[meteorite-list] Weathering Grades

From: meteorites_at_space.com <meteorites_at_meteoritecentral.com>
Date: Thu Apr 22 10:01:29 2004
Message-ID: <20020602075803.3480.h014.c000.wm_at_mail.space.com.criticalpath.net>

Robert Verish wrote

"Can you imagine a
stone with a "W6" interior having an exterior
exhibiting well-preserved, relict fusion crust?"

Yes, I think that I have not only imagined one, but
seen one-- in a recent fall no less--- Holbrook.

This meteorite offers an excellent opportunity for
those doing research into meteorite weathering.
Samples have been collected from the day of fall to the
present, and many of these have beautiful fusion crusts.

But when they are cut, the weathering of recent finds
becomes apparent (though some of these show little
weathering at all, and such are usually restricted to
pieces 1 gram or less).

In my searches of Holbrook, I have found pieces that
are near to pristine, and others just yards away from
those that are weathered to the point of being nearly
unrecognizable.

What I suspect with regards to weathering is that not
only do environmental factors play a part, but
composition of the meteorite matrix is something to be
considered.

With the fall of Monahans, a common chondrite it was
found that this meteorite contained salt crystals.

I suspect that salt is more common to chondritic
meteorites than was previously realized, and that its
presence in meteorites will greatly affect the
preservation of such after they fall to earth--
regardless of where they fall.

For as the salt leaches out, and in the presence of
oxygen and water it's corrosive action to FeNi will
eventually destroy the meteorite matrix.

Now, I suspect that the reason that the fusion crust
would for Holbrooks be so preserved is that the crust
is a glass, and if the matrix had salt this salt would
under high temperatures become a flux that would be
incorporated into that melt and react with metal and
and melted minerals. In essence it would therefore be
neutralized-- Hence, after decades of sitting on the
ground, the salty matrix would decompose but the fusion
crust remain bright and unusually fresh (as is the case
with many Holbrooks).

It would be interesting if a museum that has thousands
of Holbrook stones, such as the Museum of Natural
History, or the Smithsonian would take a random sample
of their small complete Holbrook stones, and examine
them for salt content.

I would not be surprised if salt is present in the
majority of those that were collected shortly after it
fell-- and that would partly explain why so little of
it if found today. >200 Kg (16,000 stones in 1912);
<30 Kg (several dozen stones between 1931 to 1947 by
Nininger); >1kg (several hundred small stones by me
from 1967 to presnent); and perhaps a similar amount by
the combined effort of others over that same period.

After the initial finds of 1912, I suspect that the
ever diminishing returns from Holbrook are due more to
weathering than collecting.

The study of recent falls, and the collection of
material from those falls in the years after, offer an
excellent opportunity for determining the onset of
weathering and how such progresses over a period of
time.

How long meteorites last on earth after fall is a
question that is "up in the air."

Steve Schoner
http://www.geocities.com/american_meteorite_survey

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Received on Sun 02 Jun 2002 10:58:00 AM PDT