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" <aeru...@ucla.edu>
To: <meteorite-list@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: aeru...@ucla.edu
website: http://cosmochemists.igpp.ucla.edu/Rubin.html
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