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244 Handbook on the Physics and Chemistry of Rare Earths Substitution (4) is also well documented in natural REE-bearing apatites (Comodi et al., 1999; Roeder et al., 1987; Rønsbo, 1989) and is supported by a complete solid solution between hydroxyapatite and britholite-(Y) (Ito, 1968; Khudolozhkin et al., 1973). This substitution leads to the endmember Ca4REE6(SiO4)6F2, which has been synthesized for compositions involving La, Ce, Nd, and Y (Ito, 1968; Mayer et al., 1974). As mentioned earlier, belo- vite is only LREE type of La- and Ce-type, but Ce- and Y-type for britholite have been reported. Therefore, there are both LREE-rich and HREE-rich types of apatite formed via substitution (4). Britholite typically occurs in alkaline rocks and pegmatite in the metamict state. It has been reported that britholite contains up to 21 wt.% ThO2 and 3.5 wt.% UO2 (Table 6). Compo- sitional data (Baumer et al., 1983; Hughson and Sen Gupta, 1964) have shown that Th4+ ions substitute for Ca2+ via: Th4+ +□ðvacancyÞ1⁄42Ca2+ (5) Similarly, Clarke and Altschuler (1958) suggested that U in apatite is mainly tetravalent and that U4+ ions occupy the Ca sites via a similar substi- tution (Baumer et al., 1983): U4+ +□ðvacancyÞ1⁄42Ca2+ (6) Therefore, apatite with britholite substitution (4) contains relatively high Th contents compared to apatite with belovite substitution (3), but U is below detection limit by EMPA (Table 16). Incorporation of REEs into natural apatite is caused by belovite-type and/or britholite-type substitution (Pan and Fleet, 2002) and may be related to the alkalinity and the silica saturation in melt (Roeder et al., 1987; Watson and Green, 1981). Britholite substitution is important for HREE, Th, and U incorporation into apatite. 4.3.2 Apatite REE Deposits Most of phosphate fertilizers are produced from apatite ores. As mentioned earlier, apatite deposits are divided into S- and I-types which are distributed all over the world (Fig. 46). Sedimentary apatite deposits are exploited to pro- duce more than 80% of the total world production of apatite rocks (Ihlen et al., 2014; Van Kauwenbergh et al., 2013). Sedimentary phosphate deposits occurred throughout the long geological time scale from Precambrian to Recent, but most of them are located in the great Cenozoic deposits of North Africa including Morocco, Tunisia, and Algeria (Van Kauwenbergh, 2010), and the United States and southeast Atlantic (Emsbo et al., 2015, 2016; Manheim and Gulbrandsen, 1979; Van Kauwenbergh, 2010). Most sedimen- tary phosphorite deposits were apparently formed in continental and ocean basin margins (Fig. 46). Their phosphorous were mainly derived from deep oceanic source associated with upwelling currents of cold, nutrient-rich watersPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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