From uranothorites to coffinite: a solid solution route to the thermodynamic properties of USiO4

Experiments on the solubility of intermediate members of the Th(1-x)U(x)SiO4 solid solution were carried out to determine the impact of Th-U substitutions on the thermodynamic properties of the solid solution and then allow extrapolation to the coffinite end member. The ion activity products in solutions equilibrated with Th(1-x)U(x)SiO4 (0 ≤ x ≤ 0.5) were determined by dissolution experiments conducted in 0.1 mol·L(-1) HCl under Ar atmosphere at several temperatures ranging from 298 to 346 K. For all experiments, dissolution was congruent, and a constant composition of the aqueous solution was reached after 50-200 days of dissolution. The solubility product of thorite was determined (log *K(S,ThSiO4) = -5.62 ± 0.08) whereas the solubility product of coffinite was estimated (log *K(S,USiO4) = -6.1 ± 0.2). The stoichiometric solubility product of Th(1-x)U(x)SiO4 reached a maximum value for x = 0.45 ± 0.05. In terms of the standard Gibbs free energy of dissolution, solid solutions dissolve more spontaneously than the end members. The standard Gibbs free energy associated with the formation of thorite, coffinite, and intermediate members of the series were then evaluated. The standard Gibbs free energies of formation were found to increase linearly with the uranium mole fraction. Our data at low temperature clearly show that uranothorite solid solutions with x > 0.26, thus coffinite, are less stable than the mixture of binary oxides, which is consistent with qualitative evidence from petrographic studies of uranium ore deposits.