Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from freshwater to marine systems, aquifers to hydrothermal hot springs and scales, soils, and areas affected by mining. It can be precipitated directly from oxygenated iron-rich aqueous solutions, or by bacteria either as a result of a metabolic activity or passive sorption of dissolved iron followed by nucleation reactions. Ferrihydrite also occurs in the core of the ferritin protein from many living organisms, for the purpose of intra-cellular iron storage.
Ferrihydrite only exists as a fine grained and highly defective nanomaterial. The powder X-ray diffraction pattern of Fh contains two scattering bands in its most disordered state, and a maximum of six strong lines in its most crystalline state. The principal difference between these two diffraction end-members, commonly named two-line and six-line ferrihydrites, is the size of the constitutive crystallites. The six-line form has been classified as a mineral by the IMA in 1973 with the nominal chemical formula 5Fe2O3•9H2O. This formula can be recast as FeOOH•0.4H2O to account for the FeOOH stoichiometry of the core of the dominant structures. However, its formula is fundamentally indeterminate, because Fh is not a single phase and its water content is variable. The two-line form is also called hydrous ferric oxide.
According to the standard structural model established by X-ray diffraction , Fh is a multiphase material with three components: defect-free crystallites (f-phase) with double-hexagonal stacking of oxygen and hydroxyl layers (ABAC sequence) and disordered octahedral Fe occupancies, defective crystallites (d-phase) with a short-range feroxyhite-like (δ-FeOOH) structure, and subordinate ultradisperse hematite (α-Fe2O3). The f- and d-phases have been confirmed by neutron diffraction, and all three components observed by high-resolution transmission electron microscopy, including single crystal electron nanodiffraction. (HRTEM) Nanohematite can coexist with a spinel-type phase (maghemite γ-Fe2O3 or magnetite Fe3O4), in proportions that vary from sample to sample. A new structural model was proposed in 2007, but shown to be incorrect.
Because of the small size of individual nanocrystals, Fh is nanoporous yielding large surface areas of several hundred square meters per gram. In addition to having a high surface area to volume ratio, Fh also has a high density of local or point defects, such as dangling bonds and vacancies. These properties confer to this mixed phase a high ability to adsorb many environmentally important chemical species, including arsenic, lead, phosphate, and organic molecules. Its strong and extensive interaction with trace metals and metalloids is used in industry, at large-scale in water purification plants, as in North Germany and to produce the city water at Hiroshima, and at small scale to clean wastewaters and groundwaters, for example to remove arsenic from industrial effluents and drinking water. Its nanoporosity and high affinity for gold can be used to elaborate Fh-supported nanosized Au particles for the catalytic oxidation of CO at temperatures below 0°C.
Ferrihydrite is a metastable mineral. It is known to be a precursor of more crystalline minerals like hematite and goethite by aggregation-based crystal growth. However, its transformation in natural systems generally is blocked by chemical impurities at its surface, for example silica as most of natural ferrihydrites are siliceous.
1. ^ J. L. Jambor, J.E. Dutrizac, Chemical Reviews, 98, 22549-2585 (1998)
* Handbook of Mineralogy