Polythermal studies of weeksite, a microporous uranyl silicate, and its synthetic analogues

We investigated crystal structures and the mechanism of thermal expansion of weeksite and its synthetic analogs (K-, Rb-, Cs-) using a combination of geometricaltopological analysis and empirical methods (powder X-ray diffraction, infrared spectroscopy, scanning electron microscopy, single-crystal and powder X-ray variable-temperature diffraction).The weeksite sample studied herein was collected at the Anderson mine, Yavapai County, Arizona, U.S.A.Its synthetic analogs were prepared using high-temperature approaches in sealed silica tubes.Natural weeksite is stable until 860 10 ; it dehydrates between 100 -200 .Its synthetic analogs with Rb and Cs are stable at least until 1000 .Their thermal expansion is strongly anisotropic due to shear deformations of the crystal structure.The framework in the structure of weeksite can be regarded as a sequence of uranyl silicate layers linked by SiO 4 tetrahedra.With increasing temperature, the angles at the Si-O-Si "hinges" change essentially, which causes the shear deformations.The differences in the thermal behavior, including expansion anisotropy, are likely due to the nature (size) of the alkali cations occupying the cavities in the framework.The partial or complete replacement of Rb + by Cs + illustrates the zeolite-like nature of the uranyl silicate framework in weeksite.Therefore, its structure can be considered a possible candidate for the selective