Stony meteorites

The majority of meteorite falls are stony meteorites consisting mainly of silicate minerals. There are two main types of stony meteorite.

  • Chondrites have never significantly melted and have compositions similar to the sun and the solar system as a whole.
  • Achondrites have melted and are similar to igneous rocks on Earth. These include rocks from Mars and the moon as well as from melted asteroids. Achondrites have compositions different from the sun as they have been changed by melting and crystallisation.

Both chondrites and achondrites are split into many sub-groups based on their compositions, minerals and structures.

A cut slab of a chondrite meteorite.

A cut slab of a chondrite meteorite.

Chondrites

Chondrites are some of the most primitive rocks in the solar system. These 4.5-billion-year-old meteorites have not changed much from the asteroid they came from. Because they have never really got hot, they have not melted. This means that they have a very distinctive appearance made from droplets of silicate minerals, mixed together with small grains of sulphides and iron-nickel metal. This structure of millimetre-sized granules also gives chondrites their name, which comes from the Greek for sand grains 'chondres'.

Bringing the conditions for life

There are many varieties of chondrite. The differences in mineralogy relate to the type of asteroid the meteorite has come from and have occurred due to heating or contact with water. The most basic chondrites, known as carbonaceous chondrites, are rich in water, sulphur and organic material. They are thought to have brought volatile material to Earth when it was newly formed, helping to establish the atmosphere and the right conditions for life.

Building blocks of the solar system

Chondrites are the materials from which the solar system formed and have been little changed compared with rocks from the larger planets, which have been subjected to billions of years of geological activity. They are very similar in composition to the sun and can tell us a lot about how the solar system formed.

A slab of a cumulate eucrite, a type of achondrite.

A slab of a cumulate eucrite, a type of achondrite.

Achondrites

Achondrites include meteorites from asteroids, Mars and the moon. They are igneous, meaning that at some point they melted into magma. When the magma cools and crystallises, it creates a concentric layered structure. This process is known as igneous differentiation. 

The rocky planets of Mercury, Venus, Earth and Mars, have also been formed by igneous differentiation, giving them planetary crusts, mantles and cores. Achondrites can, therefore, tell us a lot about the internal structure and formation of the planets.

Cartoon image of a stegasaurus disappearing through closing door

The first collected specimen of Theobroma cacao, the plant from which chocolate is made, is kept in the Museum.