Hydrolysis Products of Fe(III)‐Si Systems With Different Si/(Si + Fe) Molar Ratios: Implications to Detection of Ferrihydrite on Mars.

Ferrihydrite, a nanocrystalline iron (oxyhydr)oxide mineral, is widely distributed in soils and sediments on Earth and is probably an important component and/or precursor of widespread nanophase iron minerals on Mars. Terrestrial ferrihydrite often co‐occurs with amorphous silica and/or contains a certain amount of Si in its structure. However, it remains ambiguous how environmental Si concentration affects the formation‐evolution and structure‐spectral features of ferrihydrite in the Fe(III)‐Si systems. To this end, hydrolysis experiments were carried out for Fe‐Si systems at an unprecedentedly wide range of initial Si/(Fe + Si) molar ratios (0–0.80), followed by characterizing the products detailly. X‐ray diffraction, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, Mössbauer spectroscopy, and transmission electron microscopy results showed that at Si/(Fe + Si) molar ratios ≤0.30, the main phase of the products was ferrihydrite, of which the unit cells enlarged, the crystallinity decreased, and the existing state of Fe changed with increased Si contents; at Si/(Fe + Si) molar ratios ≥0.40, ferrihydrite was no longer formed and a novel amorphous Fe‐O‐Si phase was instead obtained, with the excess Si forming amorphous silica. The visible and near‐infrared spectroscopy, the most powerful tool to detect hydrous minerals on the surface of Mars at global or regional scales, showed weakness in identifying ferrihydrite‐like materials obtained in the Fe‐Si systems. Raman spectroscopy can identify ferrihydrite and Si‐containing ferrihydrite but cannot differentiate between them. Mössbauer spectroscopy showed great potential in both identifying and differentiating between ferrihydrite and Si‐containing ferrihydrite, and thus can be used to characterize the poorly ordered iron (oxyhydr)oxides on Mars. Plain Language Summary: On the surface of ancient Mars where liquid water was available, Fe‐Si hydrolysis systems were likely common as a result of the high abundances of Fe and Si in the crust of Mars. By performing hydrolysis experiments of Fe‐Si systems at an unprecedented wide range of Si/(Fe + Si) molar ratios, we found that the hydrolysis products of Fe‐Si systems varied among combinations of ferrihydrite, Si‐containing ferrihydrite, an unknown Fe‐O‐Si phase, and amorphous silica. We also found that the visible and near‐infrared reflectance spectroscopy, the most powerful tool to remotely detect hydrous minerals on the surface of Mars at global and regional scales, showed weakness in identifying ferrihydrite, Si‐containing ferrihydrite, and the novel Fe‐O‐Si phase; Raman can be used to identify ferrihydrite‐like materials but cannot be used to differentiate between ferrihydrite and Si‐containing ferrihydrite; mid‐infrared spectroscopy can differentiate ferrihydrite from Si‐containing ferrihydrite and the novel Fe‐O‐Si phase; Mössbauer spectroscopy (recorded at varied temperatures) provided clues to identifying and further differentiating between ferrihydrite‐like materials and thus holds great potential for detecting Fe/Si‐rich phases on the surface of Mars. Key Points: Hydrolysis experiments of Fe(III)‐Si systems were performed at an unprecedentedly wide range of initial Si/(Fe + Si) molar ratios (0∼0.80)The environmental Si concentration controls the formation and evolution of ferrihydrite in the Fe(III)‐Si systemsMössbauer spectroscopy shows great potential in identifying and differentiating between ferrihydrite‐like materials [ABSTRACT FROM AUTHOR]