How meteorites are classified

Tens of thousands of meteorites are sorted into a system of classes and groups. This lesson explains the main branches and what the labels on a meteorite mean.

The big branches

The first division is between meteorites from bodies that never melted and those that did. Chondrites come from unmelted asteroids and are made of chondrules, metal and fine-grained matrix. They make up the large majority of falls.

Meteorites from melted, or differentiated, bodies include achondrites (igneous rocks, such as the basaltic eucrites from Vesta, and rocks from the Moon and Mars), iron meteorites from metal cores, and stony-irons: pallasites and mesosiderites. Between the two sit the primitive achondrites, which were partly melted.

Groups and codes

Within each branch are groups defined by chemistry and oxygen isotopes. Ordinary chondrites are H, L and LL, from high to low total iron. Carbonaceous chondrites form groups named after a type specimen, such as CI (Ivuna), CM (Mighei) and CV (Vigarano).

A chondrite's label adds a petrologic type, 1 to 2 for water-altered and 3 to 6 for increasingly heated. Many classifications also give a shock stage (S1 to S6) and a weathering grade (W0 to W6). 'L6, S4, W2' means an L chondrite, strongly heated, moderately shocked and moderately weathered.

From find to official name

A new meteorite is classified by a specialist laboratory, which examines a polished section under the microscope and measures mineral compositions with an electron microprobe; oxygen isotopes may be needed. The results are submitted to the international committee that approves names, and published in the Meteoritical Bulletin database. A type specimen is deposited in a recognised collection.

Names come from the nearest locality. In dense collection areas, such as parts of the Sahara and Antarctica, names add a number, for example 'Northwest Africa 7034' or 'Allan Hills 84001'.

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