Accurate thermodynamic data for uranyl minerals are needed to understand their role in uranium deposit formation and subsequent alteration, transport of uranium in contaminated subsurface environments, and for predicting the performance of a geological repository for nuclear waste. Three members of the uranyl sulfate zippeite group, natrozippeite, cobaltzippeite, and zinczippeite, were synthesized and characterized prior to determination of their standard state enthalpies of formation from binary oxides and from elements using high-temperature drop-solution calorimetry. Calculated standard-state enthalpies of formation from the elements, ΔHf°, at 298 K, were found to be –18015.84 ± 28.76, –4362.09 ± 10.44, and –4698.22 ± 13.68 kJ/mol for natrozippeite (Na5[(UO2)8(SO4)4O5(OH)3]•12H2O), cobaltzippeite (Co[(UO2)2(SO4)O2]•3.7H2O), and zinczippeite (Zn[(UO2)2(SO4)O2]•4.1H2O), respectively. Current results, together with the standard-state enthalpy of formation value for zippeite (containing K) reported earlier, allows investigation of the role of acid-base interactions between the structural unit and interstitial complexes relative to the enthalpy of formation of zippeite-group minerals. There is a positive linear relationship between the formation enthalpies from oxides and the acidity of cation oxides. There is also a linear correlation between the ionic radius of charge-balancing alkali cations and the enthalpy of formation of the studied compounds from their binary oxides, indicating the importance of the nature and coordination environment of the interstitial complex in determining the thermodynamic properties of these minerals.

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