Understanding Iron Meteorites
by Frank Stroik

Perhaps the most complicated, and least understood classification system in science today(with the possible exception of crinoid classification in paleontology) is Iron meteorite classification. I have spoken to a few researchers, and few understand it completely. I am going to try to explain it so that when you visit a museum, read a book, or, perhaps, look at your own specimens, you will be able to realize what exactly the iron meteorite is telling you.

First, a knowledge of the minerals, Kamacite and Taenite are needed to understand the Widmanstaten structure. By understanding this you will be able to tell how hot the meteorite was, and how long it took to cool. If you have slice of an Iron meteorite, go get it, and refer to it as you read this, as it always helps to look at what someone is writing about.

Kamacite and taenite are minerals that compose the majority of iron meteorites. They are formed by cooling of a magma (molten material). In this case the magma is composed mostly of two elements Iron (Fe) and Nickel (Ni). As the magma cools, these elements begin to bond together.

The definition of the Widmanstatten structure is as follows:The true Widmanstatten structure forms by a diffusion-controlled nucleation that is slow, even on a geologic time scale(Buchwald 1975). What does this mean? In simpler terms, it means it takes a long time to cool Fe and Ni, and even longer to bond them together. It is the amount of time for cooling that generates the size of the bands in the Widmanstatten structure.

Ni would like to bond as soon as possible. At about 1100 degrees C. Ni atomic structure is stable enough to allow bonding. Fe, however is still reluctant to bond. So as time passes more Ni atoms bond than Fe atoms bond. This continues until most of the Ni is gone from the melt.

Now here is an implication for the different types of iron meteorites. Since a it is generally accepted that iron meteorites are derived from an asteroid core, this allows us to model a planetary core in some detail. In doing this, it gives us a look at how the Widmanstatten structure clues us in on where a particular meteorite formed. Ataxites are iron meteorites without structures. These formed first in a melt, as they are composed of mostly Ni in their mineralogy.

Did you see the another connection? The more Ni you have, the smaller the Widmanstatten structure, until, finally, it is not visible to the naked eye. Ataxites are composed mostly of the mineral taenite, which is rich in Ni, and demonstrate what an outer core of a planet must look like.

On the other end of the spectrum we have Hexahedrites.These are composed mostly of the mineral Kamacite, which is the Fe rich counterpart to taenite. Hexahedrites are the last to cool, and they represent the inner core of an asteroid, which can be extrapolated to other planetary cores. These meteorites are basically one large Widmanstatten band.

The other Iron meteorites fall somewhere in between these two extremes. Remember this, the larger the band width, the longer it took to cool, and the closer it is to the inner core. Here is where a structural classification comes into play: by measuring bandwidth in an iron meteorite, we can assign them to particular classes. These classes are chemical in nature, and will be discussed (at a later date). Suffice to say, bandwidth is a function of chemical classification, and the two are dependent upon each other.

Bandwidths range from .006mm to 3.1mm. In each type of meteorite there is a very small tolerance in the width . For example, the group IIIF has a bandwidth of 1.3-1.6mm if it was any higher, or lower, it would indicate a different group, or a possible anomalous structure. I will continue to discuss this classification in more detail (at a later date). I hope this will help those interested in Iron meteorites.

Frank Stroik
Department of Geology and Geophysics
The University of Wyoming

Buchwald Vagn F. Handbook of Iron Meteorites Vol. 1 pp243
University of California Press, 1975