Meteorite classification

The ultimate goal of meteorite classification is to group all meteorite specimens that share a common origin on a single, identifiable parent body. This could be a planet, asteroid, Moon, or other current Solar System object, or one that existed some time in the past (e.g. a shattered asteroid). However, with a few exceptions, this goal is beyond the reach of current science, mostly because there is inadequate information about the nature of most Solar System bodies (especially asteroids and comets) to achieve such a classification. Instead, modern meteorite classification relies on placing specimens into "groups" in which all members share certain key physical, chemical, isotopic, and mineralogical properties consistent with a common origin on a single parent body, even if that body is unidentified. Several meteorite groups classified this way may come from a single, heterogeneous parent body or a single group may contain members that came from a variety of very similar but distinct parent bodies. As such information comes to light, the classification system will most likely evolve.

Terminology

Beyond the assignment of meteorites into groups (see above), which is essentially universally accepted, there is no consensus among researchers as to what hierarchy of classification terms is most appropriate. For chondrites, groups may be divided into subgroups where there are features that distinguish certain meteorites from the others in the group, but it is thought that all still come from a single parent body. It is also fairly common for groups that seem to be closely related to each other to be referred to as clans. In turn, groups or clans that appear to be loosely related are often referred to as chondrite classes (e.g., carbonaceous chondrites, enstatite chondrites, and ordinary chondrites). But higher order terms for aggregating groups of meteorites tend to be somewhat chaotic in the scientific and popular literature. There is little agreement on how to fit nonchondritic meteorite groups into an overall scheme.

Several other classification terms are in widespread use:

Traditional classification scheme

Meteorites are often divided into three overall categories based on whether they are dominantly composed of rocky material (stony meteorites), metallic material (iron meteorites), or mixtures (stony–iron meteorites). These categories have been in use since at least the early 19th century but do not have much genetic significance; they are simply a traditional and convenient way of grouping specimens. In fact, the term "stony iron" is a misnomer as currently used. One group of chondrites (CB) has over 50% metal by volume and contains meteorites that were called stony irons until their affinities with chondrites were recognized. Some iron meteorites also contain many silicate inclusions but are rarely described as stony irons.

Nevertheless, these three categories sit at the top of the most widely used meteorite classification system. Stony meteorites are then traditionally divided into two other categories: chondrites (groups of meteorites that have undergone little change since their parent bodies originally formed and are characterized by the presence of chondrules), and achondrites (groups of meteorites that have a complex origin involving asteroidal or planetary differentiation). The iron meteorites were traditionally divided into objects with similar internal structures (octahedrites, hexahedrites, and ataxites), but these terms are now used for purely descriptive purposes and have given way to modern chemical groups. Stony–iron meteorites have always been divided into pallasites (which are now known to comprise several distinct groups) and mesosiderites (a textural term that is also synonymous with the name of a modern group).

Below is a representation of how the meteorite groups fit into the more traditional classification hierarchy:{[1] }

Rubin classification

A. E. Rubin (2000) classification scheme:

Alternative schemes

Meteorite classification after Weisberg, McCoy and Krot 2006.[1]

Two alternative general classification schemes were recently published by Krot et al. (2003)[4] and Weisberg et al. (2006),[5] illustrating the lack of consensus on how to classify meteorites beyond the level of groups. In the Krot et al. scheme, the following hierarchy is used:

And, in the Weisberg et al. scheme, meteorites groups are arranged as follows:

where irons and stony–irons are considered to be achondrites or primitive achondrites, depending on the group.

History

Modern meteorite classification was worked out in the 1860s.[1] It is based on Gustav Rose's and Nevil Story Maskelyne's classifications. Gustav Rose worked on the meteorite collection of the Museum für Naturkunde, Berlin and Nevil Story Maskelyne on the collection of the British Museum, London.[6][7] Rose was the first to make different categories for meteorites with chondrules (chondrites) and without chondrules (nonchondrites). Maskelyne differentiated between siderites (now called iron meteorites), siderolites (now called stony-iron meteorites) and aerolites (now called stony meteorite).[1]

In 1872 Gustav Tschermak published his first meteorite classification based on Gustav Rose's catalog from 1864:

In 1883 Gustav Tschermak modified Gustav Rose's classification again.[9]

Further modifications were made by Aristides Brezina.[1]

The first chemical classification was published by Oliver Cummings Farrington 1907.[10]

George Thurland Prior further improved the classification based on mineralogical and chemical data,[11][12] introducing the terms mesosiderite, lodranite and enstatite chondrite.[13] In 1923 he published a catalogue of the meteorites in the Natural History Museum (London). He describes his classification as based on Gustav Tschermak and Aristides Brezina with modifications by himself. His main subdivisions were:

  1. Meteoric Irons or Siderites
  2. Meteoric Stony-irons or Siderolites
  3. Meteoric Stones or Aerolites.

He subdivides the "Meteoric Stones" into those that have chondrules (Chondritic Meteoric Stones or Chondrites) and those that don't (Non-chondritic Meteoric Stones or Achondrites). The iron meteorites are subdivided according to their structures as ataxites, hexahedrites and octahedrites.[14] A complete overview of his classification is given in the box below:

Brian Harold Mason published a further revision in the 1960s.[15]

See also

References

  1. 1 2 3 4 5 Michael K. Weisberg; Timothy J. McCoy; Alexander N. Krot (2006). "Systematics and Evaluation of Meteorite Classification". In Lauretta, Dante S.; McSween, Jr., Harold Y. Meteorites and the early solar system II (PDF). Foreword by Richard P. Binzel. Tucson: University of Arizona Press. pp. 19–52. ISBN 978-0816525621. Retrieved 15 December 2012.
  2. ↑ Wasson, J. T.; Kallemeyn, G. W. (July 2002). "The IAB iron-meteorite complex: A group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts". Geochimica et Cosmochimica Acta 66 (13): 2445–2473. Bibcode:2002GeCoA..66.2445W. doi:10.1016/S0016-7037(02)00848-7.
  3. ↑ Norton, O. Richard (2002). The Cambridge encyclopedia of meteorites (1. publ. ed.). Cambridge [u.a.]: Cambridge Univ. Press. ISBN 0-521-62143-7.
  4. ↑ Krot, A.N.; Keil, K.; Scott, E.R.D.; Goodrich, C.A.; Weisberg, M.K. (2003). "Classification of meteorites". In Holland, Heinrich D.; Turekian, Karl K. Treatise on Geochemistry, 1. Elsevier. pp. 83–128. doi:10.1016/B0-08-043751-6/01062-8. ISBN 978-0-08-043751-4.
  5. ↑ Weisberg et al. (2006) Systematics and Evaluation of Meteorite Classification. In, Meteorites and the Early Solar System II, 19-52 (D.S. Lauretta and H.Y. McSween, Eds.), Univ. Arizona press
  6. ↑ Rose, Gustav (1864). Beschreibung und Eintheilung der Meteoriten auf Grund der Sammlung im mineralogischen Museum zu Berlin (in German). Berlin: Königlichen Akademie der Wissenschaften: in Commission bei F. Dümmler's Verlags-Buchhandlung Harrwitz und Gossmann. p. 161.
  7. ↑ Maskelyne, Nevil Story (c. 1863). Catalogue of the Collection of Meteorites exhibited in the Mineral Department of the British Museum. London: Woodfall & Kinder.
  8. ↑ Arestides, Brezina (1885). Die Meteoritensammlung des k. k. mineralogischen Hofkabinetes in Wien am 1. Mai 1885.
  9. ↑ Tschermak, Gustav (1883). Die Mikroskopische Beschaffenheit der Meteoriten erläutert durch photographische Abbildungen. Stuttgart: E. Koch.
  10. ↑ Farrington, Oliver Cummings (1907). "Analysis of iron meteorites, compiled and classified". Geologic Series 3: 59–110. Retrieved 16 December 2012.
  11. ↑ Prior, George Thurland (1916). "On the genetic relationship and classification of meteorites". Mineralogical Magazine 18: 26–44. doi:10.1180/minmag.1916.018.83.04.
  12. ↑ Prior, George Thurland (1920). "The classification of meteorites". Mineralogical Magazine 19: 51–63. doi:10.1180/minmag.1920.019.90.01.
  13. ↑ Mason, Brian (1966). "The enstatite chondrites" (PDF). Geochimica et Cosmochimica Acta 30: 23–39. Bibcode:1966GeCoA..30...23M. doi:10.1016/0016-7037(66)90089-5. Retrieved 16 December 2012.
  14. 1 2 Prior, George Thurland (1923). Catalogue of meteorites : with special reference to those represented in the collection of the British Museum (Natural History). Trustees of the British Museum. p. 196.
  15. ↑ Mason, Brian Harold (1967). "Meteorites". American Scientist 55: 529–455.

External links

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