Solution-chemistry control of Mg2+-calcite interaction mechanisms: Implication for biomineralization.

We investigated the effect of Mg²⁺ on calcite hillock growth over a broad range of solution conditions in terms of supersaturation (Ω_calcite) and Mg/Ca ratios using atomic force microscopy and secondary ion mass spectrometry. We found that both the incorporation pattern/incorporated Mg²⁺ quantity in the hillock structure and the Mg²⁺-induced morphological change of the hillock surface showed strong dependence of the growth conditions. Specifically, when Mg/Ca was high (i.e., >5) and Ω_calcite was low (i.e., ~0.45), Mg²⁺ was predominantly incorporated into the negative sectors of the hillock structure, resulting in gradual loss of step structure and morphological amorphism on these vicinal surfaces. When Mg/Ca and Ω_calcite were in intermediate ranges (i.e., Mg/Ca < 5, and 0.45 < Ω_calcite < 1), the originally straight edges of the hillock steps exhibited curvatures of varying degrees and formed "tear-drop" morphologies. It is noted that such "tear-drop" morphology was stable within the duration of the experiments and did not evolve into other surface patterns. By contrast, when both Mg/Ca and Ω_calcite were high (i.e., Mg/Ca > 5, and Ω_calcite > 1.1), the growing hillocks experienced two phases of morphological changes, initiated with the formation of "tear-drops" followed by the development of linear ruptures along [481] and [441] directions. And the occurrence of these ruptures segmented the hillock surface effectively into multiple isolated plateaus. Significantly, we revealed the underlying mechanisms for these condition-specific effects of Mg²⁺ on calcite growth, which mainly resulted from the interplay among three major factors: (1) the size-mismatch between Mg²⁺ and Ca²⁺ that causes structural strains in magnesian calcite and leads to morphological amorphism in high-Mg²⁺ carbonate; (2) the asymmetry of the calcite crystal structure that sets a physical limitation for Mg²⁺ incorporation patterns in the hillock structure; and (3) the step advancing rate (i.e., the calcite growth kinetics) that affects both Mg²⁺ incorporation and the accommodation of Mg²⁺-induced structural strains in the hillock structure. Detailed discussions were given for each growth scenario. The results of our study provide a theoretical base to decipher the roles of Mg²⁺ in CaCO₃ mineralization, and thus, have important implication for a range of processes that involve the growth of Mg-Ca-CO₃ systems, such as biomineralization, carbon capture and storage, and scale controls in industrial settings. [ABSTRACT FROM AUTHOR]