Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite

The solubility products and standard molar Gibbs free energies of formation were obtained from solubility measurements at 303.15 K for the following synthetic uranium-silicate phases: soddyite, (UO 2 ) 2 SiO 4 ·2H 2 O; uranophane, Ca(H 3 O) 2 (UO 2 ) 2 (SiO 4 ) 2 ·3H 2 O; sodium boltwoodite, Na(H 3 O)(UO 2 )SiO 4 ·H 2 O; and sodium weeksite, Na 2 (UO 2 ) 2 (Si 2 O 5 ) 3 ·7H 2 O. These phases were identified as uranium precipitates likely to form near a high-level nuclear waste repository. The experimental standard molar Gibbs free energies of formation for soddyite, uranophane, sodium boltwoodite, and sodium weeksite are: −(3658.0±4.8) kJ·mol −1 , −(6210.6±7.6)kJ·mol −1 , −(2966.0±3.6)kJ·mol −1 , and −(9088.5±18.4) kJ·mol −1 , respectively. The value for sodium boltwoodite is a lower bound and therefore should be used with caution.