Is It A Meteorite?

Matthew Genge

The features that identify a rock as a meteorite differ depending on the type of meteorite and on whether it is fresh or whether it has been altered on Earth. Certain terrestrial rocks and artificial materials which are commonly mistaken for meteorites (nicknamed pseudometeorites or meteor-wrongs) are, however, easy to distinguish from meteorites.

Surface features
The external surfaces of meteorites experience intense heating during atmospheric entry and a melted rim known as a fusion crust is formed. The fusion crust tends to be black or dark grey in colour and is very thin, usually being no more than a millimetre in thickness. Within the fusion crust, small gas bubbles can sometimes be seen. In unweathered, recently fallen, meteorites the fusion crust is easy to recognise and very small 'dribbles' can often be seen on the external surface where the molten rock has been dragged by the air flowing around the meteorite. In weathered meteorites the fusion crust may be absent (such as on most iron and stony iron meteorites) and where it is present much of the detail can be lost and the crust is no longer black.

Industrial slag also has a melted appearance and usually contains bubbles. It is, therefore, commonly mistaken for meteorites. Slag, however, has melted all the way through, rather than having a very thin crust, and contains large gas bubbles.

Another characteristic feature of meteorites, seen on their exterior surface, are thumb-shaped imprints known as Regmagpyts. These features are formed by sculpting of the surface of the stone by the air flow during atmospheric entry and are particularly common on iron meteorites.

Internal features
Iron meteorites are the easiest to distinguish from terrestrial rocks since they are strongly magnetic and very dense (8 g/cc). It is, however, very difficult to distinguish between industrial iron and iron meteorites and this usually involves a chemical analysis.

The commonest type of stony meteorites are the chondrites and these can be identified by milimetre-sized spherical objects they contain, known as chondrules within a finer-grained matrix which often includes metal. Fresh stony meteorites are usually slightly magnetic due to the presence of iron-nickel metal, however, weathering removes the metal and these can become non-magnetic. There are many terrestrial rocks that broadly resemble stony meteorites and often chemical analysis of their minerals are needed to prove they are meteorites.

Pseudo-Meteorites
Perhaps the commonest objects that are mistaken for meteorites are marcasite nodules. Thesre are spherical metallic or rusty looking objects consisting of the iron sulphide mineral pyrite. Often their surface is covered with either crystals or rounded protrusions that give them a melted appearance. They are quite dense (around 5 g/cc), but not as dense as iron meteorites. Marcasite nodules form within sediments and are common in chalk. Within nodules, the pyrite is frequently arranged as radiating crystals.