American Mineralogist, Volume 81, pages 595-602, 1996

A synchrotronradiation, HRTEM, X-ray powder diffraction, and Raman spectroscopicstudy of malayaite, CaSnSiOt

Lnn A. Gnoerrl SrnrlN Knn 2 Ur,nrcn Brstv.lyonr3 Clnuorl Scunnrotra H,lNs Gnonc Kru.Nnr5 HrNnrcn MEyERr3 LnoN,l NrsroRr6 lNo Gusralr VaN TrNonr,oo6

'Department of Geological Sciences,University of British Columbia, Vancouver, British ColumbiaY6T lZ4, Canada 'zFRKristallographie, Universidt des Saarlandes,D-66041 Saarbriicken,Germany 3Mineralogisch-PetrographischesInstitut, Universitlit Hamburg, D-20I46 Hamburg, Germany 4lnstitut {iir Mineralogie, Universitiit, Welfengarten l, D-30060 Hanover, Germany 5lnstitut fiir Kristallographie, Universidt, Kaiserstrasse12, D-76128 Karlsruhe, Germany 6Universiteit Antwerpen (RUCA), Groenenborgerlaan17 I , B-2020 Antwerp, Belgium

AssrRAcr Synchrotron radiation, high-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction, and Raman spectroscopywere used to study the structure and thermal behavior of malayaite, CaSnSiOr. No indications of deviation from A2/a sym- metry and no structural transitions were observed between 100 and 870 K. HRTEM revealed that the material is free of domains and antiphase boundaries. However, the lattice constants,cell volume, and Raman-active phonons show a thermal discontinuity near 500 K, which is possibly related to variation of the coordination spherearound the highly anisotropic Ca position.

IxrnooucrroN rameter (Saljeet al. 1993a).In perfect agreementwith observationsby Zhang et al. (1995), Kek et al. (199a) Malayaite, CaSnSiOr,is a rare mineral found in skarn recently showed that in synthetic between 850 deposits.The structure of malayaite was solved by Hig- and 496 K the Ca atoms are displaced parallel to a and gins and Ribbe (1977) in spacegroup A2/a and was re- c, so that the true symmetry of the intermediate regime fined to R: 4.'lo/ofrom data collectedwith conventional is P2r/n. However,the averagesymmetry is A2/a because Zr-fltered Mo radiation. They showed that the structure of antiphasedomains. consistsofcorner-sharing SnOu polyhedra that form chains The situation in natural titanite is more complex. Hig- parallel to a. These are linked to sevenfold-coordinated gins and Ribbe ( I 976) found that in natural samples,both Ca atoms by SiOu tetrahedra. Malayaite is isostructural Al3* and Fe3+substitute for Ti. At levels of substitution with titanite, CaTiSiOr. Takenouchi(1971) showedthat greater than approximately 5 molo/o(Al + Fe) titanite there is complete solid solution between the two phases showsdiffuse k + l: 2n + | reflections,which disappear at elevatedtemperatures. The solvus is asymmetric, with at approximately 15-20 molo/o(Al + Fe). Higgins and a maximum of 888 + l5 K at a compositionof TiorrSnorr. Ribbe (1976) suggestedthat octahedralsites containing Synthetictitanite undergoesan antiferrodistortive lran- Al or Fe serye as boundaries on either side of domains sition at 496 K from a monoclinic high-temperaturephase of Ti octahedra with atoms displaced in opposite direc- (A2/a) to a monoclinic low-temperaturephase (P2r/a). tions. The greater the amount of substitution, the more As describedby Taylor and Brown (1976)and Ghoseet abundant are the domains, which results in more ditruse al. (1991), the transformation in titanite involves dis- k+l:2n+lreflections. placement of the Ti atoms from the centers of the TiOu In their study of the malayaite structure Higgins and octahedraparallel to the a (relatively large shift), b, and Ribbe (1977)found no evidenceofviolating reflections, c axes (both relatively small shifts). This displacement even on veryJong-exposureprecession photographs. They pattern leads to antiparallel sublattices of octahedral suggestedthat the formation of domains leading to an chainsbecause within individual chainsthe Ti atoms shift averageA2/a symmetry in titanite is dependent on the in the samedirection. This transition in synthetic titanite substitution of cations smaller than Ti. They proposed has been studied in great detail becauseofthe antiferroel- that becauseSn is approximately 0.085 A larger than Ti ectric distortion pattern (Taylor and Brown 1916 Zhang it is more likely to occupy the center of the octahedra; et al. 1995),the nonclassicalcritical behavior (Ghoseet therefore,the symmetry of malayaite is more likely to be al. l99l; Bismayer et al. 1992), the mobile antiphase A2/a. No information on the thermal behavior of the boundariesabove ?""(Van Heurck et al. l99l), the pseu- malayaite structure and related physical properties was do-spin characteristics(Bismayer et al. 1992), and the available. correspondingeffective critical exponent ofthe order pa- This study was undertaken to determine whether (l) 0003-004x/9610506-0595$0s.00 595 596 GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE

TABLE1. Crystallographicdata and refinementinformation for malayaite a (A) 7.153s(6) Crystal size (mm) 0.19x011x0.06 b (A) 8.8933(8) Wavelength(A)/mono 0.5597(1yS(111) c (A) 6.6674(6) Reflectionscollected 4017 B (') 113342(7\ Unique data (2/m) 1384 v(A') 389.s0 Unobsewed(lF"l< 3") 13 Space group A2la R(%l 151 z 4 R* (./"1 231

there are reflections violating extinction conditions for Data reduction included normalization to the primary A2/ a IhaI are too weak or diffuse to be seenon precession beam intensity monitor and an on-line correction for the photographs;(2) the apparentA2/asymmetry is the result (measured)beam polarization, following the procedureof of a domain structure; and (3) the thermal development Kirfel and Eichhorn (1990). The data were corrected for of the structure or its metric shows anv discontinuities. absorption by approximating the shapeof the crystal with a polyhedron (eight faces).The unique data set (R,n,: Sl'trnpr,r 2.90lo)comprised 1384 reflections (13 with l,Fl < 3o'",). The samples used in this study are from a skarn ap- With no evidenceto suggestviolation of the A center- proximately 4 km north of Ash Mountain, near Mc- ing, the structure was refined in spacegroup A2/a. Start- Dame, in northern British Columbia, Canada. Electron ing values for positional and thermal parameters were microprobe analyseswere performed with a CamecaCa- taken from Higgins and Ribbe (1977). Least-squaresre- mebax instrument, operating at 20 kV and 25 nA. The finementswere based on lltl with weights: IoI + Q.02' t, following standards were used: AlrO3 (Al), (Si), n'l chosen to minimize variation of mean c^rA2as wollastonite (Ca), FerO, (Fe), and SnO, (Sn).Wavelength functions of ,F. and (sin d)/L An isotropic extinction cor- scanswere used to search for additional elements;none rection (Beckerand Coppens1974a,1974b) was applied were detected. The results were processedwith the Ca- using a type-l Lorentzian distribution. The refined pa- meca PAP program. They show 21.82 wto/oSiOr, 21.06 rameterwas g: 0.61(2) x l0a; the most stronglyaffected wto/oCaO, 0.17 Wo/oFerOr, and 56.86wto/o SnO, (average reflectionswere 2Il (y : 0.45),220(0.59), and 020 (0.73), of eightanalyses), corresponding to 0.98 Si, l.0l Ca, 0.01 where y is the intensity reduction relative to the kine- Fe, and I .01 Sn atoms per formula unit (renormalized on matic value. Scatteringfactors for the neutral atoms were the basis of five anions). Hence, the malayaite is almost taken from the International Tablesfor X-ray Crystallog- pure CaSnSiOr,with a negligible amount of Fe substitut- raphy, volume 4 (Ibers and Hamilton 1974).Anomalous ing for Sn. dispersion corrections were taken from Cromer and Lib- erman (1970). Computer programs used in the refine- SrNcr,B-cnysrAr, X-RAv DTFFRAcToMETRy ments include modified versions of ORFLS (Businget al. Single-crystalX-ray diffraction data were collected at 1962),ORFFE (Businget al. 1964),and ORTEP (John- 295 K with the four-circle diffractometer at HASYLAB son 1965). beam line D3 during dedicatedexperiments (4.5 GeV) of Refinement with anisotropic displacement factors for Doris IIL The wavelength used throughout the experi- all atoms led to convergenceat R : l.7o/o(R* : 2.80/o). ment was 0.5597(l) A. ttre sampleshowed sharp Bragg Attempts to split the strongly anisotropic Ca site were reflections with full-width at half-maximum (FWHM) unsuccessfulbecause the positions convergedand the R values(0.012-0.016', c,r scan) typical of goodquality mo- values increased dramatically. Refinement with anhar- saic crystals.The lattice parameters,refined from 16 cen- monic displacement parameters (i.e., higher cumulants tered reflections,are a : 7.1535(6),b : 8.8933(8),c : up to third order, "gtensor") for the Ca site led to a large 6.6674(6)A, P : 113.342(7).A continuousscan mode decreasein R*. Only one tensor element (c"r) was signif- was usedwhere the

Trale 2. Finalatom parametersfor malayaite ur U"" us Utz

lt 0.68196(4) Y2 685(12) 409(8) 420(10) 476471 0 1s1(s) 0 '/2 0 663(8) 362(6) 417(6) 494(5) -18(1) 143(4) 4(4) 5n V2 -129(6) Ca V4 0.66262(4) Y2 1289(4) 3240(10) 486(8) 753(7) 0 0 o1 Vq 0.58681(4) 0 s44(8) 540(10) s70(10) 1340(1 0) 0 470(9) 0 o2 0.91140(1 3) 0.56776(3) 0.67552(4) 883(8) s56(9) 797(10) 561(8) 298(71 154(6) 154(6) o3 0.37224(41 0.71195(3) 0.89032(4) 895(8) 910(10) 600(10) 758(9) 252(61 414(7) 72(6\

Nofe.'4 values have been multipliedby 10s.

The refined structure is as described by Higgins and peratures and were able to refine a-split Ca position at Ribbe (1977). However, details are presentedhere be- S:O f [Ca-Ca distanceof 0.17(5) A1. ff tne strong an- causeofthe increasedprecision in values obtained using isotropy of the Ca position is due to antiphase domains synchrotron radiation. As shown in Figures I and 2, the the apparentA2/a symmetry of malayaite may be an av- malayaite structure consists of corner-shanng [SnOr]6- eragesymmetry similar to that seenby Kek et al. (1994) chains that extend parallel to a ar'd are further linked in synthetictitanite above 496K. To answerthis question along their lengths by SiOo tetrahedra to form type-I we decided to use transmission electron microscopy (APBs). X-ray [Sn(SiO.)Or]6chains (Moore 1970).The tetrahedralver- (TEM) to look for antiphase boundaries tices not linked to the central octahedralchain cross-link powder diffraction (XRD) and Raman spectroscopywere to adjacentchains to form a mixed tetrahedral-octahedral used to study the responseofthe structure to heating. framework. Within this framework are large cavities con- taining Ca atoms in irregular sevenfold-coordinatedpoly- Tru.NspgssroN ELECTRoN MrcRoscoPY hedra. Samples for transmission electron microscopy were The most remarkable feature of the malayaite structure prepared by crushing the crystallites and suspendingthe is the extreme anisotropy of the Ca position. Because fragments on copper grids covered with a holey carbon none of the other atoms show this degreeof distortion it film. A double-tilt heating holder was used for the in situ is not likely the result ofan error in the absorption cor- electron diffraction experimentsin a Philips CM 20 elec- rection. The anisotropy of the Ca position may represent tron microscope. High-resolution transmission electron true thermal vibration or it may be due to antiphasedo- microscopy (HRTEM) was performed at room tempera- mains. The irregular coordination spherearound the Ca ture in a JEOL 4000EX microscoPe. atom (Fig. l) suggeststhe former becausethe thermal vibration ofthe Ca atom is largely constrained by Ca-O bonds to be parallel to a. However, the study by Kek et al. Q99$ of the structureof synthetictitanite at 100,295, and 530 K suggeststhat the anisotropy may be related to antiphase domains previously observed by Van Heurck et al. (1991).They found that the thermal parametersof the Ca position were strongly anisotropic at all three tem-

TABLE3. Selectedinteratomic distances (A) and angles(") in malayaite

Si-O2 x2 1.6320(3) O2-Si-O2b 103.03(3) Si-O3" x2 1.6421(41 O2-S|-O3" xz 112.86(1) (si-o) 1.6371 02b-si-o3" x2 109 07(1) O3a-Si-O3s 109.86(3) Sn-O1 x 2 1.9479(1 ) (o-si-o) 109.46 Sn-O2b x 2 2.0878(2) Sn-O3" x 2 2.0961(3) O1-Sn-Ozb x2 89.07(1) (sn-o) 2.0439 O1-Sn-O2n xz 90.93(1) O1-Sn-O3" x2 85.24(1) Ca-O1d 2.2284(51 O1-Sn-O3' x2 94.76(1) Ca-Oz" x 2 2.4145(4) O2b-Sn-O3" x2 90.76(1) Ca-O3r x 2 2.4344(2) O2b-Sn-O3' x2 89.24(1) Ca-O3" x 2 2.7415(3) (O-Sn-O) 90.00 (ca-o) 2.4870 Note:a:1 - x,'l - y +1/2,'l- z+V2ib:y2- x+ 1,y,1 - zic - - th,l/z;e: - - - :lz : x,f, 1 + ztd:1h,1 y + 1 x,1 y'1 z;l -1 y2i 1/z-r - - x, y, 1 - zi g : y2 + x, 1 - y + y2, + z + and h : x Frcun-r l. ORTEP plot of the characteristicstructural units 1,1-y,-1+2. in malayaite (990/oprobability ellipsoids). 598 GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE

asinB

D Frcunp2. Polyhedralrepresentation of the malayaitestruc- ture,projected onto (001). Frctnr 3. Selected-areadiffraction (SAD) patterns along the (a) [001]zone at roomtemperature (295 K); (b) [001]zorre at Someresults of the TEM study are shownin Figure 3. -490 ( (c) [001] zor'eat -580 K; (d) U0ll zoneat room Figures 3a and 3d show selected-areadiffraction (SAD) temperature. patternsalong the [001] and [101] zonesat room tem- perature (295 K). All the diffraction spots are sharp and round, and only those correspondingto k + l: 2n arc were used as temperature calibration standards.Experi- present.This showsthat the symmetry is A2/a at room mental detailsare given in Saljeet al. (1993b). temperature.Figures 3b and 3c depict SAD patternsalong Lattice parameterswere refined using the LCLSQ 8.4 the [001] zonenear 490 and 580 K. There is little appar- program (Burnham 1991).The resultsare shown in Fig- ent difference between the patterns recorded at room ure 4. The graphs of a, b, and c vs. temperature show temperature and 490 K. The only modification seen in slight changesin slope at approximately 473 K. This is the pattern recorded at 580 K is broadening of the dif- much less apparent in the graph of B vs. temperature fraction spotsbecause of increasedDebye-Waller factors. becauseofscatter in the data.However, the graph ofcell Patternsrecorded between 580 and approximately870 K volume vs. temperature shows a distinct break in slope show no structural changes.Apparently no phasetransi- at approximately 493 K. Extrapolation of the high-tem- tion occursfrom room temperatureto 870 K. At all tem- peraturecurve gives a volume of approximately388.3 A3 peraturesthe sample was observedto be stableunder the at 300 K as opposedto an apparentvolume of 389 Al electron beam. for the low-temperature curve extrapolated to the same Unlike in riranite,CaTiSiO, (Van Heurck et al. 1991), temperature. no superstructurereflections were observedin the room- Becauseno APBs were seenin the HRTEM experiment temperature[001]and [101]zones. Consequently, no do- it is unlikely that the XRD results indicate a structural main boundaries are expectedin real-spaceimages. Nei- phase transition. It is more likely that they represent a ther low magnification nor high-resolution observations slight readjustment of the averagestnrcture with increas- showedany evidencefor suchdomain formation, not even ing temperature; this may primarily involve the volume on a nanometeror subnanometerscale. ofthe coordination spherearound the highly anisotropic Ca position with its largest anisotropic displacementpa- rameter U,, (Table 5). X-nq.y powDER DIFFRACToMETRy In preparation for X-ray powder diffractometry the sample was crushed,ground to a fine powder, and mixed R.q,Nr.c,NsPEcrRoscoPY with Si as an internal standard. The powder was spread Raman spectroscopicmeasurements were performed over a platinum heating element in an evacuatedheating with a spectrometerconsisting of a 4W argon laser light cell equipped with Kapton windows. The furnace was source(I : 488 nm) with continuously variable polarrza- mounted on a diffractometerwith a monochromatic CuK, tion directions. The beam was doubly focusedon a sam- X-ray beam (incident diameter 0.1 x 10 mm) and an ple mounted in a furnace. The light scatteredat 90'to INEL 4 K-PSD detector.The temperaturewas monitored the sample was focused on the entrance slit of a double with ultra-thin thermocouples(Pt-Rh alloy) welded to the monochromator and detectedby a Peltier-cooledphoto- heating element. Quartz and langbeinite [KrCdr(SOo).] multiplier attached to a phonon counter. The fit proce- GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE s99

l'l-l-l 7.185 aa 113.34 7.180 oo ao 7.175 O 113.32 ta "!z.rzo o o ..t.Q ooo G cl 7.165 a o 113.30 oaa o o ooi .o-a a 7.160 oaa t' 113.28 a a a od 7.155 a r-l-l.l

8.93 395

394 8.92 o 393 ooo o o E 3s2 X e.sr a = a o ooo o' 391 a 8.90 '.o. 390 :o/ ." b i.7'''/. e 389 - r - r - r

0.5 'o 6.70 0.4 '& o o 0.3 .o ^ 6.69 o o a o 0.2 o o oo o o .a 6.68 aa 0.1 o Cr aa Oa F 0.0 t oo o 6.67 .f, t -0.1 a 400 600 800 1000 400 600 800 1000 T (K) T (K) Frcunr 4. Temperaturedependence of (t) a, (b) b, (c) c, @)A, and (e)cell volume.(f) Differenceof linearlyextrapolated high- temperaturedata and measuredvolume. duredescribed in Bismayeret al. ( 1986)was used for line- between470 and 620 cm-' collectedat temperaturesbe- profile analysis. tween 300 and 953 K are shown in Figure 6. The optically The A2/a phonon spectrum(A, symmetry) of malay- active representationsare I,o,: 9,\ + l2BE + l1A'-+ : aite at 300 Kis shown in Figure 5,-andA. Raman spectra l3B" and I"*,", A" + 2B". Twenty-one active modes 600 GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE

r 000 TABLE7. Frequencies(cm-') of the most prominentRaman signalsof malayaiteat 110 K

Frequencies lntensities Frequencies lntensities

/c 328 109 S 340 vs 142 s 412 176 VS 450 197 510 227 520 s 250 S 595 vs 280 802 295 837 S 305 895 300 400 s00 600 700 r 000 vs in Wavenumber(cm 1) Note: E(or is a1 cm-', intensitiesare scaled,vw, w, s, and increasingstrength Frcunr 5. ,\ Ramanspectrum of malayaiteat 300K.

are expectedin the first-order Raman spectrum. The fre- ening of the density of statesowing to disorder phenom- quenciesof the most intense bands are given in Table 7. ena or perturbation of the translational symmetry. On The line widths of all phononsare approximately l0 cm ' heating, the frequency of the mode near 560 K (Fig. 7b) at room temperature. At 0 K the FWHM of the mode decreasesby approximately5 cm-r from 300 to 800 K. near560 cm-l extrapolatesto approximately4 cm 1(Fig. Coupling betweenthe volume changeand the optical AB 7a), which indicates that there is practically no broad- phonon leads to the change of slope of the phonon fre- quency near 500 K.

DrscussroN The evolution of lattice parameters at high tempera- tures shows a change in slope at approximately 500 K, but this does not seem to correspond to the same sym- metry-breaking processseen in synthetic titanite. In ma- layaite the thermal changein slope of the lattice param- eters and the volume is probably due to an anomaly involving the highly anisotropic Ca position. In the case 2500 of a true phase transition the excessvolume is propor- tional to the squareof the order parameter of the system, o which displaysthe thermodynamicanharmonicity. In fact, the differencebetween the measuredand the extrapolated 2000 volume (Fig. af) decreasesprogressively as temperature $ approaches500 K. If a thermally controlled collapse of the coordination sphere around the Ca site is the mech- a anism responsiblefor the measuredchanges, the overall c) 1500 volume couples with this local effect, which reflects the thermal evolution of the structure without leading to a changeof the global symmetry. We statethat in this sense the excessvolume cannot be correlated with a classical order parameterbut with the triggering distortion such as found in langbeinite (Percival 1990). Further work is un- derway to characterizethis event. The integrated inten- sities (Fig. 7c) obtained from the Raman spectra (Fig. 6) 500 show that the macroscopicvolume changeis compatible with the weak anharmonicity of the structure on a local 480 520 560 600 Iength scale. Our work has shown that there is no evidence of do- Wavenumber(cm-1) main structure in malayaite. Therefore, the A2/a sym- Frcunr 6. A" Raman spectra between 470 and 620 cm-, metry must reflect that the large Sn atoms are already at collected at the following temperatures(bottom to top; in K): the centersof the octahedral sites and are not displaced, 300,38 5, 391, 453,47 3, 483,5r3, 523,533, s43,553, 563, 583, as are the Ti atoms in primitive titanite. As suggestedby 603,623, 643, 663, 683, 7 13,7 43,7',7 3, 803,833, 873, 9 I 3, 953. Higgins and Ribbe (1977), it would be interesting to ex- GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE 60I

AcxNowr,nncMENTs The authors thank B.S. Wilson for providing the samplesand acknowl- 20 edge the technical assistanceof A. Graeme-Barber,G. Adiwidjaja, and K.H. Klaska. The authors also thank T.S. Ercit for his constructivecom- ments. The researchwas supported by grants from the National Science 16 and Engineering Research Council (Canada) to L.A.G. and from the - ta 'E ao a DeutscheForschungsgemeinschaft and the Bundesministerfilr Forschung t. und Technologieto S.K.,C.S., H.M., and U.B. 9 12 .tj.tt = E 1f". RnnnnpNcns crrnn iL Becker,P., and Coppens,P. (l 974a)Extinction within the limit ofvalidity of the Darwin transfer equations: I. General formalisms for primary 4 and secondary extinction and their application to spherical crystals. Acta Crystallographica,A30, 129-147. - (1974b\Extinction within the limit ofvalidity ofthe Darwin trans- 0 fer equations:II. Refinement of extinction in sphericalcrystals of SrF, and LiF Acta Crystallographica,A30, 148-153 s61 Bismayer, U., Salje, E.K.H., Jansen,N., and Dreher, S. (1986) Raman 1..., scatteringnear the structural phasetransition ofAsrOr: Order param- 560 eter treatment. Journal ofPhysics, Cl9, 4537-4545. 'E Bismayer, U., Schmal, W., Schmidt, C., and Groat, L.A. (1992) Linear t phase 559 O1 and X-ray diffraction studies on the structural tran- () sition in titanite, CaTiSiO,. Physics and Chemistry of Minerals, 18, 558 260-266. o Bumham, C.W. (1991) LCLSQ version 8.4. kast-squares refinement of tr s57 crystallogxaphiclattice parameters.Harvard University, Cambridge, o oa a f b Massachusetts. E 556 a Busing, W.R., Martin, K.O., and krry, H.A. (1962) ORFLS: A FOR- o least-squaresprogram. Oak Ridge National a o TRAN crystallographic lr 555 Laboratory, Repo( ORNL-TM-305. - (1964) ORFFE: A FORTRAN crystallographicfunction and error 554 program. Oak Ridge National Laboratory, Report ORNL-TM-3OS. Cromer, D.T., and Liberman, D.A. (1970) Relativistic calculation of 553 anomalousscattering factors for X-rays Journal ofChemical Physics, 53,1891-1898. Ghose, S., Ito, Y., and Hatch, D.M. (1991) Paraelectric-antiferroelectric 20 3O phase transitions in titanite, CaTiSiOr: L A high temperature X-ray o . diffraction study of the order parameter and transition mechanism. E ..t 1t1.. of Minerals, 17 604-610. c Physicsand Chemistry , f Higgins, J.B., and Ribbe, P.H. (1976) The crystal chemistry and space 15 O1 groups of natural and synthetic titanites. American Mineralogist, 61, ri 878-888 G -(1977) The structure of malayaite, CaSnOSiOo,a tin analog of titanite. American Mineralogist, 62, 801-806 .= 10 Ibers,J.A., and Hamilton, W.C., Eds. (1974)Intemational tablesfor X-ray o p. E oo crystallogaphy, vol. 4,366 Kynoch, Birmingham, U.K. o Johnson, C.K. (1965) A FORTRAN thermal-ellipsoid plot program for tr 5 illustrations. Oak Ridge National Laboratory, Report oRNL-3794. Kek, S., Aroyo, M , Bismayer, U., Meyer, H., Schmidt, C., Eichhorn, K., and Krane, H.G. (1994) Synchrotron radiation study of the crystal 0 structureof titanite (CaTiSiOJ at 100 K, 295 K and 530 K: Model for 400 600 800 1000 a two step structural transition. HASYLAB/DESY, Annual Report, p. 453-454. (K) Kirfel, A., and Eichhorn, K. (1990) Accurate structure analysiswith syn- T chrotron radiation: The electrondensity in AlrO, and CurO. Acta Crys- Frcunn 7. (a) FWHM of the 560 cm I A, mode as a function tallographica.A46, 27 l-284. ( oc- of temperature(error + 1.5cm-t). (b) Frequencyof the 560 cm ' Moore, P B I 970) Structural hierarchiesamong minerals containing tahedrallycoordinating oxygen: I. Stereoisomerismamong corner-shar- mode as a function of temperature (error + l cm r). (c) Inte- ing octahedral and tetrahedral chains. Neues Jahrbuch fiir Mineralogie grated I function intensity of the 560 cm mode as a of temper- Monatshefte, 163-173. (error + ature l0o/o). Percival, M.J.L (1990) A trigger mechanismin an improper ferroelastic: The langbeinite structure. In E.K.H. Salje, Ed, Phase transitions in p. Press, amine single crystals of titanite containing small amounts ferroelasticand co-elasticcrystals, 296. CambridgeUniversity Cambridge. of Sn or other cations larger than Ti to see what effect Salje, E.K.H., Schmidt, C., and Bismayer, U. (1993a) Structural phase these would have on domain formation and thus the transitions in titanite, CaTiSiOr: A Raman spectroscopicstudy. Physics overall svmmetrv. and Chemistry of Minerals, 19, 502-506. 602 GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE

Salje, E.K.H., Graeme-Barber,A., Carpenter, M.A., and Bismayer, U. Electron diffraction and electron microscopic studies of transition dy- (1993b) Lattice parameters,spontaneous strain and phase transitions namics. Physicsand Chemistry of Minerals, 17, 604-610. in Pb,(PO),. Acta Crystallographica,B49, 387-392. Zhang,M., Salje, E.K.H., Bismayer, U., Unruh, H.G., Wruck, B, and Takenouchi, S. (1971) Hydrothermal synthesisand consideration ofthe Schmidt, C. (1995) Phasetransition(s) in titanite CaTiSiO': An infrared genesisof malayaite. Mineral Deposrta,6, 335-347 spectroscopic,dielectric responseand heat capacity study. Physicsand Taylor, M., and Brown, G.E. (1976) High-temperaturestructual study of Chemistryof Minerals,22,4l-49. the P2t/a = A2/a ph^se transition in synthetic titanite, CaTiSiOr American Mineralogist, 61, 435-44'7. Van Heurck, C., Van Tendeloo, G., Ghose,S., and Amelinckx, S. (1991) MeNuscnrpr REcETvEDJule 20, 1995 Paraelectric-antiferroelectricphase transitions in titanite, CaTiSiOi: II. Meruscnrrr AccEprEDDecsr.ansR 29, 1995