alphaFeOOH

Alpha-FeOOH, commonly known as goethite, is an iron oxyhydroxide mineral and one of the polymorphs of FeOOH. It has the chemical formula FeO(OH) and crystallizes in the orthorhombic system.

In nature it forms as a weathering product of iron-bearing minerals and as a coating in soils and sediments. Goethite occurs as acicular or prismatic crystals and aggregates, typically yellow-brown to brown in color, and it often displays a relatively high surface area compared with other iron oxides.

Goethite forms by oxidation of ferrous ions under near-neutral to alkaline conditions and is widespread in soils, sediments, and oxidized ore deposits. It can be synthesized by precipitation from ferric salts under basic conditions or by hydrolysis of ferric solutions. Upon heating, goethite dehydrates to hematite (Fe2O3), a transformation that is commonly observed during drying or thermal treatment.

Applications of goethite arise from its color and surface properties. It has historically been used as a yellow ochre pigment. In environmental and industrial contexts, its high surface area and reactivity make it useful as an adsorbent for contaminants such as arsenic and phosphate, and as a catalyst support in various chemical processes.

Goethite is the most common naturally occurring FeOOH mineral. Other FeOOH polymorphs include beta-FeOOH (akaganéite) and gamma-FeOOH (lepidocrocite), which are less prevalent in natural systems.