Compositional data for Bi-Pb tellurosulfides

Abstract

Compounds containing essential Bi, Pb, Te and S are rare in nature. Aleksite, PbBi2Te2S2, is known from less than ten localities worldwide, and the single other recognized mineral, saddlebackite, Pb2Bi2Te2S3, is known only from the type locality, the Boddington Au deposit, Western Australia. Another phase, unnamed PbBi4Te4S3, had earlier been recognized as homogeneous grains and lamellar intergrowths within an assemblage consisting of aleksite, tellurobismuthite and tetradymite from St. David’s mine, Clogau, Wales, U.K. Re-investigation of this assemblage, including careful micro-analysis to avoid obvious intergrowths of phases, reveals an almost continuous range of compositions between tetradymite and aleksite. Investigations of complex sulfosalt–telluride assemblages from Iilijärvi, a satellite deposit within the Orijärvi orefield, southwest Finland, have revealed compositions approximating to the range Pb5Bi4Te4S7 – Pb7Bi4Te4S9. These occur as fine intergrowths, rarely as larger single lamellae, also with aleksite, within a matrix of giessenite, galena and cosalite. The samples contain abundant gold, seen also as symplectite intergrowths with rutile. Investigation of the microparagenesis of precious-metal-bearing galena – chalcopyrite – pyrite mineralization in quartz veins at Fragant ("Langenleiten"), Carinthia Province of Austria, revealed the presence of several compositionally different Bi–Pb tellurosulfides. Aleksite is the most abundant, followed by unnamed phases with compositions close to Pb3Bi4Te4S5, Pb5Bi4Te4S7 and Pb6Bi4Te4S8. They occur as inclusions in galena and are variably associated with sulfosalts such as lillianite, cosalite, felbertalite and aikinite. The compositional dataset from the above occurrences is difficult to interpret without accompanying structural data. The data, however, suggest the existence of an incremental chemical series with the general formula PbNBi4Te4SN+2. Alternatively, these are not discrete, essentially immiscible phases within a chemically defined modular series, but are simply compositions across a continuous compositional series. If the existence of a series can be proven, tetradymite, Bi4Te4S2, would correspond to N = 0, unnamed PbBi4Te4S3, to N = 1, aleksite, to N = 2, unnamed Pb3Bi4Te4S5, to N = 3, and saddlebackite, to N = 4, where the N values reflect chemical composition, rather than structurally defined homologous order. Indirect support for such a hypothesis comes from the recognition in the literature of four phases (N = 1, 2, 3 and 4) as synthetic products (phases D, E, F and J) obtained at 500 °C. The unnamed phases from Iilijärvi and Fragant may correspond to unspecified higher members of the same series. The lamellar banding with galena and tetradymite, and extended compositional fields observed in the Clogau and Iilijärvi specimens, are highly reminiscent of similar issues in Bi-sulfosalt series, allowing us to speculate that we may be looking at a typical accretional homologous series, with incremental growth in the thickness of layers. In such a scenario, random sequences of stacking of discrete members of the series at the lattice scale are considered to apply, causing chemical variation.