Dear Zelimir:
Schreibersite (an iron-nickel phosphide) occurs
in iron meteorites and it also occurs in some
lunar impact-melt breccias, particularly those
from Apollo 16. There is evidence that a subset
of the Apollo 16 impact-melt breccias was formed
by impact of an iron meteorite (the breccias
contain 0.5-2.0% metal, which is quite a lot), so
it is no surprise that the breccias contain
schreibersite associated with metal blebs.
I should emphasize that the metal was melted and
part of the mainly-silicate impact melt. The
metal resolidified as tiny blebs as the melt
cooled. So, the breccias do not actually contain
"fragments" of iron meteorite. The silicate melt
was moderately rich in phosphorous, so it's been
argued that most of the P in lunar schreibersite
is from the Moon whereas the Fe and Ni are from
the impactor. Schreibersite can only form under
reducing conditions, like on the Moon.
NWA 5000 is the only lunar meteorite of which I'm
aware for which schreibersite has been reported
(A. Irving and S. Kuehner, in MetBull writeup):
http://tin.er.usgs.gov/meteor/metbull.php?sea=nwa+5000&sfor=names&ants=&falls=&stype=contains&lrec=50&map=ge&browse=&country=All&srt=name&categ=All&mblist=All&phot=&snew=0&pnt=no&code=45986
That's no surprise because NWA 5000 is loaded
with FeNi metal of meteoritic origin (that is,
ASTEROIDAL meteoritic origin). Another lunar
meteorite where I might expect schreibersite is
NWA 4936 because that meteorite is a (and the
only) compositional match to Apollo 16 soil,
including high concentrations of siderophile
(metal-loving) elements like Ni, Ir, and Au. I
suspect the meteorite comes from near the Apollo
16 site and, therefore, likely contains a small
component of the same iron meteorite that is
found in Apollo 16 rocks and soil.
The NWA 773 clan of meteorites is one of the last
places I'd expect schreibersite because the
breccia portions that we've analyzed are low in
Ir and, we infer, low in metal. But, FeNi metal
has been observed in NWA 773, so I can imagine
that if one looks hard enough, there may be some schreibersite.
As it turns out, I was unaware of NWA 3186 until
your message. I checked with Ted Bunch, who
classified the stone and submitted the writeup to
the Nomenclature Committee (not A. Greshake). I
just added the stone to my lunar meteorite list,
along with a great photo taken by Stefan Ralew
and Martin Altmann (I hope they don't mind!).
http://meteorites.wustl.edu/lunar/stones/nwa0773.htm
I don't know whether schreibersite in lunar
meteorites differs in composition from schreibersite in OC's.
Randy Korotev
At 16:26 12-08-08 Tuesday, you wrote:
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 <[EMAIL PROTECTED]> a écrit :
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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