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

[Alan Rubin]

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 700ºC.
(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 <400ºC 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