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Fivefold-Coordinated Ti4* in Metamict Zirconolite and Titanite

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Fivefold-Coordinated Ti4* in Metamict Zirconolite and Titanite American Mineralogist, Volume 82, pages 44-50, 1997 Fivefold-coordinatedTi4* in metamict zirconolite and titanite: A new occurrenceshown by Ti K-edgeXANES spectroscopy FneNqorsFlncns Laboratoire de physique et m6caniquedes g6omat6riaux,Universit6 de Marne-la-vall6e,URA CNRS 734 and LURE (and Stanford University), 2 rue de la butte vette,93166 Noisy 1eGrand cedex, France AssrRAcr The coordination environmentsof Ti in two fully metamict zirconolite samplesand two partially metamict titanite sampleswere determined using high-resolution, X-ray absorp- tion near-edgestructure (XANES) spectroscopyat the Ti K edge. Fivefold-coordinatedTi is the dominant Ti speciesin the zirconolite samples(-80 t lU%oof the total Ti atoms). This unusualTi coordinationis also possiblein the titanite samples.No signif,cantevidence for I41Tiwas found in any of the samplesstudied. Comparison with other amorphous materials, such as other metamict minerals (aes- chynite and pyrochlore) and titanosilicate glassesand melts, suggeststhat fivefold coor- dination is rather common for Ti4* in aperiodic structures.However, the metamict stateis characterizedby the presenceof unusual trigonal bipyramids around t5rTi4*. INrnooucrroN terpretationfor the coordination of Ti in these metamict Titanite and zirconolite (ideally CaTiSiO, and Ca- phasesis proposed,which suggests,for the flrst time, the t5rTi ZrTi.Or, respectively) are important phases in various presenceof major amounts of in the radiation-dam- (and crystalline titanate-phaseassemblages proposed as nucle- aged portions of zirconolite possibly of titanite) and ar wasteforms (seeLutze and Ewing 1988;Ringwood et in severalother complex metamictoxide minerals,such as al. 1988).These phases can host significantamounts of aeschynite,pyrochlore, and euxenite. high-activity radionuclidesand, consequently,are subject ExppnrrrnNrAr, METTToDS to radiationdamage (Ewing et al. 1987, 1995). Metamict minerals such as zirconolite and titanite can Samplesstudied be used to investigate the effect of radiation damage in Two natural samples of zirconolite from Sri Lanka thesephases (Higgins and Ribbe 1976;Ewingetal.1982; were usedin the presentstudy (no. 111.35from the Mu- Fleet and Henderson 1985; Vance and Metson 1985; seum National d'Histoire Naturelle. Paris: no. 8-20392 Lumpkin et al. 1986,1991; Ewing et al. 1988;Hawthorne from the National Museum of Natural History, Washing- et al. l99l). Indeed,some natural actinide-bearing titanite ton, DC). Theseminerals have been describedin detail and zirconolite becomeprogressively amorphous to X-ray elsewhere (Ewing et al. 1982; Lumpkin et al. 1986). diffraction because of an increasing amount of defects These two Sri Lankan zirconolite specimens are black that are mainly related to the o-recoil atoms, displacing and fully amorphous in X-ray diffraction (XRD). They up to several thousandsof atoms for each cr-decayevent have received an accumulatedradiation dose ofup to 10'z6 (Ewing et al. 1987,1995). a-decayevents/m3 over -550 m.y. [equivalentto about To investigatethe effects of radiation damage on the two displacementsper atom (dpa)1.One chip of sample structureof metamict minerals,extended X-ray absorption 111.35was annealedat 1100'C for 4 h in a platinum fine-structure (EXAFS) spectroscopyhas been used to crucible in air, resulting in the crystallization of mono- probe the coordination environment around a variety of clinic zirconolite. These three zirconolite samples have elements,such as Ti (Greegoret al. 1984a,1984b; Lump- also been studied at the Zr K, Th lar, and U 2., edges kin et al. 1986;Ewing et al. 1988;Hawthorne et al. 1991). (Fargeset al. 1993). In these studies, a decreasein the average Ti-O bond Thrce titanite sampleswere selected(all from the Stan- length between the crystalline and the metamict modifi- ford University mineral collection). One is a crystalline, cationswas attributedto the presenceof minor amountsof olive green sampleof gem quality from Caperlibas,Minas t4rTiin the lafter form. A complementaryhigh-resolution, Gerais,Brazil (sampleno. 51,938).The other two are par- X-ray absorptionnear-edge structure (XANES) spectros- tially metamict: a slightly damagedtitanite from Egans- copy study at the Ti K edge in crystalline and metamict ville, Ontario, Canada(referred to here as Egansville; no. titanite and zirconolite is presentedhere. The advantageof 6,620), and a more damaged sample from an unknown the XANES method is its much greater sensitivity to the locality in Ontario (referredto here as Ontario; no. 6,310). coordinationenvironment of Ti. Indeed, an alternativein- The latter two titanite specimensare black and opaquewith o003404x/97l0102-0044$05.00 44 FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE 45 Taele 1. Preedgeparameters for selectedTi-bearing model meallic Ti Ti(O) compoundsand crystallineand metamictzirconolite o and titanitesamoles K{rzOt z JE B-BarTiO. X Avg Ti Preedgefeature Niru'Iioro, 5 coordi- q nation Position Normalized KNaTiO3 I (10.1) (eV) height- barrtsl ,ua a Kifr rO, 3 Ni26Tio7O4 (synthetic) 4 4969.7(1) 0 94(5) N a'-Ba"TiOo(synthetic) 4 4969.5 100 j neptuite .5 GTi,O? (synthetic) 4 4969.7 0.93 ua@se x YrTiMo06(synthetic) 4 4969.9 074 Entrotre Rb,TiO3(synthetic) 4 4969.6 100 z KNaTiOo(synthetic) 5 4970.6 073 Rb2Ti4Oe(synthetic) 5 4970.5 047 K"Ti,O, (synthetic) 5 4970.6 051 Fresnoite(synthetic) 5 4970.6 0.65 4960 4970 4980 4950 Ba"TiGerO"(synthetic) 5 4970.6 uoc ENERGY(eV) Rutile (Brittany) 6 4971.6 022 Anatase(synthetic) 6 4971.4 017 Frcunr 1. Ti K-edge XANES specffa collected for various Perovskite(synthetic) 6 4971.4 011 model compounds.From top to bottom: metallic Ti (O) and com- Benitoite(California) 6 4971.2 004 Neptunite(Caliiornia) 6 4971.1 032 pounds in which Tia* is fourfold, fivefold, and sixfold coordi- llmenite(Brittany) 6 4971.7 022 nated by O. The maximum of the preedgefeature for metallic Ti TiZO4 (synthetic) 6 4971.2 021 is used for monochromatorenergy calibration (maximum set at Titanite 4966.8 ev). Caperlibas(Brazil) 6 4971.4 018 Egansville(Canada) 5.9-' 4971.3 022 Ontario(Canada) 5.7* 4971.1 027 Glassy(synthetic) 5.3* 4970.7 050 XANES spectroscopy Zirconolile To calibrateTi K-edge XANES spectrafor the metamict 11135 (annealed) 57 4970.9 017 t4rTi,t5rTi, 11 1 35 (Sri Lanka) 51* 4970.4 039 minerals, many model compounds containing B 20392 (Sri Lanka) 5 2" 4970.5 0.36 and I6rTiwere investigated(a selectlisting of them is given " UsingSi(31 1) on EXAFS4beamstation (LURE) The full listot model in Thble 1). Detailed information about their structuresand compoundsstudied is in Fargeset al. (1996a) (Farges -- origins (or synthesis) is given elsewhere et al. Estimatedaverage Ti coordinationbased on crystallinezirconolite (see Fig 5) 1996a). Ti K-edge XANES spectra were collected at the LURE facility (Laboratoire pour l'Utilisation du Rayon- nementElectromagn6tique, Orsay, France).The DCI stor- age ring was operatedat 1.8 GeV and 10G-300mA pos- itron current.XANES data were collectedat the Ti K edge conchoidalcleavage. The XRD spectrafor the two mod- (4966 eY) using a high-resolution transmission mode erately radiation-damagedtitanite samples still show a tS(31l) double-crystalmonochromator and 0.3 mm vertical Bragg component(seven diffraction peaksobserved for 20 slit before the monochromatorl on beam station EXAFS4. values between 20 and 60' in the Egansville titanite, but The maximum of the preedge for metallic titanium was only three lines measuredfor the Ontario titanite). How- used for monochromatorenergy calibration following in- ever, the X-ray diffuse-scatteringcomponent of the XRD ternationalrecommendations (maximum set at 4966.8 eY: patternfor thesesamples is significant:The intensity of the seeFig. l). Energy resolutionwas estimatedto be -1.2 -40 311 line normalizedto that for the diffuse scatteringis eV at 5 keV in these conditions (see details in Fargeset and 10 for the Egansville and Ontario titanite samples, al. 1996a). Preedge parameters (absolute position and respectively(-70 for Caperlibastitanite). The samplesare height relative to the edge jump; Table 1) was extracted similar to thosefrom other localities in Ontario (e.g.,from from the normalized spectraby fitting Lorentzian profiles the Cardiff mine), which are describedin detail by Fleet to these features. and Henderson(1985), Hawthorne et al. (1991), and Lumpkin et al. (1991). These titanite samplesare mostly RBsur-rs from the Grenville Province of the CanadianShield (-950 Preedgefeatures for model compounds m.y. old). They have a low actinide content (ThO, + UO, The Ti K-edge XANES spectracollected for oxide-type - -0.3 0.1 wtvo), which producesa total radioactivity of model compounds show some preedge features, located -2-3 mrad/s (calculateddoses of x 10,5ct-decay events/ -18 eV beforethe main-edgecrest (Fig. 1). Thesefeatures -0.4-0.1 rrf; dpa; seeHawthorne et al. 1991;Lumpkin et are commonly attributed to electronic transitions from ls al. 1991).Finally, a glass of titanite composition(referred energy levels of Ti to the Ti 3d or O 2p molecularorbitals to as glassy titanite) was synthesizedby melting a sample (Waychunas1987; Fargeset al. 1996a).A ls -+ 3d tran- of Caperlibas titanite at 1700 K for 4 h in a platinum sition is forbidden by dipole selectionrules but is allowed crucible and quenchingthe melt by placing the bottom of when p-d orbital mixing occurs,such as when Ti is located the crucible in water. in a noncentrosymmetricsite (e.g., a TiO, tetrahedron).At 46 FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE + 100:0 v eo: 1o lsl Ti: t6l Ti x 50:50 mrxtures o l0: 90 Er o 0:100 o ta,fi 6i o & to tr go lvEo I4l Ti ooo N +tr oo o F1 ooo 90?o [6t Ti " 3 oo r tr_o z tr 4968 4970 4972 ENERGY (eV) Frcunr 2. Strengthsand limitations of the protocol used in chanical mixture of -10 atVotarTi and -90 at%oof t6rTi.Note that this study. (left) Measured variation of the preedgefeature for the preedgefor the mixture has an energy po'sitionthat is close severalmechanical mixtures of prTi (as Ba,TiGerOr) and r6rTi(as to that for I5rTi(see also Fig.
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