THE IMPORTANCE OF ACCURATE CRYSTAL-STRUCTURE DETERMINATION OF URANIUM MINERALS .1. 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.