[meteorite-list] "flow lines" on weathered irons (was "question on cleaning irons")

From: Piper R.W. Hollier <piper_at_meteoritecentral.com>
Date: Mon, 28 Sep 2009 09:47:08 +0200
Message-ID: <200909280749.n8S7nAqn046224_at_smtp-vbr14.xs4all.nl>

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
Received on Mon 28 Sep 2009 03:47:08 AM PDT