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

• Published in: The American mineralogist Vol. 96; no. 7; pp. 983 - 991
• Main Authors: Plasil, Jakub, Dusek, Michal, Novak, Milan, Cejka, Jiri, Cisarova, Ivana, Skoda, Radek
• Format: Journal Article
• Language: English
• Published: Washington Mineralogical Society of America 01.07.2011; De Gruyter; Walter de Gruyter GmbH
• Subjects: Atoms & subatomic particles; Bohemia; Cations; Central Europe; chalcopyrite; crystal chemistry; crystal form; Crystal structure; Crystals; Czech Republic; Diffraction; Europe; formula; Gypsum; Hydrogen bonds; Jachymov Czech Republic; Jáchymov; metal ores; Mineralogy; new mineral; new minerals; nonsilicates; oxides; pseudojohannite; rabejacite; refinement; sejkoraite; Sejkoraite-(Y); spectra; sulfarsenites; sulfides; sulfosalts; tennantite; Topology; Ultraviolet radiation; uraninite; uranium ores; uranopilite; uranyl; uranyl ion; valency; vibration spectroscopy; vibrational spectra; X-ray diffraction; X-ray diffraction data; Yttrium; zippeite; zippeite group; Czech Republic
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am.2011.3713

Sejkoraite-(Y), the triclinic (Y1.98Dy0.24)Σ2.22H0.34+ [(UO2)8O88O7OH(SO4)4](OH)(H2O)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 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), 2Vcalc=79°. 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)Σ1.90H0.54+ (UO2)8.19O7OH(SO4)3.81](H2O)26.00. Sejkoraite-(Y) is triclinic, space group P1̄, 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) Å3, Z=2, Dcalc=4.04 g/cm3. The seven strongest reflections in the X-ray powder diffraction pattern are [dobs in Å (I) (hkl)]: 9.28 (100) (100), 4.64 (39) (200), 3.631 (6) (1̄42), 3.451 (13) (1̄44), 3.385 (10) (2̄4̄2), 3.292 (9) (044), 3.904(7) (300), 2.984 (10) (1̄4̄2). The crystal structure of sejkoraite-(Y) has been solved by the charge flipping method from single-crystal X-ray diffraction data and refined to Robs=0.060 with GOFobs=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 Jiri Sejkora, a Czech mineralogist of the National Museum in Prague.