Goethite
| Goethite | |
|---|---|
 (size: 5.8 × 4.8 × 3.3 cm) |
|
| General and classification | |
| other names |
|
| chemical formula | α-Fe 3+ O (OH) |
|
Mineral class (and possibly department) |
Oxides, hydroxides - hydroxides and oxidic hydrates |
|
System no. to Strunz and to Dana |
4.FD.10 ( 8th edition : IV / F.06) 01/06/01/02 |
| Crystallographic Data | |
| Crystal system | orthorhombic |
| Crystal class ; symbol | orthorhombic-dipyramidal; 2 / m 2 / m 2 / m |
| Space group | Pbnm (No. 62, position 3) |
| Lattice parameters | a = 4.62 Å ; b = 9.95 Å; c = 3.01 Å |
| Formula units | Z = 4 |
| Physical Properties | |
| Mohs hardness | 5 to 5.5 |
| Density (g / cm 3 ) | measured: 4.28 (1); calculated: 4.18 |
| Cleavage | perfect after {010}, imperfect after {100} |
| Break ; Tenacity | uneven; brittle |
| colour | light yellow to dark brown |
| Line color | yellow-brown |
| transparency | opaque, translucent edges |
| shine | Diamond luster, metallic luster, silk luster |
| magnetism | antiferromagnetic |
| Crystal optics | |
| Refractive indices |
n α = 2.260 to 2.275 n β = 2.393 to 2.409 n γ = 2.393 to 2.409 |
| Birefringence | δ = 0.133 to 0.134 |
| Optical character | biaxial negative |
| Axis angle | 2V = 20 ° (calculated) |
| Pleochroism | Strong: X = yellow to colorless Y = yellowish brown to reddish orange Z = yellow orange to dark orange red |
Goethite , also known as needle iron ore or brown glass head , is a widespread mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the orthorhombic crystal system with the chemical composition α-Fe 3+ O (OH) and develops mostly needle-like to radial-rayed or prismatic crystals , but also grape-like to kidney-like aggregates from black-brown to light yellow due to weathering with yellow-brown streak color . Fresh, crystalline or grape-like goethite samples show a metallic sheen , weathered or fine-needle aggregates, on the other hand, have a velvety sheen ( velvet cover ).
As the main component of limonite , this name is often used synonymously for goethite.
Etymology and history
Johann Georg Lenz first used the term goethite in 1806 for the mineral named after the German poet (and mining official) Johann Wolfgang von Goethe . The name was given through the mediation of Ludwig Wilhelm Cramer at the suggestion of the pastor Heinrich Adolf Achenbach (1765-1819) and the miner Johann Daniel Engels (1761-1828), both from Siegen, who suggested the name Goethenite for the mineral. Friedrich Wilhelm Riemer caused Johann Georg Lenz to change the name to Goethite.
classification
In the meanwhile outdated, but still in use 8th edition of the mineral systematics according to Strunz , goethite belonged to the mineral class of "oxides and hydroxides" and there to the department of "hydroxides and oxidic hydrates", where together with akaganeit , boehmite , diaspore , feitknechtite , Feroxyhyt , Groutit , Lepidokrokit , Manganit , Schwertmannit and Tsumgallit formed a separate group.
The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns goethite to the class of "oxides and hydroxides" and then to the "hydroxides (without V or U)" category . However, this section is further subdivided according to the possible presence of crystal water and according to the crystal structure, so that the mineral is classified according to its composition in the sub-section “Hydroxides with OH, without H 2 O; with chains of edge-linked octahedra ”, where it forms the unnamed group 4.FD.10 together with Bracewellite , Diaspor, Groutite, Guyanaite , Montroseit and Tsumgallite .
The systematics of minerals according to Dana also assigns goethite to the class of "oxides and hydroxides" and there in the department of "hydroxides and oxides containing hydroxides". Here he is together with the eponymous Diaspor and the other members Groutit, Montroseit, Bracewellit and Tsumgallit in the "Diaspor group (Orthorhombic, Pnma or Pnmd)" with the system no. 06.01.01 to be found within the sub-section of " Hydroxides and hydroxide-containing oxides with the formula: X 3+ O OH ".
Crystal structure
Goethite crystallizes orthorhombically in the space group Pbnm (space group no. 62, position 3) with the lattice parameters a = 4.62 Å ; b = 9.95 Å; and c = 3.01 Å and 4 formula units per unit cell .
properties
Goethite has an iron content of up to 62%, which, however, decreases when water of crystallization is absorbed. It has a Mohs hardness of 5 to 5.5, a density of 4.3 g / cm³ and a yellow-brown line color . Goethite dissolves weakly in hydrochloric acid but well in nitric acid .
The mineral is antiferromagnetic in its normal state . When heated in front of the soldering tube , it gives off water and becomes magnetic. It turns red and converts to α-Fe 2 O 3 .
Modifications and varieties
The compound Fe 3+ O (OH) is trimorphic , so besides the orthorhombically crystallizing goethite it also occurs as a trigonal crystallizing feroxyhyte and also as orthorhombic, albeit with a different space group and different cell parameters, crystallizing lepidocrocite .
A chestnut-brown to ocher-yellow variety of goethite , which forms spherical aggregates with a velvety surface, is called a velvet cover .
The term Eisenoolith (also Eisenrogenstein ) means generally an oolithic iron ore and in particular a bulbous-peeled goethite or limonite - floor ore .
Education and Locations
Goethite is usually secondary to the 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 can also primarily form in hydrothermal veins , in which case it is usually found in cavities in volcanic rocks such as pegmatite . As swamp and brown iron ore (limonite), it also occurs in sedimentary ore deposits. Normal rust also consists mainly of goethite.
So far (as of 2011) goethite has been found at around 5000 sites worldwide. Sites in Germany include Siegen and Horhausen , in England Bottalack , Redruth and the county of Cornwall , in Mexico Santa Eulalia , in the Czech Republic Příbram and in the US state Colorado Florissant .
Goethite was also detected on the surface of Mars in December 2004 by the “ Spirit ” space probe . NASA scientists rate this as one of the most reliable proofs of formerly liquid water on the red planet, since goethite only forms in connection with water.
use
Goethite is no longer of outstanding importance as a raw material; historically it played a role as iron ore . In the form of limonite , it is still used today as a color pigment ( brown ocher ).
See also
literature
- Petr Korbel, Milan Novák: Mineral Encyclopedia (= Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 110 .
Web links
- Mineralienatlas: Goethite , Mineralienatlas: Mineralienportrait / Goethit (Wiki)
- Mineral Lexicon - Goethite
- Webmineral - Goethite (English)
- RRUFF Database-of-Raman-spectroscopy - Goethite (English)
- American-Mineralogist-Crystal-Structure-Database - Goethite (English)
Individual evidence
- ^ A b c Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 235 .
- ↑ a b c d e Goethite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 69 kB ; accessed on April 12, 2018]).
- ↑ a b c d e Mindat - Goethite (English)
- ↑ Uni Bonn - Geophysical Basics, Le Borgne Effect
- ^ Kremer pigments: Goethite
- ↑ Otto Lueger (Ed.): Lexicon of the entire technology and its auxiliary sciences . 2nd Edition. tape 3 . Deutsche Verlags-Anstalt, Stuttgart, Leipzig 1906, p. 368 ( available online at zeno.org [accessed January 24, 2019]).
- ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .