HIGH-PRESSURE BEHAVIOR OF KYANITE - DECOMPOSITION OF KYANITE INTO STISHOVITE AND CORUNDUM

The pressure stability of kyanite was experimentally reversed with the use of a multi-anvil apparatus. Kyanite was found to decompose into i ts oxides stishovite and corundum between 14 +/- 0.5 GPa (at 1000 degr ees C) and 17.5 +/- 1.0 GPa (at 2000 degrees C). Reliable thermodynami c calculations can be performed to temperatures of approximately 1500 degrees C. Up to this temperature, the location of the equilibrium kya nite = corundum + stishovite, determined in this study, constrains the equilibrium coesite = stishovite. A set of thermodynamic data was cal culated by linear programming from the kyanite breakdown reaction and the coesite = stishovite equilibrium. Feasible values for the fitted t hermodynamic properties are -28.5 to -26.3 MPa/K for the temperature d erivative of the bulk modulus [(dK/dT)(p)] of kyanite, -815254 to -813 635 J/mol for G((1298))(0) of stishovite, and 24.6 to 26.3 J/mol . K f or Sp((1298))(0) of stishovite. The experimental results indicate (1) that in peraluminous eclogites of basaltic or sedimentary origin, stis hovite may coexist stably with corundum at a depth greater than 420-45 0 km and (2) that in an inhomogeneous Al-enriched mantle, corundum cou ld be a minor constituent in the lower mantle.