Stability of the assemblage cordierite—Corundum in the system MgO-Al2O3-SiO2-H2O

The assemblage Mg-cordierite — corundum is formed stably through the reaction chlorite+Al-silicate=cordierite+corundum+H₂O at 535° C, 2kb; 615°, 5 kb; and 665° C, 7 kb water pressure. In the order of increasing pressure andalusite, sillimanite, and kyanite participate as stable phases in this equilibrium. A spinel-Al-silicate tie-line is only stable at high temperatures not likely to be attained in rocks. The natural assemblage spinel-Al-silicate is, however, to be explained by the additional presence of FeO in these rocks.