Achondrites: Meteorites Lacking Chondrules

Achondrites are nothing more than rocky meteorites that lack the gray spherical chondrules found in chondrites. There are a variety of different types, but in terms of origin all share one important distinction from chondrites: they have undergone melting, differentiation, and recrystallization, most likely on one of the many growing protoplanets that were present in the early Soloar System, or even on the Moon or Mars. Achondrites are thus igneous rocks and can look very similar to volcanic rocks on Earth. They have chemical and isotopic differences that clearly distinguish them from Earth rocks, but visually the most distinctive feature would be a fusion crust.

The photo above is an achondrite found in Antarctica that was blasted from the surface of the Moon. The dark areas in the top photo are glassy (quickly cooled rock) while the white angular grains are shattered pieces of anorthosite. Anorthosite is the white rock that dominates the light areas of the Moon. The cube with the N on it is 1 cm on a side.

This view of the lunar meteorite shows a brown frothy fusion crust. The dark gray interior is revealed to the lower left.

More will be said about achondrites in the section on iron meteorites!

Next: Iron meteorites

Image credits: The Antarctic Meteorite archive hosted at NASA's Johnson Space Center. Meteorites are so scientifically interesting that the National Science Foundation has funded years of efforts aimed at finding meteorites in Antarctica. The famous Martian meteorite (ALH84001) with putative evidence for ancient life on Mars was found in Antarctica.