The nucleation ofCaCO₃ crystals has been an important research subject in the field of crystal growth, physics, chemistry, mineralogy and biomineralization from long ago. Especially, the effect of Mg²⁺ ions on the nucleation of CaCO₃crystals has attracted great interest, because Mg²⁺ ions affect CaCO₃ polymorphism [1]. That is, normally, the nucleation of vaterite or calcite occurs. However, the nucleation of other polymorphs, such as aragonite and monohydrocalcite, occurs if Mg²⁺ ions exist. The cause of the polymorphism in the presence of Mg²⁺ ions still remains unclear.

Recently, several experimental studies have reported amorphous CaCO₃ (ACC) precursors formed in solution, and the nucleation of CaCO₃ crystals from the precursors [2]. It has been proposed that the structure of ACC precursors may determine which CaCO₃ crystal structure is formed preferentially. Hence, the changes in the atomic-scale structure ib ACC precursors by adding Mg²⁺ ions is particularly relevant to CaCO₃ polymorphism in the presence of Mg²⁺ ions.

In this study, molecular dynamics simulations were conducted to elucidate the effect of Mg²⁺ ions on the structure of ACC [3]. Simulation results indicated that although ACC did not have a long-range ordered structure, it did have a weakly ordered local structure resembling the local structure of a CaCO₃ crystal. The local structure of pure ACC resembled that of vaterite. However, the formation of the vaterite-like local structure was hindered by Mg²⁺ ions. Moreover, when the fraction of water molecules in ACC was high, the formation of a monohydrocalcite-like local structure was promoted by Mg²⁺ ions.

The effects of Mg²⁺ ions on the structure of ACC were verified using the size of Mg²⁺ ions, and the interaction among Mg²⁺ ions, H₂O and CaCO₃. The simulated structure of ACC obtained in the simulation was compared with the structure of CaCO₃ crystals nucleated from ACC precursors in real systems. At the presentation, we will discuss the role of Mg²⁺ ions in CaCO₃ polymorphism.

References

[1] E. Loste, R. M. Wilson, R. Seshadri, and F. C. Meldrum, J. Cryst. Growth 254 (2003) 206.

[2] E. M. Pouget, P. H. H. Bomans, J. A. C. M. Goos, P. M. Frederik, G. de With, and N. A. J. M. Sommerdijk, Science 323(2009) 1455.

[3] H. Tomono, H. Nada, F. Zhu, T. Sakamoto, T. Nishimura, and T. Kato, to be published.

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