The classification framework

Meteorite classification is a hierarchy built on petrology, bulk chemistry and isotopes. This lesson sets out the modern framework that every later lesson refers back to.

From three families to a hierarchy

The traditional division into stones, irons and stony-irons is useful for display cases but says little about origin. A pallasite and a mesosiderite, for example, are both stony-irons yet probably formed in different ways and on different parent bodies.

Modern schemes, such as that set out by Michael Weisberg, Timothy McCoy and Alexander Krot in 2006, divide meteorites first by the degree of processing on their parent body. Chondrites are undifferentiated. Primitive achondrites experienced partial melting or extensive recrystallisation but retain some chondritic chemical or textural features. Achondrites, irons and most stony-irons derive from bodies that underwent substantial melting and differentiation.

Within the chondrites, the main classes are carbonaceous, ordinary and enstatite, alongside the R and K chondrites. Achondrites are divided into asteroidal groups, such as the HEDs, angrites, aubrites and ureilites, and planetary groups from the Moon and Mars.

Classes, clans, groups and grouplets

Below the class level, the key unit is the group: a set of at least five meteorites that are not paired and that share petrologic, chemical and isotopic properties consistent with a single parent body. Sets of two to four such meteorites are called grouplets, and meteorites that fit nowhere are ungrouped.

The clan is an intermediate level linking groups that share chemical, mineralogical and isotopic traits suggesting formation in a similar nebular region, but with differences implying separate parent bodies or locations. The CR clan includes CR, CH and CB chondrites; the CM and CO chondrites are often grouped as a clan, as are the CV and CK chondrites.

This framework is evolving. New groups are occasionally defined as additional members are found, and others are merged or questioned as data improve. Definitions are statements of current understanding, not fixed truths.

Official recognition

A meteorite becomes official when its name and classification are approved by the Nomenclature Committee of the Meteoritical Society and published in the Meteoritical Bulletin and its database.

The submission must include the location of the fall or find, recovered mass, number of pieces, classification data and the names of the classifiers. A type specimen must be deposited in an approved repository: under the committee's guidelines, at least 20 per cent of the total mass or 20 grams, whichever is less. The committee also rules on names in dense collection areas and on pairing.

The data required depend on the type. Equilibrated ordinary chondrites can often be classified from mineral compositions and petrography. Unusual or ungrouped meteorites, achondrites and planetary meteorites generally need additional evidence, such as oxygen isotopes or bulk chemistry. Throughout the remaining lessons, you will see how each of these tools contributes.

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