Several methods of estimating thermodynamic data of natural phosphates were unable to predict with enough accuracy stability fields of such minerals and the parageneses observed in natural conditions. Detailed observations made on lateritic weathering profiles, from the bottom to the top, yield the following mineral associations:

Fluor-carbonate-apatite + montmorillonite + calcite → fluor-carbonate-apatite + montmorillonite → fluor-carbonate-apatite + kaolinite → crandallite + kaolinite → wavellite + kaolinite → augelite → gibbsite.

A refined method for estimating Gibbs free energies of formation of these phosphates is proposed. A first step of estimation is based on the summation of the Gibbs free energies of formation of simple components and an additional term which depends on the nature of cations in each compound. A second calibration step is based on the correction of the additional term so that the Gibbs free energies of formation and the fields of stability of the phosphates and clay minerals would be in good agreement with the natural observed sequence. These estimations yield the following values (kcal/mole):

Fluor-carbonate-apatite, Ca10(PO4)5CO3F3:ΔGf0=2983.3Crandallite, CaAl3(PO4)2(OH)5.H2O:ΔGf0=1336.7Wavellite, Al3(PO4)2(OH)3(H2O)4.H2O: ΔGf0=1330.0Augelite, Al2PO4(OH)3: ΔGf0=660.9Variscite, Al2PO4.2H2O: ΔGf0=498.8Ca-millisite, Ca1.5Al6(PO4)4(OH)9.3H2O: ΔGf0=2614.0
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