The crystal structure of natural zippeite, K1.85H+0.15[(UO2)4O2(SO4)2OH2](H2O)4, from Jáchymov, Czech Republic, has been determined by single-crystal X-ray diffraction and refined to R1 = 0.0367. Zippeite is monoclinic, space group C2/m, with a 8.7802(6), b 13.9903(12), c 8.8630(6) Å, β 104.524(7)°, V 1053.92(12) Å3 and Z = 2. The structure consists of structural sheets of the zippeite uranyl anion topology and an interlayer, in which split K+ atoms and disordered O atoms (of the H2O groups) are located. The structure unit [(UO2)4O2(SO4)2OH2]2− is novel for both natural and synthetic compounds, but generally consistent with the known zippeite-type structures. The structural formula is in agreement with bond-valence analysis and the results of an electron-microprobe study. The composition of the zippeite specimen studied is assessed from the point of view of the bond-valence approach. Further, we comment on morphology of zippeite, its optical properties, and its relationship to the crystal structure.

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