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

From: Randy Korotev <korotev_at_meteoritecentral.com>
Date: Sun, 10 Aug 2008 15:30:42 -0500
Message-ID: <200808102029.m7AKTrn20386_at_levee.wustl.edu>

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
Received on Sun 10 Aug 2008 04:30:42 PM PDT