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REE Mineralogy and Resources Chapter 279 217 Gromet and Silver (1983) indicated that allanite and titanite together account for 80–95% of REEs in monazite-free granodiorite. Bea (1996) analyzed accessory minerals of variable granitoids and crustal protoliths using laser- ablation inductively coupled-plasma mass-spectrometry (LA-ICP-MS) and EMPA, and indicated that strongly peraluminous granites contain monazite, xenotime, apatite, zircon, Th-silicates (thorite and/or huttonite), and pyro- chlore, and that metaluminous and weakly peraluminous granites contain alla- nite, titanite, apatite, zircon, monazite, and Th-silicates. In his study, a percentage of LREEs (except for Eu) of each mineral relative to whole-rock LREE is mostly dominated by monazite in the strongly peraluminous allanite-free granites, and by allanite in the other monazite-free granites. Pre- dominant HREE-bearing minerals are zircon, apatite, and xenotime in the strongly peraluminous allanite-free granites, and are zircon and titanite in the other monazite-free granites. As the quantitative data of Bea (1996) indi- cated, magmatic allanite and monazite rarely coexist in granitoids except in the case of crystallizing before equilibrium of melt (Broska et al., 2000). Allanite and titanite are the dominant REE-bearing minerals in granitoids, whereas the occurrences of these minerals are not common in strongly differ- entiated granites underlying ion-adsorption ore bodies (Table 13 and Fig. 40). Previous studies in southern China and the following studies suggested that crystallization of allanite and titanite becomes unstable during magmatic differentiation and this is probably due to fluorine in highly differentiated melt. Allanite in granitoids commonly alters to secondary minerals including fluorite and REE fluorocarbonates, such as bastnäsite and synchysite (Bea, 1996; Berger et al., 2008; Caruso and Simmons, 1985; Giere, 1996; Giere and Sorensen, 2004; Lira and Ripley, 1990). Titanite is also transformed to secondary minerals including allanite, monazite, xenotime, REE fluorocarbo- nates, anatase, rutile, quartz, calcite, and/or chlorite (Middleton et al., 2013; Pan et al., 1993). Titanite is stable in the oxidized felsic magma, whereas it is unstable in the reduced magma, because in the reduced condition, ilmenite becomes stable instead of titanite (Wones, 1989). An experimental study indi- cates that titanite is unstable in fluorine-rich melts because it reacts with fluo- rine to form fluorite (Price et al., 1999). The reduced residual melt including a certain amount of fluorine may become enriched in HREE progressively by fractional crystallization because titanite, a HREE-bearing mineral, hardly crystallizes and zircon is not common due to shortage of zirconium in melt. Eventually, HREEs are more incorporated into other minerals which probably include synchysite-(Y) and xenotime. REE-bearing phosphates consisting of apatite, and lesser amounts of mon- azite and xenotime, in granites have an important role in immobilizing REE during weathering (eg, Aide and Aide, 2012; Aubert et al., 2001; Laveuf and Cornu, 2009; Sanematsu et al., 2015; Sanematsu and Watanabe, 2016). Apatite is altered and replaced by secondary REE-bearing phosphate mineralsPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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