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

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, ΔHr-ox, 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 Δ Hr-ox for andersonite is .710.4 ± 9.1 kJ/mol for the reaction Na2O (xl, 298 K) + CaO (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + 5H 2 O (l, 298 K) = Na 2 CaUO 2 (CO 3 ) 3 (H 2 O) 6 (xl, 298 K). The Δ Hr-ox for grimselite is .989.3 ± 14.0 kJ/mol for the reaction 1.5 K2O (xl, 298 K) + 0.5Na 2 O (xl, 298 K) + UO 3 (xl, 298 K) + 3CO 2 (g, 298 K) + H 2 O (l, 298 K) = K 3 NaUO 2 (CO 3 ) 3 H 2 O (xl, 298 K). The standard enthalpies of formation from the elements, ΔHfº 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