Examining the Effect of Water on the Strength of Quartz Using Polycrystalline Quartz Aggregates

Abstract:

Quartzite rheology has been extensively studied to model the strength of continental crust. Previous studies have shown that the presence of water in fluid inclusions weakens polycrystalline quartz, and this weakening is usually related to water fugacity. However, no attempt has been made to determine the effect of water content on the strength of quartz.

We have deformed hot-pressed quartz aggregates with low water contents at a pressure of 1.5 GPa, a temperature of 1200°C, and strain rates of 10^-4 to 10^-6/s. Fine synthetic quartz powders were hot-pressed at 1120°C and 1.5 GPa for 24 hours to create quartzites with a grain size of ~20 microns and water contents of <150 H/10⁶Si. The water band in FTIR spectra collected from the hot-pressed quartz aggregates is similar to the broad water band observed in natural quartzites (i.e. free water in fluid inclusions) rather than the spectra observed in synthetic quartz crystals. Results of deformation experiments indicate that the strain rate sensitivity of the strength of these quartz aggregates is consistent with deformation by dislocation creep (n~3.5). Microstructures observed in samples from these experiments include undulatory extinction, flattened grains, and bulging grain boundaries, which are also consistent with dislocation creep.

The strength of these quartz aggregates deformed with low water contents (<150 H/10⁶ Si) are an order of magnitude greater than the strengths predicted by polycrystalline quartz flow laws derived from quartzites with high (>2000 H/10⁶Si) water contents. Our results indicate that quartz strength is dependent on water content, in addition to being dependent on water fugacity.