrespondingto that in theopal-CTsamplesfrom the 6r,emines;(3) opal-Cftom SEMimageis composedof aggregatesof zuchthinsrystals,ftsAkass mins; the X-ray powder-diffractionpatternof which are roughly parallel in orientationto eachotherthis sampleis thatof highly orderedlow crisobalite with in theblade.Thethin crystalis severaltensof nanometersa very weakshoulderat 43 A.in thicknessand less than 0.5 pm in length and width,whichagreeswiththeplatycrystalobsenedin en[101]*directionsoftwo crossedbladesis about70",whichcorrelatesveiftfts anglebetween11e sampleswere cut in the orientationnormal or two { 101} planesof low cristobalite.The cross-sectionparallel to the horizontallayer of the silica-rich geode, of the lepisphereis similar in textureto that of opal-CTandrhinsectionsweremadefor observationwith anoptical crystalsfrom sedimentaryrocks, as studiedby Caty etmicroscope(polarizedlighQ.Thecut faceswerepolished al. (L996).
OPALCTANDOPAI-C INVOrcANICR@KS951Frc.3. SEM photomicrographsofcolumnartexu[e ofopal-CT cut (a) paralleland(b) normalto the horizontal layer of a silica-rich geode.Photographswere taken on the etchedsurfaceafter polishing. The column cut normal to the layer is irregularly circular tooblong,with fine sniations.Vermicularpauern(anow A) andboundary(anow B) in @)correspondto the boundariesshownby arrowsA andB in (a), respectively.Thedirectionofopticalextinctionis parallelto thesfriation.The directionsof striationsin two adjacentcolumnsareFigureI showsanopticalmicrophotographof a layer inclinedin oppositesidesto eachother,sothatthepairedof opal-CTshowingthe typically colnmnartexture.The columnsshow a relationshiplike in the caseof a twinthin sectionis normal to the horizontal layer of the seenin thin sectionundertheopticalmicroscope.Figuresilica-rich geode.Figure 3a showsan etchedpatternof 3b showsan etchedpatternofthe sectioncut paralleltothecolumnartexture,in which fine striationsareinclined the horizontal layer of the silica-rich geode.The crossd ahnlrt?M" with respectto theelongationof thecolumn- sectionshowsan irregularly circular texture with fineColumnartextureof opal-CT
952F\c. 4. Schematic illustration of threedimensional conical formof a pair of columns.stiations.Ave,micularpattemshownby rrow Ain Figure3b correspondsto boundariesofthe columns,shownbyanowAin Figure3a-Also,theboundary(shownbyarrowB in Fig. 3b)of theineguladycircularfeanrecorrespondsto a distinct or indistinct boundary(shown by arrow Bin Fig. 3a). The pairedcolumnsas shownin Figure 3acorrespondto onecircular-featuredomainio pi*rO,suchthat it is not a twin. From theseobservations,wesuggestthat the three-dimensionalform of the pairedcolumnsis conical,asillustratedin Figure4.Figure 5a showsa TEM photomicrographof theintemal textureof the columnin a sectioncut normaltothehorizontallayerof thesilica-richgeode.The columnexhibitsafibroustexturecomposedofthin crystals,whosethicknessis severaltensof nanometersandwhoselengthis lessthan 1 m. The fibers correspondto the striationsshownin theSEMimage(Fig.3a)andareroughlyparallelto eachother.Along the boundaryof the columns,thedirectionsof the fibers seemrandomly oriented.Fromoptical and TEM observations,the fiber is opticallylength-slow.The fiber axis is parallelto (101) of lowcristobaliteand(001) of low tridymite. The sftuctureoftheopal-CTcrystalis describedin detailin thefollowingsection.A similar fibroustextureis observedin a sectioncutparallelto thehorizontallayerofthe silica-richgeode.FIc. 5. TEM images of opal-CT crystals in the columnartexture. a) Fibrous texture, which is inclined to the thinand platy crystals. b) Lattice image and conespondingelectron-diffraction pattem of the opal-CT crystal. Thehorizontal direction of the photograph (b) is parallel to thecrystal elongation in (a). The a*-c* plane oflow cristobaliteis parallel to the photograph, and the orientation of thesheets of SiO tetrahedra is horizontal. The stackingsequence indicated by AA in (b) contains antiphase domainboundaries. OC : ordered cristobalite.
OPALCT AND OPAI{Althoughthecrystalsseemfibrous (Fig. 5a),thecrystalsthat give an [010]*-[102]* diffraction patternof lowcristobalitearenotfibrous,but rarlerpolygonalandplaty.Thereforeowe suggestthatthecrystalsin thecolumnarevery thin andplaty, as shownin Figure 2a.The sectioninclined to the platesshowsa fibrous texturebecauseofthe parallel growth of extremelythin crystals.The opal-CTy layer (optically massivelayer in Fig.1), below the layer showingthe columnartexture,iscomposedof intersectingaggregatesof blades,accordingto TEM observations.The cross-sectionof intersectingaggregatesin the opal-CTy layer is similar in texturetothat ofthe lepisphereshownin Figure 2b, andthebladealsoconsistsof a parallelgrowthof thethin platycrystals.However, in the caseof the opal-CTy, the interstitialspaceamongthesphericalaggregafesis filledby orderedcristobalitecrystals;thisfeafire willbe descdbedin detailin from arandomorie,lrtationin themassive-texfi[ ddsmainIN VOLCANIC ROCKS953crystalsarein roughly parallelorientationto eachotherwittr thein the blade (or bundle).From measurementswithTEM, the sizeof ftsthin crystalsslightly decreasesthechangein textne, from thecolumnartothepatchworkpattern.The blade is lessthan 0.2 tm in thicknessand1-3 pm in length.The orientationof the bladesis moreinthe refringentpart; in fact, the bladescrosseachother.Strucrureof opal-CTfrom theHosakaopal minzFigure5b showsaHRTEM imageandcorrespondingelecfron-diftaction pattemof a crystalin the columnartexture of opal-CT from the Hosakaopal mine. Thehorizontaldirection of the photographis parallel to thecrystal'selongationshownin Figure5a.The spacing9fthe horizontalfringe in the lanice imageis about4 A,which agreeswith dror of low cristobalite.The verticaldirectionis normalto the sheetsof SiO tetrahedraandFlc. 6. Sim{ated images of (a) low cristobalite, (b) Iow ridymite and (c) antiphase domain boundary in low cristobalite. The imagesare projected on the (010) of low cristobalite and the (100) of low tridymite, and the vertical is the stacking direction of thesheets. The arrows in (c) show antiphase twin boundaries (APB).to aroughly parallelorientationin the domainshavingacol rmnartexture.Thecolumnsshowinga stong birefringencegnduallybecomeslenderduing gowtb, md moreweaklybireftingentparts,which arehomogeneouslyetched develop2mongthe slendercolu'nns.This texturewas describedas a"patchworkpattern" by Fldrke et al. (1991).Our TEMobservationsreveal that the part showing a weakbirefringenceconsistsofplaty crystelsa5q7sll.J[e thinto the (010) projectionof lowthe flow hidymite.Thediftaction patternis heavily streakedalongthe 101 *andthesteakingsdirectionofthelow cristobalitesequence,imFly that the crystal includesnumerousplanar faultsparallelto (101)of low cristobalite.HRTEMimagesona correspondingprojection were simulatedfor lowcristobaliteandlow tridymite (Fig. 6). As shownin thearedenotedsimulatedimagesothesestackingsequenc s
954FIG. 7. TEM images of opal-CT crystals in the patchwork pattern t xture. The dark areas,shown by A in (a), consist of well-ordered stackings of cristobalite as shown in (b). Theorientation in (a) is the same as in Figures 5 and 6.by riple (ABQ anddouble(AB)periodicities,respectively.A careful comparisonof the simulatedimageswith theHRTEN4imagesof opal{T showsrharmostof houghthecrystalincludesnumerousstackingfauls. The { 101
947 Thz C anadian M ine r ab g ist Vol. 35, pp. 947-958 (1,997) TEXTURE AND STRUCTURE OF OPAL-CTAND OPAL.C IN VOLCANIC ROCKS TOSHIRO NAGASE 1 AND MIZUHIKO AKIZUKI Institate of Mineralogy, Petrolagy and Economic Geology, Farulty of Scienc
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