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REE Mineralogy and Resources Chapter 279 219 4.2.4 Mobility and Fractionation of REE During Weathering The degree of weathering, that is influential to REE mobility, is closely asso- ciated with the pH of soil and soil water and they were measured at different REE deposits in southern China. The moderately low pH values in the surface soils are attributed to humic substances and to CO2 in the atmosphere. The pH values of water associated with weathered granites of the Zudong and Guanxi intrusions vary from a moderately low pH of 4.4–5.2 in the surface soil, to a pH of 5.5–6.0 in the strongly weathered zone that includes the ion-adsorption ores, and to a pH of 6.3 in the weakly weathered zone on parent granites (Wu et al., 1990). Similar pH values were reported in other mining sites of Xunwu, Dingnan, and Gan Counties in Jiangxi Province, consisting of 4.13–4.80 in the surface soil, 4.36–4.84 in the strongly weathered zone, and 5.01–5.77 in the moderately weathered zone (Wei et al., 2001). At other mining sites of Guangdong Province, Yang and Xiao (2011) reported similar pH values consisting of 4.8–5.96 in the surface soil, 5.22–6.80 in the strongly weathered zone including ion-adsorption ores, and 6.23–6.48 in the moder- ately weathered zone on parent rocks. These results suggest that the strongly weathered zone occurs in a pH range between 4 and 6 and the underlying moderately weathered zone occurs in a slightly higher pH range between 4.5 and 6.5, approximately (Sanematsu and Watanabe, 2016). The increase of pH is due to rock-forming minerals or ground water. Vertical variation of REE contents in a weathering profile is strongly influenced by the degree of weathering which is controlled by the depth from the surface. Soil water is generally acidic at the surface, because of the pres- ence of CO2 dissolved from the air and/or humic substances (humic and fulvic acids), which are produced by biological degradation of organic matter if the surface is covered by vegetation. Rock-forming minerals and REE-bearing minerals are altered and dissolved by reacting with acidic soil water with the exception of minerals that have very low solubilities. A weathering profile including ion-adsorption type ore bodies has been classified into different zones based on the degree of weathering reflected by constituent minerals, textures, and colors (Wu et al., 1990; Yang et al., 1981; Zhang, 1990). The weathering profile may be classified into the surface soil zone (A), strongly weathered zone (B), moderately weathered zone (C), weakly weathered zone (D), and basement rock (E) from top to bottom (eg, Wu et al., 1990; Zhang, 1990) as shown in Fig. 42. However, the classification criteria are ambiguous and weathered zones may be classified differently by different geologists. REEs are generally concentrated in the moderately weathered zone (C); this zone can be recognized as an ore body if it is thick enough and contains suf- ficient amount of ion-exchangeable REEs. Ion-adsorption ores typically show over $50% of ion-exchangeable REEs which are primarily present on the surface of clays such as kaolinite and halloysite (Bao and Zhao, 2008; Chen, 2013; Chi and Tian, 2009; Chi et al.,PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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