Thermodynamics of uranyl minerals: Enthalpies of formation of rutherfordine, UO₂CO₃, andersonite, Na₂CaUO₂(CO₃)₃ (H₂O)₅, and grimselite, K₃NaUO ₂(CO₃)₃H₂O

Enthalpies of formation of rutherfordine, UO₂CO₃, andersonite, Na₂CaUO₂(CO₃)₃ (H₂O)₅, and grimselite, K₃NaUO ₂(CO₃)₃(H₂O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, ΔH_r-ox, is -99.1 ± 4.2 kJ/mol for the reaction UO₃ (xl, 298 K) + CO₂ (g, 298 K) = UO₂CO₃ (xl, 298 K). The ΔH_r-ox for andersonite is - 710.4 ± 9.1 kJ/mol for the reaction Na₂O (xl, 298 K) + CaO (xl, 298 K) + UO₃ (xl, 298 K) + 3CO₂ (g, 298 K) + 5H₂O (1, 298 K) = Na₂CaUO₂(CO₃)₃ (H₂O)₆ (xl, 298 K). The ΔH_r-ox for grimselite is -989.3 ± 14.0 kJ/mol for the reaction 1.5 K₂O (xl, 298 K) + 0.5Na₂O (xl, 298 K) + UO₃ (xl, 298 K) + 3CO₂ (g, 298 K) + H₂O (1, 298 K) = K₃NaUO₂(CO₃)₃ H₂O (xl, 298 K). The standard enthalpies of formation from the elements, ΔH_f^o, are -1716.4 ± 4.2, -5593.6 ± 9. 1, and -4431.6 ± 15.3 kJ/mol for rutherfordine, andersonite, and grimselite, respectively. Energetic trends of uranyl carbonate formation from the binary oxides and ternary carbonates are dominated by the acid-base character of the binary oxides. However, even relative to mixtures Of UO₂CO₃, K₂CO₃, and Na₂CO₃ or CaCO₃, andersonite and grimselite are energetically stable by 111.7 ± 10.2 and 139.6 ± 16.1 kJ/mol, respectively, suggesting additional favorable interactions arising from hydration and/or changes in cation environments. These enthalpy values are discussed in comparison with earlier estimates.