Analyses of many clays by X-ray, optical, chemical and dehydration methods have shown that clays generally are aggregates of extremely minute crystalline particles of one or more species of a small group of minerals known as the clay minerals. There are three important groups of clay minerals: namely, kaolinite, illite, and montmorillonite. The optical, chemical, base-exchange, dehydration, and lattice structural characteristics of the clay minerals are considered.

Replacements within the lattice of the illite and montmorillonite minerals, chiefly Al⁺⁺⁺ for Si⁺⁺⁺⁺ and Mg⁺⁺ for Al⁺⁺⁺, are considered in relation to base-exchange capacity and particle size characteristics. Physical properties of clays are considered also in relation to the structure of their constituent clay minerals and the character of the exchangeable bases that are present. In general plasticity, bond strength, and shrinkage are relatively high for montmorillonite clays and low for kaolinite clays. In illite clays these properties vary from low to moderately high. Comprehensive information on the influence of specific exchangeable bases and physical properties remains for future researches. Present data are scant and somewhat conflict ng.

The distribution of clay minerals in various types of argillaceous sediments is considered. Also the origin of the clay minerals based on synthesis experiments and weathering studies is reviewed.

Clay mineralogy must be considered in making and interpreting mechanical analyses of argillaceous materials, because the clay minerals are affected differently by diagenetic processes and break down on disaggregation with different degrees of ease.