PARAMAGNETIC FE3- A SENSITIVE PROBE FOR DISORDER IN KAOLINITE( )

The Fe3+ substituted for Al3+ at the 2 octahedral positions is one of the most common impurities in the kaolinite structure detected by elec tron paramagnetic resonance (EPR). Evidence has been provided for a re lationship between the shape of EPR spectra for structural Fe and the structural disorder in kaolinite. It is proposed that the structural F e be used as a sensitive probe for the degree of disorder of natural k aolinites. With this aim in view, an EPR disorder index (E) is defined from the width of selected EPR lines. Using reference kaolinites, it is shown that this index can account as well for long-range disorder d etected by means of X-ray diffraction (XRD) as for local perturbations such as radiation-induced defects (RID). It is shown that the disorde r observed through EPR has some points in common with the XRD-measured one. The influence on E of the presence of RID is shown by the study of artificially and naturally irradiated kaolinites.