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Published February 2001 | public
Journal Article

A re-evaluation of the structure of weeksite, a uranyl silicate framework mineral

Abstract

The structure of weeksite, K_(1.26)Ba_(0.25)Ca_(0.12)[(UO_2)_2(Si_5O_(13))]H_2O, orthorhombic, a 14.209(2), b 14.248(2), c 35.869(4) Å, V 7262(2) Å^3, space group Cmmb, has been solved by direct methods using data collected with MoKα X-radiation and a CCD-based detector and refined by full-matrix least-squares techniques, on the basis of F^2 for all unique reflections, to an agreement factor (R_1) of 7.0% and a goodness-of-fit (S) of 1.04, calculated using the 1565 unique observed reflections (F_o ≤ 4σ_F). The structure contains four unique U^(6+) positions, each of which is part of a nearly linear (UO_2)^(2+) uranyl ion. The uranyl ions (Ur) are further coordinated by five atoms of oxygen arranged at the equatorial corners of UrO_5 pentagonal bipyramids. There are ten silicon atoms, each of which is tetrahedrally coordinated by oxygen atoms. The uranyl polyhedra share equatorial edges to form chains, which in turn share polyhedron edges with silicate tetrahedra. The uranyl silicate chains are linked to crankshaft-like chains of vertex-sharing silicate tetrahedra, resulting in layers that are connected by the sharing of vertices between silicate tetrahedra to form an open framework. Potassium, barium, calcium and H_2O are located in the channels within the uranyl silicate framework. Displacement of some atoms from their corresponding special positions indicates that the results represent an average structure.

Additional Information

© 2001 Mineralogical Association of Canada. Received August 19, 2000, revised manuscript accepted December 30, 2000. The crystals of weeksite used in this study were provided by Dr. Mark Feinglos. We thank Bob Gault at the Canadian Museum of Nature, who provided results of preliminary electron-microprobe analyses, and Ian Steele at the University of Chicago for the microprobe data reported in this manuscript. We thank Paulus Moore and David Wronkiewicz for their comments and helpful suggestions on the manuscript, and Bob Martin for providing beneficial annotations. This research was funded by the Environmental Management Sciences Program of the United States Department of Energy (DE–FGO7–97ER14820).

Additional details

Created:
August 21, 2023
Modified:
October 23, 2023