Crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4; comparison with gustavite

Abstract

The crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4, from hydrothermal veins at Kutna Hora in central Bohemia, Czech Republic, has been refined from single-crystal X-ray data to final R w = 5.08 %. The symmetry is orthorhombic, space group Cmcm , with a = 4.2539(8), b = 13.3094(8), c = 19.625(1) A, V = 1111.1(2) A3, Z = 4. On the basis of electron-microprobe analysis, the formula is Ag0.69Pb1.56(Bi1.32Sb1.37)∑=2.69(S6.04Se0.01)∑6.05 with N chem = 3.94, Bi/(Bi + Sb) = 0.49 and a gustavite substitution percentage L% = 70.5, which characterize the phase as a natural orthorhombic Sb–Bi 4,4L member of the lillianite homologous series. The structure contains seven independent positions: three metal sites M 1, M 2 and M 3 and four sulphur positions. The crystal structure consists of blocks of diagonal chains of four octahedra M 1 (Bi/Ag/Sb) – M 2 (Sb/Pb/Bi) – M 2 – M 1 separated by Pb atoms in bicapped trigonal prismatic coordination of the site M 3. The marginal octahedron of the chain of four octahedra is formed by one mixed site M 1 containing 52 % Bi, 35.6 % Ag and 12.4 % Sb while the central M 2 octahedron is a mixed site containing 60.1 % Sb, 25.9 % Pb and 14 % Bi. The mixed population of the M 1 site results in orthorhombic symmetry, space group Cmcm , with a 4 A a -repeat while in natural gustavite, the alternating occupation of the M 1 site by Ag and Bi atoms (which are thus two sites M 1a and M 1b) reduces the symmetry to monoclinic, space group P 21/ c , with an 8 Arepeat. The structure is similar to that of Ag-free lillianite.