X-ray inverse fluorescence approach for accurate determination of Fe valence in Fe-rich geological materials

Iron exhibits variable redox states between Fe2+ and Fe3+, governing key processes from Earth’s deep interior to surface environments. Quantitative evaluation of Fe valence is therefore essential in geoscience; however, partial fluorescence yield XANES (PFY-XANES), a widely used nondestructive technique, often suffers from thickness or self-absorption effect in samples with high Fe concentrations. In this study, we applied the inverse PFY (IPFY) method, to geological samples to assess its reliability in Fe valence determination. Iron LIII-edge IPFY-XANES and conventional PFY-XANES analyses were performed on olivine, wadsleyite, bridgmanite, and two biotite samples with FeO contents of 6.6–37.7wt.%. Conventional PFY-XANES overestimated Fe3+/ΣFe by approximately 10% relative to Mössbauer spectroscopy, whereas IPFY-XANES yielded values differing by less than 3%, showing excellent agreement. The inverse spectra also exhibited reduced pre- and post-edge tailing, confirming suppression of thickness effects. These results demonstrate that IPFY-XANES provides more accurate determination of the valence state of Fe, particularly for Fe-rich or thick samples. Because many geoscientific specimens such as asteroidal return samples, meteorites, or recovered high-pressure samples cannot be destructively prepared, the IPFY approach offers a powerful and reliable method for nondestructive Fe valence analysis across a wide range of natural materials.