Calcite Growth Kinetics: Dependence on Saturation Index, Ca2+:CO32- Activity Ratio, and Surface Atomic Structure

It is becoming increasingly clear that the rate of crystal growth, even at constant saturation, varies with pH, ionic strength, and solution stoichiometry. Here, we contribute to the limited data set on experimentally obtained calcite step velocities from solutions with strictly controlled parameters. We measured growth on obtuse and acute edges in solutions with five Ca2+:CO32– activity ratios (r): 0.1, 1.0, 10, 25, and 50, at three saturation indices: SI = 0.6, 0.8, and 1.0. The curve describing rate as a function of r is not centered at r = 1, and the maximum velocity is different for each step. Obtuse steps generally grow faster in Ca2+-rich solutions, i.e., r > 1, whereas the acute rates dominate when CO32– is in excess, i.e., r < 1. We show that the local arrangement of every second carbonate at the acute steps can help explain these rate differences. To further analyze the differences in growth rates, we fitted four currently used growth models to our data set, to derive growth parameters for each type of step. Some of the models worked for some of the conditions, but none could describe the results over the full range of our experiments