Hydroxy-interlayered vermiculite genesis in Jiujiang late-Pleistocene red earth sediments and significance to climate

To understand the paleoclimatic significance of the red earth sediments in Jiujiang, southern China, hydroxy-interlayered vermiculite (HIV) in the deposits was investigated using X-ray diffraction (XRD), chemical extraction, and high resolution transmission electron microscopy (HRTEM). The XRD results indicated that the clay mineral assemblages of the sediments are mainly illite, kaolinite, HIV, vermiculite, with minor illite-HIV mixed-layer clays. The 1.41-nm reflection did not expand after Mg saturation and glycerol solvation, but it shifted to 1.38 nm and 1.20 nm when the samples were heated to 350 °C and 550 °C, respectively. The 1.41-nm spacing showed partial collapse after K+ saturation at 80 °C in association with the reinforcement of the 1.0 nm reflection. Collapse of the HIV (001) spacing was related to the release of Al, Mg, and Ca from the interlayer after KCl extraction. The Al was probably present as hydroxy-Al in HIV interlayers before being released. The HRTEM image revealed 1.4-nm lattice fringes interstratified with illite 1.0 nm fringes and two illite lattice fringes merged into one HIV fringe. This suggests that the HIV clays in Jiujiang soils resulted from illite weathering. The shift of a small X-ray diffraction reflection between 1.0 and 1.4 nm for an air-dried and glycerol-solvated sample to 0.997 nm after heating at 550 °C indicated the presence of an irregularly interstratified illite–HIV mineral. The common occurrence of HIV minerals in the Jiujiang red earth sediments suggests a climate with frequent wet and dry cycles prevailed during late-Pleistocene time.

► HIV was determined using XRD in the upper portion of Jiujiang section. ► We found that the intercalated Al of HIV is probably present as hydroxy-Al hydroxide. ► Two of the illite layers merging into one HIV layer was observed by HRTEM. ► The genesis of HIV provides deep insights into its palaeoclimate significance.