THE CRYSTAL STRUCTURE OF SKIPPENITE, BI2SE2TE, FROM THE KOCHKAR DEPOSIT, SOUTHERN URALS, RUSSIAN FEDERATION

Abstract

Skippenite, Bi2Se2Te, was found in a sample from the Kochkar deposit, southern Urals, Russian Federation. The sample forms part of the mineralogical collection of the Natural History Museum of the University of Florence. Electron-microprobe analyses gave the chemical formula (Bi2.06Cu0.05Sb0.03)� 2.14Se2.00(Te0.89S0.11). The mineral occurs as anhedral grains up to 0.3 mm across. Skippenite is steel-grey in color, shows a black streak, and has a perfect basal cleavage. In reflected light, the mineral is greyish white, weakly bireflectant, nonpleochroic, moderately anisotropic in yellowish tints. Reflectivity values (Rmin and Rmax, in %) are 47.8, 48.1 (471.1 nm), 49.0, 50.2 (548.3 nm), 49.1, 50.3 (586.6 nm), and 49.3, 50.7 (652.3 nm), respectively. Skippenite is trigonal, space group R¯ 3m, with the following unit-cell parameters: a 4.183(1), c 29.137(3) Å, V 441.5(2) Å3 and Z = 3. The crystal structure has been refined to R = 2.93%. It consists of hexagonal close-packed sheets of Bi, Se and Te, respectively. The unit-cell of skippenite contains 15 layers, and the stacking sequence is: A'cBAc - B'aCBa - C'bACb, where A', B', C' = Te and S, A, B, C = Se, and a, b, c, = Bi.