[M-IS36_1AM2] Interface- and nano-phenomena on crystal growth

[MIS36-06] Theoretical analysis on the stability of divalent cations in the surface sites and clusters of calcium carbonate

*Jun KAWANO¹, Hiroshi SAKUMA², Takaya NAGAI³ (1.CRIS, Hokkaido Univ., 2.NIMS, 3.Grad. School of Science, Hokkaido Univ.)

Keywords:calcium carbonate, impurity, crystal growh

Formation process of calcium carbonate polymorphs, calcite, aragonite and vaterite has been extensively investigated, and impurity effect has been proposed as controlling phenomena in order to account for the formation of a particular polymorph. For example, it has been reported that an addition of Mg²⁺ in a solution inhibits calcite formation and promotes aragonite formation, however incorporation mechanism of this kind of impurities is poorly understood.In general, smaller divalent cations than Ca²⁺, like Mg²⁺, cannot form solid solution with aragonite. However, the structure of a crystal surface or small cluster forming at an initial stage of crystal growth can be different from the bulk crystal because of its flexibility, and it can act as the site for incorporation of ions which is unstable in the bulk structure. In the present study, the stability of divalent cations, especially Mg²⁺, (1) on hydrated aragonite surface and (2) in the cluster forming in an early stage of nucleation was investigated by quantum-chemical calculations, and the impurity effects on the formation of polymorphs were discussed.The calculation results show that Mg²⁺ is easier to be incorporated into a small cluster, while the hydration energy of Mg²⁺ is higher than that of other divalent cations. This indicates that Mg²⁺ is difficult to be released from hydration shell, however, once released, it is easy to incorporate into the cluster. Atomic arrangement of these clusters including Mg²⁺ is different from that of additive-free CaCO₃ clusters. Furthermore, Mg²⁺ on the aragonite surface considerably affects the surface structure and has an influence on the stability of aragonite. Thus, incorporation of Mg²⁺ into the clusters and surfaces sites should play an important role on the formation of the crystalline nuclei and the consequent crystal growth.