THE EFFECT OF DISLOCATION DENSITY ON THE DISSOLUTION RATE OF QUARTZ

In order to determine the effect of crystal dislocations on quartz dissolution rates, the dissolution rates of i) synthetic quartz with a dislocation density of <105 cm-2 and ii) plastically deformed quartz from the same crystal with a dislocation density of ~5 × 1010 cm-2 were measured in distilled water at 80°C and in 0.2 M HF at 22°C. All of the experiments were conducted in highly undersaturated solutions. The high dislocation density in the deformed sample dramatically increased the extent of etch pit development during dissolution in 0.2 M HF. However, the dissolution rates of the high and low dislocation density quartz were indistinguishable in both distilled water at 80°C and in 0.2 M HF at 22°C. This indicates that the dissolution from etch pits formed at dislocations is a small contribution to the overall quartz dissolution rate, and implies that dislocations are not an important factor in controlling quartz dissolution rates in highly undersaturated geochemical environments such as soils. Theoretical arguments indicate that dislocations have a larger potential for accelerating dissolution rates in solutions near equilibrium, and this may be an important factor in the kinetics of many diagenetic and metamorphic reactions. © 1990.