IRON

    Class : Elements
    Subclass : Metals
    Crystal system : Cubic
    Chemistry : Fe
    Rarity : Rare


Very rare in terrestrial rocks, iron is mainly present in metallic meteorites. Terrestrial iron is a rare primary mineral of basalt flows, and a secondary mineral formed by reduction of iron-bearing minerals in contact with organic matter. It frequently contains nickel : rarely more than 3% in "terrestrial" irons, up to 7% in meteoritic irons. It then gradually passes to another mineral : kamacite. Iron crystals are very rare and poorly formed, iron is mainly in the form of grains or flakes, sometimes in plates and masses weighing up to 25 tonnes, covered with an oxidized crust. Native terrestrial iron is very rare on the Earth's surface, however meteoritic iron represented the first source of iron available to Men who made tools and weapons. Currently native iron is only a mineralogical curiosity, too marginal to constitute an iron ore, a role which is fulfilled almost entirely by magnetite and hematite. The iron industry is inseparable from that of steel, the king alloy of our society and which appeared among the Hittites around 1500 BC.

Main photo : Metallic meteorite (Muonionalusta, Sweden) with Widmanstätten figures © R. Tanaka

Iron in the World

The best known deposit of terrestrial iron is Disko Island (Greenland), discovered in 1870 but known to Eskimos for much longer ; the latter made harpoons, and other hunting and fishing objects with them. Masses exceeding 20 tonnes have been exploited there. The German deposit of Bühl also provided masses of several kilos. Metallic meteorites ("siderites or "iron meteorite") are made up of native iron (rare), but above all of iron-nickel alloys (mainly kamacite and taenite). In polished plate, after acid attack, these meteorites often show geometric arrangements, "Widmanstätten figures" that result from the regular growth of taenite around kamacite lamellae. The famous meteorite from Meteor Crater in Arizona, also called Canyon Diablo, is the most famous example and also the largest known mass of siderite.

Right photo : Native iron from Disko Island, Qeqertalik, Greenland © Marc Gravel

Iron in France

In France, native iron is reported in an enclave of graphitic schists of a basanite at Mas Aubert (Gard) as well as in iron meteorites that fell on the territory.

Twinning

Twins are known on (111) and lamellar according to {112}.

Fakes and treatments

No fake listed for this mineral species.



Hardness : 4.5
Density : 7.3 to 7.87
Fracture : Irregular
Streak : Gray


TP : Opaque
RI : -
Birefringence : 0
Optical character : None
Pleochroism : None
Fluorescence : None


Solubility : Hydrochloric acid


Magnetism : Ferromagnetic
Radioactivity : None

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