Experimental Study on Adsorption of Rare Earth Elements on Kaolinite and Halloysite

The weathered crust rare earth deposits is one of the most significant rare earth deposits, which are mostly distributed in southern China, especially in the south of Jiangxi province (Chi et al., 2007). The compositions of these deposits,which are weathered from igneous rocks such as granite through biological, physical or chemical processes in open systems (Chi et al., 2012),are quartz, feldspar, and mainly clay minerals (40%-70%, Chi et al., 2007). And the clay minerals in the deposits include halloysite (25%-50%), illite (5%-20%), kaolinite (5%-10%) and montmorillonite (< 1%) (Chi et al., 2007). Besides, not only the original rocks, but also the properties of clay minerals could influence the distributional patterns of the rare earth elements (Fan et al., 2015). * Halloysite and kaolinite are the most common clay minerals in the weathered crust rare earth deposits, both of which have the TO layered structure and similar chemical compositions. The chemical formula of halloysite is Al2[Si4O10](OH)2, while the formula of kaolinite is Al2[Si4O10](OH)8. Unlike kaolinite, the unit layers in halloysite are separated by a monolayer of water molecules. As a result, hydrated halloysite has a basal (d001) spacing of 10 Å which is ~3 Å larger than that of kaolinite (E. Joussein et al., 2005). Because of the stronger bonds between the alumino-silicate layers, the water molecules can be driven off without changing the structure, easily and irreversibly (Hendricks S B et al., 1938). The particles of halloysite can adopt a variety of morphologies, commonly elongated tubule, when the kaolinite shows platy particle shapes (E. Joussein et al., 2005). Ma. & Eggleton (1999) suggested that the slightly