Ameican Mineralogisl, Volume 76, pages 257-265, 1991 Crystal chemistry and crystallography of some minerals in the tetradymite group Ptrrn Berr-rss Department of Geology and Geophysics,The University of Calgary,Alberta T2N lN4, Canada AssrRAcr Selen-tellurium is a mixture of the minerals selenium and tellurium. The atoms in stibarsenare ordered(completely or partially) so that the correct chemical formula is AsSb. Matildite AgBiSr, bohdanowicziteAgBiSer, and volynskite AgBiTer, show atomic ordering in space group P3ml with unit-cell dimensions similar to those of tsumoite, BiTe. The S and Te atoms are apparently disorderedin ingodite to yield the chemical formula Bi(S,Te) and in sulphotsumoite to yield the chemical formula Bi(Te,S). The unnamed mineral of Aksenov et al. (1968) seemsto be tellurian tsumoite, (BiorrTeoor)Te.The synthetic com- position of Godovnikov et al. (1966) is equivalent to bismuthian tsumoite, Bi(Teo"Bio,'). Platynite is possibly plumbian sulfurian nevskite, (Bio65oPbo 34s)(Seo.urrSo ror). Kawazulite, BirSeTer,is isostructural with tetradymite. Csiklovaite is a mixture of te- tradymite and bismuthinite. The chemical formula of skippenite is probably Bir(Se,Te,S)r, like that oftellurian paraguanajuatite.The Se and S are disordered in laitakarite to yield the chemical formula Bio(Se,S)..As S and Te ordering was not observed in josEite or josgite-B,there is possiblya completesolid solution seriesfrom tellurian ikunolite, Bio(S,Te)r, to sulfurian pilsenite, Bio(Te,S)r.The unnamed mineral of Yingchen (1986) is apparently tellurian ikunolite. Rucklidgeite, BirTeo, has an antipilsenite, BioTer, structure type. The unnamed mineral of Haraflczyk (1978) is rucklidgeite. INrnonucrroN in order to investigate some of the problems associated group. The principal objectiveswere The narrow definition of the tetradymite group was with minerals in this given by Strunz (1970) to include only Bir(S * Se + Te), as follows: minerals. These hexagonalor trigonal minerals have an l. To obtain X-ray powder-diffraction data for minerals approximately cubic closest-packedlayer crystal struc- where no published data exist, or discredit the mineral ture (Pauling, 1975) in which the five planes within the (e.g.,selen-tellurian, csiklovaite). tetradymite layer are Te-Bi-S-Bi-Te. The axis [0001] of 2. To use published X-ray powder-diffraction data to de- the hexagonal cell correspondsto [1 1l] of the approxi- termine if the crystal structuresare ordered, partially mately cubic closest-packed crystal structure, where the ordered, or disordered in order to ascertain the correct planes are stackedperpendicular to [11]. The relation- chemical formula (e.g.,stibarsen, volynskite, ingodite, ship between different ratios of Bi:(S + Se + Te) and sulphotsumoite, kawazulite, skippenite, laitakarite, Te:S with respect to the d-value with the strongestcor- jos6ite,jos6ite-B). responding intensity in X-ray diffraction patterns was 3. To reindex the published X-ray powder-diffraction data discussedby Spiridonov (1981). The definition ofthe te- and obtain refined unit-cell dimensions (e.g., matil- tradymite mineral group was widened later by Bayliss et dite, bohdanowiczite). al. (1986) to include as essentialthose chemical elements 4. To identifi unnamed minerals, incorrectly named from group Va (As, Sb, and Bi) and group VIa (S, Se,and minerals, and synthetic phaseswithin the tetradymite Te) to produce eight subgroups each with a different num- group. ber ofplanes in the basic layer repeat unit layer. Table I 5. To review the literature and ascertain the status of lists minerals within the tetradymite group, which may platynite. be subdivided on the basis of the number of approxi- mately cubic closest-packedplanes in the layered repeat Mrrrroos unit, where each plane contains one chemical element or In the approximately cubic closest-packedlayered me- a random distribution of two or more chemical elements. tallic minerals, the number of possible crystal structure Differencesin atomic size and electronegativityallow or- types is limited. All of the crystal structure types have dered crystal structures at low temperature, but there are atoms with symmetry-fixed values of coordinates x and disordered crystal structures with extensive solid solution y, whereasthe coordinate z may or may not have a fixed at high temperature. value. Some atoms may have ordered, partially ordered, The purpose of this paper is to compare and contrast or disordereddistributions. From thesepossible ordered, the various mineral specieswithin the tetradymite group partially ordered, or disordered crystal structure types with 0003-004x/9l /0 l 024257$02.00 257 258 BAYLISS: TETRADYMITE GROUP TABLE1. Subgroups within the tetradymite group TABLE2. Unit-celldimensions obtained by least-squaresrefine- ment No. of atom Space Subgroup Powder planes group name Chemicalformula diffrac_ 1 nel Te selenium Se tion file tellurium Te or spect- 2 R3m As antimony sb Chemicalformula men no. a (A) c (A) arsenic As stibarsen AsSb 31-80 4.025s(11)10.837(9) bismuth Bi matildite AgBiS, 24-10314.0662(21) 18.958(17) stibarsen AsSb bohdanowiczite AgBiSe, 29-14414.2049(14) 19.650(11) frm1 tsumoite tsumoite BiTe volynskite AgBiTe, 18-11734.468(7) 20.7s(5) sulphotsumoite Bifie,S) ingodite Bi(So*Teo*Seoo,)37 4.2477(24)23.075(221 nevskite BiSe ingodite Bi(Teo,.So*) platynite 77 4.246(3) 23.26(3) (Bi,Pb[Se,S)(?) sulphotsumoite Bi(Teo6?50s) 38-442 4.304s(18) 23.47q141 ingodite B(S,Te) tsumoite (Bio 22-117 4.36413) 24.32(4) bohdanowiczite 56Teon2)Te AgBiSe, tsumoite Bi(feo 19-176 4.4sq3) 23.90(3) malildite ?sBio25) AgBiS, laitakarite Bi.(Seo",Soj. 1+220 4.2239(14)39.94(3) volynskite AgBiTe, ikunolite Bi3 4.258(3) 39.s4(4) frm tetradymite tellurantimony ss2oTels Sb2Te3 Bi3 22-364 4.2402(12)39.77411s1 tellurobismuthite iosEite es2seorTer o BiJe3 joseite Bi415, 12-735 4.241s(10)39.76906) paraguanaiuatite Bi2Se3 eTe,o joseitsB Bi43s' 'so Je' o 9-43s 4.3i'26(25)40.92(5) skippenite Bi,(Se,Te). rucklidgeite (Bi,Pb)sTe4 29-234 4.4180(13)41.513(15) tetradymite Bi,sTe, rucklidgeite BeJe4 4.389(16) 42.0(3s) kawazulite BirSeTe. protoioseite BissTe1sS16 4.3386(11)57.804{18) F3n joseite ikunolite Bils3 aleksite Bi2PbSrTe2 29-765 4.2423(25)79.73(5) jos6ite Bi.(S,Te)o laitakarite Bi.(Se,Te)3 pilsenite BilTe3 ioseite-B Bi/Te,S)g poubaite (Bi,Pb)3(Se,Te)r rucklidgeite BiJei been found naturally, even though at least five Se-Te 9 frm1 protojosaite protojoseite Bis(fe,S)4(?) phases have been synthesized.The crystal structure of 14(?) P3m1 aleksite aleksite BirPbSrTe,(?) 2O(?) ? hedleyite hedleyite Bi?Tes(?) both minerals (i.e., selenium, tellurium) in spacegroup P3,21 has Se or Te in 3(a) at (x,O,Yt)with x about 0.25. A complete solid-solution seriesoccurs at moderate tem- peratures(200-450 "C) from selenium(a: 4.37,c: 4-95 values for z estimated from similar crystal structures, A, PDF 6-362)ro tellurium (a: 4.46, c : 5.93 A, pof X-ray powder-diffraction patterns were calculated with 4-554) without superstructure reflections (Lanyon and the program of Smith (Pennsylvania State University). Hockings, 1966).Appreciable segregationoccurs if a melt The number of crystal-structure models may vary from is not cooled instantaneously.A phasewith both Se and two for a two-plane structure type such as stibarsen,to Te in fixed proportions would require a crystal structure ten for a seven-plane structur€ type such as jos€ite. type different from that of selenium and tellurium. As yet The calculated intensities for each possible crystal- such a phasehas not been synthesized,and it is unlikely structure model were compared to the observedintensi- to occur becauseof the difference in atomic radii of Se ties of the published X-ray powder-diffraction data. The and Te. observed X-ray powder-diffraction data were reindexed, Enquiries for selen-tellurium at eight large mineralog- and improved unit-cell dimensions were obtained by least- ical museumsrevealed only one specimen,NMNH Rl86 squaresrefinement of unit-cell parameters.If the calcu- (Smithsonian Museum). This specimen, from the only lated intensities are similar to the visually estimated ob- known locality, Honduras, was initially examined with a served intensities in the literature, then a qualitative Kevex solid-state detector on an SEM. Distinct grains conclusion may be made on the order-disorder to yield were identified as quartz, barite, selenium,tellurium, and probable chemical formula, structure type, and unit-cell selen-tellurium. The mineral assemblageof the specimen dimensions. The calculated X-ray powder-diftaction data is identical to that describedby Dana and Wells (1390), of the crystal-structuremodels that give the best possible so that this specimen probably comes from the type lo- fit to the observedX-ray powder-diffraction data are giv- cality. The sp€cimenitself has not been establishedcon- en in the tables in order to show the goodness-of-fi1. clusively to be a type specimen; however, none of the X-ray powder-diffraction patterns of the very small type specimenappears Io exist. specimens available were obtained with a 114.6 mm The grains were too fine for mounting in a polished Gandolfi camera (Fe radiation; Mn filter). The intensities block for electron-probeanalysis. An X-ray powder-dif- were measuredwith an automated densitometer. fraction photograph taken of the fine grains containing both Se and Te was identified as a mixture of selenium Tn suscnoup and tellurium. The unit-cell dimensions of selenium and Selen-tellurium, (Se,Te) with Se:Te : 2:3, was origi- tellurium are similar to those of pure Se and Te, so that nally describedfrom the El Plomo silver mine, Ojojoma negligible substitution of Te occurs in selenium and neg- District, Tegucigalpa, Honduras by Dana and Wells ligible substitution of Se occurs in tellurium.