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. 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.