The shift of the α-β transition temperature of quartz associated with the thermal expansion of granite at high pressure

Abstract

Measurements are presented of volume changes in granite during room-temperature compression to 100, 200 and 300 MPa confining pressure followed by temperature increase to 900°C. Comparison with thermal expansion and compressibility data for the constituent minerals allows changes in porosity to be estimated. Under confining pressure, porosity is found to decrease with heating to 200°C through expansion of the minerals into cracks which are thought to be related to the geological cooling history of the rock. Between 200°C and 840°C porosity increases as a result of differential thermal expansion of the constituent minerals, but crack opening is increasingly suppressed at higher confining pressures. Extrapolation of the results indicates that differential thermal expansion can no longer cause crack opening in dry granite at confining pressures in excess of 450 MPa. The quartz α-β transition temperature in granite is marked by a kink in the thermal expansion curve of the rock, and it is found to increase by 60°C–70°C per 100 MPa confining pressure, as opposed to the published value of 26°C per 100 MPa for single crystals of quartz. Equations are presented which allow calculation of the effects of confining pressure and temperature on the stresses and displacements in and around a spherical inclusion embedded in a matrix of different elasticity and thermal expansion. The theory, together with a simple self-consistent model for granite, accounts semiquantitatively for the observations of thermal expansion and the effect of confining pressure thereon, and for the observed α-β transition temperatures for quartz in granite.