Bulletin Mineralogie Petrologie

Škácha Pavel, Plášil Jakub, Sejkora Jiří, Čejka Jiří, Škoda Radek, Meisser Nicolas

Volume 22, issue 2 (2014), pages 240-248

bayleyite, uranyl carbonate, crystal structure, electron microprobe data, Raman spectroscopy, Jáchymov

A rare supergene uranyl carbonate mineral, bayleyite, was found on a single specimen originated from the 3rd level of the Barbora shaft, the Jáchymov ore district, Czech Republic. Bayleyite occurs there as a yellow very small, imperfect crystals of glassy lustre forming crystalline aggregates up to 3 mm on a small fragment of the rock. Liebigite, schröckingerite, rabbittite and natrozippeite were found in the mineral association. According to single-crystal X-ray data bayleyite is monoclinic, space group P2₁/a, with a = 26.535(1), b = 15.244(2), c = 6.497(3) Å, β = 92.942(10), and V = 2624.5(13) ų. Crystal structure was refined to R₁ = 2.68% on the basis of 5008 unique observed reflections [I_obs.>3σ(I)] collected on a Oxford Diffraction Gemini diffractometer with a Atlas CCD detector and MoKα radiation. Chemical analyses of bayleyite yielded the average composition MgO 12.68, FeO 0.06, Al₂O₃ 0.05, SiO₂ 0.45, UO₃ 51.97, CO_2calc. 21.27, H₂O_calc. 29.46, total 115.94 wt. % corresponding to the empirical formula (Mg_1.73 Al_0.01Fe_0.01)_Σ1.75(UO₂)_1.00[(CO₃)_2.66(SiO₄)_0.04] •18H₂O calculated on the basis of U = 1 apfu (CO₃ content derived from the charge-balance). Raman spectrum of bayleyite is dominated by the symmetric stretching vibration of UO₂²⁺ ion, and stretching and bending modes of planar CO₃ groups. Inferred uranyl bond-lenghts and possible hydrogen bond lenghts obtained from the Raman frequencies of corresponding vibrations, based on the empirical relations, are consistent with values from the refined structure.

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