ATOMIC-SCALE IMAGING OF CALCITE GROWTH AND DISSOLUTION IN REAL-TIME

We present a new experimental technique for real-time observation of aqueous mineral growth and dissolution at the atomic scale using an atomic-force microscope (AFM) equipped with a flow-through fluid cell. We applied this technique to observe changes in surface topography on the (1014BAR) cleavage plane of calcite during alternating episodes of growth and dissolution. Growth occurred in a layer-by-layer fashion by the forward motion of monomolecular steps (0.3 +/- 0.1 nm high) lying parallel to the edges of the cleavage face. Under all conditions studied, the velocities of positive [481BAR] and [441BAR] steps were the same; velocities of negative [481BAR] and [441BAR] steps were undetectably small, less than 0.1 nm s-1. Steps were straight passing above perfect crystalline material, but roughened into two-dimensional dendrites above defective material. Dissolution nucleated shallow (< 5 nm deep) etch pinholes in defective material and faceted existing surface voids into > 90-nm-deep rhombic etch cores. Growth into these etch cores was impeded so that steps moved around them. AFM images of the surface atomic structure revealed rows of atoms along [010] spaced by 0.39 +/- 0.05 nm with a periodicity along the rows of 0.43 +/- 0.05 nm.