The effects of the α‐β phase transformation on the creep properties of hydrolytically‐weakened synthetic quartz

Abstract

Nine rectangular prisms of hydro‐thermally‐grown synthetic quartz crystals with 900 atomic ppm H + were loaded in compression at 1400 bars stress and temperatures between 403 and 764°C. The a and c directions were at 45° to the compression direction, and the slip system appears to operate over the entire range of temperatures. The strain vs. time curves were sigmoidal in shape; an incubation stage of accelerating creep rates was followed by a hardening stage in which creep rates decreased with time. The creep rate (⋵) vs. temperature (T) data were found to fit very well an Arrehenius law of the form where E* is the activation energy for creep. E* is 38.9 ± 2.1 kcal/mole in the α field and 14.3 ± 4.7 kcal/mole in the β field. An offset in the maximum strain rates occurs at the transition temperature, with the β phase creeping at less than half the rate of the α phase. It is suggested that the changes in creep parameters are due to the effects of the inversion on the rates of oxygen diffusion associated with the dissolved water, which hydrolytically weakens the crystals. The α‐β transformation involves rather subtle structural changes. No bond breaking occurs and the change in density is rather small. The transformations responsible for the transition zone of the mantle are reconstructive (involving extensive bond breaking and complete atomic reorganization) and the density changes are large. Thus the potential for large effects of these transformations on creep properties is even greater.