[meteorite-list] thin section waste?-an explaination

From: E. L. Jones <jonee_at_meteoritecentral.com>
Date: Wed Jun 8 23:03:23 2005
Message-ID: <42A7B15C.6090803_at_epix.net>

Ok folks... a little knowledge is a dangerous thing- and I prefer a safe
list.

Thin sections: Why?

Thin sections are made for petrological studies-- and of course for we
Obsessive Compulsive Collectors just because they exist and no fancy
meteorite specimen is complete with out metal label and thin section to
go with the (ahem) --main mass. The way originally used--(and still
are) is for identifying the composition of individual grains of fine
grained specimens. Today, they are also used in conjunction with the
microprobe. Technology is slowly supplanting classic thin sections as a
research tool, but a majority of geological students are still required
to learn how to make thin sections.

In a very precise and laborious process , sections are ground
exceptionally thin and polished to microns of perfect flatness and small
"thinness" (e.g.<.03 mm) so that opaque grains are thin enough to allow
passage of light though the slide. Why go to all the effort?

Viewed through cross polarizing filters, minerals refract light unique
to their crystal structure--they give off different colors etc. By
determining those colors and "angles of extinction", a competent
operator using a polarizing microscope can determine the mineral ID for
different grains, etc. --Thus the composition of the sample. ( How do
you suppose they are able to report percentages of Fa, Fo, and etc. in
meteorites or identify individual chondrules?)

Slides are made "covered" or "uncovered". "Uncovered" requires extra
effort to avoid contamination but allows the slide to be used in
conjunction with a microprobe. Scientific specimens are made to a
specific size in order for one researcher to index specific grains and
allow another researcher to later find them.

As mentioned before, the "chip" must be ground to very narrow margins of
flatness. Both sides are ground. One is epoxied to a ground glass slide
and used to handle the sample as it is ground. On completion, the
remaining surface is epoxied to a glass slip to preserve the thin
sections from oxidation/ deterioration and prevent contamination.
Standardized production thickness, among other things, makes for
consistency with other thin sections , and to control--in a manner,
"birefringence" . In certain crystals, this is a vibration of light and
cross interference of colors. To a point, the thinner the section, the
better result under the scope.

Ok so what happens to the thickness (10mm+) of the original chip? (Well
it isn't returned to the owner unless it is in dust form). It is ground
away in manufacturing process. But you ask "Why start so thick if you
are just going to grind it away?" Well , in thin slices, mineral chips
can flex or, hidden cracks can give way. If this happens, the section
is ruined, other sections being ground may also be ruined, and one has
to stop the whole process and clean out the thin section grinder/lap.
When the entire chip is embedded in epoxy, it stabilizes the specimen.
Starting out thick and grinding slowly gently releases internal stresses
and in the long run makes for fewer disasters.

Any thin section for research --where possible, has a standardized area
of material surface for statistical analysis.- rare material can be
smaller but the researcher runs the chance of getting a
non-representative sampling of the material at hand. To use "less" is
the call of the researcher when the sample is especially rare. So be
carefull when lifting procedures for normal mineral studies to what goes
on with meteorites in thin sections.

(NOTE: for a view of minerals in thin section:
<http://sorrel.humboldt.edu/~jdl1/petrography.page.html#anchor748621>

Or how to make them for the techno buffs
<http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/thinsections.htm>)


EJ
Received on Wed 08 Jun 2005 11:02:52 PM PDT