Ferrihydrite

Ferrihydrite is a ubiquitous iron oxyhydroxide mineral. Its chemical formula is generally presented as Fe₅HO₈•4H₂O, also written as 5Fe₂O₃•9H₂O or as Fe₂O₃•2FeOOH•2.6H₂O [J. L. Jambor and J. E. Dutrizac, Chemical Reviews 98, no. 7, 2549-2585 (1998)] . It is found throughout soil and water systems [U. Schwertmann, L. Carlson, E. Murad, Clays Clay Miner. 35, 297 (1987) ] and is important to many industrial applications [G. P. Huffman et al., Energy Fuels 7, 285 (1993). ] [P. A. Riveros, J. E. Dutrizac, P. Spencer, Can. Metall. Q. 40, 395 (2001). ] . It can even be found within many living organisms, including humans [A. Lewin, G. R. Moore, N. E. Le Brun, Dalton Trans. 2005, 3597 (2005). ] In particular, it is found in the iron core of the ferritin protein, with serves as an intra-cellular iron storage.

Two types of material are commonly called ferrihydrite: 2-line ferrihydrite (sometimes misleadingly termed protoferrihydrite) and 6-line ferrihydrite. The distinction refers to the number of X-ray diffraction lines in their respective X-ray patterns. While 6-line ferrihydrite has been classified as a mineral by the IMA since 1973 [F. V. Chuckrov, B.B. Zvyagin, A.I. Gorshov, L.P. Yermilova and V.V. Balashova, International Geology Review 16, 1131-1143 (1973)] [M. Fleischer, G.Y. Chao and A. Kato, American Mineralogist 60 (1975)] , its structure is still a subject of debate. Recently, a structural model for 6-line ferrihydrite has been published by "Science" [F.M. Michel et al., Science 316, 1726-1729 (2007)] , but its validity has been contested. [D.G. Rancourt and J.-F. Meunier, American mineralogist 93, 1412-1417 (2008)] The less crystalline 2-line ferrihydrite, on the other hand, is not considered a mineral. Compared to most minerals, both 2-line and 6-line ferrihydrite show very broad diffraction lines.

Ferrihydrite only exists as a nanomaterial, showing crystals 2-4 nanometers wide for 2-line ferrihydrite and 5-6 nanometers wide for 6-line ferrihydrite [D.E. Janney, J.M. Cowley and P.R. Buseck, Clays and clay minerals 48, no. 1, 111-119 (2000)] [D.E. Janney, J.M. Cowley and P.R. Buseck, American mineralogist 85, 1180-1187 (2000)] [D.E. Janney, J.M. Cowley and P.R. Buseck, American mineralogist 86, 327-335 (2001)] .

With its high surface area per volume [J. L. Jambor and J. E. Dutrizac, Chemical Reviews 98, no. 7, 2549-2585 (1998)] , ferrihydrite is a very reactive mineral. It can interact, either by surface adsorption or by co-precipitation, with a number of environmentally important chemical species, including arsenic, heavy metals like lead or mercury, phosphate, as well as many organic molecules.

Ferrihydrite is a metastable mineral. It is known to be a precursor of more crystalline minerals like hematite and goethite [H. Stanjek and P.G. Weidler, Clay minerals 27, 397-412 (1992)] .

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