Anaerobic Microscopic Analysis of Ferrous Saponite and Its Sensitivity to Oxidation by Earth’s Air: Lessons Learned for Analysis of Returned Samples from Mars and Carbonaceous Asteroids

Abstract

Ferrous saponite is a secondary mineral that can be used to reveal the redox state of past aqueous environments on Mars. In mineralogical analyses for ferrous saponite formed in laboratory simulations or contained in future returned samples from Mars, its oxidation by the Earth’s air could be problematic due to the high redox sensitivity. Here, we performed micro X-ray diffraction and scanning transmission X-ray microscopy analyses for a single particle of synthesized ferrous saponite without any exposure to air. The sample was reanalyzed after air exposure for 10–18 h to assess the adequacy of our anoxic preparation/measurement methods and the impacts of air on the sample. We found that the crystal structures agreed with ferrous saponite, both before and after air exposure; however, ferrous iron in saponite was partially oxidized, at least until 0.1–1 μm from the surface, after air exposure at the submicron scale, forming micro-vein-like Fe(III)-rich features. Together with our results of infrared spectroscopy of ferrous saponite, we showed that oxidation of octahedral iron occurred rapidly and heterogeneously, even in a short time of air exposure without any structural rearrangement. Since ferrous saponite is expected to exist on carbonaceous asteroids and icy dwarf planets, our methodology is also applicable to mineralogical studies of samples returned from these bodies.

Ultraviolet Photooxidation of Smectite‐Bound Fe(II) and Implications for the Origin of Martian Nontronites

“…(2021) was shown to accommodate ferric iron while maintaining trioctahedral structure within the 10–18 hr of exposure. The single particle saponites had Fe 3+ /ΣFe of ∼15% and formed ferric iron micro‐vein‐like features after air exposure (Noda et al., 2021). Chemtob et al.…”

“…Air exposed ferrous saponite by Noda et al (2021) was shown to accommodate ferric iron while maintaining trioctahedral structure within the 10-18 hr of exposure. The single particle saponites had Fe 3+ /ΣFe of ∼15% and formed ferric iron micro-vein-like features after air exposure (Noda et al, 2021). Chemtob et al (2017) found XRD hematite peaks after hydrothermally recrystallizing H 2 O 2 oxidized, Fe 2+ -rich smectites; they concluded that the hematite represented the recrystallization product of poorly crystalline Fe oxyhydroxides formed by iron ejection during oxidation.…”

“…Farmer et al (1971) noted that this process can precede iron ejection in high iron biotites. Air exposed ferrous saponite by Noda et al (2021) was shown to accommodate ferric iron while maintaining trioctahedral structure within the 10-18 hr of exposure. The single particle saponites had Fe 3+ /ΣFe of ∼15% and formed ferric iron micro-vein-like features after air exposure (Noda et al, 2021).…”

Spectral Analysis of Clay-bearing Outcrops in Northern Xanthe Terra, Mars: Comparison with Oxia Planum, the Landing Site for the ExoMars Rover Mission

“…Nontronite (NDJB26), saponite (LASA53), and Fe-bearing saponite (LASA59) are obtained from the Reflectance Experiment Laboratory (RELAB) spectral library, which is regularly updated by the CRISM team in the CAT ENVI tool (Morgan et al 2017, and references therein). Fe-bearing saponites (e.g., Griffith saponite or Griffithite) have been suggested for clays detected at Yellowknife Bay in Gale crater and elsewhere on Mars (e.g., Treiman et al 2014;Chemtob et al 2015;Fox et al 2021;Noda et al 2021); they have characteristic absorptions near 1.40-1.42 μm, 2.30-2.32 μm, and 2.40 μm. Pure vermiculite (GDS469L) and hydrobiotite (GDS469M) are handpicked ground flakes of a sample collected near Libby, Montana (Swayze et al 2018).…”

“…Besides the noise and dust cover, the lack of absorption near 2.4 μm could be due to compositional properties of Fe-saponites and pure vermiculites, the two possible candidate minerals. Fe-rich saponites have a very weak overtone near 2.4 μm, only recognizable after continuum removal (Figure 4(c); see, e.g., Treiman et al 2014;Chemtob et al 2015;Fox et al 2021;Noda et al 2021). Although the large variety of spectra for vermiculite ores implies diverse band depths and centers in the near-infrared spectral range (e.g., Swayze et al 2018;Krzesińska et al 2021;Cuadros et al 2022), pure vermiculite also has a 2.4 μm absorption relatively weaker than in hydrobiotite and other clay minerals (saponites and nontronites; Figure 4(c)).…”