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Identification of Soil Iron Oxide Minerals by Differential X-ray Diffraction¹

ABSTRACT

If a step-scanned x-ray diffraction pattern of a sample obtained before selective dissolution of all or part of the Fe oxide minerals is substracted from one obtained after dissolution, a differential x-ray diffraction (DXRD) pattern of the Fe oxide fraction that was dissolved can be obtained. After the selective dissolution treatment, the diffraction pattern usually has a greater overall intensity than before, so that all of the points in this pattern must be multiplied by a scale factor less than unity before making the subtraction. With a suitable scale factor, found by trial and error subtractions, a DXRD diagram essentially free of peaks due to minerals other than Fe oxides can be obtained. If DXRD is used, the presence of goethite, hematite, or both can be detected in a sample containing as little as 1.8% Fe_d (dithionite-citrate-bicarbonate soluble Fe). For samples containing approximately 5% or more Fe_d, DXRD patterns are suitable for the determination of the goethite/hematite ratio and accurate measurements of peak positions and peak widths. Differential x-ray diffraction clearly showed the material dissolved from a natural sample by oxalate treatment to be ferrihydrite.

¹ Contribution from the Institut für Bodenkunde der Technischen Universität München, 8050 Freising-Weihenstephan, GFR. Paper presented before Div. S-9, Soil Sci. Soc. of Am., Fort Collins, Colo., August 1979.

² Graduate Research Assistant.

Received for publication March 26, 1980. Accepted for publication October 17, 1980.

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