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

[Dennis Miller]

It was good to finally hear how Widmanstatten, Taenite and Moreonionsatleast
are correctly pronounced. I'm sure it takes a liter of scotch to get it right!
Dennis



> From: aerubin at ucla.edu
> To: meteorite-list at meteoritecentral.com
> Date: Wed, 15 Dec 2010 09:54:14 -0800
> Subject: [meteorite-list] iron meteorite cooling rates and Meteorite Men
> 
> 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
> 
> 
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