The influence of metamorphic environment upon the deformation of minerals

Abstract

This review examines the manner in which an imposed metamorphic environment, defined by the pressure, temperature and the thermodynamic activities of various chemical components such as O2, Si, Al, H2O, etc., can control the concentrations and mobilities of point defects in minerals. These defects include vacant atomic sites, interstitial or substitutional atoms, or complexes of these defects. The paper reviews the manner in which these imposed variables influence the diffusion and dislocation motion which control the deformation of minerals. The concentrations and mobilities of charged defects (point defects, jogs on dislocations or ledges on grain boundaries) depend upon the electronic structure of a mineral, and in particular on the width of the intrinsic band gap, the energy levels of defects within the band gap, and the position of the Fermi level or electron chemical potential relative to these levels. The relative position of the Fermi level may be altered by changing the imposed metamorphic environment and particularly by introducing trace amounts of suitable impurities. Impurities that act as acceptors increase the concentration and mobilities of positively charged defects and decrease these quantities for negatively charged defects. Impurities that act as donors have the opposite effects. (OH) would appear to be of fundamental importance in this regard in that it acts as a relatively shallow acceptor in quartz and so greatly enhances the diffusivity of oxygen or of positively charged complexes in quartz. (OH) also promotes the formation of negatively charged jogs on dislocations. This could well be the origin of the “water-weakening” effect in quartz. The available data also suggest that the rate-controlling process in olivine deformation experiments is the formation of positively charged kinks on dislocations. Since the relative concentrations of point defects can be changed by altering the metamorphic environment, the rate-controlling point defect involved in diffusion can change from one condition to another; in PbS the rate-controlling atom at low activities of sulphur is lead, whereas at high activities of sulphur it is sulphur. Little attention has been paid to these effects in the past by experimentalists and there is a need for careful control of the chemical environment in future experiments. Above all, control of the point defect chemistry offers a powerful tool for understanding mechanisms of deformation.