[meteorite-list] A question about an iron fleck in NWA 2977 Lunar

[Randy Korotev]

Dear Tom:

All brecciated lunar meteorites contain some FeNi metal (<<1%), but 
you may have to look hard in some.  (In others, like NWA 5000, you 
don't have to look hard at all.)  The metal derives from impacts of 
asteroidal meteorites with the Moon.  If the meteorite is an 
impact-melt breccia, the metal probably melted and resolidified on 
the Moon.  Regolith breccias, on the other hand, may contain FeNi 
metal that hasn't been highly reprocessed.

http://meteorites.wustl.edu/lunar/stones/nwa0773.htm

NWA 2977, however, isn't a breccia.  It's an igneous rock (a cumulate 
olivine gabbro), if your sample is like mine.  Lunar igneous rocks 
contain very small amounts of metal, but the metal is indigenous to 
the Moon and doesn't have the composition of meteoritic metal.  I see 
that one report on NWA 733 (almost-for-sure a pair to NWA 2977) did 
mention "grains of Fe,Ni metal also occur in residual pockets but are 
rare."  Another says "Metal grains occur in very small masses with 
troilite and are Ni-rich (55.5 wt.% Ni, 40.9% Fe, 1.5% Co, 0.03% 
P)."  That composition isn't meteoritic (in meteorites, the Ni/Co 
ratio is nearly always in the 10-24 range).

When you say "The thin [section] is polished to 1/4 micron," do you 
mean the section is only 1/4 micron thick (amazing!) or the final 
polish was done with 1/4 micron abrasive?  In a standard thin section 
(30-35 microns), metal is totally opaque, so I don't see how it shows 
up in polarized light (?)  How does it look in reflected light?

Sincerely,
Randy Korotev




At 17:41 09-08-08, you wrote:
>Hi list,  I had a question about an iron  fleck I found in a thin section of
>NWA 2977 Lunar.  Jim Strope sent it to  me.
>
>I plan to use this as next months Meteorite Times Micro Vision and  want to
>be accurate.
>
>The thin is polished to 1/4 micron.  This  sometimes has the same effect as
>etching but on a much finer scale.  I have  observed it in other 
>materials that
>get this kind of polish.
>
>There is a  fleck of iron in this material.  In this fleck is what looks like
>micro  Widmanstatten pattern.
>
>Can this pattern be called  Widmanstatten?  If not, are the creation
>processes the same as with full  sized Widmanstatten?  How would it 
>be still present
>in a lunar?  Could  the pattern survive a meteor collision with the moon and
>not be heated to the  point of destruction?
>
>I would like to email micrographs to any one who is  interested or, even
>better, might have the answers.
>
>The images are taken  in incident cross polarized light and I am using a
>Glan/Thompson style polarizer  that allows me near total 
>extinction.  I pull up
>the changes in the pattern  by slight rotation of the polarizer.  The
>magnification of these images is  1600X.
>
>Thanks,  Tom Phillips