Effects of loading path on the shock metamorphism of porous quartz: An experimental study

Samples of porous quartz with a density of 1.42 cc/g were experimentally shock loaded via a variety of direct and reflected loading paths using the single stage powder gun at Tokyo Institute of Technology. Loading paths in the sample material were varied by using materials with a range of shock impedances as the sample containers. For example Polyethylene has a matched impedance with the sample material and produced a single shock-loading path in the sample. Stainless steel, a traditionally used sample container material, has a much higher shock impedance than the porous quartz, with the result that peak pressure in the sample is reached via a sequence of shock reflections. Shock compression is an irreversible process. There is a net gain in internal energy of the specimen as a result of shock compression and release to ambient pressure. The magnitude of this gain in net internal energy depends on loading path. Samples shocked to the same peak pressure via different loading paths can have very different internal energy gains. Shock conditions in the samples were characterized by calculated loading path and peak pressure, peak pressure duration and calculated net internal energy increase. The shock features formed by shock metamorphism were examined using optical microscopy. X-ray diffraction and transmission electron microscopy. A number of shock metamorphic features were found to be sensitive to loading path rather than to peak shock pressure. That is, the features were evident at relatively low peak pressures in samples that experienced a large increase in net internal energy, whereas much higher peak pressures were required to produce the same features in samples that experienced smaller increases in net internal energy. These loading path-sensitive features include PDF formation parallel to the {l012} plane, spatial density of PDFs, appearance of crystalline material within PDFs, presence of amorphous silica at grain boundaries, and presence of amorphous silica with grains. There is a strong indication that the pressures of formation of other orientations of PDFs and 'ladder structure' PDFs are also dependent on the net internal energy increase of the sample. These results imply that it may not be appropriate to use shock pressure "calibrations" determined in laboratory reflected shock loading experiments to evaluate the pressure of samples from natural impact craters