Crystal structures of two uranyl selenite minerals derriksite, Cu4 [(UO2)(SeO3)2 ](OH)6, and demesmaekerite, Pb2 Cu5 [(UO2)2 (SeO3)6 (OH)6 ](H2 O)2, which structures are based on uranyl selenite 1D structural units, were studied employing single-crystal X-ray diffraction analysis at various temperatures. The refinement of their crystal structures reveals the detailed dynamics of the interatomic interactions during the heating process, which allows describing the thermal behavior. Uranyl selenite chains and their mutual arrangement mainly provide the rigidity of the crystal structure. Thus the lowest expansion in the structure of derriksite is observed along the direction of uranyl selenite chains, while the largest expansion occurs in the direction normal to chains, with the space occupied by lone electron pairs of Se4+ atoms and low covalent bond distribution density. The maximal expansion in the crystal structure of demesmaekerite is manifested approximately along the , which matches the direction of chains of less strongly bonded Cu-centered octahedra, and gaps between Cu chains occupied by the Pb cations. The crystal structure of demesmaekerite undergoes contraction in the direction of the space between the U-bearing chains with the deficiency of strong covalent bonds. Contraction of the structure can also be attributed to the orthogonalization of the oblique triclinic angles of the unit cell. It is demonstrated that the assignment of U-bearing units during structure description is reasonably justified since, regardless of their dimensionality, these substructural units are one of the most stable and rigid blocks in the structural architecture, and they govern the thermal behavior of the entire structure.
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