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

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.