Abstract
Until recently the influence of solution stoichiometry on calcite crystal growth kinetics has attracted little attention, despite the fact that in most aqueous environments calcite precipitates from non-stoichiometric solution. In order to account for the dependence of the calcite crystal growth rate on the cation to anion ratio in solution, we extend the growth model for binary symmetrical electrolyte crystals of Zhang and Nancollas (1998) by combining it with the surface complexation model for the chemical structure of the calcite–aqueous solution interface of Wolthers et al. (2008). To maintain crystal stoichiometry, the rate of attachment of calcium ions to step edges is assumed to equal the rate of attachment of carbonate plus bicarbonate ions. The model parameters are optimized by fitting the model to the step velocities obtained previously by atomic force microscopy (AFM, Teng et al., 2000; Stack and Grantham, 2010). A variable surface roughness factor is introduced in order to reconcile the new process-based growth model with bulk precipitation rates measured in seeded calcite growth experiments. For practical applications, we further present empirical parabolic rate equations fitted to bulk growth rates of calcite in common background electrolytes and in artificial seawater-type solutions. Both the process-based and empirical growth rate equations agree with measured calcite growth rates over broad ranges of ionic strength, pH, solution stoichiometry and degree of supersaturation.
Highlights
The effects of physico-chemical parameters, including temperature, pressure, pH, ionic strength, degree of supersaturation and the presence of inhibitors, on the precipitation of calcite have been studied extensively
A variable surface roughness factor is introduced in order to reconcile the new process-based growth model with bulk precipitation rates measured in seeded calcite growth experiments
Key model parameters are calibrated by fitting the model to a selected set of step velocities on calcite surfaces measured by atomic force microscopy (AFM)
Summary
The effects of physico-chemical parameters, including temperature, pressure, pH, ionic strength, degree of supersaturation and the presence of inhibitors, on the precipitation of calcite have been studied extensively (see for example the review by Morse et al, 2007). Larsen et al (2010) and Stack and Grantham (2010)investigated the effect of raq on the step velocity of a calcite growth hillock using atomic force microscopy (AFM) Both groups observed a strong dependency of the velocities on solution stoichiometry. We derive a new process-based growth model for calcite, by combining the surface structural model for divalent metal carbonates of Wolthers et al (2008)(Table 1) with the growth model for binary electrolyte crystals of Zhang and Nancollas (1998) The latter authors showed that the growth rate of a binary crystal from aqueous solution cannot be defined solely in terms of the ion activity product, but is a function of the solution activity ratio. The Z&N model is extended here by explicitly accounting for the different molecular configurations of surface sites at the carbonate mineralaqueous solution interface
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