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REE Mineralogy and Resources Chapter 279 223 because F is released by degradation of REE fluorocarbonate, fluorite, and biotite. However, the carbonate and bicarbonate complexes or free REE3+ are more dominant than fluoride and other inorganic complexes in solutions with pH over 6.5 at low temperature (Cantrell and Byrne, 1987; Wood, 1990). Phosphate complexes of REE3+ may be significant in high phosphate concen- tration at a pH range of 7–9 (Byrne et al., 1991; Lee and Byrne, 1992), though these complexes are insignificant in ground water with low phosphate concen- tration, and REE–carbonate complexes are predominant at the same pH range (Johannesson et al., 1996). Carbonate and bicarbonate are the most important inorganic ligands for REEs in natural waters, and experimental studies suggest that HREEs are more easily complexed than LREEs (Cantrell and Byrne, 1987; Lee and Byrne, 1993; Pourret et al., 2007a). The stability constant of bicarbon- ate increases from La to Lu (Cantrell and Byrne, 1987; Lee and Byrne, 1993), suggesting the enrichment in HREEs in CO2-rich solution. Enrichment in HREEs and/or U was found in CO2-rich solutions even at low temperatures in carbonaceous metasedimentary rocks (McLennan and Taylor, 1979), in CO2-rich natural water (Michard et al., 1987), and in weathered schist (Koppi et al., 1996). Humic substances are critical for leaching and complexing with REEs dur- ing granite weathering as well as carbonate and bicarbonate ions. Chen et al. (2001) leached REEs by 0.01 mol/L organic acids from weathered granites in Guangdong Province and results indicated that HREEs are more selectively leached than LREEs by citric and acetic acids compared with those leached by ammonium sulfate solution. Results of thermodynamic studies indicated that amount of REEs complexed with humic acid are strongly dependent on pH (Glaus et al., 2000; Pourret et al., 2007b; Sonke and Salters, 2006; Takahashi et al., 1997, 1999), and the carboxyl of humic acid is the main bind- ing site of REEs (Pourret et al., 2007b; Takahashi et al., 1997). Fulvic acid is also important for complexation with REEs and has similarity with humic acid (Pourret et al., 2007b). Binding between REEs and humic substances becomes more stable with decreasing ionic radii from La to Lu (Sonke and Salters, 2006; Takahashi et al., 1997), and this suggests that HREEs are more strongly bound to humic substances than LREEs. Fractionation of LREEs/HREEs is influenced by complexation with car- bonate ions, bicarbonate ions, and humic substances during weathering, however, this is less significant than the primary fractionation caused by weathering susceptibilities of initial magmatic and hydrothermal minerals (REE fluorocarbonates, allanite, titanite, etc.) as previously explained. Chondrite-normalized REE patterns of ion-adsorption ores typically reflect those of parent granites (Fig. 41) and fractionation of REEs may occur due to dissolution of specific minerals. Ion-adsorption ores can become enriched in HREEs relative to the parent granite in Longnan because of the dissolution of HREE-rich fluorocarbonate of synchysite-(Y) (Bao and Zhao, 2008; Huang et al., 1989; Wu et al., 1990). In contrast, ion-adsorption ores becomePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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