COMPARATIVE CRYSTAL-STRUCTURE STUDY OF Ag-FREE LILLIANITE AND GALENOBISMUTITE FROM VULCANO, AEOLIAN ISLANDS, ITALY

Abstract

The crystal structures of natural Ag-free lillianite of fumarolic origin from Vulcano, Aeolian Islands, Italy, Pb 2.88 Bi 2.12 (S 5.67 Se 0.33 ) ∑6.00 , with a 13.567(1), b 20.655(2), c 4.1216(4) A, V 1155.0(2) A 3 , space group Bbmm , Z = 4, and galenobismutite also from Vulcano, Pb 1.00 Bi 2.03 (S 3.87 Se 0.11 ) ∑3.98 , with a 11.815(2), b 14.593(2), c 4.0814(6) A, V 703.7(2) A 3 , space group Pnam , Z = 4, were refined from single-crystal X-ray data. The refinements converge to R = 3.40% [for 745 reflections with F o > 4σ(F o )], and R = 2.96% [for 607 reflections with F o > 4σ(F o )] for Ag-free lillianite and galenobismutite, respectively. In Ag-free lillianite, the trigonal prism M 3 is occupied only by Pb, whereas both the octahedrally coordinated M 1 and M 2 sites are mixed (Pb,Bi) positions. The same octahedra incorporate both the surplus of Bi and vacancies (□) created by the substitution 3Pb 2+ → 2Bi 3+ + □, which allows for the observed deviations from the ideal composition, Pb 3 Bi 2 S 6 . Selenium is preferentially ordered at the six-fold and five-fold coordinated anionic sites, whereas the four-fold coordinated site S3 remains free of Se. The known structural arrangement of galenobismutite was confirmed in this work. The octahedrally coordinated M 1 gb site was found to be a full Bi position. Some evidence of Pb–Bi disorder has been observed at both the M 2 gb (Bi-dominated) and M 3 gb (Pb-dominated) positions. A heterogeneous distribution of Se in the anionic S sites was observed in galenobismutite. Atoms of Se are mainly concentrated at the site S1, and less so in the other anionic sites.