Goethite

• Limonite
• Brown iron

Goethite, also known as limonite or brown iron, is a common mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the orthorhombic crystal system with the chemical composition of α - Fe 3 O ( OH), and developed mostly needle - up radial- or prismatic crystals, but also racemose to low one aggregates of black -brown to weathering pale yellow color with yellow-brown stroke color. Fresh, crystalline or racemose Goethitproben show a metallic luster, weathered noble or fine aggregates, however, a velvety gloss ( Samtblende! ).

As a major constituent of limonite this term is often used synonymously for goethite.

Special Features

Goethite has an iron content of up to 62%, but will drop when shooting crystal water. It has a Mohs hardness from 5 to 5.5, a density of 4.3 g / cc and a yellow-brown stroke color. Goethite dissolves in weak hydrochloric acid, but good in nitric acid.

The mineral is antiferromagnetic in the normal state. When heated before the blowpipe it gives off water and becomes magnetic. It turns red and transforms to α -Fe2O3.

Etymology and history

The mineral was first recognized in 1806 as an independent type and named after the German poet Johann Wolfgang von Goethe, who was also a collector of minerals.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the goethite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " hydroxides and oxide hydrates ," where he, along with akaganeite, boehmite, diaspore, Feitknechtit Feroxyhyt, Groutit, lepidocrocite, manganite, schwertmannite and Tsumgallit formed a distinct group.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the goethite in the class of " oxides and hydroxides " and then in the Department of the " Hydroxide (without V or U) " a. This division, however, is further subdivided according to the possible presence of water of crystallization as well as the crystal structure, so that the mineral according to its composition in the subsection " hydroxides with OH, without H2O; can be found with chains of edge -sharing octahedra ", where it forms the unnamed group 4.FD.10 together with Bracewellit, diaspore, Groutit, Guyanait, montroseite and Tsumgallit.

The classification of minerals according to Dana assigns the goethite in the class of " oxides and hydroxides " and there in the department of " hydroxides and hydroxy- oxides " one. Here he is with the eponymous diaspore and the other members Groutit, montroseite, Bracewellit and Tsumgallit in the " Diasporgruppe ( Orthorhombic, Pnma or Pnmd ) " with the system no. 06:01:01 within the sub-division of: to find " hydroxides and hydroxy- oxides having the formula X3 O OH ".

Modifications and varieties

The connection Fe3 O ( OH) trimorph, so come next to the orthorhombic crystallized goethite nor as trigonal crystallizing Feroxyhyt and also as orthorhombic, albeit with a different space group and other cell parameters, crystallizing lepidocrocite ago.

The only known variety is Samtblende. It forms auburn to ocher yellow balls with a velvety surface.

Education and Locations

Goethite usually formed secondarily by weathering of iron minerals such as magnetite or pyrite and is therefore often found in the form of corresponding pseudomorphs after these iron sulfides, but also other minerals.

Goethite but can also form primarily in hydrothermal veins, it is usually found in cavities of volcanic rocks such as pegmatites. The bottom and brown iron ore ( limonite ) it also occurs in sedimentary ore deposits. Normal rust is also mainly of goethite.

Worldwide, goethite far (as of 2011) are detected at around 5000 localities. Locations in Germany are among other victories and Horhausen, in England Bottalack, Redruth and the county of Cornwall, in Mexico Santa Eulalia, Pribram in the Czech Republic and in the U.S. state of Colorado Florissant.

Goethite was detected by the spacecraft "Spirit" on the surface of Mars in December 2004. NASA scientists regard this as one of the surest evidences of former liquid water on the red planet since goethite is formed only in connection with water.

Crystal structure

Goethite crystallizes in the orthorhombic space group Pbnm ( Raumgruppen-Nr. 62) with the lattice parameters a = 4.62 Å; b = 9.95 Å; and c = 3.01 Å and four formula units per unit cell.

Use

Goethite has as a raw material are no longer outstanding importance, historically it has played a role as an iron ore. In the form of limonite, it is still used as a pigment ( brown ocher ).