Reversal of the metastable kyanite + corundum + quartz and andalusite + corundum + quartz equilibria and the enthalpy of formation of kyanite and andalusite

The metastable equilibrium kyanite + corundum + quartz has been reversed with a H₂O flux at 800 °C and 7.75 ± 0.25 kbar in the piston-cylinder apparatus with a NaCl pressure medium, and at 700 °C and 5.17 ± 0.32 kbar in an internally heated pressure vessel. The metastable equilibrium andalusite + corundum + quartz was reversed at 700 °C and 7.75 ± 0.25 kbar in the piston-cylinder apparatus. The reversals yield enthalpy of formation values from the oxides of −8.563 ± 0.189 kJ/mol for kyanite and −4.499 ± 0.125 kJ/mol for andalusite at 298 K. For reasons not fully understood, these values are systematically about 3 kJ more negative than values measured by high-temperature oxidemelt solution calorimetry. The kyanite + andalusite equilibrium determined by the present work satisfies all available experimental reversals. Remaining ambiguities concerning sillimanite prevent construction of a definitive Al₂SiO₅ triple point diagram.

Addition of Fe to the kyanite + corundum + quartz assemblage, buffered with magrretitehematite f_O2, did not detectably shift the equilibrium. We conclude that coexisting corundum and quartz found in some high-grade rocks are metastable relative to sillimanite.