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

Abstract

Enthalpies of formation of rutherfordine, UO 2 CO 3 , andersonite, Na 2 CaUO 2 (CO 3 ) 3 (H 2 O) 5 , and grimselite, K 3 NaUO 2 (CO 3 ) 3 (H 2 O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, ΔH r - o x , is -99.1 ′ 4.2 kJ/mol for the reaction UO 3 (xl, 298 K) + CO 2 (g, 298 K) = UO 2 CO 3 (xl, 298 K). The ΔH r - o x for andersonite is -710.4 ′ 9.1 I kJ/mol for the reaction Na 2 O (xl, 298 K) + CaO (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + 5H 2 O (1, 298 K) = Na 2 CaUO 2 (CO 3 ) 3 (H 2 O) 6 (xl, 298 K). The ΔH r - o for grimselite is -989.3 ′ 14.0 kJ/mol for the reaction 1.5 K 2 O (xl, 298 K) + 0.5Na 2 O (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + H 2 O (1, 298 K) = K 3 NaUO 2 (CO 3 ) 3 H 2 O (xl, 298 K). The standard enthalpies of formation from the elements, ΔH o 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 UO 2 CO 3 , K 2 CO 3 , and Na 2 CO 3 or CaCO 3 , 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.