KINETICS OF TRANSFORMATIONS OF POLYMORPHIC MODIFICATIONS OF AL2SIO5 IN THE PRESENCE OF A WATER-BASED FLUID

Abstract

We measured the kinetics of the transformation of kyanite to andalusite and sillimanite and of andalusite to sillimanite in order to determine the activation energies of the transformations and to estimate the absolute growth rates of the crystals of the stable phase. The rates of the above mineral transformations at 500 to 700°C are high in geological terms if the system contains an excess of water-based fluid and if the difference P-P* is at least 0.5 kbar. Factors capable of greatly hindering the process include, of course; the absence of a water-based fluid, a diffusion-limited process, difficult nucleation, and constrained growth of crystals in a rigid matrix as well as crystal growth under isochoric or partially isochoric conditions, when the crystals are confined within the elastic but rather rigid matrix into which the rock is converted at relatively low temperatures in the case of a deficiency of water.