RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS, ITALY. III. WITTITE AND CANNIZZARITE

Abstract

Wittite and cannizzarite, two rare and structurally related nonstoichiometric sulfosalts belonging to the system Pb‐Bi‐S‐Se, were found together around high-temperature fumaroles at La Fossa crater, Vulcano island, Italy. This occurrence makes a comparative study by different methods possible (scanning electron microscope, electron microprobe, and X-ray and electron diffraction). We discuss once more the question whether or not they are distinct mineral species. Cannizzarite was found in samples collected from 1990 to 1995 in assemblages with bismuthinite and galenobismutite (mostly), in some cases with lillianit e, heyrovskiite, Se-bearing galena, kirkiite, and the new species mozgovaite. Wittite was revealed only in samples collected in 19 95 around the fumarole vent F11 in association with bismuthinite. Both minerals form aggregates of very tiny sheaves of slightly divergent, bladed crystals (cannizzarite up to 0.5 mm in length and 0.07 mm in width; wittite up to 2 mm in length). The electr onmicroprobe data obtained (141 analyses) show significant variations in proportions of the main elements. In terms of Bi/(Bi + P b), the range of composition is 3.14 at.%; the Se content varies from less than 1 to 14.65 wt%. The composition field of the minera ls under consideration may be described by the general empirical chemical formula Pb 3+xBi4‐x(S9‐ySe y)9‐x/2 or Pb3(Bi4‐xPbx)4 (S9‐ySey)9‐x/2, with 0.04 ≤ x ≤ 0.28, and 0.5 ≤ y ≤ 3.5. Electron-diffraction data and X-ray investigations of a sample of wittite containing ~8.5 wt% Se confirm that cannizzarite and wittite have the same structure. Measured unit-cell parameters show that volumes of the H and Q subcells both vary in direct proportion to Se content. No chemical discontinuity between cannizzarite an d wittite series has been observed up to 40 at.% Se; consequently, wittite may be validated or discredited as a distinct mineral species only when a definitive crystallographic work will prove that Se exceeds S in the H layers.