A fundamental approach has been attempted in the study of the temperatures of formation and limits of stability of certain clay minerals and metamorphic rock assemblages. This has been done by studying the phase equilibrium relationships in the system Al2O3-SiO2-H2O under conditions of independently controlled temperature and water-vapor pressure. The data from a large number of runs are presented in a series of compatibility triangles for various temperatures and pressures. The equilibrium decomposition temperature of the kaolinite minerals has been placed at 405° C. (at 10,000 psi water pressure) and that of pyrophyllite at 575° C. Two new clay-phases exist in the system and are described in detail, one a pure Al-Si montmorillonite and the other herein named hydralsite, one of the decomposition products of kaolinite. The application of the results to geological and ceramic problems is discussed.