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

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. It was first described in 1806 based on samples found in the Hollertszug Mine in Herdorf,

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

. The mineral was named after the German

polymath A polymath or polyhistor is an individual whose knowledge spans many different subjects, known to draw on complex bodies of knowledge to solve specific problems. Polymaths often prefer a specific context in which to explain their knowledge, ...

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 crystals ("needle ironstone") 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. Goethite has the same crystal structure as diaspore, the analogous aluminium oxide-hydroxide mineral. Oxygen and hydroxide ions form a hexagonal close-packed structure, with iron ions filling octahedral sites between the anions. The sites filled by iron ions form paired chains running the length of the crystal, with the two chains in each pair joined by hydroxide ions. File:Goethite c.png, The goethite crystal structure viewed along 01 Red ions are iron, white are oxygen, and yellow are hydroxide. File:Goethite b.png, The goethite crystal structure viewed along 10

Formation

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

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

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. Common goethite pseudomorphs include pyrite, 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 Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

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 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 Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

of sulfide ore deposits is formed of goethite along with other iron oxides and quartz. Significant deposits of goethite are found in

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;

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; and

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, and

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, in the

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. 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 Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...

of 3.5–6.0 GPa and

elastic modulus An elastic modulus (also known as modulus of elasticity (MOE)) is a quantity that describes an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Definition The elastic modu ...

of .

Usage

Its main modern use is as an iron ore, being referred to as ''brown iron ore''. Goethite is an important component of ochre pigments, and has been heat-treated for use as a red pigment since

Paleolithic The Paleolithic or Palaeolithic ( years ago) ( ), also called the Old Stone Age (), is a period in human prehistory that is distinguished by the original development of stone tools, and which represents almost the entire period of human prehist ...

times. 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. In a royal tomb of the ancient kingdom of

Phrygia In classical antiquity, Phrygia ( ; , ''Phrygía'') was a kingdom in the west-central part of Anatolia, in what is now Asian Turkey, centered on the Sangarios River. Stories of the heroic age of Greek mythology tell of several legendary Ph ...

, a body was found believed to be King

Gordias Gordias (; also ) was the name of at least two members of the royal house of Phrygia. The best-known Gordias was reputedly the founder of the Phrygian capital city Gordium, the maker of the legendary Gordian Knot, and the father of the legendary ...

, father of the legendary King

Midas Midas (; ) was a king of Phrygia with whom many myths became associated, as well as two later members of the Phrygian royal house. His father was Gordias, and his mother was Cybele. The most famous King Midas is popularly remembered in Greek m ...

. 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
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and th
Wayback Machine

Gallery

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

Minas Gerais Minas Gerais () is one of the 27 federative units of Brazil, being the fourth largest state by area and the second largest in number of inhabitants with a population of 20,539,989 according to the 2022 Brazilian census, 2022 census. Located in ...

, 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,

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

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 Minerals described in 1806