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Metamict and Chemically Altered Vesuvianite

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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, One RobertsonDrive, 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 vesu- vianite 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
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