PLUMBIAN BAKSANITE FROM THE TYRNYAUZ W Mo DEPOSIT, BAKSAN RIVER VALLEY, NORTHERN CAUCASUS, RUSSIAN FEDERATION

Abstract

A sample of Pb-rich baksanite was recovered in a magnetite‐andradite skarn from the Tyrnyauz W‐Mo deposit, in the northern Caucasus, Baksan River valley, Kabardino‐Balkaria Republic, Russian Federation. The rock sample forms part of the historical mineralogical collection of the Natural History Museum of the University of Florence. Associated minerals are bismuthinite, tetradymite, joseite-A and ingodite; gangue minerals are calcite and andradite. Electron-microprobe analyses gave the chemical formula (Bi4.94Pb0.96)5.90(Te2.03S3.06). The mineral occurs as anhedral grains up to 0.5 mm across. Plumbian baksanite is dark steel-grey, shows a black streak, and has a perfect basal cleavage. Its calculated density (for Z = 3) is 7.44 g/cm 3 for the empirical formula and 7.51 g/cm 3 inferred for (Bi5Pb)(Te2S3). The mineral is greyish white in reflected light, weakly bireflectant, nonpleochroic, faintly anisotropic in yellowish gray tints. Reflectivity values (Rmin and Rmax in %) are 47.2, 50.3 (471.1 nm), 47.9, 50.7 (548.3 nm), 48.2, 51.3 (586.6 nm), and 48.5, 51.4 (652.3 nm), respectively. Indexing of the X-ray powder pattern indicates trigonal symmetry, space group P¯ 3m1, with a 4.251(1), c 64.19(3) A, V 1063.4(6) A 3 . The theoretical structural model proposed for the original baksanite is confirmed, and a comparison of intensities in the X-ray-diffraction patterns of the two samples strongly indicates that the Bi‐Pb and Te‐S atoms are disordered at the same positions. Therefore, even if a nearly perfect stoichiometry among the elements is observed, this material is not a new species but merely a plumbian variety of baksanite.