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 geff = 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.

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SUMMARY. Two clay samples originating in the kaolinized granite area near St. Austell, Cornwall, have been examined by MSssbauer spectroscopy. The iron-containing impurity in the sample with a red discoloration (sample A) has, by measurement of the hyperfine magnetic field, been identified as hematite. In the case of sample B, which was yellow in colour, preliminary MSssbauer work indicated fi-FeOOH as the impurity, but detailed measurements of hyperfine field and N6el temperature, when compared with similar results for a synthetic/3-FeOOH sample, suggested otherwise. X-ray and chemical work suggested that the iron-containing phase in sample B is goethite, ~-FeOOH in which some substitution of Al+8 for Fe z+ has occurred; the MSssbauer data are consistent with this conclusion. Both clay samples exhibited superparamagnetic behaviour and this has been utilized to obtain rough estimates of the mean radius of the particles of the iron-containing impurity. These were 117/~ for the ~-Fe~O8 in sanaple A and 270/~ for aluminian c~-FeOOH in sample B.IT has been established by Malden and Meads (I967) by Mtissbauer exp ~'riments on a natural kaolinite purified by magnetic separation that Fe z+ substitutes, almost certainly in the octahedral cation site, in the kaolinite lattice. It is important to distinguish this situation from another that commonly exists, namely the occurrence of iron in separable impurities in the clay, usually in mica, or as oxides and oxy-hydroxides of iron. The experiments here described illustrate the usefulness of M6ssbauer work in the identification of impurities of the latter type. Some recent Mossbauer experiments by Janot et al. (I973) on kaolinites of tropical soil origin have yielded results that are generally similar to those reported here, but the measurements were restricted to two temperatures, 85 K and 3o0 K. As will be shown, more information regarding the iron-rich impurity phase can be obtained by experiments at a number of intermediate temperatures.The association of iron hydroxides with the surfaces of kaolinite particles has been studied by chemical, X-ray, and electron microscopic methods by Greenland and Oades (I968). Under certain circumstances the iron compound may be rather poorly crystallized.Sample description and preparation. Two samples of kaolinite from a clay pit in the St. Austell area of Cornwall were studied: both showed coloration, sample A having a red tint and sample B a yellow tint. Each clay contained about 2 % by 9

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