Pedogenic alteration of illite in subtropical China W. HAN 1 , H. L. HONG 1,2, *, K. YIN 1 , G. J. CHURCHMAN 3 , Z. H. LI 4 AND T. CHEN 5 1 Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei, 430074, China, 2 Key Laboratory of Geobiology and Environmental Geology, the Ministry of Education, China University of Geosciences, Wuhan, Hubei, 430074, China, 3 School of Agriculture, Food and Wine, The University of Adelaide, 5005 Australia, 4 Geosciences Department, University of Wisconsin À Parkside, Kenosha, WI 53141-2000, USA, and 5 Institute of Gemology, China University of Geosciences, Wuhan 430074, China (Received 25 October 2013; revised 13 April 2014; editor: George Christidis) ABSTRACT: Pedogenic alteration of illite from red earth sediments in Jiujiang in subtropical China was investigated using XÀray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Illite, hydroxy-interlayered vermiculite (HIV), kaolinite and mixed-layer illite-HIV (I-HIV) are present in the soils. The characteristic reflections of the clay phases were 14 A ˚ , 10À14 A ˚ , 10 A ˚ , and 7 A ˚ , respectively. After Mg-glycerol saturations, the 14 A ˚ peak of the samples did not expand, and after heating at 350ºC and 550ºC it shifted to 13.8 A ˚ and 12 A ˚ respectively, with no residual 14 A ˚ reflection, suggesting the occurrence of hydroxy-interlayered vermiculite. The randomly interstratified I-HIV clays were characterized by a broad peak at 10À14 A ˚ , which did not change its position after Mg-glycerol saturation, but collapsed to 10 A ˚ after heating at 350ºC and 550ºC. HRTEM analysis showed different lattice fringes of 12 A ˚ , 10 A ˚ and 7 A ˚ . Mixed-layer I-HIV, HIV-K and illite-kaolinite (I-K) were observed in the HRTEM images which represented the intermediate phases during illite alteration. The merging of two 10 A ˚ illite layers into a 12 A ˚ HIV layer, lateral transformation of one HIV layer into one kaolinite layer and alteration of one illite layer into two kaolinite layers illustrated the mechanisms of illite-to-HIV, HIV-to-kaolinite and illite-to- kaolinite transformation, respectively. The proposed pedogenic alteration of illite and the weathering sequence of the clay minerals in Jiujiang is illite ? I-HIV ? HIV? HIV-K ? kaolinite. In addition, illite may transform directly to kaolinite. KEYWORDS: pedogenic alteration, illite, mixed-layer clay, high-resolution transmission electron microscopy (HRTEM), red earth. Red earth is distributed widely in subtropical China in the lower and middle reaches of the Yangtze River (25ºNÀ31ºN). The sediments are continuous and may originate from the well-known Chinese Loess Plateau (Xiong et al., 2002; Hu et al., 2010). They are accretionary in nature and syndepositional pedogenesis proceeded during weathering under subtropical climatic conditions (Zhao & Yang, 1995; Hong et al., 2007). During the conversion of the pre-existing clay minerals into stable phases by weathering, a sequence of intermediate inter- stratified clays may develop in the pedogenic process. The alteration is controlled by the nature of parent minerals, the chemical composition of solutions and climatic conditions (Singer, 1980). Dioctahedral aluminous illite is abundant in the red earth sediments, which is considered to undergo weathering in sequence illite ? vermiculite ? smectite ? kaolinite, and the intermediate steps * E-mail: honghl8311@aliyun.com DOI: 10.1180/claymin.2014.049.3.03 Clay Minerals, (2014) 49, 379–390 # 2014 The Mineralogical Society