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REE Mineralogy and Resources Chapter 279 259 prospects are augite and ferro-edenite–actinolite which are mafic silicate minerals. Apatite accompanying diopside as main minerals in Hoidas Lake deposits contains high LREE contents, but is not IOA deposit. These suggest that magma crystallizing REE-rich apatite (over several wt.%) and mafic sili- cate minerals, as main minerals, from the above three prospects, and these deposits have very high silica activity compared to carbonatite, alkaline rocks, and other IOA deposits. As mentioned earlier, incorporation of REEs into apa- tite is related to silica activity, and the high activity in the above three pro- spects and deposits may be a major cause of occurrence of apatite having high REE contents. Apatite in the Hoidas Lake deposit have LREE-rich distri- bution (Table 16 and Fig. 50) and form accompanying lamprophyres and/or carbonatites. Apatite crystallization having LREE-rich distribution from other IOA deposits are also related to intrusions of carbonatite and felsic magma. On the other hand, HREE-rich apatite crystallization in Benjamin River and Blockspruit prospects are accompanied by intrusion of mafic magma like dio- rite and ferro-edenite magma. These differences of intrusive rocks are related to those of REE distribution (LREE-rich or HREE-rich) (Fig. 50). The Block- spruit and the Benjamin River IOA prospects are promising as HREE resources. In particular, existences of many similar type of prospects in Bush- veld complex, South Africa are expected, so further explorations are needed. Sedimentary apatite rocks generally contain low REE contents compared to igneous ones (Table 14), so in the past it has been thought that their REE economic potentials are low. Recently, some sedimentary apatite rocks having HREE potential were reported by Emsbo et al. (2015). REE contents in sedi- mentary apatite rocks are quite different based on the difference of occurrence compared to those in igneous apatite rocks. Detailed studies concerning min- eralogy of sedimentary apatite rocks are also rare (eg, with respect to REE substitution mechanism). Further studies about the relationships between REE concentration and sedimentary environment (eg, sea composition and age) of sedimentary apatite are needed. 4.3.3 REE Production from Apatite as By-Product of Phosphoric Acid: Leaching Method and Adsorbent for REEs As mentioned earlier, the largest advantage of apatite as REE resources is in the promising possibility for the production of REEs as a by-product of phos- phoric acid production. Although this is not achieved in plant-scale at the present time, recent years have brought significant progress for this topic. In this section, we first describe typical chemical processes for phosphoric acid production from apatite ore concisely, and then introduce several recent inves- tigations aiming the REE production as a by-product of the mineral proces- sing. Particularly, it is notable that Ogata et al. (2014) synthesized a noble adsorbent for selective recovery of REEs (especially HREEs) in solution with considerable impurities. These recent advancements would enable one to make better use of a vast amount of phosphate rocks in the near future.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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