Effects of Environmental Fe Concentrations on Formation and Evolution of Allophane in Al‐Si‐Fe Systems: Implications for Both Earth and Mars

Abstract

Identifying the mineralogy on the surface of Mars helps to deduce the formation and evolution of sediments and the possible paleoenvironments involved (e.g., Bishop et al., 2018; Fraeman et al., 2013). At present, based on the visible and near-infrared (VNIR) spectroscopic data collected by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter and Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) onboard the Mars Express orbiter, the global mineralogy on Mars, including mainly the distribution of phyllosilicate (Poulet et al., 2005) and hydrated sulfates (Gendrin et al., 2005), has been recorded. Analyses of these data suggest that distinct aqueous geochemical environments existed on Mars, including warm surface waters (Bishop & Rampe, 2016), subsurface hydrothermal systems (Ehlmann et al., 2011) and hot springs (Al-Samir et al., 2017). For instance, the phyllosilicate outcrops on the surface of Mars mainly include Fe-Mg smectite (Ehlmann & Edwards, 2014); on the basis of the fact that smectite minerals typically occurs in surficial soils or sediments on Earth (Chamley, 1989), these outcrops might have formed in warm waters (25°C-50°C) on early Mars. Apart from the above mentioned well-crystallized minerals, data from both orbital spectroscopy and in situ rover investigations suggest the wide distribution of poorly crystallized and amorphous hydrated minerals on the surface of Mars, such as opal-A, allophane, and ferrihydrite (e.g., Rampe et al., 2012; Vaniman et al., 2014). Among them, allophane and allophane-like materials are considered to be common Abstract Allophane, a common component on Earth and a probable constituent of the amorphous component on Mars, is closely associated with Fe in the form of structural Fe and/or iron (oxyhydr)oxide coatings. However, until now, the formation and evolution of allophane as products of environmental Fe concentrations have rarely been studied. We investigated allophane precipitation from gels with different Fe/(Fe + Al) molar ratios (n, 0 ≤ n ≤ 1.0). X-ray diffraction patterns and Fourier transform infrared spectra of the products showed that allophane was nearly the only product at n ≤ 0.2 and that the crystallinity of Fe-rich allophane decreased with increasing n. Combined with the results of transmission electron microscopy and Mössbauer spectroscopy, Fe was found to be mainly incorporated into the gibbsite-like sheet of allophane, forming clusters; the highest Fe-for-Al substitution content was roughly estimated to be 20 mol.%. As n increased further, the formation of allophane was increasingly suppressed, and the Fe phases began to separate from the Al-Si phases, resulting in mixtures of incipient allophane and incipient akaganeite and finally akaganeite. The near-infrared spectroscopic data (1.2-2.6 μm) of the products showed incapability in identifying poorly ordered minerals in Al-Si-Fe systems, while the features in the range of 0.4-1.2 μm were powerful for studying iron occurrence in t...

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

“…In the TEM images of Fe 0.60 Si 0.40 (Figures 5d and 5e), two phases with different morphologies were observed, one appearing as roughly rounded individual crystals with a narrow size distribution (40∼50 nm) and the other as aggregates of nanoparticles with much smaller sizes (7∼10 nm). The finer phase was more Fe‐rich than the phase with a larger size because Fe has a much larger atomic number than Si and thus can be identified with ease by its significantly higher contrast in the TEM images (blacker in the bright field images and whiter in the dark field images) (Du et al., 2020). This assumption was further supported by the EDS mapping where the former phase was rich in only Si and the latter was rich in both Fe and Si (Figure 5e), to which a possible explanation is that the former phase is amorphous silica while the latter is a novel Fe‐O‐Si phase.…”

“…These materials likely account for about 45% of the sediments on the martian surface (Blake et al, 2013). Due to their structurally unstable nature and sensitivity to environmental changes, these materials may serve as robust indicators in interpreting the details of the paleoenvironment changes (Bishop & Rampe, 2016;Du et al, 2020).…”

Imogolite: Curvature‐Induced Hospitality for Trivalent Dopants

“…In addition, our study points to the curvature of aluminosilicate layers as a collateral factor potentially affecting geochemical circulation of cations in natural clay minerals containing imogolite or proto‐imogolitic allophane. [ 36,37,56 ] We hope that our preliminary insight would stimulate a further quantum‐chemical research on the metal dopants of imogolite with other valencies as well as on the possible routes of their charge compensations (vacancies, extraprotons, etc.). Seemingly, synthesis or minerogenesis of a metal‐doped imogolite is restricted only by the overlap of the phase stability windows for precursor reactants or by kinetic factors, which is beyond the capabilities of quantum‐chemical level of theory.…”

“…[31] However, such functionalization modulates polarity of imogolite wall, potentially enhancing the surface reactivity of imogolite as lipophilic substrate. [32] The examples of the second and third strategies for functionalization are rather scarce and are realized so far for isomorphic substitution of Si on Ge [33] and only for partial isomorphic substitution of Al on Fe, [31,[34][35][36][37] enabling regulation of the morphology, the dispersity, the wall thickness and the reactivity of imogolite and protoimogolite nanoparticles. Moreover, these strategies could lead to fabrication of many other families of imogolite-like nanotubes, yet, only the aluminogermanate imogolites have been profoundly identified and characterized to date.…”

Chemical weathering over hundreds of millions of years of greenhouse conditions on Mars

“…The upper unit has a broader asymmetric 2.20 μm absorption, suggesting the presence of opaline silica or allophane/imogolite common in weathered volcanic ash 41 . The presence and preservation of amorphous materials imply a cold and waterlimited environment at that time 33,52 . An important caveat to consider is that it might not be possible to distinguish between the age of deposits in which a weathering profile occurs and the age of the chemical weathering event itself.…”