NWA 5764 LL6 Polymict breccia with L4 clasts
NWA 5764 was my first Meteoritical Society (Nomenclature Committee) approved meteorite.
And a special meteorite it turned out to be!
The unclassified 502g Acfer I bought at the 2003 Gifhorn Meteorite Fair (Germany), was given an NWA number due to the unavailability of find coordinates. NWA 5764 was classified as an LL6 Polymict breccia with L4 clasts.
For Meteoritical Bulletin details see: http://tin.er.usgs.gov/meteor/metbull.php?code=48957
Special thanks go to J. Gattacceca (CEREGE) and M. Denise (MNHN) for their classifying effort.
The photographs below show the stone after cleaning, and sanding and polishing of the fractured end in two plane surfaces.
At that point in time I had no plans to try to have the stone classified.
|The large contrasts
in the stone’s
interior, which became visible in the planed and polished surfaces,
made me curious
about its class.
A posting to Meteorite Central's mailing List, resulted in a contact with Jérôme Gattacceca (CEREGE), who was willing to perform a magnetic susceptibility measurement on the stone.
The result was a magnetic susceptibility logχ 3.97, indicating type LL in case of an ordinary chondrite.
For logχ graph see: logχ (c) Jérôme Gattacceca
Jérôme kindly offered to classify the stone, and cut a type specimen. This type specimen incorporated the dark inclusion marked by the blue dot in the photograph to the right.
As will become clear below, this turned out to be a very lucky cut...
the stone had become rather facetted, I decided to cut some slices, to
end up with a nice half stone, with only one cut face. This action
resulted in four slices, and gave us the opportunity to try to check
for more L4 clasts in the LL6 matrix.
At first the idea was that all of the dark clasts represented L4 material. This turned out not to be true though...
The slices with sample ID's #2 and #4 in adjacent photograph were used to perform magnetic susceptibility measurements on all dark clasts visible in the NWA5764 cuts.
The conclusion was that the dark clast visible in the type specimen was the only L4 clast in the slices.
The other dark clasts turned out to be dark LL material.
So "only" one multiple-cm sized L4 clast.
Truly a lucky cut!
As will become clear below though, indications for more L4 clasts were found...
The picture to the right shows a magnetic susceptibility map of
the largest slice (ID #2) with the L clast
(indicated red) in LL matrix.
The lower right is weathered LL material and thus less magnetic
Jérôme Gattacceca submitted an abstract of his findings for the MetSoc meeting of 2009 in Nancy (France):
A scan of the poster for his poster session, with much more detailed research information about NWA 5764, can be found in the following link (2.3Mb):
click to open in a new window
We decided to try to map the main
mass with a small magnetic susceptibility probe, to check for more
The technique is able to "see" about 1cm deep. The results showed two hot spots (most likely L clasts) at the end, opposite to the cut face, indicated with white stickers in the photograph to the left. Trying to expose them would mean, "mutilating" the main mass by cutting off the end, or completely slicing up the mass (with the possibility of finding even more L material).
After judging scientific relevance of exposing more L type material, we decided to leave the main mass intact for now.