10:15 AM - 10:30 AM
[SMP26-06] In-situ observation of crystallization of amorphous calcium carbonate under a humid condition
Keywords:amorphous calcium carbonate, crystallization, humid condition
Amorphous calcium carbonate (ACC), a metastable form of calcium carbonate with the chemical formula CaCO3 nH2O, plays an essential role in biomineralization and crystallization of calcium carbonate. ACC is known to crystallize into calcite at high temperatures (Koga et al., 1998). ACC transforms to a mixture of vaterite and calcite under high pressure less than 1 GPa. The crystallization pressure depends on the H2O contents of ACCs and the ratio of vaterite to calcite depends on the pressure (Yoshino et al., 2012). ACC also transforms into crystalline phases of calcium carbonate, depending on the relative humidity in the air (Xu et al., 2006). Moreover, ACC has been focused on as an intermediate material to introduce large incompatible ions such as Ba2+ into calcite (Saito et al., 2020). In this study, we observed the crystallization process of ACC under a humid condition by in-situ X-ray diffraction measurements.
ACC was synthesized by mixing ice-cooled ten-milliliter aqueous solutions of Na2CO3 (0.1 M) and CaCl2 (0.1 M). The precipitates were filtered immediately using a membrane filter (φ 0.45 μm) and washed twice with 10 mL of ice-cooled acetone. The obtained samples were dried in a desiccator evaluated with a diaphragm pump for one day. Water contents in the synthesized ACC samples were determined from weight loss in TG-DTA measurements. Ba-doped ACC samples were synthesized using blended solutions of CaCl2 and BaCl2 with varying Ba/(Ba+Ca) ratios (Saito et al., 2020). XRD measurements were conducted using a synchrotron X-ray radiation source (BL-18C, Photon Factory, KEK). An ACC sample was attached on a piece of Kapton tape and air saturated with water vapor was continuously pumped into the sample chamber. X-ray with a wavelength of 0.619 Å was collimated to a diameter of 100 μm and X-ray diffraction patterns were obtained every minute using a flat panel detector.
All the pure ACC samples were found to transform into crystalline phases of calcium carbonate, calcite and vaterite, within two hours. Crystallization behavior of ACC observed from time-resolved X-ray diffraction patterns showed notable differences among the samples. The crystallization induction time ranged from ten minutes to one hour, and the ratios of vaterite to calcite were also different among the samples. Possible mechanisms that explain the behavior are differences in the water contents in ACC and crystallization mechanism: solid-solid or dissolution-recrystallization.
On the other hand, 25% and 65 % Ba-doped ACC samples exhibited no crystallization within the experimental time (two hours). This behavior suggests that ACC containing heavy Ba ions requires much higher activation energy to crystallize than pure ACC.
The crystallization behavior of ACC under humid conditions will contribute not only to the understanding of biominerallization, but also to the understanding of the intrinsic and thermodynamic properties of ACC.
ACC was synthesized by mixing ice-cooled ten-milliliter aqueous solutions of Na2CO3 (0.1 M) and CaCl2 (0.1 M). The precipitates were filtered immediately using a membrane filter (φ 0.45 μm) and washed twice with 10 mL of ice-cooled acetone. The obtained samples were dried in a desiccator evaluated with a diaphragm pump for one day. Water contents in the synthesized ACC samples were determined from weight loss in TG-DTA measurements. Ba-doped ACC samples were synthesized using blended solutions of CaCl2 and BaCl2 with varying Ba/(Ba+Ca) ratios (Saito et al., 2020). XRD measurements were conducted using a synchrotron X-ray radiation source (BL-18C, Photon Factory, KEK). An ACC sample was attached on a piece of Kapton tape and air saturated with water vapor was continuously pumped into the sample chamber. X-ray with a wavelength of 0.619 Å was collimated to a diameter of 100 μm and X-ray diffraction patterns were obtained every minute using a flat panel detector.
All the pure ACC samples were found to transform into crystalline phases of calcium carbonate, calcite and vaterite, within two hours. Crystallization behavior of ACC observed from time-resolved X-ray diffraction patterns showed notable differences among the samples. The crystallization induction time ranged from ten minutes to one hour, and the ratios of vaterite to calcite were also different among the samples. Possible mechanisms that explain the behavior are differences in the water contents in ACC and crystallization mechanism: solid-solid or dissolution-recrystallization.
On the other hand, 25% and 65 % Ba-doped ACC samples exhibited no crystallization within the experimental time (two hours). This behavior suggests that ACC containing heavy Ba ions requires much higher activation energy to crystallize than pure ACC.
The crystallization behavior of ACC under humid conditions will contribute not only to the understanding of biominerallization, but also to the understanding of the intrinsic and thermodynamic properties of ACC.