goethite

{{Infobox mineral | name = Goethite | category = oxide minerals hydroxide subgroup | image = Goethite-169735.jpg | caption = | formula = α-FeO(OH) | IMAsymbol = Gth | molweight = | strunz = 4.FD.10 | system = Orthorhombic | class = Dipyramidal (mmm) H–M symbol: (2/m 2/m 2/m) | symmetry = Pbnm | color = Yellowish to reddish to dark brown or black | habit = Radial acicular, mammillary, botryoidal, stalactitic, massive, as encrustation, as pseudomorph; may be banded or iridescent | twinning = | cleavage = Perfect {010} | fracture = Uneven to splintery | mohs = 5.0–5.5 | luster = Adamantine to dull | refractive = Opaque to sub-translucent | opticalprop = | birefringence = | pleochroism = | streak = Brown, brownish yellow to orange yellow | gravity = 3.3–4.3 | density = | melt = | fusibility = Fusible at 5–5.5 | diagnostic = | solubility = HCl soluble | diaphaneity = | other = weakly magnetic | references = }} thumb|upright|Unusual specimen of goethite replacing a gypsum [[stalactite; the center is hollow. From Santa Eulalia, Chihuahua, Mexico.]] Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α-polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient times for its use as a pigment (brown ochre). Evidence has been found of its use in paint pigment samples taken from the caves of Lascaux in France. It was first described in 1806 based on samples found in the Hollertszug Mine in Herdorf, Germany. The mineral was named after the German polymath and poet Johann Wolfgang von Goethe (1749–1832).

== Composition == Goethite is an iron oxyhydroxide containing ferric iron. It is the main component of rust and bog iron ore. Goethite's hardness ranges from 5.0 to 5.5 on the Mohs Scale, and its specific gravity varies from 3.3 to 4.3. The mineral forms prismatic needle-like crystals ("needle ironstone") but is more typically massive.