Mechanisms of metal ion sorption on calcite: Composition mapping by lateral force microscopy

We show that lateral force microscopy (also known as frictional force microscopy) can differentiate between substrate and overlayer phases during an inorganic surface reaction. A calcite substrate is imaged in situ, while immersed in aqueous solutions of pH ∼ 6 9 containing metal ions (Cd²⁺, Sr^2-, and La³⁺) at concentrations of 10^-5 to 10^-3 M. Cd²⁺ and Sr^2- passivate surface steps, initiating overgrowth only in solutions already supersaturated relative to their respective carbonates. In contrast, La³⁺ initiates overgrowth even in undersaturated conditions and carries the reaction to completion by scavenging carbonate anions directly from the dissolving calcite surface. Monomolecular surface steps play a central role, serving as both dissolution sites for the substrate and nucleation sites for the overgrowth.