A novel arrangement of silicate tetrahedra in the uranyl silicate sheet of oursinite, (Co (sub 0.8) Mg (sub 0.2) )[(UO (sub 2) )(SiO (sub 3) OH)] (sub 2) (H (sub 2) O) (sub 6)

Oursinite is a rare Co-bearing uranyl silicate of the uranophane group. The structure of oursinite, (Co (sub 0.8) Mg (sub 0.2) )[(UO (sub 2) )(SiO (sub 3) OH)] (sub 2) (H (sub 2) O) (sub 6) , is orthorhombic, space group Cmca, a = 7.0494(5), b = 17.550(1), c = 12.734(1) Aa, V = 1575.4(2) Aa (super 3) , Z = 4. It was solved by direct methods and refined on the basis of F (super 2) for all unique reflections using least-squares techniques to an agreement index (R1) of 2.66%. The structure contains an approximately linear (UO (sub 2) ) (super 2+) uranyl ion that is present as a uranyl pentagonal bipyramid, one symmetrically distinct SiO (sub 3) OH acid silicate group, and one M (super 2+) (OH,H (sub 2) O) (sub 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 M (super 2+) (OH,H (sub 2) O) (sub 6) octahedra located in the interlayer, and by hydrogen bonds. Each M (super 2+) (OH,H (sub 2) O) (sub 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.