The role of dislocations and surface morphology in calcite dissolution

We have measured the dissolution rate for undeformed (ρ ~ 10³ dislocations · cm^-2) calcite to be 3.1 × 10^-10 mol · cm^-2 · s^-1 in free-drift rotating disk experiments at 1160 rpm, 25°C and pH 8.6 in 0.7 M KCl solution far from equilibrium. The rate increased by a factor of ~2.3 for a strained sample (ρ = 6 × 10⁸ · cm^-2). Dissolution rates of calcite far from equilibrium were observed to depend on surface preparation and surface morphology resulting from defects outcropping at the crystal surface, but large rate increases due to surface roughness were not observed. The high dissolution rate for mechanically polished surfaces is attributed to enhanced dissolution at cracks and dislocation loops produced in the grinding process. The initial dissolution rate for cleaved surfaces depends on the surface morphology, but reaches a reproducible steady state value when a constant bimodal size distribution of intersecting pits with time-independent wall slope (θ ~ 3°) is achieved (t > 3 h). Steady state is also characterized by a constant ratio of sloped to relatively flat surface. The two populations of etch pits consist of abundant, short-lived, small etch pits (attributed to nucleation at impurity or point defect clusters) and long-lived, larger point-bottomed pits (attributed to dislocations). Consistent with this interpretation, significant dissolution at an abundance of nondislocation nucleation sites in undeformed calcite explains the relatively small increase in dissolution rate for strained samples. Simulation of bulk crystal dissolution based on etch pit growth rates is in reasonable agreement with observed dissolution rates and surface morphology and indicates that discontinuous dissolution at dislocations is necessary to explain the observed steady state surface morphology. Activation energies for pit deepening and widening were measured between 5 and 50°C as 27 ± 5 and 37 ± 3 kJ · mol^-1, respectively. These values are lower than the measured activation energy for bulk dissolution (59 ± 12 kJ · mol^-1).