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

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 geff = 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 geff = 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.