Illite–Smectite Mixed-Layer Minerals in the Alteration Volcanic Ashes Under Submarine Environment

The clay mineralogy of the clay intervals interbedded with siliceous mudstones across the Permian–Triassic boundary (PTB) in Pengda, Guiyang, Guizhou province, was investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The clay mineral assemblages of the sediments are mainly I/S clays and minor smectite, kaolinite, and illite as revealed by XRD analyses. The peak-shaped parameters BB1 and BB2 of I/S clays of the representative clay bed PL-01 are 4.7° and 4.4°, respectively, and the peak position of the low-angle diffraction is at 6.5° 2θ (13.6 Ǻ), suggesting that the I/S clays have a IS type of ordering. However, multi-order diffractions and their intensities are different from those of completely ordered 1:1 mixed-layer I/S clay rectorite, indicating that I/S clays of the Pengda section have partially ordered IS structures. HRTEM observations show that most of the I/S clays exhibit a IS stacking ordering. However, in some areas within a IS particle, smectite layer is observed in doublets, triplets, and quartets, which are interstratified by various amounts of illite layers, suggesting the presence of other irregular stacking in addition to the major 1:1 IS-ordered stacking. Transformation of smectite layer into illite layers is also observed in the I/S clays, suggesting that the Pengda I/S clays are derived from smectite illitization, in good agreement with the clay mineral assemblage. The I/S clays of the Pengda section contain up to 45–95 % smectite layer, the notably higher contents of smectite layer relative to those of other PTB stratigraphic sets in south China can be attributed to the difference in alteration and smectite illitization processes due to different sedimentary environments.