Fe2O3xH2O

Fe2O3xH2O denotes a family of hydrated ferric oxide phases rather than a single compound. The formula expresses iron(III) oxide with variable water content and is used to describe crystalline oxyhydroxides and amorphous hydrated forms. It is commonly applied as a general representation for hydrated iron oxides found in nature and in industrial contexts.

Crystalline members include goethite (FeO(OH), alpha-FeO(OH)) and lepidocrocite (FeO(OH), gamma-FeO(OH)), both of which are iron(III) oxyhydroxides. These minerals can be viewed as iron oxide lattices with hydroxide groups and water-derived protons. Less crystalline or amorphous hydrated ferric oxides are exemplified by ferrihydrite, often described by formulas near Fe2O3·0.5H2O, and by limonite, a historical term for a variable mixture rich in hydrated iron oxides and oxyhydroxides.

Occurrence and formation: Fe2O3xH2O phases form through oxidation and hydrolysis of ferrous iron in aerated environments, as well as by weathering of iron-containing minerals. They are common constituents of soils, sediments, and corrosion products, and can dominate the iron mineralogy in the near-surface environment. Water content in these phases can vary from near dryness to substantial hydration, influencing structure and reactivity.

Properties and roles: These compounds are typically insoluble in water and exhibit reddish-brown to yellow-brown colors. They have a high capacity to adsorb contaminants and nutrients, affecting soil chemistry and contaminant mobility. The more amorphous ferrihydrite phase is generally more reactive than well-crystallized goethite or lepidocrocite and can transform to more crystalline forms over time.

See also: hematite, goethite, lepidocrocite, ferrihydrite, limonite.