Thtr CRYSTAL STRUCTURE of TALNAKHITE, Cursferossz.' S. R

A:merican Mineralooist Vol. 57, pp. 368-380 (1972')

THtr CRYSTAL STRUCTURE OF TALNAKHITE, CursFeroSsz.'

S. R. Helr, AND E. J. Genn,' Department of Energg, Mines and, Resontrces,Mines Branch, Ottawa, Canada KIA 0G1

Assrnect Talnakhiteis a Cu-Fe sulphidefirst reportedby Bud'ko and Kulagov (1963)as having a compositionof CuFeSr.eand a cubicunit cell with o : 5.28i" They pro- posedthat this wasthe naturalequivalent of synthetichigh-temperature chalcopyrite (a : 5.26A). Subsequentwork by Genkinet al. (1966)-andCabri (1967)suggested that tatnakhitehas a largercubic unit cellwith o : 10.6;', similarto that of slmthetic B-phase(IIiIler and Probsthain,1956). This study showstalnakhite to have a cell with o : 10.593A, spacegroup I43m and a partially-orderedstructure which is consistentwith both the p-phaseand the metal-richstoichiometric composition of CursFeroSez(Cabri and Harris, 1971).

INrnooucrtoN Talnakhite is a Cu-Fe sulphide occuffing, in concentrations of up to 70 percent, in the vein ores of the Noril'sk and Talnakh deposiis, Western S beria, U.S.S.R. The new mineral was firct discovered by Bud'ko and Kulagov (1963) following difficulties with the extraction of chalcopyrite by the flotation process.They reported that talnakhite was distinguishable from chalcopyrite (CuFeSr) by its different optical properties and X-ray powder data, and the sulphur-deficient composition CuFeS1.s. Bud'ko and Kulagov described the crystal cell as cubic with o : 5.28A and considered it the natural equivalent to the synthetic high-temperature form of chalcopyrite with a = 5.264 (Donnay and Kullerud, 1958). This conclusion was of minerz16*1.u1 importance as high-temperature chalcopyrite is generally considered unstable at room temperature unless rapidly quenched from above 550'C (Yund and Kullerud, 1966). Although the presence of zinc was postulated as a possible reason for the unusual stability of "small- cell" talnakhite, the occurrence of such a large deposit at Noril'sk im- plied a rather extraordinary geological history. These conclusions prompted further investigation into the nature of talnakhite. Genkin et al. (1966) reported that the X-ray diffraction powder pattern of talnakhite was also consistent with the synthetic - B-phase (Curz*'Ferz*nSsz,n 0.6) of Hiller and Probsthain (1956) and appearedto form an intergrowth with chalcopyrite in the ore body.

t Mineral SciencesDivision, SulphideResearch Contribution No. 38. sPresentaddress: Division of Chemistry,National Research Council of Can- adaOttawa. STRUCTURE OF TALNAKHITE 369

They concludedtherefore that talnakhite was & parlially ordered phaseintermediate betweeri the normal orderedchalcopyrite (Pauling and Brockway, 1932) and a disorderedsmall cubic cell phaseunstable at room temperature.The electron microprobe analysesof Genkin et aL. (1966) and Cabri (1967) failed to detect the presenceof zinc in the new mineral. The authors pointed out that the presenceof sphaleritewould accountfor the zinc reportedin the original analysis. Cabri (1967) found that the Noril'sk ore ,samplecontained two minerals with the (Cu,Fe):S ratio closeto unity. When physically separated,one of these was chalcopyriteitself and the other gave a more extensive diffraction pattern, particularly in the low-angle region,than the data of .eitherBud'ko and Kulagov (1963) or Genkin et aI. (1966).All the lines on this pattern could be indexedassuming the body-centredcell of B-phase except for two weak reflections due to a small chalcopyriteimpurity. Cabri thereforeproposed that the mineral talnakhite was in fact the natural form of the B-phase rather than of high-temperaturechalcopyrite. Electron microprobe analyses first suggestedthat the stoichiornetry for this mineral was Cu reFereSszsimilar to that stated for the B-phase.Subsequent analy- seson a largernumber of natural and syntheticsamples indicate, however, that the compositionis closerto the stoichiometryCu1sFe16Ss2 (Cabri and Harris, 1971). The formula may also be written as CugFeeSro.

Expnnrnrpwner,

The crystals of talnakhite were obtained from Dr. L. J. Cabri of the Mineral SciencesDivision. These occur as irregular fragments which show no observable twinning effects but, have a relatively high mosaic spread. Attempts to grind these fragments to spheres were unsuccessful and a crystal approximating a parallelpiped was selectedfor further work. The dimensions were approximately 0.1 X O.Z X O.2 mm. Precession films of the 0/cl, llct, 2ttl, h}l, hll, h2l, atd. h3l levels show only reflections satisfying the condition h + k + I : 2n andLaue s5rmmetry zr3zr.. This permits three possible space groups; 1432,I43nx, atd ImBm. The films also show a marked superlattice pattern, in which reflections with h, k, I : 2n ar;d h + Ic,h + I : 4n ate the most intense and this suggests the presence of a sphalerite-like subcell with a : 5.3 i.. fle futrahedral coordination in such a subcelUis not permitted for the spacegroups 1432 and Imtsm. The spacegroup I4Bm was therefore selected as the correct one on the basis of the superlattice reflections. The final cell'parameters wereierived using a least squa-resmethod applied to the diffractometer angles 20, g, y, arLd.o, and assuming a triclinic cell. These angles were obtained from an automatic alignment process(Busing, 1970) on the 24 equivalents of the 0, 4, 12 reflection. This gave the values o, : b : c : 10.5g3(3) A and o : : 'y I - 90.00(3)0. The cell contents were established by electron probe analysis (Cabri and Harris, 1971)as: 370 8. R. HALL AND E. J. GABE

Ctr:27.10 (13) atomicPercent Fe : 24.15 (15) atomicPercent Ni : 0.59 (05) atomic Percent S : 48.16 (13) atomicPercent giving a chemical formula of Cur"n(Feru.nNL.,)Sosand the stoichiometry Cu'a Fe'uS.r.The calculated density of 4.28 g cm-", assuming one formula per unit cell, compares favoura.bly with the value 4.24(5) g cm-" measured pycnometrically (Cabri, 1967). Wiih this formula and density, the linear absorption coefficient p - 142.8cm-'fo MoKa radiation. The initial intensity data of talnakhite were collected some: two years ago and have been recollected on a number of occasions since as the data collection facilities improved. The nature of the superlattice means that the intensities of a,bout 75 percent o{ the reflections are relatively weak, and thi's necessitated more than usual care with the counting statistics. In the initial measurement of intensities only about 12 percent of the reflections were considered significant at the 10 percent statistical level |i,.e.,I(net) ) 1.65o(1)1.Subsequent measurements increased the proportion of observable reflections until finally 189 of the 528 examined in the 2d range from 0' to 90' (MoKo radiation) were accepted using the above criterion, and subsequentlyconsidered'observed'. The final measurements were made on an automatic 4-circle diffractometet' using graphite monochromatized MoKa radiation. The 0/20 scan width (3-3.5") was adjusted for dispersion.Background counts were measured each side of the peak, for approximately the same length of time as that taken for the scan. The intensities of three reference reflections were recorded after every 25 measurements in order to monitor the crystal alignment and insfrumental stability. No significant variation in the intensities of the reference reflections was observed during the data collectionprocess. Absorption correctionswere computed (Gabe and O'Byrne, 1970)by Gaussian integration for the crystal shape and p-value described above and applied to the net intensities averagedfrom three separatemeasurements. The normal Lorentz and polarization factors vsere aFiplied to produce a set of obserwedstmcture fastorsfrom thebevalues. "i

Srnucrunp DprenurNerroN .

Although there were significant differences in composition between the synthetic B-phase (Hillef and Probsthain, 1956) and talnakhite, it was anticipated from diffraction data that their structures would be very similar. However, becausethe model for the B-phasewas proposed from limited data and not refined, the present determination was carried out independently. The distribution of diffraction intensities indicated that talnakhite, to a first approximation, could be consideredinitially in terms of a disoldered sphalerite-like (Cu, Fe)S cell,! which was related to the 10.64cell by the diagonaltransformation matrix ll/2,l/2,1/21 and the

'Gabe, E. J., G. E. Alexa.nder,and R. E. Goodman (1971).The Mines Branch on-line single crystal diffractometer.Mineral SciencesDiuision Report MS 71-54. STRACTURE OF TALNAKHITE 37r

O METALr (o,o,o)

O METAL2 (X,O,O)

O METAL3 TN,I/4,O|

O METAL4 (X,X,X) (iiffi).r.t"r^ | (x,x,x) O .r.trr* 2 (x,Y,Y)

Frc. 1. Unit cell model showing the crystal structure of talnakhite determined in this analysis. It is essentially equivalent to the strrrcture of the synthetic p-phase (Hiller and Probsthain, 1g56.) origin translalian (l/4, l/4, l/4). Structure factors were calculated assumingthis model,x'ith 32 S atoms aLtJneI43m sites8c(1/8, L/8,l/8) and249(L/8,3/8, 3/8); and32 (Cu,Fe) atomsat the sitesl2e(l/4,0,0), lzd(I/4,1/2,0), and8c(l/4, I/4, t/4). The overallisotropic temperature factor was 1.14'z.This antt all subsequentcalculations were per- formed on a Univac 1180 computer using the programs in the XRAY crystallographicsystem.' The Cu, Fe, and S atomic scattering factors were those of Cromer and Waber (1965), calculatedusing relativistic Dirac-Slater wave functions. The scattering factor for the (Cu, Fe) atoms was approximatedas the averageff(Cu) * I$e)1/2. The agreement between observedand calculated structure factors resulted'in a .R-valueof 0.31.A difference(lf',| - lF"l) synthesiswas calculatedto determine the difference between the sphalerite-Iike model and the true structure. The only major electrondensity residualin the synthesis was at the cell origin. This suggestedthat the site 2a(0, 0, 0) was occupiedby the two additional metals which had been excludedfrom the sphalerite-like model. Agreement between the structure factors, calculatedwith the two (Cu, Fe) atoms at the2a sites,and the observed data improved considerablyand resulted in an ft-value of 0.22. This showedthat the structure of talnakhite was essentiallvconsistent with the model proposedfor the B-phaseby Hiller and Probsthain (1956). The structure is shown diagrammaticallyin Figure 1. 'Stewart, J. M., F. A. Kundell, and J. C. Baldwin (1970) The XRAY system of crystallographic programs. Computer Sciance Centre Report, U. ol Margland. 372 S. R. HALL AND E. J. GABE

With the inclusionof the additionaltwo metal atoms [the (Cu, Fe) atomswill subsequentlybe referredto as metal or M atorns],the most significantfeatures in the differencemaps could be attributed either to shifts of certainmetal and sulphur atomsfrom the ideal sphalerite- like sitesor to changesin individual atomictemperature factors. These featureswere well-definedso that manual adjustmentof the coordi- natesand isotropicthermal parameters,in a seriesof structurefactor- difference synthesis calculations,quickly improved the agreement .R-valuefor the observedreflections to 0.071.At this stagethe only significantfeatures in the differencemap wercnegatiue residual shells, roughly sphericalin shape,surrounding each of the metal sites.This indicated that the calculatedradial distribution of electron density was excessivelyhigh beyondabout 0.2A from the atom centerandt in turn, that the calculatedscattering was too large at low Bragg angles. Thesefeatures appeared to result either from using an atomic scattering curve which was excessivelyhigh at low sin d/,\ values,or from a combinationof too high a site occupancyand too high a teinpera- ture factor. The latter of thesetwo possibilitieswas lesslikely however,,because the averagetemperature factor of the metals was not high (-1.3112 and the site enclosedin the most pronouncednegative shell had the lowesttemperature factor (0.80A)'. In addition,the postulatedmodel of.34 M atoms and 32 S atoms agreed closely with the actual composition and, apart frorn sorneresidual about,the metal sites, there wereno other featuresin the differencemap to suggestthat the metals were situated in other sites.O'n the other hand, a neutral scattering euwe had been used for the metal atoms up to this point whereas Mtissbauer and magnetic susceptibility measurements(Cabri and Goodman,1970; Townsend ef aL, l97l\ indicate talnakhite is essentially antiferromagneticsimilar to chalcopyrite(Donnay et al., 7958). This implies that, like chalcopyrite,the metals are probably in the form of Cu* and Fe3*ions. A scatteringculve was thereforeprepared from the averageof the Cu* and Fe3*cutves of Cromer and Waber (1965). S;bructurefactors were calculatedusing the averageionized curve for the metal atoms and the S?- curve of Tomiie and Stam (1958),and this improvedthe agreementE-value to 0.059. Although this lent support to the concept of positivily ionized metal atoms, no attempt had been made up to this point to establish the ordering of the copper or iron atoms at specificmetal sites. The symmetry of the spacegroup I43m doesnot permit the completeoldering of Cu and Fe atoms in a sphalerite-like structure while still maintaining a stoichiometry of CuraFeroSsr.This inherent requhement for disorder STNUCTURE OF TALNAKHITE of the metals was also recognisedby Hiller and Probsthain in deriving the structure of B-phasewith the compositionCurz.oFerz.uSrr. This, of course,does not excludethe possibility that someof the metal sites are occupiedby one metal type. In fact, the conceptof both order and disorderat the metal sites of talnakhite must be favoured over complete metal disorder for two reasorlsi.First, the formation of ^the large cubic cell (10.64), oyer the smaller disordered.cubic cell (S.B,E)prop-osed. for high-temperaturechalcopyrite (Donnay and Kullerud, lg58; Yund and Kullerud, 1966),implies a longer-rangeordering of the metals. Second, the information availableto date on the magneticstructure of talnakhite, suggestsan ordering of metals not dissimilar to that of chalcopyrite. A differencesysthesis calculated with the metals as averaged(Cu., Fe3*) atoms was examinedfor featuresthat might indicate specific ordering of these atoms.Two of the sites did appear to have residual features in their environment which were different from the rest.The 2a site was still enclosedin a roughly sphericalshell of nega- tive residual density which indicatedthat the atom at this site may require a higherpositive ionization than that providedby the scattering curve [l(Cu.) * l(Fes.)l/2.The other site, gc, was enclosedin a similar buLp,ositiue shell of residual,suggesting that this atom should have less positive ionization. Structure factors calpulatedas before, but with an Fe*Satorn at site 2o and a Cu* atom at site 8c, decreased the R-value to 0.054.This improvementin agreementcould not in itself be consideredvery significant but certainly this configuration now had to be consideredon equal footing with the completedisorder structure for the remainder of the refinement.The differencemaps now containedno pronouncedfeatures that could be attributed to coordinateor isotropicthermal parametershifts. However,there were asymmetric residual features about some sites that indicated the possibility of thermal anisotropy.There was also a positive peak of about 5eA-3aL (l/2,0, 0) indicating that the metal atoms partially occupythe site 6b. This is similar to the B-phasestructure. The differencemap gaveno further evidenceof metal orderingat spticificsites. rn addition, a interatomic distancecalculation showed no significant differencesin the various metal-sulphur distancesthat could be related directly to specificFe-s and cu-s bondsas found in chalcopyrite (Paulingand Brockw ay, tg}2) . All subsequentrefinement of coordinateand thermal parameters was performedby structure factor least squares,using the program CRIT,SQ written by F. A. Kundell for the XRAY system. Ttre two atomic arrangements(models ,4 and B) which were consideredare describedin Table 1. Each modelwas refined with B cyclesof isotropic 374 S. R. HALL AND E. J. GABE

T.AB LE I

Each modet was refibed using 3 rounds of isotropic full-matrix full-hatrix SFLS (50 parameters, t3 variables) and 3 rounds oI anisotroPic SFLS (85 parameters, 20 Yariables).

Scatterine Facto!s Atom Sjte

u(l) tl(cu+) +!(Fe3+) l/2 f(Fe- ) + 2 v(2) [!(cu+)+ !(Fe3+)]12 t j(cu+) J{F.3+)l/ rl(Fe3+) l/2 u(3) 12d r(co+) + l(r"3+)y2 tl(cu+) + f(cu ) v(4) 8c IJ(co+)+ J(r.3+tllz + (Fe3+) ] /2 \4(5)+ t !(cu')+ ItFe' ) )/z [ !(cu+) f(s- s(r) 8c f(s- ) )

s(2) 24E r(s-) f(s- )

*TheoccupancyolMetal5wasrelineddulingthc3Iounds.ofisoiroPic]east 01i2 squares with a fixed isotropic tempe!ature factor of 1

and 3 cyclesof anisotropicfull-matrix leastsquares using unit weights. The isotropicthermal parameterof metal 5 at site 6b was held con- stant at 1.0A, but its occupancywas refined during the initial three cycles.with the inclusionof a fractional metal at this site there ap- pealeclno further evidenceof atomsat the unoccupiedinterstitial sites. The refinedparameters for models,4 and B are'shownin Table 2 and the structure factor magnitudesand phasesare listed in Table 31 Anomalousdispersion corrections Af' and Lf" werenot includedin

TSLE Z

Th. Theanlsotlopictemp€r.tulefacrorsaleexpressedintheformT=eryt-zfl(u,,aoz5z129,r1*qo45+....)1. tso6oDicte.Deratu;eractois(atniz)vereobiatn;dfromthetnltialrou'dsoftsoticiptcsFLsre'finpmeht' Th'ette were not reflned exceDt in the case of detal M{5) ".."".""r"" l;*;;"; ;= ,r, _ ", -;; 0 0 0 1000'91{14) M(l) zs 4 o o o 1.08(17)l.o8{1?) 108(1?) DOoo0.83(1?)0.83{17)0.83(l?)0000'??(15) o -'01(43)100130(5) !!(z) 129 I .2570(4) o o 1.39(16)1?6(9) t.?6(9) o 1 23( 5) E .25?t(4) o o l.32(15) r.56( 9) 1.66( 9) o 0 .16(34) Mt3) izd A l/z l/4 o t. ?o(15) 1.55( 9) 1.55{ 9) 0 0 '00 | oo t'z1l 5) - l' 18{ 5) B t/z r/1 0 1.58(15) r.47( 8) 1.4?( 8) 0 o 0 00 1 33{ 6) M(4) 89 4 .2522191.25?zl9t .252zlgl l.?r( 8) 1.?t( 8) 1.?l( 8) .OZ{rl) oz(ll) '02(11) t -.02(11)-.02(11) -'02(11) t 58( 6) D -252816J.25Z816l ,Z5Za\6\ 2.04( 8) 2.04( 8) z.04( 8) o04{2) r.00 l4(5) 6b 4 r/z o o 1.00 DL/zoo ' s(1) 89 A . r250(9) 1250(9) 1Z5o(9) r. 80(35) t. 80(35) t 80{35) - 29(20) - z9lZO) Z9l29l 1.00 r 55(20) -. - - t'36122) E .t24sl9) .lz45lg\ 1245{9) 1.54(35) 1,54(35) I 54(3s) l8(26) 18(26) 18(26) --04(14)-,O4lt4\ O3lZ7l i 00 0 94( 6) slzl zas L .120717) .3?40(5) .3740(5) 1.32(36) 1.20(20) 1.20(20) a .rzos(z) 373716).3737161 1,23\31, r.zglzl) 1.29(21)-.03(t5) - 03(16)-.09(28) 0 95( 7)

1To obtain a copy of this material, order NAPS Document No' 01717from National Auxiliary Publications Service of the A.S.I.S., c/o CCM Information Corporation, 866 Third Avenue, New York, N. Y. 100"2; remitting $2'00 for microfiche or $5.00for photocopies,in a.dvance,payable to CCMIC-NAPS. STRUCTUNE OF TALNAKHII'tr) 375

the least squaresprocess but wereapplied to eachmodel in a structure factor calculation on completionof the refinement.The corrections usedwere A.f'"o : 0.36,A/'F" - 0.37,A./'* = 0.13,Af"c" - 1.36,A!"u"= 0.92,and af"s = 0.16 (Cromer,1965). The flnal li-values were 0.053 for Model ,4 with no anomalo,usdispersion corrections, 0.060 for model ,4 with a,ft and Af" corections, and 0.066for the inverse of model ,4 with Af' and,A/" corrections.The R-values for the equivalentcalcu- lation on modelB were0.045,0.051, and 0.060.Correction for secondary extinction was not applied to the Fo_dalaboth becauseof the high mosaicityof the talnakhite crystalsand the lack of consistentevidence for this effectin lhe Io/1" ratios.

DoscnrprroNoF THE Srnucrunp The atomic coordinatesof talnakhite, listed in Table 2, do not vary significantly for the two atomic arrangements.These values provide, to reasonablyaccuracy, the displacementsof the atomsfrom the idealized sphaleritesites. On the other hand the thermal motion and occu- pancyparameters, also listed in Table 2, areof questionablereliability. This is not only to the interactionof thesetwo types of paqametersin an X-ray structurerefinement b'ut also to the lack of conclusivein- formation about the orderingand ionization of the Cu and Fe atoms. Thereforecomparison of the two refinementmodels can only be, at best,qualitative, and no attempt has beenmade to, accuratelybalance the total chargeof the anionsand cations.Also, in the absenceof con- clusiveX-ray evidence,it should be stressedthat models refined in this analysisrepresent only two of a number of possibleatomic ar- rangementsand that conclusiveidentification of the true metal ordering and ionizationmust await additionalinformation from other tech- niques. The structureof talnakhite is similar, both in cell and atomic ar- rangement,to that of binnite CurzAs+Srs(Wuensch et al., lg65) and of tetrahedrite Cur2sb4sra(Wuensch, 1964). The similarity of these minerals in diffraction aspect had been reported previously (Cabri, 1967).This study shows,however, that t"hetalnakhite structurediffers in two important respectsfrom the structuresof binnite and tetrahedrite. First, metal atomsare locatedat site 2o in talnakhite while there are sulphurs in binnite and tetrahedrite. Second,the eight sulphur atoms at site 8c in talnakhite are absentin the other two structures. Thesedepariures give rise to a significantlydifferent distortion of the atoms from the sphalerite-likesites and make further comparisonof thesestructures unwarranted. The interatomic distancesand angles,calculated using the coordi- 376 S. R. HALL AND E. J. GABE nates from models A and B in Table 2, do not differ significantly. These values are listed for model ,4 in T'able 4 and are shown diagram- matically in Figure 2. Three atoms show substantial shifts away from the idealized sphalerite sites, with M(2) having the largest displacement of 0.075A. This consistent with the inclusion of the atom M(1) at, site 2o, which is intersitial to the sphalerite structure. Despite this shift, the M (l)-M (2) distance of.2.72A' is significantly shorter than a close Fe-Fe contact of. 2.82L in cubanite CuFezSs (Fleet, 1971) but longer than a comparable (Fe,Ni)-(Fe,Ni) distance of. 2.52F in pentlandite (Fe, Ni)eSs (Pearson and Buerger, 1959). Comparison of the metal-metal distances between these structures is difficult, how- errer, due to the substantially different environments of the metal atoms. In the structure of cubanite, the FeS+tetrahedra share one com- mon edge but, in pentlandite, the (Fe, Ni)S+ tetrahedra share three edges.In talnakhite, each M(2)S+ tetrahedron shares an edge with one M(l)Se tetrahedron, and as a consequence,the latter shares all sr,r of its edges. This results in a close second-neighbour octahedral coordination of M (2) atoms about the M (1) atom. Therefore the struc-

TABLE 4

Interatomic Distances and Ansles

Calculated from cooldinates iD Table 2, model A. standard deviations are in parentheses-

Distances (in ringstroms) Angles (id degrees)

s(r) - !4(r) 2.293 (13) M(r) - s(r) - I4(z) 1z.o (3) s(r) - M(2) 2,337 (r3) M(l) - s(1)- !!(4) 180.0 (3) s(t) - l4(4) 2.331 (13) M(2)- s(t) - M(z) 1r0.9 (3) M(z) - s(l) - l4(4) ro8.o (3) s(2) - M(2) 2.289 (13) s(2) - M(3) 2.268 (13) l4(2)- s(z)- M(3) 108.3 (3) - s(2) - I4(4) 2.295 (13) !4(2) - s(2) !4(4) 108.z (3) s(2) - M(5) 2.2a0 (r3) M(2)- s(2)- M(s) 68.6 (3) l4(3)-s(2) -M(3) tlt.3 (3) s(r) - s(r) 3.744 (19) M(3)- s(z)- y(4) 110.3 (3) s(t) - s(z) 1,731 (t9) I4(3)- s(z) - M(5) 1r.z (3) s(l) - s(2)* 3.185 (19) l4(4)- s(2)- V(5) t?6.8 (3)

s(z) - s(z) 3.794 (t9) s(1)- M(r)- s(1) r09.5 (3) s(2) - s(z)* 3.775 (19) s(2) - s(z)* 3.697 (i9) s(1)- y(2) - s(1) r06.5 (3) s(2) - s(z)+ 3.706 (19) s(r) - M(2)- s(z) 109 I (3) s(2)- \4(2)- s(2) t11.t (3) M( t) - M(z) z- 7zz ( l8) s(2) - l4(3) - s(2) ro9.z (3) l4(2)-19(2) 3.850 (19) s(2)- M(3)- S(2)'3 109.6 (3) M(z)-M(3) 3.693 (t8) !4(2) - I4(4) 3.114 (19) s(r) - M(4)- s(2) ro7.4 (3) M(2) - M(4)+ 3. ?78 (t9) s(2)-M(4)-s(z) 111.s (3) !4(2) - M(s) 2.574 (18) s(2)- l4(5)- slz) t08.3 (3) !4(3) - I4(3) 3.715 (19) s(z)- M(5)- s(2)* 111.8 (3) M(3) - !4(1) 1.74s (19) !4(3) - \4(5) 2,61a (18)

*The precedine intcratomic distance on anqic in the Iist is bcts'een atoms aL diffcrcnt bqt cqilivalent sites. STRACTURE OF TALNAKHITE

(t\ \-a d1"oj\ \

\-,N: \ -dr\ loe \

M(5)l \ \

Frc. 2. A partial structure model of talnakhite showing the metal coordination about the two independent sulphur atoms with selected interatomic distances (in Angstroms) and angles (in degrees) calculated for model ,4. ture of talnakhite, unlike that of either cubaniteor pentlandite,con- tains both asymmetric(about lhe M(2) atom) and symmetric (about the M(1) atom) second-neighbourmetal coordination.The nature of the electronic interaction betweenthese atoms, particularly in the absenceof real evidenceto their identity, is not understood. The fivefold coordinationof the metals about the S(1) atom can be describedas a distofted trigonal bi-pyramid (seeFigure 2). Four of theseatoms, Lhree M(2) and one M (4), form anglesclose to tetra- hedraland the fiffh, M(I), forms a bond parallelto S(1)-M(4) and a M(1)-S(1)-M(2) angle of 72.0'. This compareswith a similar Fe-$-Fe angleof 74.7" in cubanite (Fleet, 1971). The other two major displacementsof atoms from the sphalerite- like sites can also be attributed to the inclusion of.M (L\ at the site 2a. The S(2) aioms, which sharethe edge of lhe M(2\ tetrahedra, are 0.0454 from the idealizedsite. The S(2) atom has essentially shifted in a parallel directionto the displacementof M (2), but slightly away. This results in a S(2)-M(2) distanceof 2.2894 closeto the 378 8. R, HALL AND E, J. GABE

M0)

M(3)

Fra. 3. A partial structural model of talnakhite sholving the atomic thermal ellipsoids of model ,4 plotted at the 90 percent probability level.

"ideal" distance of. 2.293L. As S(2) shifts towards the tetrahedron edge boundedby M(3) this reducedthe S(2)-M(3) distanceto 2.268Fr. The only other sipificant atomic changefrom the idealizedstruc- ture is the 0.087Ashift of M (4), along the 3-fold rotor axis of the cell, towards the tetrahedronface boundedby three S(2) atoms.The shifts of the S(2) atoms in the plane of this face but directly away from the center,appears sufficient to move the equilibrium centre of the tetrahedronand, consequently,Lhe M (4) atom towards this face. The anisotropic Uq thermal paramentersare listed in Table 2 and displayed diagrammically in Figure 3 as thermal displacement STRUCTARE OF TALNAKHITE 375 ellipsoids.The ellipsoids were drawn using the program ORTEP (Johnson,1965). These show that the thermal motion of both metals and sulphur atoms is essentiallyisotropic. Both the M(3) and S(2) atoms have thermal ellipsoidsflattened in the direction of their dis- placementfrom the idealizedsphalerite sites but this cannot be con- sideredsignificant with respectto the standarddeviations. Similarly, the elongationof the M(3) thermal ellipsoidsis probably not real, as suggestedby the reversalof anisotropiccomponents in the refinement, of modelsA and B. It is most probable,particularly as the ratio of reflectiondata to atomicvariables in the leastsquares refinement proc- essis not high (seeTablel), that someanisotropic parameters are com- pensatingfor errors in the data, or for asymmetricelectron residual in the structurenot necessarilydue to thermal motion effects.Varia- tions in isotropic temperature factor are probably not reliable for similar reasons.This is especiallytrue becausethe effects of site occupancyand its interactionwith ionizedscattering curves constitute a symmetric effect that is difficult to distinguish from variation in isotropic thermal motion. Becauseof this, no attempt was made to refine site occuBancy,except in the case of M(5) where the value is low (4 percent). Similarly, the low thermal parametersf.or M (l) do not appearsignificant, though they would be consistentwith the constraintsplaced on this atom by its closely packed environment.

Acrwowr,nlcnlrnr.rrs The authors are grateful to Dr. L. J. Cabri of the Mineral SciencesDivision for suggesting talnakhite for study and for supplying the crystals. Thanks are due to J. !'. Rowland and J. M. Stewart of this laboratory for assistancein the preliminary X-ray survey of this structure and to a number of members of the Mineral SciencesDivision, particularly Dr. E. H. Nickel for their discussion and suggestions during the course of this analysis. We were grateful to Dr. C. P. Huber of the'National ResearchCouncil of Canada for her assistancein the use of the NRC version of the ORTEP program. The authors are aiso grateful to Prof. G. Donnay for her constructive critique of the manuscript, and to Prof. B. J. Wuensch for several valuable suggestionson reviewing this paper.

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Manuscript receiued,,September 13, 1971; acceptbd lor publi,cati,on,Nouember 23, 1971.