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

[Jeff Grossman]

I'd like to correct what I said two weeks back about alloys.  I've 
talked with a specialist (Joe Goldstein) and he clarified the 
terminology for me as material scientists would use it.

An "alloy" refers to a batch of metal containing more than one element 
(only one of which needs to be a metallic element).  The term carries no 
implications about how the mixture formed or what its structure is. 

The term "alloy" should not be used to refer to a specific phase that 
crystallizes from an alloy or is present in the alloy.  Taenite and 
kamacite are not alloys.  They are phases (minerals). Therefore, 
hundreds of websites (including some professional ones) use the term 
incorrectly.  An iron meteorite IS an alloy, even if it only contains 
one phase like taenite. Alloys can and often are composed of crystalline 
phases.

Jeff

Mr EMan wrote:
> --- On Mon, 9/7/09, Jeff Grossman <jgrossman at usgs.gov> wrote:
>  <<Most of the metallic minerals in iron meteorites are described as
> "alloys" in that they are composed of various metals combined together.>> 
>
> I understood the distinction was that the Fe Ni formed a "chemical compound" not merely a mixture like copper and tin to make brass but even brass can form crystalline plates so that may be a bad example. It was my understanding that were it not for the mineral structure The Fe-Ni would be called an alloy.  Agreed that it is frequently discussed in terms of alloy.
>
>  <<It does not happen as the metal cools from the liquid state and solidifies.>>
>
> I stand corrected, 30 years is a lot of facts to keep in just one's head. I did recall correctly that there is a temperature range and below which all translocation stops. Seems off the top of my head it is 800°C.
>
> <<Goldstein and coworkers have shown that the process is controlled by the Fe-Ni-P phase diagram>>. 
>
> I have long suspected that phosphorus was a key component in the process--likely as a catalyst.  The Schreibersite seems to exist largely at the boundaries in thin laminae even surrounding trolite nodules.  I look forward to reading Goldstein's paper.
>
> Thanks again,
> Elton
>
>   


-- 
Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA