Mechanism of palygorskite formation in the Red Clay Formation on the Chinese Loess Plateau, northwest China Qiaoqin Xie a, ⁎, Tianhu Chen a , Hui Zhou a , Xiaochun Xu a , Huifang Xu b , Junfeng Ji c , Huayu Lu d , William Balsam e a School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, PR China b Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA c School of Earth Science and Engineering, Nanjing University, Nanjing 210093, PR China d School of Geographical and Oceanographical Sciences, Nanjing University, Nanjing 210093, PR China e Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, TX 76019, USA abstract article info Article history: Received 16 May 2011 Received in revised form 11 July 2012 Accepted 26 July 2012 Available online 16 November 2012 Keywords: Chinese Loess Plateau Red Clay Formation Palygorskite Smectite transformation Rearrangement of smectite structure Chemical precipitation 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 in- vestigated 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 anal- yses 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 crystalli- zation. There is evidence to show that these two mechanisms took place synchronously in the Red Clay For- mation, 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 be- tween 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 envi- ronment 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. © 2012 Elsevier B.V. All rights reserved. 1. Introduction It has long been recognized that clay minerals play a major role in the physical and chemical properties of soils; consequently, questions concerning the origin, distribution, and formation of these minerals are prominent in soil research (Owliaie et al., 2006; Wilson, 1999). Palygorskite is a Mg-rich phyllosilicate with fibrous morphological characteristics, and it can be approximated by the formula yMg 5 Si 8 O 20 (OH) 2 ·(1 -y)[xMg 2 Fe 2 ·(1 -x)Mg 2 Al 2 ]Si 8 O 20 (OH) 2 (Chryssikos et al., 2009). It occurs in diverse geological situations, including marine, bay, lagoon, and lake environments, in dry-region soils, and in hydrothermal zones (Bonatti and Joensuu, 1968; Bouza et al., 2007; Lopez et al., 1996). Natural occurrences of palygorskite in the sediments and soils of arid re- gions have been widely reported, and nearly all palygorskite-bearing soils are found in arid and semiarid regions (Neaman and Singer, 2000, 2004). On a global scale, palygorskite is found mainly in Tertiary sediments, and the mineral is widely regarded as a proxy for arid and semiarid climates (Rodas et al., 1994). The occurrence and genesis of palygorskite in arid soils have been in- vestigated by numerous researchers (Daoudi, 2004; Gürel and Kanir, 2006; Hong et al., 2007; Owliaie et al., 2006), and two principal processes have been proposed to account for authigenic palygorskite: (1) the transformation of pre-existing smectite, mixed interlayer illite–smectite type aluminosilicates (Chahi et al., 1999; Chen et al., 2004; Sancho et al., 1992; Yaalon and Wieder, 1976), or the transformation of chlorite type Geoderma 192 (2013) 39–49 ⁎ Corresponding author. Tel.: +86 551 2904061. E-mail address: qqxie204@sina.com (Q. Xie). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2012.07.021 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma