How meteorites are classified

Every newly recognised meteorite is given a classification, a short code that says what kind of rock it is, where it probably came from and what has happened to it since. The system can look baffling, but it follows a sensible order of questions, from what you can see with the naked eye down to isotopes measured in a laboratory. Here is that order, step by step.

1. Step 1 of 7

   Stone, iron or stony-iron?

   The first question is the simplest: how much metal is there? Stony meteorites are mostly silicate minerals, with no more than scattered grains of metal. Iron meteorites are almost entirely iron–nickel metal. Stony-irons are roughly half and half. A magnet, the weight in the hand and a cut, polished face usually answer this at once.

   Stones make up about 94 per cent of meteorites seen to fall, irons about 5 per cent and stony-irons about 1 per cent. Among chance finds irons are far more common, because they survive weathering better and look so obviously unusual.

2. Step 2 of 7

   Chondrite or achondrite?

   Stony meteorites split next into two very different families. Chondrites are full of chondrules, tiny once-molten beads, and still contain metal and sulphide grains. They come from asteroids that never melted as a whole, so they preserve the raw material of the Solar System. They are by far the most common meteorites.

   Achondrites have no chondrules. They are igneous rocks, such as basalts and cumulates, made when a parent body melted and separated. Some come from asteroids such as Vesta; others come from the Moon and Mars. A group in between, the primitive achondrites, were only partly melted.

3. Step 3 of 7

   Which chondrite class and group?

   Chondrites are sorted into classes: ordinary, carbonaceous, enstatite, and the smaller R and K classes. Within each are groups, believed to come from separate parent bodies. Ordinary chondrites, the commonest, form the H, L and LL groups, standing for high iron, low iron and low iron, low metal.

   The groups are told apart by measurements. For ordinary chondrites, the key one is how much iron sits in the olivine crystals, given as the fayalite content: roughly 16 to 20 per cent in H, 23 to 26 in L and 27 to 32 in LL, as metal gives way to oxidised iron. Carbonaceous groups are separated by their chondrule sizes, matrix, water-bearing minerals and chemistry, and every class has its own range of oxygen isotopes, the most powerful fingerprint of all.

4. Step 4 of 7

   Petrologic type: how cooked or soaked

   Next comes the petrologic type, a number that says what happened to the chondrite inside its parent body. Type 3 is the least changed. Types 1 and 2 were soaked in liquid water, which turned much of the rock into clay minerals: type 1 completely, type 2 partly. Types 4, 5 and 6, and the rare 7, were heated: as the number rises, minerals even out in composition and chondrules blur into the surrounding rock.

   The type is written after the group, giving familiar codes such as CM2 for Murchison, CV3 for Allende and L6 or H5 for many ordinary chondrites. Unequilibrated type 3 chondrites are subdivided further, from 3.00 to 3.9, because they are so useful for studying the early Solar System.

5. Step 5 of 7

   Shock and weathering

   Two further grades are often added for ordinary chondrites. The shock stage, from S1 to S6, records damage from collisions in space, read from the microscopic effects in olivine and feldspar, from unshocked grains up to rock that partly melted and turned some feldspar into glass.

   The weathering grade, from W0 to W6, applies to finds and records what has happened since landing on Earth: W0 is fresh, with bright metal; by W4 the metal has completely rusted; W5 and W6 show the silicate minerals altered too. A full classification might read L6 S4 W2.

6. Step 6 of 7

   Achondrites and irons

   Achondrites are grouped mainly by the body they came from, identified by their minerals, chemistry and oxygen isotopes. The HED meteorites, howardites, eucrites and diogenites, come from the asteroid Vesta. Others are lunar or Martian, and there are smaller groups such as angrites, aubrites and ureilites, each from its own parent asteroid.

   Iron meteorites get two descriptions. Structural names come from the etched pattern: hexahedrites, octahedrites of various band widths, and ataxites. Chemical groups, such as IAB, IIAB, IIIAB and IVA, are set by measuring trace elements, especially nickel, gallium, germanium and iridium. Each well-defined group is thought to come from one parent body; about one iron in ten fits no group and is called ungrouped.

7. Step 7 of 7

   Naming and approval

   Every meteorite is named after the place where it fell or was found, such as Allende, Murchison or Winchcombe. Where many are found in one area, such as Antarctic ice fields or the deserts of north-west Africa, they get the area's name or abbreviation and a number: ALH 84001 came from the Allan Hills and was collected in the 1984–85 season; NWA 7034 is north-west Africa find number 7034.

   To be recognised, a meteorite must be studied by a qualified laboratory, and a type specimen, usually at least 20 grams or 20 per cent of the mass, whichever is less, must be deposited in an approved collection. The Meteoritical Society's Nomenclature Committee then approves the name and classification, which are published in the Meteoritical Bulletin, the official record of every recognised meteorite.

Related pages

See where these events sit in deep time on the 4.6-billion-year timeline.