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Publication
Natural nanomaterials : reappraising the elusive structure of the nano-sized mineral ferrihydrite through X-Ray absorption spectroscopy at the iron K-edge
Advisor(s)
Abstract(s)
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.
Description
Keywords
Nanominerais Ferrohidrite XANES K-edge
Citation
Figueiredo, M. O.; Silva, T. P.; Veiga, J. P. - Natural nanomaterials : reappraising the elusive structure of the nano-sized mineral ferrihydrite through X-Ray absorption spectroscopy at the iron K-edge. In: Materials Science Forum, vols. 730-732, (2013), p. 931-935