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

Peter C. Burns

Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556-0767, U.S.A.

E-mail address: pburns{at}nd.edu

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) Å, V 2344.3(7) ų, 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 (|F_o| 4_F) collected using MoK 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.

Keywords: haiweeite, uranyl silicate, structure determination.

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