Concentration of Iron Oxides from Soil Clays by Density Gradient Centrifugation

High speed centrifugation with a large‐scale zonal rotor and a linear, tetrabromoethane‐ethanol density gradient (1.0–2.8 Mg m−3) was utilized to fractionate the <0.2µm clay materials from three soils with differing Fe‐oxide mineralogy. Hematite, goethite, and lepidocrocite were concentrated 4‐ to 10‐fold in the high density range (>2.4 Mg m−3) while the low density fractions (<2.2 Mg m−3) were depleted in reductant‐soluble Fe. Density gradient centrifugation also produced useful mineralogical separations of the soil phyllosilicates. In particular, smectite was concentrated in the 1.8 to 2.1 Mg m−3density range whereas the 2.1 to 2.3 Mg m−3density range was enriched with kaolinite, vermiculite, and clay mica. Electron microscopy and x‐ray diffraction analyses indicated that Fe‐oxides were present in the materials studied as discrete particles rather than as “coatings” on associated minerals.