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

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.