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

From: Dennis Miller <astroroks_at_meteoritecentral.com>
Date: Wed, 15 Dec 2010 14:34:19 -0600
Message-ID: <BAY152-w47A6352F0C032E75CFEB14B1140_at_phx.gbl>

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 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
>
>
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Received on Wed 15 Dec 2010 03:34:19 PM PST