The combination of TEM, based on focused ion beam (FIB) sample preparation, and high-resolution SEM allows the investigation of grain and phase boundary networks from nanometer to centimeter scale, i.e., over about 8 orders of magnitude. Recent studies show that the boundaries of various minerals in different metamorphic and magmatic rocks are lastingly open on the nanometer scale, due to the elastic response of crystals to temperature and pressure decrease during exhumation of rocks, and can be partly to totally filled with secondary minerals.
SEM measurements on square centimeter large areas in granite indicate that the boundaries between feldspars, quartz and biotite are nearly continuously and up to several hundred nanometer open and partly filled with secondary minerals. It is most likely that the boundaries form networks in even larger parts of the granite, which allow fluid flow. The occurrence of newly grown biotite indicates that open grain and phase boundaries are not just a phenomenon in rocks at uppermost crustal levels but can occur at depths of at least 10-15 km. Open and partly filled boundaries do not only control various physical properties of crystalline material and govern its behavior during different natural and technical conditions as well as in experiment. Such boundaries potentially affect the migration of materials even over larger distances in rocks, for example of radionuclides released from nuclear waste in deep geological repositories. Alternatively, fillings of boundaries by secondary minerals increase the absorptive capacity and, consequently, the bedrock's retention capability of fluid-carried materials.