Goethite

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

crystal

s ("needle iron ore") but is more typically massive.

Feroxyhyte and lepidocrocite are both polymorphs of the iron oxyhydroxide FeO(OH) which are stable at the pressure and temperature conditions of the Earth's surface. Although they have the same chemical formula as goethite, their different crystalline structures make them distinct minerals.

Additionally, goethite has several high-pressure and high-temperature polymorphs, which may be relevant to the conditions of the Earth's interior. These include ε-FeOOH, which has an orthorhombic crystal structure, a cubic

pyrite

-type polymorph with or without losing hydrogen and an ultradense hexagonal structure.

Formation

Goethite often forms through the weathering of other iron-rich minerals, and thus is a common component of

soil

s, concentrated in laterite soils.

nanoparticulate

authigenic goethite is a common diagenetic iron oxyhydroxide in both marine and lake sediments. The formation of goethite is marked by the oxidation state change of Fe²⁺ (ferrous) to Fe³⁺ (ferric), 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.

Distribution

Goethite is found all over the planet, usually in the form of

concretion

s,

stalactitic

formations,

oolite

s (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.

Significant deposits of goethite are found in

England

,

Australia

,

Cuba

, and

Michigan

,

Minnesota

,

Missouri

,

Colorado

,

Alabama

, Georgia,

Virginia

, and

Tennessee

, and Florida caves in the

United States

.

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.

Limpets' teeth are composed of about 80% goethite fibres of only tens of nanometers in diameter, small enough to be flaw-insensitive, which accounts for their extreme tensile strength of 3.5–6.0 GPa and elastic modulus of 120±30 GPa.

Usage

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.

Perhaps the oldest known use of goethite comes from the ancient kingdom of Phrygia. In a royal tomb a body was found believed to be King Gordias, father of the legendary King

Midas

. The burial shroud had been colored with a dye containing goethite, which in its original unfaded state would have made the shroud look like it was woven from gold. Historians speculate that the legend of King Midas' golden touch might have originated from Phrygian royalty wearing clothes made from such golden-colored textiles.Archived a
Ghostarchive
and th
Wayback Machine

Gallery

File:Goethite-242608.jpg, Iridescent goethite, Filón Sur Mine, Tharsis, Huelva, Spain
File:Goethite3.jpg, Goethite from

Minas Gerais

, Brazil
File:Goethite-Calcite-Feldspar-Group-215498.jpg, A sharp, disc-shaped calcite has here been completely replaced by goethite, which preserved the original form perfectly
File:PyOx.JPG, Goethite coating/replacing rusted

pyrite

cubes
File:Goethite-162473.jpg, Finely crystallized specimen of goethite from Lake George, Park County,

Colorado

, USA
File:Harvard Museum of Natural History. Goethite. Negaunee, Marquette Co., MI (DerHexer) 2012-07-20.jpg, Goethite from Negaunee, Marquette County, Michigan

See also

*Ochre
*List of minerals

References

External links

*

{{Authority control

Iron(III) minerals
Iron oxide pigments
Hydroxide minerals
Johann Wolfgang von Goethe
Orthorhombic minerals
Minerals in space group 62
Iron ores
Magnetic minerals