Hydrolytic Weakening of Quartz and Other Silicates*

Abstract

Rocks at high pressure and temperature become weak when exposed to an aqueous environment. Four different weakening processes have been found: (1) Increase in pore pressure reduces the effective confining pressure, thus negating the strengthening effect of confining pressure (Terzaghi). (2) Penetration of water into the intergranular boundaries reduces the cohesive strength. This is believed to have occurred in the dehydration weakening found in gypsum (Heard & Rubey) and in serpentinite (Raleigh & Paterson). (3) Water promotes recrystallization, greatly reducing the strength at low strain-rates at temperatures above the critical temperature for recrystallization. (4) Water permeates most silicate crystals so far tested and causes hydrolytic weakening of the component crystals of rocks. The most revealing experiments on hydrolytic weakening have been those on single crystals of synthetic quartz with varying water content (Griggs & Blacic). The water content inferred from 3 micron infra-red absorption varies from 0·0015 to 0·13 wt% (100–9000 H/106 Si) in the nine crystals tested. All crystals became weak at a critical temperature dependent on the water content, varying from 380°C at 0·13% to 1070 °C at 0·0015%. The weakening process is thermally activated, is reversible, and is rate dependent. The mechanism of deformation is intracrystalline glide. MLaren has shown by transmission electron microscopy that deformed weakened synthetic quartz contains dislocation densities similar to those found in strong dry quartz deformed in the ductile regime. Annealing of water-weakened quartz removes most of the dislocations and causes the formation of bubbles. These are believed to be water bubbles formed by the reaction Si(OH)4→ SiO2+2H2O, the water migrating through the lattice. The Frank-Griggs hypothesis of hydrolytic weakening is that easy slip occurs only when Si-O-Si bridges adjacent to a dislocation are hydrolyzed by the migration of water. No Si-O bonds need be broken, for dislocation motion can occur by hydrogen bond exchange. All the evidence in hand suggests that hydrolytic weakening is general in silicates containing very small amounts of water. This phenomenon is expected to be important in the mantle, leading to a rate-dependent strength.