The tendency of preferentially forming aragonite in aqueous solutions (hydrothermal synthesis) contradicts the stable relationship in the CaCO3 system. Aragonite is thermodynamically unstable at room temperature, and becomes even more so with increasing temperature (where it tends to convert to calcite even more). Therefore, the formation of aragonite in lower stability and higher solubility conditions can only be explained in terms of nucleation and crystallization rates in comparison to calcite. The fact that the formation of aragonite to be predominant in supersaturated solutions in relation to both of the phases, is an indicator of a higher rate of crystallization rather than the conversion from aragonite to calcite. This relationship of crystallization rates doesn’t apply to just hydrothermal solutions; aragonite can form a single phase, also at ambient temperature, when aragonite crystal seeds are added in solutions in condition that, in the opposite case, would form calcite. On the contrary, seeding with calcite is ineffective in conditions that would normally produce aragonite.
Calcium capture happens more easily with aragonite nuclei in comparison to calcite nuclei. Once calcium is absorbed they are also more easily retained. The same thing applies to the entrance of CO3 groups as well when you consider that thermal vibrations are more contained in aragonite than calcite. This factor would imply the capture and retention are more efficient of both CO32+ and Ca2+ by active nucleation sites in aragonite. The growth yield of aragonite with an increase in precipitation temperature implies, along with a higher rate of crystallization, that the rate of spontaneous nucleation has a sharper increase with the increase in temperature than does calcite. With this, an inversion of relative importance of the rate of nucleation should occur between ambient temperature and 100°C. This phenomenon should also be susceptible to interpretation in terms of crystalline structure: a proposed possible explanation is based on ionic hydration, where the nucleation of both, calcite and aragonite, is obstructed by water dipoles. The molecular action of water on the nucleus is supposedly more critical in aragonite’s case, where only a small portion of ion aggregation of ions will be effective as a nucleus (hydrated aragonite). Sparse aggregates could be sufficient as calcite with its lower density, and the water molecules contained in these aggregates are more easily liberated in the open structure. The ion-dipole bond is undone by the thermal movement with the increase in temperature, and thus, the nuclei of aragonite can easily form in aqueous solutions.
