Rare sulfosalts from Vulcano, Aeolian Islands, Italy. IV. Lillianite

Excellent samples of lillianite, a rare Pb-Bi sulfosalt, have been found ar ound a high-temperature fumarole at the La Fossa crater. Vulcano Island, It aly, The mineral is associated with cannizzarite and, subordinately, with g alenobismutite and rarer sulfosalts, as well as sphalerite and selenian gal ena. Lillianite occurs as laths and fibers up to 100-130 mum in length, and about 20-30 and 1 mum in width, respectively. Electron-microprobe analyses show that the lillianite from Vulcano has a composition close to the ideal Pb3Bi2S6. Particularly interesting is the absence of Ag and Cu, which make s the lillianite from Vulcano unique in its purity, with only minor incorpo ration of Cd. Like in other sulfosalts from Vulcano, significant Se concent rations are invariably present (1.38-2.22 wt. %). Traces of chlorine were f ound in some crystals due to the high Cl activity in the fumaroles, and are noted for the first time. The general empirical formula, Pb3-xBi2+2x/3(S(6 -y)Sey)(Sigma6), reflects the narrow composition field for the lillianite f rom Vulcano. The X-ray powder-diffraction data, as well as microhardness an d reflectance measurements, are given for distinct crystals with a differen t Pb/Bi value. The unit-cell constants are a 13.576(9), b 20.606(8), c 4.11 9(2) Angstrom for the crystal with Pb/Bi approximate to 1.50, and a 13.56(1 ), b 20.57(1), c 4.115(2) Angstrom for the one with Pb/Bi approximate to 1. 41. In both films, h + l = 2n reflections correspond to the space group Bbm m. The lower value of microhardness of the Bi-rich sample may reflect the l ess well-ordered structure and the effect of the heterovalent substitution 3Pb(2+) --> 2Bi(3+) + rectangle. A comparison between the lillianite from V ulcano arid the synthetic homologous Phase III is also given.