The first Italian occurrence of the rare lead-antimony sulfosalt robinsonite, Pb4Sb6S13, is reported from the Pitone marble quarry, Apuan Alps, Tuscany. Robinsonite is monoclinic, space group I2/m, a = 23.637(7) Å, b = 3.969(1), c = 24.406(7), beta = 93.73(3)°. A morphological study, based on 45 indexed faces, led to the following parametric ratios: a:b:c = 5.821:1:6.018, with a beta value of 93.70°. A microprobe analysis indicated for robinsonite a chemical composition very close to the ideal stoichiometry, with only minor substitution of Sb by As, resulting in the formula Pb4Sb5.69As0.31S13. The X-ray single crystal structural study of robinsonite (R = 0.065, Rw = 0.061 for 1753 reflections) confirmed the isostructurality of robinsonite with its synthetic analogous. Pb and Sb are fundamentally coordinated by 5 S in a square pyramidal configuration; pyramids share faces to give rise to cross-linked ribbons with different width (either 4 or 12 pyramids) which develop along b.
|Autori:||FRANZINI M; ORLANDI P; PASERO M|
|Titolo:||Morphological, chemical and structural study of robinsonite (Pb4Sb6S13) from Alpi Apuane, Italy|
|Anno del prodotto:||1992|
|Appare nelle tipologie:||1.1 Articolo in rivista|