Lead-Antimony Sulfosalts from Tuscany (Italy). XX. Members of the Jordanite-Geocronite Series from the Pollone Mine, Valdicastello Carducci: Occurrence and Crystal Structures

A crystal-chemical study of historical specimens as well as new ones belonging to the jordanite-geocronite series from the Pollone baryte + pyrite +/- (Pb-Zn-Ag) ore deposit (Valdicastello Carducci, Apuan Alps, Tuscany, Italy) has been performed. These crystals were collected in quartz extension veins embedded in three different occurrences: (i) baryte + pyrite orebodies; (ii) schist layers interbedded between baryte + pyrite orebodies; and (iii) schists at the contact with pyrite-poor baryte orebodies. Electron-microprobe data indicated the occurrence of three distinct groups of compositions within the sample suite. These correspond to As-bearing geocronite, Sb-rich jordanite, and Sb-bearing jordanite, with mean compositions Pb14Sb3.8As2.2S23, Pb14Sb2.9As3.1S23, and Pb14Sb2.6As3.4S23, respectively. Crystals representative of these different compositions have been investigated through single-crystal X-Ray diffraction studies and their crystal structures have been solved to R-1 = 0.078, 0.069, and 0.033, respectively. The unit-cell volume decreases passing through As-bearing geocronite (V = 2149.5(3) angstrom(3)) to Sb-bearing jordanite (V = 2132.3(3) angstrom(3)). The As-to-Sb substitution takes place preferentially at the Sb4 site; through the increasing of the Sb content, Sb can substitute As also at the As6 site. According to the structural study of the ore deposit, formation of jordanite-geocronite is subordinated to a late Alpine deformative D-2 stage, which permitted in situ remobilization of preexisting sulfide ore in small quartz extension veins. Such a local recrystallization would explain the variability of the As/(As + Sb) ratio of the members of the jordanite series, reflecting the heterogeneity of the orebody.