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Larisaite, Na(H₃O)(UO₂)₃(SeO₃)₂O₂·4H₂O, a new uranyl selenite mineral from Repete mine, San Juan County, Utah, U.S.A.

¹ Institute of Problems of Chemical Physics, Chernogolovka, Moscow region, 142432, Russia, Corresponding author: e-mail: chukanov{at}icp.ac.ru
² Faculty of Geology, Moscow State University, Vorobievy Gory, Moscow, 119992, Russia
³ Dipartamento di Science della Terra, Universita di Pisa, Via S. Maria 53, I-56126 Pisa, Italy
⁴ Alpha-geophysik in Sachsen, Neudorfer Str. 18, D-09629 Burkersdorf, Germany
⁵ NPO Regenerator, Skladochnaya 1, Moscow, 127018, Russia
⁶ All-Russian Scientific-Research Institute of Mineral Resources (VIMS), Staromonetnyi per., Moscow, 109017, Russia

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 (–), 1.597(2), ß 1.770(5), 1.775(5); –2V = 20°. Dispersion is strong, r<v. Orientation: X = b, Z coincides with the direction of elongation. Pleochroism: (yellow) = ß (yellow) > (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: Na_0.81K_0.18Ca_0.05(H₃O)_0.73(UO_sb>2)_3.09(SeO₃)₂O₂4.1H_20.1-(K_p/K_c) = 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, = 90.039(4)°, V = 920.64 ų, ^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).

Key-words: larisaite, new mineral, uranyl selenite, Repete mine, Utah, hydrogen bonds.

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