Mineralogy, VNIR (Visible Near-Infrared), and Fe XAS (X-ray Absorption) Spectroscopy of Terrestrial Hematite Analogs: Implications for Iron Speciation on Mars

Hematite is a widespread iron oxide on both Earth and Mars, and its detection on Mars has made it a key target in the search for past water activity and potential biosignatures. On Earth, hematite occurs in a range of geological settings. In this study, we investigate hematite from three contrasting terrestrial environments: Archean (>2.5 Ga) banded iron formations (BIFs), laterites from the Cretaceous–Paleogene (∼65 Ma) Deccan Traps, and Jurassic (∼145–201 Ma) sedimentary concretions from Kutch, Gujarat, India. A multitechnique approach combining petrography, X-ray diffraction (XRD), Visible and Near-Infrared (VNIR) spectroscopy, and X-ray Absorption Spectroscopy (XAS) reveals distinct mineralogical and structural features across these samples. XANES data confirm Fe3+ as the dominant oxidation state, although subtle edge shifts in certain samples suggest incomplete oxidation. EXAFS analysis reveals Fe–O distances near 1.95–2.00 Å and variable Fe–Fe interactions, indicating differences in crystallinity and structural order. In particular, the superior type BIF sample exhibits diminished Fe–Fe coordination and shortened Fe–O paths, suggestive of ferrihydrite-like precursors and potential microbial mediation. WT-EXAFS mapping further highlights structural differences, with BIF samples displaying stronger midrange order compared to more disordered hematites in laterite. VNIR spectra show diagnostic absorption features near 0.65 and 0.85 μm, with weaker signals in BIF (superior type) possibly reflecting biogenic or diagenetic influences. These results underscore the value of integrated mineralogical and spectroscopic methods in distinguishing hematite formation pathways and provide a terrestrial analog framework for interpreting Martian hematite deposits and assessing possible biosignatures.