SLAVKOVITE, Cu13(AsO4)6(AsO3OH)4•23H2O, A NEW MINERAL SPECIES FROM HORNI SLAVKOV AND JACHYMOV, CZECH REPUBLIC: DESCRIPTION AND CRYSTAL-STRUCTURE DETERMINATION

Slavkovite, Cu-13(AsO4)(6)(AsO3OH)(4)center dot 23H(2)O, is a newly discovered supergene mineral from the Geschieber vein, Svornost mine at the Jachymov (St. Joachimsthal) ore district, Czech Republic, commonly associated with lavendulan, geminite, lindackerite and ondrusite. It forms coatings of pale green rosettes up to 1 mm across or individual spherical aggregates up to 5 mm across. Individual acicular to lath-like crystals are up to 1 mm long and 0.05 mm thick, and are colorless with a greenish tint. Slavkovite is pale green, translucent (aggregates) to colorless with a greenish tint, transparent (crystals). It has a white streak and a vitreous luster, and does not fluoresce under both short-and long-wave ultraviolet light. The cleavage on {011} is perfect, and on {010}, it is good. The Mohs hardness is similar to 3.5-4; slavkovite is very brittle, with an irregular fracture. Its measured density, 3.05(1) g/cm(3), is identical to the calculated one. Slavkovite is biaxial positive; the indices of refraction are alpha ' 1.591(2), beta ' 1.620(2), gamma ' 1.701(2), and the 2V (calc.) is approximately 64 degrees. It is moderately pleochroic (X light gray to colorless, Y very light greenish gray, Z light green). Slavkovite is triclinic, space group P (1) over bar, a 6.408(3), b 14.491(5), c 16.505(8) angstrom, alpha 102.87(3), beta 101.32(5), gamma 97.13(3)degrees, V 1442(1) angstrom(3), Z = 1, a: b: c = 0.4422:1:1.1390. The strongest eight lines in the X-ray powder-diffraction pattern [d in angstrom(I)(hkl)] are as follows: 15.70(3)(001), 11.98(100)(0 (1) over bar1), 6.992(3)(0 (2) over bar1, 020), 5.992(6)(0 (2) over bar2), 3.448(5) (040), 2.967(5)(0 (3) over bar5), 2.4069(4)(1 (5) over bar4), 2.4002(4)(115, (1) over bar 35, 0 (4) over bar6, 0 (6) over bar2). The chemical analyses made with an electron microprobe yielded FeO 0.12, CuO 39.93, Al2O3 0.13, As2O5 44.71, H2O 17.31, total 102.20 wt.%. The resulting empirical formula on the basis of 63(O, OH, H2O) anions is (Cu12.96Al0.07Fe0.04)Sigma(13.07)(AsO4)(6.11) (AsO3OH)(3.93)center dot 22.83H(2)O. The ideal end-member formula, Cu-13(AsO4)(6)(AsO3OH)(4)center dot 23 H2O, requires CuO 39.26, As2O5 43.36, H2O 17.10, total 100.00 wt. %. The crystal structure of slavkovite has been solved by direct method and refined to a final R-obs of 4.37% on the basis of 6613 observed reflections collected on a single-crystal diffractometer with MoK alpha X-radiation. The crystal structure is based upon sheets consisting of copper polyhedra linked by arsenate and hydrogen arsenate tetrahedra. The sheets are linked by bridging Cu6-Phi polyhedra. In the asymmetric unit of the slavkovite unit-cell, seven symmetrically independent Cu2+ atoms, five As5+ atoms and 34 O atoms were found. Two of the oxygen atoms belong to OH-groups and fourteen to H2O molecules. Slavkovite possesses a unique crystal structure that has not been found in any mineral or synthetic compound before. It was named after its first occurrence in the Krasno Sn-W ore district, near Horni Slavkov, Czech Republic. Selected data for slavkovite from this locality are also given.