Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 24 Part 2: 57-73 - July 1962

A publication of the National Speleological Society

The Aragonite-Calcite Problem
Rane L. Curl

Abstract

Aragonite is about 16 percent more soluble than calcite and hence unstable with respect to it. However, both are stable with respect to the ions in solution during precipitations when the supersaturation with respect to calcite exceeds the solubility of aragonite. Under this condition, the relative rates of crystal nucleation and crystal growth determine which polymorph will dominate. In the literature may be found a great deal of information on how temperature, trace impurities and rate of precipitation promote one or the other polymorph, but there is essentially no information available on the mechanisms of nucleation and growth. Most conclusions are inferential.

It is proposed here that aragonite occurs much more frequently than is usually believed and that while aragonite is never stable with respect of calcite, unless the latter is only present as minute crystals, various factors can produce an apparent or kinetic stability which is responsible for the precipitation and preservation of aragonite crystals. The calcite-aragonite problem is discussed in the light of the Theory of Crystal growth. It is suggested that the effects of foreign ions and molecules are due to interactions on both aragonite and calcite with the origin and spreading of the crystal growth steps. While growth layers and spirals arising from screw dislocation in the crystal lattices have never been observed on calcite or aragonite, they are probably responsible for the growth of these materials as they are for other crystals. In this view strontium, magnesium and other ions and molecules may induce the precipitation of aragonite or calcite either by providing isomorphous nuclei, by absorbing on either calcite or aragonite growth steps and modifying relative growth rates, or by causing lattice distortions after adsorption which lead to the creation of the dislocations which aid growth. Temperature modifies the rates of all these mechanisms and, in addition, probably plays some primary role involving solvation of the surface or motion of dislocations.

Further understanding of the aragonite-calcite problem now rests upon study of the fundamental mechanisms of nucleation and growth of these polymorphs in the absence or presence of foreign substances. Further studies of what substances are able to promote aragonite or calcite precipitation under different conditions will not be nearly as useful as would be kinetic studies on single crystals of the two polymorphs under carefully controlled conditions of temperature, supersaturation, and impurity ion type and concentration.

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