Free energies of formation of liebigite, Ca₂UO₂ (Co₃)₃·10H₂O, swartzite, CaMgUO₂(Co₃)₃·12H₂O, bayleyite, Mg₂UO₂(CO₃)₃·18H₂O, and andersonite, Na₂CaUO₂(CO₃)₃·6H₂O have been determined from solution studies at various temperatures. ΔGf(298.2K)° values for the above minerals are −6226±12, −6607±8, −7924±8 and −5651±24 KJ mol⁻¹ respectively. ΔH⁰_f(298.2K) values respectively are −7037±24, −7535±20, −9192±20, and −5916±36 kJ mol⁻¹. These results have been used to construct the stability diagram for the four minerals shown in fig. I. Andersonite can only form when the activity of the sodium ion, a_Na⁺, is relatively high and a_Ca²⁺ and a_Mg²⁺ are small. The interconversions of the sodium-free species are defined. When a_Mg²⁺/a_Ca²⁺ < 0.32 liebigite is the stable phase. If a_Mg²⁺/a_Ca²⁺ > 7.94, bayleyite forms preferentially. One might therefore expect the bayleyite-andersonite association to be more common than liebigite-andersonite, but this is entirely dependent upon the relative concentrations of Mg²⁺_(aq) and Ca²⁺_(aq) in the solutions from which the minerals form.