[meteorite-list] iron meteorite cooling rates and Meteorite Men

From: Alan Rubin <aerubin_at_meteoritecentral.com>
Date: Wed, 15 Dec 2010 09:54:14 -0800
Message-ID: <D606A5ED835C49E18CC75DE451806AA9_at_igpp.ucla.edu>

On last night's Meteorite Men show, the narrator was attempting to explain
that the Widmanstatten pattern is caused by kamacite and taenite cooling at
different rates. This is incorrect. How could two intergrown metal grains
buried deep inside a core cool at different rates? The Widmanstatten
pattern forms in the following manner:
(1) At high temperatures (but below the solidus), metallic Fe-Ni exists as a
single phase -- taenite. (2) As the metal cools, it eventually reaches the
two-phase field (or solvus) on the phase diagram. For metal containing 90%
iron and 10% nickel, it reaches this boundary when temperatures cool to
about 700?C.
(3) At this point, small kamacite grains nucleate inside the taenite. With
continued cooling, the kamacite grains grow larger at the expense of
taenite, but both phases become richer in nickel. This is possible because
the low-Ni phase (kamacite) is becoming increasingly abundant.
(4) At low temperatures, say <400?C or so, diffusion becomes so sluggish
that the reaction essentially stops.
These meteorites are called octohedrites because solids have
three-dimensional structures and the kamacite planes are oriented with
respect to each other in the same way as the faces of a regular octahedron.


Alan Rubin
Institute of Geophysics and Planetary Physics
University of California
3845 Slichter Hall
603 Charles Young Dr. E
Los Angeles, CA 90095-1567
phone: 310-825-3202
e-mail: aerubin at ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
Received on Wed 15 Dec 2010 12:54:14 PM PST