[meteorite-list] nwa869

From: almitt <almitt_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:05 2004
Message-ID: <3D28DB83.5F906B4E_at_kconline.com>

Hi John,

Probably in the past when ordinary chondrites were classified by oxidized iron to iron
metal percentages there was no doubt room for more error. Today using the microprobe
for chemical analysis, scientists are looking at about ten different items and trace
chemical signatures to distinguish the minute variation of the common chondrites. To
further help this out and isolate it more, the shock metamorphism is also taken into
consideration. Although no process is perfect I think there is a pretty good handle on
being able to distinguish individual falls these days using the processes stated. Also
weathering comes into play on these.

With that said, I would think the bigger problem on a fall coming out of the Sahara
desert is whether some nomad decided to throw (or perhaps the rocks just get mixed up
in camel transit) into the lot and so we have a mixture of different falls stated to
be from one fall. As with meteorite dealers it is a matter of trust on whom you are
dealing with and if they are being honest with you or not. This isn't meant to be a
grind to those going over there an collecting these items.

I don't think it is always a good idea to go by looks on meteorites (I have many in my
collection that are similar in appearance but from discrete falls and finds) and until
a detail analysis is done then it is really hard to say. As a rule with time when
these meteorites are looked at they may refine the classification. With the break up
of Hebe and back into a rubble pile we sample a variation of classes in a single
chondrite sometimes. I trust the researchers to be able to distinguish the bulk matrix
of these finds and produce a accurate classification. It is to their own advantage
when they do research on these to know the accuracy of classification on the
meteorites they are dealing with so they don't go the wrong direction on understanding
them. All my best!

--AL

John Divelbiss wrote:

I just looked at the group I have and quite frankly it is not easy to say
they are different materials. Even under the scope. Yet I see the reports of
L4 or L5 for NWA 869, L6 for 787, now L3.4/4 for 900, and similar
suspicions for 995 (not 905 that I listed in earlier message). Thin sections
of each one may help...but as Dean has stated, his section may have thrown
off the evaluation of his sample relative to its true petrologic type.

All this makes me wonder about the process and "accuracy" of identifying a
particular fall or find. As I understand it, the boundaries between H's and
L's can be somewhat blurred when metal and iron contents are in the
transition percentage levels. All the more bewildering.
Received on Sun 07 Jul 2002 08:23:31 PM PDT