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

• Published in: Physics and chemistry of minerals Vol. 50; no. 1; p. 5
• Main Authors: Yamamoto, Gen-ichiro, Kyono, Atsushi, Okada, Satoru
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-03-2023; Springer Nature B.V
• Subjects: Aqueous solutions; Basic magnesium carbonate; Carbon sequestration; Carbonates; Cations; Crystallization; Crystallography and Scattering Methods; Decomposition; Distribution functions; Earth and Environmental Science; Earth Sciences; Geochemistry; Magnesium carbonate; Magnesium chloride; Mineral Resources; Mineralogy; Nucleation; Original Paper; Stirring; Thermal decomposition; X-ray diffraction; Amorphous magnesium carbonate; Atomic pair distribution; Dypingite; Nesquehonite; Hydromagnesite
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-022-01231-4

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 MgCl
2
and Na
2
CO
3
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 Rb
2
CO
3
, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb
2
CO
3
solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs
2
CO
3
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.