Oxyhydroxides

Oxyhydroxides are a family of compounds that contain both oxide (O2−) and hydroxide (OH−) groups bound to metal cations. In mineral terms they are often described by formulas such as MOOH or M(O,OH), reflecting mixed oxide–hydroxide structures. Typical examples include iron and aluminum compounds, but the concept applies to other metals as well. Common minerals and minerals-like phases include goethite (alpha-FeOOH), lepidocrocite (gamma-FeOOH), and akaganeite (beta-FeOOH with chloride), as well as aluminum oxyhydroxides such as boehmite (AlO(OH)) and diaspore (AlOOH). Ferrihydrite is a poorly crystalline iron oxyhydroxide/hydrate that frequently appears in natural environments.

Natural occurrence and formation are tied to weathering, oxidation, and hydrolysis of parent minerals. Oxyhydroxides form as secondary products during the weathering of iron- and aluminum-bearing minerals in soils and sediments, and they commonly occur as coatings or precipitates in aquatic systems. They are typically highly reactive, with large surface areas and a capacity to incorporate water, hydroxide, and various ions (such as carbonate or chloride). Under changing environmental conditions, these phases can transform into more stable oxides seperti hematite or other minerals through dehydration and restructuring.

Applications and significance include their roles as iron ore components, natural adsorbents for nutrients and contaminants, and catalysts in chemical processes. Their behavior in soils and sediments influences nutrient cycling, metal mobility, and the redox chemistry of environments. The classification of oxyhydroxides intersects mineralogy, geochemistry, and materials science, reflecting a spectrum from well-ordered minerals to poorly crystalline, hydrous phases.