Vurroite, ideally Pb 20 Sn 2 (Bi,As) 22 S 54 Cl 6, is a new mineral species from the modern Pb-Bi epithermal deposit of La Fossa crater, on Vulcano Island, Italy. It occurs as a volcanic encrustation deposited around high-temperature fumaroles (400-600°C) located both on the rim and on the inner slope of the active crater. The mineral consists of slender, needle-shaped crystals up to 0.4 mm long and 0.01 mm across, which generally form fibrous aggregates. Associated minerals are bismuthinite, lillianite, kirkiite, heyrovskýite, galena, and other less-well-characterized sulfochlorides of Pb(Bi), some of which contain traces of Tl, Sn or As. The color of vurroite is silver-gray, with a metallic luster. In reflected plane-polarized light, it is white, without distinct bireflectance or pleochroism. Anisotropism is weak, without color effects. The measured values of reflectance in air for a single grain are 34.25 (470 nm), 32.95 (546 nm), 32.60 (589 nm), 31.05% (650 nm); values for 400-800 nm are tabulated. Electronmicroprobe analyses yield the following ranges of concentrations (wt.%):
The crystal structures of two very close, but distinct complex minerals of the lead sulfosalt group have been solved: sterryite, Cu(Ag,Cu)(3)Pb(19)(Sb,As)(22)(As-As)S(56), and parasterryite, Ag(4)Pb(20)(Sb,As)(24)S(58). They are analyzed and compared according to modular analysis. The fundamental building block is a complex column centred on a Pb(6)S(12) triangular prismatic core, with two additional long and short arms. The main chemical and topological differences relate to the short arm, which induces a relative a/4 shift (~2 Å along the elongation parameter) of the constitutive rod layers, as illustrated by distinct cell settings within the same space group (P2(1)/n and P2(1)/c, respectively). Selection of the shortest (i.e. strongest) (Sb,As)-S bonds permitted to enhance the polymeric organization of (Sb,As) atoms with triangular pyramidal coordination. These two quasi-homeotypic structures are expanded derivatives of owyheeite, Ag(3)Pb(10)Sb(11)S(28). The hierarchy of organization levels from zero- to three-dimensional entities is subordinated to building operators, which appear as the driving force for the construction of such complex structures. Minor cations (Ag, Cu) or the As-As pair in sterryite secure the final locking, which favours the formation of one or the other compound.
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