Octahedrites are the most common structural class of

iron meteorite Iron meteorites, also known as 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 ...

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 ir ...

has a certain

nickel Nickel is a chemical element with 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 slow ...

concentration that leads to the

exsolution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

kamacite Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites. According to the International Mineralogical Association (IMA) it is considered a proper nickel-rich variety of the mineral native iron. The proportion iron: ...

out of

taenite Taenite is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with a chemical formula of and nickel proportions of 20% up to 65%. The name is derived from the Greek ταινία for "band, ribbon" ...

while cooling.

Structure

Octahedrites derive their name from the crystal structure paralleling an

octahedron In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

. Opposite faces are parallel so, although an octahedron has 8 faces, there are only 4 sets of

plates. Due to a long cooling time in the interior of the parent asteroids, 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 nickel–iron crystals, found in the octahedrite iron meteorites and some pallasites. They consist of a fine interleaving of kamacite and taenite bands or ribbons ...

s of intersecting lines of lamellar kamacite, are visible. In gaps between the kamacite and taenite lamellae, a fine-grained mixture called

plessite Plessite is a meteorite texture consisting of a fine-grained mixture of the minerals kamacite and taenite found in the octahedrite iron meteorites. It occurs in gaps (its name is derived from the Greek "plythos" meaning "filling"according tMinda ...

is often found. An iron nickel phosphide,

schreibersite Schreibersite is generally a rare iron nickel phosphide mineral, , though common in iron-nickel meteorites. It has been found on Disko Island in Greenland and Illinois. Another name used for the mineral is rhabdite. It forms tetragonal crystals ...

, is present in most nickel-iron meteorites, as well as an iron-nickel-

cobalt Cobalt is a chemical element with the 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. The free element, p ...

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 Cohenite is a naturally occurring iron carbide mineral with the chemical structure ( Fe, Ni, Co)3 C. This forms a hard, shiny, silver mineral which was named by E. Weinschenk in 1889 after the German mineralogist Emil Cohen, who first described ...

Graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

and

troilite Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency whic ...

occur in rounded nodules up to several cm in size.

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 structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other polymorphs (or mineral form ...

, one of the three known

titanium Titanium is a chemical element with the Symbol (chemistry), 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, resista ...

dioxide

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

References

External links

Webmineral
{{Meteorites

Structure

Subgroups

Mineral

See also

References

External links