Metamict and Chemically Altered Vesuvianite

357

Canadian Mineralogist Vol.31, pp. 357-369(1993)

METAMICTAND CHEMICALLYALTERED VESUVIANITE

RAY K. EBY', JANUSZJANECZEI( ANDRODNEY C. EWING Departmentof Geology,University of NewMexico, Albuquerque, New Mexico 87I 3 I , U.S.A.

T. SCOTIERCIT

Mineral Scierrces Section, Canadian Mweumof Nanre, Ottawa, Ontario KIP 6P4

LEEA. GROAT Depanmentof GeologicalSciences, IJniversity of BritishColumbia" Vancouver, British Columbia V6T 284

BRYAN C. CHAKOUMAKOS SolidState Division, Oak Ridge Nationnl laboratory, P.O. Box 2008, Oak Ridge, Tennessee 378i 14056, U.S.A,

FRANKC. HAWTHORNE

Department of Geolngical Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2

GEORGER.ROSSMAN Divisionof GeologicalSciences, Califurnia Institute of Technology,Pasadena, Califomia 91 125, U.S.A.

ASSTRAC"I

Partly metamictvesuvianite samples from two localities wereexamined and compared. The unit cell of oneis enlargedowing to volume expansioncaused by the buildup ofradiation damage.The other samplesustained enough damage to excludeaccurate determination of unit-cell parameters.BSE imaging shows that boti sampleshave undergonechemical alteration, and electron-microprobedata indicate that the alteration has resulted in a heterogeneousdistribution of radionuclideson ttremicrometer scate.HRTEM and SAED analysesreveal a wide variation in the extent of alpha-recoildamage, which correspondsto the heterogeneousdistribution of the radionuclides.The progressivestages of metamictizationalso are observedin detail with TEM. Partly metamictvesuvianite recrystallizes over the range6G850'C, which is bnoaderthan the rangefound in other metamict silicates. Combinedthermogravimetric analysis and isothermal annealingshow that, upon heating (in N2 or Ar), metamict vesuvianitebegins to recrystallizeat 6@oC,and at 9@'C decomposesinto the multiphaseassemblage gtossular + gehlenite+ wollastonite.Unpolarized IR spectraof both vesuvianitesamples are similar andalso resemble those of radiation-damagedzircon and titanite, suggestingthat the major structuralfeatures of the aperiodic stateare similar for complex ceramicsof different composition.

Keywords: vesuvianite, metamict, radiation damage,alteration, transmissionelectron microscopy, back-scatteredelectron imaging,infrared spectroscopy,thermogtavimetric analysis.

SovnrenB

Nous avons6tudi6 et comparddes 6chantillons de v6suvianitepartiellement mdtamicte provenant de deuxendroits. La maille 6l6mentairedans uncas estagrandieparleseffets dudommage structural d0hl'irradiation. L'autre6chantillonasubi suffisammem de dommagepour rendreimpossible la d6terminationdes paramdtres du r6seau.Les imagesobtenues avec dlectrons r6trodiffirs6s montrentquedans les deuxcas, la vdsuvianitea subiune alt6ration chimique. ks donn6esd lamicrosonde6lectronique d6montrent

I Presentaddress: TopoMenix, OneRobertson Drive, Suite 18, Bedminster,New Jersey07921, US.A. 2 Presentaddress: Department of Earth Sciences,Silesian University, Bedzinska60 , 4l-200 Sosnowiec,Poland. 358 TTIE CANADIAN MINERALOGIST une distribution h6tdrogbnedes radionucl6idesi l'6chelle microm6trique.lrs analysespax microscopie dlectroniquepar transmissioni hauter6solution, avec diffraction des6lectrons sur domainesrestreints, rdvdle une grandevariation dans la portde du dommaged0 au resacdes nucl6uspar particulesalpha qui correspondi la distribution h6t6rogdnedes radionucl6ides.La progressionde la m6tamictisationpeut 6tre document6een d6tail par microscopiedlectronique par transmission.On peut recristalliserla v6suvianitepartiellement m6tamicte en la chauffantentre 600 et 850'C, un intervalleplus grandque pour lesautres silicatesm6tamictes. Unecombinaisond'analyse thermogravim6trique etde traitementparrecuit isothermal montre qu'enprdsence de N2 ou Ar, il y a un ddbutde recristallisation I 600'C, et qu'i 900oC,la vdsuvianitese ddcompose en un assemblagemultiphasd degrossulaire, gehlenite et wollastonite.l€s spectresd'absorption infra-rouge non polarisds des deux 6chantillons sont semblables, et ressemblenti ceux de zircon et de titanite endommag6spar irradiation.Ces rdsultats font penserque les 6l6mentsstructuaux typiques de 1'6tat apdriodiqueddveloppd aux d6pensde mat6riaux cdramiquescomplexes de compositionsdiff6rentes se ressemblent.

(Traduit par la Rddaction)

Mots-cl6s:vdsuvianite, mdtamicte, dommage par inadiation, altdration,microscopie 6lectronique par transmission,images par 6lectronsr6trodiffrrs6s, spectroscopie d'absorption infra-rouge, analyse thermogravim6trique.

INTRODUCIIoN displacements(l'.e., a collision cascade)and limited ionization-relatedeffects. Alpha-recoil collision cas- Radiation damage resulting from alpha-decay cadesresult in localizedaperiodic regions that have (metamictization)is rare in vesuvianite,because U and recentlybeen observed ryith HRTEM in titanite (Lump- Th are not usually presentin appreciableamounts. The kin et al. l99l). For extensivereviews of radiation amountof U and Th found in partly metamictminerals effectsin mineralsand ceramics,see Wang & Ewing is typically severalhundred ppm, and fully metamict (1992),White at al. (1989),Ewingetal. (1987),Clinard mineralsmay contain severalthousand ppm of U and & Hobbs(1986) and Thompson (1981). Th. Two samplesof vesuvianiteexhibiting metamict characteristicswere examined using high-resolution ExpSRTMENTALMETHoDS transmissionelectron microscopy (IIRTEM), selected- areaelectron diffraction (SAED), powderX-ray diffrac- Elect ron-mic ro probe analys is tion QGD), electron-microprobeanalysis (EMPA), back-scattered electron (BSE) imaging, thermo- gravimetric analysis(TGA) and infrared spectroscopy Electron-microprobeanalyses (EMPA) were caried (IR). These samplesof vesuvianitewere previously out with a JEOL 733 instrument(at Ottawa);experimen- characterizedby Groat et al. (1992), and the same tal detailsare givenby Groat et al. (1992).T'hefollowing samplenumbers are usedhere. Sample V53 is from the standardswere used:riebeckite (F), Na-bearingamphi- SewardPeninsula Alaska whereit occursas large (up bole (Na), almandine(Mg, Al, Fe), diopside (Si), to 1.5 cm) brown crystals in a highly radioactive anhydrite(S), tugtupite(Cl), syntheticgehlenite (Ca), nephelinesyenite of mid-Cretaceousage (Himmelberg titanite (Ti), rephroite(Mn), LaPo4 (La), cePoa (ce), & Miller 1980,Miller & Bunker1976). Sample V73 is hPO4 (h), ThO2 (Th) and UO2 (U). The following a lwge (2 cm) black crystal with conchoidalfracture, elementswere sought, but not detected:B, K, Cr, Zn, Sb, from a skam near Lake George,Colorado. The only Nd, Sm, Pb and Bi. Back-scatteredelectron (BSE) other repor0edlocality for metamictvesuvianite is an imaging was done with the JEOL 733 electronmicro- alkaline pegmatitein the Tuva district, in the former probeand with a Nanolab7 scanningelectron-micro- USSR. Samplesfrom this locality were studied using scope(at UBC). thermalanalysis methods (Kononova 1960). The vesuvianite from Tuva shows an exotlermic peak and X-ray dffiaction weight-lossover the range 650 - 750'C (conesponding to recrystallization),and then decomposesinto garnet Unit-cell dimensionswere determined with a Nicolet above900oC. R3n single-crystaldiffractometer (at the University of Metamictizationresults from thealpha-decay process Manitoba);experimental details are given by Groatetal. vla the UlTh decayseries. An alpha-eventproduces two (1992). PowderXRD patternsof annealedsample V73 particles:i) a4.0-5.5 MeV alphaparticle,whichtravels were acquiredwith CuKcl radiation using SCINTAG relatively far throughthe structure(-20 pm) andcauses diffractometers,one equippedwith a monochromator negligible amorphizationby elastic coUision,but pro- and scintillation detector (at the University of New duces significant ionization effects in the form of Mexico) and oneequipped with an intrinsic Ge detector isolatedpoint defectsand associated structural strain; ii) (at Oak RidgeNational Laboratory). Owing to the small a 75 - 90 keV alpha-recoil nucleus, which travels amount of material, samplemounts consistedof thin approximately20 nm, andresults in 1500- 2000atomic slurrieson a zero-backeroundouartz holder. METAMICT VESWIANTIE 359

Transmission electron micros c opy REsuLTsAND Dtscusstox

Vesuvianitesamples were prepared for TEM analysis Chernicalcompositions and BSEimaging usingthe crushed-grain method. Sample fragments were groundinto I - 5 pm powder and dispersedin acetone Results of chemical analysesof V53 and V73 are ontoholey-carbon Cu grids.One sample of V73 wasalso given in Table 1. Unit formulaewere recalculated on the preparedby ion-milling, whichpermitteddetailed analy- 6asis of 50 cations and were assignedto specific sisof in sit, features.Specimens were examined with a structuralsites, as recommended by Groatet al. (1992). JEOL2000FX TEM operatedat 200keV, usingstandard Himmelberg& Miller (1980)reported that vesuvian- axial illumination and phase-contrastimaging for high- ite from the SewardPeninsula is optically zoned,with resolutionwork and SAED. A preliminaryexamination an isotropic(1e., metamict) core and anisotropic(i.e.' of nonmetamictvesuvianite indicates that electron-beam crystalline) rim. No optical or chemical zoning was radiation damageis easily avoidedduring high-resolu- observedin sampleV53; however,BSE imaging of the tion imaging,thereby justifying the useof HRTEM for chemicallyanalyzed region (Fig. l) showsthree areas of the examinationof amorphizationfeafires. All SAED varying contrast(i.e., different averageatomic number). patternswere focused on the objectiveaperture (aligned Comp6sitionsV53-1 to V53-3 (Tablel) perain to the with theback focal plane)in orderto providea consistent light, medium and dark grey areasofFigure 1, respec- representationof broadeningof the diffraction maxima. tively (eachof theseareas has a unique composition). The different compositions,combined with differing Annealingand. therma gravimetric analysis BSE contrast" suggest that chemical alteration has occurred.The mediumgrey zonecontains no U, andless Annealing of TtsM-analyzedsamples was done in a sealed,inert, Ar atmospherefor 2 hours each,at 650, 700, 750, and 800'C with sampleV73, and at 650'C 'I. with sampleV53. Sampleswere crushedand encapsu- TABLE CHEMTCALCOMPGMON OF PARILY METAMPT VESIJVIANITE latedin gold envelopesfor heating,thus preventingany reactionwith the capsulewalls. Annealingof XRD-ana- g.O7 was doneusing a Deltechprogrammable SO2 {t7c 34.01 4.73 39.tts 34.SO 34.6 lyzed samples 16,f Al2Q 17,' 1259 rog/ t&05 laoS furnace.Isothermal annealing of sample,VT3was done IW 2&. 279 aa 1A 0.s2 045 1.70 MgO l.g) t.85 r.3r 221 lt8 400, 600, 800, 900 and l000oC, fgr 24 hours at each og at MnO 0,11 o10 ooo 0.68 0.41 further confirm the 7,4 6.@ 7.55 4@ 4Ag 4.1r temperature,in flowing nitrogen.To 3035 CsO 30,q1 3t.3 r6.dt 1t5.26 34.98 ot8 iderrtity of the decompositionproducts of vesuvianite Na, 0.30 0.36 0.16 00 0@ o@ LaZ9A 1'g7 o.g7 1.34 000 0@ crystallinereference samples, 0.@ above900oC, two highly Ce2Og zg 1.78 2A o(D om Przog 0,4 024 oa 0.00 co om one (deep coffee brown) from a skarn in Phillips, o@ Nd2Q O8a c46 0,7! o(I) o@ o0 Franklin County, Marne,and the other (yellow brown) ThO2 l.e 1.97 t.@ om o(D 0J3 UOz 038 0.m 0.34 o.@ o49 at 0.00 from Iake Jaco"Chihuahua. Mexico, were heated 9o3 0.15 o@ 0.13 0@ o@ F 1.76 1,54 1.OO ZQ a75 Z4s 1000'C in flowing Ar for 15 hours.The powdered 0.5S o 0.@ o.o0 o,oo o14 021 1.6 samplesrested on a platinumfoil to preventcontamina- H2O 1.71 1.94 -1.97 z@ t.73 tion and reactionwith the substrate. e€-4, s&34 85.59 Sg05 9&87 $.44 1.aE 1A 1.16 Thermogravimetricanalyses (TGA) of sampleV73 OEF,CI O.74 o05 o.e. g'@ gam s.gl and the two referencesamples were done at a heating TOrAL g/.€e 89,17 9r.64 rateof 10'C/minto 1000"C,in flowing argon,using a 1&17 1&12 23.55 17.54 1?.94 t&52 7f,4 7,72 8.e au e?8 Perkin-Elmer TGS-2 system. AP+ 7,1o (I39 rP+ l.oi t.@ 1.4S O4a 035 Mf+ t.€ 1.44 1.16 1.68 1'47 138 M;2+ 06 oo4 t).(p .oa o18 0.18 Infrared spectroscopy F&+ 3.s4 ze2 3.81 2@ 1,87 1.97 c€F+ 17.19 t7.s1 1Q74 18.@ l&s 17.68 Na+ o3l 036 o.19 06 016 o.l7 High-resolution infrared spectra of single-crystal ta3+ ozl o.17 o.30 oal, om o@ vesuvianitesamples were recordedon a Nicolet model co3+ o5o os4 (}40 0.@ oqt o@ PP+ oo4 (l(E 0(E 0.00 o(l, o@ 60 SX Fourier-transforminfrared spectrophotometer. Nds+ o.tg o(a ot7 o.ct) od) o@ oa org ozt om o@ cm Th4+ (l.@ Single crystalswere sawninto slices,anached to glass u4+ oo4 o@ o.o5 0'@ 06 slideswith Crystalbond@,ground to lffi pm thick and s8+ o(b odt oo8 0.@ om o@ polished on both sides.Spectra were taken through a F 2.5? 2,9 t.gl a85 4l}9 421 cr 0.@ oo o.(x) u.12 (It8 049 100-pm brass aperture that was positioned to avoid olf a08 a78 .5.51 ?.aD 6A1 974 cracksand visible inclusions.The specnalcontribution a, 1&69 18.64 l22l r&00 18.9t 17.81 't324 'la4l of the glass slide and Crystalbondwere subsequently fl la14 14.1S t&43r 13.tt z ta17 1&12 23.58 17ja 17.U la32 removedduring spectral processing, and spectra forboth sampleswere normalized to a thicknessof 100pm. rv6uvta$e @lys6 m mrmlzed m 50 btal a€doB HzO b @lqilalgd lot dEg€ b8b@ 360 THE CANADIAN MINERALOGIST

L4 Ce and Nd, than the other areas.The darkestzone hasa low oxidetotal (85.17Vo).Severe depletion of Ca is responsiblefor this low total,indicating that the dark region has sustainedappreciable chemical alteration. Overall, sampleV53 contains3.35 - 4.84 tat.Voof rare-earthelements, expressed as oxides(0.65 - 1.02 atomsper formulaunit, apfu), 1.07- 1.99 wt.VoTltO2 (0.13- O.27apta) and 0.0 - 0.36 wt.VoUO2 (0 - 0.05 aptu).Himmelberg & Miller (1980)found 0.72 - 2J0 wLVoTl'rO2and0.39 - 0.84wt.VoUO2in a samplefrom the same locality. The discrepancyin UlTh contents betweenthe valuesof this study and thoseof Himmel- berg& Miller (1980)is not surprising,because chemical alterationcan produce highly variableresults. Neverthe- less,these UlTh valuesare unusually high for vesuvianite. V53 alsocontains up to 0.36 wt.7oNa2O and7.93 wt.VoFeO1' these values also are high. BSE imaging and phaseidentification with energy- Ftc. 1. Back-scafteredelectron image of V53. Chemical dispersionspectrometry (EDS) indicate that sampleV73 alterationis represented by areasof differing contrast(1e., is a polycrystalline aggregateof vesuvianite(Fig. 2), different atomic mass);the light, medium grey and dark with minor inclusions of titanite, uraninite, quartz, areaseach have a different composition,corresponding to calcite, apatite,chlorite, magnetite, compositionsV53-1, V53-2 and V53-3, respectively,in barite and zircon. Table1. Textures seen in BSE and optically in thin section suggestthat these minerals crystallized at the sametime. As with sampleV53, areasof differing BSE contrast showa correlationwith their composition;however, the

Ftc.2. Back-scatteredelectron image ofv73. chemical alterationis evidentby the areasof differelt contrast.Regions labled as a, b, and c conespondto U-poor, U-rich, and altered-rimU-rich compositions(V73-1, y73-2,y73-3,respective1y, in Table 1). METAMICT VESUVIANNE 361

Frc. 3. Back-scatteredelectron image of V73. Sharply defined linear featureshave beenfilled in with titanite, uraninite, and vesuvianite.Their origin is uncertain.(a) A linearzone is offsetby laterfracnring. (b) Notethe symmetrical pattem of alteration. compositionaldifferences and the degreeof contrastare (Groat et al. 1992).Cell dimensionsfor V73 were not moresubtle in V73. In general,those regions with a high obtained,because ofthe high degreeofmetamictization. U content are usually metamict and have an altered border. Chemical analysesClable 1) were done on a Transmission electron micro s copy U-poor region,a U-rich region,and the alteredborder of theU-rich region, respectively. Regions ofdark contrast, SamplesV53 andV73 arenonunifonn in their degree labeledas (c) in Figure2 and V73-3 in Table l, are of radiationdamage from onecrushed-grain fragment to depletedin Ca. However,the U content of the altered the next.This is the resultof small-scalevariations in border (V73-3) has not changedappreciably from that concentrationsof radionuclides, as establishedwith of the associatedU-rich region(V73-2). EMPA. Each crushed fragment has, within itself, a A seriesof subparallel,linear featuresin V73 were uniform amountof radiationdamage, indicating that the observedusing BSE imaging.The linear featureshave size of similarly damagedregions is on the sameorder beenfractured and offset by later deformation(Fig. 3a), as that of the crushedgrains used for TEM work (l-5 and appear to be leached by fluids becauseof the pm). A micrograph of undamagedvesuvianite from symmetricallybanded pattern along their lengfh @g. Brooks Mountains, Alaska is shown in Figure 4 for 3b). Within thesefeatures are titanite, vesuvianiteand comparison with subsequentmicrographs of partly uraninite. Very straight edgessuggest that they were metamictvesuvianite. once crystallographicallyconrolled fractures.Consid- A seriesof micrographs(Fig. 5) illustrate the tull ering that BSE is a surfaceimaging technique, the linear rangeof alpha-recoildamage observed in sampleV53. featuresmust actually be planarin their true dimensions, Figure 5a shows a lattice image with fringes that are becauseit is unlikely that we havecross-cut an entireset largely continuous,but that have mottled contrastand of one-dimensionalfeatures along the planeof observa- one localized amorphousregion (indicated with an tion. This rulesout the possibilitythat they arefission arrow). The SAED pattern gives no hint of radiation tracks(also, they aremuch too long, >100 pm, andtheir damage,because of the small volume of aperiodic subparallelorientations do not resemblethe orientations material. The small amount of damageand lack of a of randomtracks found in other track-bearingphases, diffr.rseelecron-scattering halo in SAED arc typical of i.e.,zirconorapatite). Chemical leaching of the"planar" a low alpha-decaydose of lessthan 0.05 dpa (displace- featuresimpedes their positiveidentification. mentsper atom)(less than 100ppm). Low-dosedamage has previouslybeen examined in titanite with HRTEM Unit-cell dimcnsions (Eby & Ewing 1990). Calculationof dpa for both vesuvianitesamples is not possible,as the age of the The cell dimensions of V53 were determined:a chemicalalteration event(s) is unknown.An intermedi- 15.792(5)and c 11.993(8)A. fhe a cell dimensionis ate amountof damageis shown in Figure 5b. Alpha- large fqr vesuvianite;most sampleswith a larger than recoil cascaderegions are sufficiently numerousfor 15.62 A either contain boron or are partly metamict overlap,resulting in the formation of continuousamor- 362 THE CANADIAN MINERALOGIST

Fro.4. A high-resolutionelectron micrograph of nonmetamictvesuvianite in [ 1I 0] zone-axis orientation.

phousregions which separatelarge, isolatedregions of in Figure 5b, owing to the volumetric dominanceof crystallinematerial. A diffirse electron-scatteringhalo is amorphousmaterial. The single-crystalcomponent of visible with SAED. The diffraction maximaare broad- the SAED patternhas missingdiffraction maxima(not ened and have unequalintensities relative to those in due to extinctionby symmetry),and relative intensities Figure 5a. The appearanceof a diffuse halo coincides of the maxima vary considerably.The fully metamict with the onsetof discontinuityin the latticefringes, both stateis shownin Figure 5d. No lattice fringesare visible of which signif the beginning of overlap between in the phasecontrast image, and no diffraction maxima amorphousregions (adjacent amorphous regions expand are observedby SAED. Minute amountsof remaining and coalesce).This observationis consistentwith an crystalline matter may still exist, but are difficult to earlier study in which Ga2Ti2O7and CaZrTi2OTwere detect in SAED, since the strong intensity of the dopedwith Cm ($leberet al. 1986).The coalescence of electron-scatteringhalo obscuresthe visibility of any amorphousregions in theseoxide phasescoincides with weak diffraction-maximathat may be present. the appearanceof a diffuse halo in SAED. In the A progrcssivesequence of increasingradiation dam- advancedstate of radiationdamage (Fig. 5c), the small agein sampleV73 is shownin a seriesof SAED patterns crystalline regions that remain are surroundedby an (Fig. 6). Figure 6a hassharp diffraction maximaand no amorphousmatrix. Lattice fringes are barely visible halo, indicalive of little or no alpha-decaydamage. becauseof the large amountof imageinterference from Figure6b showsa single-crystalpattern with a superim- theamomhous material. The diffuse halo is stronserthan posedweak halo. The stateof damageis intermediate. METAMICT VESI.IVIAMIE 363

Frc.5. A seriesofelectron micrographs,each ofadifferentcrushed grain ofsample V53. (a) A low amountofalpha-recoil damage; SAED is a [001] zone-axispattem (ZAP). (b) A moderateamount of alpha-recoildamage; t00ll ZAP. (c) An advancedstate of alpha-decaydamage. (d) The fuUy meamict state.

An advancedstate of alpha-recoildamage is shown in gadolinite(Janeczek & Eby 1991);ii) the materialwas Figure 6c, in which only a few diffraction-maxima initially polycrystallineand has since sustained radiation remain and an intense halo is present. The grain damage.Either casewould resultin the superpositionof examinedin Figure 6d is a mixture of polycrystalline, a halo onto a diffraction-ring pattern. randomly oriented grains (producing the diffraction A further exampleof the heterogeneouslydistributed rings) andamorphous material (producing thehalo). The alpha-decaydamage is shownby in sin observationof diffraction rings wereindexed and march thed-values of adjacentregions that have variable damagewithin an vesuvianite.The polycrystalline state may be due to ion-milled single crystal (Frg. 7). Two regions of severalcauses; for example,i) high alpha-decaydoses vesuvianite,marked with arrows(Fig.7a), are separated leadto subgrainformation, as observed in ion-in"adiated by anothersilicate phase(confirmed by EDS analysis). neptunite (Eby et al. 1991) and naturally metamict At higher magnification, these two regions clearly 364 TTIE CANADIAN MINERALOGIST

Ftc. 6. A seriesof SAED of V73, with varying amountsof metamictization.(a) [01l] ZAP of seeminglyundamaged material. (b) An intermediateamount of alpha-recoildamage. (c) An advancedstate of alpha-decaydamage. (d) A polycrystallinegrain wift a significantamount ofdamage as well (notethe halo). Ring d-valuescan be indexedas belongingto vesuvianite.

Ftc. 7. (a) Electronmicrograph of an ion-milled single crystal of V73 in [0] 1] orientation.(b) A lattice imageof the arca in (a) nearthe left.arrow. (c) A lanice imageof the areain (a) nearthe right arrow. Note the differencein radiationdamage between (b) and (c). METAMICIVESWIANITE 365 exhibit different degreesof radiation damage;the area in the rate of weight loss in the TGA analysis, at in Figure 7b is undamagedand showsclean, unmottled approximately600'C @g. 9). After heating V53 to lattice fringes; the areain Figure 7c has mottled,partly 650oC,and V73 to 650"C and700oC (in argon),TEM discontinuouslaftice fringes with coalescedamorphous analysisshows no evidenceof recrystallization.Figure areas. lOa showsthat manyamorphous regions are still present in V73. Apparently,TEM is not sensitiveto the onsetof Annealing recrystatlization,which most likely involves a subtle increasein the long-rangeperiodicity ofthe structureas point-defectswithin )(RD analysesof V73, following eachheating inter- a result of annealingthe isolated (or more) sequence val of 200'C (in flowing nitrogen),indicate that recrys- lessdamaged regions. A two-stage Theseobservations and taltzation beginsat approximately600"C (Fig. 8). A few of recrystallizationis thuslikely. with similar work on of the smaller diffraction-peaksare more intense.This interpretations are consistent (l minor increasein crystallinity coincideswith anincrease allaniteby Janeczek& Eby 993). At 800"C, XRD patternsindicate a significant im- provement in the crystallinity of V73. Background intensity has decreased,and resolution of diffraction peakshas increased.The TGA curve (Fig. 9) flattens somewhatbetween830'C and900'C. Observation with TEM, after annealingto 750 and 800'C, indicatesthat much of the alpha-recoil damagehas been annealed g (there is no observabledifference in recrystallization features between these two temperatures).Lattice o fringes are continuous,but some grains still display o mottled contrastbetween fringes. The variation in the o state of repair between different grains presumably existsbecause the moredamaged areas require a higher activationenergy for recrystallization.Grains with little of newly crystal- L damagedo not require the nucleation o line material,and their localizedamorphous regions can li regrow epitactically at 800"C. The requirementof a : higher temperatureof annealing for more damaged o materialhas beenproposed for anothersilicate, titanite L (Hawthorneet al. l99l). The amorphousmaterial that o remainsin the more highly damagedareas implies that anotherthermal stage is necessaryfor full restorationof the structure. Anotherfeature appears at 750 and800'C. Recrystal- lization is not yet complete,and heatingof the material : with the electronbeam results in damageto someof the o grains.Figure 10bshows the effectsofbeam damage, E andshould not be mistakenfor naturalradiation damage, o most of which has been annealed.The beam-damaged fJ g&ins vibrate intensely.Grain E were also observedto - vibration was also observedduring annealingstudies of titaniteand gadolinite (Eby & Ewing 1990,Janeczek & Eby l99l), but wasnot previouslyreported. For melamictvesuvianite, three stages of weight loss canbe identifiedin theTGA curveGig. 9). Initially,the samplegradually dehydrates over the temperaturerange I 50- 600"C,losing I wt.7o. Then, the rate of weightloss 30 40 50 60 increasesover the interval 600 - 850'C, in which the sampleloses an additional2wt.Vo. Beyond 900'C, the 2-theta(') rate of weight loss increasesagain (an additional I until the final measuredtempera- Frc. 8a.Powder XRD pattem(CuKcl radiation) of sampleV73 wt.Vo),and continues as found and annealedin nitrogen(24 hourseach) at 400', ture.Further explanation ofthe stagesin weightloss can 6@", 800"and 900"C. The stick pattemat thebottom shows be made basedon the XRD patternsof isothermally the peakpositions and relative intensities typical ofcrys- annealed samples, the differential thermal analysis talline vesuvianite. (DTA) dataof Kononova(1960), and the TGA dataof 366 THE CANADIAN MINERALOGIST

b o 9q c J o o 2 No

5o I gehlenlle lll .= ,ll | ' r lJ I | | t l [r I an wollastonite tr o .,,,,,.,,,,1,t ! ..1 il .1, . , , n . I-. tr I tr 1 grossular I 'l l,lrr lr , l, I r, 20 40 50 60 2-theta (')

FIc 8b. PowderXRD pattem (Cu.Kcrradiation) of sampleV73 decomposedat 1000"C(24 houn in nitrogen),with the major productsof decompositionshown by stick pattems.

crystallinevesuvianite. Metamict vesuvianite begins to sular, gehleniteand wollastonite.The decomposition recrystallizeat 600'C (in N2orAr). At 900.Cand above, phasesare not unique to the peculiar chemistryof the therecrystallized vesuvianite decomposes. This is con- metamictvesuvianite, as two crystalsof highly crystal- sistentwith the DTA data which showa broadexotherm Iine vesuvianitefrom other localitiesproduce the same between600 and 800'C and an additional exotherm products.A nearlybalanced reaction converts vesuvian- beginningat 870'C. The productofdecomposition is a ite into 2 grossular + gehlenite + 2 wollastonite. multiphaseassemblage containing predominantly gros- Accordingto the TGA curvesof the two highly crystal-

10 crystalline vesuvianlte

t I o metamlct vesuvlanlte 3 I 97

0 200 400 600 800 1000 Temperature("C)

Ftc. 9. Thermogravimetricanalysis curves for v73 anda sampleof crystallinevesuvianite from Phillips,Maine, heated at 10oC/minin flowing argonto 1000.C. METAMICTVESWhNTTE 367

hasnot Frc. l0. (a) Electronmicrograph of V73 heatedto 650oCand corresponding [001] ZAP. Note that the alpha-recoildamage numerouscircularfeatures beenremoved. O) Electronmicrograph ofV73 heatedto 775oCand corresponding [l I l] ZAP. The arenot resultsof naturalradiation damage, but insteadare due to damagecaused by the electronbeam.

line samples,the reaction probably evolves (OH)' minerals.Thus, the similarity in the spectraof metamict beginningat 900'C (Fig. 9). minerals suggeststhat the distribution of major structural featuresof the aperiodic state (as probed by the Inf'raredspectroscory strengthofthe O-H bondand its correspondingrange.of nearestneighbors) is similar, even for materialswith The infrared spectraof fully crystalline samplesof different bulk-comPosition. vesuvianiteare typically rich in detail within the princi- Furthermore,the individual bandsare strongly aniso- palOH-stretching region of 3800-2500cm(Groatet al.' tropic, indicating prefenedcrystallographic orientation in prep.).In contrast,none ofthis detailremains in the of the individual OH environments. pri- spectraof the partly metamictvesuvianite. Spectra of The integratedintensity (Paterson1982) of the samplesV53 andV73 (Fig. l1) showa sharpincrease mary OH band at 3475 cm-t indicatesthat sampleV53 in absomtionat 3700cm-r, which reachesa maximum coniainshydrous components (OH andHrO) equivalent new 34i5 cm-l and gxaduallydecreases on the low-en- to 2.O wt% H2O, in reasonableagreement with the ergy sidedown to 2500cm-'. This type of absotption estimatedwater contents of Table l. The intensityof the cm-r also is observedin metamictzircon (Aines & Rossman H2O combinationmode (stretch+ bend) at 5200 in 1986)and titanite (Haw0rorne et al. l99l).However,the inlicates that only about 5Voof the total water is present OH. conespondingspectra of nonmetamictvesuvianite, t! molecularform asH2O. The remainderis as tanite and zircon are quite different from one another, The dominanceof OH was also observedin metamict owing to the major structuraldifferences between these zircon(Woodhead et al. 1991). 368 THE CANAD1AN MINERALOGIST 2.0 Vesuvianite

(l) 1.5 () trl (ll -o 1.0 o v) v73 € 0.5 v53 0.0 3500 3000 2500 Wavenumbers(cmr )

Ftc. I l. Single-crystalinfrared specraof V53 and V73 in theprincipalOH-strerching region. Both spectrashow the low-energytailing absorptioncommon in metamictminerals.

CoNcLUSIoNS alpha-recoildamage in minerals.Any attemptto calcu- latedpa in alteredmaterials is futile unlessthe timing of the alterationevent(s) is fully understood,because TEM analysesindicate that alpha-decaycan leave alterationalso disturbsthe structuralstate. vesuvianiteintact, or makeit partly metamict,or fully Alpha-decaydamage in vesuvianiteis annealedin at metamict,all within localized regions of the same least two different stages.The proposedstages are: i) sample.The variableconcentrations of radionuclides 600-750oC,the repairof point defects in partlymetamict thataccompany the micrometer-scale zones of chemical regions,which slightly improvesthe long-rangeperi- alteration(ascertained by BSE imagingand EMpA) are odicity (detectedby XRD andTGA, bur not TEM); ii) responsiblefor the different extentsof alpha-recoil 750-800"C,the full repair of alpha-decaydamage in damageobserved in bothsamples of thevesuvianite. The partly metamictareas (detected by )RD and TEM, but relativedifferences in radiationdamage were classified notdistinguished from thefirst stageby TGA); iii) above by observing:i) the variationof discontinuityin the 850oC,material that was highly metamictis not recrys- latticefringes; (ii) the proportionof aperiodicarea to tallized, but instead,vesuvianite decomposes into a lattice fringe area; iii) rhe intensity of the diffr:se multiphaseassemblage of grossular,gehlenite, and electron-scatteringhalo relative to the intensity of wollastonite.The high structural diffractionmaxima. and compositional complexityof vesuvianite,relative to mostother miner- Localized concentrationsof alpha-recoildamage als, may accountfor its high andbroad thermal range of indicate that U and Th are not uniformlv distributed recrystallizationas comparedto the temperatureranges throughoutthe specimens.The presenceol uraninitein of other silicates,such as allanite and gadolinite,at V73 indicatesan abundance ofU availableforproducing 600-700'C (Janeczek& Eby 1991),REE-bearing epi metamictization.Whether or not U was Dresentin dote,at 600"C(Proshchenko et al. 1987),and titanite, at vesuvianitebefore alteration is unknown.A conserva- 400-600'C (Eby & Ewing 1990). Although temperarure tive estimateof the UlTh contentsthat are necessaryto ranges for the recrystallizationof metamict minerals producethe observed variations in themetamict state car show some variation, the structural fea.turesof the be calculated.First, assume that 0.1 dparepresents the aperiodic state appearto be similar, at least as they lowest amount of alpha-recoildamage that can be pertainto the detailsof OH stretching. observed,and that at least I .0dpa is requiredfor thefully amorphousstate. Such a variationin dpa requiresa variation by one order of magnitudein U and Th ACKNOWLEDGEMENTS concentrations.EMPA analysesof the nvo samplesof vesuvianiteshow such a range,from lessthat 0.1, wt.Vo SampleV73 was collectedby a prospector,the late (i.e., below detectionlimits) to 2.33 wt.VoU and Th. Bob Beal" who then broughtthe sample1o Jack A. Therefore, chemical alteration can have a orofound Adams,formerly of the U.S. GeologicalSurvey, for effect on distributionof radionuclidesand subseouent identification.By XRD of tle as-foundand heated ME"TAMICTVESUVIANITE 369 sample,Adams identified the sample as metamict vesu- SHERRIFF,B.L., Flnnr, M.L. & McCAMltoN,C. (1991): vianite and later donated it to the Department of Alpha-decaydamage in titznite.Am. Mineral. 76, 370-376. Geology,UNM. RogerAines providedus with assis- HTMMELBERc,G.R. & MILLER,T.P. (1980): Uranium-and tancein preliminaryIR measurementsof sampleV53. thorium-richvesuvianite from theSeward Peninsula, Alas- Usefulcomments and suggestions were provided by Joe ka.Am. M ineral.65, 1020-1025. Blatz andFred Shmuck, who reviewedthe manuscript. NSERC of Canadaprovided assistancein the form of JANIECzEK,J. & Esv, R.K. (1991):Annealing of the radiation fellowshipsto RKE, operatinggrants to FCH andLAG, damagein gadolinite.Trans. Am. Geophys.Union72,554 and an infrastructuregrant to FCH. This work was also (abstr.;. supportedby Basic Energy Sciencesof DOE, grant #DE-FGo4-84ER45099to RCE.TEM wasdone in the - & - (1993):Annealing of radiationdamage in ElectronMicrobeam Analysis Facility of the Depart- allaniteand gadolinite. Piys. Chem.Minerals 19"3 43-356. ment of Geology, UNM, supportedin part by NSF, vesuvianite NASA. BES and the stateof New Mexico. Research KoNoNovA,V.A. (1960):On a metamictvariety of alkalinepegmatite in southwestTuva. Dokl. Acol. done at ORNL was supported by the Division of from an Sci. USS& Earth-Sci.Sect. 130, 129-132. Materials Sciences. DOE contract #DE-AC05- 84OR21400with Martin MariettaEnergy Systems, and LuMpKrN,G.R., Esv, R.K. & EwtNc,R.C. (1991): Alpha-recoil at Caltechby NSF grant#EAR-8916064. damage in titanite (CaTiSiO5): direct observation and annealingstudy using high-resolution transmission electron REFBRENCES microscopy.J. Mater. Res.6,560-564.

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