ELECTRON DIFFRACTION AND THE STUDY OF FERRIHYDRITE COATINGS ON KAOL IN ITE

A B S T R A C T: Selected-area electron diffraction has been used to examine f rrihydrite coatings on kaolinite crystals and is shown to provide asensitive means of detection. It also gives better diffraction patterns of ferrihydrite han does XRD and the patterns sometimes give additional indications of the crystal size. It is suggested that he technique may be useful in examining other coatings on soils and clays. Elucidating the nature of poorly crystalline ferric oxides and ferrihydrite, and detecting their presence amongst other soil clay minerals, is notoriously difficult (Brown, 1980) because the XRD pattern of even a synthetic ferrihydrite is characterized by only five or six broad maxima (or peaks), the equivalent spacings of which, at 2.54, 2.24, 1.98, 1.73, 1-51 and 1.47 A, broadly coincide with common spacings of phyllosilicates and other iron oxides and hydroxides. To meet this problem, differential X-ray diffraction (DXRD) methods have been developed which depend on examining the effect of the removal of ferrihydrite and its broad diffraction peaks from the total diffraction pattern of a soil clay by ammonium oxalate treatment (Schulze, 1981; Schwertmann et al., 1982; Brown & Wood, 1985). Even this elegant method may be relatively insensitive in a situation where small amounts of ferrihydrite or other iron oxide/oxyhydroxide minerals affect he surfaces and surface behaviour of other soil minerals. Other means of determining ferrihydrite suc