Layered manganese oxides structures: Micro-Raman and selected mobile Raman spectroscopic studies

The main structural building blocks that form manganese oxides are MnO₆ octahedra; these share corners and edges to construct specific structures, which can either be tunneled or layered. In the layered structures, that is, phyllomanganates, the MnO₆ octahedra form sheets, which, in turn, alternate with sheets of metal oxides and H₂O. These metal ions can vary (Zn, Co, Ni, Al, Li, …) and give rise to an entire range of different metal oxides. The characterization of these layered materials is important as they have various economical/industrial applications. Birnessite-type materials, a specific type of layered manganese oxides, are widely studied for their use as cathode materials in alkali-ion batteries. Phyllomanganates are also commonly found as constituents in sediments and soils or as coatings on rock surfaces. Their natural occurrence as black colored components have ensured that these minerals were also applied as pigments in archaeological and historical contexts. They are, for example, often found in rock art paintings and on pottery. As the oxides are used in unique archaeological objects, Raman spectroscopy is an evident choice for characterization due to its non-destructive nature of analysis. In the current study, five mineral samples of (layered) manganese oxides are analyzed with different Raman instrumentations, including mobile systems and a benchtop micro-Raman setup. The characterization of each selected manganese oxide and their comparison with literature data is discussed for the micro-Raman instrumentation. In addition, the ability of identifying and characterizing layered manganese oxides and the possible challenges when using mobile instrumentation are discussed as well.