A novel arrangement of silicate tetrahedra in the uranyl silicate sheet of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6

Oursinite is a rare Co-bearing uranyl silicate of the uranophane group. The structure of oursinite, (Co0.8Mg0.2)[(UO2) (SiO3OH)](2)(H2O)(6), is orthorhombic, space group Cmca, a = 7.0494(5), b = 17.550(1), c = 12.734(1) angstrom, V = 1575.4(2) angstrom(3), Z = 4. It was solved by direct methods and refined on the basis of P for all unique reflections using least-squares techniques to ail agreement index (RI) of 2.66%. The structure contains ail approximately linear (UO2)(2+) uranyl ion that is present as a Uranyl pentagoonal bipyramid, one symmetrically distinct SiO3OH acid silicate group, and one M2+(OH,H2O)(6) octahedron (M is dominated by Co). The uranyl pentagonal bipyramids and silicate tetrahedra are linked by the sharing of edges and vertices, giving a sheet based upon the uranophane anion topology. Adjacent sheets are linked by M2+(OH,H2O)(6) octahedra located in the interlayer, and by hydrogen bonds. Each M2+(OH,H2O)(6) octahedron contains two OH groups that are apical ligands of silicate tetrahedra in adjacent uranyl silicate sheets. Although several uranophane-group minerals contain sheets that are based upon the uranophane anion topology, the oursinite sheet involves novel orientations of silicate tetrahedra.