Mössbauer and X‐Ray Evidence for the Pedogenic Transformation of Hematite to Goethite

Abstract

AbstractSoils of the Appling series (Typic Hapludults; clayey, kaolinitic, thermic) are extensive in the Piedmont Province of the southeastern United States. Typically, these soils exhibit colors grading from brown (7.5YR) in the upper B horizons to red (2.5YR) in the saprolite. Fine clay separates (<0.2µm) from the brown (B21t horizon) and red (C2 borizon) zones of a representative pedon of the Appling soil were analyzed to determine if the color transition reflected a change in iron mineralogy. Concentration of secondary iron forms by alkali dissolution of gibbsite and silicate clay minerals enabled the x‐ray identification of aluminous geothite and hematite in the C2 clay but only aluminous goethite in the B21t material. These results were substantiated by Mössbauer spectroscopic analyses of the natural clays, which gave spectra characteristic of finely divided hematite and/or goethite. Subnormal effective internal magnetic fields (relative to standard goethite and hematite) were also measured and were in keeping with the effect of Al substitution. Presence of both goethite and hematite in the saprolite but only goethite in the B21t material suggests a pedogenic transformation of hematite to goethite.