Rare sulfosalts from Vulcano, Aeolian Islands, Italy. VII. Cl-bearing galenobismutite

Cl-bearing galenobismutite (with Cl ranging from 0.12 to 1.45 wt.%), discovered among high-temperature fumarole incrustations at "La Fossa" crater, Island of Vulcano, Italy, has been chemically and structurally investigated. The mineral is commonly associated with bismuthinite, cannizzarite, lillianite, heyrovskýite and galena, and rarely with kirkiite, vurroite and other less well characterized Pb(Bi)-sulfochlorides. Electron-microprobe investigations show that Cl-bearing galenobismutite from Volcano has a wide compositional field (Pb:Bi atomic ratio ranging from 0.52 to 0.72). The statistical analysis of chemical data shows correlations between the main elements and chlorine, positive Pb versus Cl and negative Bi versus Cl, according to the coupled heterovalent substitution scheme: Pb2+ + Cl- ⇌ Bi3+ + S2-. The compositional variation of Cl-bearing galenobismutite can be represented by the general empirical formula Pb1+xBi2-x (S4-x-yClxSey), with 0 4σ(Fo)] and 1.49% for 376 reflections [Fo > 4σ(Fo)], respectively. The structure of Cl-bearing galenobismutite is topologically identical to that of Cl-free galenobismutite. The excess of Pb observed in Cl-bearing galenobismutite is incorporated in the trigonal prismatic M2gb and M3gb sites. No Pb-for-Bi substitution was observed at the octahedral position M1gb. We do not find evidence of a preferential distribution of Cl among the anion positions