Impurity Effects on Habit Change and Polymorphic Transitions in the System: Aragonite-Calcite-Vaterite

Crystal growth experiments carried out in aqueous solutions, supersaturated with respect to calcite (CaCO3) at room temperature and pressure, show that the lithium ion added to growth solutions not only acts as a habit modifier of calcite, but promotes the nucleation and growth of the two other CaCO3 polymorphs: aragonite and vaterite. This behavior is interpreted on the grounds of foreign adsorption going beyond Langmuir's isotherm model: two-dimensional (2D)-adsorbed nuclei of Li2CO3 (mineral zabuyelite) can change, under varying Li+ concentrations in solution, the growth habit of calcite, promoting as well both nucleation and growth of aragonite and vaterite, through the mechanism of 2D-epitaxy. Calculation proves how the epitaxy aragonite/2D-zabuyelite reduces the average value of the specific surface energy of aragonite, allowing the nucleation of aragonite and then the coexistence of calcite and aragonite in the same batch. Finally, it is outlined that the 2D lattices fulfilling the epitaxy constraints for the couples calcite/zabuyelite and aragonite/zabuyelite show, as a common feature, a remarkable pseudohexagonality.