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

Dissolution kinetics at the aqueous solution-calcite ( 1 0 1 ¯ 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 Mg 2 + displayed negligible inhibitory effects on calcite dissolution even at concentrations of 10 - 4 molal ( m ) . Upon the introduction of 10 - 3 m Mg 2 + , the solution saturation state with respect to calcite, Ω calcite ( = a Ca 2 + · a CO 3 2 - K sp ( calcite ) ) , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω calcite < 0.2 , whereas a sudden inhibition from Mg 2 + was activated at Ω calcite ⩾ 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 - 3 m Mg 2 + 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 Mg 2 + .