AL-O--AL PARAMAGNETIC DEFECTS IN KAOLINITE

Trapped holes located on Al-O-Al bonds in kaolinite were studied by el ectron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz app lied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at g(xx)=2.040/-0.0005, g(yy)=2.020+/-0.0005 and g(zz)=2.002+/-0.001. In each of the se groups, the 11 superhyperfine lines exhibit intensities according t o the ratios 1:2:3:4:5:6:5:4:3:2:1. An angular dependence of the Q-ban d EPR spectrum with respect to the magnetic field is demonstrated by m easurements on oriented films of kaolinite. An appropriate numerical t reatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X- and Q-band spectra have also been simulated. It is concluded from these experiments that only one t ype of center is present. This center, labelled the B-center in the li terature, is very probably a hole trapped on oxygen (O- center) atoms coupled to two octahedral aluminium.