A number of natural kaolinites from a variety of world sources have been investigated using electron spin resonance at both X-band and Q-band. The results show systematic differences some of which are related to the crystallinity of the material, some to the presence of transition metal ions other than Fe3+ and hole-trapping defects.
The results provide evidence for at least three sites for substitution of Fe3+ having large crystal fields : (i) a site with near maximum rhombic character having crystal field parameters D > 1·2, λ = ⅓ the occurrence of which is correlated with lack of crystalline perfection due to stacking defects or to the proximity of surfaces, (ii) a site with less rhombic character with λ = 0·234, D = 0·585 as likely parameters, and (iii) a site with parameters near the values λ = 0·207, D = 0·322.
Mössbauer and ESR evidence suggests that the principal sites of Fe3+ substitution are octahedral. The presence of Fe3+ in adjacent cation sites leads to a very broad resonance centred near g
eff = 2. Other resonances in the spectra are attributed to the effect of trapped holes, some at least of which are situated at oxygen sites adjacent to Al3+ ions substituting in sites normally occupied by Si4+.
Hyperfine effects due to the presence of the transition ions Mn2+ and (VO)2+ are also observed.