Zalesiite, a new mineral of the mixite group, from Zalesi, Rychlebske horyMts., Czech Republic

Zalesiite, CaCu6[(AsO4)(2)(AsO3OH)(OH)(6)] . 3 H2O, is hexagonal, with spac e group P 6(3)/m inferred from isotypic agardite-(Y) and mixite, and with a = 13.571(1) Angstrom, c = 5.880(1) Angstrom, V = 937.8(2) Angstrom(3), Z = 2, D-calc. = 3.50 and D-meas. = 3.49(2) g/cm(3). The strongest lines in XR D pattern are (d in Angstrom/I/I-o (hkl)): 11.64/100 (100), 4.431/41 (111, 210), 2.935/42 (221, 400), 2.693/29 (112, 320, 230), 2.562/30 (140). Zalesi ite forms transparent minute acicular crystals up to 0.1 mm in length and i n 10 mu m in width, which produce crystalline hair-like coatings up to 1 cm (2) on quartz gangue, cross-fiber veinlets and filling of small (up to 5 mm ) cavities of gangue. It has a pale green color, white streaks, vitreous lu stre; Mohs' hardness is about 2-3. No fluorescence at 254 and 366 nm of UV light was observed. Zalesiite is uniaxial positive, with refraction indices omega = 1,688(2) and epsilon = 1,765(2) at Na light (589 nm) and dichroic: pale yellow green (O) - pale green (E) with epsilon > omega. Chemical anal ysis yields an average composition of CaO 5.46, CuO 46.46, Y2O3 1.50, Al2O3 0.26, La2O3 0.10, As2O5 34.27, P2O5 0.37, H2O 11.95, total 100.37 wt.%, le ading to a formula CaCu6[(AsO4) (AsO3OH)(OH)(6)] . 3H(2)O. Zalesiite origin ated by weathering of chalcopyrite and cobalt arsenides in conditions of su pergene zone in-situ. Zalesiite was found in association with chalcopyrite, chrysocolla, clinoclase, conichalcite, erythrite, malachite, metanovacekit e, metatorbernite, novacekite, quartz, tyrolite, uranophane and zeunerite. The name is for the locality Zalesi. Zalesiite is compared with other miner als of the mixite group, and its X-ray powder diffraction data, thermal ana lysis and infrared spectra are given.