Laboratory reflectance spectra of clay minerals mixed with Mars analog materials: Toward enabling quantitative clay abundances from Mars spectra

Quantitative estimates of clay minerals on the martian surface, via remote sensing observations, provide constraints on activity, timing, duration, and extent of aqueous processes and the geochemical environment in martian history. We describe an analytical study to begin enabling quantitative estimates of phyllosilicates when mixed with martian analog materials. We characterize the chemistry, mineralogy, particle size distribution, and reflectance spectra of the end-member materials: saponite, montmorillonite, pyroxene, and palagonitic soil. Reflectance spectra were obtained for physical mixtures of saponite and montmorillonite with pyroxene, and saponite with palagonitic soil. We analyzed the diagnostic phyllosilicate spectral signatures in the 2.2-2.4 μm wavelength region in detail for the mixtures. This involved fitting the observed ∼2.3 or ∼2.2 μm band depth, associated with the presence of saponite and montmorillonite, respectively, as a function of the abundance of these materials in the mixtures. Based upon the band depth of the spectral features we find that 3-5 wt.% of the clay minerals in the mixture with pyroxene can be recognized and at 25 wt.% their presence is indisputable in the mixtures. When the saponite is mixed with the lower albedo palagonitic soil, its presence is clearly distinguishable via the 1.4 and 2.3 μm features at 25 wt.% abundance. These relationships, between abundance and band depth, provide an ability to quantitatively address the amount of these materials in mixtures. The trends described here provide guidance for estimating the presence of phyllosilicates in matrices on the martian surface.