IIG meteorites are a group 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 met ...

s. The group currently has six members. They are

hexahedrite Hexahedrites are a structural class of iron meteorite. They are composed almost exclusively of the nickel–iron alloy kamacite and are lower in nickel content than the octahedrites. The nickel concentration in hexahedrites is always below 5.8% and ...

s with large amounts of

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 w ...

. The meteoric iron is composed of

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:ni ...

Naming and history

Iron meteorites are designated with a Roman numeral and one or two letters. Classification is based on diagrams in which

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 to ...

content of meteoric iron is plotted against trace elements. Clusters in these diagrams are assigned a row (Roman numeral) and a letter in alphabetical order. IIG meteorites are therefore from the second row, cluster G. The

Bellsbank Bellsbank is a village half a mile away from the market town of Dalmellington in East Ayrshire, Scotland. It is the second-highest place in East Ayrshire East Ayrshire ( sco, Aest Ayrshire; gd, Siorrachd Àir an Ear) is one of thirty-two c ...

, La Primitiva and Tombigbee meteorites were

s that were found to have chemical and structural similarities in 1967. Further descriptions were made in 1973 and in 1974 it was proposed that the three meteorites should be grouped into the "Bellsbank Trio" grouplet. The group status, that requires five specimen was filled in 1984 by the

Twannberg meteorite The Twannberg meteorite is a hexahedrite iron meteorite. It is the only meteorite of the IIG meteorites, IIG group found in Europe and the largest meteorite ever found in Switzerland. Discovery and naming The first fragment () was found on 9 May ...

and in 2000 by the

Guanaco meteorite The guanaco (; ''Lama guanicoe'') is a camelid native to South America, closely related to the llama. Guanacos are one of two wild South American camelids, the other being the vicuña, which lives at higher elevations. Etymology The guanaco g ...

Description

IIG meteorites are

s. The meteoric iron has a low concentration of

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 to ...

(4.1 to 4.9%) and is exclusively

. IIGs contain large amounts of

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

in the form of

and very low concentrations of

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

Parent body

Trace elements of IIAB and IIG meteorites are offset, which was interpreted as the two groups forming on a separate

planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System a ...

. Other explanations for the offset are melt inmiscibility. This process took place while the planetesimal was cooling off. First meteoric iron crystallized into a network of cavities and channels. Eventually crystallization cut off the channels and made cavities of trapped melt. When the remaining melt reached the eutectic point, the cavities crystallized a mixture of schreibersite and meteoric iron. The matrix of this process would form the IIAB meteorites, while the cavities would form the IIG meteorites.

Specimen

The IIG group currently has 6 meteorites that are assigned to it. The

, La Primitiva, Tombigbee, Twannberg, Guanaco and the Auburn meteorite.

References

{{meteorites

Naming and history

Description

Parent body

Specimen

See also

References