Evaluation of scanning transmission X-ray microscopy at the Mn L2,3-edges as a potential probe for manganese redox state in natural silicates

Abstract

Determining the Mn valence variation at the nanometer scale will be an important advance in the study of heterogeneous natural silicates. Here, the potential of the scanning transmission X-ray microscopy at the Mn L2,3-edges (640–655 eV) as a probe for manganese redox state is evaluated. For this purpose, several natural Mn-silicates (rhodonite, ardennite, piemontite, Mn4+-silicate, jacobsite) were analysed to identify the spectral parameters most sensitive to the Mn valence, regardless of the coordination environment, the crystal field strength, the nature and the length of the metal–ligand bonds, and the intra-atomic Coulomb and spin–orbit interactions. Two suitable spectral empirical calibrations are thus proposed, linking the Mn valence to two peak intensity ratios: one ratio of intensities from two energy points of the L2 peak (at 651.7 and 655.2 eV), and one ratio of intensities from one energy point of the L2 peak (at 655.2 eV) and one of the L3 peak (at 641.6 eV). Thank to them, the first quantitative Mn valence maps are constructed, with a high spatial resolution (<40 nm pixel size), opening the way to exhaustive crystallochemical studies of silicates containing Mn with different valences.