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. Application of the experimentally determined phase relations to more complex natural pelitic rocks suggests that reaction (1) represents maximum temperatures for the disappearance of chlorite from pelitic assemblages containing muscovite and quartz, whereas reaction (2) gives maximum water pressures for the disappearance of cordierite from these rocks.
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F. Seifert (1970) studied this question.