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Kaolin in the net-like horizon of laterite in Hubei, south China

¹ Faculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei, China 430074,² Geosciences Department, University of Wisconsin – Parkside, Kenosha, WI 53141-2000, USA,³ Department of Earth Sciences, National Cheng Kung University, 1 University Road, Tainan, 70101, Taiwan, and⁴ School of Geographical Sciences, Guangzhou University, Guangzhou, Guangdong, China

^* E-mail: hong18311{at}yahoo.com.cn

(Received 10 February 2007; revised 26 June 2008)

The clay mineralogy and chemical composition of the white veins, red matrix and both Fe- and Mn-bearing nodules occurring in a laterite profile in Hubei, south China were investigated using X-ray diffraction, scanning electron microscopy equipped with an energy-dispersive spectrometer, and high-resolution transmission electron microscopy. The results show that the mineral components of the red matrix are mainly quartz, kaolinite, halloysite, goethite and minor illite, whereas the white net-like veins contain mostly quartz, kaolinite, halloysite, and illite. In the net-like horizon, the chemical index of alteration (CIA, the ratio of Al₂O₃/(Al₂O₃+CaO+K₂O+Na₂O)) and the TiO₂/Al₂O₃ ratio are 89.8% and 0.021 for the white vein and 90.7% and 0.025 for the red matrix, respectively. Both white-vein and red-matrix components have similar TiO₂/Al₂O₃ ratios, and are similar to the ratio 0.027 of the unaltered bedrock. The similarity in TiO₂/Al₂O₃ values indicates that all three portions of the laterite soil share the same origin. Also, although the white-vein and red-matrix components differ in Fe₂O₃ abundance, the similar CIA values do imply similar degrees of alteration. The Fe-bearing and Mn-bearing nodules were produced by the local accumulation of Fe₂O₃ and MnO, respectively. Halloysite in the weathering profile occurs in two different morphologies, tubular and platy crystals. Tubular halloysite occurs both in the red matrix and the Fe-bearing nodule whereas platy halloysite occurs only in the white vein and Mn-bearing nodule assemblages. Crystallization of small tubular halloysite from Si and Al concretions in the red matrix is observed, indicating that the morphology of these crystals in the weathering environment is mainly controlled by Fe³⁺ cations, whereas platy halloysite may be derived from the hydration of kaolinite.

KEYWORDS: net-like soil horizon, laterite, weathering, Fe oxides, kaolin, Hubei, south China