Recent experimental investigations in the system Al2O3-SiO2 provide sufficient data to re-evaluate the heats and free energies of formation of the Al2SiO5 polymorphs (andalusite, kyanite and sillimanite) and Al6Si2O13 (mullite). The calculated free energy of formation from the elements at 298.15° K, δG°f.298.15, is —583,897 cal/gfw for andalusite; —582,737 cal/gfw for kyanite; —583,351 cal/gfw for sillimanite; and —1,545,093 cal/gfw for 3:2 mullite. These results clearly indicate that previously accepted free energy values of the Al2SiO5 polymorphs are approximately 40 kcal/gfw too negative with respect to quartz and corundum.
The available experimental data almost certainly preclude the compatibility of quartz and corundum in igneous and metamorphic rocks or in the synthetic system. However, the assemblages midlite-corundum, mullite-andalusite, and mullite-sillimanite may be more common in highly aluminous metamorphic rocks than previously supposed.