RARE SULFOSALTS FROM VULCANO, AEOLIAN ISLANDS, ITALY. IV. LILLIANITE
2001; Mineralogical Association of Canada; Volume: 39; Issue: 5 Linguagem: Inglês
10.2113/gscanmin.39.5.1383
ISSN1499-1276
AutoresY. S. Borodaev, Anna Garavelli, Carlo Garbarino, Silvana Maria Grillo, N. N. Mozgova, T. Yu. Uspenskaya, Filippο Vurro,
Tópico(s)Solid-state spectroscopy and crystallography
ResumoExcellent samples of lillianite, a rare Pb–Bi sulfosalt, have been found around a high-temperature fumarole at the La Fossa crater, Vulcano Island, Italy. The mineral is associated with cannizzarite and, subordinately, with galenobismutite and rarer sulfosalts, as well as sphalerite and selenian galena. Lillianite occurs as laths and fibers up to 100–130 � m in length, and about 20–30 and 1 � m in width, respectively. Electron-microprobe analyses show that the lillianite from Vulcano has a composition close to the ideal Pb 3Bi2S6. Particularly interesting is the absence of Ag and Cu, which makes the lillianite from Vulcano unique in its purity, with only minor incorporation of Cd. Like in other sulfosalts from Vulcano, significant Se concentrations are in variably present (1.38–2.22 wt. %). Traces of chlorine were found in some crystals due to the high Cl activity in the fumaroles, an d are noted for the first time. The general empirical formula, Pb 3–xBi2+2x/3(S6–ySey)� 6, reflects the narrow composition field for the lillianite from Vulcano. The X-ray powder-diffraction data, as well as microhardness and reflectance measurements, are given fo r distinct crystals with a different Pb/Bi value. The unit-cell constants are a 13.576(9), b 20.606(8), c 4.119(2) A for the crystal with Pb/Bi ≈ 1.50, and a 13.56(1), b 20.57(1), c 4.115(2) A for the one with Pb/Bi ≈ 1.41. In both films, h + l = 2n reflections correspond to the space group Bbmm. The lower value of microhardness of the Bi-rich sample may reflect the less well-ordered structure and the effect of the heterovalent substitution 3Pb 2+ → 2Bi 3+ + . A comparison between the lillianite from Vulcano and the synthetic homologous Phase III is also given.
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