An unusual radiation dose dependent EPR line at g_eff=2.54 in feldspars: possible evidence of Fe³⁺ O²⁻↔Fe²⁺O⁻ and exchange coupled Fe³⁺−Fe²⁺−nO⁻

Abstract

We report electron paramagnetic resonance (EPR) results on natural feldspar grains extracted from sediment samples of three widely different source regions. The XRD analysis indicated that the samples had albite, microcline and Ca/Ba-orthoclase as the major constituents. The EPR spectrum of all the samples exhibited an intense line around g_eff=4.3. This is characteristic of Fe³⁺ with a zero field splitting that is significantly larger than the microwave quantum. More interesting was the presence of a strong EPR signal in the region of g_eff=2.54-2.7. On irradiation with gamma rays the position of the line shifted to higher magnetic field. The possible origin of this line and its unusual behavior with dose is explained as being due to the interaction of a hopping 'hole' on oxygen with iron centers. The possibilities considered include (1) the valence fluctuation of an Fe³⁺ center at a tetrahedral site, (2) oscillation of FeO₄⁻ between two resonant forms, Fe²⁺O⁻ and Fe³⁺O²⁻, and (3) exchange coupling between Fe³⁺, Fe²⁺ and O⁻. The concentration of the latter is dose dependent and, consequently, it results in the observed dose dependent line shift.