Gibbs free energies of formation (ΔG°r) were calculated for representative clay minerals equilibrated in aqueous solution using specific chemical compositions, and taking into account the different species of hydrated Al and Fe species produced during reaction. Results in kcal/mole are: Bueker flint clay, —893.8; Keokuk kaolinite, —902.7; Georgia kaolinite, —903.5, but —902.5 for ideal structural formula; Wyoming (Na-rich) montmorillonite, —1248.2; Arizona (Ca-rich) montmorillonite, —1252.1; Beavers Bend (Na-rich) illite, —125O.5; Fithian (Ca-rich) illite,—1282.1.

Ca-rich varieties of certain clay minerals, plagioclases, and pyroxenes had higher negative ΔG°r values than did Na-rich varieties.

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