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222 Handbook on the Physics and Chemistry of Rare Earths 10 1 0.1 0.01 0 20 40 60 80 100 REE ion-exchangeable/REE whole-rock × 100 (%) FIG. 44 Plots of Ce anomalies vs PERs (percentages of ion-exchangeable REEs relative to whole-rock REEs) of weathered granites including ion-adsorption ores in southern China (dia- mond) and Thailand (circle). Data from Sanematsu, K., Kon, Y., 2013. Geochemical characteris- tics determined by multiple extraction from ion-adsorption type REE ores in Dingnan County of Jiangxi Province, South China. Bull. Geol. Surv. Jpn. 64, 313–330; Sanematsu, K., Kon, Y., Imai, A., 2015. Influence of phosphate on mobility and adsorption of REEs during weathering of gran- ites in Thailand. J. Asian Earth Sci. 111, 14–30. 4.2.5 Complexing of REEs Mobile elements including REEs in soil water migrate downward from the upper zone to lower zone in a weathering profile of granite. In this process, REEs are mobile in soil water as ions or by forming complexes, and they are subsequently immobilized by adsorption or incorporation into secondary minerals (or leached out from the weathering profile). This REE immobiliza- tion is caused by an increase of pH resulting from the contact of low-pH soil water with rock-forming minerals or ground water. Ground-water levels gen- erally decrease in the weathering profile due to uplift of the basement in the temperate areas, but these levels have larger fluctuations in tropical areas due to a change between dry and rainy seasons. This leads to a fluctuation of the contact zone in which REEs are immobilized. Previous studies including thermodynamic experiments and analyses of natural water suggest that REEs are primarily complexed with carbonate and bicarbonate ions in soil water or ground water at circumneutral pH (5 pH 9), and that the predominant ligand is dependent on pH and the con- centration of the species. Experimental and theoretical studies suggested that REEs are dissolved in solution by forming carbonate and bicarbonate com- plexes (REECO3+ and REEHCO32+, respectively) at pH over 5, and free REE3+ ions at pH below 5–6 (Cantrell and Byrne, 1987; Liu and Byrne, 1998; Wood, 1990), which is a common value in a weathering profile on granite. REE fluoride complexes such as REEF2+ and REEF2+ may be present, CeN Ce/Ce* = LaN PrNPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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