Enstatite chondrites and other rare classes

Beyond ordinary and carbonaceous chondrites lie smaller classes formed in very different chemical conditions. They show how varied the early Solar System was.

Enstatite chondrites

Enstatite chondrites, or E chondrites, formed in conditions with extremely little oxygen. Their main silicate is enstatite, a magnesium pyroxene with almost no iron. Instead of being oxidised, iron is present as metal and sulphides, and even silicon is partly dissolved in the metal.

This highly reducing environment produced minerals almost never found on Earth, such as oldhamite, a calcium sulphide, and niningerite, a magnesium sulphide. Elements that normally bond with oxygen were forced to bond with sulphur instead.

There are two groups: EH, with high iron, and EL, with low iron. They make up roughly 2 per cent of falls. Abee, which fell in Alberta, Canada, in 1952, is a well-known EH chondrite and an impact breccia. Like ordinary chondrites, they span petrologic types 3 to 6.

A link to Earth?

Enstatite chondrites are especially interesting because their oxygen isotopes are almost identical to those of Earth and the Moon. Several other isotope systems, including those of nitrogen, titanium and chromium, are also very similar.

This has led some scientists to propose that material similar to enstatite chondrites made up much of the building blocks of Earth. However, Earth's bulk chemistry differs in some respects, for example in its ratio of magnesium to silicon. Most researchers now think Earth formed from a mixture of materials, with an important component that was isotopically similar to enstatite chondrites.

The related achondrites called aubrites are also highly reduced and share similar oxygen isotopes, suggesting they formed on a melted body made of similar material.

R and K chondrites

At the other extreme are the R chondrites, named after Rumuruti, which fell in Kenya in 1934. They are highly oxidised, with almost no metal, and their olivine is rich in iron. Their oxygen isotopes are distinctive too, among the heaviest of any chondrite group. Many R chondrites are breccias that contain fragments of different petrologic types, and some contain evidence of water.

The K chondrites, or Kakangari-type chondrites, are a small grouplet named after Kakangari, which fell in India in 1890. Their oxidation state lies between H and E chondrites, and they contain an unusually large amount of matrix.

A grouplet is a set of fewer than five related meteorites. When a meteorite does not fit any group or grouplet, it is called ungrouped, and these can be among the most interesting rocks of all.

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