[M-ZZ45] Frontiers in geochemistry: discussing its appeal and future prospects

[MZZ45-11] A new application of XANES by HERFD-XANES measurement and simulation focusing on the weathering reaction of biotite

★Invited Papers

*Akiko Yamaguchi¹, Yoshio Takahashi², Masahiko Okumura¹ (1.Japan Atomic Energy Agency, 2.The University of Tokyo)

Keywords:XANES, ab initio calculation, biotite

The environmental behavior of various elements is affected by chemical reactions, such as adsorption, precipitation, and oxidation-reduction reactions. Therefore, chemical states and local structures of target elements are needed to understand their environmental behaviors. As a tool to investigate the chemical states and local structures of various elements, including trace elements, X-ray absorption fine structure (XAFS) is beneficial because of its high sensitivity and selectivity. The XAFS is divided into two structures depending on the energy of the incident X-ray; one is X-ray absorption near edge structure (XANES), and the other is extended X-ray absorption fine structure (EXAFS). The former mainly reflects the electronic states of the target element. On the other hand, the latter reflects the interaction between the atoms of the target element and their neighboring atoms. An EXAFS spectrum is usually analyzed by fitting with the EXAFS theoretical formula, which provides information about the local structure, such as distances and coordination numbers. However, purely theoretical interpretation of XANES spectrum is difficult at present. Thus, an XANES spectrum is usually analyzed by linear combination fitting (LCF) with some spectra of standard samples. There are some problems in the LCA-XANES analysis. One is the difficulty obtaining appropriate XANES spectra for the target sample. For example, the XANES spectrum of iron (Fe) in biotite easily changes by weathering. Another one is that some XANES spectra of standard samples, such as Fe in ferrihydrite and goethite, show similar spectra.
In recent years, the high-energy resolution fluorescence detection (HERFD)- X-ray absorption near edge structure (XANES) method has been applied to Fe in some minerals [1], showing that HERFD-XANES can detect minor differences that cannot be found in normal XANES . In addition, advances in software for ab initio calculation have made it possible to simulate XANES more accurately. The combination of the HERFD-XANES measurement and the XANES simulation can provide more accurate fitting and more information of Fe speciation than ever before.
This study focused on the weathering reaction of biotite. The biotite, a mica mineral, is ubiquitous and often weathered at the Earth’s surface. The weathered biotite has a character like vermiculite and can adsorb cations, which is essential for understanding environmental behavior of various cations, including radioactive cesium emitted from the nuclear power plant accident. The weathering of biotite seems to oxidize Fe(II) to Fe(III), but the details remain unclear. This study aims to investigate the effect of the weathering reaction on biotite by using XANES, HERFD-XANES, and ab initio calculation.
Weathered biotite samples with several degrees of weathering were artificially produced based on a previous study [2]. Their XANES and HERFD-XANES spectra at Fe K-edge showed that the spectra were systematically changed. The HERFD-XANES spectra had sharper peaks, although HERFD-XANES spectra can be affected by oxidation reactions by incident X-ray. In addition, the XANES spectrum of pure biotite was simulated by using the Vienna Ab initio Simulation Package (VASP) and WIEN2k. The characteristics of the simulated spectra were consistent with the XANES and HERFD-XANES results, which can bring us clues to understanding the effect of weathering on Fe in biotite.

References:
[1] Natori et al., Minerals, 12, 536 (2022)
[2] Kitayama et al., European Journal of Soil Science, 71, 1-13 (2020)