Mineralogical constraints on the high-silica martian surface component observed by TES

The Thermal Emission Spectrometer (TES) has observed a high-silica material in the dark regions of Mars that is spectrally similar to obsidian glass and may have a volcanic origin. An alternate interpretation is that the spectrally amorphous material consists of clay minerals or some other secondary material, formed by chemical alteration of surface rocks. The regions where this material is observed (e.g., Acidalia Planitia) have relatively high spectral contrast, suggesting that the high-silica material exists as coarse particulates, indurated soils or cements, within rocks, or as indurated coatings on rock surfaces. The geologic interpretation of this spectral result has major implications for understanding magmatic evolution and weathering processes on Mars. One of the complications in interpreting spectral observations of glasses and clay minerals is that both are structurally and compositionally complex. In this study, we perform a detailed spectroscopic analysis of indurated smectite clay minerals and relate their thermal emission spectral features to structural and crystal chemical properties. We examine the spectral similarities and differences between smectite clay minerals and obsidian glass from a structural-chemical perspective, and make further mineralogical interpretations from previous TES results. The results suggest that neither smectite clays nor any clay mineral with similar structural and chemical properties can adequately explain TES observations of high-silica materials in some martian dark regions. If the spectrally amorphous materials observed by TES do represent an alteration product, then these materials are likely to be poorly crystalline aluminosilicates. While all clay minerals have Si/O ratios ≤0.4, the position of the emissivity minimum at Mars suggests a Si/O ratio of 0.4-0.5. The spectral observation could be explained by the existence of a silica-rich alteration product, such as Al- or Fe-bearing opal, an intimate physical mixture of relatively pure silica and other aluminosilicates (such as clay minerals or clay precursors), or certain zeolites. The chemical alteration of basaltic rocks on Mars to phyllosilicate-poor, silica-rich alteration products provides a geologically reasonable and consistent explanation for the global TES surface mineralogical results.