Randomly interstratified illite–vermiculite from weathering of illite in red earth sediments in Xuancheng, southeastern China.

Mixed-layer illite–vermiculite (I/V) clays of the Xuancheng red earth were investigated using X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectrometer, and high-resolution transmission electron microscopy (HRTEM) methods. The XRD results indicated that the mixed layer I/V clays were characterized by the broad 10–14Å peak with a maximum intensity at ~12Å in air-dried sample, which decomposed into two peaks at 10Å and 14Å peaks respectively after ethylene glycol treatment. After K+ saturation, the 14Å peak changed to 10Å, which reverted back to 14Å after saturation with Mg2+ with a notable decrease in intensity of the 10Å peak. The broad 10–14Å peak collapsed to 10Å on heating to 400°C. These suggested that the broad 10–14Å peak resulted from randomly interstratified illite–vermiculite clays together with discrete illite and also vermiculite. HRTEM observation showed that straight 10Å illite layers were interstratified with 12Å vermiculite layers, suggesting that transformation of illite to vermiculite involves an exchange of K+ by hydrated cations associated with a swelling of the illite. The (060) spacing of I/V clay is 1.50Å, indicating a dioctahedral precursor of illite and thus reinforcing the transformation from illite to vermiculite. FTIR analysis suggested that most band positions of vermiculite and illite in mixed-layer I/V did not change in comparison with those of the discrete clay species, indicating that the basic crystal structure of mixed-layer illite–vermiculite remains the same as the discrete clays. The interlayer cations in the interlayer region of vermiculite layers of the I/V clays are mainly Ca and minor Mg, Al, and Fe. The abundant Ca makes the I/V clays to behave like I/S on glycolated and thermal treatment, while the Mg, Al, and Fe complex ions in the interlayer region cause the vermiculite layer to lose its expandability and to behave like chlorite. The mixed-layer I/V clays were widely formed in late Quaternary alluvial fans in subtropical to tropical areas, where the sediments experienced synchronous pedogenic modification. The syndepositional pedogenesis process leads to an incomplete transformation of illite to vermiculite and causes the formation of metastable intermediates of mixed-layer illite–vermiculite. [ABSTRACT FROM AUTHOR]