Sejkoraite-(Y), a new member of the zippeite group containing trivalent cations from Jachymov (St. Joachimsthal), Czech Republic: Description and crystal structure refinement

Sejkoraite-(Y), the triclinic (Y1.98Dy0.24)(Sigma 2.22)H-0.34(+) [(UO2)(8)O88O7OH(SO4)(4)](OH)(H-2)(26) is a new member of the zippeite group from the Cervena vein, Jachymov (Street Joachimsthal) ore district, Western Bohemia, Czech Republic. It grows on altered surface of relics of primary minerals: uraninite, chalcopyrite, and tennantite, and is associated with pseudojohannite, rabejacite, uranopilite, zippeite, and gypsum. Sejkoraite-(Y) forms crystalline aggregates consisting of yellow-orange to orange crystals, rarely up to I mm in diameter. The crystals have a strong vitreous luster and a pale yellow-to-yellow streak. The crystals are very brittle with perfect {100}cleavage and uneven fracture. The Mohs hardness is about 2. The mineral is not fluorescent either in short- or long-wavelength UV radiation. Sejkoraite-(Y) is yellow, with no visible pleochroism, biaxial negative with alpha' = 1.62(2), beta' = 1.662(3), gamma' = 1.73(1), 2V(calc) = 79 degrees. The empirical chemical formula (mean of 8 electron microprobe point analyses) was calculated on the basis of 12 (S + U) atoms: (Y1.49Dy0.17Gd0.11Er0.07Yb0.05Sm0.02)(Sigma 1.90)H-0.54(+) [(UO2)(8.19)O7OH SO4)(3.81)](H2O)(26.00). Sejkoraite-(Y) is triclinic, space group P (1) over bar, a=14.0743(6), b=17.4174(7), c = 17.7062(8) angstrom, alpha = 75.933(4), beta = 128.001(5), gamma = 74.419(4)degrees, V = 2777.00(19) angstrom(3), Z = 2, D-calc = 4.04 g/cm(3). The seven strongest reflections in the X-ray powder diffraction pattern are [d(obs) in angstrom (I) (hkl)]: 9.28 (100) (100), 4.64 (39) (200), 3.631(6) ((1) over bar 42), 3.451 (13) ((1) over bar 44), 3.385 (10) ((2) over bar(4) over bar2), 3.292 (9) (044), 3.904(7) (300), 2.984 (10) ((1) over bar(4) over bar2). The crystal structure of sejkoraite-(Y) has been solved by the charge flipping method from single-crystal X-ray diffraction data and refined to R-obs = 0.060 with GOF(obs) = 2.38, based on 6511 observed reflections. The crystal structure consists of uranyl sulfate sheets of zippeite anion topology, which alternate with an interlayer containing Y3+(H2O)(n) polyhedra and uncoordinated H2O groups. Two yttrium atoms are linked to the sheet directly via uranyl oxygen atom, and the remaining one is bonded by hydrogen bonds only. In the Raman and infrared spectrum of sejkoraite-(Y) there are dominating stretching vibrations of SO4 tetrahedra (similar to 1200-1100 cm(-1)), UO22+ stretching vibrations (similar to 900-800 cm(-1)), and O-H stretching (similar to 3500-3200 cm(-1)) and H-O-H bending modes (similar to 1640 cm(-1)). The new mineral is named to honor Jiri Sejkora, a Czech mineralogist of the National Museum in Prague.