| Goethite | |
|---|---|
 | |
| General | |
| Category | oxide minerals hydroxide subgroup |
| Formula | α-FeO(OH) |
| IMA symbol | Gth[1] |
| Strunz classification | 4.FD.10 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H–M symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Identification | |
| Color | Yellowish to reddish to dark brown or black |
| Crystal habit | Radial acicular, mammillary, botryoidal, stalactitic, massive, as encrustation, as pseudomorph; may be banded or iridescent |
| Cleavage | Perfect {010} |
| Fracture | Uneven to splintery |
| Mohs scale hardness | 5.0–5.5 |
| 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 |
| Solubility | HCl soluble |
| Other characteristics | weakly magnetic |
| References | [2][3][4][5] |
Goethite (/ˈɡɜːrtaɪt/,[6][7]US also/ˈɡoʊθaɪt/[8][9]) is a mineral of thediaspore group, consisting ofiron(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 apigment (brownochre). Evidence has been found of its use in paint pigment samples taken from thecaves of Lascaux inFrance. It was first described in 1806 based on samples found in the Hollertszug Mine inHerdorf,Germany.[4] The mineral was named after theGermanpolymath and poetJohann Wolfgang von Goethe (1749–1832).
Goethite is aniron oxyhydroxide containingferric iron. It is the main component ofrust andbog iron ore. Goethite's hardness ranges from 5.0 to 5.5 on theMohs Scale, and itsspecific gravity varies from 3.3 to 4.3. The mineral forms prismatic needle-likecrystals ("needle ironstone"[3]) but is more typically massive.[2]
Feroxyhyte andlepidocrocite are bothpolymorphs 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.[5]
Additionally, goethite has several high-pressure and high-temperaturepolymorphs, which may be relevant to the conditions of the Earth's interior. These include ε-FeOOH, which has an orthorhombic crystal structure,[10] a cubicpyrite-type polymorph with[11] or without losing hydrogen[12] and an ultradense hexagonal structure.[13]
Goethite has the same crystal structure asdiaspore, 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.[14]
![The goethite crystal structure viewed along [001]. Red ions are iron, white are oxygen, and yellow are hydroxide.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGoethite_c.png)
![The goethite crystal structure viewed along [010]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGoethite_b.png)
Goethite often forms through theweathering of other iron-rich minerals, and thus is a common component ofsoils, concentrated inlaterite soils.Nanoparticulateauthigenic goethite is a commondiagenetic iron oxyhydroxide in both marine and lake sediments.[15] 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 apseudomorph. 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 includepyrite,siderite, andmarcasite, though any iron(II)-bearing mineral could become a goethite pseudomorph if proper conditions are met. It may also be precipitated bygroundwater or in other sedimentary conditions, or form as a primary mineral inhydrothermal deposits. Goethite has also been found to be produced by the excretion processes of certain bacteria types.[16]
Goethite is found all over the planet, usually in the form ofconcretions,stalactitic formations,oolites (a form consisting of tiny round grains cemented together),[4] 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. Theboxworks orgossan resulting from theoxidation of sulfide ore deposits is formed of goethite along with other iron oxides and quartz.[17][2]
Significant deposits of goethite are found inEngland;Cuba; andMinnesota,Missouri,Colorado,Alabama,Georgia,Virginia, andTennessee, in theUnited States.[17][2]
Deposits significant in location, if not in abundance, have been found in theMartian craterGusev by NASA'sSpirit rover, providing strong evidence for the presence of liquid water on the planet in an earlier stage of its evolution.[18]
Limpets' teeth are composed of about 80% goethite fibres of only tens of nanometers in diameter, small enough to beflaw-insensitive, which accounts for their extremetensile strength of 3.5–6.0 GPa andelastic modulus of120±30 GPa.[19][20]
Its main modern use is as aniron ore, being referred to asbrown iron ore.[4] Goethite is an important component ofochre pigments,[21] and has been heat-treated for use as a red pigment sincePaleolithic times.[22] Iron-richlateritic soils that have developed overserpentinite rocks in tropical climates are mined for their iron content, as well as other metals.[23]
Fine goethite specimens are rare and therefore are valued collectibles.[17] Banded or iridescent varieties are cut and polished intocabochons for jewelry making.[24]
In a royal tomb of the ancient kingdom ofPhrygia, a body was found believed to be KingGordias, father of the legendary KingMidas. The burial shroud had been colored with a dye containing goethite, which in its original un-faded state would have made the shroud look as though it were 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.[25][26]
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