Fig. 6: Neodymium X-ray absorption spectra.

a Neodymium L₃-edge X-ray absorption near-edge structure (XANES) for mineral standards and Nd in solution, vertically arranged by decreasing coordination number (CN). Cerium L₂-edges observed in the Nd pre-edge demonstrate variable Ce oxidation states (Ce³⁺ at 6168 eV and Ce⁴⁺ at 6179 eV) in the natural mineral standards. The main Nd absorption features are marked by grey lines at 6215 eV (Nd³⁺, main peak), at 6248 eV (Feature A) and 6286 eV (Feature B). b Nd XANES spectra for clays and relict minerals in the Chinese and Malagasy samples. Kaolinite spectra from both sites demonstrate variable peaks for Ce³⁺ and minor Ce⁴⁺. Absorption of Ce⁴⁺ (6179 eV) is most prominent in the zircon and the Mn oxyhydroxide, the latter containing traces of cerianite-(Ce). Grey lines are as in a.