Mechanical properties and recrystallization of quartz in presence of H<inf>2</inf>O: Combination of cracking, subgrain rotation and dissolution-precipitation processes

Abstract

Natural quartzite samples, as-is and with 0.1 wt% of added H₂O, have been deformed up to ∼30% bulk strain in axial shortening experiments with constant strain rate of ∼10⁻⁶ s⁻¹ at 900 °C and 1 GPa, and in strain rate stepping ∼10⁻⁵ to ∼10⁻⁷ s⁻¹ at 900 °C and 1–1.5 GPa, in order to investigate the role of H₂O in deformation and recrystallization of quartz. H2O-added samples showed ∼30 MPa lower mean strengths than as-is samples. Samples weaken slightly after 15% strain with mean flow stresses in the range of 154–227 MPa, and stress exponent (n) values between 1.45 and 2.13. The original quartz grains have been deformed plastically (dislocation glide). Discrete mode I cracks without detectable offset have developed in addition to plastic strain. Deformation was associated with recrystallization of up to 20% of the material in the most deformed parts of the samples. New grains were nucleated by both cracking and subgrain rotation, and were largely reconstituted by H₂O-promoted grain boundary migration, related to dissolution-precipitation processes. This reconstitution of material is documented by a change in luminescence to blue, caused by trace elements exchange in quartz structure. The blue luminescence is prominent along healed cracks and high angle grain boundaries while it was not observed along the low angle boundaries formed by subgrain rotation. Compared to the as-is samples, the crack-related recrystallization is more frequent in the H₂O-added samples. The low stress exponent values may indicate dissolution-precipitation and grain boundary sliding processes to accommodate incompatibilities at grain boundaries arising from an insufficient number of active slip systems. We suggest that the ubiquitous presence of H₂O in nature may promote recrystallization of quartz by combinations of cracking, dislocation glide and creep and dissolution-precipitation processes.