the hydrothermal recrystallization of metamict …

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The Canadian MineralogistVol.48,pp.513-521(2010)DOI:10.3749/canmin.48.3.513

THE HYDROTHERMAL RECRYSTALLIZATION OF METAMICT ALLANITE-(Ce)

AndreAČOBIƧ,VlAdImIrBermAneCAndnenAdTOmAŠIĆ

Institute of Mineralogy and Petrology, Faculty of Science, University of Zagreb, Horvatovac 95, HR–10000 Zagreb, Croatia

rAdekŠkOdA

Institute of Geological Science, Masaryk University, Kotlarska 2, 61137 Brno, Czech Republic

ABsTrACT

Samples ofmetamict allanite-(Ce) originating fromgranite pegmatiteswere recrystallized by heating in air and underhydrothermalconditions.Recrystallizationinairresultedinpartialrecoveryofthecrystalstructureat650°C.Heatingathighertemperatures resulted in decomposition of the allanite structure to amultiphase assemblage containing hematite, cerianite,feldspar,britholiteandthorianite.Toourknowledge,thisisthefirstreportofthorianiteasabreakdownproductofallaniteinliterature.ThecollapseofthestructureismainlyrelatedtotheremovalofstructuralH2O.Recrystallizationunderhydrothermalconditions yielded amore rapid and complete recovery of the structure at lower temperatures,without the appearance ofadditional phases.The stability of the allanite structure during hydrothermal recrystallization is related to the retention ofOH–groups,availablebecauseoftheaqueousenvironmentofrecrystallization.Also,H2Opromotesamorerapidandefficientrecrystallizationdue to the enhanceddiffusionof cations.Crystallite size and strain calculationalso record evidenceof therecoveryofthestructureofdamagedallanite.

Keywords:allanite-(Ce),metamictization,hydrothermalrecrystallization,X-raydiffraction,graniticpegmatites.

sOmmAIre

Nous documentons la recristallisation de l’allanite-(Ce) prélevée de pegmatites granitiques par chauffage dans l’air etsous conditions hydrothermales.Dans l’air, la recristallisation permet de récupérer partiellement la structure à 650°C.Unchauffageàtempératuresplusélevéesmèneàladéstabilisationdelastructurepourdonnerunassemblagemultiphasécontenanthématite,cérianite, feldspath,britholiteet thorianite.Anotreconnaissance,ceserait lapremièrefoisque la thorianitefigureparmilesproduitsdedéstabilisationdel’allanite.Ladestructiondelastructureestsurtoutliéeàl’éliminationdeH2Ocommecomposantstructural.Untraitementhydrothermalmèneàunerecristallisationplusrapideetpluscomplètedelastructure,etàdestempératuresplusfaibles,sansformationdephasesadditionnelles.Lastabilitédelastructuredel’allanite-(Ce)aucoursdelarecristallisationhydrothermaleestliéeàlapréservationdesgroupesOH–danslastructure,résultatdumilieuaqueuxdelarecristallisation.Aussi,laprésencedeH2Ofavoriseunetransformationplusrapideetefficaceàcausedesonrôleàfaciliterla difusiondes cations.La taille des domaines cristallins et l’importancededéformationdémontrent aussi le progrès de larecristallisationstructuraledel’allaniteendommagée.

(TraduitparlaRédaction)

Mots-clés:allanite-(Ce),métamictisation,recristallisationhydrothermale,diffractionX,pegmatitesgranitiques.

§ E-mail address:ancobic@geol.pmf.hr

InTrOduCTIOn

Allanite,whichbelongstotheepidotegroup,hasthegeneralformulaA2M3[T2O7|TO4](O,F)(OH,O),withA=Ca,REE,Mn,Fe2+,Pb,Sr,ThandU,M=Al,Fe2+,Fe2+,Mn2+,Mn3+,TiandMg,andT=Si,Al(Deeret al.1986,Armbrusteret al.2006).Allaniteiscommonlymetamict.Themetamict state of allanite and other

metamictmineralshavebeeninvestigatedextensively(Ewing1987,1994,Janeczek&Eby1993)inordertoexplainthemechanismsofamorphizationaswellastoreconstitutetheoriginalcrystal-structure.Therecoveryofthecrystalstructureisusuallycarriedoutbyheatingmineralsinacontrolledoruncontrolledatmosphere.

Themajorityofmetamictminerals recrystallizeattemperaturesgreaterthan600°C.Therecrystallization

514 TheCAnAdIAnmInerAlOgIsT

ofallaniteiscomplicatedbythepresenceofstructurallyboundhydroxylgroups,whichescapeduringrecrystal-lization inducedbyheating.Also, in an uncontrolledatmosphere,Fe2+canoxidize toFe3+.This inevitablyresultsindecompositionoftheallanitetomoresimpleoxidesandsilicates(Berman1955).Inordertopreventthese problems, hydrothermal recrystallization offersa reasonable approach to recrystallize thosemineralscontaining structuralwater. Janeczek&Eby (1993)reportedon thesuccessfulhydrothermalrecrystalliza-tionofgadolinite,amineralthatdoesnotcontainH2Oinitsstructure.Inthecurrentstudy,thisapproachwillbe applied to allanite-(Ce) from granitic pegmatitessampledinBulgaria,Mexico,twolocalitiesinNorwayandthreeinCanada.

BACkgrOundInfOrmATIOn

Metamictizationisusuallyattributedtoa-decayofradionuclides, andadestructive impactofa-particlesanda-recoilnucleitothestructureofminerals(Ewing1994,Weberet al.1998).Manymineralsthatcontainradionuclidesexhibitradiationdamagetotheircrystalstructures.However, there aremineralswith a rela-tivelyhighamountof radionuclides thatdonotshowproperties ofmetamictminerals, for instance apatiteandmonazite. In contrast, there areminerals thatcontainrelativelylowamountofradionuclides,butaremetamict,e.g.,mineralsoftheallanitegroup.

Apart from the leading role of radionuclides inthemetamictizationofminerals,Graham&Thornber(1974) proposed that chemical composition andcomplexityofthecrystalstructurecanbesignificantlyresponsible formetamictization.We thussuggest thatchemical complexity (particularly ofNb–Ta oxides)

duetosubstitutionofalargenumberofhighlychargedcationswithdifferentionicradiicouldleadtodispropor-tionationandsegregationtomicroregionsanddomainsofdifferentcompositionsandstructure.

experImenTAl

The samples of allanitewere examined using astereomicroscope in order to remove visible impuri-ties.Thepurifiedsampleportionswereusedforfurtherexaminations.Theorigin of the samples is describedinTable1.

Prior to X-ray powder-diffraction analysis, thesamplesweregroundinanagatemortar.X-raypowder-diffraction datawere collected using a Philips PW3040/60X’PertPROdiffractometerwithaCutubeusedat45kVandatacurrentof40mA.Thestepsizewas0.02°andthecountingtimewas2sperstep.Analysesof theX-ray-diffraction patterns and calculations ofcrystallitesizeandstrainwereperformedusingHigh-ScorePlus software (Panalytical 2004).Theunit-cellparameterswerecalculatedusingunITCell(Holland&Redfern1997).

Thechemicalcompositionofnatural sampleswasestablished using both energy-dispersive (EDS) andwavelength-dispersive systems (WDS).AnOxfordInstrumentsEDScontrolledbyanINCA250systemismountedontoa scanningelectronmicroscopeTescanVegaTS5136MMusedforqualitativeanalysis.Abeamvoltageof20kVandcurrentof10nAwereapplied.

Quantitative chemical analysesweremadewith aCAMECASX100 electronmicroprobe in theWDSmodeatthefollowingconditions:accelerationvoltage15keV,beamcurrent20nA,andbeamdiameter2mm.Forquantification,thefollowingstandardswereused:sanidine(SiandAl),albite(Na),andradite(CaandFe),Mg2AlO4(Mg),spessartine(Mn),titanite(Ti),ScVO4(Sc), fluorapatite (P), CeAl2 (Ce), LaB6 (La),YAG(Y),asetofREEfluorides(Nd,Sm,Pr),GdPO4(Gd),DyPO4(Dy),Th(Th)andtopaz(F).TheabundancesofK,U,Er,YbandZrarepresentbelowdetectionlimits.Detectionlimitsfortheelementssought(inppm)are:Si440,Al385,Na569,K344,Ca576,Fe920,Mg278,Mn578,Ti342,Sc257,P222,Ce861,La836,Y446,Nd1735,Sm767,Pr1624,Gd1179,Dy1623,Er700,Yb1102,Th1027,U612,F491andZr415.

Allsampleswereheatedat650and1000°Cfor24hinair.SamplesMEX1andMEX2werealsohydrother-mally treatedinamicrowaveovenMicrowaveAntonPaar3000(rotor16HF–100).Acuvettewith500mgof the samplewasfilledupwithwater andheated to200°Cover20minutes.For thenext20minutes, thetemperaturewasmaintainedbetween200and260°C.Afterward, the temperaturewas decreased to roomtemperatureover15minutes.

SamplesMEX2,CDN2,CDN3andN1wereinaddi-tiontreatedinaPerkin–Elmerautoclave.Thesampleswereheated inaTeflonvessel togetherwith5mLof

ThehydrOThermAlreCrysTAllIzATIOnOfAllAnITe-(Ce) 515

waterforeach200mgofthesample.Thevesselwasenclosedinanaluminumhousingandheatedat150°Conahotplatefor2,5and24h.DetailsofthesamplethermaltreatmentarepresentedinTable2.

resulTs

X-ray-diffraction data

X-raydiffractionpatterns of all unheated samplesare characterized by a high background and broaddiffraction-linesoflowintensity.Allobserveddiffrac-

tion-linesbelongtoallanite.Annealingthesamplesinairat650°Cfor24hintensifiesthediffractionlinesanddecreasestheirwidth.Atthesametime,afewadditionaldiffraction-lines of allanite appear,whichwere notobservedatroomtemperature.

Asaresultofheatingat1000°C,thecrystalstructureof allanite is destroyed, giving rise to simple oxideslike hematite, cerianite and thorianite, and silicateslike feldspar and britholite.The occurrence of thesephaseshadbeenpreviouslyobservedin therecrystal-lizationofallanite(Lima-de-Faria1958,Mitchell1966,Vance&Routcliffe1976,Janeczek&Eby1993)withtheexceptionof thorianite,probablydue toproblemsin identification due to its low concentration in theresultingmultiphasesystem.Theproductsofheatingofarepresentativesampleofallanite,MEX1,annealedat650and1000°C,arepresentedinFigure1.

Thesamplehydrothermallytreatedat150°Cinanautoclave for5h showsmorepronounceddiffractionlinesanda lowerbackgroundthanthesampletreatedthesamewayfor2h(Fig.2).Ontheotherhand,thesample hydrothermally treated for 24 h but at lowertemperature(130°C)yieldslesspronounceddiffractionlinesthaneitherofthepreviouslymentionedsamples.Diffractionlinesofallthreesamplesallbelongtoalla-nite,withouttheappearanceofanyotherphases.Thisis also true for the sampleshydrothermally treated inamicrowave oven (Fig. 2).There is no large differ-enceamongthesamplestreatedinautoclaveandthosetreatedinamicrowaveovenexceptforaslightdiffer-ence in intensity and resolution of diffraction lines,whichisprobablyduetoadifferenceindurationofthehydrothermaltreatment.Theunit-cellcalculationsshowincreasingcrystallinity(Table3).Theresultsofacrys-tallitesizeandstrainanalysisarepresentedinTable4.

Chemical composition

BothWDS andEDS analyses show evidence ofheterogeneityofthenaturalsamplesinvestigated.MostoftheanalyzedspotsdepictcompositionsintherangeAln0.25Ep0.75–Aln0.89Ep0.11.Themost abundantREEinallthesamplesisCe,followedbyLa(Table5).Theactinidecontentisslightlyaboveorbelow1%,mainlyduetoThO2,whereasuraniumisbelowthedetectionlimit.ThesumofREE+Thinallthesamplesis>0.5apfu, except in sampleMEX2; thus the investigatedsamples belong to the subgroup allanite (Armbrusteret al. 2006). SampleMEX2, with the calculatedformula (Ca0.97Mn0.03)S1.00(Ca0.70REE0.27Th0.02)S0.99(Al0.96Fe3+0.02)S0.98Al(Fe2+0.85Mg0.03Fe3+0.12)S1.00[Si1.03O4|Si2O7]O(OH0.77O0.23),isasolidsolutionbetweenepidote(dominant)andallanite.

Formula calculations for all natural samples ofallanitewereperformedinagreementwiththerecom-mended nomenclature of epidote-groupminerals(Armbrusteret al. 2006).Normalization of electron-microprobedatawasdoneonthebasisofS(A+M+T)

516 TheCAnAdIAnmInerAlOgIsT

fIg.1. XRDpatternsofallanitesampleMEX1atroomtemperature,650and1000°C(BRI:britholite,Fds:feldspar,HEM:hematite,CER:cerianite,TOR:thorianite).

fIg.2. XRDpatternsofsampleMEX2atroomtemperature(MEX2_RT)andheatedinamicrowavedigestor(MEX2_MD)at200–260°Cfor20minutesandinanautoclaveat150°Cfor2(MEX2_AK_1)and5h(MEX2_AK_2).

ThehydrOThermAlreCrysTAllIzATIOnOfAllAnITe-(Ce) 517

=8apfu.ThecontentofFedeterminedbyEMPAwasinitiallyassumedtobeFe2+.However,thisresultedinanexcessofoxygeninformulacalculation,whichwasassignedtoFe3+intheallanitesamples.TherecalculatedformulaethuscontainbothFe2+andFe3+.

As observed in back-scattered electron (BSE)images,thesamplesinvestigatedbelongtoaparagen-esiswith otherminerals that occur as nests, lamellaeandveinlets in allanite (Fig. 3).Alongwith lamellaeofepidoteenrichedinREE,othersilicateslikethorite(Table6)occurinsomesamples(Figs.3a,b,c).Also,xenotime-(Y) (Fig. 3b, Table 7), veinlets of REEcarbonate,most likely bastnäsite-(Ce), and calciumcarbonate(Fig.3a)areobserved.

dIsCussIOn

Thesamplesofallanitethatwechoseforstudyaremetamict.The chemical analysis confirmed the pres-enceofallanitewithacomplexcompositionandwith

518 TheCAnAdIAnmInerAlOgIsT

a largenumberofcationshavingdifferent ionic radiiand charges (Th,Na,Mg,Mn,REE).Thedifferencein ionic radius and charge ofmany different cations

in a complex crystal structure favorsmetamictization(Wanget al.1991).

Radionuclides play a significant role inmetamic-tization;a decay of the radionuclides present in the

fIg.3. (a)SampleN2,severalgenerationsofallanitewithislandsofthorite;(b)sampleN2:Thsilicateandxenotime-(Y),(c)sampleMEX2:severalgenerationsofallanite–epidoteserieswithvariouscompositions;thewhitespotsrepresentaThsilicate(thoriteorhuttonite);(d)BG:lamellaeofepidoteinallanite.

ThehydrOThermAlreCrysTAllIzATIOnOfAllAnITe-(Ce) 519

mineral structure is considered tobe themain reasonformetamictization(Ewing1994,Weberet al.1998).In all the samples, radionuclides are present,mainlyTh (Table5).By thoroughanalysisofXRDpatterns,wefoundthatsampleN1showsthehighestdegreeofmetamictization,whichcorrespondswellwithasignifi-cantcontentofTh(Fig.4).

TherecrystallizationofmetamictmineralscontainingstructuralH2OiscomplicatedbytheescapeofH2Oinroutinerecrystallizationexperimentsinair(Bonazzi&Menchetti1994).TheremovalofstructuralH2Ofromallanite,i.e.,OH–groups,isfavoredbyasimultaneousoxidation of divalent cations in octahedral positions(Bonazzi&Menchetti 1994).The charge balance ismaintained byH+ removal during oxidation of, forinstance,Fe2+andMn2+.Thisreactioncanbedescribedbyequation

2OH––e–!H2O+½O2,

whichliberatesoxygenfortheoxidationofironandthebalancingofchargesdescribedbyfollowingequation:

2Fe2++½O2!2Fe3++O2–.

However, the recrystallization in this case is limitedby dehydroxylation and consequent destruction ofthe crystal structure.Thus, allanite is decomposed tohematite, cerianite, feldspar, britholite and thorianite.Thesamplesthatare lessstronglymetamictaremore

easily recrystallized (Janeczek&Eby1993) becausethey containmore relics of the original structure.Indeed,therecrystallizationat650°Cinairshowedthatthesampleswithalowerdegreeofmetamictizationaremorereadilyrecrystallized.Thepreservedrelicsoftheoriginalstructureserveascentersofepitacticrecrystal-lization(Tomašićet al.2006).Therecrystallizationisalsoaccompaniedbyadecreaseinunit-cellvolume,ashasbeendemonstrated in aprevious investigationbyJaneczek&Eby(1993),whoshowedthatthevolumeofunit-cellincreasesduringmetamictizationanddecreasesasthemineralrecrystallizes(Table3).

During the recrystallization of allanite in air attemperatures around 1000°C, new phases appear,mainlysilicatesandsimpleoxides.Thus,silicateslikefeldspars(Lima-de-Faria1958)orbritholite(Mitchell1966, Janeczek&Eby 1993) have been observed.Heatingexperimentsinanoxidizingatmosphereyieldoxideslikecerianiteandhematite(Vance&Routcliffe1976).Unlike previous studies, thorianitewas alsoobservedinourinvestigation.

Owing to the problems arising in recrystallizationexperiments ofOH-containingminerals in air, it ismore appropriate to recover crystal structures hydro-thermally.Inthisway,H2Oispreservedinitspositionsinthecrystalstructure.Astudyofanhydrousgadolinite(Janeczek&Eby 1993) showed that hydrothermalheatingresultsinadirectrecrystallizationofgadolinitewithout an occurrence of other phases.Moreover,recrystallization ofmetamict zircon in hydrothermal

fIg.4. XRDpatternsoftheallanitesamplesinvestigatedatroomtemperature.

520 TheCAnAdIAnmInerAlOgIsT

conditionsshowedthatH2Ocanplaytheroleofacata-lystincrystal-structurerecovery(Geisleret al.2003).OurXRDpatternsofallanitehydrothermallytreatedinamicrowaveovenandautoclaveshowpartialbutnever-theless significant recrystallization.The temperaturesandpressuresappliedarerelativelylow,butrecrystal-lizationoccursquickly,yieldingnoadditionalphases.

Recrystallization is alsomade evident by changesincrystallitesizeandstrainparameters.Inthecaseofhydrothermally recrystallized samples, the crystallitesizeincreases,whereasstraindecreases.After24hofrecrystallization,thereisanindicationofoxidationofFeandCe,probablyinhibitingfurtherrecrystallization.This is based on the observed increase in strain anddecreaseinthebparameter(Tables4,3).

OurchemicalanalysesshowthatCeisthedominantREE in the samples investigated. In addition toTh,whichresidesinthecrystalstructure,ourSEMinves-tigationconfirmedtheoccurrenceofthorite(Table6)inaformofveinletsandgrainsintergrownwithallanite.The distribution ofTh indicates the development ofthoritenuclei.

Bonazzi&Menchetti(1995)showedthattheunit-cellparameterbisstronglyrelatedtotheionicradiusof cations in octahedral coordination in the structureof epidote-groupminerals.With an increase of Fe2+content, an increase in theb parameter also occurs,and iron oxidation causes its decrease aswell as thedecreaseofaandbparameters(Bonazzi&Mencheti1994).ThedecreaseinthebparameterinthecaseofallanitesamplesinvestigatedthussuggestsFeoxidation.Ontheotherhand,theincreaseinthecparametercanberelatedtoareleaseofH+,i.e.,H2O(Table3)(Bonazzi&Menchetti 1995), a result of the heating experi-mentsbeingperformedinanuncontrolledatmosphere.Structurerecoveryuponheatingisalsoindicatedbyadecrease in unit-cell volume.Although these generalrelations in the behavior of unit-cell parameters areapplicabletothemajorityofourallanitesamples,somediscrepancies can be observed due to the complexchemicalcomposition,variousdegreesofmetamictiza-tion,possiblealterationandoxidation.

COnClusIOns

All of the investigated samples but one consistof allanite-(Ce), as confirmed by chemical analysis.Inclusions ofTh phases likeThSiO4 are found.Acomparison ofX-ray-diffraction data shows that thedegree ofmetamictization is related to the contentof radionuclides in the structure of the investigatedsamples.Also,severelymetamictsamplescontainlargeramount ofREE.Significant contents of large cationswithdifferentionicradiiandchargesindicatethatthedegreeofmetamictizationisfavoredbycrystalstructureinstabilitiescausedbyacomplexchemicalcomposition(andstructure).

The recrystallization experimentswere performeddry (in air) and in aqueous (hydrothermal) environ-ments. Partial recovery of the structure at 650°C inairisfollowedbyacollapseofthestructureathighertemperaturesaccompaniedbyanoccurrenceofphaseslikehematite,cerianite,thorianite,feldsparandbritho-lite.ThecollapseofthestructureismainlyassociatedwiththeremovalofOH–groupsfromthestructure,butalsowithoxidationofFeandCe.On theotherhand,hydrothermalrecrystallizationoccursmorequicklyandatlowertemperaturescomparedtodryrecrystallizationinair.Thereisnoindicationofmineraldecompositionduringaprolongedhydrothermaltreatment.Crystallitesizeandstrainanalysis indicatea fast andsignificantrecoveryofthestructureatlowtemperaturesandpres-sures.The results of hydrothermal recrystallizationsuggest thatH2O in the systemenables amore rapiddiffusionofionsinthestructureatlowertemperature,and thus promotes recovery of the structure.Recrys-tallization in an aqueous environment also enablesH2Oretentioninthestructure,whichiscrucialforitspreservation.

Recrystallization experiments in air are indicatedbyunit-cellparametersbehavior:bothunit-cellvolumeand theparameterbdecreaseduringrecrystallization,thusgivingevidenceofstructuralrecoveryandpossibleoxidationofFe2+,respectively.Duringdryrecrystalliza-tion,thecparameterincreases,asaresultoftheremovalofOH–groupfromthecrystalstructure.Hydrothermalrecrystallization yielded an evenmore significantdecreaseoftheunit-cellvolume,whichindicatesamorerapidrateofrecrystallization.

ACknOwledgemenTs

TheauthorsaregratefultotheMinistryofScience,Education and Sport of theRepublic ofCroatia forsupporting thiswork (grantsNo119–0000000–1158,098–0982934–2742).GrantGAAVno.KJB301630801of theCzechRepublicwasawarded toR.Škoda.WethankGoranDurn, fromRGNF,who supported thisinvestigationwith his autoclave. The authors alsothank the reviewers,GregLumpkin andLeeGroat,andMichaelEaston (OntarioGeologicalSurvey)andRobertLinnen, for their constructive help andusefulsuggestions.

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Received October 29, 2009, revised manuscript accepted May 28, 2010.