Crystallography, Hierarchy of Crystal Structures and Chemical Thermodynamics of Select Uranyl Phases

Over 200 minerals contain uranium as an essential structural constituent. Uranyl minerals, which contain U6+, are significant for understanding the genesis of uranium deposits, as well as the water-rock interactions that occur in uranium-rich rocks. They may impact the mobility of actinides in contaminated soils and in vadose zone sediments polluted with actinides, such as the Hanford and Savanna River sites in the United States. Uranyl minerals are important phases of alteration of nuclear waste forms under simulated geological repository conditions, such as those expected in the proposed repository at Yucca Mountain, Nevada. Despite their environmental importance, the crystallography and chemical thermodynamics of uranyl phases remains largely unexplored. The primary objectives of the research presented herein are to (1) expand the current foundation of uranyl crystal chemistry, (2) collect calorimetric data to assess the enthalpies of formation of environmentally relevant uranyl phases and (3) examine the relationship between crystal chemistry and chemical thermodynamics of select uranyl phases.

Crystal structures were determined for twenty-two uranyl phases, natural and synthetic, from single crystal X-ray diffraction data collected using MoKγ radiation and a Bruker CCD-based detector. Research efforts focus primarily on uranyl peroxides, of which thirteen crystal structures were determined. The uranyl peroxides discussed herein represent a new class of uranyl polyhedral topologies, as well as a new class of polyoxometalates with the polymerization of 20, 24, 28 and 32 uranyl polyhedra into finite clusters. In addition, crystal structures were determined for four uranyl carbonates, three uranyl oxide hydrates, a uranyl nitrate and a uranyl silicate.

Enthalpies of formation were determined using high temperature oxide-melt drop-solution calorimetry for a variety of well-characterized uranyl phases: one uranyl peroxide, three uranyl carbonates, ten uranyl oxide hydrates, a uranyl silicate and nine uranyl phosphates. Standard enthalpies of formation from the elements were used to ascertain energetic trends of uranyl mineral formation from the binary oxides. Enthalpies of formation with respect to the binary oxides are dominated by the acid-base character of the binary oxides. The energetics of these systems suggests that some additional stabilization associated with changes in crystal structure and/or hydration states are possible.