Dissolution kinetics of experimentally shocked silicate minerals

Abstract

The effect of lattice strain on mineral dissolution rates was examined by comparing the dissolution rates of shocked and unshocked minerals. Labradorite, oligoclase and hornblende were explosively shocked at mean pressures ranging from 4 to 22 GPa. The labradorite was examined with transmission electron microscopy to estimate the density of dislocations produced by the shock-loading experiment. Subsamples of the labradorite were then thermally annealed to remove some of the dislocations, and to evaluate the importance of such thermal pre-treatment in preparing mineral surfaces for experiments. The dissolution rates of these minerals were measured in batch experiments at pH-values of 2.7 and 4.0. Shock-loading did not produce extremely high dislocation densities in the labradorite. The density of dislocations in the unshocked labradorite is ≤ 10 10 m −2. After shocking, the density increases to ∼ 10 12-10 13 m −2. The distribution of dislocations is heterogeneous, and the amount of deformation does not increase substantially with shock pressure. These results are highly atypical of shock-modified minerals, where relatively low shock pressures usually induce high (∼ 10 15 m −2) densities of dislocations. Thermal annealing for 1 hr. at 900°C in a dry furnace removes many dislocations from the shocked labradorite. The difference in observed dissolution rates between shocked and unshocked minerals appears to have a weak correlation with the increase in the density of dislocations on the mineral surface. The unshocked and shocked oligoclase and hornblende samples exhibit limited dissolution enhancement at pH 4.0. Increasing the density of dislocations by several orders of magnitude with shock-loading causes a relatively small increase in dissolution rates for these silicate minerals. These results suggest that the surface dislocations produced by the shock treatment are not the primary sites for dissolution reactions.