DISSOLUTION AT DISLOCATION ETCH PITS IN QUARTZ

Abstract

Several samples of quartz were etched hydrothermally at 300°C in etchams of controlled dissolved silica concentration in order to measure the critical concentration, Ccrit, above which dislocation etch pits would not nucleate on the quartz surface. Ccrit for 300°C was theoretically predicted to be 0.6C0 and the measured Ccrit, was 0.75 ± 0.15C0 (C0 is the equilibrium concentration). Above this value, some dislocation etch pits formed, but the rate of formation significantly decreased. These results are the first experimental validation of etch pit formation theory under hydrothermal conditions. Dune sands showed a generally angular and pitted surface when etched in dilute solutions, while sands etched at C ~ Ccrit showed less angular pitting. Analysis of a soil profile developed in situ on the Parguaza granite, Venezuela, revealed a gradual change from angular, pitted grain surfaces at the top of the profile to rounded surfaces on grains sampled just above bedrock. Since quartz dissolution without surface pitting continues deep in the profile, the Si concentration must exceed Ccrit, at depth. These results indicate that for C > Ccrit, dissolution occurs at edges and kinks on the surface of quartz and very few pits form; in contrast, at C ≪ Ccrit, dislocation etch pits grow rapidly, contributing to the overall dissolution rate.