Modeling near-infrared reflectance spectra of clay and sulfate mixtures and implications for Mars

High-resolution mapping by visible and near-infrared orbital spectrometers has revealed a diversity of hydrated mineral deposits on the surface of Mars. Quantitative analysis of mineral abundances within these deposits has the potential to distinguish depositional and diagenetic processes. Such analysis can also provide important constraints on the nature of putative global and local-scale mineralogical transitions on Mars. However, the ability of models to extract quantitative mineral abundances from spectra of mixtures relevant to sedimentary rocks remains largely untested. This is particularly true for clay and sulfate minerals, which often occur as fine-grained components of terrestrial sedimentary rocks and are known to occur in a number of sedimentary deposits on Mars. This study examines the spectral properties of a suite of mixtures containing the Mg-sulfate epsomite mixed with varying proportions of smectitic clay (saponite, nontronite, and montmorrilonite). The goal of this work is to test the ability of checkerboard (linear) and intimate (non-linear) mixing models to obtain accurate estimates of mineral abundances under ideal and controlled laboratory conditions. The results of this work suggest that: (1) spectra of clay–sulfate mixtures can be reproduced by checkerboard and intimate mixing models to within 2% absolute reflectance or single scattering albedo, (2) clay and epsomite abundance can be modeled to within 5 wt.% when particle diameter is optimized, and (3) the lower threshold for modeling clay in spectra of clay–epsomite mixtures is approximately 10 wt.%, below which the models often fail to recognize the presence of clay.