Characterization of iron oxides: Valence states and naturally occurring polymorphs using chemical shift of X–ray FeLβ1 line by WD–XRF

Abstract

In X–ray spectroscopic studies of 3d transition metals, Kβ1,3 lines have been extensively used for the examination of chemical states. This is because Kβ1,3 lines are the second most intense X–ray lines and are more sensitive to changes in chemical state of 3d transition metals since the valence shell 3p is involved in the transition. In order to characterize iron oxides in natural iron ores, we determined the chemical shift, asymmetric index and FWHM of the third order FeKβ1,3 lines using WD–XRF technique. Unfortunately, none of these parameters exhibited good agreement with the chemical state of iron which was desired for characterization of iron oxides. Alternatively, FeL-lines were inspected because it involves the valence shell 3d electrons and moreover Fe(II) and Fe(III) have different number of unpaired electrons in the 3d shell. Hence, FeL-lines are anticipated to represent signature of different chemical states of iron. The study established a linear relationship between chemical shift of FeLβ1 line (δβ) and spin multiplicity (2S+ 1) in iron oxide ores having both Fe(II) and Fe(III) valence states. Since (2S + 1) is an intrinsic property of valence state, the study of δβ facilitated speciation of iron. The δβ also contained potential information on structural framework of iron oxides whereby it enabled characterization of various polymorphic phases of iron oxides irrespective of their crystalline/amorphous forms and structure. The δβ was found to increase with increase in Fe(III)/Fetotal ratio in the iron oxide CRMs. Hematite (α–Fe2O3) was observed to have higher δβ than maghemite (γ–Fe2O3) which is attributed to differences in their packing structure. Magnetite (Fe3O4), on the other hand, showed an anomalous behavior due to structural resemblance with maghemite, but both were characterized successfully.