IIG meteorites
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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 ...
s. The group currently has six members. They are hexahedrites 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 ...
. 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 ...
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: ...
.


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 ...
content of
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 ...
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, La Primitiva and Tombigbee meteorites were
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 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 and in 2000 by the Guanaco meteorite.


Description

IIG meteorites are hexahedrites. 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 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 ...
(4.1 to 4.9%) and is exclusively
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: ...
. 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
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 ...
and very low concentrations of sulfur.


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


See also

*
Glossary of meteoritics This is a glossary of terms used in meteoritics, the science of meteorites. # * 2 Pallas – an asteroid from the asteroid belt and one of the likely parent bodies of the CR meteorites. * 4 Vesta – second-largest asteroid in the asteroid b ...


References

{{meteorites