Calcite precipitation from aqueous solution: transformation from vaterite and role of solution stoichiometry

The morphology of vaterite precipitated by bubbling CO2 through a CaCl2 solution is framboidal aggregates. It is not possible, even when using the identical experimental setup and conditions, to reproduce aggregates having identical morphology. The density of the aggregates and crystallite size can vary significantly between batches. The differences between batches result in “loosely” “and “densely” packed aggregates, having different specific surface areas. For the aggregates used in this study, a transformation of vaterite aggregates into calcite may occur entirely within the aggregate itself. It was further shown that the transformation rate is not limited by the dissolution of vaterite, in contrast to some reports in the literature of a rate limiting role of vaterite dissolution. These contradicting findings are most probably caused by differences in aggregate density and surface area, of the starting materials used. This illustrates that the initial vaterite morphology can control the transformation rate of vaterite to calcite. Furthermore, it was shown that the transformation can be partly diffusion limited, which has not previously been described in the literature. When the transformation reaches ~60 wt. % of calcite, the transformation rate starts to decrease, because of annealing of the calcite crystallites into larger single crystals. This annealing causes a decrease of the calcite surface area, and, possibly, a change in growth mechanism due to the decreasing surface roughness. Seeded calcite growth experiments were conducted at fixed pH (10.2) and two degrees of supersaturation (Omega = 4, 15), while varying the Ca2+ to CO32- solution ratio over several orders of magnitude. The calcite growth rate and the incorporation of Sr in the growing crystals strongly depended on the solution stoichiometry. At constant degree of supersaturation, the growth rate was highest when the solution concentration ratio, r = [Ca2+] / [CO32-], equaled one, and decreased symmetrically with increasing or decreasing values of r. This behavior is consistent with the kink growth rate theory for non-Kossel crystals, assuming that the effective integration frequencies at kink sites are the same for the cation and anion. The Sr partition coefficient, DSr, ranged from 0.02-0.12, and correlated positively with the calcite growth rate. The effect of the [Ca2+] to [CO32-] stoichiometric coefficient helps explain large variability in calcite growth rate equations proposed in the literature. A logical continuation of the work presented here would be to conduct variable [Ca2+] to [CO32-] ratio experiments at lower pH. The role of solution stoichiometry on calcite dissolution also deserves to be investigated.