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

Abstract

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.

Epitaxy: a methodological approach to the study of an old phenomenon

“…To explain the peculiar morphology of the calcite and aragonite crystals grown in the presence of Li + , Aquilano et al 16,46 determined both the structure and energetics of the (10.4) Cc /(001) Za , (00.1) Cc /(001) Za , (001) Ar /(001) Za , (001) Ar /(1 ¯01) Za and (001) Ar /(1 ¯02) Za interfaces, through quantum-mechanical calculations at the DFT (density functional theory) level. They used the computational strategy previously described and the 2D near-coincidence meshes reported in Table 1.…”

“…They used the computational strategy previously described and the 2D near-coincidence meshes reported in Table 1. Computational details and description of the possible (00.1) Cc and (001) Ar terminations are reported by Aquilano et al, 16,46 and will not be further discussed. Here, we only take into account the epitaxial interfaces showing the lowest interfacial and the highest adhesion energy, whose values are listed in Table 1.…”

“…16 From these results, it follows that the 2D epitaxial adhesion of zabuyelite above the (00.1) Cc , (10.4) Cc and (001) Ar faces can strongly modify the equilibrium shape (ES) of calcite and aragonite. Aquilano et al 46 compared the equilibrium morphologies of calcite and aragonite determined without and with 2D adhesion of zabuyelite (Fig. 3); the ES was drawn by using the Gibbs-Wulff theorem 14 and the calculated values of the surface (interface) energies for calcite and aragonite.…”

“…3 Equilibrium shapes of calcite and aragonite at 0 K. Calcite (a) and aragonite (c) in the vacuum; calcite (b) and aragonite (d), after the 2D-epitaxial adsorption of zabuyelite above (00.1) Cc , (10.4) Cc and (001) Ar . Redrawn from Aquilano et al46…”

Solvent-Mediated Enhancement of Additive-Controlled Crystallization

“…In this way, it is possible to control the polymorph, size, and shape of a crystal. In addition, the occlusion of guest additives within crystal hosts can create composite materials with new physical and chemical properties. − Given the importance of calcium carbonate to the environment, biology, and industry, a large body of work has focused on the effects of soluble additives on its crystallization ranging from elemental ions (Mg 2+ , Sr 2+ , and Li + ) − to small molecules such as amino acids, to macromolecules including proteins, polysaccharides, and block copolymers − and even particulate additives. − These studies have provided information about the mechanisms underlying the interactions between additives and calcium carbonate crystals, where morphology changes can be rationalized in terms of preferential binding of the additives to the acute or obtuse steps on the crystal surface. Incorporation of additives within host crystals generates hybrid materials with enhanced hardness, magnetism, and optical properties. , …”