Vihorlatite, ideally Bi24Se17Te4, 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 (Rmax/Rmin in %): 52.9/49.9, 54.5/50.6, 54.6/51, 54.7/51.2. Vickers micro-hardness (VHN50 in kg/mm2) 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 Bi21.9Se17.4Te4.1S1.6. The mineral is trigonal, with space group P3m1. The unit-cell dimensions refined from powder data are a = 4.2797(9) Å and c = 87.01(2) Å with c/a = 20.332(6) and V = 1380.2(6) Å3. For Z = 1, the calculated density is Dx = 7.850(3) g/cm3, measured density Dm = 8.0(2) g/cm3. The five strongest diffraction lines are [d (Å), I/I0, (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 Bi8Se7 archetype. Vihorlatite belongs to the tetradymite group.

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