Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening
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Quartz aggregates were selected from a large collection of quartzites and novaculites on the basis of purity and grain size. Comparisons of the experimentally determined grain size effects for quartz aggregates with models of grain boundary hardening and weakening lead to the following conclusions: Grain boundary hardening by dislocation-grain boundary interactions does not appear to be important for the experimental conditions tested (grain sizes of 1. 2-211 mu m, temperatures of 700 degree -1100 degree C at a strain rate of 10** minus **6/s, and water treatments ranging from vacuum-heated to as is to water-added); no direct evidence has been found for grain boundary sliding at the experimental conditions (grain sizes down to 1. 2 mu m, temperatures up to 1200 degree C at a strain rate of 10** minus **6/s); a dependence of strength on grain size can be produced if quartz grains are initially at disequilibrium with respect to water and diffusion of water is slow relative to the deformation rates; and the localization of shear observed along mylonite zones may, under certain conditions, be promoted by fluid infiltration and bulk crystal diffusion of water into the finer-grained rocks.
