Evidence of native radiation-induced paramagnetic defects in natural illites from unconformity-type uranium deposits

This study presents the first unequivocal identification of natural radiation-induced defects in illites. Middle Proterozoic illites related to unconformity-type uranium deposits of Canada and Australia were studied using electron paramagnetic resonance (EPR) spectroscopy at X- and Q-band frequencies. The saturation behaviour of EPR spectra as a function of power demonstrates that native defects of illites are different from those known in other clays as kaolinite, dickite or smectite. Q-band spectra indicate the presence of several––at least two––native defects. The EPR signal is dominated by an axially distorted spectrum with apparent principal components as follows: g _∥ = 2.032 and g _⊥ = 1.993. The corresponding defect is named as A_i center. The study of oriented specimen confirms the strong anisotropy, and shows that the main defect has its g _∥ component perpendicular to the (ab) plane of illite. These defects in illite correspond to electron holes located on oxygen atoms of the structure and likely associated to Si, according to the lack of hyperfine structure. The A_i center in illite has similar EPR parameters to the A center in kaolinite and dickite. The isochronal annealing data suggest that illite can be used as a dosimeter in the geosphere. However, the determination of half-life and activation energy of the A_i center requires additional work.