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[SMP27-04] Crystallization of aragonite from amorphous calcium carbonate (ACC) under humid and wet conditions
Keywords:biomineralization, amorphous calcium carbonate, aragonite
Calcium carbonate is one of the main components of biominerals, for example, coccoliths and shells. Calcium carbonate has three anhydrous polymorphs: calcite, aragonite, and vaterite. Amorphous calcium carbonate (ACC, chemical formula: CaCO3･nH2O, n<1.5), a precursor of biominerals, is metastable and readily crystallizes. ACC crystallizes into calcite at around 350°C (Kojima et al., 1993) and into calcite and vaterite at pressures less than 1 GPa (Yoshino et al., 2012). It has also been reported that ACC crystallizes into calcite and vaterite under humid conditions above 60% relative humidity (Xu et al., 2006). On the other hand, crystallization of aragonite from ACC is difficult to occur and reported cases are very limited (Zhang et al., 2012). We focused on butylamine, which has been reported to promote aragonite formation in supersaturated solutions of CaCO3 (Chuajiw et al., 2013).
ACC was synthesized by mixing ice-cooled aqueous solutions of calcium chloride and sodium carbonate. The resulting white precipitate was immediately filtered, and the precipitate was dried in a vacuum desiccator at room temperature for one day. The water content of ACC was determined as n = 0.9-1.3 using TG-DTA. Powder X-ray diffraction (XRD) patterns of crystallized samples were obtained using an X-ray diffractometer (MiniFlex; Rigaku) with Cu Kα (λ=1.5418 Å) in the range of 2θ=10-70° at a scan speed of 1°/min. The polymorphic composition of calcium carbonate was determined by Rietveld analysis.
In the present study, aragonite formation from ACC was investigated under the following two different conditions. The synthesized ACC soaked in butylamine was placed in a chamber at 30°C under 90% relative humidity for 2 h. The addition of butylamine was found to produce aragonite from ACC. Increasing the amount of butylamine increased the amount of aragonite obtained.
In addition to the above experiments, ACC was dispersed in a butylamine aqueous solution at 22°C for 2 weeks. The concentration of butylamine in the solutions was prepared to be 33-90 vol%. The product was dried in a vacuum desiccator for one day. The recovered samples contained aragonite, calcite, and vaterite. The amount of aragonite increased with increasing concentration of butylamine, and the most abundant phase was aragonite. Although the details of the mechanism are unclear, it is possible that aragonite formed by dissolution and precipitation from ACC.
ACC was synthesized by mixing ice-cooled aqueous solutions of calcium chloride and sodium carbonate. The resulting white precipitate was immediately filtered, and the precipitate was dried in a vacuum desiccator at room temperature for one day. The water content of ACC was determined as n = 0.9-1.3 using TG-DTA. Powder X-ray diffraction (XRD) patterns of crystallized samples were obtained using an X-ray diffractometer (MiniFlex; Rigaku) with Cu Kα (λ=1.5418 Å) in the range of 2θ=10-70° at a scan speed of 1°/min. The polymorphic composition of calcium carbonate was determined by Rietveld analysis.
In the present study, aragonite formation from ACC was investigated under the following two different conditions. The synthesized ACC soaked in butylamine was placed in a chamber at 30°C under 90% relative humidity for 2 h. The addition of butylamine was found to produce aragonite from ACC. Increasing the amount of butylamine increased the amount of aragonite obtained.
In addition to the above experiments, ACC was dispersed in a butylamine aqueous solution at 22°C for 2 weeks. The concentration of butylamine in the solutions was prepared to be 33-90 vol%. The product was dried in a vacuum desiccator for one day. The recovered samples contained aragonite, calcite, and vaterite. The amount of aragonite increased with increasing concentration of butylamine, and the most abundant phase was aragonite. Although the details of the mechanism are unclear, it is possible that aragonite formed by dissolution and precipitation from ACC.