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, ΔHf°, 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.

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