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Re: Classifying Meteorites, and the Inherent Problems.

	I wanted to take some time out to tell everyone about what is 
involved in new meteorite anaylisis, and classification. There are many 
things that make this a difficult, and somewhat "magical" task. It is a 
very subjective process, that can yield different results if you allow 
personal judgement to get too involved. 
	To anaylyze a meteorite you must first have a thin section. This 
is how all data is obtained, and can be extremely limited if you only 
have one. The smaller the meteorite, the more difficult to anaylyze with 
	The first step is to look at the general texture of the meteorite 
under normal light in the microscope. Here you can see how weathered it 
is, and how bashed up it is. The minerals should be pretty much clear, 
but if it is weathered, they will appear brown. This can cause problems, 
as this may further add to the bashed up appearence of the meteorite.
	Next you have to cross the polars. This means simply, polarize 
the light. Minerals behave differently to polarization. This is due to a 
property known as bifringence. The atomic structure of a mineral will 
bend the light in a certain way, and create a most spectacular image. For 
example, olivine is pretty much colorless in plain light, but polarize 
it, and you get extremely bright blues, greens, and yellows. Each mineral 
behaves differently under polarization. It is these differences that help 
us decide what is in the meteorite.
	Now you must look for a mineral known to petrologists as OPX. 
This is bascialy the solid solution of enstatite, bronzite, hypersthene, 
and ferrosolite. A solid solution is a soulution that can have different 
chemical compositions depending on precentage of elements that are 
In OPX this is Fe, and Mg. If it were pure Mg, it would be enstatite, and 
if it were pure Fe, it would be ferrosolite. However it is rare that such 
a thing happens in nature, so bronzite and hypersthene are basicly 
different compositiond of Fe, and Mg, hypersthene being more Mg rich, and 
bronzite being more Fe rich.	
	When you find OPX, you must decide if it is the appropriate 
crystal system for you to use in your anaylisis. This done by turning the 
microscope stage(the place where the thin section rests) until the 
mineral is completly black. If it turns and goes black it is what you are 
looking for. If it must be turned 90 dgrees it is the wrong crystal system.
	Now you look at how many of these grains there are, and determine 
how abundant they are. All the while you are doing this you look at the 
state of the chondrules. Are they whole, twisted, degenrerated, or just 
plain gone? This gives you clues to what petrologic type it is. The less 
OPX, and the more degenerated the chondrules, the higher the petrologic type.
	Now sound judgement must be used. Did I see what I thought, did I 
miss anything? Based on few more tests you decide on the petrologic type.
	Now you are ready for the chemical classification(H,L,LL). You 
bring that sample to the Microprobe for anaylisis. A microprobe is a 
device that sends X-rays down a short tunnel. These X-rays hit a minute 
portion of the sample, and become reflected. Different mineral 
compositions reflect x-rays in different ways. This is how an elemental 
composition is derived. 
	The two minerals you use are OPX, and olivine. In both you try to 
determine how much Fe is present. This tells you what chemical class to 
put it in.H chondrites have about 18%Fe in both minerals,L chondrites 
have about 22%, and LL chondrites have about 26% . This is related to how 
much metallic Fe is in the matrix. The lower the percent in the Minerals, 
the more "free" Fe in the matrix. That is why H chondrites show more 
metal flakes than either of the L, or LL's.
	The problems are many. I spent two hrs today trying to find an 
OPX grain sutiable for microprobe anaylisis. The grains were so small, I 
kept getting too close to other minerals, that led to inaccurate results. 
At 75.00 an hour, this can be an expensive search. It takes about 12 hrs 
to due a really good anaylisis. So as you can see there is no such thing 
as a free anaylisis. 
	Next I found that what had been written about Correo(H4) is not 
what I observed. I am using Correo as a model to compare the meteorites I 
am anaylyzing to. I find Correo to be of the petrologic type 5. The 
problem here is subjectivity. All meteorites are a mixture of all 
petrologic types. Which type a meteorite contains the most of is usually 
what it is given. Now I must decide to publish the change in type, or say 
"it is just the way my research went". 
	Well I hope that everyone gets an idea of the complexities of 
meteorite classification. It looks good on paper, however it is not good 
in reality. Nature has a good way of making you think long and hard about 
things. You have to exercise sound scientific judgement, and piece of 
mind when doing classification. In the end you are just making a guess, 
hopefully it is educated. 
	If you have questions let me know, and I will be happy to answer them.

Frank Stroik
The University Of Wyoming
Department of Geology
And Geophysics 

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