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Use of an Internal Standard with Differential X-ray Diffraction Analysis for Iron Oxides¹

ABSTRACT

In the differential x-ray diffraction (DXRD) method the diffraction pattern for iron minerals in a soil or clay sample is obtained by subtracting the pattern for a sample free of iron oxides, after appropriate adjustment of intensity, from that of an untreated sample. We modified this method by using 1 µm -alumina as an internal standard to calculate the scale factor for the adjustment of intensity. The internal standard also serves as a reference for making corrections in peak position. Alumina has the advantages that it is not attacked by the citrate-bicarbonate-dithionite (CBD) treatment to remove iron oxides and its peak positions do not overlap the broad diffraction peaks of Fe oxides in the range of 2 to
. The effects of CBD treatment on soil components other than Fe oxides can also be observed. Iron minerals in 11 soils from Indiana and Goias State, Brazil, were characterized using this technique. Hematitegoethite ratios, mean crystallite dimension, and amount of Al substitution in the iron oxides were calculated from the position, height, and width at half-height of the DXRD peaks. The redder soils had more extractable Fe and larger hematite-goethite ratios than the yellower soils. The amount of Al substitution in goethite and hematite and the mean crystallite dimension of hematite was greater for the Oxisols of Brazil than the Alfisols of Indiana. Maghemite was detected in soil clay of a red Oxisol from Brazil.

¹ Contribution from the Agronomy Dep., Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907. Journal Paper no. 9004.

² Former Graduate Instructor (now Assistant Professor, Cornell Univ.); Graduate Fellow on leave from Escola Superior de Agricultura de Lavras, Minas Gerais State, Brazil (MEC/CAPES program); Associate Professor; and Professor, respectively.

Received for publication April 23, 1982. Accepted for publication September 9, 1982.