We investigated crystal structures and the mechanism of thermal expansion of weeksite and its synthetic analogues (K-, Rb-, Cs-) using a combination of geometrical-topological 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, USA. Its synthetic analogues were prepared using high-temperature approaches in sealed silica tubes. Natural weeksite is stable up to 860 ± 10°C; it dehydrates between 100–200°C. Its synthetic analogues with Rb and Cs are stable at least until 1000°C. 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 SiO4 tetrahedra. With increasing temperature, the angles at the Si–O–Si ‘hinges’ change, which causes the shear deformations. The differences in the thermal behaviour, including expansion anisotropy, are probably 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 immobilization of 137Cs+ upon storing nuclear waste with little interference from the more abundant Na+ and K+.