Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates

Abstract

The enthalpies of formation of seven uranyl oxide hydrate phases and one uranate have been determined using high-temperature oxide melt solution calorimetry: [(UO2)4O(OH)6](H2O)5, metaschoepite; β-UO2(OH)2; CaUO4; Ca(UO2)6O4(OH)6(H2O)8, becquerelite; Ca(UO2)4O3(OH)4(H2O)2; Na(UO2)O(OH), clarkeite; Na2(UO2)6O4(OH)6(H2O)7, the sodium analogue of compreignacite, and Pb3(UO2)8O8(OH)6(H2O)2, curite. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpy, ΔHds. The standard enthalpies of formation from the elements, ΔH0f, at 298 K are -1791.0 ± 3.2, -1536.2 ± 2.8, -2002.0 ± 3.2, -11389.2 ± 13.5, -6653.1 ± 13.8, -1724.7 ± 5.1, -10936.4 ± 14.5, and -13163.2 ± 34.4 kJ/mol, respectively. These values are useful in exploring the stability of uranyl oxide hydrates in auxiliary chemical systems, such as those expected in U-contaminated environments.