Goethite is a hydrated iron oxide, HFeO₂ or Fe³⁺O(OH) (known as lepidocrocite ). Its hardness ranges from 5.0 to 5.5 on the Mohs Scale, and its specific gravity varies from 3.3 to 4.3. It forms needle-like crystals, and is not generally considered very interesting as a mineral specimen; its main use is as an iron ore, being referred to as brown iron ore. It does have some use as a clay earth pigment.

Goethite often forms through the weathering of other iron-rich minerals, and thus is a common component of soils. It may also be precipitated by groundwater or in other sedimentary conditions, or form as a primary mineral in hydrothermal deposits. Goethite is found all over the planet, usually in the form of concretions, stalactitic formations, oolites (a form consisting of tiny round grains cemented together), reniform (kidney shapes) or botryoidal (globular, like bunches of grapes) accumulations. It is frequently encountered in the swampy areas at the head of spring waters, and on the bottom of lakes and small creeks.

Goethite has been well known since prehistoric times for its use as a pigment. Evidence of goethite has been found in paint pigment samples taken from the caves of Lascaux in France.

It is related to the mineral Feroxyhyte, FeO(OH).

Goethite was named after the German writer Johann Wolfgang von Goethe who was also interested in mineralogy.

Significant deposits of Goethite are found in England, Cuba, and Michigan, Minnesota, Missouri, Colorado, Alabama, Georgia, Virginia, and Tennessee in the United States.

Deposits significant in location, if not in abundance, have been found in Gusev (Mars crater) by NASA's Spirit rover, providing strong evidence for the presence of liquid water on the planet Mars in an earlier stage of its development.

See also

• List of minerals