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. 4), but the preedgefeature for the -liZrO^). The detectionlevel for eachTi coordinationis -10 at7o. mixture is much broader.This difference favors the presenceof (right) Examplesof preedgefeatures measured for tarTi,t5rTi, and t5rTiin the metamict state of zirconolite and eliminates the pos- t6rTi,in comparison with a preedgefeature measuredfor a me- sibility of a mixture between tarTiand t6rTi. constant oxidation state and experimentalresolution, the (Speer and Gibbs 1976). Preedgeparameters for crystal- height and position of the preedgefeature are direct func- line zirconolite lie near the domain for t6lTi (especially tions of the degreeof p-d mixing, i.e., the geometryof the the preedgeheight). But the preedgeposition (4910.9 ! Tia* coordination environment. Three well-separateddo- 0. I eV) is low in comparison with all other t6rTi-bearing mains for the preedgeparameters can be identifiedfor t rTi, model compoundsstudied (average is 4971.4* 0.3 eV). tsiTi,and turTi,respectively (Table l). The preedgefeatures According to Figure 4, this preedgeposition may be due are found to have the greatestheight for t4rTi(e.g., as high to the presenceof some significant amount of low coor- as l00vo of the edge jump), whereas those for t6rTiare dination aroundTi (or1i, t'rTi, or both; in this case,XANES shifted to higher energiesin comparisonwith r4rTi(by -2 cannot clearly differentiate between t5rTiand tarTi,but, in r- 0.2 eV; Table l). Preedgeenergies and heights for t5rTi all cases,t6rTi is dominant). Indeed,crystalline zirconolite fall betweenthe values measuredfor the other two Ti co- contains one-third of the total Ti as t5rTi(the other two- ordinations.The use of both preedgeparameters is neces- thirds being turTi)(Gatehouse et al. 1981). sary to determinecoordination reliably from preedgefea- The Ti K-edge XANES spectra collected for the me- tures collected in compounds containing different Ti tamict zirconolite and titanite samples are identical to coordinations.For instance,the use of both parametersal- those previously published (Ewing et al. 1988; Haw- lows the distinction between tslTi and a 50:50 mixture of thorne et al. l99l). The preedgeinformation for the two I4rTiand t6rTi(Fig. 2). Finally, r4rTiand t5rTishow just one highly metamict zirconolite samplesare significantly dif- noticeablepreedge feature. In contrast,t6iTi is characterized ferent in comparison with their crystalline counterpart by the presenceof two to three preedgefeatures (the most (especially in position): This feature falls on the lower intense feature was used to measurethe preedgeheight, side of the domain for tstTi(Fig. a), suggestingthat some following Waychunas1987). significant amount of a lower Ti coordination is present in the two radiation-damagedsamples. Preedgefeatures for crystalline and metamict The preedgeparameters for the slightly damagedtitanite titanite and zirconolite from Ontario is shifted toward lower energiesthan that The Ti preedgeparameters for crystalline titanite (Ta- measuredfor its crystalline counterpartfrom Brazil (by 0.3 ble 1; Fig. 3) are typical of r6rTi(similar to those of an- -t- 0.1 eV). Also, the normalized height increasesslightly atase), in good agreement with crystal-structure data from 0.18 to0.27 + 0.05 (Figs. 3 and 4). In comparison FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE 4'7

B C TITANITE - (J z

-4. (h

E

r-'! N I 11.35 820392 Fl

-4.

z I 11.35 annealed 4965 4970 4975 5000 5050 5000 s050

ENERGY (eV) Frcunn 3. Ti K-edge XANES spectracollected for the four titanite samples(CaTiSiO,; left) and the three zirconolite samples (CaZrTirOr; right), showing variable degreesof aperiodicity: crystalline (Brazil), radiation damaged(Egansville and Ontario), and glassy (top spectrum).The loss of long- and medium-rangestructure broadens the two edge crests(B and C), whereasthe preedge (A) shifts to a position and a height that are typical of mixtures of t6rTiand t5rTi.Metamict titanite samplesare only partially metamict, whereasthe metamict zirconolite samplesare fully aperiodic. with crystalline titanite, the changein the preedgefeature Therefore,a preedgeposition at4970.5 * 0.1 eV is indic- is insignificant for the Egansville titanite. In contrast,the ative of the presencesf tsrli++in metamict zirconolite. preedge feature for the Ontario titanite is shifted toward The calculationof the amount of t5rTirelative to t6rTiin lower energiesby 0.3 + 0.1 eV and has a greaterheight these radiation-damagedsamples is difficult becausethe (by 9 t 5Vo)than crystalline titanite (see Table 1). preedgefeatures for both t5tTiand t6rTiin metamictzircon- olite are unknown. To get some indication of the amounts DrscussroN of both coordinationsin metamictzirconolite, we usedtwo The preedgeposition observedfor the two metamictzir- methods. The first method is based on Figure 4, which conolite samples(4910.5 -r 0.1 eV) is indicative of a va- shows preedge parametersdetermined for a variety of me- riety of mixtures between r5lTiand t6rTicontaining >507o chanicalmixtures of t5rTiand t6lTi,assuming average values t5lTi(see Fig. a). However, this preedgeposition can also for the preedge parametersfor these two coordinations be explained by a particular mixture of t41Tiand t6rTi(in (4970.6 and 4971.4 eV for the energy, 0.55 and 0.20 for the fixed proportions:5:95 + l; Fig. 4). However,it would the height, respectively).On the basis of this figure, the be an exceptionalcoincidence for both samplesto show preedgeheight and position for metamict zirconolite cor- this very particular mixture (most of flre t4rTi-t6rTimixtures respond to a 60:40 + 20 mixture betweenthese two co- have a much lower preedgeenergy, i.e., well below 4970.0 ordinations.This method has the advantageof being sim- eV). Also, a mixture of t4rTiand t6rTiwould broaden the ple and is based on a statistical approach of preedge preedgefeature significantly (see Fig. 2), which was not analysis. observed.A shift in the preedgeposition can also be at- There is another method for deriving the respective tributed to the presenceof Ti3* (Waychunas1987). How- amounts of t5rTi and r6rTiin metamict zirconolite that is ever,this is unlikely becauseionizing radiationswould be probably better adaptedto zirconolite compounds.We first expectedto oxidize radioactivematerials. Also, the pres- examine the caseof crystalline zirconolite,for which it is enceof Ti3* would be expectedto shift the entire XANES known from X-ray diffraction that there are three non- spectrumtoward lower energy, which was not observed. equivalentsites for Ti: One is occupiedby tstli in a trigonal 48 FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE

1.2 ing oxide model compoundsreported in the literatureshow Ti in a squarepyramidal environment).Thus, this method 3 1.0 is particularly well suited to zirconolite, in which t5rTiis located in an unusualtrigonal bipyramid. On the basis of e 0.8 these estimatesfor t5rTiand t6rTiin crystalline zirconolite, one can reconstructthe variation of preedgeparameters for t{ 0.6 mechanical mixture of these two particular coordina- N any Fl tions (Fig. 5, right). Using this model, we derived the rel- 3 o.l ative proportionsof tsrTiand t6rTiin the metamictcounter- I part. This calculation assumesthat prTi occurs as trigonal 2 o.t bipyramidsin metamictzirconolite (we demonstratebelow that this model is plausible,on the basis of EXAFS data).

0.0+T rtrrrl On the basis of Figure 5 (right side), the preedgeheight 4969 4970 4971 4972 measuredin the radiation-damagedsamples (0.36-0.39) ENERGY (eV) suggeststhat -80-90(*5)Vo of the total Ti is presentas prTi. This estimateis consistentwith that previously cal- Ftcuno4. Normalizedpreedge height vs. preedge energy po- culatedusing a less sophisticatedmethod (60 + 2lvo).For sition for the preedgefeatures at the Ti K edgein Ti-bearing t5rTi in metamict zir- modelcompounds (Thble 1) andin thefour titanitesamples (cir- both models, is a major coordination cles:1 : crystallinefrom Caperlibas,Brazil; ).: partiallydam- conolite, if not the dominant one. agedfrom Egansville,Ontario; 3 : partiallydamaged from an of unknownlocality, Ontario; 4 : glassysynthetic titanite) and the Evidencefor trigonal bipyramidal coordination tstfi threezirconolite samples (squares: I : annealed111.35;2 : in mstarnict zirconolite fully metamict111.35; 3 : fully metamict8-20392). The do- In oxide-type compounds, t5rTi4+is present mostly in mainsfor t4rTi,t5rTi, and t6rTi are shown, labeled [4], [5], and[6]. squarepyramidal coordination (fresnoite, lamprophyllite, The variationof the preedgeparameters for mixturesof coordi- innelite, KNaTiO., Ti-bearing glasses;see review in Far- (along paths nations the shownas solid lines) andthe preedge ges et al. 1996a). Trigonal bipyramids are documented parametersfor someparticular mixtures circles;see Farges [open essentiallyin crystallinezirconolite. Because of the close et al. (1996a)for detailslare alsoshown For instance,see the differencebetween the preedgeparameters for t5rTiand those of structural relationshipsbetween crystalline and metamict a 50:50mixture of I4rTiand I6rTi. zirconolite(see discussion in Fargeset al. 1993),trigonal bipyramids may also occur in the aperiodic regions of metamict zirconolite.This suggestionis consistentwith bipyramid, and two are occupiedby turli in octahedralge- the analysisof the EXAFS data collected at the Ti K edge ometry (Gatehouseet al. 1981; Mazzi and Munno 1983; in metamict "blomstrandine" 1: aeschynite-(Y), or Cheary 1992).We previously found distinct rangesof val- (Ce,Ca,Fe,Th,Y)(Ti,Nb),(O,OH)u;Greegor et al. 1984a, ues for the preedgeenergy (4970.6 + 0.1 and 4971.4 -r 1984b1.These compoundshave preedge and XANES 0.3 eV) and height (0.55 and 0.20 * 0.15) for r5rTiand spectraat the Ti K edgethat are very similar to that col- t6rTiin a variety of model compounds(Table 1). Also, the lectedfor the fully metamict 111.35zirconolite (see Ew- preedgeinformation for eachcoordination shows relatively ing et al. 1988).Therefore, the previousdiscussion about moderatevariations: t0.1 (tsr1i;to -r0.3 eV (tor1i;for the the importance of t5rTiin metamict zirconolite may also energy and +0.15 for the height, respectively (see Table apply to the metamict titanium oxides. Also, in metamict 1). On the basisofthese rangesofvalues, the contributions blomstrandine,Greegor et al. (1984b) f,tted the EXAFS- arising from eachTi coordinationpresent in crystalline zir- derived Ti-O contribution with four shells of Ti-O dis- conolite (e.g., t.r1i arld r6rTiin the proportion l'.2) can be tancescentered around -1.81(2) (2.2 O atomsaround Ti), estimatedrelatively accuratelyby interpolation.During the l.9l (2.8 O atoms),2.01(0.3 O atoms),and2.16 A(0.2 calculation, the preedge positions for both coordinations O atoms).This set of Ti-O distancesaverages 1.89 A, were fixed at the averagevalues for IsrTiand t6tTi(49'70.6 which is typical for t5rTi (the predicted average tsrTi-O and 4911.4 eV, respectively) because these parameters distance,on the basisof ionic radii, is I .91 A). This result show the smallest variations. In contrast, the preedge confirms that t5rTi can be dominant in these metamict heights were allowed to vary within their respectivelimits minerals,in good agreementwith the preedgeparameters. (0.4-0.7 and 0.05-0.35) until the preedgefeature for the In addition, this Ti-O distancedistribution is inconsistent mixture (one-third t51Tiand two-thirds ru[i) fitted the mea- with the squarepyramidal geometry, which is character- suredpreedge spectrum for crystalline zirconolite (Fig. 5, ized by one short Ti:O double bond (bond length be- left). The refined preedge heights for rsrTi and t6tTi are tween 1.69 and 1.78A) and four longer Ti-O bonds(be- -0.45 (suchas in RbrtsrlioOn)and 0.13 + 0.05 (suchas in tween 1.9 and 2.0 A). In contrast, the Ti-O distance Car6rTiO:perovskite), respectively. The preedgeparameters distribution in metamict blomstrandine is not far from determinedfor the nonequivalentsite occupiedby tstfi in that known for the trigonal bipyramids around PrTi in crystalline zirconolite are the first estimatesfor ptTi in a crystallinezirconolite (2 x O atl.l9, I x O at 1.81, I trigonal bipyramidal geometry (indeed,all other t5rTi-bear- x O at 2.19,and I x O at 2.25A; Gatehouseet al. 1981). FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE 49

U? tsl fi: t6l Ti Fi IF !!s \J -l rv'l |'r - ^d? Eio N rl or

Fl. zo

4969 4971 4973 4969 497L 4973 ENERGY (eV) Frcunp 5. (left) The best model of mixture betweenf5rTi and preedgeposition fixed at 4970.6 eY) has a height of -045, r6rTiobtained for the most intensepreedge feature in crystalline whereas that for t6rTi(open squares,preedge position fixed at zirconolite (one site for t5rTiand two sites forturTi; solid circles). 49'11.4eY) is -0.13. (right) On the basisof the two preedges The spectrumshown for crystalline zirconolite correspondsto a determined in the figure on the left, preedge parameterswere Lorentzian simulating the most intense feature observed in the calculated for all mixtures between 10OVoFtTi (mixture 100:0) experimental preedge spectrum for crystalline zirconolite (see and lOOVorurTi(mixture 0:100). According to that particularmod- text; this protocol minimizes the influence of the other preedge el, the preedgeparameters for the two metamict zirconolite sam- componentsobservable in the experimentalspectrum for zircon- ples suggestthat >80-907o of Ti in these minerals is presentas olite; see Fig. 3). The calculatedpreedge for t5rTi(open circles, rsrTi.

A very similar conclusion can be drawn for metamict Ti K-edge EXAFS spectroscopydata for a metamict pyrochlore(XAFS data in Ewing et al. 1988).Thus, ac- titanite from the Cardiff mine (Ontario) has suggested-the cording to thesemodels, t5rTi4+is probably located within appearanceof 0.2 O-atom first neighborsnear l.8l A in trigonal bipyramids in metamict titanium oxides (zircon- comparison with crystalline titanite (Hawthorne et al. olite, aeschynite,and pyrochlore). 1991).This result is interpretedas indicativeof the pres- enceof minor amountsof r4rTi(e.g., -0.2 atomsor -37o tstTi Evidencefor some in metamict titanite? of the total Ti). For such a mixture, the preedgefeature The preedgeparameters for metamict titanite lie within should be broader and centered near 4910.8 * 0.1 eV the domain of values for t6rTi,suggesting the presenceof which was not observed (Fig. a). A Ti-O contribution significantamounts of t6rTiin these samples.In glassy near l.8l A can also result from rsrTi(a possibleshorl titanite, the preedge feature suggestsmajor amounts of Ti-O distancearound Ti'*O. trigonal bipyramids).EXAFS t'rTi (but as squarepyramids; Farges et al. 1996b).In data for highly damagedtitanite sampleswould give more comparison with crystalline titanite, the preedge param- reliable conclusions (unfortunately, such samples were eters for the metamict samplesare shifted in a direction not available for this study). (lower energy, greater height) that suggestseither the presenceof a more distorted t6rTisite (Waychunas1987) Implications for the coordination chemistry of Ti in (as in neptunite; see Fig. 1 and Table l), a greater pro- aperiodic materials portion of tsrTi,or both. With the current data, it is diffi- Fivefold-coordinatedTi4* should now be recognizedas cult to distinguishbetween the two models.However, in- an importantcoordination in radiation-damagedoxides and creasing metamictization in titanite shifts the preedge silicates.Examination of the preedgeparameters at the Ti feature in a similar way to that observed for zirconolite, K edge in a variety of highly metamict titanium-niobium- suggestingthat radiation damagealso favors the presence tantalumoxides such as euxenite,aeschynite, betafite, and of t5rTi.If this model is correct, the amount of t5rTi(over pyrochlore (Greegor et al. 1984a, 1984b; Ewing et al. t6rTi)increases clearly with increasingradiation dose (e.g., 1988) suggeststhat the coordination chemistry of Ti in in the order Caperlibas < Egansville < Ontario << thesehighly metamict minerals is similar to that observed B-20392< 111.35). in the two fully metamictzirconolite samplesstudied here. 50 FARGES: Ti COORDINATION IN ZIRCONOLITE AND TITANITE

The height of the preedgefeatures of these complex me- Ewing, R.C , Chakoumakos,B C , Lumpkin, G R., Murakami, T,, Greegor, tamict oxides (especially pyrochlore and zirconolite) is RB, and Lytle, EW. (1988)Metamict minerals: Natural analogues for in nuclear waste forms Nuclear In- much greater radiation damageeffects ceramic than that of their crystalline counterparts. strumentsand Methods in PhysicsResearch B,32, 48'7-49'1 More important, the preedge positions for the metamict Ewing, R.C , Weber,W.J., and Clinard, FW., Jr. (1995) Radiation effects modifications are all shifted toward lower energies (by in nuclear waste forms for high-level radioactive waste Progressin -0.5-1 eV, as for metamict zirconolite). These changes Nuclear Energy, 29, 63-121 can be attributedto the presenceof major amountsof ptTi. Farges, F, Ewing, R.C , and Brown, G.E., Jr (1993) The sffuchrre of metamict, aperiodic(Ca,Th).ZrrTi,O,o Journal of Material Research,8, This analysisof the preedgeparameters is then consistent 1983-1995. with the EXAFS data for metamict pyrochlore (Greegor et Farges,F, Brown, G.E., Jr., and Rehr,J.J. (1996a)Coordination chemistry al. 1984a,1984b;Ewing er al. 1988),which can also be of Ti(IV) in silicate glassesand melts: L XAFS srudy of titanium co- explainedby the presenceof t5rTi4+as trigonal bipyramids. ordination in oxide model compounds Geochirnicaet Cosmochimica Here again, large amountsfor tsrTiare found in the most Acta, 60, 3023-3038 Farges,E, Brown, G E, Jn, Navrotsky,A , Gan,H., andRehr, J J (1996b) radiation-damagedsamples. Coordination chemistry of Ti(IV) in silicate glasses and melts: III The change in coordination number of Ti from five to Glassesand melts from ambient to high temperaturesGeochimica et six that is induced by a-recoil damagein severalTi-bear- CosmochimicaActa. 60. 3055-3065. ing minerals must dramatically affect the local structure Fleet, M.E, and Henderson,G S (1985) Radiation damagein natural ti- around Ti in these radiation-damagedmaterials. In con- tanite by crystal structureanalysis Material ResearchSociety Sympo- Proceedings, trast, sium 50, 363-370 the coordinationsof Zt't and Th4t in metamict zir- Gatehouse,BM, Grey, I.E, Hill, RJ., and Rossell,HJ (1981)Zircon- conolite (sevenfold and eightfold, respectively) are little olite, CaZr,Ti.-,O,: Structure refinementsfor near-endmember com- affected by radiation damage (Farges et al. 1993). This positionswith .r : 0 85 and 1.3 Acta Crystallographica,B37 ,306-312 comparison shows some structural similarities between Greegor,R.B., Lytle, FW., Ewing, R C., and Haaker,R.E (1984a) Ti-site the crystalline and the metamict modifications (Zr, Th) geometryin metamict,annealed and syntheticcomplex Ti-Nb-Ta oxides absorptionspectroscopy Nuclear Instrumentsand Methodsin but also emphasizes (Ti). by X-ray some fundamentaldifferences PhysicsResearch B, 1, 587-594 Finally, the metamict titanium oxides may be an ex- -(1984b) EXAFS/XANES studies of metamict minerals In K.0 ample of a rare class of minerals in which trigonal pyra- Hogdson, B Hedman, and J.E. Penner-Hahn,Eds., EXAFS and near- mids around Tio* are present,as in crystalline zirconolite edge structureIII, vol 2, p.343-348. Springer Verlag, New York. (Gatehouseet al. 1981).This environmentis much differ- Hawthome, EC , Groat, L.A, Raudsepp,M., Ball, N A., Kimata, M , ent from that Spike,FD., Gaba,R, Halden,NM, Lumpkin,GR, Ewing, RC, determined for a variety of Ti-bearing oxide B Lytle, FW., T.S Rossman, R, Wicks, FJ Ra- tslTi4+ Greegor,R , Ercit, , G , glassesand melts, in which is also found but as mik, R.A., Sheniff, B.L., Fleet,M.E, and McCammon,C. (1991)Al- squarepyramids (see Farges et al. 1996b).Because glasses pha-decaydamage in titanite. American Mineralogist, 76,370-396. are quenchedfrom high+emperaturemelts, square pyra- Higgins, JB, and Ribbe, PH (1976) The crystal chemistryand space midal Ti4* may be a high-temperaturespecies that could groups of natural and synthetic titanites American Mineralogist, 61, be metastableat room temperaturein the glassy state.In 878-888. Lumpkin, G.R., Ewing, RC, Chakoumakos,BC, Greegor,R.B., Lytle, contrast,radiation damageoccurs at much lower temper- EW.,Foltyn, E M, Clinard,FW, Jr, Boatner,L A, andAbraham, M M afuresand promoteshighly disorderedstructures with com- (1986) Alpha-recoil damagein zirconolite (CaZr'ti,O,). Journal of Ma- plex medium-rangeenvironments. This observation con- terial Research.1. 564-576 trasts with that for titanosilicate glasses, in which a Lumpkin, G R , Eby, R K., and Ewing, R C (1991) Alpha-recoil damage (CaTiSiO.): and annealing study using relatively more orderedmedium-range structure can be de- in titanite Direct observation high resolution transmissionelectron microscopy Journal of Material tectedaround Ti (seeFarges et al. 1996b).Therefore, these Research.6.560-564. high-resolution Ti K-edge XANES data provide another Lutze, W., and Ewing, RC (1988) Sumary and evaluationof nuclear exampleof stnrcturalsimilarities between various types of waste forms In W Lutze and R C Ewing, Eds , Radioactive waste aperiodicmaterials but also highlight somedifferences be- forms for the future, p.7O0-740. Elsevier,Amsterdam. (1983) (new tween the glassy and the metamict states. Mazzi,F., and Munno, R Calciobetafite mineral of the pyr- ochlore group) and related minerals from Campi Flegrei, Italy, crystal polymignyte pyrochlore AcxNowr,nocMENTS structureof and zirkelite: Compmisonwith and zirconolite American Mineralogist, 68, 262-27 6 The author is indebtedto H.J. Schubnel,R.C. Ewing, A. Voileau, J.S. Ringwood,AE, Kesson,SE., Reeve,KD, Levins,DM, and Ramm, White, and G.E Brown Jr for donating the samplesused in this study, EJ. (1988)Synroc (for radwastesolidification). In W. Lutze and RC and to the staff of LURE for their help in data collection. R C. Ewing and Ewing, Eds.,Radioactive waste forms for the future, p. 233-334 North- an anonymous reviewer are thanked for their helpful reviews of this Holland, Amsterdam. manuscrlpt. Speer,J A., and Gibbs, G V (1976) The crystal structureof syntheticti- tanite, CaTiOSiO,, and the domain texturesof natural titanites Amer- RnrnnencBs crrnD ican Mineralogist, 61, 238-247 Vance,ER, and Metson,JB. (1985)Radiation damage in naturaltitan- Cheary,R.W. (1992) Zirconolite CaZr from 294 to I 173 K on,Ti, o"O, Jour- ites Physicsand Chemistry of Minerals, 12,255-260. nal of Solid StateChemistry, 98,323-329. Waychunas,GA (1987) Synchrononradiation XANES spectroscopyofTi Ewing, R C , Haaker,R F, Headley, TJ., and Hlava, PE (1982) Zircon- in minerals: Effects of Ti bonding distances,Ti valence, and site geometry olites from Sri-Lanka, South Africa and Brazil In S V. Topp, Ed., Sci- '72,89-101. on absorptionedge strucnre AmericanMineralogist, entific basis for nuclear waste management,vol. 6, p 249-256 North Holland, New York Ewing, R C, Chakoumakos,B C, Lumpkin, G R, and Murakami,T. (1987) MeNuscnrm RECEryEDJlm,nny 30, 1996 The metamictstate. Material ResearchSocietv Bulletin. 12.58-66 Mrxuscnrn AccEroD Semvsen 19, 1996