IIG meteorite

IIG meteorites
— Group —
Type	Iron
Structural classification	Hexahedrite
Class	Magmatic
Subgroups	None?
Parent body	IIG-IIAB
Composition	Meteoric iron (kamacite), nickel (4.1 to 4.9 %), much schreibersite (phosphorus), little sulfur
Total known specimens	6

IIG meteorites are a group of iron meteorites. The group currently has six members. They are hexahedrites with large amounts of schreibersite. The meteoric iron is composed of kamacite.^[1]

Naming and history

Iron meteorites are designated with a Roman numeral and one or two letters. Classification is based on diagrams in which nickel 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.^[2]

The Bellsbank, La Primitiva and Tombigbee meteorites were iron meteorites that were found to have chemical and structural similarities in 1967.^[3] Further descriptions were made in 1973 and in 1974 it was proposed that the three meteorites should be grouped into the "Bellsbank Trio" grouplet.^[4][5] The group status, that requires five specimen was filled in 1984 by the Twannberg meteorite and in 2000 by the Guanaco meteorite.^[6]

Description

IIG meteorites are hexahedrites. The meteoric iron has a low concentration of Nickel (4.1 to 4.9%) and is exclusively kamacite. IIGs contain large amounts of phosphorus in the form of schreibersite and very low concentrations of sulfur.^[1][6]

Parent body

Phase diagram showing the suspected cooling path of the parent body. While cooling the parent body reached the IIAB field. It then followed the field to the eutectic point where the remaining melt cavities formed the IIG meteorites.^[1]

Trace elements of IIAB and IIG meteorites are offset, which was interpreted as the two groups forming on a separate planetesimal. 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.^[1]

Specimen

The IIG group currently has 6 meteorites that are assigned to it. The Bellsbank, La Primitiva, Tombigbee, Twannberg, Guanaco and the Auburn meteorite.^[7]

See also

• Glossary of meteoritics

References

1. Wasson, John T.; Choe, Won-Hie (31 July 2009). "The IIG iron meteorites: Probable formation in the IIAB core". Geochimica et Cosmochimica Acta. 73 (16): 4879–4890. doi:10.1016/j.gca.2009.05.062.
2. McSween, Harry Y. (1999). Meteorites and their parent planets (Sec. ed.). Cambridge: Cambridge Univ. Press. ISBN 978-0521587518.
3. Wasson, John T.; Kimbeblin, Jerome (1 October 1967). "The chemical classification of iron meteorites—II. Irons and pallasites with germanium concentrations between 8 and 100 ppm". Geochimica et Cosmochimica Acta. 31 (10): 2065–2093. doi:10.1016/0016-7037(67)90143-3.
4. Scott, Edward R.D; Wasson, John T; Buchwald, V.F (31 July 1973). "The chemical classification of iron meteorites—VII. A reinvestigation of irons with Ge concentrations between 25 and 80 ppm". Geochimica et Cosmochimica Acta. 37 (8): 1957–1983. doi:10.1016/0016-7037(73)90151-8.
5. Wasson, John T. (1974). Meteorites : classification and properties. Berlin: Springer-Verlag. ISBN 978-0387067445.
6. Hofmann, Beda A.; Lorenzetti, Silvio; Eugster, Otto; Krähenbühl, Urs; Herzog, Gregory; Serefiddin, Feride; Gnos, Edwin; Eggimann, Manuel; Wasson, John T. (1 February 2009). "The Twannberg (Switzerland) IIG iron meteorites: Mineralogy, chemistry, and CRE ages". Meteoritics & Planetary Science. 44 (2): 187–199. doi:10.1111/j.1945-5100.2009.tb00727.x.
7. "Meteoritical Bulletin Database". Meteoritical Society. Retrieved 17 December 2012.