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Formula: Fe
Native element, iron group
Varieties
Alpha iron (α-iron). Below 912oC iron has a body-centered cubic (bcc) crystal structure and is
known as α-iron or ferrite. It can be subjected to pressures up to about 15 GPa before transforming into a
high-pressure form (Wiki).
Kamacite is a nickel-rich variety of iron
Martensite is a solid-solution of carbon in iron, found in
meteorites
Properties
In the Earth's crust native iron is very rare. Iron generally occurs in compounds in the ferrous state as Fe2+ ions, or in
the ferric state as Fe3+ ions. The Earth's core, however, is about 80% iron.
Crystal System: Isometric
Specific gravity: 7.3 to 7.87 measured 7.874 calculated
Hardness: 4½
Streak: Grey
Colour: Iron-black
Melting point: 1535.1oC
Boiling point: 2750oC
Common impurities: Ni,C,Co,P,Cu,S
Environments
Igneous environments
Sedimentary environments
Extraterrestrial environments
Abundance in the Earth’s crust: 5.6 % weight, 2.1 % by moles (ChC)
Abundance in the solar system: 1000 parts per million by weight, 30 parts per million by moles (ChC)
Abundance in the Earth's core is about 80%.
In the Earth's crust native iron occurs very rarely in igneous rocks, especially
basalt, in
carbonaceous sediments, in volcanic fumaroles and in petrified wood, mixed with
limonite
and organic matter. Associated minerals include pyrite,
magnetite, troilite,
wüstite and cohenite
(HOM). Iron occurs in iron meteorites and it is by far the most common constituent of the Earth's core.
More than 95% of meteorites observed to fall to Earth are stony meteorites, that are composed mostly of silicate minerals, but they also
contain metallic iron in small scattered grains.
Stony-iron meteorites contain about equal proportions of metal and silicate material, and are rare, less than 2% of all known meteorites.
Iron meteorites are only a few percent of known meteorites. They are composed of iron and nickel
and are extremely dense. They are
fragments of the cores of asteroids. Early in Solar System history, asteroids melted and the dense
iron-nickel metal sank to the centre to
form a core, much like the Earth has a core
(https://ares.jsc.nasa.gov/meteorite-falls/what-are-meteorites).
At the type locality, Disko Island, Qeqertalik, Greenland, a 20 tonne lump mixture of iron and iron carbide has been found. There are
only a few places on Earth where native iron is found which is not of meteoric origin
(Wiki).
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