[meteorite-list] What is and isn't a Widmanstatten Pattern was Cooling rates

From: Jeff Grossman <jgrossman_at_meteoritecentral.com>
Date: Mon, 07 Sep 2009 19:58:59 -0400
Message-ID: <4AA59E43.5090109_at_usgs.gov>

I think there are some misconceptions here, although this is not my
specialty. Most of the metallic minerals in iron meteorites are
described as "alloys" in that they are composed of various metals
combined together. These alloys have specific structures, e.g., the
metal atoms in kamacite are arranged in a body-centered cubic structure
and those in taenite are face-centered cubic. The minerals Kamacite and
taenite are solid solutions of mainly Fe and Ni which can have a range
of compositions without altering the basic structure. Tetrataenite is
another alloy, but this time with a fixed composition (FeNi) and an
ordered structure.

Formation of the Widmanstatten structure is pretty well understood. It
does not happen as the metal cools from the liquid state and
solidifies. That process leads to the formation of just taenite. Only
when the alloy cools to much lower temperature, after it is completely
solid, can the Widmanstatten pattern form. High pressures are not
involved. Goldstein and coworkers have shown that the process is
controlled by the Fe-Ni-P phase diagram. Depending on the exact
composition of the alloy, a variety of phase transformations take place
over a range of temperatures, ultimately leading to the formation of
kamacite and taenite. Composition and cooling rate play roles in
determining in the structures we now observe. You can read about it in:
http://articles.adsabs.harvard.edu/full/2005M%26PS...40..239Y

jeff

Mr EMan wrote:
> We had a metallurgist on the list a few years back that insisted Widmanstatten patterns were found everywhere and posted some micrographs supporting his assertion. As I recall he got very ill with us when we pointed out why, what he had photos of, weren't Widmanstatten patterns. It was focused on a physical "casual" similarity not "causal" chemistry.
>
> Once again Widmanstatten patterns aren't stress fractures nor alloy specific patterns. I further assert that metal in meteorites is NOT an alloy in that the nickel is in a specific locus within a molecule. It is therefore not a mixture but a compound, chemically speaking.
>
> Widmanstatten patterns are a cross-sectional view of crystal latices that result from the migration of nickel atoms over eons into two distinct unusual, zoned, crystalline arrangements. Bandwidth is actually plate thickness. The migration is chemically driven while the metal is molten and only occurs in a specific range of temperatures. This is a subtle but distinct difference. This migration may even be a molecule by molecule transfer of nickel atoms which takes millions of years to clear out a 3mm band. This is to say a nickel atom may move in one side of a molecule and forces the central nickel atom to the face and lacking stability is ejected out the other side--maybe not, as the actual displacement/sorting is still an enigma. The nickel iron content may assemble from a single form as it accretes and represent a move to homogeneity interupted when the mass ran out of thermal energy. It may all start out as taenite and part of it converts to
> kamacite or vice versa. Who really knows?
>
> I fully believe collisions would impede if not stop the process-- not speed it up. It is easy and natural to try to infer a similar pattern might be from a similar process but the only similarity is in low contrast photographs when the scale is ignored.
>
> Elton
>
> --- On Sun, 9/6/09, E.P. Grondine <epgrondine at yahoo.com> wrote:
>
>
>> From: E.P. Grondine <epgrondine at yahoo.com>
>> Subject: Re: [meteorite-list] Cooling rates
>> To: meteorite-list at meteoritecentral.com, "Steve Dunklee" <sdunklee72520 at yahoo.com>
>> Date: Sunday, September 6, 2009, 1:47 PM
>> Hi Steve, all -
>>
>> I don't think they're due to repeated collisions.
>>
>> Suppose that we have molten iron/nickle under incredible
>> compression, which is then almost instantaneously released.
>>
>
>
>> 250 parent bodies seems like a lot. Perhaps instead there
>> was more differentiation within fewer parent bodies.
>>
>> Ed
>>
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-- 
Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA

Received on Mon 07 Sep 2009 07:58:59 PM PDT