Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite (101¯4) interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg2+ displayed negligible inhibitory effects on calcite dissolution even at concentrations of 10-4molal(m). Upon the introduction of 10-3mMg2+, the solution saturation state with respect to calcite, Ωcalcite(=aCa2+·aCO32-Ksp(calcite)), acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ωcalcite<0.2, whereas a sudden inhibition from Mg2+ was activated at Ωcalcite⩾0.2. The presence of the Ω-switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with 10-3mMg2+ exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg2+.