Thermodynamic properties of phosphate members of the meta-autunite group: A high-temperature calorimetric study

Samples of synthetic analogs of uranyl phosphate minerals have been prepared at room temperature by slow mixing of reactants by a diffusion method. Reaction products were analyzed using powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), inductively coupled plasma optical emission spectrophotometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS). Calorimetric measurements have been performed in a Calvet-type twin calorimeter using sodium molybdate (3Na2O-4MoO3) solvent at 976 K as a flux. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpies, ΔHds. Calculated standard enthalpies of formation from the elements, ΔH0f, at 298 K are −3425 ± 9 kJ/mol for meta-ankoleite (KUP), −6233 ± 17 kJ/mol for meta-autunite (CaUP), −6921 ± 13 kJ/mol for meta-torbernite (CuUP), −7254 ± 17 kJ/mol for meta-saléeite (MgUP), −3264 ± 12 kJ/mol for Rb-meta-autunite (RbUP), −3580 ± 7 kJ/mol for meta-natro-autunite (NaUP), −3692 ± 11 kJ/mol for Li-meta-autunite (LiUP), −6402 ± 5 kJ/mol for meta-uranocircite (BaUP), −3277 ± 6 kJ/mol for Cs-meta-autunite (CsUP), and −7109 ± 19 kJ/mol for Co-meta-autunite (CoUP). The results exhibit trends of the thermodynamic stability of these compounds. The normalized charge deficiency per anion (NCDA) approach relates the thermodynamic stability of these compounds to their crystal structures. The thermodynamic stability of uranyl phosphate minerals is important for understanding their formation in Nature, as well as their fate in a geological repository for nuclear waste, and their existence in the subsurface of anthropogenically contaminated environments.