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Calcium carbonate biomineralization is remarkable for the ability of organisms to produce calcite or aragonite with perfect fidelity, where this is commonly attributed to specific anionic biomacromolecules. However, it is proven difficult to mimic this behavior using synthetic or biogenic anionic organic molecules. Here, it is shown that cationic polyamines ranging from small molecules to large polyelectrolytes can exert exceptional control over calcium carbonate polymorph, promoting aragonite nucleation at extremely low concentrations but suppressing its growth at high concentrations, such that calcite or vaterite form. The aragonite crystals form via particle assembly, giving nanoparticulate structures analogous to biogenic aragonite, and subsequent growth yields stacked aragonite platelets comparable to structures seen in developing nacre. This mechanism of polymorph selectivity is captured in a theoretical model based on these competing nucleation and growth effects and is completely distinct from the activity of magnesium ions, which generate aragonite by inhibiting calcite. Profiting from these contrasting mechanisms, it is then demonstrated that polyamines and magnesium ions can be combined to give unprecedented control over aragonite formation. These results give insight into calcite/aragonite polymorphism and raise the possibility that organisms may exploit both amine‐rich organic molecules and magnesium ions in controlling calcium carbonate polymorph. [ABSTRACT FROM AUTHOR]

The use of amorphous calcium carbonate (ACC) as a precursor phase affords organisms with outstanding control over the formation of calcite and aragonite biominerals. Essential to this strategy is that the ACC is maintained within confined volumes in the absence of bulk water. This ensures that the ACC undergoes a pseudomorphic transformation and that the organism can independently control nucleation and growth. However, comparable control has proven hard to achieve in synthetic systems. Here, a straightforward method is demonstrated for controlling the crystallization of ACC thin films in which nucleation is first triggered using a heated probe, and then growth is sustained by incubating the film at a lower temperature. By independently controlling nucleation and growth, sub‐millimeter calcite single crystals can be generated when and where it is desired, morphologies ranging from discs to squares to serpentine strips can be created, and arrays of crystals formed. The mechanism and energetics of crystallization of the ACC are studied using in situ transmission electron microscopy and continuity between the ACC and calcite at the growth front is demonstrated. It is envisaged that this method can be applied to the formation of large single crystals of alternative functional materials that form via amorphous precursor phases. [ABSTRACT FROM AUTHOR]

Amorphous calcium carbonate (ACC) is a common precursor to crystalline calcium carbonate, and is of particular importance in biomineralization, where its crystallization in privileged environments ensures a pseudomorphic transformation. While organisms regulate this process using organic molecules and magnesium ions to selectively form calcite or aragonite, it has proven highly challenging to replicate this polymorph selectivity synthetically. Here, it is demonstrated that remarkable control can be achieved over the chemical composition and structure of crystalline calcium carbonate by using heat to drive a pseudomorphic transformation of ACC thin films. The crystal polymorph can be tuned from low magnesium‐calcite to pure aragonite, high magnesium‐calcite, and ultimately dolomite according to the magnesium content of the ACC, and mosaics of large single crystals are generated at elevated temperatures rather than the spherulitic structures formed at room temperature. This methodology also enables an in situ investigation of the ACC crystallization mechanism using transmission electron microscopy. Finally, the approach can be combined with templating methods to generate arrays of large aragonite single crystals with preselected morphologies. These results demonstrate that exceptional control can be achieved through the solid‐state transformation of Mg‐ACC, which has relevance to both synthetic and biological systems. [ABSTRACT FROM AUTHOR]