Small-angle scattering of X rays by three kinds of clay minerals (allophane, hydrated halloysite and kaolinite) was observed over scattering angles from 8′ to 70′, in the range below 275 Å in radius of gyration, and was analyzed by Jellinek Solomon-I ankuchen s method and Porod's theory. Jellinek-Solomon-fankuchen s method for hydrated halloysite and kaolinite showed the largest value of the distribution function M (Rg) occurring in the range more than 275 Å in radius of gyration. However, for allophane the largest value of M(Rg) occurred at the smallest radius of gyration. Porod's theory was applicable to hy drated halloysite and kaolinite. but not to allophane. The average value of the radius of gyration (Rg), or the “extent of coherence” Ic. was in good agreement with particle size estimated from electron micrographs, only for allophane. For hydrated halloysite (Rg) and lc were smaller than those of heated halloysite. After grinding, the values of (Rg) and Ic of allophane increased and the “structure index” ֬ decreased.

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