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 similar to 6-9 containing metal ions (Cd2+, Sr2+, and La3+) at concentrations of 10(-5) to 10(-3) M. Cd2+ and Sr2+ 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.