Self-limiting growth on dolomite: Experimental observations with in situ atomic force microscopy

In situ Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) studies on dolomite (101̄4) were performed during exposure to supersaturated aqueous solutions (supersaturated in dolomite, calcite, aragonite, vaterite, huntite and magnesite) at pH = 9 at various Ca2+/Mg2+ aqueous ion activity ratios. At high saturation ratios, rapid growth of a single layer (∼3 Å thick) of a carbonate followed by much slower growth of a second layer was observed. Growth of the second layer was highly inhibited, suggesting that the first layer was essentially self-limited, and inhibited further layer-by-layer growth. The growth of the first layer was observed over a wide range of Ca2+/Mg2+ ratios, suggesting that the dolomite surface is favorable to formation of a range of Ca-Mg carbonates. LFM data revealed contrast in the tip-surface frictional forces on the first grown layer, but this contrast was only observed in layers grown from middle to high Ca2+/Mg2+ solutions. Thus, LFM may have detected or responded to differences in the structure and/or composition between the first layer relative and the dolomite substrate. Dissolution of the first layer occurred from significantly supersaturated solutions relative to ordered stoichiometric dolomite permitting an estimate of the excess interfacial strain energy of up to 10 mJ/m2.