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, joséite-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/cm3 for the empirical formula and 7.51 g/cm3 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) Å, V 1063.4(6) Å3. 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.
