A method to predict free energies of formation of mineral phases in the U(VI)–SiO 2–H 2O system

Abstract

A method to estimate standard-state free energies of formation (Δ f G°) for U(VI)-layered oxide hydrates and silicates is described. The Δ f G° values for these phases are estimated from the free energies for the oxide and hydroxide components in the sheet structures, defined as the solid-phase free energies (Δ f G*). Δ f G* for the component oxides and hydroxides are assumed to be constant among the various sheet structures. A derivation of the Δ f G* parameters for structural water and the UO 2(OH) 2 component is presented, and used to estimate Δ f G° for important phases in the U(VI)–SiO 2–H 2O system. Although propagated uncertainties are large, this method provides reasonable estimates of the free energies of formation for the layered U(VI) structures, but not for the infinite framework structures such as soddyite.