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

[Michael Murray]

If you don't mind my offering a possible answer to this part: what  
determines the structure from fine to course.    I would say it is the  
width of the kamacite bands.
Someone will probably correct me on that though.
Mike in CO
On Dec 15, 2010, at 11:31 AM, Arlene Schlazer wrote:

> Thank you Dr. Rubin for that explanation.  As a collector of mostly  
> iron meteorites, I've always been fascinated with the various types  
> of etch patterns.  My question is, how many years does it take to  
> cool per degree in the vacuum of space?   Secondly, what determines  
> the structure from fine to course.....is it just the nickel content  
> or does the cooling rate have anything to do with it?  Thanks in  
> advance.......Arlene
>
>
> ----- Original Message ----- From: "Alan Rubin" <aerubin at ucla.edu>
> To: <meteorite-list at meteoritecentral.com>
> Sent: Wednesday, December 15, 2010 9:54 AM
> 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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