Experiments on Al‐Si Hydrolysis Systems With Different Si/(Al + Si) Molar Ratios: Implications for Identification of Poorly Ordered Al/Si Phases on Mars

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Abstract
Poorly ordered Al/Si phases are widely distributed across the surface of Mars, among which allophane and amorphous silica are the two main constituents. Both allophane and amorphous silica can form in Al‐Si systems through surface chemical weathering or subsurface hydrothermal alteration of volcanic materials. Nevertheless, our comprehension of the products derived from Al‐Si hydrolysis systems remains poorly constrained. Hydrolysis experiments were conducted on Al‐Si systems across an unprecedentedly wide range of Si/(Al + Si) molar ratios ( n , 0 ≤ n ≤ 0.9), followed by characterizing the products using multiple techniques. At n ≤ 0.1, the products consist predominantly of Al 30 (an Al polycation with a Keggin structure) and poorly ordered Al hydroxides. The introduction of Si resulted in formation of a small amount of proto‐allophane. At n = 0.2 and 0.3, poorly ordered materials were still dominant, along with the presence of well‐crystallized bayerite and gibbsite. The proto‐allophane increased in quantity and began to assemble into allophane. At n = 0.5, well‐crystallized minerals were absent and allophane dominated the product. At n = 0.7, the amount of allophane decreased significantly and at n = 0.8 and 0.9, allophane was probably absent, although proto‐allophane still formed. Meanwhile, an increasing amount of amorphous silica was formed. X‐ray diffraction, Fourier transform infrared, and VNIR provide information for differentiating Al‐rich phases and Si‐rich phases but show limited capability in identifying poorly ordered Al/Si‐rich phases. NMR is powerful for identifying poorly ordered Al/Si phases, although widespread iron on the martian surface and the large instrument pose challenges to its application on Mars.
Funding Information
  • Science and Technology Planning Project of Guangdong Province (2023B1212060048)

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