The feldspars in Swedish glacial clays include K-feldspar, low albite and intermediate plagioclases, but not peristerites. The hydrolysis of K-feldspar leads to the formation of illite under glacial conditions of P, T and μmobile components; chlorite is the stable ferromagnesian mineral in the unweathered clays. Illite + albite + quartz represents a lower energy assemblage than K-feldspar + Na-montmorillonite + quartz.