Synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder, impure silica gel, and high‐purity silicic acid by hydrothermal isostatic pressing at 300 MPa. The resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests. The flow stress at 1200–1300 K was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity. It is questionable that the natural‐quartz‐origin specimens were fully equilibrated in respect to the activity of the water, but equilibration has probably been achieved in the amorphous‐silica‐origin specimens, which crystallized under the experimental conditions. In the deformation of the latter materials, the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kJ mol−1 for both. However, the impure gel‐origin material gave a stress exponent of about 2.3, whereas the high‐purity silicic‐acid‐origin material gave a stress exponent of about 4, in spite of the grain sizes being about 90μm and 20 μm, respectively. It is concluded that while the impure specimens have higher intragranular strengths, there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high‐purity specimens.

The problems associated with extrapolating laboratory measurements on quartzite rheotogy to geological conditions are discussed, with special reference to the question of equilibration with respect to the effect of water. Some new results on synthetic specimens crystallized from wet amorphous silica are presented in the expectation that water equilibration is more closely attained in these than in natural quartz specimens. These results and earlier ones from the literature are collated and used to arrive at limits on quartzite flow strength under geological conditions.

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In 1965 Griggs and Blacic [1,2] proposed that there is a “hydrolytic weakening” process in quartz and silicates whereby the breaking of Si-O bonds, involved in the movement of dislocations, is facilitated by the presence of water. This proposal aimed to explain the observed dramatic weakening of quartz crystals when they are exposed to water in tests at high temperature, as well as the observed strong contrast in creep strength between dry natural quartz crystals and rapidly-grown synthetic quartz crystals containing traces of water. Such a “hydrolytic” process may also underlie the observed effects of water in accelerating other phenomena such as self-diffusion of oxygen in quartz, aluminium-silicon ordering in feldspars, slow crack propagation in silicates, and recrystallization in quartz. A review of this field is given in Reference 3.