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Iron Oxide Mineralogy of Well-drained Ultisols and Oxisols: I. Characterization of Iron Oxides in Soil Clays by Mössbauer Spectroscopy, X-ray Diffractometry, and Selected Chemical Techniques¹

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.

¹ Paper no. 5509 of the Journal Series of the North Carolina Agric. Exp. Stn., Raleigh, NC 27650.

² Former Research Assistant, Research Assistant, Professor of Chemistry and Professors of Soil Science, respectively, North Carolina State University, Raleigh, N.C. The senior author is now Assistant Professor, Agronomy Dep., Ohio State University, Columbus, OH 43210.

Received for publication January 19, 1978. Accepted for publication June 6, 1978.

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