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MINERALS OF THE SYSTEM Bi–Te–Se–S RELATED TO THE TETRADYMITE ARCHETYPE: REVIEW OF CLASSIFICATION AND COMPOSITIONAL VARIATION

Nigel J. Cook^1,, Cristiana L. Ciobanu², Thomas Wagner³ and Christopher J. Stanley⁴

¹ Natural History Museum, University of Oslo, Boks 1172 Blindern, N–0318 Oslo, Norway
² South Australian Museum, North Terrace, Adelaide, S.A. 5000, Australia and Department of Earth and Environmental Sciences, University of Adelaide, S.A. 5005, Australia
³ Institut für Geowissenschaften, Universität Tübingen, Wilhelmstr. 56, D–72074 Tübingen, Germany
⁴ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

E-mail address: nigelc{at}nhm.uio.no

Our compilation of ~900 published results of analyses for minerals of the tetradymite series (tellurobismuthite, tetradymite, guanajuatite, paraguanajuatite, kawazulite, skippenite, tsumoite, hedleyite, pilsenite, laitakarite, ikunolite, joséite-A, joséite-B) allows compositional fields among naturally occurring Bi–Te–Se–S compounds to be established. New compositional data for ingodite, laitakarite, pilsenite, kawazulite and tellurobismuthite extend previously known compositional limits. Recognized minerals can, for the most part, be satisfactorily and conveniently classified according to the ratio Bi(+ Pb)/(Te + Se + S), into the subsystems (isoseries) Bi₂Te₃–Bi₂Se₃–Bi₂S₃, Bi₄Te₃–Bi₄Se₃–Bi₄S₃ and BiTe–BiSe–BiS. Most minerals show limited compositional variation, but this is generally more extensive in the Se-bearing phases (e.g., laitakarite) and in certain members of the system Bi–Te, such as hedleyite and tsumoite. Substitution of minor Pb for Bi is widespread throughout the group, especially in the Bi₄Te₃–Bi₄Se₃–Bi₄S₃ subgroup. Several possible additional minerals or compositional variants of existing minerals would appear to exist in nature, including Bi₄Te₂Se, Bi₄Te(Se,S)₂, Bi₃Te₂Se and Bi₃(Te,S,Se)₄. Within the above groups, Bi(+ Pb)/(Te + Se + S) stoichiometry is remarkably constant, in accordance with known and derived structures in which all phases (except those in which Pb is essential) can be envisaged in terms of various combinations of nonvalent five-atom Bi₂X₃ and two-atom Bi₂ layers. Deviation from Bi(+ Pb)/(Te + Se + S) stoichiometry within the isoseries may be linked to stacking disorder. Noting the appearance of many other phases and stoichiometries in experimental work in the system Bi–Te–Se–S and its subsystems, as well as the homologous character of this series, we predict that a significant number of additional mineral phases exist in nature and will be discovered in the future. Many of these, however, cannot be identified by chemical microanalysis alone.

Keywords: bismuth chalcogenides, tetradymite series, system Bi–Te–Se–S, systematics, electron-microprobe analysis.

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