Thermodynamic Properties of Uranyl Minerals: Constraints from Calorimetry and Solubility Measurements

More than 50 uranyl minerals, phases containing U as the uranyl UO2 2+ cation, and hydroxide, carbonate, phosphate, and silicate anions, H2O, and alkali and alkaline earth cations, occur in nature and as corrosion products of spent nuclear fuel. Despite their importance and the need to understand their thermodynamics to predict uranium solubility, fate, and transport in the environment, reliable thermodynamic data have only been available recently. This paper summarizes recent studies of enthalpies of formation using high temperature oxide melt solution calorimetry and Gibbs free energies from solubility experiments. Standard state thermochemical parameters (at 25 !C and 1 bar) are tabulated and the stability and transformation sequences of these phases are discussed. The enthalpies of formation from oxides are discussed in terms of crystal structure and Lewis acid!base interactions.