Abstract: New and published experimental data on hydrothermally treated (2-kbar water pressure, 300–400°C natural clay minerals is used to construct phase diagrams in composition and pressure-temperature space which define the phase relations of mixed layering solid solution in dioctahedral (illite-montmorillonite) and trioctahedral expanding chlorite and corrensite-like minerals. Three major relations are established: The R3+, i.e. Al13+ and/or Fe3+, content of the assemblage will control whether or not an expanding chlorite or corrensite phase will appear. These minerals are R3+-rich in composition as well as R2+ (Mg and Fe2+)-rich. Temperature-pressure variables control the type and composition of the mixed layered illite-montmorillonite mineral which is stable with aluminous phases. A sequence of different types of mixed layered ordering can be established which might be correlated with diagenetic or epimetamorphic grade. The presence of ‘metamorphic’ trioctahedral phyllosilicate phases, i.e. those due to the effects of earliest metamorphism, is correlated with the P-T-X variables. These phases include 7 Å chlorite (iron-rich), 14 Å chlorite and the expanding chlorite and corrensite-like minerals.
It is not possible to give absolute temperatures for the different reactions implied by the phase diagram due to the slow rate of crystallization in the experiments. However, the general sequence of assemblages and the phase relations proposed should correspond to those which are encountered in nature when pressure and temperature vary.