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Re: Iron Meteorite Classification. Part I

	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 
Nickle(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-controled 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 stucture 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 generaly accepted that iron meteorites are 
derived from and 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 finaly, it is not visable 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 counter 
part 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 ar basicly one large widmastatten band. 
	The other Iron meteorites fall somwhere 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 
classififcation comes to 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 next time. Suffice to say, bandwidth is a function of chemical 
classififcation, and the two are dependent of each other. 
	Bandwidths range from .006mm - 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 
over the next few days. If you have questions, e-mail me, and I will 
answer them. In any case, I hope this will help those interested in Iron 
meteorites. 

Frank Stroik
Department of Geology 
and Geophysics 
The University of Wyoming 


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