Goethite

Goethite,from Minas Gerais, Brazil

- align=center bgcolor="#9966FF"	Goethite		- align=center	- align=center bgcolor="#9966FF"	General		-	Category	Mineral	-	Chemical formula	Iron oxyhydroxide, FeO(OH)	- align="center" bgcolor="#9966FF"	Identification		-	Colour	Yellowish to reddish to dark brown	-	Crystal system	Orthorhombic 2/m2/m2/m	-	Cleavage	Perfect 010	-	Fracture	uneven to splintery	-	Mohs Scale hardness	5 - 5.5	-	Luster	adamantine to dull	-	Refractive index	Opaque to sub-translucent	-	Specific gravity	3.3 - 4.3	-	Streak	brown, brownish yellow to orange yellow	-	Other	Fusible at 5 - 5.5, becomes magnetic in reducing flame	-

Goethite is an iron bearing oxide mineral found in soil and other low temperature environments. Named for the German polymath Johann Wolfgang von Goethe. Goethite has been well known since prehistoric times for its use as a pigment. 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 for occurrences in the Mesabi iron ore district of Minnesota.

It is an iron oxide-hydroxide with a chemical formula FeOOH. 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 prismatic needle-like crystals, but is more typically massive. Its main modern 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, on cave floors, and on the bottom of lakes and small creeks. The boxworks or gossan resulting from the oxidation of sulfide ore deposites is formed of goethite along with other iron oxides and quartz. Iron rich lateritic soils developed over serpentinite rocks in tropical climates are mined for their iron content as well as other metals.

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 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.

Feroxyhyte and Lepidocrocite are both polymorphs of the iron oxyhydroxide FeO(OH). Although they have the same chemical formula as goethite they each have different crystalline structures making them distinct minerals.

References

Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0471805807
Webmineral data
Mindat data with locations
Mineral galleries

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References

See also