Ordinary chondrites: H, L and LL

Roughly four in every five meteorites seen to fall are ordinary chondrites. They are divided into three groups, H, L and LL, and learning to tell them apart is the first real step in classification.

Ordinary, but not boring

The name ordinary chondrite simply reflects how common these meteorites are. Among observed falls, L chondrites are the most numerous, closely followed by H chondrites, with LL chondrites well behind.

All three groups contain abundant chondrules, a fine-grained matrix, iron-nickel metal and the iron sulphide troilite. Their main minerals are olivine and low-calcium pyroxene, together with smaller amounts of feldspar. Their overall chemistry is close to that of the Sun for rock-forming elements, but each group differs slightly in how much iron it contains and in what form.

The letters describe this difference. H stands for high total iron, L for low total iron, and LL for low total iron and low metal. These are not grades of quality; they record the chemical conditions in three separate parent asteroids.

Metal and oxidation

The key difference between the groups is oxidation state, meaning how much of the iron is bound to oxygen in silicate minerals rather than present as metal.

H chondrites are the least oxidised. They contain the most metal, very roughly 15 to 20 per cent by weight, and their olivine is relatively poor in iron. L chondrites contain less metal, typically around 5 to 10 per cent, and LL chondrites only a few per cent or less, with the most iron-rich olivine of the three.

This pattern follows Prior's rules, described by the British Museum mineralogist George Prior in the early twentieth century: the less metal a chondrite has, the richer in nickel that metal is, and the richer in iron its silicate minerals are.

In practice, a magnet test gives a hint. H chondrites pull strongly on a magnet, while LL chondrites respond more weakly.

Classifying them properly

A hand-held magnet is only a first clue. Formal classification relies on measuring the composition of olivine and pyroxene with an electron microprobe, an instrument that fires a focused beam of electrons at a polished sample and measures the X-rays given off.

Olivine composition is expressed as its fayalite content, written Fa, the percentage of the iron-rich end-member. In equilibrated ordinary chondrites, olivine is roughly Fa16 to Fa20 in H, Fa22 to Fa26 in L, and Fa26 to Fa32 in LL. Pyroxene composition, written Fs for ferrosilite, rises in the same order.

Oxygen isotopes and magnetic susceptibility, a measure of how easily a sample is magnetised, give further independent checks. Together, these methods allow most ordinary chondrites to be assigned confidently to H, L or LL.

Three parent bodies

Because the groups differ in chemistry, oxygen isotopes and history, each is thought to come from a different parent asteroid or family of asteroids.

The LL link is the best established. In 2010 the Japanese Hayabusa spacecraft returned tiny particles from the near-Earth asteroid Itokawa, and their minerals matched LL chondrites. The Chelyabinsk meteorite, which exploded over Russia in 2013, was also an LL chondrite.

Many L chondrites are heavily shocked and record a major heating event about 470 million years ago. This suggests their parent body was shattered by a catastrophic collision at that time. Remarkably, fossil meteorites preserved in limestone of about that age in southern Sweden are L chondrites, showing that Earth was hit by an unusually intense shower of debris after the break-up.

The H chondrite parent body is less certain, although several asteroids have been proposed.

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