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238 Handbook on the Physics and Chemistry of Rare Earths and chlorapatite. Among the three, fluorapatite and oxy are by far the most common in nature. Fluorapatite is ubiquitous accessory phase in igneous, metamorphic, and sedimentary rocks and a major constituent in phosphorites and certain carbonatites and anorthosites (Dymek and Owens, 2001; McConnell, 1973). Of particular importance in biological systems, hydroxy- apatite and fluorapatite (and their carbonate-bearing varieties) are important mineral components of bones, teeth, and fossils (Ivanova et al., 2001; Kon et al., 2014; McConnell, 1973). It was reported that apatite crystals may incorporate up to 21 wt.% REE2O3 (Hoshino et al., 2015; Hughes et al., 1991; Roeder et al., 1987). Four main types of charge-compensating mechanisms have been proposed for the substitution of Ca2+ by REE3+ (and Y3+) in apatite (Chen et al., 2002a,b; Cherniak, 2000; Comodi et al., 1999; Felsche, 1972; Fleet and Pan, 1995; Ito, 1968; Roeder et al., 1987; Rønsbo, 1989; Serret et al., 2000): REE3+ +X2 1⁄4Ca2+ +F (1) 2REE3+ +□ðvacancyÞ1⁄43Ca2+ (2) REE3+ +Na+ 1⁄42Ca2+ (3) REE3+ +SiO44 1⁄4Ca2+ +PO43 (4) In these coupled substitutions, the mechanisms (1) and (2) are reported only in synthetic compounds with apatite structure and not in natural apatite. On the other hand, (3) and (4) are generally observed in nature. Apatite-group minerals including REEs as major elements are named belovite and britholite (Table 15); the coupled substitutions (3) and (4) are largely responsible for accommodating REEs into belovite and britholite, respectively (Pan and Fleet, 2002). This sug- gests that the coupled substitution mechanisms between the two minerals and apatite are very important for REE incorporation into natural apatite. Substitution (3) is well established on the basis of compositional data from natural apatite (eg, Comodi et al., 1999; Peng et al., 1997; Roeder et al., 1987; Rønsbo, 1989; Table 16) and is largely responsible for accommodating REEs into belovites (eg, Pekov et al., 1996; Rakovan and Hughes, 2000). Incorpora- tion of REEs into apatite by (3) results in the apatite coincidentally containing high concentrations of alkaline elements like Na. Therefore, the substitution of (3) is often observed for apatite in alkaline rocks including carbonatites. LREE-rich apatite-group minerals including belovite-(Ce) and belovite-(La) have been reported, no HREE-rich one was found in nature. Therefore, apa- tites formed as a result of coupled substitution (3) tend to enrich in LREEs. Belovite contains Sr as major element, which indicates that apatites occurring via coupled substitution (3) characteristically show substitution of Sr for Ca. Interestingly, Th content in belovite is below the detection limit by EMPA (eg, Pekov et al., 1996), which results in extremely low Th contents in the apatites occurring as a result of coupled substitution (3).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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