Magnesium Ions Direct the Solid‐State Transformation of Amorphous Calcium Carbonate Thin Films to Aragonite, Magnesium‐Calcite, or Dolomite

Abstract

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.

Additive manufacturing of porous biominerals

“…Traditional XRD con rms these results from molded samples as seen in Figure S12. This result is consistent with results obtained by forming these minerals in bulk 49,50 , suggesting that knowhow on polymorphic control of CaCO 3 formation in bulk aqueous solutions can be transferred to our process.…”

Calcium carbonate: controlled synthesis, surface functionalization, and nanostructured materials

“…388 Such an effect of Mg 2+ can be attributed to strong and well-hydrated Mg 2+ cations that can delay the nucleation and growth of calcite and can either be incorporated within the calcite lattice to create a barrier to prevent the further growth of calcite nuclei. 389,390 cystoliths. 391 In these cystoliths, a high SiO 4 4À content in the ACC correlated with high thermal stability.…”

“…388 Such an effect of Mg 2+ can be attributed to strong and well-hydrated Mg 2+ cations that can delay the nucleation and growth of calcite and can either be incorporated within the calcite lattice to create a barrier to prevent the further growth of calcite nuclei. 389,390 Anions such as SiO 4 4− , PO 4 3− , and OH − also influence the stability of ACC. The role of SiO 4 4− in stabilizing ACC has been evidenced by the presence of SiO 4 4− in cystoliths.…”

Heat-Induced Dolomitization of Amorphous Calcium Magnesium Carbonate in a CO2-Filled Closed System

“…On the other hand, ACC easily crystallizes into calcite by heating in air . This crystallization process is considered to proceed by solid-state transformation. ,, The crystallization by solid-state transformation has a variety of applications, including the study of biominerals, , selective crystallization into desired polymorphs (calcite ,, or aragonite , ), the addition of large amounts of impurities to crystals, − reference materials, and synthesis of nanoporous calcite with a high specific surface area …”

“…This thermodynamic reaction prevents the crystallization of ACMC by heating at ca. 400 °C in air, although a recent study reported the dolomite synthesis from a thin film of ACMC by heating in air . On the other hand, ACMC does not crystallize into dolomite at ambient temperature because of the kinetic barriers. , Therefore, most previous studies have conducted the dolomitization of ACMC via dissolution-reprecipitation in hydrothermal solution. , …”