A Reliable Method for Determining the Oxidation State of Manganese at the Microscale in Mn Oxides via Raman Spectroscopy

Abstract

Manganese oxides are important geomaterials, widespread in terrestrial and Martian environments. Characterisation of the oxidation state of Mn is a central issue in science; this task has been addressed up to the present by X‐ray spectroscopy or diffraction techniques. The former, however, requires access to synchrotron facilities, while the latter does not provide crystal‐chemical information at the local scale. In this work, we compare a large set of Raman data from well‐characterised samples, already published by the same authors of this paper or as found in the literature. We show a clear correlation between the oxidation state of Mn and the wavenumber of peculiar bands; octahedrally co‐ordinated Mn2+ is recognised by a band around 530 cm−1, Mn3+ by a band around 580 cm−1 and Mn4+ by a band around 630 cm−1, while tetrahedrally co‐ordinated Mn2+ is recognisable by a band around 650 cm−1. Strongly distorted Mn3+ octahedra are indicated by the appearance of Jahn–Teller modes. Our method allows a reliable, easily accessible tool to characterise the oxidation states of Mn in oxides, also suitable for microscale mapping. It provides a robust analytical basis for the use of these minerals as redox indicators in geology/geochemistry, in exoplanetary research or for monitoring technological processes.

Brattforsite, Mn19(AsO3)12Cl2, a new arsenite mineral related to magnussonite, from Brattforsgruvan, Nordmark, Värmland, Sweden

“…The peaks between 700 and 500 cm − 1 , including minor ones, for both minerals are resulting either from stretching vibrations of Mn 2+ -O(Cl,OH) bonds or from bending vibrations of (AsO 3 ) 3− . Specifically, the band at 510 cm − 1 is consistent with octahedrally coordinated Mn 2+ , whereas the one at 660 cm − 1 would be compatible with lower coordination numbers for this cation (Bernardini et al 2021). Peaks in the low-frequency region from ca.…”

Revealing High‐Manganese Material on Mars at Microscale

“…This instrument will be used to assess the geological history at Jezero crater through the identification of alteration minerals with Raman technique. Recently, Bernardini et al (2021) demonstrated that Raman is a reliable method for determining the oxidation state of Manganese at the microscale. This suggests that the Perseverance rover is expected to obtain in-situ Raman data for high-Mn material on Mars.…”

Carbon isotopic composition of Frutexites in subseafloor ultramafic rocks

“…3) that showed peaks at 580 cm -1 , which can be assigned to octrahedrally coordinated Mn 3? (Bernardini et al 2021) and/or todorokite (Ostrooumov 2017), suggesting an oxidizing precipitation environment. Several of the prominent Raman peaks in the MS spectra may be attributed to various Mn phases and species, such as the relatively large and sharp peak at 636 cm -1 , which can be ascribed to todorokite and anhydrous manganese oxide.…”

“…3c, d) and in the Raman spectra that showed peaks at 580 cm -1 , which can be assigned to octrahedrally coordinated Mn 3? (Bernardini et al 2021) and/or todorokite (Ostrooumov 2017). A few peaks coincided with sharp spectral peaks at 1082 cm -1 and minor peaks at 153, 180, 209 and 278 cm -1 corresponding to the reference RRUFF ID R050128 of calcite.…”