STABILITY OF IRON-OXIDES IN 2 SOILS WITH CONTRASTING COLORS

Two soils formed from sedimentary rocks with contrasting colors (dark reddish brown, 2.5YR 3/4 vs. olive, 5Y 5/3) were studied to assess the origin and relative stability of their constituent Fe oxides. Field morphology and total elemental analyses of the silt fractions for Ti, Zr, and K indicated that both soils had developed in uniform residuum overlain by approximately 30 cm of late Wisconsinan loess. The residual 2Bt horizons were contrasting in color but similar in clay content (50-65%), mineralogy (smectitic), and chemistry (pH = 4.5-6.0, base saturation = 50-60%). X-ray diffraction studies of Fe-oxide-enriched specimens obtained through high-gradient magnetic separations of the soil clays showed goethite to be the only detectable Fe oxide in the pedon formed from olive (5Y 5/3) shale. Red pigmentation in the companion soil was due to the presence of lithogenic hematite associated with varying amounts of goethite and lepidocrocite. Lepidocrocite occurred only in the solum and was most abundant in the clayey argillic horizon. Hematite grains from the 2C horizon consisted of thin, pseudohexagonal crystals with diameters < 0.5-mu-m, whereas those isolated from overlying horizons were rounded and exhibited dissolution cavities attributable to instability within the modern pedoenvironment. Ratios of dithionite-extractable to total Fe (Fe(d)/Fe(t)) indicated that goethite had formed primarily by pedogenic weathering of silicate minerals in the profile developed from olive shale. In contrast, sufficient Fe was apparently mobilized by dissolution of lithogenic hematite in the red soil to suppress the release of Fe from silicates and to exceed the solubility products of associated goethite and lepidocrocite on at least a seasonal basis.