The Calcite-Aragonite Problem1

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Abstract
The fundamental stability relations of calcite and aragonite are considered in the light of (a) calorimetric data, (b) solubility measurements, (c) direct studies of phase changes. The difference in free energies of calcite and aragonite are small; and the effect on stability of grain size, nonhydrostatic stress, and impurities are evaluated. These factors may be as important to polymorph stability as the difference in free energies. Consideration of precipitation reactions involving nucleation and growth indicates that there need be no direct relationship between fundamental stability and the order of appearance of possible phases. Ostwald’s law of stages implies an inverse correlation. Data on the effects of temperature and solution composition indicate that the form of calcium carbonate precipitated may frequently reflect favorable kinetics of aragonite nucleation enhanced by calcite growth inhibition. A basic problem in carbonate diagenesis involves the rate at which metastable aragonite undergoes transformation to calcite. Data on this reaction in the dry state indicate that this process is essentially zero order, but a combination of structure and habit sensitivity may lead to other apparent rate laws. Nucleation normally takes place in the prism zone and proceeds via a linear boundary migration parallel to the c axis. A lack of reactivity of (001) also appears in aqueous reactions. In general, dry rates must be of no significance at the temperatures normally encountered in diagenesis unless deformation is extensive. In aqueous solution the problem is more complex. The rate is a function of volume, pH, Pco2, salt catalysis (general) and salt inhibition (specific), and organic protection as in some fossil aragonite, and aggregate fabric (as in fossils). It is suggested that the formation of calcite from aragonite in aqueous solutions may involve homogeneous nucleation. The slow steps involve continuous nucleation and growth, one or both of which involve calcium and bicarbonate ion activities. Specific inhibition appears to involve inhibition of growth and several explanations are possible, but the ability of the inhibiting species to form an insoluble compound, not miscible with calcite, appears to be critical.

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