Ondrušite, CaCu4(AsO4)2(AsO3OH)2·10H2O, is a new supergene mineral species from the Geister vein (Rovnost mine) and the Geschieber vein (Svornost mine), at the Jáchymov (St. Joachimsthal) ore district, Czech Republic. It is commonly associated with lindackerite, geminite, lavendulan, slavkovite, strashimirite, olivenite, picropharmacolite and köttigite. 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 ~2–3, and the mineral is very brittle with an irregular fracture. The measured density, 3.26 g/cm3 (affected by a lavendulan admixture), is slightly higher than a calculated one of 3.12 g/cm3. Ondrušite is biaxial positive; the indices of refraction are α′ 1.640(2) and γ′ 1.708(2), and it is strongly pleochroic, from colorless to light yellowish green. Ondrušite is triclinic, space group P1̅, a 6.432(1), b 7.986(1), c 10.827(1) Å, α 85.75(1), β 81.25(1), γ 85.04(1)°, V 546.6(1) Å3, Z = 1, a:b:c = 0.8054:1:1.3557. The strongest eight lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 10.671(100)(001), 3.970(10)(020), 3.648(11)(021̅), 3.560(18)(003), 3.286(10)(022), 3.173(13)(013̅, 200, 201), 2.922(10)(201̅, 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)∑0.98 (Cu3.84Mg0.11)∑3.95 [(AsO4)1.73(PO4)0.05]∑1.78 (AsO3OH)2.26·9.86H2O. The ideal end-member formula, CaCu4(AsO4)2(AsO3OH)2·10H2O, requires CaO 5.43, CuO 30.83, As2O5 44.53, H2O 19.20, total 100.00 wt.%. The crystal structure of ondrušite was solved by direct method and refined to a R1 index of 9.80% based on all 2888 reflections collected on a single-crystal diffractometer with MoKα 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. Ondrušite represents the Ca–Cu-dominant member of the ondrušite – geigerite – chudobaite subgroup of the lindackerite group.
