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Abstract The iron oxides in subsoil clays (<0.2 µm) from five North Carolina Ultisols and two Brazilian Oxisols were analyzed by Mössbauer spectroscopy, X‐ray diffractometry, and selected chemical techniques. Goethite and hematite were identified as the dominant crystalline iron oxides in these clays. Partial substitution of Al(III) or other nonmagnetic ions for Fe(III) in the oxide structures caused reduced unit cell dimensions and subnormal effective internal magnetic fields. All samples exhibited strong superparamagnetic relaxation, a collapse of the Mössbauer magnetic splitting at ambient temperatures due to the ultrafine size of the oxide particles. Negligible amounts of iron were solubilized with acid ammonium oxalate; however, the Ultisol clays did contain significant amounts of silicate and/or interlayer iron that was not extractable with dithionite‐citrate‐bicarbonate.
Bigham et al. (Fri,) studied this question.