Creep deformation of polycrystalline quartz in controlled chemical environments

Creep deformation of silicate materials in different chemical environments, is of paramount importance in practical engineering applications and geotectonic evolution. Over 100 circular cylinders of a polycrystalline quartz have been deformed at constant differential stresses σ of 100–1000 MPa, temperatures T of 600–900 °C, and a confining pressure of 1500 MPa using a soft solid medium apparatus. Oxygen, water, and hydrogen fugacities (fO₂, fH₂O, fH₂) were controlled over wide ranges by a solid oxygen buffering technique. Under the experimental conditions, three different creep regimes were identified, based on mechanical data and microstructural observations: high temperature and low stress regime with a stress exponent n = 1, high temperature and high stress regime with n = 2.4, and low temperature regime with n = 3. The apparent activation energies for the n = 1 and n = 2.4 regimes were about the same (101 ~ 131 kJ/mol), but much smaller than that for the n = 3 regime (214 kJ/mol). Chemical environment had an effect on creep in all regimes. Creep rate had dependences upon \({(f\text{H}_{2}\text{O})}^{0.41}\), \({(f\text{H}_{2}\text{O})}^{0.24}\), and \({(f\text{O}_{2})}^{-0.27}{(f\text{H}_{2}\text{O})}^{1.83}\) in the n = 1, n = 2.4, and n = 3 regimes, respectively. Observations of strong grain flattening and widespread dislocation substructures led to the conclusion that deformation in all three regimes was dominated by dislocation creep processes. Specifically, it is suggested that the n = 1 behavior observed in all buffered environments may result from the operation of Harper-Dorn creep that has been demonstrated for many engineering materials. The identification of different creep regimes, especially Harper-Dorn creep, in quartz will have important implications for engineering and geotectonic applications of silicate materials.