Palygorskite in the Late Miocene red clay sediment from the Chinese Loess Plateau and its paleoclimatic implications

Palygorskite is the predominant component of the Red Clay sediment on the Chinese Loess Plateau. The morphological characteristics and microstructures of palygorskite in four sections of late Miocene Red Clay were investigated using power X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The XRD results suggest that the clay minerals in the Red Clay are mainly illite, kaolinite, chlorite, and palygorskite with smectite. Two types of palygorskite microtexture were observed in the Red Clay sediment: (1) relatively straight, fibrous crystals in bundles or intertwined aggregates or in matted, felted masses on other minerals and silky aggregates radiating from platy minerals; and (2) single crystals scattering among or coating detrital particles. Based on SEM investigations, the first aggregate is considered to be the major type of microtexture. The occurrence of this type of microtexture supports the hypothesis that palygorskite developed through pedogenesis of the red clay deposits. The authigenic palygorskites are thought to be formed through the transformation of existing aeolian mineral (e.g., smectite) and direct chemical deposition in soil pore solution. Consequently, the formation of palygorskite was largely controlled by the strength of the in situ pedogenesis process induced by the East Asian summer, and palygorskite can serve as an index mineral of arid and semiarid environment since the late Miocene for the Chinese Loess Plateau. These findings add to existing fundamental mineral data related to the climatic evolution of the northwest China arid region and of arid and semiarid zones in the Northern Hemisphere