A mechanistic model for calcite crystal growth using surface speciation

A new mechanistic model for the crystal growth kinetics of calcite is prese nted, accounting for the presence of various surface complexes. Calcite cry stal growth rates were determined with the constant composition method at O mega(c) (calcite supersaturation) values of 1.5-9.8. In general the rate in creases with Omega(c), but variations in CO2 partial pressures and the (CO3 2-)/(Ca2+) ratio also have a major effect on the crystal growth rate. These effects are eliminated by assuming that calcite crystal growth proceeds th rough three reversible reactions, in which CaCO30(aq) and Ca2+(aq) are inco rporated at specific surface complexes. The model derived rates closely fol low the experimental rates over the entire experimental range (r = 0.996, n = 23). The obtained rate constants indicate that CaCO30(aq) is approximate to 20 times more reactive than Ca2+(aq) at the calcite-water interface. Th is agrees with the fact that dehydration of metal ions precedes crystal gro wth and, in analogy with other metal-ligand complexes, the CO32- ligand wil l increase the rate of water exchange of Ca. This model is a modified versi on of a rhodochrosite crystal growth model (Sternbeck, 1997) which allows f or the comparison of reaction mechanisms and rate constants. The rate const ants for incorporation of CaCO30(aq) at the mineral surface are 55 to 270 t imes higher than for MnCO30(aq). This difference can not likely be explaine d by the water exchange rates, but may be due to the fact that ligand excha nge mechanisms for Ca and Mn differ. Copyright (C) 1999 Elsevier Science Lt d.