Although there are a large number of sub classes, meteorites are divided into three main groups: irons, stones and stony-irons. Almost all meteorites contain extraterrestrial nickel and iron, and those that contain no iron at all are so rare that when we are asked for help and advice on identifying possible space rocks, we usually discount anything that does not contain significant amounts of metal. Much of meteorite classification is based, in fact, on how much iron a specimen does contain.
Most iron meteorites are thought to be the cores of asteroids that melted early in their history. They consist mainly of iron-nickel metal with small amounts of sulphide and carbide minerals.
Stony-iron meteorites consist of almost equal parts iron-nickel metal and silicate minerals including precious and semi-precious gemstones. They are considered some of the most beautiful meteorites. There are two different types of stony-iron meteorites: pallasite and mesosiderite.
The majority of meteorite finds are stony meteorites, consisting mostly of silicate minerals. There are two main types of stony meteorite: chondrites (some of the oldest materials in the solar system) and achondrites (including meteorites from asteroids, Mars and the Moon).Both chondrites and achondrites have many subgroups based on their compositions, structures and the minerals they contain.
Mesosiderite meteorites are breccias, a variety of rock composed of broken fragments of minerals or rock cemented together by a finer material. The fragments are roughly centimetre-sized and contain a mix of 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 mixes together with solid fragments of silicate rocks. Mesosiderites can therefore both record the history of both meteorites and reveal a snapshot of the conditions required for asteroids to melt and form
Achondrites include meteorites from asteroids, Mars and the Moon. They are igneous, meaning at some point they were melted into magma. When magma cools and crystallises, it creates a concentric layered structure. This process is known as igneous differentiation. iron cores.