Mechanism of palygorskite formation in the Red Clay Formation on the Chinese Loess Plateau, northwest China

Abstract

The morphological characteristics and microstructures of the authigenic palygorskite occurring with smectite and carbonate minerals in the Late Miocene Red Clay Formation, on the Chinese Loess Plateau, have been investigated with the powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM). The XRD results indicate that the red clay sediments contain quartz, feldspar, calcite, dolomite, illite, kaolinite, chlorite, palygorskite, and smectite, and SEM analyses show that palygorskite is a common component. Delicate palygorskite fibers radiate from the margins of platy smectite, suggesting a pedogenic origin during the weathering of pre-existing smectite, a process that involved intense dissolution of the smectite and an adjustment in its structure. Moreover, palygorskite occurs as coatings, pore-fillings, and pore-bridging cements among dolomite rhombs, suggesting that the fibrous palygorskite was formed by a direct dolomite precipitation from pore fluids, mostly after dolomite crystallization. There is evidence to show that these two mechanisms took place synchronously in the Red Clay Formation, and that the two processes are related. Under alkaline conditions, with high-pH (~8.0) fluids rich in magnesium, magnesium ions occupy the interlayer positions in the smectite, and this results in a misfit between the Mg octahedral sheet and the smectite layer, thereby allowing interaction between the magnesium ions and smectite layers and a reorganization of the structure to form palygorskite. Then, with this kind of palygorskite as a crystal nucleus, more palygorskite grows through a direct chemical deposition process. According to previous studies, the red clay sediments experienced a steady dry–warm climate with periods of high rainfall and evaporation. The warm climate would have increased the rate of weathering of minerals, and subsequently enhanced the ions content and pH in the soil solutions during dry periods. This soil environment is favorable for smectite transformation and palygorskite deposition. This study has improved our understanding of the environmental and paleoclimatic changes during the time that palygorskite formed in the arid soils of the Chinese Loess Plateau.