Comparative Raman and visible near‐infrared spectroscopic studies of jarosite endmember mixtures and solid solutions relevant to Mars

The chemical, structural and spectral properties of hydrated sulfate minerals are essential for their potential identifications and better understanding of martian aqueous history. Jarosite is an indicator of wet and acidic conditions on Mars. However, whether jarosites exist as solid solution phases or physical mixtures of endmembers is still an open question. In this study, we focused on comparisons of spectral characteristics (X‐ray diffraction, Raman, Visible and Near‐infrared) between Jarosite Solid Solutions (JSS) and corresponding Jarosite Physical Mixtures (JPM) with different molar ratios of endmembers (i.e. K‐jarosite to Na‐jarosite), in order to quantitatively characterize their spectral differences and mixing effects. For both X‐ray diffraction and Raman spectra, we find JPM have more overlapped peaks than JSS due to physical mixing effects of the formers, and the relative intensity of the overlapped peaks in K–Na JPM is relevant to the percentage of endmembers. On the other hand, JSS and JPM have strikingly similar Visible and Near‐infrared spectral features except the absorption features at 1950 and 2225 nm, where JPM have larger absorption depth. The detailed spectra properties of these two series of jarosite samples would provide a valuable database for current orbital remote sensing detections and future in situ Raman investigations on Mars (e.g. ExoMars and Mars 2020). Copyright © 2017 John Wiley & Sons, Ltd. With the purpose to discriminate Jarosite Physical Mixtures (JPM) and Jarosite Solid Solutions (JSS) by spectroscopy in Mars exploration, we acquired their Raman and Visible Near‐infrared spectra and found that JSS exhibit continuous systematic Raman peak shifts varying with chemical compositions, while JPM have more overlapped peaks due to physical mixing effects. Besides, ν(OH)− modes of JSS are wider compared with JPM, indicating the locally disordered structure of JSS led by the different cation substitutions.