Contribution of Fe K-edge XANES spectroscopy to the characterization of Fe distribution in clay minerals. Application during experimental alteration of swelling clay minerals

Fe-rich clay minerals, particularly smectites, are central to many (bio)geochemical cycles in terrestrial to extraterrestrial environments. The exact crystal chemical characterization of structural iron in these natural minerals and its evolution during alteration remains a long-debated issue. This work revisited the potential of XANES spectroscopy at the Fe K-edge on a series of well-characterized synthetic Fe-rich swelling clay minerals with controlled Fe content. A decomposition of the Fe K-pre-edge feature of synthetic Fe-rich clay minerals allowed proposing a procedure to determine the tetrahedral [4] Fe(III) and octahedral [6] Fe(III) ratio in a 2:1 clay mineral structure along a large solid-solution series having 14 to 35 at.% of the total Fe in tetrahedral coordination. XANES analyses were also performed after alteration experiments in acidic and oxic conditions over a 5–1.8 pH range. The high sensitivity of XANES to the Fe(III) environment in a crystalline structure allowed the evidence of crystal-chemical changes during alteration, even with a low reaction rate (<1%). The reactions indicated that whatever the initial Fe-content, the composition of Fe-rich clay minerals tended to homogenize under acidic conditions toward a common, more stable, crystal-chemistry with lower Fe in tetrahedral sites. These results partly explain the crystal chemistry of natural smectites, for which Fe is mainly located in octahedral sheets. • Fe K-edge XANES pre-edge features can be used to determine tetrahedral vs octahedral Fe in smectite. • Fe K-edge XANES sensitivity reveals crystal chemical changes in altered smectites from pH 5. • The more Fe smectite contains in the tetrahedral layer, the less stable it is. • Fe-rich smectites that have been altered evolve toward the same crystal chemistry.