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

Abstract

The structure of haiweeite, Ca[(UO2)(2)Si5O12(OH)(2)](H2O)(3), Z = 4, orthorhombic, a 7.125(l), b 17.937(2), c 18.342(2) Angstrom, V 2344.3(7) Angstrom (3), Space group Cmcm, has been solved by direct methods and refined by full-matrix least-squares techniques to an agreement index (Rt) of 4.2% for 1181 unique observed reflections (\F-o\ less than or equal to 4 sigma (F)) collected using MoK alpha X-radiation and a CCD-based area detector. The structure contains two symmetrically distinct U6+ 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 O-Ur 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 disordered 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 between silicate tetrahedra in hydrous uranyl silicates are related to the U:Si ratio, as is the mode of polymerization between silicate tetrahedra and uranyl polyhedra. With increasing Si relative to U, there is increasing polymerization of silicate tetrahedra, and a decreasing tendency for edge-sharing between uranyl polyhedra and silicate tetrahedra.