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[MIS13-P04] Influence of the hydration states on the behavior of Mg2+ in CaCO3 formation process
Keywords:carcium carbonate, hydration, polymorph
Calcium carbonate CaCO3 exists in various polymorphs, including calcite, which is stable at room temperature and atmospheric pressure, and aragonite, a high-pressure phase. It has been suggested that the magnesium in the mother solution has a significant effect on this generation and conversion process, but the detailed mechanism has not yet been fully elucidated. Iwane suggested that hydrated Mg2+ contributes to the process of aragonite formation via the amorphous phase, by synthesizing calcium carbonate in a mixed solution of formamide and water. In this study, we used the same method to synthesize calcium carbonate under conditions of low supersaturation, where calcite nucleates directly from aqueous solution, and slightly higher supersaturation conditions, to investigate the effect of hydration on the behavior of Mg2+.
pH changes monitored during synthesis and the yield of the precipitates suggest that calcite precipitates directly under the low supersaturation conditions. While under the high supersaturation conditions, ACC precipitates first and then dissolves, and a solvent-mediated phase transition to calcite occurred. Mg2+ content of calcite synthesized under the low supersaturation conditions was estimated to be 3~6 mol%, and those under the high supersaturation conditions were 5~8 mol%. There was a little difference between the results obtained in these two formation processes. In addition, under both conditions, there was no significant change in the Mg2+ content with respect to the formamide ratio. Therefore, it is suggested that the hydration state of Mg2+ does not have a significant effect on the incorporation to the calcite crystals when calcite is directly formed from aqueous solution, including the case of the solvent-mediated phase transition. Furthermore, under the low supersaturation conditions, the amount of aragonite produced decreased as the volume ratio of formamide increased, i.e. as the amount of hydrated Mg2+ decreased, and no aragonite precipitated at 75%. This result suggests that hydrated Mg2+ contributes to the formation of aragonite even in the path where ACC does not form.
References:
[1] Xu et al., PNAS, 110, 2013, 17750–17755
[2] Iwane et al., JAMS, 2024
[3] Liu et al., PNAS, 117, 2020, 3397-3404
pH changes monitored during synthesis and the yield of the precipitates suggest that calcite precipitates directly under the low supersaturation conditions. While under the high supersaturation conditions, ACC precipitates first and then dissolves, and a solvent-mediated phase transition to calcite occurred. Mg2+ content of calcite synthesized under the low supersaturation conditions was estimated to be 3~6 mol%, and those under the high supersaturation conditions were 5~8 mol%. There was a little difference between the results obtained in these two formation processes. In addition, under both conditions, there was no significant change in the Mg2+ content with respect to the formamide ratio. Therefore, it is suggested that the hydration state of Mg2+ does not have a significant effect on the incorporation to the calcite crystals when calcite is directly formed from aqueous solution, including the case of the solvent-mediated phase transition. Furthermore, under the low supersaturation conditions, the amount of aragonite produced decreased as the volume ratio of formamide increased, i.e. as the amount of hydrated Mg2+ decreased, and no aragonite precipitated at 75%. This result suggests that hydrated Mg2+ contributes to the formation of aragonite even in the path where ACC does not form.
References:
[1] Xu et al., PNAS, 110, 2013, 17750–17755
[2] Iwane et al., JAMS, 2024
[3] Liu et al., PNAS, 117, 2020, 3397-3404