Clay minerals are commonly hydrated, and water molecules are attached in two different interlayer and interparticle hydration sites. Both sites contribute to the hydration energy involved when a dry clay aggregate is placed in contact with water or when a wet and swollen aggregate is placed in a dry atmosphere. The amounts of adsorbed water, and the hydration energy as well, vary as functions of the activity of water in which particle aggregates are placed. Integration of dehydration isotherms, established for different samples of various compositions of clay minerals, was used to evaluate the Gibbs free energies of hydration at different hydration stages, i.e., from the water-saturated phases to the dry states. The proposed model of estimating Gibbs free energies is based on the following steps and assumptions: (1) Hydrated and dehydrated clay minerals are ideal solid solutions, as presented by TARDY and FRITZ (1981); (2) In each of the series of phyllosilicates (talc, mica, and celadonites), hydrated, dehydrated, and well-crystallized Gibbs free energies of formation from the oxides are linearly dependent on the parameters DELTA-O(M(z+))2- related to the electronegativity of the cation M(z+) in the octahedral position; (3) In each of the mineral series, hydration energies are proportional to the layer charge; and (4) In each of the mineral series of the same charge and same interlayer cation, the hydration energy is also proportional to DELTA-O(M(z+))2-. The principal results of the model are (1) poorly crystallized but hydrated clay minerals of a given chemical composition; (2) poorly crystallized, dehydrated clay minerals; and (3) dry, largely sized, well-crystallized phyllosilicates of the same chemical composition, all of which differ largely in their Gibbs free energies of formation and in their stability fields in natural conditions. Furthermore, the solubility products and the corresponding cation exchange constants of these minerals are dependent on the activity of water in which the equilibrium reactions take place. These parameters also depend on the tetrahedral, octahedral, or total interlayer charge, and finally on the nature of the cation located in the octahedral layer (i.e., Mg2+, Fe2+, Al3+, or Fe3+). It is also proposed that for a given interlayer cation (Li+, Na+, K+, Mg2+, or Ca2+) and for a given octahedral composition (Mg2+, Fe2+, Al3+, or Fe3+), the hydration energy generally increases with the layer charge so that most of the minerals of high charge are hydrophylic and should hydrate spontaneously in water. However, the hydration energy of K+-exchanged nontronites (Fe3+) and beidellites (Al3+), both dioctahedral, decreases with the layer charge so that illite and glauconite, muscovite, and ferrimuscovite presumably appear as hydrophobic and should not hydrate spontaneously when placed in contact with water.
