Goethite

Goethite
Goethite, Restormel Royal Iron Mine, Cornwall, England.
General
Category	oxide minerals, hydroxide subgroup
Formula (repeating unit)	α-FeO(OH)
Strunz classification	4.FD.10
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Identification
Color	Yellowish to reddish to dark brown or black
Crystal habit	radial acicular, mammilary, botroydoal, stalactitic, massive, as encrustation, as pseudomorph; may be banded or iridescent
Cleavage	Perfect {010}
Fracture	Uneven to splintery
Mohs scale hardness	5 - 5.5
Luster	Adamantine to dull
Streak	Brown, brownish yellow to orange yellow
Specific gravity	3.3 - 4.3
Refractive index	Opaque to sub-translucent
Fusibility	Fusible at 5 - 5.5
Solubility	HCl soluble
Other characteristics	weakly magnetic
References

Goethite (FeO(OH)), (pronunciation: /ˈɡɜːrtaɪt/ GUR-tite) named after the German polymath and poet Johann Wolfgang von Goethe (1749–1832), an iron bearing hydroxide mineral of the diaspore group, is found in soil and other low-temperature environments. 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.

In 2003, nanoparticulate authigenic goethite was shown to be the most common diagenetic iron oxyhydroxide in both marine and lake sediments.

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 iron ore"), but is more typically massive.