Goethite

From Wikipedia, the free encyclopedia

Goethite
Goethite,from Minas Gerais, Brazil
General
Category	Mineral
Chemical formula	Iron oxyhydroxide, FeO(OH)
Identification
Color	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
Streak	brown, brownish yellow to orange yellow
Specific gravity	3.3 - 4.3
Fusibility	Fusible at 5 - 5.5
Other Characteristics	Becomes magnetic in reducing flame

Goethite, named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low temperature environments. 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.

Recently, nanoparticulate authigenic goethite was shown to be the most common diagenetic iron oxyhydroxide in both marine and lake sediments.^[1]

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.

[edit] References

^ C. van der Zee, D. Roberts, D.G. Rancourt, C.P. Slomp. Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments. Geology 31 (2003) 993-996.