Crystal-chemical and structural characterization of selected uranyl minerals and synthetic phases of environmental importance

The crystal-chemistry and crystal structures of uranyl minerals have not yet been sufficiently characterized, but knowledge of uranyl phases is crucial to evaluate the long-term performance of spent nuclear fuel in a proposed geological repository. A total of forty-eight single crystal structures including uranyl minerals and synthetic U6+ phases were studied from data collected using Mo Kα X-radiation and a Bruker CCD-based detector, in combination with other analytical tools.; A variety of uranyl oxide hydrates, uranyl carbonates and uranyl silicated were investigated. Research concerning Pb uranyl oxide hydrate minerals focused mainly on the elucidation of the charge balance mechanism that is responsible for the large variation of interlayer Pb2+ contents in curite and fourmarierite. Presented here also is a structure refinement of an orange crystal of richetite. Five new layered Pb-free uranyl oxide hydrates were synthesized, and their structures were determined and compared with related structures. Four uranium-based framework compounds have been discovered, and the unusual structure topologies exhibited by these materials are due primarily to octahedrally-coordinated U6+ in their structures. The structures of several uranyl carbonates were determined, including bijvoetite-(Y) with a structure based on an extraordinary (Y,REE)-bearing uranyl carbonate sheet, as well as Na₄(UO₂)(CO₃)₃, synthetic grimselite, and three new Ca uranyl carbonate compounds containing (NO ₃)− groups or Cl− with structures based on the well-known uranyl tricarbonate cluster. The synthesis of uranyl silicates followed by structure analysis revealed two uranyl silicate frameworks that are found in Na₄(UO₂)₂(Si₄O ₁₀)₂(H₂O)₄ and Na₂[(UO ₂)(Si₄O₁₀)], respectively, as well as a new uranyl silicate sheet in the system R-USiOH (R = NH₄ , Cs).; Many of the materials discussed in this thesis are expected to crystallize as spent nuclear fuel is subject to the oxidizing and moist environment of the proposed geological repository at Yucca Mountain. The results from this research have added to our understanding of structures of uranyl phases, and may have significant implications concerning nuclear waste management.