Meteoric iron, sometimes meteoritic iron,[1] is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel, mainly in the form of the mineral phases kamacite and taenite. Meteoric iron makes up the bulk of iron meteorites but is also found in other meteorites. Apart from minor amounts of telluric iron, meteoric iron is the only naturally occurring native metal of the element iron (in metallic form rather than in an ore) on the Earth's surface.[2]
| Meteoric iron (native iron) | |
|---|---|
Widmanstätten pattern on a 500g endcut from the Toluca iron meteorite
| |
| General | |
| Category | Native element mineral |
| Formula (repeating unit) | Fe and Ni in different ratios |
| Space group | Different structures |
| Identification | |
| Luster | Metallic |
| Diaphaneity | Opaque |
The bulk of meteoric iron consists of taenite and kamacite. Taenite is a face-centered cubic and kamacite a body-centered cubic iron-nickel alloy.
Meteoric iron can be distinguished from telluric iron by its microstructure and perhaps by its chemical composition also, since meteoritic iron contains more nickel and less carbon.[2]
Trace amounts of gallium and germanium in meteoric iron can be used to distinguish different meteorite types. The meteoric iron in stony iron meteorites is identical to the "gallium-germanium group" of the iron meteorites.[3]
| Mineral | Formula | Nickel (Mass-% Ni) | Crystal structure | Notes & references |
|---|---|---|---|---|
| Antitaenite | γLow Spin-(Ni,Fe) | 20–40 | face centered cubic | Only approved as a variety of taenite by the IMA |
| Kamacite | α-(Fe,Ni); Fe0+0.9Ni0.1 | 5–10 | body centered cubic | Same structure as ferrite |
| Taenite | γ-(Ni,Fe) | 20–65 | face centered cubic | Same structure as austenite |
| Tetrataenite | (FeNi) | 48–57 | tetragonal | [4] |
Meteoric iron forms a few different structures that can be seen by etching or in thin sections of meteorites. The Widmanstätten pattern forms when meteoric iron cools and kamaciteisexsolved from taenite in the form of lamellas.[5] Plessite is a more fine-grained intergrowth of the two minerals in between the lamella of the Widmanstätten pattern.[6] Neumann lines are fine lines running through kamacite crystals that form through impact-related deformation.[7]
Before the advent of iron smelting, meteoric iron was the only source of iron metal apart from minor amounts of telluric iron. Meteoric iron was already used before the beginning of the Iron Age to make cultural objects, tools and weapons.[8]
| Iron in hieroglyphs | ||||||
|---|---|---|---|---|---|---|
bjꜣ-n-p.t literally "metal of the sky" | ||||||
Many examples of iron working from the Bronze Age have been confirmed to be meteoritic in origin.[9]
Even after the invention of smelting, meteoric iron was sometimes used where this technology was not available or metal was scarce. A piece of the Cranbourne meteorite was made into a horseshoe around 1854.[22]
Today meteoritic iron is used in niche jewellery and knife production, but most of it is used for research, educational or collecting purposes.
Meteoric iron also has an effect on the Earth's atmosphere. When meteorites descend through the atmosphere, outer parts are ablated. Meteoric ablation is the source of many elements in the upper atmosphere. When meteoric iron is ablated, it forms a free iron atom that can react with ozone (O3) to form FeO. This FeO may be the source of the orange spectrographic bands in the spectrum of the upper atmosphere.[23]
...the blade's composition of iron, nickel and cobalt was an approximate match for a meteorite that landed in northern Egypt. The result "strongly suggests an extraterrestrial origin"
