Calcite precipitation mechanisms and inhibition by orthophosphate: In situ observations by Scanning Force Microscopy

Abstract

Scanning Force Microscopy (SFM) was used to observe near-equilibrium calcite growth processes in solutions of known composition and saturation state. Calcite seeds were reacted in solutions of known saturation state at 25°C and 0.96 atm P CO 2 for 1–2 days before transferring to a SFM fluid cell for observations of continued growth in the same solutions. We observed that when solution saturations with respect to calcite were greater than 1–2, precipitation began with the formation of surface nuclei. These nuclei spread, coalesced, and continued growing. Only after nearly two hours was there a transition to a mechanism resembling spiral growth. At these long reaction times, migrating steps assumed individual heights of 1–2 monolayers. We also observed simultaneous growth and dissolution at undersaturated conditions very near equilibrium. The influence of phosphate was also examined and observations suggested two inhibition mechanisms, depending on surface history. Phosphate (6 and 10 μmol PO 4) introduced during the nucleation stage results in the formation of nuclei with amorphous shapes. Phosphate introduced during layer growth disrupts the relatively straight steps produced during PO 4-free growth to form jagged steps. Both of the phosphate-calcite surface interactions are consistent with mechanisms proposed in previous studies. Our findings suggest that when solution saturations with respect to calcite are greater than two, precipitation always begins with the formation of surface nuclei with a later transition to mononuclear growth mechanisms. These observations have implications for carbonate precipitation in natural systems and suggest that calcite growth in environments with frequent wetting and drying cycles begin each wetting event with precipitation by surface nucleation. Results of this study also suggest that experimental investigations of calcite precipitation kinetics and interpretations of growth mechanisms must account for this early stage contribution by nucleation. Otherwise, such rates of calcite growth may not reflect the overall slower rates that occur in continuously wet environments.