Crystal structure, cation ordering, and polytypic character of diaphorite, Pb2Ag3Sb3S8, a PbS-based structure
Armbruster, Thomas; Makovicky, Emil; Berlepsch, Peter; Sejkora, Jiří
European Journal of Mineralogy Volume 15 Number 1 (2003), p. 137 - 146
published: Feb 17, 2003
ArtNo. ESP147051501013, Price: 29.00 €
The crystal structure of diaphorite, Pb2Ag3Sb3S8, from hydrothermal veins at Příbram in central Bohemia (Czech Republic) was solved and refined from X-ray data of a crystal twinned by pseudo-merohedry. Diaphorite is monoclinic, space group P21/c, a = 17.852(4), b = 5.887(1), c = 15.809(3) A and β = 116.17(3)°, Z = 4, ρ = 6.06 g/cm3. The structure can be derived from the one of galena, PbS, by the substitution Ag+ + Sb3+ ↔ 2Pb2+. In addition to eight S, three Sb, two Ag,and one Pb positions, the average structure exhibits also two mixed (Pb2+, Ag+) sites (M1, M2). The different coordination requirements for Pb2+ and Ag+ make a complete miscibility between these ions rather unlikely. The observed structure is a family or superposition structure and four possible, maximally Pb2+, Ag+ ordered structures can be derived with space-group symmetries (1a): P1, (1b) and (2a): P21/n, and (2b): P21. The structures (1a) and (2b) have the same cell dimensions as the family structure whereas structures (1b) and (2a) have a doubled a periodicity compared to the family structure. In addition, each of the above space groups gives rise to two different configurational variants depending on the spatial relation of homoionic occupied M1 and M2 sites.