Soil-Saprolite Profiles Derived from Mafic Rocks in the North Carolina Piedmont: II. Association of Free Iron Oxides with Soils and Clays

The association of free Fe oxides with soils and clays from two Enon sandy loam (Ultic Hapludalfs, fine, mixed, thermic) soil saprolite profiles was studied. Goethite was the dominant Fe oxide identified. Lepidocrocite was detected in trace amounts in some sam ples. Fe/clay ratios were highest in the epipedons of these soils suggesting the concentrating of Fe oxides as a result of alumino silicate mineral weathering. External (BET-N2) surface area mea surements of non-deferrated and deferrated clays were analyzed in conjunction with electron micrographs of selected clay fractions to determine the association of free Fe oxides with aluminosilicate clays as a function of depth in the profile. Free Fe oxides were found to exist mainly as small, discrete clusters in the A and B horizons of both profiles and specific of the clay surface decreased as a result of treatment for Fe removal. However, external surface areas in creased for the saprolite (Cr) horizon clays after deferration. One subfraction identified as having an increase in surface area after deferration was fine clay from the Cr2 horizon, Enon (metagabbro) profile. Chemical data and electron micrographs suggest that either partial dissolution of small, poorly crystalline aluminosilicate clays or removal of some Fe or non-Fe oxide aggregating agent results in breakdown of the fine clays into smaller particles of higher net spe cific surface. Additional Index Words; weathering, gabbro, Alfisols, Argillic horizon. Rice, Jr., T.J., S.B. Weed, and S.W. Buol. 1985. Soil-saprolite pro files derived from mafic rocks in the North Carolina piedmont: II. Association of free iron oxides with soils and clays. Soil Sci. Soc. Am. J. 49:178-186. I is THE FOURTH most abundant element in the earth's crust (after O, Si, and Al), comprising 5% by weight (Brownlow, 1979). During weathering of Febearing primary minerals, the Fe, predominantly bound in the reduced (Fe) state, is released through 1 Paper no. 9109 of the North Carolina Research Service, Journal Series, Raleigh, NC 27650. Received 23 Feb. 1984. Approved 11 Aug. 1984. 2 Former Graduate Research Assistant and Professors, respec tively, Dep. of Soil Science, North Carolina State Univ. Senior au thor is now Assistant Professor, Soil Science Dep., California Po lytechnic State Univ., San Luis Obispo, CA 93407. a combined hydrolytic and oxidative reaction of the following type: -Fe -0-Si+H2O = -Fe -OH + OH — Si ± e~ in which atmospheric oxygen takes up the electron (O2 + 4e~ 20~) (Schwertmann and Taylor, 1977). Due to the extremely low solubility of Fe"" oxides in the normal pH range of soils, the Fe released is precipitated as an oxide or hydroxide (Garrels and Christ, 1965). (In this paper, "Fe oxides" will represent oxides, hydroxides and oxyhydroxides, and free Fe oxides will refer to reductant soluble Fe). Oades (1963) divided the occurrences of free Fe oxide accumulations in soils into four general categories: i) Fe concretions; ii) illuyial deposits in lower B horizons of podzolic soils; iii) laterite crusts; and iv) Fe oxides of agricultural soil and brown earths, which are not found as local concentrations due to thorough mixing of the soils. The free Fe oxides include nine crystalline oxides of which goethite, hematite, and lepidocrocite are the most common forms. Of these, goethite (a-FeOOH) is the most frequently occurring form of Fe-oxide in soils. Thermodynamically, goethite is the most stable under most soil conditions (Schwertmann and Taylor, 1977). Also, "amorphous" free Fe oxides have been reported (Mackenzie, 1949). Recently, Schwertmann et al., (1981) have identified a poorly crystalline Fe oxide, ferrihydrite (5Fe2O3 • 9H2O), which they believe is the so-called "x-amorphous" form, soluble in acid ammonium oxalate. Moreover, they have identified several diffuse x-ray diffraction (XRD) reflections for ferrihydrite thus establishing that the material is not truly amorphous. Iron concretions in soils are normally Fe-Mn concretions. Their properties have been reported in detail by numerous authors including Gallaher et al., (1973a, 1973b), Schwertmann and Fanning (1976) and Ciric and Skoric (1973). These workers have generally observed that Fe-Mn concretions occur between the winter ano^ summer water table levels in hydromorphic soils with impeded internal drainage. The presence of alternate oxidation-reduction conditions in that zone suggests that concretion precipitation is physicor