THE CRYSTAL STRUCTURE OF WATKINSONITE, Cu2PbBi4Se8, FROM THE ZALESI URANIUM DEPOSIT, CZECH REPUBLIC

Abstract

The crystal structure of sulfur-free watkinsonite, ideally Cu2PbBi4Se8, monoclinic, a 12.952(4), b 4.152(1), c 15.155(5) A, β 108.93(1)°, space group P 21/ m , Z = 2, part of the selenide mineralization of the uranium deposit of Zalesi, Czech Republic, has been solved by direct methods and refined to R 1 = 4.61% on the basis of 1287 unique reflections [ F o > 4σ( F o )] collected on a Bruker P3 diffractometer with a CCD detector and Mo K α radiation. The crystal structure contains one Pb site, four Bi sites, two Cu sites and eight Se sites. Partial replacement of one Cu (in a linearly coordinated site) by Ag is observed. The crystal structure consists of (100)PbS slabs four atomic planes thick of a galena-like structure alternating with single layers of hexagonally packed selenium. The Pb atoms form asymmetric bicapped coordination prisms embedded in the surfaces of the galena-like pseudotetragonal slabs; atoms Bi1–3 occur as irregular coordination octahedra in the interior and at the surfaces of this slab. In addition, Bi4 forms a coordination prism similar to that of lead. Tetrahedrally coordinated copper Cu2 caps triangular voids of the hexagonal layer, whereas linearly coordinated Cu1 (with minor Ag), with the opposing Cu–Se bonds equal to 2.36 and 2.40 A, respectively, forms foreshortened coordination octahedra, with four additional Cu–Se distances in the range 3.05–3.17 A. The crystal structure of watkinsonite is closely related to that of berryite, Cu3Ag2Pb3Bi7S16, in which the foreshortened coordination octahedra are occupied by Ag, and Cu resides in the hexagonal layer.