Cohenite

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Cohenite
General and classification
other names
chemical formula Fe 3 C
Mineral class
(and possibly department) 		Elements - Metallic carbon, nitrogen and phosphorus compounds
System no. to Strunz
and to Dana 		1.BA.05 ( 8th edition : I / A.09)
01.01.16.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m 2 / m 2 / m
Room group (no.) Pnma (No. 62)
Lattice parameters a  = 5.09  Å ; b  = 6.75 Å; c  = 4.52 Å
Formula units Z  = 4
Physical Properties
Mohs hardness 5.5 to 6
Density (g / cm 3 ) measured: 7.20 to 7.65 D (calc.) = 7.68
Cleavage according to {100}, {010} and {001}
Break ; Tenacity clamshell; very brittle
colour tin-white to light bronze-colored to golden yellow
Line color dark gray
transparency opaque
shine Metallic luster
magnetism strong magnetic

Cohenite is a rather seldom occurring meteoritic mineral from the mineral class of "element minerals". It crystallizes in the orthorhombic crystal system with the idealized composition Fe 3 C. Since the mineral usually contains small amounts of nickel and cobalt in nature , the formula is often also given as (Fe, Ni, Co) 3 C. The elements indicated in brackets can represent each other, but are always in the same proportion to the carbon .

Cohenite is also opaque in thin layers and develops only millimeter-sized, imperfectly tabular to needle-shaped crystals or eutectic dendritic adhesions with iron . Fresh samples are tin-white in color with a metallic sheen , but the crystal surfaces oxidize after a while and the color takes on a light bronze to golden yellow hue.

In metallurgy the compound Fe 3 C is known as cementite (formerly: cementite ) or iron carbide and is an important component in white raw and cast iron and steel .

Special properties

Cohenite is a very brittle and strongly magnetic mineral.

Under terrestrial conditions, the mineral is only stable at pressures above 40 k bar , which corresponds to a depth of around 120 km. On the surface it breaks down very slowly into the minerals kamacite and graphite .

Etymology and history

Cohenite was first discovered in the Magura meteorite in Arwa County, which was formerly Hungarian and now belongs to Slovakia . The mineral was described in 1889 by Ernst Weinschenk , who named it after the German mineralogist and petrographer Emil Cohen . His work was mainly devoted to the structure of iron meteorites and the minerals they contain.

classification

In the meantime outdated, but still in use 8th edition of the mineral classification by Strunz of cohenite belonged to the department of " metals , alloys and intermetallic compounds " where he along with haxonite , Hongquiit , Isovit , Khamrabaevit , Niobocarbid , tantalum and Tongbait the unnamed group I / A.09 formed.

In contrast , the 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), classifies cohenite in the newly defined division of “Metallic carbon, nitrogen and phosphorus compounds”. This is further subdivided according to the connection partner characterizing the connection, so that the mineral can be found according to its composition in the subdivision of " Carbides ", where it is the only member of the unnamed group 1.BA.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , also assigns Cohenite to the class of "element minerals" and there in the department of the same name. Here it can be found together with Haxonit and Isovit in the unnamed group 01.01.16 within the sub-section "Elements: Metallic elements other than the platinum group".

Education and Locations

Cohenite is a component of iron meteorites and enstatite chondrites . On earth it is only found in igneous rocks that have been reduced secondarily by penetrating coal seams, such as. B. at Bühl near Kassel in Germany and at Qeqertarsuaq in Greenland. As Begleitminerale among other dignified occur iron , schreibersite , Troilite and wustite on.

The mineral was found in various meteorites , including the Allan Hills 84001 in Antarctica, Campo del Cielo in Argentina, Cranbourne meteorites in Australia, Uruaçu in the Brazilian state of Goiás , Ulasitai meteorites in the autonomous Chinese district of Changji , and Toluca meteorite in Mexico, South Dahna meteorites in the Rub al-Chali in Saudi Arabia, Nagyvázsony meteorites in the Hungarian Bakony forest and in several meteorites found in the United States of America ( Canyon Diablo , Goose Lake Pulaski County , Las Vegas and Odessa ).

Cohenite was also found near Bukowno and in the Morasko meteorite in Poland, near Chatanga ( Khatanga ) and in the coal mine “No. 45 “at Kopeisk in Russia.

Cohenite could also be detected in rock samples from the moon .


Crystal structure

Cohenite crystallizes orthorhombically in the space group Pnma (space group no. 62) with the lattice parameters a  = 5.09  Å ; b  = 6.75 Å and c  = 4.52 Å as well as 4 formula units per unit cell .

See also

literature

  • Ernst Weinschenk : About some parts of the meteor iron from Magura, Arva, Hungary , in: Annalen des KK Naturhistorisches Hofmuseums , Volume 4 (1889), pp. 93-101 ( PDF 755.1 kB )

Web links

Individual evidence

  1. ↑ Webmineral - Cohenite
  2. ^ A b c Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  46 .
  3. ↑ a b c John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols: Cohenite , in: Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 65.8 kB )
  4. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 5th completely revised and supplemented edition. Weise, Munich 2008, ISBN 978-3-921656-70-9 .
  5. ^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 107-109 .
  6. ↑ Mindat - Cohenite