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The Crystal Ghemistry of Complex Niobium and Tantalum Oxides Ll. Composition and Structure of Wodginite

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The Crystal Ghemistry of Complex Niobium and Tantalum Oxides Ll. Composition and Structure of Wodginite American Mineralogist, Volume 59, pages 1040-1044, 1974 TheCrystal Ghemistry of ComplexNiobium and Tantalum Oxides ll. Compositionand Structure of Wodginite J. Gnermu, axo M. R. TnonNsnn Diaision of Mineralogy, CSIRO, Floreat Park, Iilestern Australia, 6014 Abstract The structure of wodginite (non-centrosymmetric monoclinic space group Cc; ideal formula, MnXtX[lao,, where site Xg contains 5-valent ions (and all the niobium) and the average valency of cations in site X, is 4) was determined by film methods to be an ordered form of tho ixiolite structure. The a and D cell edges and angle B increase with manganesb content, whereas c remains constant for wodginites from five localities in Western Australia. Composition and Lattice Parameters from Wodgina, hand picked from a homogeneous The charasteristicsof the mineral wodginite were region of a polished section.Electron probe analysis clearly describedby Nickel, Rowland, and McAdam and powderX-ray diffraction (Hiigg Guinier focussing (1963) for samplesof the mineral from Wodgina, camera) gave the cell dimensibnsand composition Australia, and Bernic Lake, Manitoba. The s,truc- usedin the calculationsshown in Table l. Data were ture was not fully defined, and the efiect of com- collectedphotographically along two axes([010] and positionalvariations on the st,ructurecould not there- Ul2l) using a Nonius Integrating Weissenberg fore be discussed. camera, equiinclination geometry,and CuKa radia- Table 1 shows the compositions and lattice tion. After the applicationof Lorenz and polarization parametersof wodginitesfrom five localitiesin West- corrections, these two sets of data were inter-cor- ern Australia and the averageparameters quoted by related, and symmetrically similar reflections were Grice, eerny and Ferguson (1972) fbr the wod- averagedto give 555 independentobservations. No ginites from the Tanco pegmatite at Bernic Lake, correction was applied for absorption due to the Manitoba. Thesevalues are by no meansrepresenta- difficulty of precisely describing the shape and tive of the wodginitesfor the various localities, but orientation of such a small crystal with the apparatus they are quoted to demonstratethat there is a con- available. This correction would have a small but siderablevariation in cell volume correlatingto some significant effect (pR - 4), which would be partly degreewith the Mn/(Fe * Mn) ratio, as shown offset, however,by the averagingof up to 8 separate graphically in Figure 1. There is also a strong cor- intensity measurementsto give a single observation. relation betweenmanganese content and the a and b Structure refinementwas carried out in both the centrosymmetric C2 c parametersand the monoclinic angle B; the chain / and the non-centrosymmetric length,c, remainsconstant. space group ,Cc using the full matrix least-squares Figure 2 demons,tratesqualitatively how the metal program of Busing, Martin, and Levy (1962) and content can vary within a small crystal fragment the scattering factor data of Cromer and Weber from a wodginite sample. This could indicate un- (1965). The result was confirmed by Fourier mixing taking place at low temperatures,and makes methods. Refinementin the centrosymmetricspace it clear that a well-characterizedstructure may be group gave an R factor of 14.3 percent with 21 difficult to achieve,particularly in a large crystal. variables, while the non-centrosymmetric space Crystals of wodginite have been located that have group gaveR = 13.1 percentfor 41 variables.The small nuclei of cassiteritedotted throughout. improvement is highly significant using Hamilton's (1965) criteria. The positional parametersof the Structure Determination atoms for the two refinementswere not significantly A structure determination was carried out on a different,but the absence,of centrosymmetryallowed wodginitecrystal fragment (-0.03 X 0.04 X 0.06mm) an ordering of the metal atoms that was not possible 1040 COMPLEX NIOBIUM AND TANTALUM OXIDES 1041 Tlsln l. Compositionand Unit Cell Dimensionsof VariousWodginites speci[Fn l]ocallty* cell ?ilareters (8) Vol (83) lGtal content Nqlqlized to 32 oxygetls ltl,/Fe+Mr abcB Ta Nb l{n Fe sn El 2A7a Geenbushes 9.-468(5) Lf.432(5) 5.r10(2) 90.74(s) s53.1(r) 7.70 1.2s 2.11 2.72 2.Ol o.2l .437 l,tDC1453 l.{t. MaLtherv 9.448(5) 1r.435(4) s.os9(3) 90.76(s) 5s0.8(r) 2.L4 2.52 2.OO 0.30 ;459 s 388 B wodgina 9.530(7) 1r.490(s) s.oe8(4) 9r.2r(6) ss8.1(2) 8.48 1.r. 3.83 o.2l 1.87 nll .947 s 3315 Tabba Tabba s:szs(gl L1.479(s) 5.rrs(4) 91.2s(6) 559.1(2) 8.13 0.68 4.24 o.05 2.64 0.26 .988 4292 Marble Bar 9.537(9) 1r.47(r) 5.rr2(6) 9L.24(91 5s9.r(4) 8.56 0.6L 4.06 0.52 1.80 o.d6 .885 eice Bernic Lake 9.50f(9) 1I.453(2) s.r13(s) 9l.oo(6) 556.3(2) 7.94 0.35 3.33 0.98 2.54 0.95 et aL * A}l fril vlestern Awtralia qcept Bernic Iake Uditoba. in the centrosymmetricspace group no matterwhere the origin was placed. Electrostatic calculations (Part V) also showedthat the non-centrosymmetric metal ordering is the most favored. The ordering was determinedby using an average o scatteringfactor for the metals, and refining an oc- r + cupancy factor and positional parameters.The fact that one fourfold metal site was occupiedby Mn became immediately apparent. In the non-centro- .g symmetric space $oup, this rhethod also indicated s o that one fourfold site was occupied exclusivelyby E a Ta while the other two fou'rfold siteswere occupied E by a mixture of the remaining cations. Table 2 showsthe positional parametersfor the atoms, and unit A3 the bond lengths are shown in Figure 3. Oxygen "ett'lltum" parameterswere not accura'telyobtainable in view Frc. l. Variation of unit cell volume of wodginite asa Fe). of the large scattering factor of tantalum, so the function of the atomic ratio Mn/(Mn * Mn counts Sn counts 3 Nbcounts x 3 grain from a sample of Frc. 2. Qualitative variation in composition of an inhomogeneous 200rr wodginite. Electron microprobe step scan of XRF counts for Mn, sn, and Nb over a interval of wodginite from sample S 388 B from Wodgina. lo42 I. GRAHAM, AND M. R, THORNBER Teurr 2. Atomic Positional Parameters accuracyof thesebond lengthsis low (probably less than the computed accuracy,which was 0.08 A for M-O bonds and 0.12A for the O-O bonds). The 0.37s(3) 0.414(1) o.2so(4) 0.4 (1) O-O distancesfall in the normal range. The average XI 0.375(3 ) o.089(1) 0.7s0(4) 0.8(1) 0.125(3) 0.16e(r) 0.250(6) 0.8(1) metal-oxygendistances are Ta-O = 2.09A, Xr-O = Mn o. 125(6) 0. s43(r.) 0.7s0(e) o.7 l2') 2.08A, Mn-O : 2.13A, and Xr-O = 1.97A. Ob- OI 0.25s(e) o.085(8) 0.088(16) servedand calculatedstructure factors are shown in o2 o.so7(9) 0.438(8) 0.908(16) Table 3. 03 o.266(e' 0.428(8) 0.5ss(16) o4 0.4er(e) 0.0s1(8) 0.448(16) The metal temperaturefactors were only refined 0.2s6(9) 0.296(8) o.102(16) subsequentto the determination of the occupancy, o,s25(9) 0,183(8) o.884 (16) o7 0.247(9) 0.1e2(8) o.577 lL6) and becauseof the high correlationbetween scale ( 08 0.s2s(9) 0.312(8) 0.406 16) factor, occupancy,and temperaturefactor, they are c Wxr @u" @ro Oxz Qoxyg"n Frc. 3' Idealized projections of structure along the a axis from a : O to 0.5 showing oxygen-oxygen(top) and metal-oxygen (bottom) bond lengths in A. Tasrs 3. Observed and Calculated Structure Factors for Wodginite ir k I r(obs) r(calc) h k 1 F(obs) F(calc) I F(obs) E(carc) h k r F(obs) E(caic) I f(cbs) F(calc) r4-2 12.2 6a 10.8 t r.3 4 0 o 40.5 49.5 -6 2 51 -2 2 a2 89 -4 11,1 14.6 -68 14.7 11.0 6 0 0 11 7 14.6 10 2 16.1 14 9 1 ; 2 28 2 28.4 6 42,A 4I.1 88 26.2 8 o o 4r.2 -to 2 r1-2 r5 2 -4 4 2 346 241 66 -66 55.6 43.4 27.9 IO 0 o 8.9 11.6 0 3 14.3 13.2 6 4 2 6.7 I L 12.3 0 9.9 L2 0 0 2r.2 21 5 2 3 49.6 15 I -6 4 2 86 -8 lr.f 9-2 2 7.1 5.6 0 o 2 58.2 54.2 -2 3 465 494 a 2 2A2 211 6 33.I 29 1 2 -4 5.6 1.4 2 o 2 6.1 3.5 1 3 I4.1 13 0 -a 4 2 28 5 22.4 to6 -r0 io. o 4 4 I4-7 2 o -2 24.2 28.0 3 rt.6 rt.8 r0 2 85 1a 6 32.5 3.4 4 -4 I0.9 4 2 65.6 624 6 3 39.3 31.O -lo 4 2 6.a 6.2 06 6 1.3 -2 54.8 62.1 -6 3 40 9 35.2 0 4 3 26 6.1 5.2 -26 36 6 -4 6.7 6 2 21,8 2r.2 8 3 9.2 lO 8 2 4 3 130 107 94 3.7 0 24.
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