The pressure stability of kyanite was experimentally reversed with the use of a multi-anvil apparatus. Kyanite was found to decompose into its oxides stishovite and corundum between 14 ± 0.5 GPa (at 1000 °C) and 17.5 ± 1.0 GPa (at 2000 °C).
Reliable thermodynamic calculations can be performed to temperatures of approximately 1500 °C. Up to this temperature, the location of the equilibrium kyanite = corundum + stishovite, determined in this study, constrains the equilibrium coesite = stishovite. A set of thermodynamic data was calculated by linear programming from the kyanite breakdown reaction and the coesite = stishovite equilibrium. Feasible values for the fitted thermo-dynamic properties are –28.5 to –26.3 MPa/K for the temperature derivative of the bulk modulus [(dK/dT)P] of kyanite, –815254 to –813635 J/mol for of stishovite, and 24.6 to 26.3 J/mol · K for of stishovite.
The experimental results indicate (1) that in peraluminous eclogites of basaltic or sedimentary origin, stishovite may coexist stably with corundum at a depth greater than 420–450 km and (2) that in an inhomogeneous Al-enriched mantle, corundum could be a minor constituent in the lower mantle.