Grain-boundary migration of quartz during annealing experiments at high temperatures and pressures, with implications for metamorphic geology

Annealing experiments on agate, quartz schist and metachert at 800–1000 °C, confining pressures of 400 and 800  MPa, and annealing times of 6.0×10–3.6×10⁵s revealed normal grain growth of quartz in the agate, grain-size increasing with time, but the rate of grain growth decreasing with increasing grain size.
The boundaries of agate with quartz schist and metachert migrated into the agate at the expense of fine-grained quartz in the agate. The driving force for the migration appears to be the reduction of surface energy associated with removal of fine-grained quartz in the agate. Assuming the activation energy as Q _m =11  kcal mol^?1, a general expression for the relationship between velocity of boundary migration ( V   ) and driving force ( P ) is where γ is the specific surface energy of quartz, R is the gas constant and T  is the absolute temperature. The velocity is relatively fast at high temperatures on a geological time-scale. The expression assists a quantitative understanding of the microstructural development of quartz at metamorphic conditions.