A systematic spectroscopic study of laboratory synthesized manganese oxides relevant to Mars

Abstract Mn‐rich minerals have been found by Mars rovers Curiosity and Opportunity in multiple sedimentary and basaltic outcrops at Gale and Endeavour craters, respectively, though the exact Mn‐bearing mineral species and the related geologic processes still need to be confirmed and elucidated. In the Kimberley region at Gale Crater, the Mn content of fracture‐filling materials that crosscut sandstone was evaluated to be higher than 25 wt%, indicating the existence of Mn oxides rather than evaporites or other salts. Furthermore, hydrated manganese oxides were also observed in Martian breccia meteorites Northwest Africa 7034 and 7533. However, due to the chemical complexities and often poor crystallinity of Mn‐oxide group, there are still difficulties for the quick and accurate phase identification by any single spectroscopic method. In this work, we focus on the systematic spectroscopic studies of seven pure Mn oxides (α‐MnO 2 , β‐MnO 2 , γ‐MnO 2 , δ‐MnO 2 , α‐Mn 2 O 3 , α‐Mn 3 O 4 , γ‐MnOOH) synthesized in laboratory, which were grown using a set of hydrothermal reaction and solid‐state calcination methods. Scanning electron microscope (SEM), energy‐dispersive spectrometer (EDS), and laser‐induced breakdown spectroscopy (LIBS) were used to obtain the morphology and chemical compositions. X‐ray diffraction (XRD) was employed for the phase identification and structural characterization of Mn oxides. Various spectroscopic techniques such as Raman, mid‐infrared (MIR), and visible and near‐infrared (VNIR) were used to demonstrate their spectral characteristics. We thus established a diverse spectral library including Raman, MIR, VNIR, and LIBS datasets of Mn oxides, facilitating for the phase identifications of Mn‐bearing minerals for future applications in Mars explorations missions (e.g., Tianwen‐1 orbiter and Zhurong rover and Perseverance rover) as well as terrestrial and other planetary explorations.