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 (such as a planet, asteroid, or moon) 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 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:
Chondrites
Carbonaceous chondrite class
CI chondrites (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-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 group Diogenite group Angrite group Aubrite group (enstatite achondrites) Ureilite group Brachinite group Lunar meteorite group Martian meteorite group (sometimes called "SNC meteorites") Shergottites Nakhlites Chassignites Other Martian meteorites, e.g., ALH84001