Ferrihüdriit

Ferrihydrite is a poorly crystalline iron oxyhydroxide mineral that is widespread in natural environments. Its idealized chemical form is often written as Fe5HO8·4H2O, and in oxide terms it is sometimes described as Fe2O3·0.5H2O. It is one of the most common iron oxide phases in soils, sediments, and natural waters, but its structure is only short-range ordered, giving it an amorphous appearance under many analytical techniques.

The mineral consists of nanoscale particles with a very high surface area, which makes ferrihydrite highly reactive. Because of this, it can adsorb and slowly release a variety of elements, including nutrients and trace metals, as well as contaminants. In practice, two spectroscopic variants are commonly distinguished: 2-line and 6-line ferrihydrite, referring to differences in crystallinity observed by Mössbauer spectroscopy.

Ferrihydrite forms rapidly via oxidation of Fe2+ or hydrolysis of ferric ions in mildly acidic to neutral waters, such as groundwater, pore waters, or iron-rich surface waters. It is metastable and tends to transform into more crystalline iron oxide minerals over time, most commonly goethite or hematite, during aging, dehydration, or heating. The transformation alters its physical and chemical properties, including solubility and adsorptive behavior.

Ferrihydrite occurs in soils, lake and river sediments, hydrothermal environments, and other iron-rich settings. Its high reactivity and large surface area control key biogeochemical processes, notably nutrient cycling and contaminant mobility. Because of its rapid formation and tendency to transform under changing conditions, ferrihydrite is a central subject in studies of soil chemistry, environmental remediation, and paleoredox conditions.