The solubility and thermodynamic stability of a synthetic liebigite was investigated in NaCl and NaClO₄ solutions within a wide range of ionic strength (0.03 m ≤ I_m ≤ 5.61 m), pH (7 ≤ pH_m ≤ 9, with pH_m = –log [H⁺]) and temperature (22 °C ≤ T ≤ 80 °C) conditions. A comprehensive characterization of the synthetic solid phase using XRD, quantitative chemical analysis, TG–DTA, SEM–EDS, IR and Raman spectroscopy confirmed the stoichiometry of Ca₂UO₂(CO₃)₃·10H₂O(cr). At room temperature, liebigite remains stable and controls the solubility of U(VI) in the investigated NaCl and NaClO₄ systems with I_m ≤ 0.51 m. For the same temperature but high ionic strength (5.61 m NaCl), liebigite transforms into andersonite (Na₂CaUO₂(CO₃)₃·6H₂O(cr)). This solid phase transformation results in a decrease in solubility of approximately 2 log₁₀-units at pH_m ≈ 8. Solubility data in combination with solid phase characterization (XRD, quantitative chemical analysis) likewise confirm the transformation of liebigite into CaU₂O₇⋅xH₂O(cr), Na₂U₂O₇⋅xH₂O(cr) and/or other sub-stoichiometric Na-uranate compounds in all systems investigated at T = 80 °C.

On the basis of solubility data at room temperature determined in this work, in combination with thermodynamic and activity models available in the literature for the aqueous speciation in the system Ca–U(VI)–carbonate, solubility products for liebigite and andersonite are derived:

These results complement previously reported thermodynamic data, now allowing complete thermodynamic and geochemical calculations for the system UO₂²⁺–Ca²⁺–Na⁺–H⁺–CO₂(g)–HCO₃^––CO₃^2––H₂O(l), including U(VI) aqueous species and solid compounds, in the context of environmental uranium chemistry and nuclear waste disposal.