Stony-irons: pallasites and mesosiderites

Stony-iron meteorites are rare mixtures of metal and rock. They include some of the most beautiful objects that fall from the sky.

Pallasites

A pallasite is made of olivine crystals embedded in iron-nickel metal. Olivine is a magnesium iron silicate, and in pallasites it is usually yellow-green to amber. When a thin slice is polished and held to the light, the crystals glow like stained glass.

Pallasites are named after the German naturalist Peter Simon Pallas. In 1772 he examined a large mass of metal and olivine found near Krasnoyarsk in Siberia, which became known as the Krasnojarsk meteorite. Its unusual appearance helped convince scientists that some rocks really do fall from space.

Famous pallasites include Imilac from Chile, Esquel from Argentina, Fukang from China and Seymchan from Russia. Seymchan was first classified as an iron and only later found to contain olivine in some parts.

Where did pallasites form?

For a long time, scientists thought pallasites formed at the boundary between the metal core and the olivine-rich mantle of a melted asteroid. That idea explains why they mix the two materials.

More recent research has suggested that the story is more complicated. Studies of tiny magnetic signals recorded in some pallasites show that their parent body had a magnetic field, generated by a molten core. Some researchers have proposed that pallasites formed nearer the surface, where molten metal from an impact was injected into the mantle rock. Different pallasite groups may also have formed in different ways on different bodies.

This is a good example of science in action: a simple explanation is tested, new evidence appears, and ideas are refined.

Mesosiderites

Mesosiderites are the other main kind of stony-iron. They are breccias made of roughly equal parts metal and silicate rock, but the rock is very different from the olivine in pallasites. It is mostly basalt-like material and pyroxene, similar to the eucrites and diogenites from Vesta.

A polished mesosiderite looks like a chaotic mixture of shiny metal and grey or dark stony fragments. This jumble suggests a violent history in which crust and core materials were mixed together, possibly when a differentiated asteroid was struck by a large metal-rich body, or broke apart and reassembled.

One well-known mesosiderite is Vaca Muerta, found in the Atacama Desert of Chile, where many pieces have been recovered. Mesosiderites also tend to have cooled extraordinarily slowly at low temperatures, another puzzle researchers are still working on.

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