Synopsis: 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. ΔH0f(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, aNa+, is relatively high and aCa2+ and aMg2+ are small. The interconversions of the sodium-free species are defined. When aMg2+/aCa2+ < 0.32 liebigite is the stable phase. If aMg2+/aCa2+ > 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.