The first-order phase transitions among the hydration states in both Na and Ca vermiculites are studied by water-adsorption isotherm and specific-heat measurements. These compounds have three kinds of hydration states defined by the number of water layers in the interlamellar space between the silicate layers (0-, 1-, and 2-water-layer hydration states). The steplike increases in the water adsorption with increasing water vapor pressure are discussed in terms of thermodynamical parameters. The Gibbs free energy is composed of (1) electrostatic interaction between cations and negative charge of the silicate layer and (2) hydration energy.

• Received 20 March 1987

DOI:https://doi.org/10.1103/PhysRevB.36.2844