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

[Count Deiro]

Dr. Rubin said " ...the narrator was attempting to explain that the Widmanstatten pattern is caused by..."

And, of course, Dr, Rubin's very succinct correcting explanation is, by the standards of today's media, three paragraphs too long. 

The time and space requirements of electronic and print media prevent intelligible descriptions of scientific processes. How many times have we seen, or heard, ridiculous and missleading accounts made by reporters and pundits based on their refusal to use sweat equity to get some facts straight? I found this only to true whilst trying to describe the physical reasons for an airplane to have had a stall/spin crash to a reporter for a local television station. The story came out that the plane's "motor" had stalled and stopped the plane in midair causing it to fall to the ground. I had said nothing of the kind.    

There was a time when major media employed experts in the sciences, so that what was published had some veracity. Now, restraints in time and money and a "what the hell...this stuff is too complicated attitude." leave the public in their ignorance.

Happy Holidays to all...and thank you Dr. Rubin.

Count Deiro
IMCA 3536     

-----Original Message-----
>From: Alan Rubin <aerubin at ucla.edu>
>Sent: Dec 15, 2010 9:54 AM
>To: meteorite-list at meteoritecentral.com
>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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