The importance of accurate crystal structure determination of uranium Minerals. I. Phosphuranylite KCa(H3O)3(UO2)7(PO4)4O4.8H2O

On the basis of accurate crystal structure determination, the mineral phosphuranylite corresponds to the chemical formula KCa(H3O)3(UO2)7(PO4)4O4.8H2O. Cmcm, a = 15.778 (3)-15.899 (2), b = 13.702 (2)-13.790 (5), c = 17.253 (3)-17.330 (3) angstrom, Z = 4, D(x) = 4.575-4.631 g cm-3, mu = 287.6-291.1 cm-1. The presence of potassium (about 1.80 wt% K2O), over-looked until now, has been confirmed by microprobe analysis on samples from four different localities. The best data for structure determination have been obtained by single-crystal X-ray diffraction on specimens from Capoterra, Sardinia, and Bois Noirs, France; here 1453 and 1254 independent reflections, respectively, were used in the refinement, and the corresponding final R index is 0.036 and 0.048. The structure consists of layers of phosphate groups connected with hexagonal, pentagonal and tetragonal dipyramids centered on the U atoms. The Ca and K atoms are located within channels in the uranylphosphate framework, together with water molecules, with statistical distribution of the Ca atoms over two symmetrically related sites. The U - O distances are of two kinds: two bonds are very short (1.77 to 1.80 angstrom) and make an angle close to 180-degrees, in agreement with the conformation of the uranyl ion UO2(2+); the others range from 2.23 to 2.68 angstrom.