Marécottite, ideally Mg3[(UO2)4O3(OH)(SO4)2]2(H2O)28, a triclinic, Mg-dominant member of the zippeite group, was described originally from a small uranium deposit at La Creusaz in Wallis (Switzerland). It has recently been found at Jáchymov (Czech Republic), where it forms exceptional crystals, up to 0.3 mm across. According to an electron microprobe study of these crystals, marécottite from Jáchymov is chemically similar to the material from the La Creusaz deposit. However, the Jáchymov crystals exhibit more cation substitution (Zn2+ and Mn2+ for Mg2+). The chemical composition of marécottite from Jáchymov corresponds to the empirical formula [(Na0.05K0.07)Σ0.12 (Mg1.83Zn0.41Mn0.41Cu0.15Ni0.08)Σ2.88Al0.07]Σ3.07(UO2)8[(SO4)3.77(SiO4)0.21]Σ3.98O6(OH)1.84·28H2O (the mean of four representative spots; calculated on the basis of eight U atoms and 28 H2O per formula unit and 1.84 OH for charge balance). According to single-crystal X-ray diffraction, marécottite from Jáchymov is triclinic, P1İ, a = 10.8084(2), b = 11.2519(3), c = 13.8465(3) Å, α = 66.222(2), β = 72.424(2), γ = 70.014(2)°, V = 1421.57(6) Å3 and Z = 1. The crystal structure was refined from a highly redundant dataset (30,491 collected reflections) to R1 = 0.0367 for all 7042 unique reflections. The refined structure confirms the previously determined structure for the crystal from the La Creusaz deposit. An extensive network of hydrogen bonds is an important feature that keeps the whole structure together, but the positions of H atoms had not been determined previously. The H-bond scheme proposed based on a detailed bond-valence analysis and the role of different types of molecular H2O in the structure is discussed.

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