I. Introduction. Choice of information. Clay mineral names and structure. Chemical systems. Stability and meta-stability. Stability and reaction rate. Geological environments. Chemical coordinates for phyllosilicates assemblages. Summary. II. Minerals and Mineral Groups. Silica. Kaolinite, pyrophyllite, gibbsite and talc. Chlorites. Zeolites. Sepiolite-palygorskite. III. General Phase Diagrams for Some Clay Mineral Assemblages. Introduction. General systems. Weathering. Hydrothermal alteration. Deep sea basalt alteration. Mixed layered minerals in sequences of buried rocks (P-T space). Conclusions. References. Index.