What does the classification of a meteorite mean?
A meteorite's classification is a short code that tells scientists what kind of rock it is, which group it belongs to and, for many stones, how strongly it was heated, altered, shocked or weathered.
Every officially recognised meteorite gets a name and a classification. The name usually comes from the place where it fell or was found; the classification describes what it is. Think of it as a compact label on a specimen drawer that tells experts at a glance where the meteorite fits in the family tree. For chondrites, the label normally has two parts. Letters give the chemical group: H, L and LL for the ordinary chondrites, E for enstatite chondrites, and CI, CM, CV and others for the carbonaceous chondrites. A number then gives the petrologic type, which shows how much the rock was heated or altered by water on its parent asteroid. Chelyabinsk is an LL5, Murchison a CM2, and Allende a CV3.
Other meteorites use group names instead. Achondrites are classified by rock type, such as howardite, eucrite or diogenite for the Vesta-linked HED family, or shergottite and nakhlite for Martian meteorites. Iron meteorites are assigned to chemical groups such as IAB, IIIAB or IVA, often alongside a structural description such as fine octahedrite. Some meteorites do not fit any existing group and are labelled ungrouped.
Classifications can carry extra information. A shock stage, from S1 to S6, describes how strongly impacts affected the minerals, and a weathering grade, such as W0 to W6, records rusting and alteration on Earth. A label may also note that a meteorite is a breccia, or that it is a melt rock produced by an impact.
These details come from careful laboratory work on a sample, usually including thin sections examined under a microscope and analyses of mineral compositions. Once approved, the name and classification are published by the Meteoritical Society, so scientists worldwide use the same terms.