Effectiveness of Shearing Stresses in Accelerating Solid Phase Reactions at Low Temperatures and High Pressures

The use of displacive-shearing stresses in the study of phase equilibriums in opposed-anvil high-pressure apparatus has been of value in resolving the relationship of the anomalous polymorphic thorite-huttonite phases of ThSiO₄ and in achieving the first reversibility of the high-low-temperature forms of Nb₂O₅. Such stresses do not alter noticeably equilibrium, as shown previously by the examples of the calcite-aragonite and quartz-coesite transitions, among others, but do contribute markedly to increasing reaction rates at low temperatures. This is further illustrated in a kinetic study of the PbO₂-I→II transition at 40 kb. from 25 to 400° C.