THE IMPORTANCE OF ACCURATE CRYSTAL-STRUCTURE DETERMINATION OF URANIUM MINERALS .2. SODDYITE (UO2)2(SIO4).2H2O

(UO2)2(SiO4).2H2O, M(r) = 668.17, orthorhombic, Fddd, a = 8.334 (2), b = 11.212 (5), c = 18.668 (6) angstrom, V = 1744 (1) angstrom 3, Z = 8, D(x) = 5.088 g cm-3, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 353.38 cm 1, F(000) = 2256; T = 293 K, R = 0.014, wR = 0.018 for 335 independent reflections with I > 3-sigma(I). A least-squares refinement of the structure of soddyite from a new set of X-ray data shows that accurate results can be obtained even for crystals containing very heavy elements; the strong absorption can be satisfactorily accounted for if appropriate procedures are used. Here, in particular, it was even possible to locate the water H atoms and refine their coordinates and thermal parameters. The structure shows chains of pentagonal bipyramids centred on the U atoms, joined by edge sharing: the chains are parallel to [110] and are cross linked through SiO4 tetrahedra. The cohesion within the resulting three-dimensional structure is further enhanced by a pattern of hydrogen bonds involving the water molecules and the uranyl O atoms.