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Structural variations of amorphous magnesium carbonate during nucleation, crystallization, and decomposition of nesquehonite MgCO3·3H2O

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Abstract

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl2 and Na2CO3 solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb2CO3, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb2CO3 solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs2CO3 solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure.

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Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors would like to thank two anonymous reviewers for their constructive comments that helped improve the manuscript. PDF measurements performed at BL22XU of SPring-8 were approved by the Photon Factory Program Advisory Committee (Proposal No. 2021A3737). This work was supported by JSPS KAKENHI Grant number JP20K04124).

Funding

This work was supported by Atsushi Kyono: Supported by JSPS KAKENHI (, Ggrant number JP20K04124) granted to Atsushi Kyono.

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GY: Sample preparation, Synchrotron X-ray measurement, Data analysis, and Writing; AK: Supervision, Synchrotron X-ray measurement, Data analysis, Funding acquisition, and Writing—review and editing; SO: Synchrotron X-ray measurement; All authors reviewed and approved the manuscript.

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Correspondence to Atsushi Kyono.

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Yamamoto, Gi., Kyono, A. & Okada, S. Structural variations of amorphous magnesium carbonate during nucleation, crystallization, and decomposition of nesquehonite MgCO3·3H2O. Phys Chem Minerals 50, 5 (2023). https://doi.org/10.1007/s00269-022-01231-4

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  • DOI: https://doi.org/10.1007/s00269-022-01231-4

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