FeOOHnH2O

FeOOHnH2O is a general chemical formulation describing iron(III) oxyhydroxide hydrates. The notation FeOOH·nH2O represents a family of compounds in which iron is coordinated to oxide and hydroxide groups with variable amounts of water in the structure. Crystalline members of this family include goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), while ferrihydrite refers to a poorly crystalline, highly hydrated form in the same broad category. The value of n is variable and depends on formation conditions, leading to a spectrum from almost anhydrous to highly hydrated phases.

Occurrence and formation often begin with the oxidation of Fe2+ in aqueous environments. This can occur during the weathering of iron minerals, in rusting steel and abandoned infrastructure, or as precipitation from groundwater and surface waters. In soils and sediments, FeOOHnH2O serves as a principal iron(III) phase, organizing the sorption of nutrients and contaminants.

Properties and behavior are governed by crystallinity, surface area, and hydration. The minerals are typically opaque to X-rays when poorly crystalline, have high specific surface areas, and exhibit strong adsorption characteristics for phosphate, arsenate, fluoride, and various heavy metals. Their stability is pH-dependent and they tend to transform to more crystalline forms under aging or changing redox conditions; ferrihydrite, for example, can recrystallize to goethite or hematite over time.

Applications and significance include roles in geochemical iron cycling, soil fertility, and water treatment, where FeOOHnH2O phases act as important sorbents and catalysts. Analytical identification often relies on diffraction, infrared spectroscopy, Mössbauer spectroscopy, and thermal analyses to assess crystallinity and hydration level.