Telluronevskite, Bi3TeSe2, a new mineral

The new mineral telluronevskite, ideally Bi3TeSe2, has been found as irregu lar grains disseminated in quartzite from the volcanic Vihorlat Mts near Ko sice in eastern Slovakia, Slovak Republic. The mineral forms massive aggreg ates up to 2 mm in diameter and/or individual disseminated tabular crystals flattened along (0001). Telluronevskite is megascopically steel grey in co lour, with a metallic lustre and black streak. In reflected tight, it is wh ite with a yellow tint, very weak pleochroism and moderate anisotropy. Maxi mum and minimum values of reflectance measured in air for 470, 546, 589 and 650 nm are (R-max/R-min in %): 48.5/46.6; 51.1/48.5; 51.9/49.5; 52.8/50.5. Vickers micro-hardness (VHN10 in kp.mm(-2)) varied in the range 63-137 wit h mean value of 100. Microprobe analyses yield the empirical formula (Bi2.9 2Pb0.02)Te-1.01(Se1.73S0.32) It is trigonal with space group P (3) over bar ml. The unit-cell dimensions refined from X-ray powder microdiffraction da ta are a = 4.264(6) Angstrom and c = 23.25(3) Angstrom with c : a = 5.453 : I. For Z = 2 the calculated density is D-x = 8.08(2) g.cm(-3); measured de nsity D-m = 8.1(2) g.cm(-3). The five strongest lines in the diffraction pa ttern are (d (Angstrom), I/I-0, hkl): 4.66, 19, 005; 3.12, 100, 104; 2.28, 33, 108; 2.13, 36, 110, 109; 1.355, 18, 1.0.16, 214. Constrained structure refinement from combined powder and precession data revealed ordering of Te and Se atoms within the telluronevskite crystal structure. Telluronevskite belongs to the tsumoite subgroup of the tetradymite group.