..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@meteoritecentral.com
Subject: Re: Classifying Meteorites and the Inherent Problems
From: [EMAIL PROTECTED]
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
