Possible confusion between so-called ferrihydrites and hisingerites

A B S T R A C T: The term ferrihydrite, originally defined as a poorly ordered hydrous ferric oxide exhibiting five or more distinctive X-ray diffraction maxima, has come to be extended to include species that exhibit only 4, 3 or 2 of these maxima. It is pointed out here that hisingerites may exhibit a similar 2-line diffraction pattern. The laboratory s nthesis ofhisingerites show that hey can form under soil conditions. Acid oxalate buffer of pH 3, a reagent considered a fairly specific solvent for ferrihydrite, xtracts he iron content of these synthetic hisingerites quantitatively. Ferrihydrite, as defined by the International Mineralogical Association (Fleischer et al., 1975) is a hydrous ferric oxide with a characteristic X-ray diffraction (XRD) pattern of five broad diffraction maxima in the range 0-14 to 0-26 nm. Material of this type has been synthesized in the laboratory (Towe & Bradley, 1967), and identified in nature in weathering crusts on basic rocks (Jackson & Keller, 1970), as ore alteration products (Chukhrov et al., 1973), as stream deposits (Carlson & Schwertmann, 1981) and in soils (Schwertmann etal., 1982). Some better-ordered synthetic preparations how further resolution of two of the broad diffraction peaks to give six or seven diffraction maxima (Brown, 1980)