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

[Piper R.W. Hollier]

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