Cl-bearing galenobismutite (Cl ranging from 0.12 to 1.45 wt.%), discovered among high-temperature fumarole incrustations at "La Fossa" crater (Vulcano Island, Italy), has been chemically and structurally investigated. The mineral is commonly associated with bismuthinite, cannizzarite, lillianite, heyrovskýite and galena; rarely with kirkiite, vurroite and some other less characterized Pb(Bi)-sulfochlorides. Electron-microprobe investigations show that Cl-bearing galenobismutite from Vulcano 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 vs Cl and negative Bi vs Cl, according to the coupled heterovalent substitution scheme: Pb²⁺ + Cl^- « Bi³⁺ + S^2-. The compositional variation of Cl-bearing galenobismutite can be represented by the general empirical formula Pb_1+xBi_2-x(S_4-x-yCl_xSe_y) with 0 < x < 0.3 and 0 < y < 0.2, indicating the extent of departure from the ideal composition PbBi₂S₄ toward an increase of the Pb content and a decrease of Bi. The phase IV, synthetic analogue of galenobismutite in the system Pb-Bi-S, has the compositional field extending from ideal composition toward decreasing Pb (Pb/Bi atomic ratio 0.50 - 0.39). It reflects another type of substitution: 3Pb²⁺ à 2Bi³⁺ + ¿. X-ray single crystal investigations were performed on two selected crystals of Cl-bearing galenobismutite from Vulcano. The composition of galenobismutite controls the a and b unit-cell parameters, which increase with the increase of the Cl content. The structural refinements converge to the agreement R values of 3.10 % for 840 reflections [F_o> 4s(Fo)] and 1.49 % for 376 reflections [F_o> 4s(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 M2_gb and M3_gb sites. No Pb for Bi substitution was observed for the octahedral position M1_gb. No evidence of a preferential distribution of Cl among the anion positions was observed.

Yuri S. Borodaev

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