Raman analyses of Al and Fe/Mg-rich clays: Challenges and possibilities for planetary missions

Clays are fine-grained rocks that form in the presence of water. Their occurrence typically depends on a combination of geological setting and geochemical conditions. The widespread and diverse clay deposits identified on Mars indicate that the planet experienced multiple episodes of water activity, especially in the early period. The characterisation of Martian clays will reveal key information on geological history, the past climatic conditions and the overall habitability of Mars. Among the analytical techniques typically utilised for such mineralogical characterisation, Raman spectroscopy will be applied for the first time during several robotic exploration missions of Mars in the 2020s. In this study, various clay-rich samples from Westerwald, Germany were analysed to help inform mission operations and interpretation of data returned by the miniaturised Raman instruments. The samples, as verified by infrared spectroscopy, constitute kaolinites, Al-rich smectites and Fe/Mg-rich smectites, which are relevant to the clay mineralogy expected at Martian landing sites (Jezero crater and Oxia Planum). Despite the levels of fluorescence experienced during Raman measurements of the clay-rich powder samples when using a 532 nm laser, the Raman signatures of phyllosilicates were identified for kaolinite and several different types of smectite (e.g. montmorillonite, nontronite, saponite) in these complex dispersions. Following these measurements, identification of various accessory minerals disseminated in the clay matrix was also achieved using the micro-Raman system. Finally, a brief discussion concerning the analytical strategies for successful clay analyses using miniaturised Raman instruments is provided.