Larisaite, a new uranyl selenite with the idealized formula Na(H3O)(UO2)3(SeO3)2O2 · 4H2O, has been found in a sedimentary rock from Repete mine near Blanding, San Juan Co., Utah, U.S.A., in association with quartz, haynesite, andersonite, wölsendorfite, uranophane, gypsum, calcite and montmorillonite. The mineral is named in memory of Russian mineralogist and crystallographer Larisa Nikolaevna Belova (1923–1998) who made a significant contribution to the knowledge on the uranium minerals. Larisaite forms coarse lamellar crystals up to 1 mm and radial aggregates up to 2 mm. It is transparent or translucent, yellow, lustre vitreous, streak yellow. Fluorescence under the UV light is green (wavelengths of excitation 250 nm). Larisaite is sectile, with Mohs' hardness 1, perfect cleavage on (010) and uneven fracture across the cleavage direction. Calculated density is 4.50 g/cm3 from the crystal structure refinement and 4.46 g/cm3 from the empirical formula. Optically biaxial (−), α 1.597(2), β 1.770(5), γ 1.775(5); −2V = 20°. Dispersion is strong, r<v. Orientation: X = b, Z coincides with the direction of elongation. Pleochroism: γ (yellow) = β (yellow) > α (light greenish-yellow). IR spectrum is given. Average values for 3 point microprobe analyses (wt.%, ranges are given in brackets) are: Na2O 2.04 (1.82–2.32), K2O 0.69 (0.62–0.76), CaO 0.23 (0.17–0.30), UO3 72.19 (71.77–72.64), SeO2 18.12 (17.83–18.48); H2O content determined by Penfield method is 7.64; total 100.91 wt.%; contents of Mg, Sr, Ba, Pb, Zn, Mn, Ni, Co, Cu, Fe, Al, Si, S, As, Cl, F are lower than detection limits i. e. < 0.02–0.05%. The empirical formula based on (SeO3)2O2 is: Na0.81K0.18Ca0.05(H3O)0.73(UOsb>2)3.09(SeO3)2O24.1H20.1-(Kp/Kc) = 0.013 (“superior”). The crystal structure has been determined (R=0.067). Larisaite is monoclinic, space group P11m; a = 6.9806(9), b = 7.646(1), c = 17.249(2) Å, γ = 90.039(4)°, V = 920.64 Å3, Z = 2. The strongest lines in the powder diffraction pattern [d, Å (I, %) (hkl)] are: 8.63 (43) (002), 7.67 (100) (010), 3.85 (40) (−113, 020, 113), 3.107(77) (211), 2.874 (53) (006, −115). By the U: Se ratio, the values of unit cell parameters and the structure type, larisaite is related to haynesite, guilleminite and piretite. In common with guilleminite, uranium polyhedra and SeO3 triangles form the sheets, however the distribution of interlayer cations and H2O molecules is different. Holotype specimen is deposited in the Geoscientific Collections of Freiberg University of Mining and Technology, Faculty of Geosciences, Geotechnics and Mining, Freiberg, Germany (the inventory number 80251).

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