[meteorite-list] Could it be.... Frank's Article Dated 20 Mar 1997

From: bernd.pauli at paulinet.de <bernd.pauli_at_meteoritecentral.com>
Date: 24 Jan 2007 21:02:23 UT
Message-ID: <DIIE.000000BA00001514_at_paulinet.de>

..should be of interest to all of us, especially to those who are into
thin sections (under crossed polars) and classification of meteorites!

Cheers,

Bernd


To: meteorite-list at meteoritecentral.com
Subject: Re: Classifying Meteorites and the Inherent Problems
From: fes at UWYO.EDU
Date: Thu, 20 Mar 1997 20:56:10 -0700 (MST)

Classifying Meteorites and the Inherent Problems

I wanted to take some time out to tell everyone about what is involved in new meteorite
analysis 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 judgment to get too involved. To analyze 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 analyze with accuracy.
The first step is to look at the general texture of the meteorite under normal light in the micro-
scope. 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 differ-
ently 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 basicly the solid solution
of enstatite, bronzite, hypersthene, and ferrosilite. A solid solution is a solution that can have
different chemical compositions depending on precentage of elements that are available.
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 ferrosilite. However it is rare that such a thing happens in nature, so bronzite and hypersthene are
basicly different compositions 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
analysis. This is done by turning the microscope stage (the place where the thin section rests) until
the mineral is completely black. If it turns and goes black it is what you are looking for. If it must be
turned 90 degrees 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, degenerated,
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 a 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 analysis. 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 LLs. The problems are many. I
spent two hours today trying to find an OPX grain suitable for microprobe analysis. 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 do a really good analysis. So
as you can see there is no such thing as a free analysis. 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
analyzing 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
Received on Wed 24 Jan 2007 04:02:23 PM PST