Laboratory Raman and VNIR spectroscopic studies of jarosite and other secondary mineral mixtures relevant to Mars

Abstract Jarosite is an indicator of ephemeral, acidic fluids in Martian history, which often occurs with some other secondary minerals based on Mars in situ and remote sensing detections. However, detailed Raman and visible and near‐infrared (VNIR) spectroscopic studies of jarosite and other secondary mineral mixtures relevant to Mars still lack, which are essential for jarosite identifications and hence a better understanding of Martian aqueous history. In this study, we prepared 44 binary mixtures of jarosite with four other secondary minerals (alunite, gypsum, kaolinite, and montmorillonite), and characterized their detailed Raman and VNIR spectral features. We find the Raman spectral patterns of jarosite, alunite and gypsum are distinctive and ready for their identification in mixtures, although mixtures with montmorillonite and kaolinite show high fluorescence background and their Raman spectral features are obscured by strong and sharp peaks of jarosite. The normalizing strengths of their characteristic Raman spectral features (e.g., ν 1 for jarosite) change with jarosite mass fractions, and mineral distribution maps and volume percentages of endmembers are acquired using Raman imaging method. The band depths of absorption features in VNIR spectra are also correlated with jarosite mass proportions, and four models are proposed to evaluate jarosite mass fractions in the mixtures using a partial least squares algorithm. These detailed spectral properties of mixtures and models would provide valuable information for in situ Raman investigations on Mars (e.g., ExoMars and Mars 2020) and orbital VNIR remote sensing detections (e.g., Compact Reconnaissance Imaging Spectrometer for Mars).