[meteorite-list] How to Polish Stoney Meteorites?

[STARSANDSCOPES 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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