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Subdivision of the Pyrite Group, and a Chemical and X Canadion Mineralogist Yol.24, pp. 27-33(1986) SUBDIVISIONOF THE PYRITEGROUP, AND A CHEMICALAND X.RAY-DIFFRACTIONINVESTIGATION OF ULLMANNITE PETERBAYLISS Department of Geologt and Geophysics, university of Calgary, Calgary, Alberta T2N IN4 ABSTRACT tre de S de I'ullmannitea €t6dercrminee pr€c6demment. Cesobservations peuvent s'expliquer par une macle sextu- Minerals in the pyrite group, with the chemicalformu- ple decristaux orthohombiques-Pca21 qui s'interp6ndtrent lae of AMXY and a pyrite-type crystal structure, are divided autourd'un axeternaire de macle 3 suivantI I I I ] ennota- into three categories:pyrite (cubic space-groupPa3), tion cubique. ullmannite (cubite space-group P213) and cobaltite (Traduit par la R6daction) (orthorhombic space-gxoupPca21). Willyamite CoSbS belongsto the cobaltite subgroup.The speciesnames of Mots-clds:groupe de la pyrite,ullmannite, willyamite, gers- gersdorffite-Pa3, gersdorffite-P213, and gersdorffite- dorffite,relation ordre-d6sordre, analyses i la micro- Pca2lhave beenapproved by IMA, Ullmannite, with the sonde,anisotropie optique, diffraction X, macle. end-membercomposition NiSbS, is isotropic and proba- bly belongs to cubic space-groupP213. Electron- microprobe analysesof nine ulhnannite specimensshow INrnonucuoN substitutionsolid-solution of Sb by up to 10.390As and 4.690Bi, and of Ni by up to 3.390Co and 0.270Fe, but almost no substitution solid-solutionfor S. The strength Ullmannite (Ramsdell 1925, Peacock & Henry of the optical anisotropism in ullmannite is correlatedwith 1948,Bokii & Tsinobev 1954,Tak6uchi 1957'Pratt the molecularpercentage of (Co,Fe)AsSas well as the inten- & Bayliss 1980)has a pyrite-typecrystal structure. sity of the reflections forbidden by cubic space-groupP213 In the chemicalformula of ullmannite 4NiSbS,the (ft00 with & odd). Partial ordering of As within the four Sb is ordered in one set of four sites,whereas the Sb sites and four S sites in ullmannite has been previously S is orderedin the other set of four sites.A crystal- determined.These observations may be explainedby a sex- stnrctur€ refinement of arsenianullnannite by Bay- tuplet of interpenetrating, twin-related, orthorhombic liss (1977)showed that As is probably concentrated (spacegroup Pca2) domainsof (Co,Fe)AsSabout a 3- in one of the four Sb sitesand in one of the four twin axis I I l] within the cubic (spacegroup P213)struc- ture of ullmannite. S sites.Such ordering would reducethe symmetry from cubic space-groupP2,3, allow reflectionsfor- Keywords: pyrite group, ullmannite, willyamite, gersdorf- bidden by cubic space-groupP2r3, aloidexplain why fite, order-disorder, microprobe analyses, optical ilsenian ullmannite is anisotropic. anisotropism,X-ray diffraction, twinning. King & Prewitt (1979)suggested that the results ofFinklea et al. (1976\and Bayliss(1977)'on pyrite disagree.Finklea et al. (1976)have refined the crys- SoMMAIRE tal structure of a pyrite specimen in cubic space- group Pa3. Their specimenwas obtained from the Les min€raux du groupe de la pyrite, de formule chimi- CommercialOre kyanite mine from the metamorphic que du type 4MXIet de structure cristalline du type pyrite, belt in South Carolina. Sincekyanite is the product se subdivisent en trois tlpes structuraux: pyrite (cubique of medium-graderegional metamorphism(Berry & (cubique (orthorhom- Pa3), ullmannite P2r3) et cobaltine Mason 1959),their pyrite specimenwas formed at bique Pca21).La willyamite CoSbSappartient au sous- and pressure.Evans el groupe de la cobaltine. Les noms d'espbcesgersdorffite- moderately high temperature Pa3, gersdorffite-P2{, et gersdorffite-Pca21,ont €t€ al. (L982)showed by Mdssbauerspectroscopy that approuv6spar I'IMA. L'ullmannite, dont la composition thereare two distinct typesof pyrite. Pauling(1978) est celle du p6le NiSbS, est isotrope et fait probablement suggesteda chemicalreason for the two crystal struc- partie du groupe spatial P213. L'analyse de neuf spdcimens tures of pyrite basedupon the pf,esenceor absence d'ullmannite i la microsonde dlectronique a 6tabli la solu- of rbsonancebetween two typesof S bonding. Since tion solide de substitution de Sb par As fiusqu'd 10.390 the pyrite of Finklea et al, (1976), formed at a high As) et Bi (4.6t/o),arnsi que celle de Ni par Co (3.390)et temperature,is expectedto be cubic, and the pyrite (0.290); par pres- Fe la m6meanalyse, contre, n'a trouv€ of Bayliss (1977), formed at low temperature, is que pas I'ull- de substitution de S. L'anisotropie optique de there is no dis- mannite est en corr6lation avecle pourcentagernoldculaire expected.tohave a lower symmetry, (1976) de (Co,Fe)AsScomme I'est aussil'intensit6 desr€flexions agreementbetween the resultsof Fnuea et al. interditespar le groupe P2P (hM d & impair). L'ordon- and Bayliss (1977). nancepartielle de As parmi les quatre sitesde Sb et les qua- If the intensity of a Bragg reflection should 27 28 THE CANADIAN MINERALOGIST becomeunobservable at any angle as the crystal is nite and cobaltitesubgroups is now possible,a clas- rotated about a scatteringvector, then the observed sification of theseminerals is now appropriate. intensityis attributedto multiple diffraction. There- fore, it is relatively easyto prove multiple diffrac- tion by finding a single angular position with an SunorvrsroNoF THE Pvnrre MXY GproUP unobservableintensity. On the other hand, King & Prewitt (1979)showed that many multiple reflections Mineral specieswith a 4MXY chemical formula occur from a CuS2sample with a pyrite structure, and a pyrite-type crystal structure have four metal and concludedthat it is extremelydifficult with four- atoms (4M) that occupy similar atomic positions in circlegoniometry to prove that the extra reflections all three subgroups(Tossell et al. l98l). Thesesub- are not causedby multiple diffraction. groups are based upon the disorder or different Optical anisotropism may be either intrinsic to the ordering-schemesof the eight nonmetal atoms crystal structure or a surface feature. Besidesthe @X + 4n in eight atomic positions. The cobaltite microscope considerations of perfectly crossed group of Fleischer(1983), which containsmineral nicols, exact centring of light, and aberration-free specieswith different ordering-schemes,should be optics, the polishedblock should be free from sur- subdivided. face featuressuch as relief, scratches,strains and The pyrite subgroupin the cubicspace-group Pa3 etching. The membersof the pyrite group that dis- has eight nonmetal atoms (4X+4I') equally dis- play orthorhombic symmetry and space group tributed (disordered)over the eight atomic positions Pco2y such as cobaltite and cobaltian gersdorffite on the threefold axes(Fig. l), The disorderand high (Bayliss 1982a,b), are optically anisotropic owing symmetryare shownby the absenceof the 010 and to crystal structure. King & Prewitt (1979)found that 0ll reflections. CuSr, which is a cubic memberof the pyrite group The ullmannite subgroup in the cubic space-group (spacegroup Pa3), is optically anisotropic. On the P2r3 has eight nonmetal atoms (4X + 4Y), where other hand, all sevenspecimens of sperrylite,which the 4X atoms are orderedin one set of four atomic is a cubic memberof the pyrite group (space-group positionson the threefold axesand the 4latoms are Pa3), were found to be optically isotropic by Klemm orderedin the other set of four atomic positionson (1962).Therefore, it is possibleto preparea polished thethreefold axes(Fig. l). The order andreduction block of a mineral in the pyrite group without sur- in symmetry from cubic space-groupPa3 to cubic face features(relief, scratches,strains and etching). space-groupP2s3 allow each set of four atoms to Optical observationsby Klemm (1962)indicate that move along the threefold axes.The lower symmetry sevenout of eight specimensof ullmannite contain of cubic space-groupP2p is shown by the absence anisotropic areasin a polished block, and suggest of the 010 reflection and the presenceof the 0ll that the optical anisotropism of ullmannite is caused reflection. by chemical variations. The cobaltite subgroupin the orthorhombic space- Since the existing evidencefor deviations from grotp Pca2, is pseudocubic.The eight nonmetal cubic symmetryby ullmannite is not substantial, fur- atoms(4X + 4Y) areordered, with 4Xatoms in one ther specimensof ullmannite wereinvctigated. Since setof fow atomic positionsand 4Yatoms in the other many new minerals with a 4MXY cherrical formula setof four atomic positions(Fig. l). Figure I shows and a pyrite-type crystal structure have been the crystal structure as it appearsprojected along the described,and a clear distinction betweenthe ullman- three pseudocubicaxes. The order and reductionin @ @ @ B @ @ B B w @ @ w w w w w iw @ \@ 6 i@ @ \@ @ i.'@ w i''@ w 7r@ EI r'@ @ P213 I-Pco21 tr-Pco21 II[-Poo2q Frc. l. Atomic positions in pyrite-group minerals. The circles and squaresare used to denote the elementsX aad Y, respectively, in the general formula 4MXY. A line connects every nonmetal pair X and Y. l-Pca2y ll-Pca21 and lll-Pca21 representdifferent orientationsof the pseudocubicstructure related by a 3 twin axis Ulll. SUBDIVISION OF THE PYRITE GROUP 29 TABLE ].. CRYSTAL.STEOCIIBE CI.AASISICATION OT TEE PIRITE MXY GROII? Pyllte aubgroup Ull@1te subgloup cobaltlto subg!@p Pa3 PaA. NtAaS G€rsilo!ff,lts-Pa3 NIABS eolsdo.ffito-P2-3 (Nt'co)AsS cslsdorf,f,lte-Pca21 RbAES f,o!.1hgrc!thlt6 NIEbS U1lfunnlte CoAsS Cobaltlte IrAaS Iraraito Itsbs Tol@hlte CoSbS WllLy@lte PtAsg Platardl,te Pdgbl€ feotlblopallaalLte PrBisb I@ldalte PdBtTe !.llch@elLto PrBIT€
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