Revision of the symmetry and the crystal structure of cejkaite, Na4(UO2)(CO3)3

The crystal structure of cejkaite, Na-4(UO2)(CO3)(3), from the type locality, was determined for the first time by single-crystal X-ray diffraction. In contrast to the previously reported pseudohexagonal triclinic symmetry, the current data indicate cejkaite is monoclinic, triply twinned, and belongs to the space group Cc. Refined unit-cell parameters are a = 9.2919(8), b = 16.0991(11), c = 6.4436(3) angstrom, beta = 91.404(5)degrees, and V = 963.62(12) angstrom(3). The monoclinic unit cell is also supported by the good fit to the powder diffraction data. The structure of cejkaite consists of uranyl tricarbonate clusters, forming sheets sub-parallel to (001) by sharing edges with (Na Phi) polyhedra. Sheets are interconnected through the uranyl O atoms and columns of (Nal Phi) polyhedra that share their trigonal faces. All Na atoms in the structure are in sixfold coordination. The structure refinement yielded R-obs = 0.0424 for 1687 observed reflections [I-obs > 3 sigma(I)] and 0.0538 for all 2016 unique reflections. Refinement and bond-valence analysis of the structure confirmed the previously proposed formula Na-4(UO2)(CO3)(3), Z = 4.