Canadian MineralogistVol. 14, pp. 550-560 (1976)

ABSTRACT

The crystal structure of wodginite from the Tancopegmatite, Bernic Lake, Manitoba has been analyzedusing MoKa radiation and 3-dimensional 4-circlediffractometer data. Monoclinic, a 9.489(5), bll.429?), c 5.105(3)4, p 91.10(5)'. Symmetricallyit is possible to choose any orre of four possiblepositions in tho parent ixiolite structure as aoorigin for wodginite, and these possible origrng co6-bined with the two possible space €roups, centricC2/c and, acentric Cq result in eieht closely relatedbut different possible structures for wodginite. Re-finement was carried out for all eight using aprocedure that assumed no particular cation or-dering scheme. The wodginite structure based onan origin in ixiolite at (0,0,0)r and space groupC2/c refined to R=6,tVo with reasonable aniso-tropic temperafure factors for all atoms and isclearly the correct structure. The cations are ess€n-tially ordered: Z-4lACBrO;l-4[Mn1.sn(Sn6.su"Ta6.256Tie.13Fee+o.oro)Cfao,asrMo.ror)Oel. Mean ogta-hedral cation-oxygen distances and radii sunu areA-2.185(8), 2.19; 8=2.003(8), 2.00; C=2.027(8),2.044. Botrd strengths distributed inversely as the4th power of the cation-orygen distances yiel<led2.O2n, 2.008, 1.978, 1.984 v.u. to O(1), O(2), O(3),O(4) respectively. Two previously described wod-ginite structures, one by Grice (1973) and Grice &Ferguson (1974), and the other by Graha:n & Thorn-ber (1974) are based on ixiolite origins (0,14,0)r and(0,0,0)1 and on space groups C2/c and Cc re-spectively. Both have hieher R factors (-tl/6)and different cation ordering patterns from ours,and show poor agreement between mean octahedralcation-oxygen distances and calsrrlated radii sums.

SOMMA,IRB

La structure cristallino de la wodginite a 6t6analys6e, sur un cristal provenant de la pegmatitede Tanco, Bernic Lake, Manitoba, i laide de don-n6es tridimensionnelles obtenues en rayonnementMoKo au diffractombtre i 4 cercles. La wodginiteest mo_noclinique, a 9.489(5), b ll.4290), c5.105(3)4, p 91.10(5)'. Au point de vue- sym6trie,on peut trouver, dans la structure type de fixiolite,

*Present address: Mineral Sciences Division, Na-tional Museum of Natural Sciences, Ottawa, On-tario, KlA 0M8.

THE GBYSTAL STRUCTURES OF TANTALITE, IXIOUTE ANDWODGINITE FROM BERNIC LAKE, MANITOBA

II. WODGINITE

R. B. FERGUSON, F. C. HAWTHORNE eNp J. D. GRICEE

Department of Earth Sciences, University of Manitoba, Winnipeg, Manitoba RsT 2N2

quatre origines possibles pour la wodginiie; com-bin6es aux deux groupes spatiaux possiblas centr6(2/c et non-centr6 Cc, ces quatre origines condui-sent i huit structures possibles, diff6rentes quoiqueapparent6es, pour la wodginite. L'affinement deshuit structures a 6t6 cotrduit I l'aide d'une m6thodequi ne fait aucutro hypothdse sur la mise en ordredes cations. La structure de la wodginite fond6esur utre origine de I'ixiolite I (0,0,0)r et groupespatial C2/c, affin6e jusqu'au r6sidu R=6,IVo avecdes coefficients d agitation thermique aniss66peraisonnables pour tous les atomes, est 6videmmentla bonne. Essentiellement, les cations sont ordon-nds: Z=4lACBzOl=4[Mn1.66(Sn6.567Tas.gsTi..11tFee+o.ozo)Clao.ss1Nbq,1ss)2o1. I.es longueurs moyeiltesles liaisons octa6driques cation-oxygdne et les som-mes des rayors sont: A-2.185(8),2.19; 8:2.003(8),2.00; C=2.027(8), 2.04L. La valence de liaison6tant prise inversement proportionnelle i la qua-tridme puissance de la distance cation-oxygdne, onobtient les sommes 2,020, 2,008, 1.978, 1.984u.v. I O(1), O(2), O(3), O(4) respec'tivement. Deuxstructures ant6rieures. de la wodginite, I'une parGrice (1973) et Grice & Ferguson (1974), tattrepar Graham & Thornber (1974), sont fond6es surles origines (O,/z,O\ et (0,0,0)r de fixiolite et surles groupes spatiaux C2/c et Cc respectivement.Elles possldent chacrrne un r6sidu R *l2Vo plns6lev6 que le n6tre et une ordonnance des cationsdiff6rente de la n6tre; de plus, l'accord entre leslongueurs moyennes des liaisons octa6driques ca-tion-oxygdne et les sommes calcul6es des rayonslaisse i d6sirer dans ces deux structures.

Cfraduit par la R6dactibn)

INrnooucrtoN

In Part I of this pair of papers on ttre struc-tures of the tlree principal tantalum ore min-erals from the Tanco pegmatite, Bernic Lake,Manitoba, the Introduction gives a generaldescription of tle occurrence, nature and inter-relationship of the tantalite and ixiolite and, ina general \ilay, of the wodginite. The Introduc-tion, Table 1, and Figure 1 in that paper pro-vide a background to the more detailed de-scription of the nature of wodginite that followshere.

Wodginite was first characterized as a dis-tinct mineral phase by Nickel et aI. (I963a),

550

CRYSTAL STRUCTIJRE OF WOMINITE 551

whb showed that it occurred at Wodgina, Aus'tralia (the type locality) and Bernic Lake, Mani-toba. Detailed descriptions of the chemistryand mineralogy of wodginite and its associatedminerals are given by Cernf & Turnock (1971)and Grice et

-al. (1972). Turnock (1966) showed

that wodginite forms over a small compositionalrange in the system MnTarOo-FeTaOa, and hefound that it could not be synthesized in theabsence of Fe3+. However, Komkov (1970)showed that wodginite can be formed in theabsence of Fe8+ if Sna+ is present in the system.These experimental results are supported by thechemical variation exhibited by natural wod-ginites (Grice a al. 1972; Graham & Thornber1974), and suggest that polyvalent cation sub-stitution and ordering play an important part incontrolling the stability of this phase. TWo dif-ferent structures have been reported for wod-ginite, one by Grice (1972) and Gice et aI.(1974), and the other by Graham & Thornber(1974); the structure reported in the present

study is significantly different from either ofthese. Although ail tlree structures are topo-logically equivalent, the cation distribution pat-tefo is iignificantly different in each. As it ap-pean that wodginite forms a distinct pta,se -be-cause of polyvalent cation ordering' it .i! im-portant to-thi understanding of the stability ofihis phase that its cation ordering pattern becorreitly characterized. Consequently, -the var-ious ordering schemes possible have been ex-tensively investigated in this study, and arereported in detail.

The following conclusions may be drawnfrom the cell geometry presented in Table Iand Figure 1 of the Part I paper as well asFigure 1 of this paper:

(1) monoclinic-wbdginite with p-91o has astrong subcell corresponding to' the cell ofixiolite (a-PbOn structure type); as the symmetryof tle subcell is P2/ c (Elphick 1972), there arctwo cation sites rather than one as in ixioliteand thus even the subcell of wodginite could

t/t

1;t,o F

Il-Io

I t r r : t r N I N!__!__\z-!..ry

_- SHARED oqISEOR{ EDGES

. SHARED OXYGEN

. . . . . . . . c - c u D E-.-.- N- 6UDE+ 2

A c c a r o N ( 5 n , T o , T l lO A c a r o r ( M n )r B c a r r o n ( T o )

o o x Y G E N

Frc. 1. Projection of the structure of wodginite along z. Prototyfre atoms are labelled A(=Mn), B(=Ta,Nb),C(-Sn,*,Tt, O(1), O(2), O(3), O(4). Shown are chains of'staggered. (A,^Cy")4 octahedra parallel to< seen end-on: B-O octahedral chains are not shown.

.to'

552

00.35mRadlatton/i lon. Mo/CNo. of equly. lrobsl>3o Zr9

THE CANADIAN MINERALOGIST

be more cation-ordered than the cell of ixiolite;(2) relative to the subcell, the true cell of

wodginite has doubled a and b dimensions, butthe same c dimension, and it has three (C2/ c)or four (Cc) cation sites; it is therefore possiblefor wodginite to be much more ordered thaneither ixiolite or tantalite;

(3) the true cell of wodginite has the follow-ing relation to the true cell of tantalite:4-*).J lp2/3, ati b.-11.5 A-2bti c*-5.t4- ctThus the cell content of wodginite is 4/3that of tantalite, namely 4/3 l(A,B)nOzl=(A,B)nOae.

E><peRrMeNrer,

The qualify of the wodginite crystals wasgenerally poor, but because specimen G69-L7(Grice er al. 1972) is relatively well-crystallized,it was chosen for the single-crystal study. Thechemical, physical and crystallographic data forthis specimen, including a new electron micro-probe analysis of the crystal used for the struc-ture analysis, are given in Table 1.

Because the reflections that differentiate thetrue cell from the subcell (see Introduction) areweak, a fairly large crystal was used initially tocollect the intensities, namely a sphere of-O.45mm diameter. Ilowever, the quality ofthis crystal was poor and the intensities werenot satisfactory. A second data set was collectedon a second smaller sphere of -0.35mm diame-ter using a Syntex Pi automatic diffractometer.The cell dimensions given in Table 1 were

TABLE I. CHE}iICAL, PHYSICAL AND CRYSTALLOGRAPH1C DATA FORI,IODGINITE

Speclren no. (Grlce et aL. 1972\ A69-17Crystal dlareter

derived during the automatic orientation pro-cedure; they are slightly different than thosederived earlier by Grice et al. (L972). Intensitieswere collected over two asymmetric units oufto 2A(L'4.oKq.) 60o. For a few of the symmetry-equivalent pairs of reflections, the agreementwas poor; the reason for this was not appaient,but where it occurred thelarger value wasuiled.The data were corrected for abso4fion, Lorentzopolarization and background effects ffl aver-aged to produce an asymmetric set. A reflec-tion was considered as observed if its magrlitudeexceeded 3 standard deviations based on count-ing statistics; this resulted in 814 rEflections ofwhich 718 were considered as observed.-

Preliminary considerations: posrible origins lorthe wodginite unit cell in the ixiolite cell,' and.possible cation distibutions in wodginite

Nickel et aL (I963a) concluded that wod-ginite, because of the general similarity of itschemistry, powder pattern and unit-cell charac-teristics to those of tantalite (as described in the-Introduction), must have a closely related'struc-ture. They pointed out that, for the two pos-sible space groups for wodginite, C2/ c and Cc,and with Z-MuOsz (Table 1 in Part I), the 16cations could occupy either two 4-fold positionsand one 8-fold position in C2/ c, or four 4.foldpositions in Cc. From their chemical analyses;they were unable to group the nletallic ele-ments to give whole numbers of particular ca-tions in one site, and they thus-concluded thatthe 16 cations may be randomly distributed inthe 16 available sites in either space group,'

In a detailed analysis of the structure of wod-ginite, an obvious starting-point is the structureof tantalite. Ilowever, as we describe in thePart I paper, the cation-ordered tanl*lite stnrc'ture is based on a supercell of the ca*ioii-dis-ordered ixiolite structure whose structute LWe.is that of a-PbO, (Wyckoff 1963). The wod-ginite structure is based on another and differ-ent supercell of ixiolite, and so it is more ap-propriate to relate the structure of wodginiteto that of ixiolite than to that of tantalite. Start-ing with the known ixiolite structure, it is neces-sary to make two initial decisions: first, whichof the two possible space groups to assume forwodginite, and secondo which of several pos-sible origins in the ixiolite unit cell should bechosen as origin for the wodginite structute.

Regarding possible origins for the wodginitestructure in the ixiolite structure. we considerfirst the more restricted case of the centric spacegrotp C2/ c for wodginite. The symmetry char-acteristics of. C2/ c (No. 15) are given in theInternational Tables, Vol. I (1952) and moet

F (obserwd)F (a'11 data)ar(observed)4d(al l data)

Chemlcal Cel lqnalysisl _Soj&A 22Ta206 68.64 wt.? Ta B.2oNbio; 3.96 Nb o. 79T i 0 2 1 . 4 4 1 1 0 . 4 7sn02 13 .00 Sn- 2 .27FeZ03 0.79 Fer+ 0.29l'1n0 10,74 l{n 3.98rotur ge:t (o 32)

6 . 16 . 67.5 Space7 , 6

a s . a89( 5 )nb 11.429(7)o 5 . 1 0 5 ( 3 )I s r . r Q l s ) 'l/ 553.54rqrcup c2 le (No.15)

, (obs) 7 .195 s /cc, ( c a l c . ) 7 . 8 0r 491 m-l

Fomula used S lmples t s t ruc tu ra lln reflnerent3 io*1u4 (z=4)specles content site content

tln 3.98Mn*4.74 Fe 0.29 a ytn t.oo

Tt 0 .47

Ta 8 .20 Ta 8 .20 U

. Sn 2 ,27 cSn-3 .05 Nb 0 .79

Ta 0 .991Nb 0 .1 09Sn 0 .567Ta 0,250T.t 0. I l3Fe3+ o.o7o

0 8n(z)=>( lrousi- lo""r. l )Zlrourl ,

o t '

FD.FD( lFobs| - Jpca.1sly'5"rs6zlh, r=trmp'eraiure factor fom used: dxp

l- i, ,1, ,uo, 'o)rElectrcn mlcrcprcbe analysls by E.E. Foord (pr ivate com. to. ! . C e m y , ] 9 7 5 ) . z B a s e d o n 1 6 C a t l o n s p e r f o m u l a u n l t .JSee text fof further detal ls. Frcm the present structure

a n a l y s l s . , F M N l c k e l e t a Z . ( 1 9 6 3 a ) .

CRYSTAL STRUCTURE OF WODGINITE 553

are 6hown in Figure 1 (of the final structure).From these it may be seen that Ts occur at(0,0,0), (Yz,O,O), etc., 2-fold axes at (0,y,/+),

{Vz,y,Y+), and c-glide and n-glide planes at(x,O,z),(x,t/z,z)and(x,/q,z),(x,t/+nz\respectively.The relevant symmetry elements in the parentixiolite structure with space group symmetryPDcn (No. 6O) Qntemational Tables, Vol. /, andFig. 2c in Part I) are T's at (0,0,0), (Vz,O,O), etc.n2-fold axes at (O,y,r/e), (Vz,y,Vq), and c-glideplanes al (x,O,z), (x,Vz,z). Further considerationof the structure of ixiolite shows that, in rela-tion to 2at*a*, planes which are c-glides inixiolite become n-glides in the doubled cell. Acareful comparison of the $ymmetry require-ments for wodginite (C2/ c) with the symmetrycharacteristics of ixiolite, considered in the lightof the relative cell dimensions of the two struc'tures (a*-Zar, b*n2b\ c*-c), teveals that thesymmetry requirements for wodginite would besatisfied if an origin for wodginite lrere chosenat any of four different positions in the ixiolitestructure, namely: (0,0,0)r, (Vz,O,0)4 (0,/z,O)u(Yz,Vz,0)r. By comparing the structure of tan-talite with that of ixiolite @igs. la and lc re-spectively in Part I) and taking account of thefact that certain symmetry elements in cation-disordered ixiolite degenerate to only pseudo-symmetry elements in cation-ordered tantalite,it may be seen that the four possible origins forwodginite in ixiolite have their equivalent inthe tantalite structure at, respectively, (0,0,0)r,(1 / 6,0,0)', (0,V2,0)t, (I / 6,Vz,O)t

Wodginite structures whose origins are takenat these four different positions in the ixiolite(tantalite) strucfure would be similar topo-logically but slightly different structurally, andin particular the four different structures wouldeach be compatible with a different kind 61cation distribution scheme as described below.The conclusion may be drawn that one mustrefine wodginite structures based on all fourpossible origins in order to be sure that oneof these is more satisfactorv than the otherthree.

In addition to taking account of the fourpossible origins, one must also consider thatthe space group may be Cc rather than C2/ c.Because Cc is acentric. the constrarnt that theorigin of wodginite must be at aT in ixioliteno longer holds, and furthermore, the 2-foldaxes no longer exist; however, the c- and z-glides do remain in Cc, and they must coincidewith equivalent or pseudo-equivalent glideplanes in ixiolite. In this case the wodginiteorigin could be taken, from a symmetry pointof view, at any position in either of planes

(x,0,2), or (x.l/z,z)r, In these circumstance, itwould be necessary to' arbitrarily define theorigin in lbe xz plane by fixing one atom, andfor wodginite, the most appropriate atom wouldbe the heavy Ta (or the cation site that containsthe most Ta). Although for Cc, the wodginiteorigin could be chosen at any position in(x,0,2), or (x,l/z,z)r in practice because of thecloseness of the symmetry to C2/ c even if itreally were Cc, one would choose the origin at

a t in the ixiolite structure, that is, at one ofthe four origins possible for C2/ c. Because oftle lower symmetry of Cc in relation to C2/c,the cation distribution schemes possible for wod-ginite in Cc are different from those n C2/c'and we may therefore draw a second conclu'sion, namely that one must reline wodginitestructures for both possible space Sloups, C2/cand Cc.

We now consider possible cation distributionschemes for wodginite. As described below, ourresults indicate that the most satisfactory struc-ture is one based on space grotp C2/ c rathettlan Cc, and on ixiolite origin (0,0,0)r, ratherthan one of the other three. For this reason'we consider here mainly the distributionschemes for C2/ c, and our particular interestis in the relationship between the cation dis'tribution scheme for the correct strucfure basedon origin at (0,0,0)r and such schemes for struc-tures based on the other three possible origins.The possible cation distribution schemes forwodginite in C2/ c for the four possible originsin ixiolite are given in Table 2 where it maybe seen that two of the possible origins'(V2,0,0)t and (Vz,/2,0)r, lead to the same cationdistribution scheme (4N-4R|'-4P, 4Rt=4Rt'-4Q, GP *4Q)-(4P" +4Qt)=8R. Ilowevet,the three cation sites for one of these originshave different parameters from those of theother, and thus the two distribution schemes,although the same, correspond to differentstructures.

For the acentric space group Cc, the cationdistribution schemes (which are not includedin the Table) differ from those for sentricC2/ c for any given origin by the fact that theone 8R-type site in C2/ c degenerates to two 4R-type sites in Cc. This increases the number ofpossible cation distribution schemes in Cc overthose possible in C2/c.

A relevant question is whether the knowncation-ordering scheme in the tantalite structure(Part I) can give any clue to the likely cationdistribution scheme in wodginite. It may beshown that, regardless of which of the fourpossible origins is chosen for wodginite andusing the symbols of Table 2, for space group

554 TIIE CANADIAN MINERALOGIST

TAILE 2. POSSTBLE ORIGINS TOR WODCINITE IX DNOLITf,lND POSSIBLE CATION DISTRIII'IION SCun{ES IN

IIODGMIIE Fof,. SPACE GIOUP e2ld

the bulk composition as a linear constraint inthe refinement of site-populations (Finger1969b). Because of the similarity in scatteringpowers .be.t-ween Mn, Fe and Ti, these tbreespecies were combined and expressed as Mn*;similarly, Sn and Nb were combined and. ex-pressed as Sn*. The chemical formula used forconstraining the site-populations is given in Ta-ble 1 in terms of Mn*,. Sn't' and Ta. R-fastorsquoted below are of the forms given in Table 1,expressed as percentages.Provision for all possible cation distributions.In order to fulfill the requirement describedearlier that the solution for the possible site-ordering schemes for the various possible struc-tures be the most general, the site occupancyrefinements were handled in the following man-ner. In C2/ c, one 4-fold cation site wasdesignated A, the other C, and the 8-fold site81. For origins (0,0,0)r and (Yz,Vz,O\, site ./corresponded to P and P' and site C to I andQ/ respectively in Table 2, and th9 two siteswere assigned parameters close to those givenin Table 2. For origins (r/2,0,0\ and (O,Yz,O)tA corresponded to grr and Qtt' and C toP// and P"' respectively in Table 2 and werealso assigned parameters close to those in theTable. For all four origins, site B correspondedto the R-type sites (RnR',R",Rtt\ and was givenparameters close to those in the Table for theparticular origin. For all four origins, appropri-ate approximate parameters were deduced foreach of the four oxygen sites from the knowntantalite structure.

Regarding assumed occupancies for the ca-tion sites. the 4-fold u4 site was assumed to beoccupied by Mn'o and Sn* with the initial oc-cupancies set at L.0Mn* and 0.0Sn*. The 8-foldB site was assumed to be occupied by Ta andSn* with initial occupancies of 1.0Ta and0.0Sn'&. The other 4-fold site, C, was split intotwo half-occupied sites with the positional para'meters and temperature factors of the secondsite constrained to be equal to those of the firstsite, and with the two half-occupied sites com-bined constrained to be occupied by the cationsnot already occupying the other two sites, thatis initially by Ya (O.7 4Mn&*3.06Snt +0.20Ta)(Table 1). With this arrangement of sites, itwas possible to refine the Mn* occupancy (withconstraints) over the A and C sites, and the Taoccupancy over the B and C sites (with con-straints). In this manner, all possible ordering

'These designations were adopted after the com-pletion of the refinements so that they would con-form to those given for these tantalum oxide min-erals in Table 1 of Part 1.

Par@ter6 (!,yrz) glv@ bele ale apprctl@te.

ii"rli:" to,o,orl totz,rt2,or!

13fi* (o,o,o), (Lt4,Lt4,or,

Iat catioo

ajEibol 4P(r 'y 'z)s

2nd catl.otr

sFbol 4q(e,y,z)r

3rd catlon

ByEbol( : , y , 2 ) w

8R 8RI

0.25 ,0 ,41 ,0 .25

(1/2,0,0)r (0,1/2r0)r

(u4,o,o)r (0,1/4,0)r

4P" 4P'r0 , 0 . 0 9 , 3 / 4

4q" 4qrt0,0.58,314

8Bu 8R'"0 , 2 5 , 0 . 1 6 , 0 . 2 5

4ni

4Pq.4Q' 48il'

4P'o,o. t7,L l4

4a'0,0.66,L14

Eqol.val6t61te6.:

4Q8R

4!r 4R4q' 4R8Rr 4P+4q

4P" 4R4q" 4R8Rl 4F!4q

4P', 2P+2Q4q" 2P+2Q8Rt 8R

48'4R'

4Pt+4a'

*ths orl.gtu for rod8lnlte at (0,0,0)r

p roved to be the cor rec t oae, a ld -

tbelefole the equLval@t poattl66

fot tbe otbei or{glas are glv6 oolylelattye to thla alid mt to @chothe!.

For "Equlvalent sltoe", last three ontrlos Bhould hav6trlpls prlmes.

C2/c 4P-=4Q-=(1-1l3Mn * 2-2/3Ta)t and8R-=(2-2/3Mn a 5-1l3Ta)t. The known ca-tion ordering in tantalite can give therefore noclue to the likely cation distribution in wod-ginite.

We may now draw a third conclusion namely,that in analyzing the structure of wodginite,allowance must be rnade for all reasonable ca-tion distribution schemes.

The consequence of the these three conclu-sions is that, in order to arrive at the correctwodginite structure, analyses would have to becarried out for both the space groups C2/ c andCc in relation to all four possible origins,(0,0,0)r, (Vz,O,0)u (O,Vz,O)u (l/z,Yz,O)y that is,for eight different structures, and the analyticalprocedure for any one would have to be suchas to allow for all reasonable cation distribu-tion schemes. We have followed this procedurein our analysis of the wodginite structure.

REFTNEMENTs oF THE Possrnr,s Srnucfl.rnss

Etlective scattering factors. Scattering curvesfor neutral atoms were taken from Cromer &Mann (1968) with anomalous dispersion correc-tions from Cromer & Liberman (1970). Therefinements were carried out using the least-squares program RFINE (Finger 1969a) using

CRYSTAL STRUCTURE OF WODGINITE 55s

schemes (with the exception of Mn3+ substr-tuting for Tau+ and vice versa, which may beconsidered as extremely improbable on crys-tallochemical grounds) were encompassed, andthe total composition of the cation sites was con-strained to the bulk composition of the crystal.

For refinement in space group Cc, all thestructural sites that are in 8-fold generalequivalent positions in C2/ c, namely cation Band the four oxygens, will occupy pairs of 4-fold equivalent positions, and so positionalparameters had to be chosen for one additionalof each of these five atoms (sites) that were nolonger equivalent to the prototypes. The samekind of occupancy, Ta*Sn*, was assumed forthe two 4-fold B-type sites in Cc as for the one8-fold B site in C2/ c, bat the occupancies ofthese two 4-fold sites were now free to varyindependently. All other cation occupancy con-ditions were kept the same as for C2/ c.(l) Oriein at Q,0,0)t; space group C2/c. Usnepositional parameters close to those given inTable 2, full-matrix refinement of all possiblevariables for an isotropic thermal model resultedin convergence at an R-factor of 7.2%. \\eisotropic temperature factors were completelysatisfactory at this stage; all the temperaturefactors for the oxygens were statistically equal(-1.0A'a) reflecting the similarity in their en-vironments, as were the temperature factorsfor the cation sites. The occupancy refinementshowed that the r4 site was completely occupiedby Mn'r; howevero the B site was found to beoccupied by 0.89(1) Ta*0.11SnE. Because itis possible that anisotropic vibration of the Taatom could significantly affect the parametersof the much lighter oxygen atoms, the cationsites were allowed to vibrate anisotropically.Itefinement of all variables for this model re-sulted in convergence at an R-factor of 6.6Vo.Subsequent conversion of the temperature fac-tors of the remaining atoms to anisotropic, fol-lowed by full-matrix refinement of all variablesresulted in convergence at an R-factor of 6.L%6.(2) Origin at (0,0,0)t; spqce group Cc. For anisotropic thermal model, initial attempts to re-fine this structural variant were not successful,as wild oscillations occurred in the temperaturefactors of the anions related by a pseudo-centreof symmetry. This problem was overcome byconstraining all oxygen temperature factors tobe equal. With this constraint applied, refine-ment of all variables resulted in convergence atan R-factor of. 7.LVo. Because of the extremecorrelations between anion temperature factorsthat necessitated the use of the constraints de-scribed above, an anisotropic thermal model wasnot attempted.

Q) origin at (t/2,0,0)i space group C2/c. Full'matrix refinement of all variables for an iso-tropic thermal model resulted in convergenceat an R-factor of 2O.2Vo. The following occu-pancies were obtained: A-0.77Q)Mu*+b.23sn*; B-0.82(3)Ta*+0.18sn*; c=0.50Sn*+0.41(7)Mn**0.09(3)Ta. As with most of theprevious refinement$, the oxygen temperaturefactors were extremely large, all being between7 and 9A', although reasonable temperature fac-tors were-obtained for the metal sites. In viewof the high R-factor and the large anion tem-perafure factors, this model cannot be con-sidered as satisfactory.(4\ orisin at (/2,0,0); space group Cc' Full-matrix refinement of all variables for an iio-tropic thermal model with the constraints thatthJ cation temperature factors were equal andthe anion temperafure factors were equal re-sulted in the matrix becoming singular.

6l Oriein at (0,/2,0h; space group C2/c. FulI-matrix lefinement of all possible variables foran isotropic thermal model converged at anR-factor of. 13.L%. At this stage, the occupanoyrefinement showed that the A and I sites werecompletely occupied by Mn* and Ta respective-ly within one standard deviation. However,there were several unsatisfactory features to therefinement. The isotropic temperature factorsof the u{ and B sites were both zero (within onestandard deviation). In addition, one of theoxygen isotropic temperafure factors was threetimes as large as the other three. It was pos-sible that some of these features arose from astrongly anisotropic vibration of the Ta atom,and thus the 8:Ta and C sites were convertedto anisotrdpic temperature factors. Upon re-finement, one of the diagonal temperature fac-tor coefficients of Ta became negative, and theanisotropic model was abandoned. Because ofthe poor temperature factors, the refinementof this model cannot be considered as satis-factory.(6) Origin at (0,r/2,0)r; spqce group Cc. Nl at'tempts to refine in the space group Cc wereunsuccessful, all refinements leading to a dra-matic increase in the R-factor. Parametersoscillated wildly and there was extreme correla-tion between pseudo-centrically related variables.Q) Ori?in at (/2,1/2,0)r; space group C2/c. Full-matrix refinement of all possible variables foran isotropic thermal model resulted in con-vergence at an R-factor of LI.6Vo. The occu-pancy refinement showed that the .r{ site wasfully occupied by Mn*; the occupancies of theB and C sites were 0.89(2)TaE+0.11Snt and0.54Sn{'*0.28Ta* *0.18Mn* respectively. Theisotropic temperature factors for all three ca-

556 THE CANADIAN MINERALOGIST

tion sites were satisfactory; however, the oxy-gen temperature factors were very large, allbeing equal to 8.0A! within one standard devia-tion. In view of this, the refinement eannot beconsidered as satisfactory.(8) Oigin at (/z,Yz,O)q; space group Ca Full-matrix refinement of all variables for an iso-tropic thermal model with the constraints thatthe cation temperafure factors were equal andthe anion temperature factors were equal re-sulted in the matrix becoming singular.Conclusion; the most satisfactory structure. Ta-ble 3 gives a summary of the R-factors for tleeight different possible structures, and theseR-factors as well as the temperature factorsgiven in the descriptions of the results indicate

TABLE 3. LOI{ESI R FACTORS TOR TEE EIOTT POSSIBI.E WODGINITE

sTRUClt'RES.

DetaLLa ale gi.ven 1n the teat.

Si$i i . : ' (0,0,0) l (u2,o,o)1 (0,1/2,0) l e l2,r /2,o) i

space group C2/c Cc C2lc Cc C2/c Cc c2/c cc

R ( 3 ) 6 . 7 7 . J , 2 0 . 2 t 1 2 . 1 * 1 1 . 6 &

deglgnates non-convelgence!

tlat model (1) based on origin (0,0,0)r andspace group CZ/.c is clearly the most satis-factory in every respect, and we thus considerit to be the correct structure. In terms of thejoint scattering species Mn*n Sn* and Ta, thefollowing cation site occupancies were obtained:

,4=1.OMn*B-0.891(9)Taf 0.109Sn*C=0.565Sn* *0.250Ta*0. 1 85Mn*

On the basis of ionic radius and ionic charge,the minor constituents were assigned as follows:Nb was assigned to the B site, the amount ofNb available being almost exactly equal to theSn* occupancy of that site; all tle remainingminor cations were assigned to the C site. Theresulting site-populations are grven in Table 1and may be expressed by writtng the chemicalformula as ACBzOa=Mnr.oo(Sno.ssy'Tao.rsoTio.tuFee+o.oro)(Tao.eerNbo.roe)sOa. Final positional para-meters and equivalent isotropic temperaturefactors are given in Table 4 and anisotropictemperature factor coefficients in Table 5.*Interatomic distances and angles were calcu-

*Complete set of structure factor tables is avail-able, at a nominal aharge, from the Depository ofUnpublished Data, CISTI, National Research Coun-cil of Canada, Ottawa, Canada, KtA 0S2.

TSLI 4. POSITION& PM N EOUIVINT ISMOPIC MMMIMOS rcI TMINIq

e z s - , . ( 8 2 ,' 4urv.Sl !e

c 0 0 . 1 6 8 9 ( 2 )

0 0.6575(4)

n 0.2498(1) 0.4r31(r)

01 0.1334(20) 0.0577(15)

02 0.1453(19) 0.4504(17)

03 0.1215(19) 0.3060(16)

04 0,1161(19) 0.1834(17)

i l i 0.52(6)

l lL 0.63<1)

0.2472(2) 0.61(3)

0.0833(37) 0.89(27)

0.5597(39) t .12(28)

0.0984(38) 1.01(27)

0.5758(37) r .@(27)

rsu 5. tfrso;T;ii

omrcrms r& mrerf,m

!t, /3 zt /ss / t" 2r3 /23

c

B

0 1

02

03

0l

13(2) ro(2) 53(8) o 16(3) o

12(4\ 13(3) 7J(15) o 21(7) o

15(1) 12(r) 64(4') -1(1) 13(1) r(r)

31(20) 1l(11) 85(62) 0(r2) 19(27) r(21)

2r(19' 22(13) rr9(70) 4(12' 21(28) -5(24'

20(16) 16(r2) 141(65) -15(u) n(21' -2(23)

23(16) 19(13) 142(66) 3(12) 33(27) 5(24)

lated with the program ERRORS (L. W. Fioger,pers. comm.) and are listed in Table 6. A pro-jection of the structure is shown in Figure 1.

DlscussroN

Comparison of models (1) and. (7).For space group C2/ c, structures based on

origins (0,0,0)r and (Y2,1/z,O)r would have ca-tions in the same relation to the origin Cfable 2)but oxygens that differ by *(/z-z). The resultsbear out the nature of the difference betweenthese two structures (models (1) and (7) re-spectively): model (7) has cation site occu-pancies which are essentially the same as thoseof model (1), the most satisfastory structure,but the oxygens of model (7) show very hiChisotropic temperature factors (-8.041 whereasfor the correct structure represented by model(1), the isotropic temperature factors of theoxygens are reasonable (Table 4r, B-t.0A1.Comparison to lhe tantalite structure. The ca-tion ordering scheme in wodginite is differenlfrom that in tantalite; the latter has one 4-foldcation (.,{) site occupied by Mn and one 8-foldcation site (B) occupied by Ta whereas wod-ginite with a cell of different symmetry, celldimensions and volume. has one 4-fold cationsite (u{) occupied by Mn and one 8-fold cationsite (B) occupied by Ta but another 4-fold site(C) occupied by (SnfTa*Ti); furthermore, theA and B sites in wodginite do not correspondstructurally to the A and B sites in tantalite.The chains formed by the linking of staggeredcation-oxygen octahedra running parallel to z

TSG 6. 6m INlmTmC DISmCS ND Atffs 1!

{omnru.

CRYSTAL STR.UCTTJRE OF WODGINTTB

Equlvaleo! Posltlo6

ternate with layers of B(=Ta) chains (Fig. 1).Bond lmgths. Table 6 shows the followingvariation and mean bond lengths (A) for thethree cation-oxygen octahedra: .,{(:Mn),2.1.01(19)1.313(18), 2.185(8); B(=Ta\, 1.877(17)-2.I62(LS), 2.003 (8); C(-Sn,Ta), 1.984(19)-2.10I(17), 2.027(8). Using the derived cationsite contents Clable 1) and the ionic radii ofShannon & Prewitt (L969, 1970), the mean ca-tion-oxygen radii sums are for site l=Mn,Z.l9A; for B=(Ta,Nb), 2.00A; and for C=(Sn,Ta,Ti,Fes+), 2.044 Clable 7). The observedvalues agree well with these calculated values.In addition, the mean octahedral bond lenglhsobserved for the Mn-rich z{ sites in tantalite andin wodginite, 2.1.5(2) and 2.185(8)A respectively,agree well with each other as do the Ta-rich Bsites in both minerals with respective values of2.01(1) and 2.003(8)A.

Both the individual cation-oxygen distancesand the oxygen-cation-oxygen angles in Table 6for the three octahedra reveal that all three areappreciably distorted from ideal octahedral con-figurations with the C octahedron showing theleast distortion. The Table also shows ttrat theoxygen-oxygen distances for the shared octa-hedral edges are the shortest for all three octa-hedra, and that the oxygen-cation-oxygenangles for the shared edges are the secondsmallest. the smallest and the second smallestfor the A, B and C octahedra respectivelyn soit may be said that the wodginite structure con-forms well to Pauling's third rule regarding thelinking of coordination polyhedra.Comparison with previous wodginite structures.Two previous structure analyses of wodginitehave been described, one by Grice (1973) andGrice & Ferguson (1974), and the other byGraham & Thornber (L974). In neither casewere refinements carried out for all four ofthe structures it is possible to derive from ixiolite

5f7

e -O(r)c 2.064(rg) i-0(r)d 2.162(r8)-0(2)a 1.942(19)-o(2ro 2.t74(79)-0(3)s 1.877(17){(4)f 1.899(19)

2.003(8)

@-0(1)a,b 1.996(10) i 2-0(3)a,b 2.101(17) x 2-0(4)6,b r .984(r9) a 2

2.027 (8)

Aa-0(2)e,s 2.rot(Ig, ' 2-0(3)h,r 2.142(19) x 2-o(4)e,8 !lLf(L9l x 2

2.185 (8)

I ktahedr@

0(1)c-0(1)d 2.875(15) 0(1)c-Ta-0(r)dr0(1)c-0(2)e 2,659(24) *0(1)c-ra-0(2)€0(1)c-0(3)a 2.963(26) 0(r) !-ra-0(3)a0(l)c-0(4)f 2.966(25) 0(r)c-Ta-o(4)f

.0(1)d-0(2)a 2.551(26) r0(1)d-ra-o(2ia0(1)d-0(2)e 2.757(24) o(r)d-ra-oa2ie0(l)d-0(4)f 2.761(25) 0( l)d-Ta-o(! t f0(2)a-0(2)e 2.793(16) O(2)a-ra-0(2)e0(2)a-0(3)a 2.880(24) 0(2)a-?a-0i3ia0(2)a-0(4)f 2.830(25) 0(2)a-ra-0(4i fo(2)e-0(3)a 2.721(27) 0(2)e-rs-0(3)a0(3)a-0(4)f 2.969(26) 0(3ia-rFoaajf

<0-0 t 2.810 <0-ra-0)

C Octahedror

0(1)a-0(r)b 3.078(15) 0(1)a-c-C(1)bo ( 1 ) a , b - 0 ( 4 ) a , b 2 . 9 0 4 ( 2 ? ) 0 ( r ) a , b - c { ( a ) a , b0 ( 1 ) a , b - 0 ( 4 ) b , a 2 . 8 7 3 ( 2 6 ) o ( r ) a , b - c - 0 ( 1 ) b ; a0(r)a,b{(3)a,b 2.842(25) 0(1)a,Fc-o(3)a,b0(3)a-0(3)b 2.801(17) 0(3)a-c-c(3)b

.0(3)a,b-0(4)a,b 2.813(25) O(3)a,b-c-c(4)a,ba(3)a,b-0(4)b,a 2.786(24) t0(3)a,b-c-o(4)b,a

( o-o) 2.060 (o-c{2

t ta'z*t llt -z,?a 4rt Vzr l-y z-L

\n +y r-.1-r l-z

r h \ + zr-v -z

gs. z(o)o7 6 . r ( 7 )97.4 (8)9 6 . 8 ( 7 )76.7 (8t8 r . 2 ( 7 )8 5 . 4 ( 8 )8 8 . 1 ( 6 )9 7 . 9 ( a )94.9 (8)90.2 (8)

103.7(8)

8 9 . 5

100.9 (10)9 3 . 7 ( 8 )92.4(8)8 7 . 8 ( 7 )83.6 (r0)8 7 . 0 ( 8 )8 5 . 9 ( 7 )

8 9 . 8

\ 2x 2

t 2x 2

A oclahdron0 ( 2 ) e - 0 ( 2 ) 8 3 . 4 0 1 ( I 5 ) 0 ( 2 ) e - A - o ( 2 ) s 1 0 8 . 2 ( r o )9 ! ? l e , c - 0 ( 3 ) h , t 3 . 2 r 9 ( 2 5 ) n ( 2 ) e , s - A - o ( 2 ) c , r 9 8 . 7 ( 7 )0(2)e,s-0(3)r,h 3.116(27) 0(2)e,s-A-0(3)h, i 9L.s(7)0(2)e's-0(4)t ,s 3.064(25) 0(2)e,c-A-0(4)t ,s 87.8(7)

.0(3)h'1-0(4)e,c 3.0r2(25) 0(3)h,1-A-0(4)e,s 85.0(7).0(3)brr-0(4)g,e 2.786(24) *0(3)h, i -A-O(4)s,; 17,3(1)0(a)e-0(4)s 2.8s8(15) 0(4)eA-0(4)s 76.3|J)

<O-O ) 3.055 ( FA-0> s9.3

'euarea eacos

that characterize tantalite are present in wod-ginite, but whereas in tantalite ono layer ofA=Mn chains is followed by two layers ofB=Ta chains (Fig. 2a in Part D, in wodginitelayers of l(=Mn) -f C(=Sn,Ta) chains al-

'!su ?. olPsrson 0F 6TIoN slu mTA By (;M & momER (1974) (d) $ln mT oF rusm Amos (mc)

obs. ca lc .slze Blze

s!!9 conrenr I I

c T b ! h . ^ 2 , 1 3 2 . 1 9

I H G C s D ^ . - T o ^ ^ - 2 . 0 3 2 . 0 4 o h t . o o 2 . 7 9 2 . t 9

hesfotualloo, xm-0.I25 - \qc, yg - ymc, ,m -

"mc

s!!9 corlent I I s4e cortert I I lfrs seEles! I I

P T " 0 . 7 S 0 . 3 1 . 9 7 2 . 0 O t .oo.ort. l.o, 2.08 r.95 t " t r t .O 2.O9 2.f i

oba. calc.atze slze

g49 coatat I I

x l / ra rao.71sn;.24 2.08 1.eg

" T"O.89SO.tt 2.00 2.00

'llu reo.o, E 0.,

to. xl sd plus 0 o.oJ to! :l,/Ta.

**otu t to.rrreo.oro.

558 TI{E CANADIAN MINERAI,OGIST

combined with both possible space gloups aswe have now done.

The Grice & Ferguson structure is based onixiolite origin (0,/2,0)r and space group Czlc'and thus it corresponds to our model (5). Boththe earlier and the present analysis of this mo-del met with serious problems with the tem-perature factors of most of the atoms, and bothrefined to R-t2Vo. Furthermore, it caq beshown that the mean cation-oxygen distancesobserved for the octahedra of the two cationsin 4-fold sites (l and B in Grice 1973)' agreepoorly with the weighted mean cation-oxygendistances calculated for the deduced site occu-pancies from the ionic radii. We now rejectthat structure in favor of the one described here.

The structure described by Graham & Thorn-ber (1974) was carried out on a crystal fromthe type locality, Wodgina, Australia usingCuKa radiation and photographic intensitydata. They refined their structure in both spacegroups C2/ c and Cc and concluded on the basisof a sliCbtly better R-factor (13.I%o overt 4.3Vo) and of electrostatic calculations that Ccis to be preferred.

A comparison of their cation parameten withthose relative to the possible origins n C2/ cin our Table 2 and also taking account of theiroxygeq parameters jn relation to ours showsthbt a change of their x by 4.125 will trans-form their origin to one of ours, and the parti-cular one is that for the correct structure,(0,0,0)r. It is then possible to equate their cationsites with ours, and this is done in Table 7where the contents and tle observed and cal-culated mean cation-oxygen octahedral dis-tances (sizes) are also given for both structuresfor comparative purposes.

Table 7 shows that the two structure analysesresult in somewhat different cation orderingschemes: their Mn site containing Mnt.o isstructurally equivalent to our C site containingSlb.urTao.nrTio.rrFe6.6T rather than to our Mn-richsite, A, to which their site X2 containing Tao.zNbo.g is equivalent. Their other two 4.fold sitesin Cc, Xl and Ta are together equivalent toour 8-fold site B in C2/ c, and in this case ttrereis better agreement than in the other two casesof their combined cation contents with our sin-gle content, both being Ta-rich. We cannot ex-plain why the two different structure analysesresulted in two different ordering schemes.

Graham & Thornber (1974) note that theaccuracy of their individual M0 bond lengthsis probably no better than 0.12A which corre-sponds to a a of the mean octahedral cation-oxygen distances of 0.05A whereas our ds forthe mean octahedral cation-oxygen distances are

0.00sA (table 6). Nevertheless, one rTay stillcompare their observed mean octahedral cation-oxygen distances with those calculated from theionic radii of Shannon & Prewitt (1969, l97O),and this we have done in Table 7. The agree-ment is generally poor, especially compared withthe agreement between the observed and cal-culated values for our structure which are alsogiven in the Table.

Because of the lower R-factor and the betteragreement between observed and calsulatedmean octahedral cation-oxygen distances of ourstructure relative to those derived by Graham& Thornber (1974), we conclude that ours istle preferred structure for wodginite.Bond strengtlu. Table 8 gives the bond strengthsto the oxygens from the cations, evaluated inthe same general ways as was done for tantalite(see Discussion in Part I). In wodginite the non-distance-dependent Pauling values to the oxy-gens are fairly poor but somewhat better thanthe corresponding values for tantalite (fable 6of Part I). The distance-dependent method ofFerguson (1974) in which the bond strengthsfrom the cations to the anions are distributedin amounts that are inversely proportional tothe cation-oxygen distances, that is, using anexponent of 1, yielded bond strengths (not re-produced here) that q/ere more satisfactory thanthe Pauling values but were such that a higherexponent was indicated. We accordingly carriedout calculations using the method of Pyatenko(1973) for which different exponents are ingeneral used for chernically different cations;for wodginite an exponent of 5.0 was applicableto all the elements. Bond strengths calculated inthis way are given in Table 8, and althoughthey are fairly satisfactory, they suggested thata sornewhat smaller exponent would give betterresults, particularly for O(1). The exponent wasthen changed arbitrarily to 4.0, and the bondstrengths to both the individual oxygens and tothe oxygens taken in octahedral groups of sixare better than those for the exponent of 5.0and in fact they may be regarded as reasonablysatisfactory. It is possible that still further modi-fications of the exponent embodying differentvalues for the different cations might improvethe bond strengths still further.

Barker & Graham (1974) have calculatedlattice energies and site potentials for the wod-ginite structures described by Graham & Thorn-ber (1974) and by Grice (1973) as well as forother related compounds, and their results wereimportant in the choice of space group Cc overC2/ c for the wodginite structure of Graham &Thornber. We have not attempted to assess ourtantalite or wodginite structures in this way al-

CRYSTAL STRUCTT'RE OF WODCINITE 559

See Flgste 1 sd labl.s 7. A11 bond stlatrgths (vtj) are ln valdce ult8 (v.u.)

Eethod of evalutlon

et{on A-fiq?+^^

asEmed chalge

ea@ed A-0 e:poren!

catroa t-raf89rNbf1o9

es@ed chalgesa@d B-0 @onent

Paullu (1929)

7r0*

0r

Modlfl.edPyatento (1973)/Fer@on (1974)

4 . O

Pvateuko (1973)r'

2+5 . 0

ss4e1-..q-srfi+rurrafl ,rnrrfi]rr rr.lf o,aaa@ed charge 4+€aa@ed c-0 spoBot o*

5+4 . 0

3.98r4 . 0

5+5 . 0

3.98+

LaBlod

tt to(t)

r11o(z)

rrro(:)

rrlo(q)

E I A I

Aeund D- E -lal 'rt

anlotr-catlondl,ataac6

2 . 0 6 4 ( 1 8 ) 82 . t 6 2 ( 1 8 )r.996 (18)

2.101(19)1 . 9 6 2 ( 1 9 )2.074(r9')

2.742(r9)r. .B77(17)

2.313(18)r. .899(19)1.984(19)

At 'iL

s/6

I I35/6

Ll35/62/7

113

ca!1on

BB ,c

BB ,

Bc

Bc

f t r t l v fg l a i l -

o . 7 2 12 r/3 7/3 0.598 2.020(257)

0 . 7 0 1

0 . 6 9 10.547 7,911<339' 0.05r0 . 7 L 0

0 . 3 9 60.935 2.004(309) 0.004o . 6 7 3

0 . 3 5 91.108 2.Or7(24r) 0.0170.549

o.245L.M6 2.022(262) 0.0220.731

0.3842 O 0 . 9 1 8 2 . 0 0 8 ( 2 3 1 ) 0 . 0 0 8

0 . 7 0 5

0.355r 5t6 u6 1.052 7.978(L84) 0.022

0 . 5 7 1

o.267| 516 116 1.004 1.984(198) 0.016

0.718

2 t 3 0 . 0 6 6

l d f

0.096lotal botrd str€ngths (v.u.) to octahedlal. groups of 6 onAes,

r.1 1/32 t3

t2 Ll3t/3

11.9400.060

1 2 . 0 1 80.018

u . 9 6 40.035

0 . 1 3 2

12.0860 . 0 8 6

11.9410.059

t! .9720.028

0.232

L20

X lai'tu*zprc) I L I 3

lEplylng a noB-dl6tsdce dependedt relattonshlp.t$

.Pyatenko (1973) glves no exponmt for Mn: ue hsee asswed the vahe of 5,0.r )A i - l z - a l

" . o .t i r r r r - -

I A t r l r r - a l v . u .

though it would perhaps be a valuable approach.Because of the satisfactory bond shengthsderived for our tantalite and wodginite struc-tures when evaluated according to the pyarenko(1973) or tle modified Pyatenko method, andespecially in view of the high degree of distancedistortion in at least one of the cation octahedrain each of these structures, we feel ftn1 thisprocedure provides a reasonably satisfactorycrystal-chemical interpretation of these struc-tures. Implicit in this interpretation is the con-cept that, in structures such as these, the dif-ferently-charged cations tend to order them-selves into particular sites - which h largelythe case with our wodginite - and at tle sametime the cation-oxygen bond lengths within onepolyhedron are able to vary to a fairly highdegree in order to maintain close to ideal bond

strengths to each oxygen anion. Because wod-ginite contains an appreciable proportion ofcations that are other than divalent or penta-valent, it requires an additional cation site overthe two sites in tantalite to accommodate them;the result is a strusture ttrat is topologicallysimilar to that of tantalite although lower insymmetry but still one ln which the particularcations are largely ordered and the oxygens areessentially satisfied with respect to bondstrengths.

ACKNO'TVLBI}GMBNTS

We express our thanks to the following for4-circle diffractometer data collections: C. Calvoand R. Faggiani, McMaster Univenity; M.N.G.James, Univenity of Alberta; and V. Kocman,

560 TIIE CANADIAN MINBRAI,OCIST

formerly of the University of Tpronto. E. E.Foord of Stanford University kindly carried outan electron microprobe analysis of our wod-ginite specimen. Financial assistance was pro-vided by the National Research Council.

REFERET{CEs

BARKER, W. W. & GRAHAM, I. (1974): The crystalchemistry of complex niobium and tantalumoxides: V. Electrostatic energy calculations.Amer. Mineral 59, 1051-1054.

BRowN I. D. & SHANNo\ R. D. (1973): Empiricalbond-strenglh-bond-length curyes for oxides. ActaCryst. 429,266-282,

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Manuscript received April 1976, emended Juie1976.