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. 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 systems Mössbauer spectroscopy shows great potential in identifying and differentiating between ferrihydrite-like materials 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.