234 Notes ALTERATION PROCESSES IN CLAY MINERALS

It is proposed to review here some recent Russian contributions to our understand-ing of the alterations which clay minerals undergo during natural weathering and in technological processes. These studies have indicated that the kinetics and mechanism of alteration of layer silicates are largely controlled by the structure and composition of the layers, and by the nature of the bonding forces between layers. These factors have long been known to determine such clay properties as absorption, ion exchange and rheology. But as alteration of clay minerals results from an interaction with their environment, and in particular with water, it is to be expected that their surface properties and their interlayer bonding will also be important controlling factors in this process (Kukovsky, 1965, 1968a). In the weathering of micas to hydromicas and montmorillonites, the initial step, involving loss of potassium, cannot be a simple hydration of the interlayer cations by penetration of water molecules into the interlayer space, as the energy of hydration of potassium is very much less than the energy of the interaction between potassium and the two silicate layers, both of which carry a charge. An essentia