Structure of allophanes with varied Si/Al molar ratios and implications to their differentiation on Mars

Abstract

Allophane occurs ubiquitously in soils of volcanic origin on Earth and is also a probable constituent on martian surface, serving as a natural adsorbent and/or an environmental indicator. It remains to be one of the few minerals whose structure has not been unequivocally defined, as a result of its ultrasmall particle size, extremely low structural order and highly variable Si/Al molar ratios. To gain insights into the structure and probably also the formation processes of allophane, we hydrothermally synthesized allophanes at varied initial Si/Al molar ratios (0.5, 0.57, 0.66, 0.75, and 1.0), followed by characterizing their structure using a combination of advanced techniques. X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectra, and 27Al/29Si magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectra show that bayerite was obtained at initial Si/Al molar ratios of 0.5 and 0.57. FTIR and 29Si/27Al MAS NMR spectra also suggest the presence of oligomeric silicates in all the products and the significant amount of AlIV occurring invariably in both natural and synthetic allophanes is located in these oligomeric silicates. A possible explanation for the presence of oligomeric silicates is that the formation of imogolite local structure (ImoLS, which constitutes the skeleton of allophane) and the attachment of oligomeric silicates to it occur simultaneously. The scanning transmission electron microscopy (STEM) and atomic force microscopy (AFM) images showed that these products exhibited different aggregation states, probably resulting from formation of byproducts such as bayerite and proto-allophanes. The visible and near-infrared (VNIR) spectra exhibited little obvious differences resulting from Si/Al molar ratios, indicative of the weakness of this technique in detecting the geochemistry of Al and Si in poorly ordered aluminosilicate components at Mars' surface. Instead, the significant increases of intensity ratios observed in the FTIR bands (1043 cm−1 / 972 cm−1) and the 27Al/29Si MAS NMR signals (60 ppm / 7.6 ppm and (−80 ppm − −110 ppm) / −79 ppm, respectively) with increase of the Si/Al molar ratio of allophane indicate FTIR and MAS NMR could be used in combination with VNIR on future Mars orbiters/rovers.