FeOOH

FeOOH, or iron oxyhydroxide, refers to a family of iron(III) compounds composed of FeO(OH) units. The principal natural polymorphs are goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganéite (β-FeOOH), and feroxyhyte (δ-FeOOH).

These minerals form by oxidation of ferrous iron in water and by weathering of iron-bearing minerals. Goethite is the most common FeOOH in soils and sediments, whereas lepidocrocite tends to form in oxidizing, chloride-rich environments, and akaganéite is associated with ferric oxides in seawater and some industrial settings. Feroxyhyte occurs in some hydrothermal and lateritic systems.

Structurally, FeOOH features layers of edge-sharing Fe(O,OH)6 octahedra linked through shared edges and held together by hydrogen bonds. The materials are typically insoluble in water under ambient conditions. They dehydrate on heating to form hematite (Fe2O3) and can transform slowly into other iron oxides and oxyhydroxides under changing pH and redox conditions.

Preparation in the laboratory typically involves hydrolysis or controlled oxidation of iron salts, or precipitation from ferric solutions at near-neutral pH. In soils and sediments, they commonly arise from ferrihydrite aging.

Applications and significance include use as pigments (ochre), indicators of iron cycling, and sorbents for contaminants in remediation. They are also used as precursors to other iron oxides in materials synthesis. Analytical methods such as X-ray diffraction and Mössbauer spectroscopy help distinguish the polymorphs.