Some typical microstructures in deformed rocks

Abstract

The deformation of rocks at a specified pressure and temperature is governed by mechanical characteristics of individual constituent minerals (often highly anisotropic), by differences in properties between minerals, and by chemical processes involving minerals and fluids. Major processes in deforming rocks include intracrystalline slip, recovery, recrystallization, grain boundary sliding, brittle fracture, pressure solution, and growth of new minerals. Intracrystalline slip and recovery are generally similar to those in metals. Dynamic and annealing recrystallization both occur. Recrystallization in polymineralic rocks is complicated by the inability of grain boundaries to migrate between phases. Grain boundary sliding can occur in fine-grained materials, allowing homogeneous deformation to large strains without failure. Brittle fracture can occur in strong mineral grains even when the rock as a whole deforms in a ductile manner. Pressure solution takes place when minerals preferentially dissolve in high stress areas and reprecipitate in low stress ones. Chemical reactions can lead to growth of new mineral grains before, during, or after deformation. This paper provides a list of pertinent references. Geologists try to understand the thermal and mechanical history of the earth by studying the microstructures of deformed rocks. This can be extremely difficult, particularly when the rocks have been deformed more than once. Nonetheless, microstructures visible with an optical microscope can be used to identify deformation mechanisms in rocks, and sometimes also to infer strain fields. The figures in this paper illustrate characteristic microstructures as they appear with a transmitted-light microscope.