Types of Meteorites

Since the first meteorites were collected and studied it has been clear that there are three basic types of meteorites. Iron Meteorites which consist of mostly iron with varying amounts of nickel. The second and most abundant group is the Stone Meteorites made up of silicate minerals with a small amount of metal grains. The third group is the Stony-Iron Meteorites which contain roughly half mineral and half nickel iron metal. Most are believed to originate from the asteroid belt but there are also meteorites proven to be from the Moon and Mars.

Iron Meteorite

Iron meteorites as the name states are composed of almost solid nickel-iron. Often there are mineral inclusions in iron meteorites, but they amount to a small percentage of the mass. Three main subgroups of irons have been recognized. They are the Octahedrites, the Hexahedrites and the Ataxites. The metal nickel is found in nearly all meteorites. It is the amount of nickel that was in the original bulk alloy that cooled in space that determined which of the three iron families a meteorite will be placed into. This difference in nickel chemistry results in a difference in crystalline appearance of the metal when acid etched and also determines even which nickel iron minerals will crystalize. In the Octahedrites both Taenite and Kamacite mineral crystals grow. In Hexahedrites only Kamacite can form. So Hexahedrites are essentially pure kamacite. The opposite is true with Ataxites where only Taenite was able to grow. Much more can be said about this but it is the size of the crystals in Octahedrites that determines a meteorite's placement in the next level of classification. Low nickel Octahedrites will have large crystals and are call Coarse Octahedrites, middle level nickel percentages will result in what are called Medium Octahedrites, and iron meteorites that formed where there was a higher amount of nickel will have tiny crystals and are called Fine Octahedrites.

Stone Meteorite

The largest group of meteorites by far is the stony meteorites. Made of minerals that for the most part are the same as minerals that are found on Earth. However,they have structures and textures not found here. The stony meteorite group is divided into two major subgroups; the chondrites and the achondrites. The chondrites get their name because of the spherical mineral structures they contain called chondrules. In space the molten droplets of rock material cooled in the very low gravity into tiny round spheres that crystallized into minerals. Achondrites are stony meteorites without chondrules. These meteorites have an igneous mineral structure with crystals that indicate they formed in an environment with some gravity. In most stony meteorites there is metallic nickel-iron. The metal is in the form of grains scattered throughout the matrix of the rock.

Stony-Iron Meteorite

The Stony-Iron meteorites are made of both nickel iron metal and stone and this group includes two large sub groups, the pallasites and the mesosiderites. The Pallasites are characterized by having a large portion of their mass in the form of olivine crystals with these crystals being surrounded by nickel-iron. Mesosiderites have silicate minerals found commonly in many iron meteorites but the amount of silicates gets quite high resulting in these meteorites classified as mesosiderites