A NEW URANYL SILICATE SHEET IN THE STRUCTURE OF HAIWEEITE AND COMPARISON TO OTHER URANYL SILICATES

Abstract

The structure of haiweeite, Ca[(UO2)2Si5O12(OH)2](H2O)3, Z = 4, orthorhombic, a 7.125(1), b 17.937(2), c 18.342(2) A, V 2344.3(7) A 3 , space group Cmcm, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement index (R1) of 4.2% for 1181 unique observed reflections (| Fo| ≤ 4F) collected using MoK X-radiation and a CCDbased area detector. The structure contains two symmetrically distinct U 6+ positions, each of which is occupied by nearly linear (UO2) 2+ uranyl ions (Ur) that are coordinated by five additional O atoms arranged at the equatorial corners of pentagonal bipyramids capped by the OUr anions. There are four Si cations in tetrahedral coordination, three by O atoms only, and one by three O atoms and one (OH) – group. Uranyl polyhedra share edges, forming chains parallel to [100] that are one polyhedron wide. Silicate tetrahedra share edges with the uranyl polyhedra, and are staggered along the chain length. Adjacent chains are linked through additional silicate tetrahedra, forming a sheet parallel to (001). The silicate tetrahedra form a positionally disorder ed crankshaft-like chain parallel to [100] that involves four-member rings. Edge-sharing dimers of partially occupied Ca polyhedra occur in the interlayer and, together with H bonds, provide linkage between adjacent uranyl silicate sheets. The linkages betwe en silicate tetrahedra in hydrous uranyl silicates are related to the U:Si ratio, as is the mode of polymerization between silicat e tetrahedra and uranyl polyhedra. With increasing Si relative to U, there is increasing polymerization of silicate tetrahedra, a nd a decreasing tendency for edge-sharing between uranyl polyhedra and silicate tetrahedra.