Determination of the host phase of rare earth elements in natural carbonate using X-ray absorption near-edge structure

抄録

The Y K-edge X-ray absorption near-edge structure (XANES) spectra of various materials were measured to investigate whether rare earth elements (REEs) are incorporated in carbonate phase or in other minor phases such as Fe-Mn oxides and phosphate within limestone. As reported in our previous work, a white line peak specific to synthetic Y-doped calcite is observed in the Y K-edge XANES spectrum. Natural limestone and dolomite samples show XANES spectra similar to that of the Y-doped calcite. The similarity of XANES spectra between natural and synthetic carbonates strongly indicates that Y is incorporated into the carbonate phase of natural samples, and that local coordination structures of Y in natural and synthetic carbonates are the same. Previous work reported a difference of local coordination structure between light and heavy REEs. Furthermore, we reported that local coordination structures of Y and Ho are similar. The results of this study therefore demonstrate that the Y K-edge XANES spectrum can be used as a fingerprint to trace heavy REEs including Y in natural carbonate. The information that REEs are not distributed to other minor phases is an important basis for the use of limestone to estimate REE pattern of ancient seawater coupled with the REE partition coefficient data between seawater and carbonate.