Goethite, Restormel Royal Iron Mine, Cornwall, England.
|Category||oxide minerals, hydroxide subgroup|
|Crystal system||Orthorhombic 2/m2/m2/m|
|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|
|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|
|Other characteristics||weakly magnetic|
Goethite (FeO(OH)), (pronunciation: // 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.
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.
Feroxyhyte and lepidocrocite are both polymorphs of the iron oxyhydroxide FeO(OH). Although they have the same chemical formula as goethite, their different crystalline structures make them distinct minerals.
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. Iron-rich lateritic soils that have developed over serpentinite rocks in tropical climates are mined for their iron content, as well as other metals.
Fine goethite specimens are rare and therefore are valued collectibles. Banded or iridescent varieties are cut and polished into cabochons for jewelry making.
Goethite often forms through the weathering of other iron-rich minerals, and thus is a common component of soils, concentrated in laterite soils. The formation of goethite is marked by the oxidation state change of Fe2+ to Fe3+, which allows for goethite to exist at surface conditions. Because of this oxidation state change, goethite is commonly seen as a pseudomorph. As iron-bearing minerals are brought to the zone of oxidation within the soil, the iron turns from iron(II) to iron(III), while the original shape of the parent mineral is retained. Examples of common goethite pseudomorphs are: goethites after pyrite, goethite, siderite, and marcasite, though any iron(II)-bearing mineral could become a goethite pseudomorph if proper conditions are met. It may also be precipitated by groundwater or in other sedimentary conditions, or form as a primary mineral in hydrothermal deposits. Goethite has also been found to be produced by the excretion processes of certain bacteria types.
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 also a very common pseudomorph. It is frequently encountered in the swampy areas at the head of spring waters ('bog iron'), on cave floors, and on the bottom of lakes and small creeks. The boxworks or gossan resulting from the oxidation of sulfide ore deposits is formed of goethite along with other iron oxides and quartz.
Deposits significant in location, if not in abundance, have been found in the Martian crater Gusev by NASA's Spirit rover, providing strong evidence for the presence of liquid water on the planet in an earlier stage of its evolution.
Goethite from Minas Gerais, Brazil.
Goethite coating/replacing rusted pyrite cubes.
Goethite--Leadville mining district, Colorado
- Hurlbut, Cornelius S.; Klein, Cornelis (1985). Manual of Mineralogy (20th ed.). Wiley. ISBN 0-471-80580-7.
- http://webmineral.com/data/Goethite.shtml Webmineral data
- http://www.mindat.org/min-1719.html Mindat data with locations
- http://www.galleries.com/minerals/oxides/goethite/goethite.htm Mineral galleries
- van der Zee, C.; Roberts, D.; Rancourt, D. G.; Slomp, C. P. (2003). "Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments". Geology 31 (11): 993–996. Bibcode:2003Geo....31..993V. doi:10.1130/G19924.1.
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