Chondrites: the oldest rocks

Chondrites are the most common meteorites of all, and some of the most important rocks in science. They are almost unchanged leftovers from the birth of the Solar System.

Never melted

A chondrite is a stony meteorite from a parent body that never melted completely. Many chondrites have been warmed or soaked with water inside their asteroids, but none were melted into a lava, so they keep their original ingredients.

Because of this, the overall chemistry of chondrites is very close to that of the Sun itself, once you leave out the gases like hydrogen and helium. The rare carbonaceous chondrites of the CI group match the composition of the Sun's outer layers almost perfectly for most elements. That makes chondrites a reference point: when scientists talk about how the rocky parts of the Solar System began, chondrites are the starting recipe.

About 85 per cent of observed meteorite falls are chondrites.

Chondrules

Chondrites are named after chondrules, small spheres that are usually between a fraction of a millimetre and a few millimetres across. The word comes from the Greek chondros, meaning grain.

Chondrules were once molten droplets that floated freely in the disc around the young Sun. Something heated dust clumps very quickly, to around 1,500 °C or more, and then they cooled within hours, forming crystals of minerals such as olivine and pyroxene, sometimes with glass between them. What caused these flash-heating events is still debated. Ideas include shock waves in the disc and collisions between early bodies.

Most chondrules formed during the first few million years of Solar System history. In many chondrites they make up a large share of the rock, sometimes more than half.

CAIs and matrix

Chondrites contain other ingredients besides chondrules.

Calcium-aluminium-rich inclusions, or CAIs, are irregular white or pinkish patches made of minerals rich in calcium and aluminium. These minerals are the first solids expected to condense from a hot gas of solar composition. CAIs are the oldest dated solids from our Solar System, about 4,567 million years old. Scientists use their age as time zero for the Solar System.

Between the chondrules and CAIs is a fine-grained material called matrix. It is made of tiny dust grains, and in some chondrites it contains carbon compounds and even rare grains of stardust older than the Sun, made in other stars.

Scattered through many chondrites are grains of iron-nickel metal and an iron sulphide mineral called troilite.

Did you know?