In meteoritics, a meteorite classification system attempts to group similar meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of characteristics, especially mineralogical, petrological, chemical, and isotopic properties.

Terminology

There is no single, standardized terminology used in meteorite classification; however, commonly used terms for categories include ''types'', ''classes'', ''clans'', ''groups'', and ''subgroups''. Some researchers hierarchize these terms, but there is no consensus as to which hierarchy is most appropriate. Meteorites that do not fit any known group (though they may fit somewhere within a higher level of classification) are ''ungrouped''.

Genetic relationships

Meteorite classification may indicate that a "genetic" relationship exists between similar meteorite specimens. Similarly classified meteorites may share a common origin, and therefore may come from the same

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

(such as a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...

, or

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...

) known as a ''parent body''. However, with current scientific knowledge, these types of relationships between meteorites are difficult to prove.

Traditional classification scheme

Meteorites are often divided into three overall categories based on whether they are dominantly composed of rocky material (

stony meteorite In meteoritics, a meteorite classification system attempts to group similar meteorites and allows scientists to communicate with a standardized terminology when discussing them. Meteorites are classified according to a variety of characteristics ...

s), metallic material (

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 i ...

s), 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

ataxite Ataxites (from Greek meaning "without structure") are a structural class of iron meteorites with a high nickel content and show no Widmanstätten patterns upon etching. Characteristics Ataxites are composed mainly of meteoric iron, a native metal ...

s), 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

pallasite The pallasites are a class of stony–iron meteorite. Structure and composition It consists of centimetre-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Mino ...

s (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: * Chondrites ** Carbonaceous chondrite class ***

CI chondrite CI chondrites, also called C1 chondrites or Ivuna-type carbonaceous chondrites, are a group of rare carbonaceous chondrite, a type of stony meteorite. They are named after the Ivuna meteorite, the type specimen. CI chondrites have been recover ...

s (Ivuna-like) group *** CM-CO chondrite (mini-chondrule) clan **** CM chondrite (Mighei-like) group **** CO chondrite (Ornans-like) group *** CV-CK chondrite clan **** CV chondrite (Vigarano-like) group ***** CV-oxA chondrite (oxidized,

Allende Salvador Guillermo Allende Gossens (, , ; 26 June 1908 – 11 September 1973) was a Chilean physician and socialist politician who served as the 28th president of Chile from 3 November 1970 until his death on 11 September 1973. He was the fi ...

-like) subgroup ***** CV-oxB chondrite (oxidized, Bali-like) subgroup ***** CV-red chondrite (reduced) subgroup **** CK chondrite (Karoonda-like) group *** CR chondrite clan **** CR chondrite (Renazzo-like) group **** CH chondrite (Allan Hills 85085-like) group **** CB chondrite (Bencubbin-like) group ***** CBa chondrite subgroup ***** CBb chondrite subgroup ** Ordinary chondrite class *** H chondrite group *** L chondrite group *** LL chondrite group ** Enstatite chondrite class *** EH chondrite group *** EL chondrite group **Other chondrite groups, not in one of the major classes *** R chondrite (Rumuruti-like) group *** K chondrite (Kakangari-like) grouplet (a grouplet is a provisional group with <5 members) * Achondrites ** Primitive achondrites *** Acapulcoite group *** Lodranite group *** Winonaite group ** Asteroidal achondrites *** HED meteorite clan (possibly from asteroid 4 Vesta, also called basaltic achondrites) **** Howardite group ****

Eucrite Eucrites are achondritic stony meteorites, many of which originate from the surface of the asteroid 4 Vesta and are part of the HED meteorite clan. They are the most common achondrite group with over 100 meteorites found. Eucrites consist of ...

group **** Diogenite group *** Angrite group *** Aubrite group (enstatite achondrites) *** Ureilite group *** Brachinite group **

Lunar meteorite A lunar meteorite is a meteorite that is known to have originated on the Moon. A meteorite hitting the Moon is normally classified as a transient lunar phenomenon. Discovery In January 1982, John Schutt, leading an expedition in Antarctica for ...

group ** Martian meteorite group (sometimes called "SNC meteorites") *** Shergottites *** Nakhlites *** Chassignites *** Other Martian meteorites, e.g., ALH84001 *

Pallasite The pallasites are a class of stony–iron meteorite. Structure and composition It consists of centimetre-sized olivine crystals of peridot quality in an iron-nickel matrix. Coarser metal areas develop Widmanstätten patterns upon etching. Mino ...

s ** Main group pallasites ** Eagle station pallasite grouplet ** Pyroxene pallasite grouplet * Mesosiderite group * Magmatic iron meteorite groups ** IC iron meteorite group ** IIAB iron meteorite group ** IIC iron meteorite group ** IID iron meteorite group ** IIF iron meteorite group ** IIG iron meteorite group ** IIIAB iron meteorite group ** IIIE iron meteorite group ** IIIF iron meteorite group ** IVA iron meteorite group ** IVB iron meteorite group * "Non-magmatic" or primitive iron meteorite groups ** IAB iron meteorite "complex" or clan (formerly groups IAB and IIICD) *** IAB main group *** Udei Station grouplet *** Pitts grouplet *** sLL (low Au, Low Ni) subgroup *** sLM (low Au, Medium Ni) subgroup *** sLH (low Au, high Ni) subgroup *** sHL (high Au, Low Ni) subgroup *** sHH (high Au, high Ni) subgroup ** IIE iron meteorite group

Rubin classification

A. E. Rubin (2000) classification scheme:

Alternative schemes

Meteorite Classification after Weissberg McCoy Krot 2006 Stony Iron

Two alternative general classification schemes were recently published, illustrating the lack of consensus on how to classify meteorites beyond the level of groups. In the Krot et al. scheme (2003) the following hierarchy is used: * Chondrites * Nonchondrites ** Primitive ** Differentiated *** Achondrites ***

Stony iron Stony-iron meteorites or siderolites are meteorites that consist of nearly equal parts of meteoric iron and silicates. This distinguishes them from the stony meteorites, that are mostly silicates, and the iron meteorites, that are mostly meteoric i ...

s *** Irons In the Weisberg et al. (2006) schemeWeisberg 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 meteorites groups are arranged as follows: * Chondrites * Primitive achondrites * Achondrites 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, based on Gustav Rose's and Nevil Story Maskelyne's classifications. Gustav Rose worked on the meteorite collection of the Museum für Naturkunde,

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and Maskelyne on the collection of the

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. Rose was the first to make different categories for meteorites with chondrules (chondrites) and without (nonchondrites). Story-Maskelyne differentiated between siderites, siderolites and aerolites (now called

stony-iron meteorite Stony-iron meteorites or siderolites are meteorites that consist of nearly equal parts of meteoric iron and silicates. This distinguishes them from the stony meteorites, that are mostly silicates, and the iron meteorites, that are mostly meteoric i ...

s and

, respectively). In 1872

Gustav Tschermak Gustav Tschermak von Seysenegg (19 April 1836 – 24 May 1927) was an Austrian mineralogist. Biography He was born in Litovel, Moravia, and studied at the University of Vienna, where he obtained a teaching degree. He studied mineralogy at Heidel ...

published his first meteorite classification based on Gustav Rose's catalog from 1864: In 1883 Tschermak modified Rose's classification again. Further modifications were made by

Aristides Brezina Aristides Brezina (4 May 1848 – 25 May 1909) was an Austrian mineralogist born in Vienna. In 1872 he graduated from the University of Tübingen, and afterwards taught crystallography at the University of Vienna. In 1878 he succeeded Austria ...

. The first chemical classification was published by Oliver C. Farrington, 1907.

George Thurland Prior George Thurland Prior FRS (16 December 1862 – 8 March 1936) was a British mineralogist. He made great contributions to mineralogical chemistry, petrology and meteoritics. He was born in Oxford, England, and attended Magdalen College there in ...

further improved the classification based on mineralogical and chemical data, introducing the terms mesosiderite, lodranite and enstatite chondrite. In 1923 he published a catalogue of the meteorites in the Natural History Museum (

). He describes his classification as based on Gustav Tschermak and Aristides Brezina with modifications by himself. His main subdivisions were: # Meteoric Irons or Siderites # Meteoric Stony-irons or Siderolites # 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

s, hexahedrites and octahedrites. A complete overview of his classification is given in the box below:

Brian Harold Mason Brian Harold Mason (18 April 1917 – 3 December 2009) was a New Zealand geochemist and mineralogist who was one of the pioneers in the study of meteorites. He played a leading part in understanding the nature of the Solar System through his stu ...

published a further revision in the 1960s.

References

External links

Meteorite frequencies by group
- National History Museum
Meteorite "family tree"
- National History Museum
Meteorite Classification List
- Meteorites Australia {{Meteorites it:Meteorite#Classificazione