Microbeam X-ray analysis of Ce³⁺/Ce⁴⁺ in Ti-rich minerals: A case study with titanite (sphene) with implications for multivalent trace element substitution in minerals

Cerium L₃ absorption edge (L₃-edge) X-ray absorption near edge structure (XANES) spectra were obtained from ∼7 × 5 μm areas on green titanite and brown titanite (both with total Ce ∼ 0.6 wt%) using the X-ray microprobe at the Pacific Northwest Consortium-X-ray Science Division (PNC-XSD) Insertion Device (ID) line of the Advanced Photon Source (APS). Using a wavelength-dispersive X-ray (WDX) fluorescence detector with a bent LiF (220) crystal monochromator (E/ΔE ∼ 1000), we have overcome the challenge of having to measure trace amounts of Ce in a Ti-rich sample of which the energy of the fluorescence X-rays from Ce L₃-edge and Ti K-edge excitation cannot be resolved with solid-state detectors. We show that both Ce³⁺ and Ce⁴⁺ are present in our titanite samples by examining the Ce L₃-edge XANES spectra. Our results show that to correctly determine trace element substitution mechanisms in titanite (and other minerals), it is necessary to determine multivalent element concentrations, including Ce³⁺/Ce⁴⁺. We present a new approach for predicting and evaluating multivalent trace element substitution in titanite and other minerals.