Types Of Meteorites
Since the first meteorites were collected and studied it has been clear that there are three main families. But, over time the differences seen among the ever growing number of these visitors has led to many more categories than three. At the present there are approximately fifty types of meteorites recognized. They still fall mainly into the three original families; the irons, the stony-irons and the stones. There are now a host of sub-groups within each main type.
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.
The second main group of meteorites is the Stony-Iron group. These meteorites have much higher amounts of mineral in them than the small inclusions found in the irons. The mineral portion of stony-irons is more like half the mass. It is mixed with the metal usually.
The mineral portion may be olivine crystals as in the Pallasite family of stony-irons. Or as in the Mesosiderite group the mineral portion is silicate minerals mixed in a matrix of metal.
The last and largest main group the Stony meteorites are as their name states made of rock. The two largest subgroups of the Stony family are the Chondrites and the Achondrites.
Chondrites get their name from the small spherical structures that they are composed of called chondrules. As with many words adding an “A” in front indicates the negative or in this case not having chondrules.
The drops of molten mineral that originally were in space aggregated together into larger and larger masses and with the aid of tiny dust particles to fill the pore space and some heat and pressure chondrite material formed.
Achondrites in general are meteorites that have experienced greater amounts of heat. Enough heat to either melt away their prior chondrite characteristics or enough to form original igneous rocks.
These igneous rocks are not that dissimilar to igneous rock from Earth, but come from the larger asteroids, Mars and the Moon.
This has been a very brief summary of a rich and interesting area of study. In our galley of photographs there are pictures of nearly all the types of meteorites. We hope that you will enjoy spending some time there seeing the beautiful diversity in meteorites.
Author: James Tobin [Google]