[meteorite-list] How to Polish Stoney Meteorites?

From: STARSANDSCOPES at aol.com <STARSANDSCOPES_at_meteoritecentral.com>
Date: Mon, 4 Aug 2008 15:03:40 EDT
Message-ID: <c6b.399abe30.35c8ac8c_at_aol.com>

Hi Pete,

I am frequently polishing samples for incident light microscope work. I
often examine meteorites at 1600X and the results are cool! This requires a
high polish. If you haven't seen my micrographs they are at
http://www.meteorite.com/meteorite-gallery/

The most frequent mistake is to polish to fast. Previous rougher grits leave
deep cracks and tend to pluck out crystal fragments. Each polish step must
be given time to cut through the micro damage caused by the last step. It may
look good with a loupe but get it on a scope at 400X+ and you will see. I use
a six step process with 5 diamond grit disks and finish with a 1/4 micron
Diamond slurry on a felt pad. This final step makes all the difference. You
will notice it on a scope and it also gives that high polish "wet look" that
really enhances a hand sample.

When I say "to fast" I am not talking about platter speed but rather the
time spent at the wheel. I bring music and sometimes a magazine (never a book
I don't want damaged). Even on small 20-40mm samples I may spent a half hour
on one of the finer grit levels. The rough grind takes only a few seconds
but the fine grit is time consuming.

I am sure you can get a beautiful hand sample finish with less time than
what is need for scope work.

The August 2006 Meteorite Magazine had an article (From the Strewn Fields)
by Martin Horejsi. He came out to my place and we put in many hours polishing
some of his historical meteorites. (He has an awesome collection!) He
wrote a cool article describing the process. I don't know if Meteorite articles
are available on line.

Tom

In a message dated 8/4/2008 12:22:26 P.M. Mountain Daylight Time,
rsvp321 at hotmail.com writes:



Greetings, fellow Meteoriteheads,

Here is a topic rarely discussed on the List, but one that I think many of
us could benefit from.
The only references I find on the web are for polishing irons.

I have been trying to mechanically polish some of my stones and I just can't
get the same glassy finish which I see on some of my purchased samples, or
even close to what I have done by hand with 1500 grit wet paper as the
finishing stage.

I am using four diamond polishing discs: 100, 600, 1600, and 3000 grit,
using distilled water for lubrication.

After I cut the stones open they are quite smooth, and I only use the 100 to
get rid of the small nub when it breaks off at the end, or the most visibly
non-level surfaces, if necessary.

I hold the stones on the remaining three discs for about five minutes each -
when it feels like there is no more abrasion taking place.
The finished product is a glassy-smooth surface to the touch, but without
the glassy-smooth, shiny appearance.
Under the microscope I can see very fine scratches from the discs, which I
don't see when I polish by hand.


A while ago, someone (I think our Dean Bessey) once made mention of a rule
of thumb for mechanical polishing as, "grind fast, polish slow". (I hope I'm
not recalling this backwards!)


My questions are; 1. How slow? I don't expect an RPM answer, but would it
be the same relative speed that the hand would be moving, if the (final) disc
was stationary?

2. Am I just being impatient, and it takes a lot more than five minutes at
each stage, even though there doesn't appear to be polishing taking place?
How long should each stage realistically take?

3. Is there another finishing step after the diamond discs?

4. Is there too much of a leap between the grits I have?

5. Is there such a thing as too much liquid lubrication which would reduce
the desired grinding effect from the discs?

For the sake of argument, let's use NWA 869 as the meteorite to polish.

Kind regards,
Pete






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Received on Mon 04 Aug 2008 03:03:40 PM PDT