In the past, two factors have impeded the quantitative estimate of Fe-oxides in soils by X-ray diffraction. First, Fe-oxides are still quite often considered X-ray amorphous, although numerous results, e.g. a low ratio of oxalate- to dithionite-soluble Fe, have indicated the opposite. Second, even if crystalline, the cocentration of Fe-oxides in many soils is low, thereby complicating their identification by XRD. Recently, however, more sensitive methods such as Mössbauer spectroscopy and Differential-XRD (Schulze, 1981) have been introduced, which substantially reduce the lower limit of detection. Because these two methods are not generally available and, especially in the case of M&oum;ssbauer spectroscopy, are rather time consuming, ordinary XRD should be adapted for quantitative estimation of Fe-oxides.
Determination can be facilitated by using samples in which the Fe-oxides are concentrated by particle-size separation and a 5 M NaOH boiling treatment (Norrish & Taylor, 1961). The latter treatment is particularly suitable for kaolinitic soils as the Fe-oxides are unaffected―provided certain precautions are taken (Käimpf & Schwertmann, 1982a). This paper gives details of a procedure for the quantitative estimation of goethite (Gt) and hematite (Hm) by XRD.