Crystallographically controlled void space at grain boundaries in the Harkless quartzite

Abstract

Electron backscatter diffraction analysis of quartz grains and imaging of void spaces at grain boundaries in two orthogonal sections of a sample from the plastically deformed and dynamically recrystallized Harkless quartzite reveals the presence of void spaces at quartz-quartz grain boundaries. The distribution of these voids correlates with the crystallographic orientation of neighboring quartz grains relative to the grain boundary. Void spaces are more prevalent along grain boundaries where the a-axis of quartz is normal to the grain boundary. These void spaces likely formed during the exhumation (decrease in temperature and pressure) of the Harkless quartzite as a consequence of the volume change associated with an anisotropic elastic change in quartz crystal lattice dimensions, in which the length of the a-axis decreases by more than that of the c-axis. This volume change results in void space preferentially forming at grain boundaries neighbored by quartz with the a-axis normal to the grain boundary. The preferred orientation of the void spaces thus matches the preferred crystal orientation of quartz in the samples, suggesting that the preexisting rock microstructure and crystal fabric plays an important role in controlling the development and distribution of porosity during exhumation, which more generally may have important implications for fluid migration and the strength of exhumed rocks.