Single-crystal EPR and ENDOR study of an AlO center in prehnite: implications for aluminum-associated oxyradicals in layer silicates

Abstract

Single-crystal electron paramagnetic resonance (EPR) spectra of gamma-ray-irradiated prehnite (Jeffrey mine, Quebec, Canada) measured at 298 and 160 K reveal an aluminum-associated oxygen hole center (Al—O − ). Spin Hamiltonian parameters g and A ( 27 Al) fitted from the 298 K spectra suggest that this Al—O − center represents hole trapping on an apical hydroxyl oxygen atom (after removal of the proton) coordinated to an octahedral Al 3+ ion ( i.e. , an [ • OAlO 4 (OH)] center from the [(OH)AlO 4 (OH)] precursor, where • denotes the unpaired spin). Pulsed electron nuclear double resonance (ENDOR) spectra measured at 25 K allow the identification and quantitative analysis of two sets of 27 Al hyperfine structures and five proton hyperfine structures, which are all consistent with the proposed structural model. Isothermal and isochronal annealing experiments show that this center is bleached out completely at 375 °C, but can be readily restored by gamma-ray irradiation, and exhibits second-order decay kinetics. These results from the Al—O − center in prehnite provide support for and new insights into Clozel et al . (1995)’s VI Al—O − — VI Al model for B-centers in kaolinite.