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Incorporation of Mg2+ in surface Ca2+ sites of aragonite: an ab initio study

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Abstract

First-principles calculations of Mg2+-containing aragonite surfaces are important because Mg2+ can affect the growth of calcium carbonate polymorphs. New calculations that incorporate Mg2+ substitution for Ca2+ in the aragonite {001} and {110} surfaces clarify the stability of Mg2+ near the aragonite surface and the structure of the Mg2+-containing aragonite surface. The results suggest that the Mg2+ substitution energy for Ca2+ at surface sites is lower than that in the bulk structure and that Mg2+ can be easily incorporated into the surface sites; however, when Mg2+ is substituted for Ca2+ in sites deeper than the second Ca2+ layer, the substitution energy approaches the value of the bulk structure. Furthermore, Mg2+ at the aragonite surface has a significant effect on the surface structure. In particular, CO3 groups rotate to achieve six-coordinate geometry when Mg2+ is substituted for Ca2+ in the top layer of the {001} surface or even in the deeper layers of the {110} surface. The rotation may relax the atomic structure around Mg2+ and reduces the substitution energy. The structural rearrangements observed in this study of the aragonite surface induced by Mg2+ likely change the stability of aragonite and affect the polymorph selection of CaCO3.

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Kawano, J., Sakuma, H., & Nagai, T. (2015). Incorporation of Mg2+ in surface Ca2+ sites of aragonite: an ab initio study. Progress in Earth and Planetary Science, 2(1). https://doi.org/10.1186/s40645-015-0039-4

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