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REE Mineralogy and Resources Chapter 279 165 (Toyoda and Tokonami, 1990). Extraction experiments targeting REEs from the pelagic sediments in different sampling sites of the Pacific indicate that phosphate phases are high in REEs and are characterized by a negative Ce anomaly (Toyoda and Masuda, 1991; Toyoda et al., 1990). More detailed studies are necessary to determine the origin of the REEs in such sea mud. 2.7 Weathering Weathering is one of the important processes that concentrates and/or frac- tionates REEs. This process particularly effective for carbonatite deposits, because high concentrations of REEs, which may reach 10–25 wt.%, are expected for laterites that are developed above the carbonatites and alkaline complexes (Morteani and Preinfalk, 1996). The high REE concentrations result from the high concentrations in carbonatites and from a significant amount of carbonates which can dissolve in acidic water during weathering. Another important process is the adsorption of REEs on clays during weathering of granite to form ion-adsorption type deposits. This process will be described in detail in Section 4. 2.7.1 Lateritic Profile and Behavior of REEs Laterites are near-surface weathering products, which are strongly enriched in Fe2O3 and Al2O3, and depleted in SiO2 (Ba ́rdossy and Aleva, 1990). They can be distinguished from the underlying rocks, saprolites, which are more enriched in SiO2 and preserve original rock textures. Laterites are generally distributed in near-equatorial belts characterized by the present tropical climate. Bauxites are included in laterites, which are or will become economically min- able, containing not less than 45–50% Al2O3 and not more than 20% Fe2O3 and 3–5% SiO2 (Valeton, 1972). In saprolites, original rock textures are largely preserved, but most weath- erable minerals such as mafic silicates and feldspars are altered to clay miner- als such as smectites and kaolinite and other weathering products including amorphous materials. Quartz and other less weatherable minerals such as Fe oxides and Ti oxides are largely preserved. The upper pedolith is composed of clay zone, ferruginous zone, laterite, and soil in ascending order. In the clay zone, textures of the original rock are overprinted by homogeneous masses of clays, typically dominated by kaolin-group minerals. The ferruginous zone is formed of various concentrations of ferric iron oxides and hydroxides near the water table, and this zone may be cemented by precipitation of silica and iron hydroxide minerals to form a duricrust that is resistant to further erosion. The laterite is loose Fe-rich subsoil rock formed as a result of residual enrichment and precipitation of iron hydroxide and oxide minerals from iron-derived deeper in the profile. This horizon can be composed largely of pisolites, gravel-sized subspherical accretionary concentrations (Ridley, 2013).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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