[B-CG07] Geo-Bio Interactions and its Applications

[BCG07-P04] Transformation of Ferrihydrite to Crystalline Fe Oxides in Salt Solutions: Implications for Water Chemistry on Early Mars

*So Fukaya¹, Keisuke Fukushi², Yoshio Takahashi³ (1.Graduate School of Natural Science and Technology, Kanazawa University, 2.Institute of Natural and Environmental Technology, Kanazawa University, 3.Department of Earth and Planetary Science, The University of Tokyo)

Keywords:Ferrihydrite, Hematite, Goethite, Mars

The presence of iron oxides formed by water/rock interactions on early Mars were found by analyses of sediments in the Gale crater. Ferrihydrite is a common low crystalline iron oxide formed by the hydrolysis of dissolved Fe³⁺ and is metastable phase with respect to hematite and goethite which are the stable crystalline products. It has been generally believed that the transformation of ferrihydrite to hematite and goethite is mainly controlled by water chemistry. Hematite predominates at neutral pH, while goethite predominates in acidic or alkaline conditions (Schwertmann and Murad, 1983). Fukushi et al. (2022) showed that the pH of liquid water from the last aqueous event at Gale Crater was acidic at about pH 3-5. Although goethite must be the predominant iron oxide under these pH conditions, goethite has not been identified in the Gale sediments. It has also been reported that hematite formation is predominant in water with neutral pH and high salinity (ionic strength) conditions (Torrent and Guzman, 1982). Therefore, it is hypothesized that goethite, which should form competitively with hematite, is not recognized in Martian sediments due to the high ionic strength conditions. However, the transformation of ferrihydrite under conditions where both pH and ionic strength are controlled has not been studied, so a systematic understanding of alteration behavior is needed. In this study, we examined the transformation of ferrihydrite as function of ionic strength and pH with NaCl and MgCl₂ media by means of powder X-ray diffraction and X-ray absorption spectroscopy. The transformation mechanism was further examined by a monitoring of the aggregation and dissolution behaviors of ferrihydrite.