Crystal Chemistry and NIR Spectral Responses of Fe/Mg-Rich Smectites: Implications for Clay Detection on Mars

Over the past decade, remote sensing exploration has revealed widespread detection of clay minerals in Noachian terrains (>3.7 Ga) on Mars, suggesting a period of active near-surface water–rock interaction. The in-depth characterization of the crystal structure and chemistry of laboratory-synthesized smectite minerals with varying compositions offers a unique perspective to further constrain their formation mechanism, environmental conditions, and the climate of early Mars. This study synthesized smectites with varying Fe/Mg ratios to compare their near-infrared (NIR) spectral characteristics with those of representative areas on Mars. The study revealed that the characteristic metal–OH (M–OH) band in the range of 2270–2320 nm shifted toward longer wavelengths as the octahedral Fe³⁺ content decreased. When the octahedral Fe/Mg ratio was close to 1, the position of M–OH was similar to that of Mg-smectite (saponite), indicating that although the M–OH band of smectite is located at ca. 2308–2312 nm, it may still contain a significant amount of octahedral Fe³⁺. This suggests that there are likely still many Fe-smectites in Mg-rich smectite outcrops (ca. 20% as in the literature) on the Martian surface. Furthermore, correlation analysis between the smectite layer stacking order index (V/P) and the second-order derivative band intensity ratio (Deriv2 ratio) showed that when the Deriv2 ratio is small (generally <8), smectite has a relatively high degree of layer stacking order. When the Deriv2 ratio is >10 and tends toward infinity (+∞), the layer stacking order of smectite is very low. The above results suggest that the crystal chemical characteristics of smectites should be carefully studied when analyzing the mineral composition of the Mars surface using remote sensing or in situ NIR spectra. Smectites on Mars generally exhibit a high layer stacking order, indicating the presence of a relatively long-term warm and humid climate in Martian history. However, later alteration processes can significantly reduce their crystallinity.