The effect of dislocation density on the aqueous solubility of quartz and some geologic implications: A theoretical approach

Abstract

The internal energy associated with dislocations is calculated in quartz as a function of dislocation density. These calculations show that at dislocation densities of 109, 1010, 1011, and 1012 cm dislocation/cm3 the energy stored per mole of quartz is 2.2, 19.7, 170, and 1426 J, respectively. The density of free dislocations in natural quartz rarely exceeds 109 line cm/cm3, and the resulting increase in free energy is probably too small to affect any geochemical processes significantly. This applies to the overwhelming volume of rocks in the earth's crust. The density of dislocations in tangles may exceed 1011 line cm−2 which is significant, but the volume of quartz containing tangles is a very small fraction of most quartz grains. Nevertheless, where fluid/rock ratios are low, the aqueous activity of SiO2 may be significantly increased by the dissolution of quartz containing dislocation tangles. Such dissolution could be geologically significant in active greenschist facies fault zones and mylonites where repeated cold working would produce a sustained source and large population of dislocation tangles. In such a setting the locally high internal energies of quartz containing dislocation tangles would lead to strain solution. The resulting high activity of aqueous silica would enhance the stability of feldspars and would establish chemical potential gradients of SiO2 which would drive the diffusive mass transfer of SiO2 out of the fault zone.