Meteorite classification key

1. Families, classes, clans and groups

Meteorites are sorted in layers, much as living things are sorted into kingdoms, families and species. The broadest split is by how much metal they contain and whether they have ever melted:

• Chondrites: stony meteorites that never melted, full of chondrules. They are about 86% of meteorites seen to fall.
• Primitive achondrites: chondrite-like rocks heated to the brink of melting.
• Achondrites: stony meteorites that crystallised from melts, from asteroids, the Moon and Mars.
• Stony-irons: roughly half metal, half stone.
• Irons: almost entirely iron-nickel metal, mostly from the cores of asteroids that melted.

Chondrites are split into classes (carbonaceous, ordinary and enstatite), and classes into groups. A group needs at least five unrelated meteorites that share their chemistry, minerals and isotopes, and so probably come from the same parent body. Two to four such meteorites make a grouplet; a meteorite that fits nowhere is ungrouped. Groups that look related are gathered into clans.

2. The full name of a chondrite

A classification such as "L6 S4 W1" packs in four pieces of information. The letters give the group. The number is the petrologic type: how much the rock was changed inside its parent asteroid.

After the type come optional codes for shock, from S1 (unshocked) to S6 (partly melted by impacts), and for weathering on Earth, from W0 (fresh) to W6 (almost all metal and sulphide rusted away). So an L6 S4 W1 is a strongly heated L chondrite, moderately shocked and only lightly weathered.

3. Oxygen isotopes: a fingerprint for each parent body

Oxygen has three stable isotopes, ¹⁶O, ¹⁷O and ¹⁸O. Ordinary processes such as melting and evaporation change the ratios of ¹⁷O and ¹⁸O to ¹⁶O in a fixed proportion, the ¹⁷O change being about 0.52 times the ¹⁸O change, because ¹⁷O is about half as different in mass. Every rock from one well-mixed body therefore lies on one line on a graph of the two ratios. Scientists measure how far a sample sits off Earth's line:

Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O

Earth and the Moon both give zero, Mars about +0.3 parts per thousand, the HED meteorites from Vesta about −0.24, ordinary chondrites +0.7 to +1.3 and CV and CO chondrites about −4. Because no ordinary process on a parent body can move a rock from one line to another, Δ¹⁷O shows which body a meteorite came from, and is often the deciding test for a new achondrite or pallasite.

4. Sorting irons by trace elements

The Widmanstätten pattern gives an iron's structural class, but irons from different asteroids can look alike. Their chemical groups come from measuring nickel together with tiny amounts of gallium, germanium and iridium, often only a few parts per million. Plotted against nickel, irons from the same core fall along one trend, because the core crystallised step by step and each element was shared out between solid and liquid metal in its own way. There are 13 recognised chemical groups of irons, and roughly one iron in seven fits none of them, so the metal of perhaps 50 or more different asteroids is represented in collections.

5. Who decides

Anyone can propose a classification, but a new meteorite only gets its official name and class once a laboratory has studied it and the nomenclature committee of the international society of meteorite scientists has approved the description. A type specimen must be deposited in a recognised collection. Classifications are revised as new data arrive: groups are split, merged or renamed, and some meteorites move from "ungrouped" into a new group once enough relatives are found.