[B-BG01] Earth and Planetary Science Frontiers for Life and Global Environment

[BBG01-02] Finding calcium carbonate hemihydrate in alkaline lakes based on the experimentally estimated Solubility

*Takuma KITAJIMA¹, Mai Suyama¹, Keisuke Fukushi², Baasansuren Gankhurel¹, Davaasuren Davaadorj³ (1.Kanazawa university, 2.The Institute of Nature and Environmental Technology, Kanazawa University, 3.National University of Mongolia)

Keywords:calcium carbonate, alkaline saline lake, calcium carbonate hemihydrate

Calcium carbonates are ubiquitous in the Earth's surface. Calcium carbonates are of interest because of their importance for paleoclimate reconstructions, ocean acidification, and biomineralization. Calcium carbonate hemihydrate (CCH), the focus of this study, is the first synthesized in 2019 (Zou et al., 2019). CCH is known to transform from amorphous calcium carbonate while it transforms to monohydrocalcite (MHC). It is very unstable phase at ambient temperature and pressure, the phase transition to MHC occurs within tens of minutes after formation (Zou et al., 2019). As CCH has only just been identified, thermodynamic data have not yet been collected. Solubility is an inherent parameter that quantitatively measures the saturated state of a material in solution. When minerals precipitate from solution, this suggests that the solution is saturated with minerals. In other words, the water environment with a water chemistry close to the solubility of CCH is likely to be precipitated by CCH. In this study, the solubility of CCH was experimentally estimated from a supersaturation method from Na₂CO₃, CaCl₂ and MgCl₂ mixed solutions. Based on the data obtained, analysis of suspended solids in alkaline saline lake was also conducted to detect CCH in the natural environment. The water chemistry of alkaline saline lakes approximates the estimated CCH solubility. Furthermore, MHC, an altered product of CCH, has been detected in alkaline saline lakes. Therefore, we thought the presence of CCH in these lakes. X-ray diffraction analysis of the suspended solids from two alkaline saline lakes in Mongolia found small peaks that could be attributed to CCH. However, additional verification is needed to conclude the formation of CCH. The discovery of new calcium carbonate phase in the Earth's surface environment may contribute to the understanding of the biomineralization of crystalline calcium carbonate and the control of environmental variability.