[S-MP26] Physics and Chemistry of Minerals

[SMP26-05] Structural changes in magnesium carbonate hydrates during crystallization and dehydration processes

*Atsushi Kyono¹, Gen-ichiro Yamamoto¹, Satoru Okada¹, Yasuhiro Yoneda², Yoshitaka Matsushita³ (1.Division of Earth Evolution Sciences, Faculty of Life and Environmental Sciences, University of Tsukuba, 2.Materials Sciences Research Center, Japan Atomic Energy Agency, 3.Materials Analysis Station, National Institute for Materials Science)

Keywords:Nesquehonite Mg(CO3)·3H2O, dypingite Mg5(CO3)4(OH)2·5H2O, hydromagnesite Mg5(CO3)4(OH)2·4H2O, Amorphous Magnesium Carbonate, Polyamorphism

Carbon dioxide Capture and Sequestration (CCS) technology, in which atmospheric carbon dioxide is fixed in the ground as carbonate minerals, has been attracting attention as a promising method for long-term stable sequestration of carbon dioxide. In this study, the structural changes in magnesium carbonate hydrates from crystallization to dehydration were investigated in detail using PDF and XAFS methods. Nesquehonite Mg(CO₃)·3H₂O (NSQ), dypingite Mg₅(CO₃)₄(OH)₂·5H₂O (DYP), and hydromagnesite Mg₅(CO₃)₄(OH)₂·4H₂O (HMG) are known to crystallize through Amorphous Magnesium Carbonate (AMC) at 20, 60, and 80 ^oC, respectively. In this study, we first investigated the structures of AMCs prepared at 20, 60, and 80 ^oC, respectively. Thermal analysis showed that the amount of water molecules in AMC decreased with temperature, which was the same trend as the crystalline phase. PDF analysis revealed that the short-range structure of AMC had no temperature dependence, and its structure was identical to the short-range ordered structure of HMG. Next, the effect of alkali metal ions in aqueous solution on the crystallization of NSQ from AMC was investigated. AMCs were prepared using four different aqueous solutions, Na₂CO₃, K₂CO₃, Rb₂CO₃, and Cs₂CO₃. As a result, NSQ crystallized easily when Na₂CO₃ or K₂CO₃ was used as the starting material, but when Rb₂CO₃ or Cs₂CO₃ was used, the crystallization of NSQ was apparently suppressed. The results of PDF analysis furthermore showed that AMC immediately after AMC formation had different structure from that immediately before NSQ crystallization. In the study, the two different structures were called as AMC-I and AMC-II structures, respectively. This result suggested that the existence of “polyamorphism” in AMC. Finally, the structure of AMC appearing during the thermal decomposition of magnesium carbonate hydrates were investigated. The results showed that AMC with AMC-I structure appeared during the thermal decomposition of NSQ and DYP, whereas different amorphous structure from neither AMC-I nor AMC-II, appeared during the thermal decomposition of HMG. We called the third amorphous structure as AMC-III structure.