Kinetics aragonite, calcite solubility

Plummer. L.N. and Wigley, T.N.L. Critical review of the kinetics of calcite dissolution and precipitation, in Jenne, E.A., ed., "Chemical Modeling. Speciation, Sorption, Solubility and Kinetics in Aqueous Systems," Amer. Chem. Soc. Symp. Series, Washington, D.C. (this volume), de Kanel, J. and Morse, J.W. The chemistry of orthophosphate uptake from seawater onto calcite and aragonite, Geochim. Cosmochim. Acta 1335-1340 (1978). 

Mucci A., Canuel R. and Zhong S. (1989) The solubility of calcite and aragonite in sulfate-free seawater and the seeded growth kinetics and composition of precipitates at 25°C. Chem. Geol. 74, 309-329. 

Marine carbonate minerals have both biotic (dominant) and abiotic (minor) sources. Their formation is often controlled by kinetic factors or biomediated processes in organisms. Surface seawater is most highly supersaturated (the ion activity product (lAP) is much greater than the solubility product) with respect to dolomite ( 50X), followed by pure calcite ( 6 X), then by aragonite ( 4x). It may be close to... 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

In defence of his solubility results Berner makes two arguments with which we would like to take issue. First he states that since he calculated the solubility of calc it e in sea water from measurements of the solubility of aragonite in sea water and the ratio of the solubility of calcite to aragonite as measured in fresh water that he avoids the serious kinetic effects associated with calcite in sea water. Whereas we do not challenge the legitimacy of Berner s approach, we do challenge his contention that direct measurements of the solubility of calcite in sea water cannot be made because of kinetic effects which are not shared by aragonite. In this connection we make four observations. 

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