[meteorite-list] "Internal pressure that can lift the Widmanstatten plates, " ie. Re: "Sharp Protrusion"

From: Peter Richards <pedrichards_at_meteoritecentral.com>
Date: Fri, 23 Aug 2013 13:14:30 -0500
Message-ID: <CAENymwBDDsjHnLJxiKqhF6sRx7iC+GSazEOV2ZYsD6-ws+cWmQ_at_mail.gmail.com>

I never saw any solution to our dear friend, the estimable Randy
Korotev's question posted publicly to the list some time ago
(Meteorite-list Digest, Vol 117, Issue 55 Wednesday February 27,
2013), so apparently those who realize what was going on withheld that
information for some reason (and of course we were distracted then,
somehow).
Here is the solution (imo):
>From O. Richard Norton's "The Cambridge Encyclopedia of Meteorites,"
(Cambridge University Press of the U.K. in [March 11, 2002]) on p. 62:
"Cl, now understood to be from the terrestrial environment, can
seep into cracks in the meteorite causing a reaction. "another
consequence of these reactions is that space is consumed by the
forming minerals causing an internal pressure than can lift the
Widmanstatten plates above the level of the slab, or even completely
out of the specimen!" The details of the reactions are reported on
page 62 and 63 thusly: "along the low iron-nickel plate
boundaries...Kamacite is converted directly to akagan?ite. If the
corroding metal experiences moderate warming and humidity changes
through the year, the OH(-) ion can exchange with the Cl(-) ion
releasing the Cl to migrate to other sites or it could be flushed out
of the meteorite. If this occurs the mineral becomes unstable and
decomposes converting to goethite and maghemite (a hematite-like
material)."

Re:

Wed Feb 27 17:41:26 EST 2013:
"I received a well prepared letter from a fellow with a question that

I can't begin to answer. Maybe someone on the list has seen this
kind of thing before.

He bought a Baygorria (Iron, IAB complex) from a dealer 3 years ago.
He picked it up recently to find a metal protrusion sticking out of
the thing that was sharp enough to prick his thumb.

Here's a jpg of his scanned photo.

http://meteorites.wustl.edu/baygorria.jpg

What's happened here?

Randy Korotev
St. Louis "
Received on Fri 23 Aug 2013 02:14:30 PM PDT