Stony-Iron Meteorites and other Oddities

A number of meteorites are a curious mixture of both stony and iron-based meteorites. Pallasites are extraordinary, with huge centimetre-sized gems of olivine called peridot, while mesosiderites contain half and half of iron and silicate. Both flavours of meteorite are suspected of having been spliced together underneath the vicious temperatures and pressures of large impacts with asteroids.

There are a handful of meteorites that don’t fit in with any of the conventions, stubbornly refusing to be classified or their origin ascertained. Lead amongst these are the ureilites, which are made from olivine and pyroxene – fairly standard – but also contain curious veins of carbon-rich graphite and diamonds that can only form under high pressures, such as those experienced in impacts.

The problem is, impacts should transform delicate carbon-based material such as olivine. Another small group are winonaites, made mostly from low calcium pyroxene as well as a diverse mix of olivine, plagioclase, sulphides and metal. Winonaites are thought to arise from asteroids that started melting and forming differential layers of materials, before this was rudely brought to a halt by a large impact smashing the asteroid to pieces.

Finally, there are the brachinites, named after the Brachina meteorite that fell to Earth in southern Australia in 1974, and are composed mainly of olivine and basalt.