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 titanite 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), quartz (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 <oaxis was rotated at a constant speed icant; refinement with this additional parameter led to a throughout the Braggposition, with scalersrecorded and reduction of R* from 2.8 to 2.3o/o.Crystallographic data cleared at fixed time intervals. The scan speed was ad- and refinement information are given in Table l, final justed to record (after insertion ofappropriate attenuator atom parameters in Table 2, selected interatomic dis- combinations) a maximum of 2500 counts, without ex- tancesand anglesin Table 3, detailed data collection and ceedingmeasurement times of 0.1-1.0 s/step.A total of processinginformation in Table 4, details of the refine- 4017 reflections(including test reflections)were recorded ment results (including g-tensor elements for Ca) in comprisingtwo asymmetricunits with (sin 0)/)\ < 0.947 Table 5, and observedand calculated structure factors in AI. Table6.' The l-centered lattice was confirmed by measuring243 uniquereflections with k + l:2n * l; nonewere ob- 1A of Tables 4-6 may be ordered as Document AM-96- < copy served at F2 3o.2. Reflection scansat liquid-nitrogen 6 I 2 from the BusinessOffice, Mineralogical Societyof America, temperaturealso showedno significant intensity even for 1015Eighteenth Street NW, Suite601, Washington,DC 20036, countingtimes of 3.5 s/step. U.S.A. Pleaseremit $5.00 in advancefor the microfiche. GROAT ET AL.: STRUCTURE AND THERMAL BEHAVIOR OF MALAYAITE 597 Trale 2.