Fe5HO84H2O

Fe5HO84H2O is commonly written as Fe5HO8·4H2O and corresponds to ferrihydrite, a poorly crystalline iron oxyhydroxide mineral. Ferrihydrite is typically described as nanocrystalline or amorphous in structure, with iron in the Fe(III) state and a higher water content than more crystalline iron oxides. The mineral is notable for its high surface area and reactivity.

In terms of composition and structure, ferrihydrite consists of iron oxyhydroxide layers linked with hydroxide groups and structurally bound water. It shows short-range order rather than long-range crystalline order, which gives rise to broad features in diffraction patterns. Depending on crystallinity, ferrihydrite is sometimes classified as two-line or six-line ferrihydrite in X-ray diffraction analyses, reflecting different degrees of structural organization.

Ferrihydrite forms through the hydrolysis and oxidation of soluble iron under neutral to slightly acidic conditions. It is widespread in soils, sediments, and near iron-rich groundwater discharge, where it precipitates rapidly from solution. Microbial activity can influence its formation and aging, contributing to its ubiquity in natural environments.

Physically, ferrihydrite appears as fine, reddish-brown to yellow-brown particles with a very high specific surface area. It is metastable and prone to transformation into more crystalline iron oxides such as goethite or hematite upon aging, heating, or changes in pH and redox conditions. Because of its high reactivity, ferrihydrite readily adsorbs nutrients and contaminants, playing a key role in nutrient cycling and environmental remediation.

See also: ferrihydrite, goethite, hematite, iron oxyhydroxides.