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

[E. L. Jones]

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