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Thermodynamics of uranyl minerals: Enthalpies of formation of rutherfordine, UO₂CO₃, andersonite, Na₂CaUO₂(CO₃)₃(H₂O)₅, and grimselite, K₃NaUO₂(CO₃)₃H₂O

¹ Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, U.S.A.
² Thermochemistry Facility and NEAT ORU, University of California at Davis, One Shields Avenue, Davis, California 95616, U.S.A.

Correspondence: ^* E-mail: pburns{at}nd.edu

Enthalpies of formation of rutherfordine, UO₂CO₃, andersonite, Na₂CaUO₂(CO₃)₃(H₂O)₅, and grimselite, K₃NaUO₂(CO₃)₃(H₂O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, H_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 H_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 (l, 298 K) = Na₂CaUO₂(CO₃)₃(H₂O)₆ (xl, 298 K). The H_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 (l, 298 K) = K₃NaUO₂(CO₃)₃H₂O (xl, 298 K). The standard enthalpies of formation from the elements, H°_f, 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.

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