A METHOD TO PREDICT FREE-ENERGIES OF FORMATION OF MINERAL PHASES IN THE U(VI)-SIO2-H2O SYSTEM

A method to estimate standard-state free energies of formation (Delta( f)G degrees) for U(VI)-layered oxide hydrates and silicates is describ ed. The Delta(f)G degrees values for these phases are estimated from t he free energies for the oxide and hydroxide components in the sheet s tructures, defined as the solid-phase free energies (Delta(f)G). Delt a(f)G for the component oxides and hydroxides are assumed to be const ant among the various sheet structures. A derivation of the Delta(f)G parameters for structural water and the UO2(OH)(2) component is prese nted, and used to estimate Delta(f)G degrees for important phases in t he U(VI)-SiO2-H2O system. Although propagated uncertainties are large, this method provides reasonable estimates of the free energies of for mation for the layered U(VI) structures, but not for the infinite fram ework structures such as soddyite. (C) 1998 Elsevier Science S.A.