Natural monoclinic AgPb(Bi(2)Sb)(3)S(6), an Sb-rich gustavite

The crystal structure of the Sb-rich variety of the mineral gustavite, silver lead tris(dibismuth/antimony) hexasulfide, AgPb(Bi(2)Sb)(3)S(6), consists of blocks of diagonal chains of four octahedra, viz. M1a (Bi), M2a (Sb/Bi), M2b (Bi/Sb) and M1b (Ag), separated by Pb atoms in a trigonal prismatic coordination. Two marginal octahedral sites, M1a and M1b, where the gustavite substitution Ag(+) + Bi(3+) = 2Pb(2+) takes place, are formed by Bi and Ag, respectively. Two central octahedra, M2a and M2b, where the Bi(3+) = Sb(3+) substitution takes place, are formed by two mixed Bi/Sb sites with different occupancies of Bi and Sb. The alternating occupation of the M1 site by Bi and Ag atoms (which thus creates two distinct sites M1a and M1b) results in the monoclinic space group P2(1)/c. A statistical distribution of Ag/Bi in the M1 position (one mixed Ag/Bi site) was reported for synthetic gustavite, resulting in the orthorhombic space group Cmcm.