Abstract: Gibbs free energies for phases in the system Al2O3-SiO2-H2O have been calculated from reversed experiments in order to correct earlier values and to calculate a phase diagram consistent with more recent experiments. An internally consistent diagram could not be calculated that agreed with all published experiments, and choices of preferred data were made. The following Gibbs free energies, relative to the elements at STP (298.15 K, 1 bar), have been derived: ΔG°f(diaspore) = −921.19 kJ/mole ΔG°f(pyrophyllite) = −5266.47 kJ/mole ΔG°f(kaolinite) = −3800.18 kJ/mole ΔG°f(andalusite) = −2442.04 kJ/mole ΔG°f(kyanite) = −2443.66 kJ/mole ΔG°f(sillimanite) = −2439.63 kJ/mole ΔG°f(gibbsite) = −1154.9 kJ/mole ΔG°f(bayerite) = −1153.0 kJ/mole ΔG°f(boehmite) = −918.4 kJ/mole ΔG°f(nordstrandite) = −1154.1 kJ/mole ΔG°f(halloysite) = −3783.2 kJ/mole ΔG°f(dickite) = −3798.8 kJ/mole The above values were calculated assuming literature values for corundum, quartz, and H2O (v).
Examination of available thermodynamic, experimental, and observational data on the aluminum hydroxides gibbsite, boehmite, bayerite, and nordstrandite suggests that these minerals are metastable with respect to diaspore + water at STP and at higher temperatures. Similarly, halloysite and dickite are metastable with respect to kaolinite at these conditions. The occurrence of these minerals in soils must therefore be ascribed to nonequilibrium processes, and the use of equilibrium phase diagrams to explain their occurrence is inappropriate.