Vihorlatite, Bi24Se17Te4, a new mineral of the tetradymite group from Vihorlat Mts., Slovakia

Abstract

Vihorlatite, ideally Bi 24 Se 17 Te 4 , is a new mineral species found in the region of volcanic Vihorlat Mountains in eastern Slovakia. It occurs in quartz-opal veinlets or secondary quartzites as anhedral to subhedral grains flattened parallel to (0001) up to 8 mm in diameter or aggregates thereof. The mineral is steel grey in colour and shows a metallic lustre. It displays a perfect cleavage along (0001) and shows deformation lamellae in mechanically bent crystals. In reflected light, it is white with a yellow tint, bireflectance is not perceptible in air, and anisotropy is moderate varying from light yellow-grey to dark grey. Maximum and minimum values of reflectance measured in air for 470, 546, 589 and 650 nm are ( R max / R min in %): 52.9/49.9, 54.5/50.6, 54.6/51, 54.7/51.2. Vickers micro-hardness (VHN 50 in kg/mm 2 ) varies in the range 49.2 – 91.7 with mean value of 65.9. Average chemical composition (in wt. %) Bi 71.5, Se 21.4, Te 8.1, S 0.8, Au 0.01, Ag 0.01, Sb 0.04, total 101.86, results in an empirical formula Bi 21.9 Se 17.4 Te 4.1 S 1.6 . The mineral is trigonal, with space group P 3 m 1. The unit-cell dimensions refined from powder data are a = 4.2797(9) A and c = 87.01(2) A with c / a = 20.332(6) and V = 1380.2(6) A 3 . For Z = 1, the calculated density is D x = 7.850(3) g/cm 3 , measured density D m = 8.0(2) g/cm 3 . The five strongest diffraction lines are [ d (A), I / I 0 , ( hkl) ]: 4.55, 55.4, (0.0.19); 3.116, 100, (1.0.15); 2.282, 75.5, (0.1.30); 1.934, 42.8, (1.1.19) (0.0.45); 1.767, 31.5, (0.2.15). The crystal structure of vihorlatite is derived from that of Bi 8 Se 7 archetype. Vihorlatite belongs to the tetradymite group.