AbstractSeveral dioctahedral clay minerals are related through reaction series. These series can be produced hydro-thermally from beidellite gel and montmorillonite by making simple changes in interlayer and solution chemistry. The series are: gel-K-beidellite-random illite/smectite-K-rectorite-illiteK-montmorillonite-K-rectorite-illiteNa-montmorillonite or beidellite-Na-rectorite-paragoniteLi-montmorillonite-Li-tosudite-Li-rectorite-cookeite(?)Mg-montmorillonite-Mg-rectorite-tosudite-sudoite(?)Ca-montmorillonite-Ca-rectorite-margarite(?)Al-Ca-montmorillonite-kaolinite/smectite-kaolinite (150°C)Al-Ca-montmorillonite-pyrophyllite/smectite-pyrophyllite (320°C).Assuming stability for the mixed-layer phases, paragenesis is a function of P, T, and X conditions. If the phases are considered to be metastable, paragenesis is a function of the speed and path of reaction.