• journal_title：American Mineralogist
• Contributor：Jakub Plášil ; Michal Dušek ; Milan Novák ; Jiří Čejka ; Ivana Císařová ; Radek Škoda
• Publisher：Mineralogical Society of America
• Date：2011-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2011.3713
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：96
• issue：7
• firstpage：983
• section：Articles

Sejkoraite-(Y), the triclinic (Y_1.98Dy_0.24)_∑2.22H_0.34⁺ [(UO₂)₈O₈₈O₇OH(SO₄)₄](OH)(H₂O)₂₆, is a new member of the zippeite group from the Červená vein, Jáchymov (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 1 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 α′ = 1.62(2), β′ = 1.662(3), γ′ = 1.73(1), 2V_calc = 79°. The empirical chemical formula (mean of 8 electron microprobe point analyses) was calculated on the basis of 12 (S + U) atoms: (Y_1.49Dy_0.17Gd_0.11Er_0.07Yb_0.05Sm_0.02)_∑1.90H_0.54 ⁺ [(UO₂)_8.19O₇OH(SO₄)_3.81](H₂O)_26.00. Sejkoraite-(Y) is triclinic, space group P1̅, a = 14.0743(6), b = 17.4174(7), c = 17.7062(8) Å, α = 75.933(4), β = 128.001(5), γ = 74.419(4)°, V = 2777.00(19) ų, Z = 2, D_calc = 4.04 g/cm³. The seven strongest reflections in the X-ray powder diffraction pattern are [d_obs in Å (I) (hkl)]: 9.28 (100) (100), 4.64 (39) (200), 3.631 (6) (1̅42), 3.451 (13) (1̅44), 3.385 (10) (2̅4̅2), 3.292 (9) (044), 3.904(7) (300), 2.984 (10) (1̅4̅2). 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 (~1200–1100 cm⁻¹), UO₂²⁺ stretching vibrations (~900–800 cm⁻¹), and O-H stretching (~3500–3200 cm⁻¹) and H-O-H bending modes (~1640 cm⁻¹). The new mineral is named to honor Jiří Sejkora, a Czech mineralogist of the National Museum in Prague.