ONDRUSITE, CaCu4(AsO4)2(AsO3OH)2•10H2O, A NEW MINERAL SPECIES FROM THE JACHYMOV ORE DISTRICT, CZECH REPUBLIC: DESCRIPTION AND CRYSTAL-STRUCTURE DETERMINATION

Ondrusite, CaCu4(AsO4)(2)(AsO3OH)(2)center dot 10H(2)O, is a new supergene mineral species from the Geister vein (Rovnost mine) and the Geschieber vein (Svornost mine), at the Jachymov (St. Joachimsthal) ore district, Czech Republic. It is commonly associated with lindackerite, geminite, lavendulan, slavkovite, strashimirite, olivenite, picropharmacolite and kottigite. It forms white, whitish green, grey-green or apple-green crystalline crusts or aggregates composed of fine platy or lath-shaped translucent crystals up to 2 mm in size, on fissures of partly weathered mineralized quartz veins. It has a white to greyish white streak with a greenish tint, and a vitreous luster; it does not fluoresce under both short- and long-wave ultraviolet light. Cleavage on {001} is perfect, the Mohs hardness is similar to 2-3, and the mineral is very brittle with an irregular fracture. The measured density, 3.26 g/cm(3) (affected by a lavendulan admixture), is slightly higher than a calculated one of 3.12 g/cm(3). Ondrusite is biaxial positive; the indices of refraction are alpha' 1.640(2) and gamma' 1.708(2), and it is strongly pleochroic, from colorless to light yellowish green. Ondrusite is triclinic, space group P (1) over bar, a 6.432(1), b 7.986(1), c 10.827(1) angstrom, alpha 85.75(1), beta 81.25(1), gamma 85.04(1)degrees, V 546.6(1) angstrom(3), Z = 1, a:b:c = 0.8054:1:1.3557. The strongest eight lines in the X-ray powder-diffraction pattern [d in angstrom(I)(hkl)] are: 10.671(100)(001), 3.970(10)(020), 3.648(11)(02 (1) over bar), 3.560(18)(003), 3.286(10)(022), 3.173(13)(01 (3) over bar, 200, 201), 2.922(10)(20 (1) over bar, 202) and 2.736(10) (023). The chemical analyses by electron microprobe and the thermal analysis for H2O yielded MgO 0.42, CaO 5.27, NiO 0.03, CoO 0.08. CuO 29.90, PbO 0.16, P2O5 0.33, As2O5 44.92, H2O 19.40, for a total of 100.51 wt.%. The resulting empirical formula, calculated on the basis of 26 (O, OH, H2O) anions, is (Ca0.96Co0.01Pb0.01)(Sigma 0.98) (Cu3.84Mg0.11)(Sigma 3.95) [(AsO4)(1.73)(PO4)(0.05)](Sigma 1.78) (AsO3OH)(2.26)center dot 9.86H(2)O. The ideal end-member formula, CaCu4(AsO4)(2)(AsO3OH)(2)center dot 10H(2)O, requires CaO 5.43, CuO 30.83, As2O5 44.53, H2O 19.20, total 100.00 wt.%. The crystal structure of ondrusite was solved by direct method and refined to a R-1 index of 9.80% based on all 2888 reflections collected on a single-crystal diffractometer with MoK alpha X-radiation. The crystal structure consists of CuO6-AsO4-AsO3OH chains two polyhedra thick that form sheets connected through the bridging CaO6 octahedra. In the sheet-to-sheet bonding, CaO6 octahedra and AsO4 groups are involved, whereas the AsO3OH groups bond the CuO6 polyhedra only. One other H2O group is located between the sheets. Ondrusite represents the Ca-Cu-dominant member of the ondrusite - geigerite - chudobaite subgroup of the lindackerite group.