Discovery of ion-adsorption type deposits of rare earth elements (REE) in Southwest Japan with speciation of REE by extended X-ray absorption fine structure spectroscopy

抄録

Ion-adsorption type rare earth elements (REE) deposit in weathered granite is the main source of REE essential for high-technology industries. However, this type deposit has not been well surveyed in Japan. In this study, Bulk REE abundances (=REE_TOT), ion-exchangeable REE (=REE_EX) by ammonium chloride solution, and percentage of REE_EX relative to REE_TOT (=REE_PER) in fresh and weathered granite samples in Southwest Japan (i.e., Hiroshima and Shimane Prefectures) were determined. The REE_TOT and REE_PER were comparable to those of ion-adsorption type REE deposits in China. This finding indicates that ion-adsorption type REE deposits can also be found in Japan. Furthermore, extended X-ray adsorption fine structure (EXAFS) spectra of yttrium in original samples and samples after the extraction of ion-exchangeable REE showed that (i) REE in samples with high REE_PER mainly forms outer-sphere (OS) complexes and (ii) the remaining REE in the rocks after the extraction forms inter-sphere complexes. These findings suggest that ion-exchangeable REE forms OS complexes, which causes the high extraction rate by the ion-exchange reaction.