Rare sulfosalts from vulcano, Aeolian Islands, Italy. III. Wittite and cannizzarite

Wittite and cannizzarite, two rare and structurally related nonstoichiometr ic sulfosalts belonging to the system Pb-Bi-S-Se, were found together aroun d high-temperature fumaroles at La Fossa crater, Vulcano island, Italy. Thi s occurrence makes a comparative study by different methods possible (scann ing electron microscope, electron microprobe, and X-ray and electron diffra ction). Ne discuss once more the question whether or not they are distinct mineral species. Cannizzarite was found in samples collected from 1990 to 1 995 in assemblages with bismuthinite and galenobismutite (mostly), in some cases with lillianite, heyrovskiite, Se-bearing galena, kirkiite, and the n ew species mozgovaite. Wittite was revealed only in samples collected in 19 95 around the fumarole vent F11 in association with bismuthinite. Both mine rals form aggregates of very tiny sheaves of slightly divergent, bladed cry stals (cannizzarite up to 0.5 mm in length and 0.07 mm in width; wittite up to 2 mm in length). The electron-microprobe data obtained (141 analyses) s how significant variations in proportions of the main elements. In terms of Bi/(Bi + Pb), 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 minerals under consideration may be described by the general empirical chemical formula P b3+xBi4-x(S9-ySe (y))(9-x/2) or Pb-3(Bi4-xPbx)(4) (S9-ySey)(9-x/2), with 0. 04 less than or equal to x less than or equal to 0.28, and 0.5 less than or equal to y less than or equal to 3.5. Electron-diffraction data and X-ray investigations of a sample of wittite containing similar to 8.5 wt% Se conf irm that cannizzarite and wittite have the same structure. Measured unit-ce ll parameters show that volumes of the Il and Q subcells both Vary in direc t proportion to Se content. No chemical discontinuity between cannizzarite and wittite series has been observed up to 40 at.% Se; consequently, wittit e 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 lay ers.