A Further Crystal Structure Refinement of Gersdorffite

American Mineralogist, Volume 67, pages 105E-10&1,I9E2

A further crystal structure refinement of gersdorffite

PBrBn BR.vI-tss Department of Geology and Geophysics University of Calgary, Alberta T2N lN4, Canada

Abstract

Thirteen gersdorffite (NiAsS) specimenswere analyzed with electron microprobe, powderdiffraction, and precessioncamera techniques. For eachspecimen, a samplewith a well-developedcube form {100} was selected.Intensity data were collected from 27 reflections,which are mostly forbidden by cubic spacegroup Pa3,with a single crystal difractometer. X-ray powder diffractiondata indicatetwo pyrite subgroupspecimens, five ullmannite subgroup specimens,and six cobaltite subgroup specimens.A complete intensity data set was collectedby single crystal diffractometerfrom three of these six cobaltitesubgroup samples. Least-squares refinement shows different degrees ofapparent As-S disorder with one sample significantly ordered (predominantly three twin-related domains) and the other two samples disordered (equal amounts of six twin-related domains).These three crystal structuresare similarto thosedescribed for cobaltite.They are explainedas a sextupletof orthorhombic(PcaT) interpenetratingtwin-related domains abouta 3 twin axis [l11]. Although the chemical compositions of the three different pyrite-type crystal structures overlap, there is a tendency for the two pyrite subgroup (true As-S disorder at the atomic level, Pa3)specimens to contain more As, and a tendencyfor the six cobaltite subgroup (true As-S order at the atomic level,Pcal) specimensto containmore Co and Fe than the five ullmannite subgroup (true As-S order at the atomic level, P23) specimens' These threecrystal structuresare probablytemperature dependent with P213the low form, Pca21 the intermediate(metastable?) form, atd Pa3 the high form.

Introduction ROM M15861)with Straumanis-typepowder photographs, Cu radiation and Ni filter. They state "pat- Three crystal structure variants of gersdorffite tern similar to that of cobaltite", although they have been describedas follows: an orderedpyrite- observedthe 010 and 110 reflections in cobaltite type structure(P23, ullmannite subgroup,Bayliss from Hakensbo.These weak 010and 110reflections and Stephenson,1967); a disordered pyrite-type aredifficult to observe,since they may be hiddenby structure(Pa3, pyrite subgroup,Bayliss 1968);and the high background fluorescencefrom the Fe and a distorted disordered pyrite-type structure (Pl, Co in gersdorffitewith Cu radiation.These 010 and gersdorffiteIII, Bayliss and Stephenson,1968). An 110reflections were observedin both Debye-Scher- alternative interpretation to these diffraction data rer photographsand powder diffractometer patterns for gersdorffiteIII is suggestedby the work of in six gersdorffite specimens (USNM R830, Bayliss(1982), who interpretsdifferent As-S order- 8M1922,145, USNM R862, BMI929,12, HMM, disorderwithin cobaltite crystals as a sextupletof 8M57562) by Bayliss (1969a). orthorhombic (Pc a2 ) interpenetrating twin-related Klemm (1962), Ramdohr (1969), and Cabri and domainsabout a 3 twin axis [l11]. In view of this, Laflamme (1975)indicate that most specimensof the crystal structure of gersdorffite III is re-exam- gersdorffiteare optically isotropic, but some are ined in more detail. optically anisotropic.Bayliss (1969a)shows a grad- The 010 and ll0 reflections may be used to ual increasefrom optically isotropic for NiAsS to differentiatebetween space groups Pca2r, P2$ and the strongestoptical anisotropy for CoAsS in the Pa3. Berry and Thompson (1962)did not observe gersdorffite-cobaltitesolid solution series. Optical the 010 and 110reflections in gersdorffite,(Ni,Fe, anisotropyis most frequentlyrecognized by Klemm Co)AsS,from Sudbury,Ontario (ROM Ml2l76 and (1962)through twin lamellaeafter {l I l}? and {100}. 0003-o04x/82/09l 0-l 058$02.00 1058 BAYLISS: GERSDORFFITE 1059

Ramdohr (1969) states "all authentic specimens Table 2. Electronmicroprobe analyses examinedby the writer or by Meyer (1926)did not Specimen exhibit lamellar structure", however, Bayliss Number Fe Co Ni S As Sb Total (1969a) observed twinning in gersdorffite from '19.4 '100.0 Leichtenberg (USNM R862) and Lobenstein NiAsS 35.4 45.2 (BM57562). 57562 0.2 0.2 34.1 l 7.8 46.7 0.1 99.1 RB62 1.8 0.7 32.9 l8.B 45.0 0.4 99.6 Experimental and results 1929,12 2.3 0.5 32.1 19.2 44.7 0.2 99.0 t412176 5.3 5.4 23.6 18.2 46.6 0.1 99.2 Thirteen gersdorffite specimenswere obtained M]'t922,1455861 6.5 9.3 18.8 18.7 46.8 0.3 'I100.4 1.7 17.8 14.8 to.a cu.u for this investigation from the British Museum 00.5 'lR (BM), Royal Ontario Museum (ROM), United 1959,462'11 0.4 0.3't.2 34.I 18.3 44.s ool StatesNational Museum (USNM), 't 3044 0.9 33.6 20.0 45.4 'l 101.1 and University 434 | .0 1.4 32.6 18.9 43.6 .7 99.2 of New South Wales (UNSW). The specimens, I20381 0.6 4.2 27.6 11.0 57.5 100.9 which have previously been studied, are BM 1434, 1917,2855.6 0.f 29.3 17.1 48"7 0.1 100.9 BM 57562,BM lgl7,2g5, BM 1922,145,BM lg2g, UNSW 1.3 0.4 32.0 '10.114.2 52.5 0.4 100"8 12,BM 1933,371,BM 1959,462.USNM R862and 1933,3713.3 2.2 26.3 58.0 0.B I00.7 UNSW by Bayliss (1969a),and ROM Ml2l76 and ROM M15861 by Berry and Thompson (1962). Specimennumbers with their localitiesare listed in showeddistinct reflectionsplitting. Most specimens Table 1, and the sameorder is also usedin Tables2 of groups(2) and (3) above show sharp reflections and 3. near 0 : 90", which indicatescubic symmetry. On Powder diffractometerpatterns may be divided the other hand, most specimensof group (1) above into three groups as follows: (1) both the 010 and show broad reflectionsnear 0: 90o,which may be 110reflections are observedin six gersdorffitespec- interpretedas multiple reflectionsfrom a pseudo- imens(57562, R862, 1929,12,M12176, M15861, and cubic mineral. 1922,145),(2) the 010 reflectionis absentalthough All gersdoffite specimenswere chemically ana- the 110 reflection is observed in five gersdorffite lyzed by electron microprobe as previously de- specimens(1959,462, 113044, 1434, 120381,and scribedby Bayliss(1982). Calculation of the stoichi- 1917,285),and (3) reflections010 and I 10are absent ometry from electronmicroprobe results in Table 2 in two gersdorffite specimens (1933,371 and based upon a 4MX, structural formula indicates UNSW). The shapesof reflections010 and 110are that n varies between 1.97 and 2.03. Since the similar to those of the other reflectionsrecorded. electron microprobe data contain random errors, no The presenceofthese 010 and 110reflections have evidence is available to indicate deviations from beenconfirmed by 114.6mm Debye-Scherrerpow- stoic.hiometricMX2. The Ni-As-S system de- der photographs.Neither powder diffractometer scrrbedby Yund (1962)gives a MX2 formula for patterns nor Debye-Scherrerpowder photographs gersdorffite based upon chemical syntheses and a literature survey. A similar conclusion has been Table l. Specimennumbers and their localities deduced by Klemm (1965). Therefore it appears logicalto accepta stoichiometricMXz formula. Specimen Number Locali ty The gersdorffitespecimens (Table 2) show a wide rangeof substitutionof Ni by Co and Fe, which falls BM57562 Lobenstein,Russ, Germany within the solid solution limits of the ternary dia- USNMRB62 Leichtenberg,Fichtel gebirge, Germany BMI929,12 Mitterburg, Salzburg,Austria gram FeAsS-CoAsS-NiAsS at 500" C of Klemm ROMMl2l76 CreanHill, Sudbury,Ont., Canada (1965).Metal chemicalzoning is observedin speci- ROMMI5861 GarsonMine, Sudbury,Ont., Canada BM1922,145 Cobalt,Ont., Canada men M12176,where a9Vo Ni variation is inversely proportional to a 2VoFe and 7% Co variation. The BM1959,462 Cochabamba,Bolivia USNMI I 3044 Temagami,0nt. , Canada specimensalso show a wide rangeof substitutionof BM1434 Musen,l'lestphalia, Germany S by As, which falls within the solid solutionlimits USNMI2O38I Art Ahmane,Bou Azzer, Morocco /196D BM1917,285 Sudbury,0nt., Canada of Yund at 450' C. Non-metalchemical zon- ing is observedin specimenUNSW, where a9VoAs UNSt.l Ferro, Slovakia, Czechoslovakia variationis inverselyproportional to a 4VoS varia- BM1933,371 Farvic Mine, Gwanda,Rhodesia tion. 1060 BAYLISS: GERSDORFFITE

From each gersdormtespecimen, a samplewas cludes24 reflectionsforbidden by spacegroup Pa3, selected with an approximately equidimensional were collected. These integrated intensities were well-developedcube form {100}. The preliminary corrected for background and scaled so that the alignmentalong an a axis was made on a precession maximum observed relative intensity (I) is 1000. camera using unfiltered Mo radiation. These pre- Values of I are approximately equal to the limit of cession photographs hkn and ftOl confirmed the detectionat the 3o confidencelevel. presenceofreflections 010 and 110observed in the The crystallographic axes of each sample were powder diffractometer patterns. Simple twinning chosenso that Irzo) 1216,&nd if possibleIoro ) Ioor wasnot observedby either reflectionsplitting or the ) Iroo. This is satisfied by only one of the six presenceof strong 210 and strong 120 reflections. possibleorthogonal orientations. If I0r0 : Ioor : Strong 210 and 120 reflectionsmay result from a I1ss,then three different orthogonal orientations are (ll0) twin planeas shown by iron cross twinning in possible.Each columnof Table 3 hasbeen arranged pyrite (Tennyson, 1980).The observed reflection in setsof three reflections,which are equivalentin intensities(I) within each of the three groups de- the cubic spacegroups PZg and Pa3. The observed fined by the powder diffractometer patterns are relativeintensities (I) of the nine samples,which are similar, except for M15861where Ioro) 1661) 1166 given in Table 3, have been arranged so that sam- whereasfor all other samplesin this group Isls : ples with similar intensitiesare grouped together. Ioot : Iroo. Similar results were obtained from The valuesof F21a,F1;,sand Fs12 recorded in Table4 precessionphotographs of additional samplesfrom for two gersdorffite samples are approximately thesegersdorffite specimens. equal. The variability between the values of I2s1, Eight of these gersdorffite sampleswere selected 1126and Ie12in Table 3 for eight gersdorffitesamples for single crystal diffractometry. Each sample was is by a factor of about three, which takes into accurately centered on a manual four circle diffrac- account the fact that these observed relative inten- tometerwith reflections800, 800 and 080so that an sities have not been corrected for absorption. a axis coincided with the diffractometer @ axis. Therefore for these gersdorffte samples, the rela- Integrated intensities of 27 reflections, which in- tive intensity of a reflection from one sample must be at least triple the relative intensity of the same Table 3. Observed relative intensities from eight gersdorffite reflection from another sample to be considered samples different.With this limitation, Table 3 confirmsthe results (three groups) powder Pea?, '-l-D?a taJ obtained from diffraction patterns and precessioncamera photographs so * * * il3- 120-1917, I O1? R86212176 15861 1586] 044 1434 381 285 371 that it is not essentialto correct all the relative 010 1? 70 132 15',l uo5 intensitiesfor absorptionand calculatetheir stan- 001 15 35 37 014 I 100 1223363 0] 0 2 dard deviations. 030 3 20 27 ll 000 0 In specimen1933,371, reflections 010 and 110are 003 l0 13 7 1 130 t powder 300 4771 001 0 absentin a diffractometerpattern, Debye- Scherrerphotograph and precessioncamera photo- 050 5 19 24 12 071 1 005 6 13 I I 0 003 0 graphs.A number ofweak reflectionsare recorded 500 3 ll l0 5 I 0l0 0 in Table3, which are systematicallyabsent in space 070 4 12 ]5 I 0 0r0 0 007 485t 0 000 0 group Pa3 after the crystallographic axes are rotat- 700 4641 0 000 0 edto yxz to produceMiller indicesof khl in Table 3. ll0 69 25 53 l0 130 99 95 121 0 0ll 322916I 176 Il0 131 I2l 0 Theseweak reflections have a different shapecom- l0l 3133132 22 l8 26 58 0 paredto the other reflections.Similar weak reflec- '1000 120 l 000 37r I 000 I 000 1000 956 1000 909 tions, which have been attributed to the Renninger 012 670 1000 667 '1000 '663 370 986 937 1000 837 201 6]0 -o00 371 226 263 3',|l 484 370 effect, have been observed in pyrite-type crystal '15'l 210 8754 lr9 ]l5 224 0 structures in space group Pa3 by Pratt and Bayliss 021 55 I0 I 315 225 240 241 0 102 44 l',l I 44 26 45 106 0 (1979)and King and Prewitt (1980).Therefore the ',t7 310 l8 I 23 20 20 54 1 crystal structureof 1933,371issimilar to the gers- 031 20 ll 5 76 54 59 54 0 dorffite (disordered, Pa3, pyrite subgroup) of Bay- r03 t2 ]6 4 13 7 11 29 0 '130 liss (1968). ]4 r0 19 45 32 35 34 2 0l3 ll 1l 5 43'15 29 41 24 0 In four specimens(1917,285, 120381, 1434, and 301 6t3 13 16 19 0 113044),the 010 reflectionis absentin the powder BAYLISS: GERSDORFFITE l06l diffractometer pattern, Debye-Scherrerphotograph Table 4. Structure factors from two gersdorffite samples and precessioncamera photographs,although the 110 reflection is observed. A number of weak Mt586t y!l! Ml5861 t',t]2175 'obsLFI 'obs 'obe 'calc 'obs reflections (1x00,0l{, and 00/) are recorded in Table hkL caLc hkl 3, which are systematicallyabsent in spacegroup 100 11.7 2.7 6.3 201 97.093.2 104.5 P213.These weak reflectionshave a differentshape 0t0 24.222.6 8.1 120 96.792.7 100.5 compared to the other reflections. Similar weak 001 15.312.0 8.3 012 94.089.'t ll0.l reflections,which have been attributed to the Ren- 300 B.B 3.1 9.7 102 1.8 4.7 2.3 '18.9 qq ninger effect, have been observed in pyrite-type 030 'll lB.5 210 5.4 4.3 2.9 003 .4 ll .6 021 3.7 3.7 crystal structuresin spacegroup P213by Pratt and 3.0 .l03 Bayliss (1980).Therefore the crystal structure of 500 14.6 6.7 il.3 7.4 4.4 5.5 these four specimensis similar to the gersdorffite 050 24.621.7 9.1 310 14.812.9 5.2 005 15.3 9.5 l5.l 031 10.2 9.9 7.1 (ordered,P213, ullmannite subgroup)of Baylissand '11.7 700 l't.2 10.2 301 B.l 5.4 6.4 Stephenson(1967). qn 070 .l4.624.0.|1.521 .4 130 15.512.2 5.6 Three specimens(R862, Ml2l76 and M15861) 007 il.1 013 9.6 t0.t 9.2 have no systematicabsences, and reflections010 t0l 9.4 2.4 EA 303 l3.l 2.8 B.t and 110are also observedin a powder diffractom- 'n0 17.519.6 5.1 330 '11.8lB.318.0 7.2 eter pattern, Debye-Scherrerphotograph and pre- 0ll lt.6 4.8 6.0 033 7.5 15.6 cession camera photographs. The four samples have been arrangedfrom left to right across Table 3 with I0r0 : Ioor : Iroo (distorted and disordered cubic unit cell. Cubic unit cell differencesbetween crystal structureof Bayliss and Stephenson,1968) samplesof the same specimenare attributed to the to Is16) Ioot : Iroo (cobaltite-lowordered crystal Ni:Fe*Co or As:S chemicalzoning. For eachof the structurein Pca2l of Gieseand Kerr, 1965).Reflec- three gersdorffite samples, the angular position of tions that would be extinct in the untwinnedcrystal about 20 strong reflectionsat high 20 values were structurewith spacegrotp Pca21are 001,100,011, measured.No deviationwas detectablefroma : b 101,031,103,013, 301,033 and 303. : c and a : F : "f = 90". These measurements In order to investigatethe crystal structure of yieldeda : 5.622(!A for Ml586l . a : 5.657Q\A gersdorffitefurther, specimenR862 (I0ro : Ioor : for Ml2l76, and,a : 5.685(3)Afor R862. I1s0)was again selectedtogether with M12176(Ioro Integratedintensities of all reflectionsfrom one ) Ioor) Ilsq) and M15861(Ioro ) Ioor: Iroo)for hemispherewere collected with MoKo radiation singlecrystal structureanalysis. These are marked and a graphite 002 monochromator.For M15861, by x in Table3. The secondsample of M15861could 1151reflections within the range5' < 20 ( 65owere not be used,because it hasan irregularshape. Their scannedtwice in a 0:20 mode at l'20 per minute derived chemical formulae based upon a 4MX7 over the scanwidth of 1.9' + btan 0. For Ml2l76, structural formulae are as follows: 1017reflections within the range5' < 20 < 60"were scannedin a 0:20mode at 112"20per minute M15861 (Feq.1eCos.27Nie.5a)As1.eaS6.eu over the scanwidth of 1.7" + btan d. For R862,about 2000 Ml2l76 (Fes.16Coq.16Ni6.66)As1.s5See5 reflectionswithin the range5" < 20 < 80owere step scannedtwice in a 0:20mode at a 0.01:0.02'20step R862 (Fe6.65Cosq2Nis.e3)As1 s1Sb661Se.e3 after each second over the scan width of 2.4"20. All reflections,which were recorded by powder Backgroundcounts were measuredfor ten seconds diffractometerpatterns, Debye-Scherrerand pre- at both the beginning and end of each reflection cessionphotographs, may be indexed on a cubic measurement.A standard020 reflection, which was unit cell with a between5.63 and 5.71A. Cubic unit measuredevery 50 reflections throughout the data cells were calculatedfrom Debye-Scherrerphoto- collection, showed satisfactory experimental stabil- graphsby the Nelson and Riley (1945)extrapolation ity, so that no correctionwas appliedfor instrumen- methodto 0 : 90o.A comparisonof thesecubic unit tal drift. cells with 5.6934 of NiAsS (Yund, 1962) and Background, Lorerrz, polarization, and absorp- 5.5764of CoAsS (Bayliss, 1969b)and also with the tion corrections were made following the method of chemical analyses in Table 2 confirms that Co Wuensch and Prewitt (1965). The sample from decreasesthe cubic unit cell and As increasesthe M15861measured 108(3) pm x 44(3)pm x 30(3) BAYLISS: GERSDORFFITE lrm and has a linear absorption coemcient of 313 factors were varied to seek potential differences cm-r. Transmissionfactors of 0.36 to 0.51 were betweensimilar sites. For Ml2l76, the resultantR calculated from an absorption correction based value in spacegroup Pl was 0.09. upon 240grid pointsarranged in a gaussiandistribu- tion. This absorptioncorrection was checkedby the Discussion measurementof both the 010and 080 reflectionsat The positional parameters of atoms for Ml2l76 every 10"@about the scatteringvector by a 0:20 and M15861in space grottpPca2r are M(0,1/4,0), scanto give 36 different measurementpositions for X(3/8,5/8,3/8)and X(5/8,7/8,5/8).Only one of the six both reflections.These Fo6" values are consistentto orthogonalorientations is possiblefor M15861,but -r4Vo, and their reflection shapes are similar to three orthogonal orientations are possible for those of other reflectionsso that these reflections Ml2l76. Thesepositional parameters are then simi- are not causedby the Renningereffect. The sample lar to those generated by space group Pa3 after from M12176measured 156(3) pm x 84(3)pm x reductionby the vector (0,- 1/4,0)with axes yxzand 124(2)p,m and has a linear absorption coefficient of atomsof M(0,0,0)and X(3/8,3/8,3/8). 309cm-t. Transmissionfactors of 0.05to 0.12were Since the secondsample of R862 gave a similar calculated from an absorption correction based crystal structure refinement to that reported by upon960 grid pointsarranged in a gaussiandistribu- Bayliss and Stephenson (1968), the refinement tion. methodis not repeated.The R valueof 0. 15in space The observations of reflections forbidden by group Pl is lower than the previous determination space group Pca21 may be explained by either of 0.19. Since this result has been repeated, it complextwinning or further orderingof the Fe, Co, appearsconsistent, although the crystal structure Ni, As, Sb and S atoms amongthe twelve sites. In modelis incorrect becausethe R value is too high. order to investigate these possibilities, least- The strong correlation betweensite occupancies squaresrefinement was undertaken with the proce- and temperaturefactors for Ml2l76 indicatesthat dure describedby Bayliss (1982)and with the initial site occupancyrefinement is not possible. If site positional parameters from Bayliss (1968). The orderingdoes occur, it would be reflectedby large Sbs.61in M15861was placedin site X6, becausesite temperaturefactor differences.Since the tempera- X6 indicatedthe most atomic scatteringpower. For ture factorsof M1 to Ma and X1 to X6 lie with 3o, it Ml586l, the resultantR value was 0.08 in space is concludedthat neither metal nor non-metalsite groupPl with the Fe, Co and Ni evenly distributed orderingcan be observed.The crystal structureof aboutthe four metal sites. SampleM15861 was not Ml2l76 is similar to that of R862, and is also refinedin spacegroup Pca21, because the structure consistent with the earlier work of Bayliss and factors of Otl with / odd and h0l with ft odd, which Stephenson(1968). are forbidden in space grotp Pca2r, are clearly The crystal structures of R862 and M12176 are observablein Table 5. The strong correlation be- similar to the crystal structure of cobaltite D37829 tween site occupanciesand temperature factors for (equal amounts of twin-related domains) as de- Ml2l76 meansthat site occupancyrefinement was scribed by Bayliss (1982). In addition the least- not possible.Therefore the As and S were evenly squaresrefinements of these three crystal struc- distributedamong the eight non-metalsites (X) and tures of gersdorffite have similar unsatisfactory the Fe, Co and Ni were evenly distributed among featuresof high R valuesand strong correlation.A the four metal sites (M). Individual temperature twin analysisof M12176and R862may be undertaken, since Bayliss (1969a)observed that specimen Table 5. Site occupanciesof Ml586l R862is twinned.This twin analysisindicates similar amounts (15 to 20Vo)of all six orientations from Bayliss(1982) because Fnra: Fort - Fnot(Table 4) and As orderingwas not detectedin the non-metal 0.68 0.32(4) 1.0 sites. 0.78 0.22(s The total amount of As for M15861in the four 0.59 o.4l(5 0.32 0.68(s non-metalsites (Table 5) related by Pca2r symme- o.e9(s 0.01 try of X1 to X+ (0.95As atoms)is significantlyless 0.09 o.9l(s (3.21 0.38 0.62(5 than that in sites X5 to Xs As atoms). There- fore this gersdoffite crystal appears to be signifi- BAYLISS: GERSDORFFITE cantly ordered.The As occupancysequence ofthe trigonal prism with average interatomic angles of eightnon-metal sites is Xt : Xo > Xs : Xa > X+ : 85" (XA-M-Xa or Xs-M-Xg) and 95" (XA-M-XB). Xr > X: > X2. This As occupancy sequence The Xa atomsoccupy the S sitesand the Xs atoms correlateswith the averageinteratomic distances, occupythe As (or Sb) sitesin the P2fi orderingthat sincethe longer averageinteratomic distances cor- occurs in gersdorffite (Bayliss and Stephenson, relatewith the higherAs site occupancies.Also this 1967)and ullmannite(Pratt and Bayliss, 1980). As occupancysequence generally correlateswith This crystal structureof M15861is similar to the the temperaturefactors, since the lower tempera- crystal structure of cobaltite D24919(predominant- ture factors correlatewith the higher As site occu- ly three twin-related domains) as described by pancies.Exceptions to this As occupancysequence Bayliss(1982). A similartwin analysisof M15861 indicatedby both averageinteratomic distances and indicatesthat Fhks) Fo*t) Fnor(Table4), and three temperature factors are that X1 should contain twin orientations(II, III, and V of Bayliss, 1982)are slightlymore As, whereasX5 shouldcontain slight- not possible,because As is absentfrom non-metal ly lessAs. site X2 Gable 5). The results calculatedby these The occupancies of the four metal sites for two methodsare similar with 50% of orientationI, M15861were refinedbecause there may be a chemi- 30%of orientationVI, and 20% of orientationIV. cal reasonfor this type of As-S ordering, however Heating experiments on six gersdorffite speci- no meaningfulrefinement was expected, because mens at 550" and 600"C for one week by Bayliss the atomic scatteringfactors of Fe, Co and Ni are (1969a)indicate a crystal structurechange with the similar. The average interatomic distancesabout loss of the 010 reflectionfirst, and then the loss of the octahedralmetal sites are not only affectedby the 110reflection later as detectedwith powder X- the substitutionof Fe and Co for Ni, but alsoby the ray diffraction patterns. From these heating experi- amountof As in the six non-metalsites coordinated ments,it seemslikely that P23 is the low tempera- with eachoctahedral metal site.The total numberof ture orderedphase, whereas Pca21 is an intermedi- As atoms (Table 5) in these six non-metal sites ate (metastable?)phase, and Pa3 is the high coordinated with each octahedral metal site are temperaturedisordered phase. The disappearance 3.16(30)around Mr, 3.16(29)around Mt, 3.03(29) of the 3 axis in the transformation from P2fi to around M3, and 3.13(29)around Ma. An overall Pca21 reqtires two of the four fully ordered As analysis of the metal site occupancy refinement, pairs to reorient by 180'. temperaturefactors, averageinteratomic distances, and total number of As atoms in the six non-metal Conclusions sites coordinatedwith each octahedral metal site Similar to cobaltite, these non-cubic crystal shows no systematictrend. Therefore the Fe and structures of gersdorffite are a sextuplet of ortho- Co are randomly distributed amongthe four metal rhombic (Pca2) interpenetratingtwin-related do- sites. mainsabout a J twin axis [111].This explains(1) The pyrite-type crystal structure may be de- the trend to higher R values from apparently signifi- scribed as similar to the halite crystal structure, cantly ordered (M15861)to apparently disordered wherethe metal atom (M) replacesNa and the non- (M12176,R862); (2) why the R value of the disor- metalpair (Xz) replacesCl. A summationof the As dered crystal structureis above 0.03; (3) the pres- atoms in the non-metalpair (Xz) of Ml586l from ence of strong reflectionsforbidden by the disor- Table5 givesX1 + X5: 1.00(9),X2 * X6: dered crystal structure (Pa3); (4) why different 1.00(10),X3 + X7 : l l3(10), and Xa * Xs : samplesfrom M151861may have diferent intensi- 1.03(10),so that eachnon-metal pair containsabout ties; and (5) why the non-metalsite occupanciesof one As atom. The interatomic distancesbetween the apparently disordered crystal structure do not thesenon-metal pair sites are similar. refine. Therefore the refinement of the crystal The averageinteratomic anglesof M15861indi- structuremodel in spacegroup Pl is unjustified. cate that the coordination of the non-metalatoms The poor quality of these three crystal structure hasbeen distortedfrom a regulartetrahedron (109" refinementsis attributed to twinning; since the angles) to a trigonal pyramid with average interatomic positional parametersof the six orthorhom- atomicangles of 102'(M-X-X) and 116'(M-X-M). bic orientations are slightly different, becausenei- The coordination of the metal atoms has been ther x, y and z, nor a, b, and c are exactly equal. distorted from a regular octahedron (90' angles)to a Systematic elTors caused by twinning indicate that l0@ BAYLISS: GERSDORFFITE the standard deviations of the positional parame- Bayliss, P. (1982)A further crystal structure refinement of ters, and hence also of the interatomic distances cobaltite. American Mineralogist, 67, 1048-1057. (1967)The crystalstructure of and angles,are underestimated.Although it is pos- Bayliss,P. andStephenson, N. C. gersdorffite.Mineralogical Magazine, 36, 3842. sible to refine crystal structure models based on Bayliss,P. and Stephenson,N. C. (1968)The crystalstructure of twinned crystals, such a refinementis unlikely to gersdorffite(III), a distorted and disordered pyrite structure. provide an excellent quality refinement, because Mineralogical Magazine, 36, 940-947. the exact values of s, b and c are unknown. Berry, L. G. and Thompson,R. M. (1962)X-ray Powder Data Atlas. Geological Society of Therefore a single crystal without twinning should for Ore Minerals: The Peacock AmericaMemoir, 85. be usedto obtain an excellentquality refinementof Cabri. L. J. and Laflamme,J. H. G. (1976)The mineralogyof non-cubicgersdorffite in Pca21. the platinum-groupelements from some copper-nickel depos- Although the chemical compositions (Table 2) of its ofthe Sudburyarea, Ontario. Economic Geology,7l,ll59- the three different crystal structures overlap, there I t95. P. F. (1965)The crystal structuresof is a tendencyfor the two pyrite subgroup(true As-S Giese,Jr. R. F. and Kerr, ordered and disordered cobaltite. American Mineralogist, 50, disorder at the atomic level, Pa3) specimensto 1002-10t4. contain more As and a tendencyfor the six cobaltite King, H. E. Jr. and Prewitt, C. T. (1979)Structure and symme- subgroup (true As-S order at the atomic level, try of CuS2 (pyrite structure). American Mineralogist, 64, Pca2) specimensto contain more Co and Fe than 1265-1271 (1962) bei kubischenErzmin- the five ullmannite subgroup(true As-S order at the Klemm, D. D. Anisotropieefekte eralien. Neues Jahrbuch fiir Mineralogie Abhandlungen, 97, atomic leveI, P2g) specimens.Separate species 337-356. namesfor all three crystal structuresare not neces- Klemm, D. D. (1965)Synthesen und Analysenin den Dreieck: sary, becausethey are minor derivatives of the diagrammen FeAsS-CoAsS-NiAsS und FeS2-CoS2-NiS2. pyrite-type crystal structure based upon different NeuesJahrbuch fiir Mineralogie Abhandlungen, 103,205-255. (1945) investiga- types of As-S ordering, which are probably tem- Nelson,J. B. and Riley, D. P. An experimental tion of extrapolation methods in the derivation of accurate peraturedependent with P2fi the low form, Pca21 unit-cell dimensionsof crystals. Proceedingsof the Physics the intermediate(metastable?) form, and Pa3 the Society,57, 160-177. high form. Pratt,J. L. and Bayliss,P. (1979)Crystal structure refinement of cattierite. Zeitschrift fiir Kristallographie, 150, 163-167. Acknowledgments Pratt,J. L. and Bayliss,P. (1980)Crystal structure refinement of a cobaltianullmannite. American Mineralogist, 65, 154-156. Financialassistance was provided by the Natural Sciencesand Ramdohr, P. (1969)The Ore Minerals and Their Intergrowths. EngineeringResearch Council Canada. Mr. B. Rutherford and PergamonPress, New York. Mrs. J. L. Pratt did the electronmicroprobe analyses. Tennyson,C. (1980)Zwillingsstrukturen. Lapis,5, Nr. 2, 12-14. (1965) References Wuensch,B. J. and Prewitt, C. T. Correctionsfor X-ray absorption by a crystal of arbitrary shape. Zeitschrift ftir Bayliss,P. (1968)The crystal structureof disorderedgersdorf- Kristallographie,122, 24-59. fite. American Mineralogist, 53, 290-293. Yund, R. A. (1962)The system Ni-As-S: phaserelations and Bayliss, P. (1969a)X-ray data, optical anistropism, and thermal mineralogicalsignificance. American Journal of Science, 260, stability of cobaltite, gersdorffte, and ullmannite. Mineralogi- 't61J82. cal Magazine, 37, 26-33. Bayliss,P. (1969b)Isomorphous substitution in syntheticcobalt- Manuscriptreceived, July 21, 198l,; ite and ullmannite. American Mineralogist, 54, 426-430. acceptedforpublication, March 1, 1982.