RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS, ITALY. VII. Cl-BEARING GALENOBISMUTITE

Abstract

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 <em>vs</em> Cl and negative Bi <em>vs</em> 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 &lt; x &lt; 0.3 and 0 &lt; y &lt; 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 <em>a</em> and <em>b</em> unit-cell parameters, which increase with the increase of the Cl content. The structural refinements converge to the agreement <em>R</em> values of 3.10 % for 840 reflections [F_o&gt; 4s(Fo)] and 1.49 % for 376 reflections [F_o&gt; 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.