^1,2,3,4,5Yuhuan Yuan,^1,2,3,4Ke Wen,^1,2,3,4Yiping Yang,⁶Chaoqun Zhang,^1,2Xiaorong Qin,^1,2,3,4,5Jianxi Zhu,^1,2,3,4,5Hongping He,⁷Joseph W. Stucki
Journal of Geophysical Research: Planets (in Press) Link to Article [https://doi.org/10.1029/2026JE009683]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy ofSciences, Guangzhou, PR China,
²Guangdong Provincial Key Laboratory of Mineral Physics and Materials, GuangzhouInstitute of Geochemistry, Chinese Academy of Sciences, Guangzhou, PR China,
³Center for Advanced Planetary Science,Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, PR China,
⁴Guangdong Research Centerfor Strategic Metals and Green Utilization, Guangzhou, PR China,
⁵University of Chinese Academy of Sciences, Beijing,PR China,
⁶Key Laboratory of Deep Petroleum Intelligent Exploration and Development, Institute of Geology andGeophysics, Chinese Academy of Sciences, Beijing, PR China,
⁷Department of Natural Resources and EnvironmentalSciences, University of Illinois at Urbana–Champaign, Urbana, IL, USA
Published by arrangement with John Wiley & Sons

Nontronite, a Fe3+-rich smectite widely identified on Mars, serves as a key mineral indicator for reconstructing paleo-redox and paleo-aqueous environments. However, uncertainties in interpreting its spectral data hinder a precise understanding of its formation conditions and paleoenvironmental implications. To fill this gap, the present study investigated the controls of nontronite formation and its crystallographic-spectral relationships by synthesizing a series of Fe-Si-Al samples with varying Fe/Si molar ratios under hydrothermal conditions. Results demonstrated that crystalline nontronite forms exclusively within a Fe/Si molar ratio of 0.21–0.48 under the simulated alkaline conditions. Incorporation of Fe3+ into tetrahedral sites as [IV]Fe3+ reduced the tetrahedral-octahedral sheet mismatch, thereby enhancing the crystallinity of nontronite. This crystallographic evolution was systematically observed in Mid Infrared and Visible-Shortwave Infrared spectra: [IV]Fe3+ content negatively correlated with the Si-O vibration wavenumber (near 1,000 cm−1) but positively correlated with the 2Fe3+-OH band position (∼1,430 nm) and depth. Furthermore, band depths at ∼1,430 and ∼2,290 nm are robust proxies for the crystallinity of nontronite in the absence of byproducts. These findings constrain the formation of nontronite on Mars to oxidizing, alkaline subsurface hydrothermal environments during the early Noachian, which represents one of the possible pathways for nontronite formation. These results provide a refined framework for interpreting orbital and in situ spectral data, advancing the understanding of clay mineral formation and environmental evolution on Mars.