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

Roman Skála^1,*, Petr Ondru², Frantiek Veselovk², Zdenk Táborsk² and Rudolf ua³

¹ Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 269, 16500 Praha 6, Czech Republic
² Czech Geological Survey, Klárov 3/131, 11821 Praha 1, Czech Republic
³ Museum of Eastern Slovakia, Hrniarská 7, 04001 Koice, Slovak Republic

^* Corresponding author, e-mail: skala{at}gli.cas.cz

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 (R_max/R_min 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 Bi_21.9Se_17.4Te_4.1S_1.6. 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) ų. For Z = 1, the calculated density is D_x = 7.850(3) g/cm³, measured density D_m = 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.

Key-words: vihorlatite, new mineral, tetradymite group, crystal structure, Vihorlat Mts., Slovakia.

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				N. J. Cook, C. L. Ciobanu, T. Wagner, and C. J. Stanley MINERALS OF THE SYSTEM Bi Te Se S RELATED TO THE TETRADYMITE ARCHETYPE: REVIEW OF CLASSIFICATION AND COMPOSITIONAL VARIATION Can Mineral, August 1, 2007; 45(4): 665 - 708. [Abstract] [Full Text] [PDF]