Japan Geoscience Union Meeting 2023

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[J] Oral

M (Multidisciplinary and Interdisciplinary) » M-IS Intersection

[M-IS20] Aqua planetology

Thu. May 25, 2023 1:45 PM - 3:00 PM 304 (International Conference Hall, Makuhari Messe)

convener:Yasuhito Sekine(Earth-Life Science Insitute, Tokyo Institute of Technology), Hidenori Genda(Earth-Life Science Institute, Tokyo Institute of Technology), Keisuke Fukushi(Institute of Nature & Environmental Technology, Kanazawa University), Takazo Shibuya(Japan Agency for Marine-Earth Science and Technology), Chairperson:Yasuhito Sekine(Earth-Life Science Insitute, Tokyo Institute of Technology), Takazo Shibuya(Japan Agency for Marine-Earth Science and Technology), Hidenori Genda(Earth-Life Science Institute, Tokyo Institute of Technology), Tomohiro Usui(Japan Aerospace Exploration Agency), Keisuke Fukushi(Institute of Nature & Environmental Technology, Kanazawa University)

2:00 PM - 2:15 PM

[MIS20-02] Hematite-Goethite Problem on Mars: Transformation of Ferrihydrite to Crystalline Fe Oxides in Salt Solutions

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


Keywords:Mars, Hematite, Goethite

Sediment analysis indicates that liquid water once existed on Mars. Iron oxides found on Mars were formed by water/rock interactions on early Mars. Ferrihydrite is a common amorphous iron oxide formed by the hydrolysis of dissolved Fe3+, and is a metastable phase with respect to hematite and goethite which are the stable crystalline products. It has been generally believed that whether the ferrihydrite transforms to hematite or goethite is controlled by water chemistry. Hematite predominates when the pH is neutral or strongly acidic, while goethite predominates in acidic or alkaline conditions (Schwertmann and Murad, 1983). Fukushi et al., (2022) showed that the pH of the liquid water from the last aqueous events at Gale crater was acidic at about pH 3-5. Although goethite must be the predominant iron oxide under these pH conditions, goethite was not found in the corresponding stratigraphy of the Gale sediment. It has also been reported that hematite formation is favored in water with high salinity (ionic strength) at neutral pH conditions (Torrent and Guzman, 1982). 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. This understanding can resolve why goethite, which should form competitively with hematite, is not recognized in Martian sediments and could constrain water chemistry on Mars. 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.
Hematite was dominant at around neutral pH regardless of ionic strength; at acidic conditions, goethite was dominant at lower ionic strength conditions, but the ratio of hematite increased with increasing ionic strength, with only hematite forming at higher ionic strengths. These results showed that goethite formation must be inhibited because of the high ionic strength even under acidic condition, which can explain the absence of goethite in the corresponding stratigraphy of the Gale sediment.