Octahedrites are the most common structural class of

iron meteorite Iron meteorites, also called siderites or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron me ...

s. The structures occur because the

meteoric iron Meteoric iron, sometimes meteoritic iron, 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 ...

has a certain

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

concentration that leads to the

exsolution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" ...

of kamacite out of taenite while cooling.

Structure

Octahedrites derive their name from the crystal structure paralleling an

octahedron In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of kamacite plates. Due to a long cooling time in the interior of the parent

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

s, these alloys have crystallized into intermixed millimeter-sized bands (from about 0.2 mm to 5 cm). When polished and acid etched the classic

Widmanstätten pattern Widmanstätten patterns (), also known as Thomson structures, are figures of long Phase (matter), phases of nickel–iron, found in the octahedrite shapes of iron meteorite crystals and some pallasites. Iron meteorites are very often formed ...

s of intersecting lines of lamellar kamacite, are visible. In gaps between the kamacite and taenite lamellae, a fine-grained mixture called plessite is often found. An iron nickel phosphide, schreibersite, is present in most nickel-iron meteorites, as well as an iron-nickel-

cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...

, cohenite.

Graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

and troilite occur in rounded nodules up to several cm in size.Vagn F. Buchwald: Handbook of Iron Meteorites. University of California Press, 1975.

Subgroups

Octahedrites can be grouped by the dimensions of kamacite lamellae in the

, which are related to the

content: * Coarsest octahedrites, lamellae width >3.3 mm, 5–9% Ni, symbol Ogg * Coarse octahedrites, lamellae 1.3–3.3 mm, 6.5–8.5% Ni, symbol Og * Medium octahedrites, lamellae 0.5–1.3 mm, 7–13% Ni, symbol Om * Fine octahedrites, lamellae 0.2–0.5 mm, 7.5–13% Ni, symbol Of * Finest octahedrites, lamellae <0.2 mm, 17–18% Ni, symbol Off * Plessitic octahedrites, kamacite spindles, a transitional structure between octahedrites and ataxites,Geochimica et Cosmochimica Acta, Volume 45, Ed. 9–12 9–18% Ni, symbol Opl

Mineral

Octahedrite is an obsolete synonym for

anatase Anatase is a metastable mineral form of titanium dioxide (TiO2) with a Tetragonal crystal system, tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other Pol ...

, one of the three known

titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

dioxide

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

References

External links

Webmineral
{{Meteorites

Structure

Subgroups

Mineral

See also

References

External links