Stony-iron meteorites

Stony-iron meteorites consist of almost equal amounts of iron-nickel metal and silicate minerals and are amongst the most beautiful of meteorites. There are two different types of stony-iron, the pallasites and the mesosiderites .

Pallasites

A cut and polished slab of the pallasite, Imilac.

A cut and polished slab of the pallasite, Imilac.

Pallasites contain big, beautiful olive-green crystals, a form of magnesium-iron silicate called olivine , embedded entirely in metal. Pallasites can show big variations. Sometimes the olivine does not occur as a single crystal but as a cluster and elsewhere it can create a pattern of veins through solid metal.

There are differing opinions as to how pallasites formed. Some scientists believe that they formed from melted asteroids, like iron meteorites. When an asteroid melts, the dense iron metal sinks toward the centre to form an iron core. Pallasites are thought to be samples of the boundary between this metal core and the silicate, olivine-rich mantle around it. If this is the case, pallasites could potentially tell us a lot about the formation of the Earth and other terrestrial planets because they all have a similar structure. However, other scientists think that there are very few olivine-rich meteorites in the asteroid belt, and too many pallasites for them all to have come from a core-mantle boundary.

Mesosiderites

The Estherville mesosiderite formed after a catastrophic collision between two asteroids.

The Estherville mesosiderite formed after a catastrophic collision between two asteroids.

Mesosiderites are breccias - coarse fragments cemented together by a finer material. The fragments are centimetre-sized and contain a mixture of an igneous (solidified) silicate and metal clasts (rocks made of pieces of older rocks).

Mesosiderites form when debris from a collision between two asteroids is mixed together. In the crash, molten metal mix with solid fragments of silicate rocks. Mesosiderites may, therefore, record the history of both meteorites.

Together with the stony achondrite meteorites, stony-iron meteorites may reveal how some of the asteroids melted.

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.