Versiliaite (Fe12Sb12O32S2, Pbam, a = 8.492, b = 8.326, c = 11.938Å) and apuanite (Fe20Sb16O48S4, P42/mbc, a = 8.372, c = 17.974Å) are closely related to schafarzikite (Fe4Sb8O16, P42/mbc, a = 8.59, c = 5.91Å), from which they are derived by sulfide insertion, substitution of IVFe3+ for IIISb3+, and oxidation of VIFe2+ to VIFe3+.
Transmission electron microscopy shows in apuanite the presence of wide regular regions, scattered faults and regions with a high density of faults. The regular sequence is interrupted by insertion of a phase with 12Å periodicity (versiliaite). Usually, (001) is the boundary between the two phases, with the guest phase lamellae typically up to twelve c periods thick. Less frequently, phase boundaries lying in planes normal to (001) have also been recorded, e.g., a (001) lamella, 36Å thick, is crossed by a (110) boundary plane normal to the lamella plane; whereas a 2 × 18Å thick apuanite region occurs on one side of the boundary, a 3 × 12Å thick versiliaite region is present on the other side. The crystals of versiliaite are characterized by large grain domain structure. Wide domains of versiliaite are mixed with wide domains of schafarzikite. The results of this study, which confirm the presence of sulfur-poor phases inside both apuanite and versiliaite, lead to an understanding of the nonstoichiometry in this mineral group in terms of microstructures.