597
Canadian Mineralogist Vol. 30, pp. 597-611 (1992)
THE WODGINITE GROUP. I. STRUCTURAL CRYSTALLOGRAPHY
T. SCOTT ERCIT Mineral Sciences Section, Canadian Museum of Nature, Ottawa, Ontario KIP 6P4
FRANK C. HAWTHORNE AND PETR CERNY Department of Geological Sciences, University of Ma nitoba, Winnipeg, Ma nitoba R3T 2N2
ABSTRACT
Four new crystal-structure refinements have been done on wodginite-group minerals. Two partially ordered samples were used in single-crystal experiments, one, tantalian, the other, ferroan. The ferroan sample was heated to induce order, and its structure was re-refined. In addition to the single-crystal experiments, a Rietveld refinement from powder-diffraction data was done on a fully ordered lithiowodginite. Refinements of the partially ordered samples show that they are structurally intermediate to wodginite and ixiolite, and that heating under conventional conditions of 1000°C for 16 hours induces full wodginite-type cation order. In terms of both individual and mean bond-lengths, data for Li-rich samples deviate significantly from established trends for Li-poorer samples in a manner consistent with disorder of Li off the central A position. Schemes of cation order for potential derivatives of ixiolite have been evaluated for cells up to four times the volume of the ixiolite cell = size of the wodginite cell). For full ABC208 cation order, four most-probable schemes of order result. Three of these involve a doubled cell; one involves a quadrupled cell, relative to an ixiolite parent. Only the scheme of order with a quadrupled cell occurs naturally, i. e. , the wodginite structure; there is nothing in its scheme of order to indicate why nature prefers it over the other three schemes; more sophisticated calculations of energy seem necessary.
Keywords: crystal-structure refinement, wodginite group, order-disorder, Rietveld method.
SOMMAIRE
Nous avons effectue quatre nouveaux affinements de Ia structure cristalline de mineraux du groupe de Ia wodginite. Deux de ceux-ci, partiellement ordonnes, dont un est tantalifere et !'autre, ferreux, ont ete utilises pour des experiences sur crista! unique. L'echantillon riche en fer a ete chauffe pour promouvoir une mise en ordre, et l'affinement de sa structure a ete repete. En plus, nous avons effectue l'affinement d'un echantillon de lithiowodginite completement ordonne sur poudre par Ia methode de Rietveld. Les affinements des echantillons partiellement ordonnes montrent qu'ils sont structuralement intermediaires a Ia wodginite et a l'ixiolite, et qu'un chauffage sous conditions conventionnelles de 1000°C pour 16 heures mene a une mise en ordre complete des cations selon le modele de Ia wodginite. D'apres les longueurs des liaisons individuelles et moyennes, les donnees pour les echantillons riches en Li devient de fa�on importante des tendances etablies pour ceux qui sont plus pauvres en Li, et concordent avec !'hypothese qu'il y a desordre du Li par rapport a Ia position centrale A. Nous avons evalue les schemas de mise en ordre des cations pour les derives potentiels de l'ixiolite pour des mailles jusqu'a quatre fois le volume de Ia maille de l'ixiolite, c'est-a-dire, de Ia maille de Ia wodginite. Pour le modele d'une mise en ordre complete des cations selon ABC208, i1 en resulte quatre schemas plausibles. Trois de ceux-ci impliquent une maille doublee; un schema requiert une maille quadruplee, par rapport a Ia maille de l'ixiolite. Seul le schema de mise en ordre de Ia maille quadruplee se retrouve dans Ia nature, sous forme de wodginite. Rien n'explique le developpement preferentiel de ce schema, plutot que les trois autres. Des calculs plus sophistiques de l'energie des structures semblent indiques pour expliquer ce point.
(Traduit par Ia Redaction)
Mots-c!es: affinement de Ia structure cristalline, groupe de Ia wodginite, ordre-desordre, methode de Rietveld.
INTRODUCTION proposed a model for the wodginite structure; however, only subcell reflections were used, The crystal structure of wodginite, MnSnTa208, resulting in an erroneous space-group for the was first recognized as a superstructure of ixiolite, structure. Grice (1973) attempted a full refinement (Ta,Sn,Mn)02, by Nickel et a!. (1963) on the basis of the structure (space group C2/c). He showed of X-ray powder-diffraction data. Elphick (1972) that there are three cationic sites in the wodginite 598 THE CANADIAN MINERALOGIST structure, and that all cations are in octahedral one-half of the octahedral interstices at each level coordination; the octahedra form a-Pb02-like in the close-packed array of anions. There are three zig-zag chains via sharing of polyhedron edges. cationic sites, denoted here as A, B and C. The A Graham & Thornber(1974) published a refinement and B sites lie at the same level in the closest-pack of the wodginite structure in the space group Cc, ing, whereas the C sites lie at a different level; these which resulted in four cationic sites, two of which layers alternate along X. All coordination show incomplete order among cation occupants. polyhedra link via edge-sharing to form a-Pb02- Ferguson et a!. (1976) re-investigated the structure like zig-zag chains along Z. Chains are intercon of wodginite and showed that the results of Grice nected by comer-sharing linkages between levels of (1973) and Graham & Thornber (1974) are in error the closest-packed layering. The A and B polyhedra owing to incorrect choices of origin; C2/c, not Cc, alternate along the zig-zag chains of their level; is the correct space-group. Gatehouse et at. (1976) adjacent levels necessarily consist only of chains of showed that the compound M-LiTa308 is isostruc C polyhedra. tural with wodginite, and obtained the same Ideally, the A site contains Mn, the B site solution of the structure as Ferguson et at. (1976). contains Sn, and the C site contains Ta, resulting A later Rietveld refinement of the M-LiTa308 in the ideal formula MnSnTa208 (Z = 4) for structure using neutron powder-diffraction data wodginite. Ferguson et at. (1976) showed that Ta (Santoro et at. 1977) located the Li atom, which can be disordered over the B and C sites; however, Gatehouse et at. (1976) could not find in their X-ray as disorder between the B and C sites does not experiment. modify the space-group symmetry, this behavior Polyhedral representations of the wodginite can only be easily detected by examining results of structure are shown in Figures 1 and 2. Oxygen chemical analyses for Ta-overstuffed compositions. atoms form approximately hexagonally close-pack Powder-diffraction experiments show that some ed layers perpendicular to X. Cations occupy members of the wodginite group have X-ray-dif-
a b FIG. 1. The structure of ideal wodginite projected along X; (a) the layer of A, B octahedra; (b) the layer of C octahedra. A (Mn) octahedra are hatch-stippled, B (Sn) octahedra are densely stippled, and C (Ta) octahedra are lightly stippled. STRUCTURAL CRYSTALLOGRAPHY OF THE WODGINlTE GROUP 599
1------b ------�
a
FIG. 2. The wodginite structure projected along Z; the zig-zag chainsof the structure are perpendicular to the plane of the page. Shading is as in Figure 1.
fraction properties intermediate to those of with the wodginite structure, and does not wodginite and ixiolite (as proposed by Lahti 1982) contradict the terminology described above. or wodginite and wolframite [as proposed by In companion papers (Breit et al. 1992a,b), we Maksimova & Khvostova (1970) and Polyakov & outline variations in the chemistry and structUi'e of Cherepivskaya (1981)]. The model involving ixiolite what we now recognize to be the wodginite group. implies disorder over all cationic sites; the model The goal of the present paper is to investigate the involving wolframite implies disorder only over the structural properties of wodginite-group minerals, with emphasis on order-disorder relationships. A and B sites. In principle, it should be relatively easy to differentiate between the two models on the EXPERIMENTAL basis of site occupancies derived from crystal-struc ture refinements. To avoid any confusion regarding Single-crystal diffraction order-disorder phenomena in wodginite-group minerals, we shall use the following nomenclature. Two samples of wodginite-group minerals were For samples with diffraction properties fully examined by single-crystal X-ray diffraction; both compatible with the wodginite structure, we use the are partially ordered, one intermediate in composi term (fully) ordered; for samples with diffraction tion between wodginite and lithiowodginite (sample properties intermediate to those of the wodginite CX- 1: Coosa Co., Alabama), the other inter structure and those of some substructure (e. g. , mediate in composition between wodginite and ixiolite, wolframite), we use the term partially ferrowodginite (sample A-17: Ankole, Uganda). ordered. Disorder of Ta over the B and C sites is Sample CX-1 was donated by E.E. Foord (U.S. here referred to as Ta disorder; it is fully compatible Geological Survey), and was described as having 600 THE CANADIAN MINERALOGIST
TABLE2. CHEMICALCOMPOSmONS OF WODGINITE-GROUP MINERALS USED IN diffraction properties intermediate between those STRUCTURE REFINEMENTS of wodginite and wolframite (E.B. Foord, pers. Wodglnlto Partially Llthlo- Ferroen Ordered wodglnlte Wodglntte comm.). Sample A-17 was investigated in Breit et Wodglnlte at. (1992a); it shows diffraction properties inter (G69-1?) (CX-1) [TSE-76) (A-17) u.o (wt%) o.o Q.22 1.21 0.03 mediate between those of wodginite and ixiolite. MnO 10.7 7.7 4.6 6.1 FeO 0.0 2.3 0.0 4.7 The observed morphology indicates that both SnO 0.7 F0.03 0.8 2.6 0.1 1.8 crystals are sector-twinned, with Yas the twin axis. m. 1.4 2.5 ZrOo 1.5 In both cases, a fragment from one sector was SnO. 13.0 10.3 4.6 10.7 isolated and examined by the precession method to Nt>.os 4.0 7.5 5.2 6.2 Ta.Os .§§.& .Jill,1 83.4 65.5 confirm the absence of twinning within each 98.6 102.0 99.3 99.5 fragment. To minimize differential absorption, the cationsper 32(0) crystals were hand-shaped to approximate spheres u 0.00 0.36 2.13 0.05 Mn 3.95 2.70 1.73 2.15 prior to data collection. Following initial collection Fe2+ 0.00 0.81 0.00 1.64 of intensity data, the crystal of sample A-17 was Sn2+ 0.13 Fe3+ 0.26 0.62 0.05 0.55 heated in an attempt to induce cation order, and Tl 0.47 0.76 Zr 0.30 data were recollected on this crystal. Conditions of Sn4+ 2.25 1.70 o.81 1.n � o.ro 1M �� �64 the heating experiment were as follows: 1000°C for Ta � LZJ! 9.97 7.38 15.62 16 15.72 15.64 16 hours in an argon atmosphere, in a Ag-Pd foil G69-17: Fonnularecalculaledfromanalysis fromFerguson eta/. (1976);U.O and basket, using a vertical quench furnace. Fe2+:Fe3+ calculatedby WODGINITE program. Intensity data were collected on the single CX-1: Electronmicroprobe analysis o1 crystalIn used structureanalysis; u 2o and Fe2+:Fe3+calculated by WODGINITE prngmm; Sn2+:Sn4+by crystals with a Nicolet R3m four-circle diffrac 115Sn-MOSSbauer(pars. comm. E.E. Foord). tometer, using the experimental method of Breit et TSE-76: Electron mlcroprobsanalysis; U.O by atomic absorptionspectroaco py; Fe2+:Fe3+ calculatedby WODGINITE program. at. (1986). Fifteen to twenty-five intense reflections A-17: Electron microprobeanalysis o1 crystal used Inanalysis; structure U.O by (to 28 = 30°) were used in centering each crystal; atomic absorption spectroscopy; Fe2+:Fe3+calculated by WODGINITE least-squares refinement of the setting angles gave program. the unit-cell parameters in Table 1, and the orientation matrix used for data collection. In each case, one asymmetric unit of data was collected to the resulting compositions and unit formulae are a maximum sin()/>.. of 0.7035 to give the numbers given in Table 2. of unique reflections shown in Table 1. The data were empirically corrected for absorption (using Powder diffraction 1/t-scan data), Lep and background effects using the SHBLXTL package of programs. A reflection was One wodginite-group mineral from the Tanco considered observed if its intensity exceeded that pegmatite (sample TSB-76) was found to have a of three times its standard deviation; application high proportion of Ta at the B site according to its of this criterion resulted in the numbers of observed chemical composition, and was suspected of having reflections given in Table 1. After collection of the a large proportion of vacancies at the A site. On intensity data, the crystals used in each collection the basis of these properties, we considered a were mounted in epoxy and polished for electron structure refinementof this sample to be important; microprobe analysis. The conditions of electron however, the material was so finely crystalline and microprobe analysis and the method of calculation in such intricate patterns of parallel growth that of the formula are described in Breit et at. (1992a); single-crystal methods of investigation were impos sible. Consequently, we decided to attempt a structure refinement using X-ray powder-diffrac TABLE1. MISCELlANEOUS INFORMATION FOR SINGLE-CRYSTALSTRUCTURE tion data (Rietveld method). REFINEMENTS Partiallyordered FerroenWodglnlte Ferroan Wodglnlte The diffraction data were collected with a Philips Wodglnlte PW1710 automated powder diffractometer using (CX-1) (heated)(A-17h) (A-17} o (2 graphite-monochromated CuKa radiation and a 1 • (A) 9.500(5) 9.460(1) 9.450 ) b 11.465(6) 11.421(2) 11.442(2) c 5.139(3) 5.119(1) 5.083(1) divergence slit. The sample was ground for 15 90.51(4) 90.40 1 90.75 2) minutes in acetone prior to mounting in a sample e (AS) 559.1(6) 553.0�1! 549.5!2) SpaceGro up C2/c C2/C C2/C holder. The data were collected in a step-scanning Meanradius { mm) 0.08 0.09 0.09 mode from 10° to 90° 28. A step size of 0.08° 28 Jl (MoKa,cm-1) 414 402 402 T01aiF.,cbs. F0 862,651 815,729 810,752 and a counting time of 5 s per step were selected FinalR, wR (%) 6.0,5.8 6.5, 6.4 8.5, 6.9 so as to obtain accurate estimated standard R = E CIFoi·IFol> IE IFol deviations from count statistics (Hill & Madsen wR = [Ew(IFoi·IF01)2/EwiFoi2J112, w=1 1984). The average result of two electron- STRUCTURAL CRYSTALLOGRAPHY OF THE WODGINITE GROUP 601 microprobe analyses for sample TSB-76 is given in ferences at the cation sites were one half those of Table 2; the conditions of analysis are given in Breit the first model. The degree of order based on this et a!. (1992a). ixiolite-wodginite mixing model is 59(1)%. The maximum peak-height on the final diffen:nce REFINEMENT Fourier map was 1.8 e-I A 3 which, given the complex chemistry, high coefficient of linear Single-crystal-diffraction data absorption and somewhat irregular shape of the crystal, seems reasonable. Miscellaneous information about the refinement On the basis of the refinement of sample CX-1, of the single-crystal X-ray-diffraction data are and from precession photographs, the order-disor given in Table 1. The structures were refined using der mixing model for sample A-17 was assumed to SHBLX76 (Sheldrick 1976) in the CRYSRULBR be of the wodginite-ixiolite type. Using the same package of programs (Rizzoli et al. 1986). Scatter techniques as for sample CX-1, refinement of the ing factors for neutral atoms, together with data set for the unheated crystal converged at anomalous dispersion corrections, were taken from values of R = 6.5, wR = 6.4%, and gave a degree Cromer & Mann (1968) and Cromer & Liberman of order of 66(1)%. The refinement of the data set (1970), respectively. Observed and calculated struc for the heated crystal converged at values of R = ture-factors for each structure are available from 6.5, wR = 6.9%, and gave a degree of order of the Depository of Unpublished Data, CISTI, 97(2)%, indicating that the heating experiment National Research Council of Canada, Ottawa, induced full order among cations (in terms of a Ontario K1A OS2. wodginite-like pattern of order versus an ixiolite Structure refinement for sample CX-1 involved like pattern of order). For both data collections, a starting model with a wodginite-like pattern of the maximum peak-height on the final difference order. Starting positions were taken from Ferguson Fourier map was 3 e-I A 3• We attribute this to errors et a!. (1976), and site contents for the various in the absorption correction, due to the high value cationic constituents of this hypothetical fully of m•JL (Table 1), in conjunction with problems ordered model were assigned by applying the with approximations of the crystal shape (hand conclusions of Ferguson et at. (1976) and Breit et shaped). at. (1992a) to the results of the electron-microprobe analysis (Table 2). In the first stage of refinement, Rietveld refinement it was assumed that the style of cation disorder followed a wolframite-like model. For fully ordered The refinement of the powder X-ray-diffraction wodginite, the structural formula is ABC208; for data was done with the program LHPM (Hill & the disordered wolframite model, the structural Howard 1986), a modification of the program formula is (AB)E2C208; through the use of DBW 3.2 (Wiles & Young 1981). Scattering curves constraint equations involving site occupancies, the for neutral atoms were used. A pseudo-Voigt proportion of each component could be refined as function with a 20-dependent, refinable a single variable. Refinement of this wodginite Lorentzian:Gaussian character was used as the wolframite model resulted in the following R profile function. factors: R = 8.0, wR = 7 .60Jo. Several unsuitable The refinement was done in three stages. First features were noted in this model. Difference the scale factor, unit-cell parameters, mean back Fourier maps were found to indicate that the A ground and 0° 20 position were refined, with all and B sites were scattering more strongly, and the positional parameters fixed at values for ordered C site less strongly, than the model could wodginite (Ferguson et a!. 1976). Temperature accommodate, indicative of disorder between the factors were initially fixed at B(cation sites) = 0.6 stronger scatterers of the C site (i.e., Ta) and the A 2, B(anion sites) = 0.8 A 2, but for later sta�es of weaker scatterers of the A and B sites (i.e., Mn, refinement were fixed at values given by Ferguson Sn). Consequently, a second refinement was carried et al. (1976) for cations (anisotropic) and anions out, this time with the assumption that the style of (isotropic; set at average of B = 1.0 A 2). For the cation disorder followed an ixiolite-like model, with second stage of refinement, the peak shape, a structural formula (ABC0E408• Again, through asymmetry and half-width parameters, and coeffi the use of constraint equations involving site cients of the background polynomial (4th order) occupancies, the proportion of wodginite versus were included as variables. For this and subsequent ixiolite character was refined as a single variable. stages of the refinement, the refined 20 range for
Refinement converged at values of R = 6.0, wR each peak was set at four times the full width of
= 5.8%, a significant improvement over the earlier the peak at half-height. For the final stage of the refinement. The success of this model was further refinement, all positional parameters and the shown by difference-Fourier maps: electron dif- occupancy of the A site were added to the list of 602 THE CANADIAN MINERALOGIST
TABLE 3. FINALPARAMETERS FOR RIETVELD3TRUCTURE REFINEMENT OF TABLE4. POSmONALPARAMETERS AND DEGREESOF CATION ORDER FOR UTHIOWODGINITE TSE-76 WODGINITE-GROUPMINERALS Scanrange("29,CuKa): 101090, step0.08 She Wodglnlte Pal1!ally M-UTaaOa UthJo. Ferroan Ferman * ** Wodglnlte Wodglnhe Unitcell �: a 9.4536(6), b 11.4977(6), c 5.0775(2) Ordered wodgfnita Wodglnlte (Healed) p 91.045(3). v 551.81(5) (G69-17) (CX-1) (TSE-78) (('.-17) (('.-17h) Peakshape�: �· 0.7(1), �· 0.01 0(6), �· -0.00010(6) A X 0 0 0 0 0 0 ASYmmetryparameter: AS 0.55(6) y 0.8575(4) 0.6637(3) 0.655{2) 0.862(3) 0.6634(3) 0.6601(4) z Half-widthparameters: u 0.08(3), v -0.04(2), w 0.027(4) 1/4 1/4 1/4 1/4 1/4 1/4 Preferred parameters: 1 -2.7(4), P2 0.74(1) ortentatlon P B X Durbin-Watson 0 0 0 0 0 0 d-Sia1ist!c: 1.25 y 0.1689(2) 0.1661(2) 0.1701(1) 0.1699(5) 0.1680(2) 0.1887(2) DiscrepancyIndices (%): Rp5.6 Rwp7.3 R83.8 z 1/4 1/4 1/4 1/4 1/4 1/4 R.,.,2.e c X 0.2498(1) 0.2493(1) 0.2415(1) 0.2451(5) 0.2466(1) 0.2466(1) Expressionsused y 0.4131(1) 0.4146(1) 0.4136(1) 0.4122(3) 0.4143(1) 0.4133(1) z Peakshape: � = �· + -n2fJ� + (29)2 0.2472(2) o.2497(3) 0.2466(2) o.246(1) 0.2493(2) 0.2480(2) = 1-AS(slgn (29r29k)](29r29f 04 X 0.116(2) 0.114(2) 0.115(2) 0.119(3) 0.115(2) 0.114(2) y 0.163(2) 0.186(2) 0.179(2) 0.180(4) 0.165(1) 0.184(1) variables, resulting in RP = 6. 5, Rwp = 8.2, R8 = z 0.576(4) 0.585(3) 0.573(4} 0.584(6) 0.583(3) 0.584(3) 5.4%. At this point, the /(obs)-l(calc) difference • Fergusoneta/. (1976) •• Gatehouseeta/. (1976);U positionfrom santoroeta/. (1977) pattern showed that the intensities of a number of reflections were underestimated by the model, in a manner consistent with either minor admixture of manganotantalite in the sample, or with a slight preferred orientation of the powder. A two-phase TABLE5. ANISOTROPICDISPLACEMENT-P ARAMETERSFOR WODGINITE-GROUP MINERALS (PRESENTSTUDY) refinement involving a lithiowodginite and man She Ferman Ferroan ganotantalite admixture resulted in insignificantly Wodglnlte Wodglnlte (Heated) different R factors; consequently, contamination (('.-17) (('.-17h) by manganotantalite is judged to be unlikely. The A u, 247(14) 176(22) u.. 219(15) �:� 193(23) l(obs)-I(calc) difference pattern suggested that the Usa 261(14) 197(14) 247(25) u,. 0 0 0 {110} cleavage is responsible for the proposed u,. 36(10) 4(10) 10(16) preferred orientation; refinement of the correction u .. 0 0 0 for preferred orientation resulted in significantly B u, 240(8) 157(8) 114(8) u.. 232(8) 181(8) 112(8) = = improved R factors of RP 5.6, Rwp 7. 3, R8 Usa 220(8) 172(8) 187(9) u, = . 0 0 0 3.80Jo. The occupancy of the A site converged u,. 41(6) 8(8) 12(8) at 0.85(2), which agrees reasonably well with the u.. 0 0 0 results of the chemical analysis (occupancy of 0.96), c u, 240(5) 192(5) 181(4) u.. 226(5) 200(5) 181(5) considering the inhomogeneity of the bulk sample Usa 195(4) 201(5) 239(5) (Ercit et at. 1992a). The final values of important u,. -7(4) -5(3) -5(3) u,. 40(3) 2(3) 2(3) profile-parameters are given in Table 3. A list of u .. 1(4) 0(4) 1(4) observed and calculated profile-intensities and of 01 u, 542(123) 293(82) 283(82) observed and calculated intensities of reflections u .. 299(84) 250(74) 189(71) Usa 360(103) 297(86) 394(99) are available from the Depository of Unpublished u,. -11(83) -19(82) u,. 274(87) :,!:j 78(71) Data, CISTI. u.. -50(81) 53(82) 17(66) 02 271(77) = 249(73) =� RESULTS 260(84) 225(78) 260(83) 79(87) 31(60) 26(59) -83(86) -1l8(59) 2(83) -124(86) -28(57) -13(82) The final values of the positional parameters and 03 u, 241(70) 285{78) 241(73) equivalent isotropic displacement-factors for the u.. 225(78) 189(70) 146(86) u.. 285(74) 273(76) 278(78) four structure refinements of the present study, for u,. -13(58) u,. �Jlril� .:!:l 30(60) wodginite 069-17 (Ferguson et at. 1976) and for u.. -33(59) -24(57) ·11(59) the compound M-LiTaP8 (Gatehouse et at. 1976, 04 u, 220(88) 239(73) Santoro et at. 1977) are given in Table 4. For this u.. �fool 254(78) 222(74) u .. 361{84) 301(78) 282(76) and other tables, data for the A site in M-LiTa308 u,. 47(59) 39(81) u,. = 0{57) 36(60) are taken from the powder neutron-diffraction u.. -79(82) -4(82) 'lll(83) work of Santoro et at. (1977); data for all other Noto:an U ijare� x 104; STRUCTURAL CRYSTALLOGRAPHY OF THE WODGINITE GROUP 603 sites are taken from the single-crystal X-ray-diffrac samples is determined with a high degree of tion results of Gatehouse et at. (1976). precision. However, the mixing model used is Anisotropic displacement-factors for the single continuous and linear; if the order-disorder crystal refinements of the present study are given transformation from wodginite to ixiolite is non in Table 5. The partially ordered samples show linear or discontinuous, then the standard devia large displacement-factors compared to previous tions associated with the refined degrees of order refinements of ordered members of the wodginite would represent minimum estimates. Actual assign ments of sites based on the application of estimates group (Ferguson et at. 1976, Gatehouse et at. 1976). However, the displacement-factors for heated of degree of order are given in Table 6, and were versus unheated sample A-17 are not significantly used in all subsequent calculations involving site chemistry (e.g., bond valences). different, indicating either that (1) if the large displacement-factors represent minor positional displacements between the various constituents of TABLE 7. BOND LENGTHS (A)FOR WODGINITE-GROUPMINERALS a given site averaged over the volume of the crystal, W�e = M·Lf!...O. � ::. Ferroan Wodglnlte then heating does not significantly minimize these Wodgfnlle (Healed) positional displacements, or (2) the large displace (G69-1i') (CX-1) (TSE-76) (A-17) (A-17h) A-02e.g ><2 2.10(2) 2.04(2) 2.09(2) 2.13(4) 2.06(2) 2.10(2) ment-factors for the single crystals of the present 03h,i x2 2.14(2) 2.12(2) 2.14(2) 04e,g"" lill(2) g,gj,(2) �11 �rs! g,g&(2) �� study are indicative of an error introduced by possible to state which of these effects is dominant. C •01o 2.06(2) 2.02(2) 2.06(2) 2.04(4) 2.06(2) 2.03(2) 01d 2.18(2) 2.14(2) 2.25(2) 2.18(3) 2.15(2) 2.17(2) The degree of order for each sample, based on 02e 1.94(2) 2.00(2) 1.88(2) 1.92(4) 1.97(2) 1.93(2) 02e 2.07(2) 2.12(2) 2.02(2) 2.12(4) 2.09(2) 2.07(2) a wodginite-ixiolite mixing model, is summarized 03o 1.86(2) 1.95(2) 1.85(2) 1.92(4) 1.91(2) 1.85(2) 04! .1.l!!l(2) 1.93(2) 1J!!!(2) 1J!Z(4) .1J!g(2) .1.l!!l(2) in Table 6. Owing to the application of constraint u 0.00 0.04 o.oo 0.00 0.00 B OC!a!tedron Mn 0.00 0.28 0.00 0.18 0.02 Fe2+ 0.08 0.00 0.14 0.01 01a -01b 101(1) 103(1) 102(1) 105(2) 100(1) 102(1) So2+ 0.02 01a.b • 03a.b 94(1) 88(1) 88(1) 89(1) 88(1) 88(1) Fe3+ 0.26 0.56 0.05 0.41 0.53 -04e,b 92(1) 98(1) 93(1) 90(1) 95(1) 95(1) TI 0.47 0.57 0.75 -04b,a 92(1 91(1 94(1) 92(1) Zt D.21 03o - 03b :m 81(1! 82(1! 78(1) :m 93(1) Srt4+ 2.25 1.17 0.81 1.32 1.73 03a.b -04e.b 87(1) 88(1) 86(1) 91(1) 87(1) 87(1) Nb 0.00 0.14 0.00 0.13 0.01 -04b.a !!§(1) m(1) !!§(1) 1!!1(1) 1!!1(2) !!§(1) Ta 0.89 UQ � ;t!!1 � .M§ <()..S.O> 90 90 90 90 90 90 3.87 4.00 4.00 3.86 a98 4.00 C Oc!a!tedron 01o •01d 86(1) 86(1) 85(1) 88(1) 86(1) 88(1) - 02e 76(1) 77(1) 78(1) 75(1) 77(1) 78(1) - 03o 97(1) 91(1) 99(1 -04! 97(1) :m :m 97(1) :m 95(1! 01d • 02e 77(1) 78(1) 77(1) 78(1) 78(1) 78(1) - 02e 81(1) 81(1) 80(1) 81(1) 81(1) 81(1) -04! 85(1) 87(1) 93(1) 87(1) 85(1) 86(1) 02e -02· 88(1) 88(1) 90(1) 85(1) 89(1) 89(1) 98(1) 95(1) 98(2) 98(1) =� :m 95(1) 94(1) 101(1) 95(1) :m 02e • 03o 90(1) 90(1) 100(1) 81(2) 91(1) 92(1) 03o •04! l!M(1) �1) 1llll(1) .111(1) 103(1) �1) <().C.()> 90 90 90 90 90 90 %Order 100 59(1) 100 100 88(1) 97(2) • Ferguson et81. ( 1976) ·% ordercalculated using a wodglnltestructure. bdollte structure mixing modeL - calculatedfrom Gatehouse ot 8/. (1976)and Santoroot 81. (1977) 604 THE CANADIAN MINERALOGIST TABLE9. POLYHEDRALEDGE LENGTHS (A } FOR WOOGINITE-GROUPMINERALS 2.24 A Site Wod� PartiaDy M·ur_S.Os Ulhlo- Ferman Ferman 0 Ordered wodg!nlle Wodglnlte Wodglnlte Wodg!nlle (Heated} BSite (G8!J.17) (CX·1) (TSE·76) �·17) (A-17h) 2.20 0 AOctahedrQD CSite !:!:. 02e ·02g 3.40(2} 3.22(2} 3.39(4} 3.29(5} 3.29(2} 3.37(2} oct 2.16 02e.g • 03h.l 3.22(2} 3.14(2} 3.27 3.11(5} 3.15(2 3.23(2} .03f,h 3.12(3} 3.04(2} 3.12 � 2.85(4} 3.08(2l 3.13(2} - ·0 1,h 3.08 2. � 3.16 2. 3.05 0 4 (3} � (3}! 3.18(5} 99(2} (2}! 03h,l • 04e,g 3.01(3} 3.05(3 ! 2.98(2 .Q · 04e,g 2.79(2} �= 2. = ��� 2.77(2 0 2.12 04e · 04o g,j!§(2) 2,1§(2} llM4� 2,!!;!(6) 2.ZZ(2} g,zji(2 - <0.0> 3.08 2.97 3.08 3.00 2.97 3.00 A 0 ftOctahedr.gn I 2.08 01a ·01b 3.08 3.17 3.07(4 3.23(5} 3.07(2} 3.10(3 :E (2} (3} } } v 01a.b • 03a.b 2.90(3} 2.�2} 2.78(3} 3.01(� 2.84(2} 2.82(2} • 04a.b 2.87(3} 3.02� 2.84(3} 2.87(4 2.97(2} 2.95(2} �04b,a 2.84(3} 2.92(:1 2.79(� 2.�� 2.89!2} 2.92(2} 2.04 03a · 03h 2.80(2} 2.72( 2.88(4 2.85( 2.76 2} 2.73(2} 03a.b • 04a.b 2.81(3} 2.85(2} 2.77( 3.06(4} 2.84(2} 2.80(2} ·04b,a &1!!(2} 2.la(2} 2,1§{3} 2Jlii(4} 2,1§(2} 2.ZZ(2} <0.0> 2.86 2.89 2.80 2.97 2.87 2.86 2.00 COCtahedBm 01c ·01d 2.88(2} 2.87(3} 2.91(2} 2.91(3} 2.86(2} 2.87(3} 2.00 2.04 2.08 2.12 2.18 2.20 .Q2e 2.68(2} 2.60(3} 2.55(3} 2.53(3} 2.59(2} 2.53(2} ·00. 2.96(3! 2.98�3} 2.97(3! 2.76�4} 2.97(2} 2. 2 ·04f 2.97(3 2.933} 2.89(3 2.92 }4 2.95(2} 2.9196 !2l Interatomic distances and angles for the single crystal experiments and for M-LiTa308 were mean bond-lengths at all sites quite well. The one calculated with the program PARST (Nardelli exception to this well-behaved relationship is the A 1983) in the CRYSRULER package of programs polyhedron of M-LiTap8 and the B polyhedron (Rizzoli et a/. 1986); distances and angles for the of wodginite TSE-76. The distinguishing charac Rietveld refinement were calculated with the teristic of these two structures is the presence of program BADTEA (Finger 1969). The bond lengths significant amounts of Li. for the various polyhedra are given in Table 7, The mean bond-lengths for the B and C bond angles are given in Table 8, and polyhedral polyhedra are practically identical, which in part edge-lengths are given in Table 9. accounts for the phenomenon of Ta disorder between the B and C sites. For all of the samples, DISCUSSION the mean bond-lengths for the A polyhedron are distinctly larger than those for the B and C In the discussions that follow, ionic radii are polyhedra; however, as one would expect, this taken from Shannon (1976). Polyhedron distortion distinction is less for the partia!Jy ordered samples parameters used are � (quadratic elongation = (grand mean deviation: 0.10 A) than in ordered "£(/i-lo)/10, where li is the individual bond-length, ones (grand mean deviation: 0.16 A). !0 refers to < li> ), and a2, the angle variance ["£(8i-9W; 8 = 0-M-0 bond angle), is taken from Individual variations in bond length Robinson et a/. (1971). Figure 4 shows the relationship between the ionic Mean variations in bond lengths radii of the constituents at the A, B and C sites (calculated from Table 6) and the individual As expected for close-packed oxide structures, bond-lengths for their corresponding coordination the observed mean bond-lengths at all the sites in polyhedra. Comparison among Figures 4a, 4b and the wodginite structure are linear functions of the 4c shows that bond lengths for Li-poor members radius of constituent cations. Furthermore, induc of the wodginite group vary linearly with ionic tive effects seem negligible, and a simple hard radius. However, the bond lengths for Li-rich sphere model (Fig. 3) serves to predict the observed members of the group deviate considerably from STRUCTURAL CRYSTALLOGRAPHY OF THE WODGINITE GROUP 605 A-02 the trends shown by the Li-poor compositions in 2.40 !::::, -e the following ways: a A-03 (1) Sample TSE-76 deviates most from the trends --e- shown by Li-poor compositions, and does so for A-04 -A- all polyhedra; however, only one set of values for oct TSE-7 6 actually deviates significantly from the - trends, those bond lengths involving 03. Sample 0 2.20 I TSE-76 is compositionally intermediate to 2.30 ,..------. (2) Bond lengths for the Band C polyhedra (Figs. B- 01 4b, c) of M-LiTa308 follow the trends shown for -e • Li-poor wodginite, as do most of the bond lengths b B-03 --e- for the A polyhedron; however, the A-04 bond 2.20 B- 04 length deviates from the trend by 6a (Fig. 4a). The --8-· A octahedron shows additional anomalous be havior, which suggests that this deviation is real. - First, the anomaly in bond lengths for the A 0 2.10 I octahedron is paralleled by anomalous edge-lengths m - for the octahedron; Figure 5 shows that the mean edge-length for the A polyhedron in M-LiTap8 is 2.00 unusually large compared to the effective ionic radius of the A-site constituent (Li). As the oxygen positions reported for M-LiTa308 were taken from the single-crystal structure refinement (Gatehouse .8 .____.___ _.__ __ ...... _ __ J.._ _ 1 0 ___j et a!. 1976), deviations in 0-0 separations for the 0.84 0.85 0.88 0.87 0.88 A octahedron cannot be ascribed to failings of the 2.30 Rietveld experiment. Second, anomalous beh&.vior .------,c-o 1c 0 -e 3.10 .------, A C-01d c 0 2.20 --e- D C-02a 0 B --0-· 0 oct 2.10 c-oz. c - --8-· 3.00 b,. 0 I [r;Jr;'------=�-�JC-03a 0 --E)-· - 2.00 A C-04f 0 - -A- I - -- _Q_ ...... - -- 0 2.80 1.80 v -- -- � ------0 1.80 '---'---�---'---�---'-----1 0.84 0.85 0.88 2.80 rCA> FIG. 4. Individual bond-lengths versus constituent ionic radii for the A octahedron (a), B octahedron (b), and 0.84 0.88 0.72 0.78 0.80 0.84 the C octahedron (c); the coding of symbols is as in Figure 3. The regression curves for the A octahedron rCA> are for Li-poor samples only; regression curves for the FIG. 5. Mean edge-lengths versus constituent ionic radii B and C octahedra are based on all data except those for the A octahedron. Symbol coding is as in Figure for sample TSE-76. 3. 606 THE CANADIAN MINERALOGIST •B eq.> TABlE 10. BOND VALENCES (v.u.) FOR WODGJNITE-GROUPMINERALS Wod� Partially M-LIT&a0a Lllhfo. Ferman Ferroan (B,C) Otdered .. wodglnlle Wodglnlte Wodglnlte Wodglnlte (Heated) 0 (G89-17) (CX-1) (TSE-76) (A-17) (A-17h) A-02e.g - •B eq.> x2 0.42 0.54 0.19 0.27 0.50 0.41 03hJx2 0.38 0.43 0.16 0.41 0.40 0.34 0 (0) TA BLE 13. MULTIPLE-CELL SUBGROUPSOF Pbcn maximum symmetry consistent with its optimum pattern of order. Of course, the latter is occasional 2x,y , z Pbc2,, P2/c [Pb><, PcY,P2,z, P2,Pi, P1] ly modified by low-temperature (2nd order) dis x; 2y, z P21221, P2/C (P21•, P2,P21•, Pc, Pi, P1] placive phase-transitions, but even these commonly x;y,2z Pb2n, P2,•!b [Pb><, P2,Pnz, P2,•, Pb, Pi, P1] involve additional ordering of cations or anions. Accordingly, we will restrict our attention to those Trtple een structural possibilities that allow full cation order 3lc, y, z } of an n[ABC208] composition with maximum x;3y, z Pbcn (+ansubgroups of Pbcn, seeTable 12 ] symmetry, and will not consider any subgroup x;y,3z derivatives of these (i.e. , lower-symmetry monoclinic or triclinic symmetries). If we strictly Quadruple Cell adhere to the principle that the structure adopts the 2y, z C2ic (P2/c, C2, Cc, P2,Pc, Ci, Pi, C1, P1] 2x, maximum symmetry consistent with full order, all P2,81, P1] 2x,y , 2z Bb><, 82 [Pb, possible space-groups for double-cell schemes must x;2y,2z A2 (P2,A1,P1] have a maximum rank of an equipoint of 4; this is the case for the orthorhombic space-groups and the centrosymmetric monoclinic space-groups. Fo.r the quadruple-cell schemes, equipoints in possible the possible space-groups for a double cell (Table space-groups must have a maximum rank of 8; this 13), Pbc21 has a minimum rank of an equipoint of is the case for (2x, 2y, z) C2/c, which is the observed 4, and thus is not a candidate for a possible scheme of order in wodginite. Double- and structure. This leaves two orthorhombic (P21221 quadruple-cell schemes of order are shown in and Pb2n) and two monoclinic (P2/ c and P21x/b) Figure 8. Note that for (x, 2y, z)P21221 , there must space groups, together with all of the lower be two crystallographically distinct Ta positions; symmetries of the subgroups listed in Table 13. this is also the case for the other quadruple-cell If the prototype cell is tripled, the minimum rank patterns of order [the example of (x, 2y, z)P21221 of an equipoint required for complete order of a is shown in Fig. 8], and presumably these schemes wodginite-like composition 3[ABC208] is I. Pos are not viable on account of their uneconomical sible symmetries are Pbcn (Table 13) or any use of symmetry. subgroup of Pbcn (Table 12), but the requirement What is very interesting about the four principal in equipoint rank reduces this to P2, PT or Pl . schemes of order in Figure 8 is that they are the Note that the situation is much more convenient four simplest patterns that can occur in this type for order of a tantalite-like composition: of arrangement. We can summarize their principal 3[ABC208] - [A3B3C,P24] - [(A3B)(B2CJ024] - features very simply: [X4 Yg024], which can fit into Pbcn symmetry in an (2x, y, z)P21c: all distinct cations ordered in ordered fashion, as the minimum rank of an different chains; equipoint required for ordering is 4, as is the case (x, 2y, z)P21c: A and B (Mn and Sn) alternate along for Pbcn. chains on one level, Ta completely ordered at all If the prototype cell is quadrupled, then a larger chains on alternate levels; number of structures seems possible, as the cell can (x, y, 2z)Pb2n: only one distinct type of chain, be quadrupled in one direction or doubled in two ordered along Z in the sequence A-C-B-C. . . ; directions. If the cell is quadrupled in one direction, (2x, 2y, z)C2/c: A and B (Mn and Sn) alternate the space-group symmetries arising from this are along chains at one level, Ta completely ordered at those listed under "Double Cell" in Table 13. all chains on alternate levels. However, the cell contents are now 4[ABC208], and As there are no recorded examples of wodginite the minimum rank of an equipoint required for with P2/c or Pb2n symmetry, we must conclude complete order is 4; hence all space groups listed that (2x, 2y, z) C2/c is the most stable pattern of in Table 13 under this heading are possible for order. There do not seem to be any obvious features ordered structures. If the quadruple cell is produced of the schemes of order in Figure 8 that indicate by doubling in two directions, the possible C2/c to be the preferred arrangement. The most space-group symmetries are C2/c, BbX, B2 and A2; obvious feature on which to base an assessment of the minimum rank of an equipoint required is 4, the patterns of order is the relative cation-cation and thus all of these space groups are compatible repulsion, local association of high-charge cations with ordered arrangements of cations. presumably maximizing this particular energetic There are presumably two factors affecting the term. Arrangement (2x, y, z)P21 c has the maximum actual choice of the structure of wodginite, the association of high-charge cations, both Ta-Ta and drive for cation order, and the observation that a Sn-Sn coupling being at their maximum. Of the structure generally adopts the arrangement of other three arrangements, (x, 2y, z)P2/ c and (2x, STRUCTURAL CRYSTALLOGRAPHY OF THE WODGINITE GROUP 609 DOUBLE-CELL ORDERING (2x,y, z) P2/c (x, ?y,z)P2/c (x,y, 2z)Pbcn C - C C - C C - C B - C \ \ I \ \ B - B A - B B - A B - C I Z= 1/2, % C - C \ (x, ?y,z) P21221 A-A B - A D - C A-C \ I \ \ A - B C - D A - C Z= O,% QUADRUPLE-CELL ORDERING (2x, ?y,z) C2/c C - C C - C \ I \ B - A A - B I \ I C - C C - C \ I \ A - B B - A FIG. 8. Selected arrangements of cation order for supercells of the ixiolite structure. 2y, z)C2./c are very similar; in each arrangement, members of the wodginite group, the use of the chains are the same, the only difference being constraint equations involving a wodginite-ixiolite a l/2b displacement of the structure at each A-B mixing model can give precise measures of the 1level. Arrangement (x, y, 2z)Pb2n seems the most degree of cation order. ' favorable from the point of cation repulsion, as it (3 ) The heating of samples under conventional separates the Ta atoms from each other. Thus a conditions of 1000°C for 16 hours induces full simple argument of this sort cannot account for order of cations in wodginite-group minerals. the sole occurrence of the (2x, 2y, z)C2./c structure (4) For both mean and individual bond-lengths, a for wodginite; more sophisticated calculations of simple hard-sphere model predicts compositional energy seem necessary. These calculations, together and structural variations well. However, Li-rich with additional experimental and theoretical work samples show deviations that seem to indicate that on the order-disorder relations in this system, are Li is disordered off the A position. currently under way. (5) For a wodginite-like ABC208 composition, patterns of order involving an ixiolite subcell have SUMMARY been considered. Symmetry considerations for unit cells to four times the cell volume of ixiolite show (1) The degree of cation order for wodginite-group four most probable patterns of order. Three of minerals is variable. Partially ordered samples are these are doubled cells; one is a quadrupled cell structurally intermediate to wodginite and ixiolite. (wodginite structure). Only the quadrupled cell (2) With respect to structure refinements for occurs naturally; because there is nothing obvious 610 THE CANADIAN MINERALOGIST about its scheme of order to indicate why the wodginite from Bernie Lake, Manitoba. II. quadrupled cell is preferred, more sophisticated Wodginite. Can. Mineral. 14, 550-560. calculations of energy will be necessary to explain FINGER, (1969): why only this variant occurs naturally. L.W. U M BADTEA. Geophysical Laboratory, Carnegie Institute of Washington (un publ.). ACKNOWLEDGEMENTS GATEHOUSE, B.M., NEGAS, T. & ROTH, R.S. (1976): The We are grateful to J. Siivola and E. E. Foord for crystal structure of M-LiTa308 and its relationship to the use of some of the samples used in this study, the mineral wodginite. J. Solid State Chern. 18, 1-7. A.C. Turnock for use of the quench furnace, and GRAHAM, J. THORNBER, M.R. (1974): The crystal the Tantalum · Mining Corporation of Canada & Limited for access to the workings of the Tanco chemistry of complex niobium and tantalum oxides. Am. pegmatite during the period of study. Financial II. Composition and structure of wodginite. Mineral. 59, 1040-1044. support for the study was in the form of NSERC operating and equipment grants and a Tantalum GRICE, J.D. (1973): Crystal Structures of the Tantalum Mining Corporation of Canada contract to It Oxide Minerals Ta ntalite and Wodginite, and of NSERC operating, major equipment and in Millerite NiS. Ph.D. thesis, Univ. Manitoba, Win frastructure grants to FCH, and a NSERC nipeg, Manitoba. Postgraduate Scholarship, University of Manitoba fellowship and Canadian Museum of Nature __, FERGUSON, R.B. & HAWTHORNE, F.C. (1976): The research stipend to TSE. We thank the referees and crystal structures of tantalite, ixiolite and wodginite R.F. Martin for helpful reviews of the paper. from Bernie Lake, Manitoba. I. Tantalite and ixiolite. Can. Mineral. 14, 540-549. REFERENCES HILL, R.J. & HOWARD, C.J. (1986): A computer program for Rietveld analysis of fixed wavelength X-ray and BROWN, I.D. (1981): The bond-valence method: an neutron powder diffraction patterns. Aust. Atom. empirical approach to chemical structure and bonding. Energy Comm., Lucas Heights Res. Lab. (Menai, In Structure and Bonding in Crystals II (M. O'Keeffe NS W, Australia), Rep. M112. & A. Navrotsky, eds.). Academic Press, New York. __ & MADSEN, I.C. (1984): The effect of profile step CROMER, D.T. & LIBERMAN, D. 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Received June 26, 1991, revised manuscript accepted May SHANNON, R.D. (1976): Revised effecive ionic radii and 21, 1992.