Fine-grained quartz aggregates show significant grain growth during high-temperature (700-1000 °C) and high-pressure (400 MPa) deformation experiments when water is present. Flattening of grains that exceeds the deformation-induced flattening and a resultant clear foliation are found in the samples recrystallized at relatively low temperatures and high strain rates (i.e., high deviatoric stresses). The distribution of water as inferred from the distribution of pore space is also anisotropic in these samples. In contrast, the samples recrystallized at relatively high temperatures and low strain rates (i.e., low deviatoric stresses) show small grain flattening and almost isotropic water distribution. The observed very large flattening under high stress indicates a significant contribution of anisotropic grain growth. Because the anisotropy of grain growth is correlated with the anisotropy of water distribution, we suggest that the observed foliation under high stress is due to anisotropic grain growth caused by enhanced mass transport through water. Foliation caused by this mechanism will be related to the stress orientation.

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