Kyanite-Mullite Transformation at High Temperature - A Unique Kind of Phase Transformation

Kyanite is a plymorph of andalusite and sillimanite. Each of them was formed under different temperature-pressure regimes, but all of them can be decomposed into mullite and silica.

Same to andalusite, the crystal convertion from kyanite to mullite is an inversible phase transformation. Kyanite (Al2O3.SiO2) will decomposed into mullite (3Al2O3.2SiO2) and amorphous silica (SiO2) once temperature is raised to 1300C or above.

3 Al2O3 .SiO2 - 3 Al2O3.2SiO2 + SiO2

For more details, please visit:  H. Wang, B. Li, M. He, J.Y. Hwang. Mullitization Characteristics and Sinterability of Kyanite in Ceramic Preparation. Characterization of Minerals, Metals, and Materials 2017. New York:  Spinger, International Publishing AG, pp251-259.