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

Abstract

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.

GeoRaman 2022

“…Furthermore, the spectra are generally not easily interpretable, typically containing broad bands of complex internal structure. Vermeersch et al 4 analysed seven natural mineral manganese oxide samples with tunnel structures by micro‐Raman spectroscopy using two different laser wavelengths (785 and 532 nm).…”

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

“…minerals are composed of main structural building blocks, that is, MnO 6 octahedra, that are arranged in a particular order. This structure can either be tunneled in which cations can be encapsulated (tectomanganates) 3 or layered (phyllomanganates) in which sheets of MnO 6 octahedra alternate with metal oxide layers and H 2 O. 2 Both layered and tunneled Mn oxides became increasingly popular, owing to their widespread applications and specific properties.…”

“…The characterization of manganese oxides with Raman spectroscopy has a proven track record, 2,3,5,[26][27][28] and the technique can be used to analyze both tunnel 3,27 and layered structures 2 in natural samples 2,3,26,27 and synthetic manganese oxides. 4,29,30 From an archaeological point of view, the non-destructive nature and in situ measurement capability of Raman spectroscopy adds an extra advantage.…”

“…Secondly, manganese oxides are known to be weak Raman scatterers. 3 An increase in measuring time and/or power is then often imposed. Mobile instrumentation is, however, typically used for its relatively quick analysis.…”

“…Many of the tunneled structures (i.e., pyrolusite, hausmannite, and manganite) have already been extensively studied and characterized. 3,26,27,29,33 Due to its industrial applications, birnessite is one of the most widely studied layered manganese oxide. However, ambiguous Raman data for this mineral have been reported.…”