The precipitation of minerals from solution is a major process controlling the chemical composition of natural waters. In order to apply empirically-derived mineral growth laws to natural environments, it is necessary to account for solution compositional effects on the growth kinetics, as well as the effects of particulate processes (e.g., aggregation and agglomeration) on the reactive mineral surface area.
In this study, seeded fluorite (CaF2) growth experiments were performed at different ionic strengths, using a constant
addition method. Fluorite was chosen because of its fairly simple chemistry and crystal morphology. During experiments, the crystal size distributions were monitored by photon correlation spectroscopy (PCS), a laser-based particle-sizing technique. Scanning electron microscopy was used to obtain an independent measurement of the size distribution and to observe possible changes in morphology. By varying the fluorite seed concentration, growth rates were measured with and without aggregation taking place, hence allowing us to differentiate between the chemical effect of ionic strength and changes in reactive surface area due to aggregation.
Fluorite growth was found to follow a second-order rate law, consistent with a spiral growth mechanism where the rate is controlled by the integration of Ca2+ ions into surface lattice positions (kink sites). Aggregation does not affect the growth rate constant or the effective reaction order, at the fairly low relative degrees of supersaturation studied (0.2 - 1.5). However,
the growth rate constant decreases with increasing ionic strength. The effect is quantitatively explained by applying Transition State Theory to the lattice integration step of Ca2+, in combination with the Debye-Hückel theory of ion activities and a simple structural model for anionic kink sites. The results imply that the ionic strength dependence of the growth kinetics of salt-type minerals can be used to infer the charge characteristics of reactive surface sites.