The definition and crystal structure of clino-oscarkempffite, Ag15Pb6Sb21Bi18S72

Abstract

Clino-oscarkempffite, Ag 1.27 Pb 0.49 Sb 1.68 Bi 1.54 S 6.01 , idealized as Ag 1.25 Pb 0.5 Sb 1.75 Bi 1.5 S 6 ( Z = 12), from the Colorada vein, Animas mine, Chocaya Province, Department of Potosi, Sur Chichas, Bolivia, is an N = 4 member of the lillianite homologous series, oversubstituted according to the Ag + (Bi, Sb) ↔ 2Pb substitution to 122.8% in comparison to the formula of gustavite, AgPbBi 3 S 6 . Optical properties of clino-oscarkempffite are very similar to those of oscarkempffite and other Bi (Bi–Sb) lillianite homologues. Reflectance values in air vary between 47.5 and 35.2%. Hardness was evaluated to 3–3½, based on VHN 50 range 189–208, mean 200 kg mm −2 . Density was calculated to 6.04 g cm −3 . Minerals associated with clino-oscarkempffite are aramayoite, oscarkempffite, arsenopyrite, ferrokesterite, freibergite, miargyrite, pyrite and quartz. Clino-oscarkempffite is monoclinic, lattice parameters a 39.811(25) A, b 19.280(12) A, c 8.278(5) A, β 96.195(9)°, V 6316.7 A 3 , space group P 2 1 / c . It is a three-fold superstructure of the lillianite aristotype, with a doubled 4 A axis and, uniquely, a tripled ∼13.3 A a parameter. In the crystal structure, configured as a general N = 4 lillianite homologue, the central octahedra of the PbS-like slabs are mixed (Bi,Sb), (Sb,Bi) and Sb sites in alternation along the 8.3 A direction, whereas in the marginal coordination octahedra (Sb,Bi) positions alternate with distorted tetrahedral coordinations of Ag. The bicapped trigonal coordination prisms on meeting planes of two octahedral slabs are occupied by mixed (Pb,Bi) sites. Cation occupation schemes result in blocks with monoclinic overall symmetry which alternate along [1 0 0] with blocks with orthorhombic overall symmetry. The order–disorder implications of this division are discussed in the paper.