The purpose of this study was to investigate the impact of redox-induced changes in the organization of the clay fraction of a bulk vertisol using transmission electron microscopy. Chemical and X-ray powder diffraction (XRD) analyses indicated that the oxidized clay was composed of 32% kaolinite and 68% non-pure smectitic material, mostly a dioctahedral beidellite with octahedral Fe, according to Quantarg2 and DecompXR models.

The cation exchange capacity of the soil increased from 26.1 to 65 cmolc+ kg−1 due to structural iron (FeStr) reduction and dissolution of oxide coatings. Transmission electron micrographs revealed dramatic changes upon reduction. Oxides were dissolved and the smectite increased in particle darkness, lateral extension, thickness, compactness and stacking order. These changes were interpreted to be a consequence of sorption of ferrous Fe and reduction of FeStr, as found in previous studies on pure Fe-bearing smectites.

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