Variation in calcite dissolution rates:: A fundamental problem?

A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10^−10.95 mol/cm²/s (PCO₂ 10^−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10^−11.68 mol/cm²/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10^−12.4 mol/cm²/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm⁻¹.