The equilibrium temperatures of the reaction muscovite+chlorite+quartz = cordierite+phlogopite+H2O (1) in the pure system K2O—MgO—Al2O2—SiO2—H2O were found to be 495±10°C at 1 kb PH2O; 525±10°C at 2 kb; 610±15°C at 5 kb; 635±10°C at 6 kb. From intersection of this curve with the lower temperature stability limit of cordierite close to 645°C, 6.5 kb PH2O, a reaction cordieritc+muscovite = phlogopite+aluminum silicate+quartz+H2O (2) is generated which has a negative slope and passes through the points 645°C, 6.5 kb PH2O and 700°C, 5 kb PH2O. On the high-pressure side of this reaction curve cordierite is restricted to K2O—poor bulk compositions.