Depending on how much metal or stony silicate material is present, meteorites can be irons, stony-irons, or stones. These may represent the centre (irons), inner (stony-irons) and outer parts (stone) of small planetary bodies which collided and broke up in the asteroid belt, or material that failed to clump together to make planetary bodies when the Solar System first formed.
The minerals which make up the bulk of meteorite composition include seven common Earth minerals and three found only in meteorites:
Small amounts of other Earth minerals can be present in meteorites, but over 20 minor minerals occur only in meteorites and not on Earth. Altogether, about 295 different minerals have been found in meteorites.
Stones are the most common type of meteorite and can be of moderately large mass such as the 0.5 tonne stone from Wildara, Western Australia. They consist mainly of the silicate minerals olivine and pyroxene with feldspar and scattered nickel-iron.
Stones are further subdivided according to whether small, spherical, concretionary mineral structures (chondrules) are present (chondrites) or absent (achondrites). Most stones are chondrites, of which there is an unusual type, the carbonaceous chondrites (e.g. The 'Murchison', Victoria) which have enormous scientific and popular interest because of their strange bituminous smell, high organic content and amino acid-like chemicals. Rare classes of achondrite stones (e.g. the Shergottites) are basalt lavas thought to have come from Mars.
Irons are the second most common type of meteorite and have the largest masses, such as the 12 tonne 'Mundrabilla', Western Australia iron. Irons are made of dense silvery nickel-iron metal alloys (taenite and kamacite) and have a range of nickel contents from about 4% (Hexahedrite Class) to 6 - 12% (Octahedrite Class) to over 20% (Ataxite Class). When cut, polished and etched with an acid/alcohol solution, some (the Octahedrite Class) show a characteristic criss-cross pattern of intersecting platy nickel-iron crystals known as Widmanstatten structure.
Stony-irons are the least common type of meteorite. One type (Pallasite Class) has about equal amounts of the silicate mineral olivine, and nickel-iron metal (green glassy olivine crystals set in a continuous meshwork of silvery nickel-iron), while another type (Mesosiderite Class) is a broken up (brecciated) mixture of pyroxene with nickel-iron. A well-known New South Wales example of a Pallasite is the 'Molong' stony-iron.
Copyright © Australian Museum, 2004