abstract
| - 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 . It is found throughout soil and water systems and is important to many industrial applications . It can even be found within many living organisms, including humans 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 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 , its structure is still a subject of debate. 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 . With its high surface area per volume , 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 .
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