From: Jason Utas <meteorite...@gmail.com>
Sent: Sep 28, 2009 4:45 AM
To: Meteorite-list <meteorite-list@meteoritecentral.com>
Subject: Re: [meteorite-list] "flow lines" on weathered irons (was
"question on cleaning irons")
Hello Piper,
Of course - hence the differential weathering rates of Campos ("old"
versus "new"), to name one of many examples.
Perhaps the best example of such weathering can be seen on irons from
Gibeon. I unfortunately don't have a copy of Buchwald here, but if
anyone does have access to the second volume, if they could flip
through the Gibeon section, they would find a photograph of a
beautiful mass of Gibeon (I forget the name of the mass) on display in
a museum in Germany. It displays beautiful fusion crust and
smooth-edged, shallow regmaglypts - it looks as fresh as many Sikhotes
on the market today. Compare it to many of the larger Gibeons on ebay
today and you'll see little-to-no resemblance. If anyone out there
can scan a picture of said page, I'd be much obliged. It really is a
good example.
There are, however, a few common irons which I would never expect to
have fusion crust: Canyon Diablo, Toluca, Odessa, and Nantan, to name
a few. I've seen hundreds, if not thousands of examples of each, and
I have never seen a single one of any of them that came close to being
"fresh" enough to retain a trace of fusion crust.
Nantan is one of the most corroded and least stable iron meteorites I
have ever known, though Dronino's turning out to be about as bad.
People need to learn more in order to clear up the misconception that
all meteorites show signs of a hot, violent entry through the
atmosphere; I see NWA's on ebay all the time that are nothing but old
weathered fragments coated with desert varnish. Check out this
seller:
http://myworld.ebay.com/eegooblago/
Almost all of his stones are covered in a 'glossy fusion crust.' Oh
wait - those are just desert varnished fragments that have been
weathered to hell. Most of the melt features the seller notes are due
to sandblasting and corrosion, and s/he goes so far as to say that the
cracks in his stones formed when they hit the ground! Anyone remotely
familiar with meteorites and weathering processes knows that over
thousands of years, meteorites fracture and break apart, in a manner
completely unrelated to their having impacted the Earth.
This seems like a very similar misconception; Guido even notes finding
flow lines on the inside of the meteorite, having broken it open.
There's no way there would have been any flow lines on the surface of
the iron, never mind the inside of it. It simply isn't possible.
Regards,
Jason
On Mon, Sep 28, 2009 at 12:47 AM, Piper R.W. Hollier <pi...@xs4all.nl>
wrote:
Hi Guido, Jason, Mike, and list,
At 22:33 27-09-09, Jason wrote:
Regardless of how well you cleaned your Nantan, whatever you found
under the surface was not flow lines.
It appears that the layers of taenite and kamacite do not always
oxidize at
the same rate at the surface of a buried iron. This would make sense
intuitively, since the proportion of nickel is different. Just as
nitol has
a differential effect on taenite and kamacite in the lab, some
conditions of
soil chemistry might produce an analogous result in the strewn field,
albeit
much more slowly. What is sometimes left after a long period of
weathering
is a pattern of parallel grooves on the outer surface that might be
(mis)interpreted as flow lines.
This is an effect that I first noticed on a thick slice of Toluca from
Alain
Carion's collection that was on display at a wonderful exhibition at
the
Ecole des Mines in Paris in 1998. The correspondence between the
shallow
ridges on the oxidized edge of the slice and the Widmanstaetten
pattern of
the cut surface was rather obvious.
There might be something about the specific soil chemistry at the site
that
could make this effect more pronounced at some localities (e.g. Nantan
or
Toluca) by enhancing the difference in oxidation rate.
Piper
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