Most meteorites fall into one of four categories.. "Iron meteorites", also called "irons", are usually just one big blob of iron-nickel (Fe-Ni) metal, as if it came from a industrial refinery without shaping. The alloy ranges from 5% to 62% nickel from meteorite to meteorite, with an average of 10% nickel. While most "irons" are pure or nearly pure metal, the technical definition of an "iron" includes metal meteorites with up to 30% mineral inclusions such as sulfides, metal oxides and silicates. The irons represent the cores of former planetoids.
"Stony irons" consist of mixtures of Fe-Ni metal of between 30% and 70% along with mixtures of various silicates and other minerals. The Fe-Ni metal can be present as chunks, pebbles and granules. Stony irons resemble the outer cores or mantles of planetoids or else a mix of materials due to a collision.
"Achondrites" are silicate rich meteorites apparently formed by crustal igneous (i.e., molten or volcanic) activity in their parent bodies, and consist of a broad range of minerals. Achondrites are the result of gravitational differentiation in relatively large bodies by melting and gravitational separation of mineral phases, and most resemble the Earth's crust. Different types of achondrites average between 0 and 4% free Fe-Ni granules.
"Chondrites". Chondrites are named after the tiny pellets of rock called "chondrules" embedded in them, a result of a kind of chemical fractionation. There are different subcategories of chondrites.
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