Effect of Mg$^{2+}$ on the structure of amorphous CaCO$_3$ -- A molecular dynamics simulation

Molecular dynamics (MD) simulations of amorphous calcium carbonate (ACC) were carried out to investigate the effect of Mg$^{2+}$ ions on the structure of CaCO$_3$ crystal nucleus formed from ACC\@. Our systems contained 432 CaCO$_3$ with several concentrations of MgCO$_3$. In this study, our original ion model of Mg$^{2+}$ was developed and combined with Raiteri model of Ca$^{2+}$ and rigid CO$_3$$^{2-}$ [1]. The simulations indicated that the fraction of vaterite-like ion arrangement was much larger than those of calcite-like and aragonite-like ion arrangements in pure ACC\@. However, as the Mg$^{2+}$ concentration increased, the faction of vaterite-like ion arrangements decreased, which suggests that Mg$^{2+}$ ions play as inhibitors of vaterite nucleation. The result explains why calcite or aragonite is preferentially nucleated in the presence of Mg$^{2+}$, whereas vaterite is nucleated in the absence of them.[1] P. Raiteri, J.~D. Gale, D. Quigley, and P.~M. Rodger, J.~Phys.\ Chem.\ C 114, 5997 (2010).