Rapid stress release caused by polymorphic transformation during the experimental deformation of quartz

Cylindrical samples of polycrystalline quartz were fabricated by hot pressing of crushed Brazilian quartz at 1100 and 1200 degrees C, then deformed in extension at 1200 degrees C at a confining pressure of 300 or 350 MPa. After initial plastic yield, samples failed with sudden and total offloading. Samples could be reloaded in compression and deformed plastically to large strains. Microscopic examination showed extensional failure had occurred with the formation of 'veins', oriented normal to the extension direction, containing fibers of silica with their length and c crystal axes parallel to the extension direction. From X-ray and microstructural observations, these are inferred to have formed as mixed tridymite/cristobalite reaction products, now reverted to quartz, accommodating offloading owing to the 38% extensional strain parallel to c. Such rapid transformation and offloading can be compared to processes invoked to explain deep-focus earthquakes in appropriate rock types, but also to the slower formation of oriented, localized reaction products in crustal shear zones.