METEORITE BASICS: IRONS

Iron meteorites are largely composed of metal alloys that are predominantly iron (Fe) with variable amounts of nickel (Ni) dissolved within the iron. Two different Fe/Ni alloys commonly found in irons are kamacite (a low Ni alloy) and taenite (a higher Ni alloy). Many irons contain both of these two alloys which have crystallized together causing the geometric pattern called Widmanstatten figures. The next most common mineral in irons is troilite, an iron sulfide, uncommon on the earth, which usually occurs as nodules with the Fe/Ni alloys.

The scientific classification of irons into various types is complicated and difficult because it is based on the amounts of contained trace elements which are not apparent to the eye. A visible classification, such as fine, medium or coarse, is also used for the group called octahedrites (those with kamacite and taenite found together). The so-called scientific classification originated in an attempt to determine which irons came from the same parent asteroid.

How do these meteorites composed entirely of Fe/Ni alloys form? To understand this, consider a large mass of rock in space, an asteroid, which undergoes wholesale melting. When this occurs, the heavy metal alloys, move to the center of the mass displacing lighter matter which is moved upward and nearer the surface. As an example, think of the planet earth which has a metal core and a mantle and crust above. After this separation of heavy alloy at the core from the lighter silicate matter above, the asteroid is shattered into thousands of pieces by a collision. Some of the shattered pieces will be Fe/Ni alloys from the previous core and some will be silicate minerals which had been above the core. The result is many fragments in the asteroid belt are all Fe/Ni alloy metal.

 

[left] Sikhote-Alin, an iron meteorite that fell in Russia in 1947