Mathematical Framework for the Effects of Sample Absorption on Raman Intensity for Quantification and Detection...

On Earth and other planets, Raman instruments can characterize minerals in situ in a nondestructive and rapid manner without sample preparation. However, they commonly detect phases in mixtures because field instruments are often limited in the magnification that can be achieved (≳100 μm/spot). The effects of mixing can create challenges in detecting and quantifying the prevalence of phases of interest, particularly when there are dark minerals that attenuate photons nonlinearly with their abundance. In this work, we develop mathematical models that parameterize the effects of sample absorption on Raman intensity for three types of two-component solid mixtures: (1) low absorptivity + low absorptivity minerals, (2) low absorptivity + high absorptivity minerals, (3) high absorptivity + high absorptivity minerals. Subsequently, we verified these models through laboratory measurements of Raman peak intensities for hydrated sulfates and phyllosilicates in mixtures. We find that Raman intensity as a function of concentration is linear for mixtures made of solely low absorptivity minerals but follows a nonlinear relationship for mixtures containing one or more high absorptivity minerals. Data quantify the challenges of detection of dark minerals with Raman spectroscopy, e.g., the hydration peak of 13 vol % nontronite is detected in a low absorptivity anhydrite matrix, but three times that amount is required to detect nontronite in a high absorptivity basalt matrix. On Mars or other field settings with limited independent data, our models can be used in conjunction with microimages or other quantitative compositional instruments to estimate the detection thresholds and abundances of minerals.