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REE Mineralogy and Resources Chapter 279 167 process is found in Araxa ́ carbonatite in Brazil, where magmatic REE minerals (apatite, monazite) are replaced by gorceixite and secondary apatite in the reg- olith, of which REE concentration is $5.8 wt.%, much higher that the concen- tration (0.7–1 wt.%) of the fresh carbonatite (Morteani and Preinfalk, 1996). 3 RARE EARTH MINERALOGY 3.1 Variation of REE Minerals Associated with Their Crystal Chemistry There are more than 200 minerals that contain essential or significant REE concentrations which occur as a variety of compounds such as silicates, fluor- ocarbonates, fluorides, oxides, and phosphates (Table 6). They are included in igneous, metamorphic, and sedimentary rocks as accessory minerals. Some of carbonatites and alkaline rocks contain a major amounts of REE minerals and form REE ores (Fig. 19). REEs are divided into LREEs and HREEs by the difference of geochemical properties as previously discussed, and this leads to the two contrasting type REE minerals and deposits: LREE-rich type and HREE-rich type. There is no REE deposit producing well-balanced LREEs and HREEs, making it more difficult to develop the REE deposits. Fig. 20 shows production of REE by operation, country, and ore type in 2014 (Roskill Information Services Ltd, 2015). REE minerals frequently contain as many as 10 different rare earths as major elements and have complex chemical compositions because REEs are chemically similar. Geochemical behavior of REEs is similar to actinides, such as Th and U, so many of REE minerals contain Th and U as essential compo- nents (Table 6). A variety of chemical compositions of REE minerals are related to not only the chemical behavior but also properties of REE minerals having many isostructural minerals. Especially important among REE minerals are zircon structure (tetragonal, a1⁄46.604, c1⁄45.979, I41/amd, Hazen and Finger, 1979), monazite structure (monoclinic, P21/n, a1⁄46.7902, b1⁄47.0203, c1⁄46.4674A ̊, b1⁄4103.38 degree, Ni et al., 1995) and fergusonite structure (tetragonal a1⁄45.16, c1⁄410.89, I41/a, Komkov, 1959) (Fig. 21). Their chemical formulae can be generalized as ABO4 (Table 7). In the case of monazite, A and B correspond to LREEs and phosphorus (P), respectively. Existence of radio- active elements such as U and Th in REE minerals (Table 6), which is becom- ing a problematic issue for the development of REE deposits, is related to the fact that many REE minerals of ABO4 type are isostructural with Th or U minerals such as thorite and coffinite (Table 7). Later, we review a variety of minerals that have zircon and monazite structures. 3.1.1 ABO4 Minerals with Zircon Structure Zircon-structure minerals are zircon (ZrSiO4), thorite (ThSiO4), coffinite (USiO4), xenotime (HREEPO4), and a few other listed in Table 7. ThesePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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