Determining Behavior, Properties and Structural Relationships of Uranium Minerals and Synthetic Phases

The objective of this dissertation is to determine the fundamental properties, behaviors and structural relationships of various suites of uranium minerals. An emphasis was placed upon the crystal structures of uranium minerals, synthetic analogs of uranium minerals and synthetic analogs of uranium phases. While the instrumentation used for each research project varied depending upon the objectives of each project, X-ray diffraction was utilized to determine either the complete structure of uranium phases or aspects of uranium containing powders.

The first project focused on demonstrating that hollow uranyl carbonate cage clusters can exist in an aqueous phase as well as in solid phases. We synthesized crystals containing a [(UO₂)₂₄O₄(OH)₁₂(CO₃)₃₆]^44- nanocluster with Ca and Na countercations. A solution of dissolved crystals with a pH of 8.1 and a concentration of 1.19 ± 0.04 mmolL-1 was introduced into an electrospray ionization mass spectrometer. This produced broad m/z peak envelopes, corresponding to masses consistent with the mass of [(UO₂)₂₄O₄(OH)₁₂(CO₃)₃₆]^44- in water (8909.07 amu) with charge-balancing cations and H₂O.

The second project focused on determining the standard-state enthalpies of formation (ΔH_f­°) of five uranyl sulfate minerals. Synthetic analogs of each mineral were crystallized, characterized, then dissolved in high-temperature Calvet-type AlexSYS calorimeters. The heat of dissolution was used in thermal cycles to determine the ΔH_f­° of each phase. Two synthetic analogs were of better quality than the minerals and H positions were determined using the X-ray diffraction data.

The third chapter is a review examining the structures of the more than forty uranyl sulfates and the more than a dozen uranyl carbonates recognized by the International Mineralogical Association that have been described from 2009 to present. The review organizes the minerals into previously unrecognized groups, recognizes undescribed topological relationships, and notes which minerals have synthetic analogs.

The fourth study attempted to capture aqueous uranyl peroxide hollow cage clusters in the interlayers of pillared synthetic phases based upon the hydrotalcite structure. Solutions of a suite of uranyl peroxide nanoclusters varying in structure and chemistry were introduced into suspension with powdered layered double hydroxides. The resulting solids were examined with powder X-ray diffraction and electron microscopy. The results indicated that the nanoclusters were precipitating from solution rather than being captured by the pillared phase.