Natural Nanomaterials: Reappraising the Elusive Structure of the Nano-Sized Mineral Ferrihydrite through X-Ray Absorption Spectroscopy at the Iron K-Edge

Abstract:

Ferrihydrite is natural ferric oxyhydroxide occurring exclusively nanocrystalline. With ideal formula 5 Fe₂ O₃ . 9 H₂ O, ferrihydrite is quite abundant in sediments, weathering crusts and mine wastes, being characteristic of red pre-soils formed by loose weathered rock plus mineral debris (regoliths) and commonly designated as “2-line” or “6-line” on the basis of the broadened maxima observed in the X-ray diffraction pattern. Synthetic nanocrystalline “6-line” ferrihydrite was recently studied through methods based on atomic-pair distribution functions disclosing the possible occurrence of icosahedral clusters formed by twelve octahedra centred by an inner tetrahedron, all filled by Fe ³⁺ ions. However, Mössbauer studies were inconclusive about the existence of 4-coordinated iron, thus suggesting that the tetrahedral cation may well be Si⁴⁺. In view of such structural uncertainty, a XANES study at the Fe K-edge was undertaken on ferrihydrite from a regolith to ascertain the occurrence of tetrahedral iron. Comparison with data collected from well crystallized iron oxide and hydroxide minerals where Fe ^3+/2+ ions occur in octahedral and tetrahedral coordination is described and the results so far obtained are discussed, showing that supplementary study is needed on the elusive structure of ferrihydrite.

• Periodical: Materials Science Forum (Volumes 730-732)
• Pages: 931-935
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.730-732.931
• Online since: November 2012
• Authors: Maria Ondina Figueiredo, Teresa P. Silva, Joao P. Veiga
• Keywords: Fe K-Edge, Ferrihydrite, Nanominerals, XANES

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