American Mineralogist, Volume 67, pages 1048-1057,1982 A further crystal structure refinementof cobaltite Pnren Bevlrssr Institut filr Mineralogie und Kristallographieder Universitiit Vienna,Austria Al0l0 Abstract Fifteen cobaltite (CoAsS) specimensfrom localitiesin North America, Australia, and Sweden were analyzed with electron microprobe, powder diffraction, and precession cameratechniques. From eachspecimen, a samplewith a well-developedcube form {100} was selected.Intensity data were collectedfrom 30 reflections,which are mostlyforbidden by cubic spacegroup Pa3, with a singlecrystal diffractometer.A completeintensity data set was collectedfrom four of these l5 samples.Least-squares refinement shows diferent degreesof apparentAs-S disorderwith one samplealmost ordered in spacegrortp Pca21 (predominantlyone twin-relateddomain), one samplesignificantly ordered (predominantly two twin-related domains),one samplepartially ordered (unequalamounts of six twin- relateddomains), and one sampledisordered (equal amounts of six twin-relateddomains). Thesesamples are explainedas a sextupletof orthorhombic(Pca2) interpenetratingtwin- relateddomains about a 3 twin axis I t l]. Thereforecobaltite-low (true As-S order at the atomic level, Pca21, cobaltite subgroup)has a : b : c. No authentic mineralogical occurrenceof cobaltite-high(true As-S disorderat the atomic level,Pa3, pyrite subgroup) appearsto have been recorded. Introduction The 010 and 110 reflections may be used to The spacegroup of cobaltite (CoAsS) has been differentiatebetween space groups Pca21, P2fi and investigated by Mechling (1921), Ramsdell (1925), Pa3.Berry and Thompson(1962)observed both the de Jong (1926, 1928),Peacock and Henry (1948), 010 and 110reflections in cobaltitefrom Hakensbo Bokij and Tsenokev(1954), Onorato (1957a,1957b), (ROM M11824)with Straumanis-typepowder X-ray Winterberger(1962), and Oftedal (1963).Giese and diffraction photographs,Fe radiation, and Mn filter. Ken (1965) determined the crystal structure of Later Gieseand Kerr (1965)also observedboth the cobaltite(CU 200-83),and reportedR values of0.10 010 and 110 reflections in cobaltite from Cobalt, for hl6 and 0.l2 for h0l. Their crystal structure Ontario (CU 200-83)and Tunaberg, Sweden by shows that As occupies four non-metal sites, powder X-ray diffraction photographswith Fe radi- whereasS occupiesthe other four non-metalsites in ation and 20 hours exposure.Long exposureswere the pyrite-type crystal structure with an orthorhom- necessaryto show these weak 010 and 110 reflec- bic (pseudocubic)symmetry of Pca21. After heat tions; they were not observedin USNM 95740after treatmentat 800-850'C for two days, the crystal an 8 hour exposure.Peacock and Henry (1948)did structure model was refined in space group Pa3 to not observeeither the 010 or the 110reflection in an R value of 0.05 with the As and S disordered ROM M14499 with Cu radiation, but any weak amongthe eight non-metalsites of the pyrite-type reflectionwould be hidden in the high fluorescent crystal structure. The crystal structure determina- backgroundcaused by the Co in cobaltite. Reflec- tion of cobaltite by Le Damany (1962) shows a tions 010 and 110 were observed in both Debye- partial disorderof As and S. Cobaltitewas reinves- Scherrer photographs and powder diffractometer tigated in order to compare it with other pseudo- patternsfor all six cobaltite specimensby Bayliss cubic pyrite-type crystal structures of gersdoffite (1969a)with Co radiation. (Bayliss and Stephenson,1968), arsenian ullman- All cobaltite specimens observed by Klemm nite (Bayliss, 1977a)and pyrite (Bayliss, 1977b). (1962), Ramdohr (1969), and Cabri and Laflamme (1976)are optically anisotropic. Twinning occurs rOn sabbaticalleave from Department of Geology and Geo- extensively in all cobaltites examined by Bayliss physics,University of Calgary,Alberta T2N 1N4. (1969a).With photomicrographsof polished sec- 0003--004)vE2109r 0-1 048$02.00 1048 BAYLISS: COBALTITE 1049 tions under crossed nicols from Hakensbo speci- Table 2. Electron microprobe analyses mens,Klemm(1962, Fig. 16)shows coarse irregular zoning,whereas (1969, Specimen Ramdohr Fig. 495b) shows Number Fe Co Ni S As Total good twinning, which is comparativelyregular on a fine scale but not in a network. Although all the CoAsS 35.5 I9.3 45.2 100.0 crystal faces exhibited '| externally by cobaltite are 362 I.8 32.6 .1 't9.720.1 43.8 99.4 consistentwith the cubic system, internally they Mf4469 3.2 28.6 3.6 45.t 100.2 61 1.7 34.2 0.t 't8.920.5 43.0 99.s consistof optically anisotropiclamellae in a some- 95740 3.9 26.6 4.8 45.3 99.5 what regular arrangement and/or irregular areas. UNSW 3.0 32.6 0.6 20.3 43.7 t00.2 024919 2.2 32.6 0.7 20.1 44.0 99.6 From these data, Ramdohr (1969)speculates that 024922 2.0 32.9 0.3 20.2 44.0 99.4 cobaltiteis pseudocubicwith a sextupletof ortho- M]1824 5.0 30.9 0.9 22.3 40.6 99.7 rhombic lamellaetwinned parallel (110). D37828 2.4 32.6 0.8 20.1 44"2 100.1 to 361 2.5 3t.6 1.2 19.9 44.5 99.7 233 1.9 33.2 0.5 20.1 45.0 100.7 Experimental and results D37827 2.7 31.7 1( 20.2 44.3 100.4 D37829 2.4 33.2 0.3 20.4 44.2 100.5 Fifteen cobaltite specimenswere obtained for this investigation from the Australian Museum (AM), ColumbiaUniversity (CU), David New-Min- confirmed by 114.6.mm Debye-Scherrerpowder erals (D. New), Royal Ontario Museum (ROM), photographs.Neither powder diffractometer pat- United StatesNational Museum(USNM), and Uni- terns nor Debye-Scherrer powder photographs versity of New South Wales (UNSW). Specimens, showed distinct reflection splitting, however the which havepreviously been studied,are CU 200-83 broad reflections near 0 : 90" of all cobaltite and USNM 95740by Gieseand Kerr (1965),ROM specimensmay be interpreted as multiple reflec- M11824 by Berry and Thompson (1962),ROM tions from a pseudo-cubicmineral. M14469by Peacockand Henry (1948),and UNSW, These cobaltite specimenswere chemically ana- UNSW 61, UNSW 233,UNSW 361,UNSW 362, lyzed for 6 elements(Fe, Co, Ni, S, As and Sb) by andAM D24919by Bayliss (1969a).Specimen num- electron microprobe as previously described by bersand their localitiesare listedin Table l, and the Stout and Bayliss (1975).Standards used were Fe, sameorder is also used in Tables2 and 3. Co, Ni, As, Sb, FeS, FeS2, CoS2, NiS, Sb2S3, Powder X-ray diffractometer patterns of all 15 As2S3,(Ni,Co)As3 and FeAsS. The As standard cobaltite specimensshow both the 010 and ll0 was repolished and recoated to avoid oxidation. reflections including the six specimensof Bayliss Data were refined with a modified version of the (1969a),two specimensof Giese and Kerr (1965), computer program sLAvE (Nicholls et al., 1977). and one each from Berry and Thompson (1962) and Detection limits were determinedto be about 0.5 Peacockand Henry (1948).Their reflectionshape is wt.Vo.The analytical results are presentedin Table similar to that of the other reflections recorded. The 2. presenceof thesereflections 010 and ll0 has been Calculation of the stoichiometry from electron microproberesults in Table 2 basedupon a 4MXn Table l. Specimennumbers and their localities structuralformula indicatesthat n varies between 1.97 and,2.03.Since the electron microprobe data Specimen Locali ty Number contain random errors, no evidence is available to indicate deviations from stoichiometric MXz. UNSW362 N.S.l,i.,Australia Klemm (1965)accepts the MX2 formula for cobalt- RoMM14469 ColumbusClaim, Cobalt, Ont., Canada UNSW6I Bimbowrie,S.A., Australia ite basedupon a literaturesurvey, chemicalsynthe- cu 200-83 Cobalt, 0ntario, Canada sesand analyses.Also no deviation from stoichio- USNM95740 Cobalt, Ontario, Canada UNSh, S.Broken Hi I I , N.S. lrl. , Australi a metric MX2 is indicated by Klemm and Weiser AM024919 Mt. Cobalt,Qld., Australia (1965). Therefore it appears logical to accept a AM 024922 Cobalt, near ChartersTowers, Qld. RoMMi 1824 Hakensbo,Sweden stoichiometricMX2 formula. AMD37828 BrokenHill S., N.S.lr|.,Australia The cobaltite specimens(Table 2) show a minor uNsw361 Cloncurry,Qld., Australia D.New Idaho,U.S.A. rangeof substitutionof Co by Fe and Ni, which falls uNsw233 Torrington, N.S.l.l.,Australia within the solid solution limits of the ternary dia- At{D37827 BrokenHill S., N.S.l,l.,Australia gram FeAsS-CoAsS-NiAsS at 400'C AM037829 BrokenHill S., N.S.t,l.,Australia of Klemm (1965).Metal chemicalzoning is observedin speci- r050 BAYLISS: COBALTITE men UNSW, where a 10VoCo variation is inversely possible.Each column of Table 3 hasbeen arranged proportionalto a 9VoFe and l% Ni variation; and in setsof three reflections,which are equivalentin alsoin specimen95740, where a 6VoCo variation is the cubic spacegroups P23 andPa3. The observed inversely proportional to a lVo Fe and 5% Ni relative intensities(I) of the 15 sampleshave been variation.All other specimensshow metal chemical arrangedfrom left to right acrossTable 3 with Isle > zoning between lVa and 3%. The specimensalso Ioor) Irooto Iqls = Ioot) Ilss tO Ioto: Ioot: Iroo. show a minor range of substitution of As by S, Other similar observed relationshipsfrom left to which falls within the solid solutionlimits at 550"C right acrossTable 3 are as follows: of Bayliss (1969b).Non-metal chemical zoning is 1116) Is11> 1161to I1o: Iotr) I1e1to Irto: lorr observedin specimen362, where aZVoAs variation : Iror' is inverselyproportional to a l%bvariation. : : From each cobaltite specimen, a sample was 1316) Is31) Itor to I31e Iolt ) I1s3to Irto Iorr selected with an approximately equidimensional : Iro:. well-developedcube form {100}. The preliminary I13s) 1613) I3qtto Itro: [ot: ) I3stto Itro: Iotr alignmentalong an a axis was madeon a precession : I:or' camerausing. unfiltered Mo radiation. These pre- : : cession photographs hlfr and /r0/ confirmed the Ir:o ) Ior: ) I:o: to I33s Io:l ) I3s3to I::o Iors presenceof reflections 010 and 110, which are : I:o:. forbiddenby spacegrotp Pa3. Simpletwinning was Reflectionsthat would be extinct in the untwinned not observed by either reflection splitting or the crystal structure with space group Pca2r are 001, presenceof strong 210 and strong 120reflections.