Petrologic types: from wet to hot

The number after a chondrite's group letter, as in H5 or CM2, is its petrologic type. It tells you what happened to the rock inside its parent asteroid.

A scale from 1 to 6

In 1967 the American scientists W. R. Van Schmus and J. A. Wood introduced a numerical scale to describe how much chondrites had been changed inside their parent bodies. It is still used today, with refinements.

The central idea is that type 3 is the least changed. Moving from type 3 down to types 2 and 1 means more alteration by water. Moving from type 3 up to types 4, 5 and 6 means more alteration by heat, a process called thermal metamorphism, in which a rock is changed by heating without melting.

So the scale is not a single line of increasing change but runs in two directions from a pristine middle. Some scientists also use type 7 for chondrites heated almost to the point of melting, although this category is debated.

Types 1 and 2: altered by water

Early in Solar System history, some parent bodies contained ice. As they warmed, the ice melted, and liquid water reacted with the rock. This is called aqueous alteration.

Water converts original minerals into clay-like phyllosilicates, carbonates, sulphates, magnetite and other secondary minerals. In type 2 chondrites, such as most CM chondrites, many chondrules are still visible but are partly altered. In type 1 chondrites, such as the CI chondrites, alteration was so complete that chondrules can no longer be recognised at all.

Type 1 and 2 chondrites are mostly carbonaceous chondrites, and they are fragile, porous and rich in water bound inside minerals. They are among the most important meteorites for understanding how water and carbon compounds were delivered to the early Earth.

Type 3: the most pristine

Type 3 chondrites, often called unequilibrated chondrites, have changed the least. Their chondrules have sharp outlines, many still contain glass, and the compositions of their olivine and pyroxene grains vary widely from one grain to another, because these grains have not exchanged atoms with each other.

Because type 3 chondrites preserve so much early detail, they are subdivided further, from 3.00 for the most pristine to 3.9 for those that are almost type 4. Subtypes are measured using techniques such as thermoluminescence and the chromium content of olivine, which are sensitive to very mild heating.

Type 3 chondrites are the best places to find presolar grains, tiny crystals of stardust that formed around other stars before the Sun existed. Heating and water tend to destroy them, so they survive best in the most primitive rocks.

Types 4 to 6: cooked by heat

In parent bodies that contained more aluminium-26, the rock was heated, typically to several hundred degrees Celsius, for long periods. The higher the type, the more intense the heating.

In type 4, chondrules are still clear, but the olivine has become uniform in composition. By type 5, chondrule boundaries start to blur, and glass has crystallised into small grains of feldspar. In type 6, chondrules are hard to see, feldspar grains are larger, and the rock has a more even, recrystallised texture.

An influential model for the H chondrite parent body, the onion-shell model, suggests that the hottest type 6 material lay near the centre and the cooler type 3 material near the surface. Cooling rates measured from minerals in different types broadly support this idea, though impacts later complicated the picture.

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