Mafic silicate mapping on Mars: Effects of palagonitic material, multiple mafic silicates, and spectral resolution

The visible to near-infrared spectral reflectance properties of intimate and areal pyroxene + palagonitic material mixtures as well as pure mafic silicates (low-calcium pyroxene, high-calcium pyroxene, pigeonite, olivine) and mixtures of these minerals were analyzed at high spectral resolution (5 nm) as well as with non-contiguous band passes equivalent to recent HST observations and the Pathfinder IMP in order to determine the quality and quantity of mineralogical information (end member compositions, abundances, and grain sizes) derivable in the presence of palagonitic material. In the case of pyroxene + palagonitic material mixtures, pyroxene is detectable at abundances as low as 10 wt%, and its composition can be constrained because (a) its diagnostic absorption feature (located near 1000 nm) persists even for high palagonitic material abundances, and (b) palagonitic material does not appreciably alter the wavelength position of this band (<4 nm variation). For broad band data (such as Pathfinder IMP band passes), different mafic silicates can be discriminated and palagonitic material abundances constrained using a variety of reflectance ratios and three-point "absorption band depths." However, other properties of mafic silicate ± palagonitic material assemblages, such as mafic silicate major element compositions, grain sizes, and end member abundances, generally cannot be rigorously quantified. The use of multiple reflectance ratios can, however, be used to identify relative changes in these properties, as most changes in mafic silicate ± palagonitic material assemblage properties are characterized by a unique corresponding set of reflectance ratio variations. The observed spectral-assemblage property trends are consistent with those expected from the known spectral properties of the end members.