[S-MP29] Physics and Chemistry of Minerals

[SMP29-06] The effect of alkali metal ions on the forsterite carbonation reaction

*Atsushi Kyono¹, Hiroki Hasegawa², Satoru Okada³, Kosuke Yamaguchi² (1.Division of Earth Evolution Sciences, Faculty of Life and Environmental Sciences, University of Tsukuba, 2.Degree Programs in Life and Earth Sciences, Graduate School of Science and Technology Master's Program in Geosciences, University of Tsukuba, 3.Degree Programs in Life and Earth Sciences, Graduate School of Science and Technology Doctoral Program in Geosciences, University of Tsukuba)

Keywords:Forsterite carbonation, Geological Carbon Storage (GCS), Magnesite crystallization, Additive effects, Silica-rich passivation layer

The impact of alkali metal ions on the carbonation mechanism of forsterite was investigated to evaluate their role in geological carbon storage (GCS). Forsterite carbonation, a subset of mineral carbonation (MC), transforms Mg-rich silicates into stable carbonate minerals, offering a promising pathway for long-term CO₂ sequestration. Using San Carlos olivine as the forsterite source, this study systematically examined the effects of temperature, reaction duration, and additives on the carbonation process through hydrothermal experiments. A particular focus was placed on alkali metal ions, including Li⁺, Na⁺, and K⁺, as well as other additives such as CaCl₂ and amorphous silica. Results indicate that reaction temperature strongly influences both the rate and type of reaction products. X-ray diffraction (XRD) and thermogravimetric (TG) analyses revealed that hydromagnesite formation dominates at 140–180 °C, while magnesite becomes the principal product at 180–200 °C. While the crystallinity of magnesite reached its highest level at 200 °C, the yield of magnesite peaked at 180 °C. Beyond this range, the activities of carbonate (CO₃^2-) and bicarbonate (HCO₃^-) ions in the solution decrease, which reduces the overall reaction rate. Reaction duration also proved critical; magnesite formation increased markedly up to 10 days but declined beyond 20 days due to passivation by Si-rich layers. Additives had pronounced effects on carbonation efficiency and product selectivity. LiCl significantly enhanced magnesite crystallization, yielding well-formed rhombohedral crystals. Li₂CO₃ emerged as the most effective additive, acting as both a CO₂ source and a pH buffer, which substantially improved carbonation efficiency. In contrast, CaCl₂ preferentially promoted calcite formation, while amorphous silica addition suppressed overall carbonation by reinforcing Si-rich passivation layers on forsterite surfaces. Scanning electron microscopy (SEM) corroborated these findings, revealing differences in morphology and crystallinity among reaction products based on additive type. This study underscores the importance of reaction conditions and solution chemistry in optimizing forsterite carbonation. The findings highlight that alkali metal ions, particularly Li⁺ and Na⁺, play pivotal roles in facilitating efficient CO₂ sequestration through magnesite formation. Moreover, mitigating the inhibitory effects of Si-rich passivation layers remains a key challenge for enhancing long-term carbonation rates. These results provide critical insights into the design of effective GCS systems and the potential for leveraging mineral carbonation as a sustainable carbon management strategy. Future work should address scaling these findings to field-relevant conditions to advance the practical application of GCS technologies.