Rare sulfosalts from Vulcano, Aeolian Islands, Italy. V. Selenian heyrovskyite

Heyrovskyite, a rare Pb-Bi sulfosalt, was found around a fumarole of ca. 500degreesC at the La Fossa crater, Vulcano Island, Italy. The mineral occurs in the thin sulfosalt-rich crust covering the brecciated altered rocks of fumaroles, mainly admixed with selenian varieties of lillianite and cannizzarite, with subordinate galenobismutite and galena. Heyrovskyite occurs as thin needle-like crystals up to 300 mum in length and less than I mum in width. No traces of a breakdown texture have been noticed in any of the crystals studied. Electron-microprobe analyses show that the heyrovskyite from Vulcano has a composition close to the ideal Pb-rich composition of synthetic Phase 11, Pb6Bi2S9, obtained by Salanci & Moh in 1969. Particularly interesting is the absence of Ag and Cu, which makes the heyrovskyite from Vulcano unique, with respect to cations; only a minor amount of a Cd impurity is present. Like in other sulfosalts from Vulcano, significant Se concentrations are invariably present (0.9-2.5 wt.%). The compositional field of heyrovskyite from Vulcano, expressed in terms of the mol.% Bi2S3, ranges from similar to14.23% (the most Pb-rich composition) up to 15.24% (the most Bi-rich), averaging 14.73 mol.%. This very narrow compositional spread (similar to1 mol.%) is in the range attributable to analytical error; the composition of heyrovskyite from Vulcano is thus quite constant in terms of Pb-for-Bi substitution. The calculated average composition corresponds to the empirical formula (Pb5.95Cd0.02)Sigma(5.97)Bi(2.06) (S8.53Se0.44)(Sigma8.97), which is close to the ideal end-member, Pb6Bi2S9. The cell parameters, resulting from an X-ray powder-diffraction study, are: a 13.734(7), b 31.28(3), c 4.135(3) Angstrom. The reflections having h + I = 2n are indicative of space group Bbmm. The cell parameters of heyrovskyite from Vulcano have been compared with those in the literature for heyrovskyite from various deposits and with those of synthetic Phase II. There is no general agreement among the data reported by different authors. These data, together with the compositional differences, in terms of mol.% Bi2S3, among various natural and synthetic products, suggest that the range occupied by synthetic Phase 11 consists of many chemically and structurally very similar compounds. In particular, the Pb-rich part of the compositional field of the Phase 11 might be considered as the synthetic analogue of heyrovskyite from Vulcano.