[meteorite-list] Taenite vs. Kamacite... was RFS Picture of the Day

From: Mr EMan <mstreman53_at_meteoritecentral.com>
Date: Sun, 21 Jan 2007 12:47:50 -0800 (PST)
Message-ID: <655872.82978.qm_at_web51009.mail.yahoo.com>

Greetings Bernd, Mike, List:

--- tett <tett at rogers.com> wrote:
> How can you tell the difference between kamacite and
> taenite? Does the crystal structure give it away?

So far as I can remember...

Taenite (79.19 % Fe 20.81 % Ni) aka gamma-iron
  and
kamacite (89.54 % Fe 10.46 % Ni)aka alpha-iron,

are the two iron-nickel minerals responsible for
Widmanstaaten patterns. While they differ in molecular
composition they both have an isometric-hexoctrohedral
crystal lattice.

It is the packing of the nickel within the lattices
which make the distinction. Face loading vs Center
loading (which I shan't go into the whys here-check
the archives) makes more spaces to put nickel atoms
into a molecule. Nickel is more resistant to the acid
etching/weathering.

Therefore, the taenite lattice will stick out from the
surface in higher relief than the kamacite lattice,
and that is the only way to "visually" distinguish
them from each other in normal nickle-iron faces.


Elton
Received on Sun 21 Jan 2007 03:47:50 PM PST