Twinnite, Pb0.8Tl0.1Sb1.3As0.8S4, the OD character and the question of its polytypism

Abstract

The electron-microprobe analyses of the studied twinnite material from Jas Roux, France, show a uniform chemical composition Pb0.80Tl0.05Sb1.31As0.80S4.04. Twinnite is monoclinic; the space group is P21/n. Lattice parameters obtained from single-crystal diffractometer data are a = 7.997(2) Å, b = 19.517(5) Å, c = 8.634(2) Å, and β = equal to 91.061(4)°. The structure of twinnite contains two distinct Pb sites, two Sb positions, two mixed-occupancy (As,Sb) positions and eight anions. The short (Sb,As)—S bonds are arranged into crankshaft chains. Twinnite is a N = 3 member of the sartorite homologous series. It is related to guettardite which has different symmetry and a substantially different Sb : As ratio. These two compounds are configurational polytypes, with unit OD layers having layer group symmetry P21(n)m and P-1 in terms of twinnite lattice.