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COMPOSITIONAL DATA FOR Bi–Pb TELLUROSULFIDES

Nigel J. Cook^1,, Cristiana L. Ciobanu², Christopher J. Stanley³, Werner H. Paar⁴ and Krister Sundblad⁵

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 Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, U.K.
⁴ Institute of Mineralogy, University of Salzburg, Hellbrunnerstr. 34, A–5020 Salzburg, Austria
⁵ Department of Geology, University of Turku, FIN-20014 Turku, Finland

View larger version (111K): [in this window] [in a new window]   FIG. 1. Photomicrographs of telluride assemblages in specimen from Clogau [section E1309, The Natural History Museum, London, U.K.] in reflected light, taken in air (a–c) and oil immersion under slightly crossed polars (d–f) to accentuate differences among phases. Abbreviations: Gn: galena, Tbs: tellurobismuthite, ph C: "phase C", PbBi4Te4S3, Alk: aleksite, Ttd: Pb-bearing tetradymite. Note, in particular, the abundant symplectitic intergrowths of galena and tellurobismuthite, contrasting with the larger patches of "phase C" or aleksite (or both). Tetradymite typically occurs at the margins of larger bodies of "phase C".

View larger version (86K): [in this window] [in a new window]   FIG. 2. Back-scattered electron images of assemblages in the Clogau specimens. (a) Larger lath-like, composite grain of "phase C", PbBi4Te4S3 (ph C) and aleksite (Alk). Boundary between domains of the two minerals (white box) features fine (1–2 µm) lamellar intergrowths. (b) Sub-symplectitic intergrowth of "phase C", tellurobismuthite (Tbs), galena (Gn) and Pb-bearing tetradymite (Ttd). Note lamellar exsolution of galena within "phase C" (white box). The white areas are submicroscopic intergrowths (decomposition products?) containing bismuth and various tellurides. (c, d) Larger, homogeneous bodies of "phase C" showing sharp boundaries against tellurobismuthite and galena. The grain in (c) contains a single homogeneous lath of aleksite.

View larger version (105K): [in this window] [in a new window]   FIG. 3. Photomicrographs of telluride assemblages in the Iilijärvi specimens in reflected light, taken in oil immersion under slightly crossed polars to accentuate differences among phases. (a) "Grain 3": large single lamella of ~Pb7Bi4Te4S9 (some results closer to Pb6Bi4Te4S8) within galena (Gn) and associated with minor cosalite (Cos). (b) Narrow, deformed and fractured lamella of ~Pb6Bi4Te4S8 within galena (Gn). Note that during deformation, cleavage domains in the extremely soft tellurides have been opened. c, d) Two smaller lamellae of telluride within giessenite (Gie) and galena (Gn). Hs: hessite; JoB: joséite-B.

View larger version (108K): [in this window] [in a new window]   FIG. 4. Back-scattered-electron images of assemblages in the Iilijärvi specimens. (a) Lamellae of aleksite (Alk), galena (Gn) and ~Pb6Bi4Te4S8 within giessenite (Gie). (b) "Grain 6": body of homogeneous ~Pb6Bi4Te4S8 within galena (Gn), featuring a patchy rim of hessite (Hs). (c) "Grain 21": Coexisting lamellae of ~Pb6Bi4Te4S8 and aleksite (Alk) within galena. (d) Homogeneous ~Pb6Bi4Te4S8 within galena (Gn). Rim consists of hessite (Hs) and native bismuth (Bi).

View larger version (83K): [in this window] [in a new window]   FIG. 5. Back-scattered-electron images of assemblages in the Fragant specimens. (a) Aleksite (Alk) is associated with anglesite (black) and cosalite (Cos); the matrix is galenass (Gnss). (b) A lamellae of unnamed Pb5Bi4Te4S7 (Un) is intergrown with lillianite (4L35); the associated phases are felbertalite (Fel) and cosalite (Cos), and the matrix is galenass.

View larger version (23K): [in this window] [in a new window]   FIG. 6. Compositional plot in terms of Pb/(Pb + Bi) versus Te/(Te + Se + S) for Pb–Bi–Te–S phases in the Clogau, Iilijärvi and Fragant specimens and, for comparison, compositions of pertinent published data for "phase C", aleksite and saddlebackite. Ideal compositions for each mineral and unnamed phase in the Pb–Bi–Te–S system are represented in the diagram by black stars.

View larger version (19K): [in this window] [in a new window]   FIG. 7. Reflectivity spectra in air for tetradymite (red and pink; from Criddle & Stanley 1986), "phase C", PbBi4Te4S3 (dark and light green) and aleksite (dark and light blue) from Clogau and ~Pb6Bi4Te4S8 ("Grain 3") from Iilijärvi (black and grey). For each mineral or phase, the pair of spectra corresponds to Ro and Re.