RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS,

Heyrovskýite, a rare Pb–Bi sulfosalt, was found around a fumarole of ca. 500°C 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. Heyrovskýite occurs as thin needle-like crystals up to 300 m in length and less than 1 m in width. No traces of a breakdown texture have been noticed in any of the crystals studied. Electron-microprobe analyses show that the heyrovskýite from Vulcano has a composition close to the ideal Pb-rich composition of synthetic Phase II, Pb6Bi2S9, obtained by Salanci & Moh in 1969. Particularly interesting is the absence of Ag and Cu, which makes the heyrovskýite 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 heyrovskýite from Vulcano, expressed in terms of the mol. % Bi2S3, ranges from ~14.23 % (the most Pb-rich composition) up to 15.24 % (the most Bi-rich), averaging 14.73 mol.%. This very narrow compositional spread (~1 mol.%) is in the range attributable to analytical error; the composition of heyrovskýite 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)5.97Bi2.06 (S8.53Se0.44)8.97, which is close to the ideal end-member, Pb6Bi2S9. The cell parameters, resulting from an X-ray powder-diffrac