Characterizing the New Mineral FeSO4OH on Mars and Describing its Geochemical History and Association with Other Sulfates

Sulfate minerals are an integral component of the martian surface and understanding the formation and alteration of these minerals provides clues about their geochemical environment. One sulfate phase in particular has been intriguing Mars scientists for over 15 years. An unusual spectral band at 2.236 µm was discovered in CRISM spectra of Mars at the plateau bordering Juventae Chasma [1] and in Aram Chaos [2]. This spectral band does not line up with any known minerals, but is observed for FeSO₄OH, a new mineral formed by heating hydrated iron sulfates [3]. Crystal structure diagrams indicate that FeSO₄OH has a structure similar to that of szomolnokite (FeSO₄•H₂O), but with OH replacing H₂O [4]. Experiments heating szomolnokite, rozenite, and melanterite in the lab resulted in production of some FeSO₄OH at 150 °C from rozenite and melanterite after 30 minutes and complete transformation at 200 °C after 30 minutes, while reaction of szomolnokite required longer heating at 200 °C and/or elevated temperatures for transformation to FeSO₄OH. Additional lab experiments also demonstrate that oxygen is required for reaction of these hydrated ferrous sulfates to form FeSO₄OH [4].

Application of FeSO₄OH spectra to Mars has benefitted from improved processing techniques [5] and mapping algorithms [6] for CRISM images that has enabled characterization of smaller spot sizes with cleaner spectra. Exposures of this unusual ferric sulfate phase with spectral features near 2.23 µm at Aram Chaos closely resemble pure FeSO₄OH formed in the lab, while the thinner units on the Juventae plateau are either mixed with other components or represent incompletely formed FeSO₄OH phases. This Fe hydroxysulfate is currently associated with monohydrated sulfate (MHS) outcrops at Aram Chaos, although polyhydrated sulfate (PHS) outcrops are also present nearby. In contrast, only PHS outcrops are currently observed on the Juventae plateau.

CRISM spectra with a spectral band at 2.225-2.238 µm were observed at several locations at Aram Chaos. The spectra of these units also contain accompanying features at 1.48, 1.82, 2.19, and 2.37 µm (Fig. 1). Small variations in the ~2.23 µm band are attributed to changes in the Fe-Mg chemistry. Pure FeSO₄OH has a band at 2.236 µm, while heated FeMg-MHS has a band at 2.226 µm. The FeSO₄OH units at Aram Chaos are typically found adjacent to MHS outcrops (Fig. 2), including both Fe-rich MHS (similar to szomolnokite) with bands near 2.11-2.12 and 2.40 µm and kieserite (MgSO₄•H₂0) with spectral bands near 2.14 and 2.41 µm. Spectra of szomolnokite and kieserite measured at colder, Mars-like temperatures have bands at ~2.11 and 2.14 µm [7], similar to these observations. The MHS spectral units more consistent with kieserite are darker than the szomolnokite-like MHS units and are covered by debris and ripples (Fig. 2C). Some of the MHS units at Aram Chaos may have formerly been mixtures of szomolnokite and kieserite, where the szomolnokite transformed to FeSO₄OH and the kieserite remained. Alternatively, polyhydrated Fe and Mg sulfates may have been present that altered to form FeSO₄OH, szomolnokite, and kieserite, depending on variations in the geochemistry. The reaction to form FeSO₄OH proceeds as Fe²⁺ in szomolnokite is oxidized to Fe³⁺ while the H₂O loses a proton to form OH. Some of the MHS outcrops include weak bands near 2.23 µm, indicating partial alteration to form a mixed phase containing some MHS and some FeSO4OH (Fig. 3). The PHS spectra contain bands near 1.44 and 1.93-1.95 µm and a drop in reflectance near 2.42 µm (Fig. 3), similar to spectra of rozenite (FeSO₄•4H₂0) and starkeyite (MgSO₄•4H₂0).

Spectra at the Juventae plateau were collected from thin light-toned layered deposits, including spectral signatures due to PHS and FeSO₄OH, and pyroxene-bearing units (Figs. 3-4). The stratigraphy of the outcrops shows a pyroxene-bearing substrate below the light-toned layered materials and a different pyroxene-bearing caprock unit covering the Fe sulfates (Fig. 4). Thin units containing spectral features consistent with PHS (blue) and FeSO₄OH (red) are observed, and some of these thin units include spectral features due to both materials. Morphologies of these primary four units are displayed in Fig. 5. The pyroxene bearing substrate (dark cyan) is flatter with extensive polygonal fracturing, whereas the pyroxene-bearing caprock (green) is partially covered by ripples and appears hilly and uneven in topography due to differential erosion. The textures of the PHS- and FeSO₄OH-bearing units are distinct from those of the pyroxene-bearing units, but appear related to each other with fine-scale layering that varies in brightness, color, and fracturing.

The presence of highly pure FeSO₄OH outcrops neighboring MHS at Aram Chaos and less pure outcrops of FeSO₄OH neighboring PHS on the plateau NW of Juventae Chasma indicate an active geochemical history in Mars’ past. The hydrated sulfates likely formed in evaporative environments, while the FeSO₄OH likely formed through heating. The FeSO₄OH-bearing units at the Juventaue plateau could be mixed with spectrally neutral components that dilute the FeSO₄OH spectral features. Coordinated characterization of the near-infrared (NIR) and mid-IR spectral features of Fe sulfates (Fig. 6) is enabling a better understanding of the spectral features due to FeSO₄OH in these intriguing outcrops on Mars.

Acknowledgements: The authors are grateful for support from NASA MDAP #80NSSC21K1103, NASA SSW #80NSSC23K0032, Austrian Science Fund FWF #P34227-N, and Europlanet Transnational Access funds.

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