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FIG. 45 REE Mineralogy and Resources Chapter 279 225 because its PCZ is $2.5 (Kosmulski, 2009, 2011). On the other hand, the higher PCZ of montmorillonite (7–9; Kosmulski, 2009, 2011) indicates that montmorillonite is unlikely to adsorb REEs at basal surface because of its pos- itive variable charge. In summary, these PZC values suggest that kaolin-group minerals and/or illite are capable of adsorbing REEs in natural soil and ground water. Phosphate content strongly influences fractions of ion-exchangeable REEs in weathered granites (Sanematsu et al., 2015), because REEs in mineral phases are mostly incorporated in phosphates (eg, monazite, xenotime, rhabdophane, crandallite group) in weathered rocks. Experimental studies indicate that apatite dissolution rate increases with decreasing pH and is higher than dissolution rates of other phosphates such as monazite at ambient temperatures (Cha ̈ırat et al., 2007; Guidry and MacKenzie, 2003; Harouiya et al., 2007; Oelkers et al., 2008; Valsami-Jones et al., 1998). The apatite dissolution is strongly influenced by rhabdophane precipitation and dissolution in acidic water at room temperature (K€ohler et al., 2005). These results suggest that dissolution of apatite and immobilization of REEs by both primary phosphates (monazite and xenotime) and secondary phosphates (rhabdophane and crandallite group) during weathering have an important role in controlling REE concentration in soil and ground waters. Sanematsu et al. (2015) analyzed chemical composi- tions of weathered granites overlying the contrasting (P2O5-rich and -poor) granite suites, and they indicate that high percentages of ion-exchangeable REEs are likely derived from P2O5-poor granites (Fig. 45). 0.4 0.3 0.2 0.1 0 0 20 40 60 80 100 REE ion-exchangeable/REE whole-rock × 100 (%) Plots of P2O5 contents vs PERs of weathered granites including ion-adsorption ores in southern China (diamond) and Thailand (circle). Open circles show hydrothermally altered and weathered granites. Data from Sanematsu, K., Kon, Y., 2013. Geochemical characteristics deter- mined 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 granites in Thailand. J. Asian Earth Sci. 111, 14–30. P2O5 (%)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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