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

From: Zelimir.Gabelica at uha.fr <Zelimir.Gabelica_at_meteoritecentral.com>
Date: Tue, 12 Aug 2008 23:26:00 +0200
Message-ID: <20080812232600.45yfm9vnh7kk80ow_at_w3mail.univ-mulhouse.fr>

Dear Randy & list,

Randy, I fully appreciated your very explicit statement regarding
metallic phases in lunars.
This is actually important and fundamental regarding the orgin of
metal or the history of lunar meteorites (and other) and I am
surprised of the lack of reactions.

I have here a comment and a couple of questions

Comment:

I recently got (from the team Altmann/Ralew - Chladni'd Heirs) 2 small
pieces of NWA 3186, still not officially approved by the NomCom but
said to be a lunar olivine gabbro (classified by Greshake in Berlin)
and suspected (at least by me but also by others) to be probably
paired with NWA 773, 2727, 3333 and possibly the others ofthe series.
The texture (from the sections cut and polished on my 0.753 g and
1.132 g fragments) fully resembles the pics shown on your link.

Well I carefully examined these sections under a simple 40x
magnification (binocular with light oriented so as to have metallic
reflexions favored). On the largest section (1.132 g piece) I clearly
detected an about 0.3 mm Schreibersite area, possibly along with a
couple of 10 times smaller such spots).
This schreibersite (identified visually, thus through its special
color/shade, by comparison with the same mineral observed on many
other meteorite sections) is observed on the black breccia surface
portion (about 60%, the other 40% being almost pure olivine as in the
paired NWA's).
I neither observed more schreibersite on the other piece, nor on my
0.277 g NWA 2977 slice (pure "olivine"-like texture), nor on any other
of my other 14 lunar samples.

On examining all my lunar samples, just DAG 400 (lunar anorthosic
breccia) clearly showed about 30-35 very tiny metallic spots
(contamination totally excluded), that have the usual typical shade of
the Fe,Ni metallic spots in most meteorites (steel-gray).

2 small questions:

- Did you ever observe screibersite domains on lunar meteorites (won't
be a surprise if schreibersite originates from the impactor) and, if
so, would the (Fe,NI,Co) phosphide be also be richer in Ni than, say,
schreibersite found on other oc's ?

- Speculating that you for sure well know that NWA 3186, could you
confirm it is also be paired with the others mentioned above ?

Thanks much for any comment or answer.

Best wishes to all,

Zelimir


Randy Korotev <korotev at wustl.edu> a ??crit? :

> 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
>
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Received on Tue 12 Aug 2008 05:26:00 PM PDT