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This article in SSSAJ

  1. Vol. 50 No. 6, p. 1633-1639
    Received: Feb 3, 1986

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Concentration of Iron Oxides from Soil Clays by Density Gradient Centrifugation1

  1. W. F. Jaynes and
  2. J. M. Bigham2



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−3 density range whereas the 2.1 to 2.3 Mg m−3 density 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.

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