Crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4; comparison with gustavite
Paout, Richard; Duek, Michal
European Journal of Mineralogy Volume 22 Number 5 (2010), p. 741 - 750
published: Oct 1, 2010
ArtNo. ESP147052205012, Price: 29.00 €
The crystal structure of natural orthorhombic Ag0.71Pb1.52Bi1.32Sb1.45S6, a lillianite homologue with N = 4, from hydrothermal veins at Kutná Hora in central Bohemia, Czech Republic, has been refined from single-crystal X-ray data to final Rw = 5.08 %. The symmetry is orthorhombic, space group Cmcm, with a = 4.2539(8), b = 13.3094(8), c = 19.625(1) Å, V = 1111.1(2) Å3, Z = 4. On the basis of electron-microprobe analysis, the formula is Ag0.69Pb1.56(Bi1.32Sb1.37)Σ=2.69(S6.04Se0.01)Σ6.05 with Nchem = 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 M1, M2 and M3 and four sulphur positions. The crystal structure consists of blocks of diagonal chains of four octahedra M1 (Bi/Ag/Sb) - M2 (Sb/Pb/Bi) - M2 - M1 separated by Pb atoms in bicapped trigonal prismatic coordination of the site M3. The marginal octahedron of the chain of four octahedra is formed by one mixed site M1 containing 52 % Bi, 35.6 % Ag and 12.4 % Sb while the central M2 octahedron is a mixed site containing 60.1 % Sb, 25.9 % Pb and 14 % Bi. The mixed population of the M1 site results in orthorhombic symmetry, space group Cmcm, with a 4 Å a-repeat while in natural gustavite, the alternating occupation of the M1 site by Ag and Bi atoms (which are thus two sites M1a and M1b) reduces the symmetry to monoclinic, space group P21/c, with an 8 Å repeat. The structure is similar to that of Ag-free lillianite.