POLYMORPHIC MODIFICATIONS OF SILICA IN THE LOWER MANTLE

Abstract

The authors attempt to calculate a phase diagram for SiO2 and estimate the range in which the high-density modifications of silica with the structure of α-PbO2 and fluorite can occur. There are several ways of calculating the phase diagrams. They are usually constructed from thermodynamic calculations, using various assumptions or approximations. This approach presupposes at least a minimum amount of reliable information on the thermodynamics of the material. In the present case, no such information is available. A novel method of calculating state diagrams for one-component systems is proposed, in which a crystalline solid is treated as a system of anharmonic oscillators. Additional aspects of the subject are discussed.