Identification of tunnel structures in manganese oxide minerals using micro‐Raman spectroscopy.

Manganese oxides have attracted the interest of many researchers due to their broad application field. These oxides possess various electrochemical, adsorption and catalysis properties and are as such applied in the industrial context, for example, steel industry, catalysis and removal of toxins in the ground or in waste water. In the archaeometrical field, manganese oxides have been used as pigments on various artefacts such as pottery and rock art paintings. The diversity of manganese oxide applications highlights the need for a well‐defined Raman spectroscopic database with correct characterization of these compounds. However, identification of manganese oxides by Raman spectroscopy is challenging due to the wide variety of manganese oxides, and manganese oxides are weak Raman scatterers and are susceptible under the laser light. As such, characterization by Raman spectroscopy is not straight forward. In this research, we have focussed on seven natural, mineral (tunnel) samples that were identified by micro‐Raman spectroscopy using two different laser wavelengths (785 and 532 nm). The experimental conditions were controlled to not introduce any modifications (degradation and/or phase transition) of the manganese oxide species. Both lasers have proven to be successful and were able to retrieve well‐characterized Raman spectra for each of the manganese oxide phase, which were compared with published literature. [ABSTRACT FROM AUTHOR]