Determination of manganese valence states in (Mn3+, Mn4+) minerals by electron energy-loss spectroscopy

Abstract

Various manganese valence quantification methods using manganese L2,3 and oxygen K electron energy loss near-edge spectra (ELNES) were applied to determine the relative portions of individual valence state in a mixed (Mn3+, Mn4+) valence system. Multiple linear least-squares (MLLS) fitting of Mn L2,3 ELNES using reference spectra and Gaussian peak fitting of Mn L3 edge are newly developed and the feasibility of these methods was tested on a set of cryptomelane minerals with different valence states. The selection of appropriate standards is crucial to the success of the MLLS method. The O K-edge structures for manganese oxides can provide valuable guidance in the selection of appropriate reference spectra for quantitative determination of Mn valence state. Gaussian peak fitting, however, failed to determine the Mn valence for (Mn3+, Mn4+) minerals due to the small separation between the primary L3 peaks from Mn4+ and Mn3+ valence. As to the methods based on calibration curves, the energy difference between Mn L3 and oxygen K, i.e., ΔE (L3-O K) vs. valence, is the most valence-sensitive method in the range of Mn3+ and Mn4+ and yields good agreement with the actual values.