[meteorite-list] triolite inclusions

[MexicoDoug at aol.com]

Hola John,
 
Nice observations, though you have missed one obvious complicating fact  
among the many others.  I hope you didn't get your suspect idea from me  that an 
squeaky clean ocean of Fe-Ni alloy with Fe(II)S merrily had formed  immiscible 
spheres of troilite driven by surface tension and other important  forces like 
electrostatic interactions.  Although in the end that is what I  would wager 
exactly happened, my "simple" argument was just to give a practical  picture 
to what is probably happening here, though behind the scenes a melting  point 
at STP is like doing particle physics with stone tools.
 
The major oversight I respectfully feel you have made is that you have  
failed to consider the effect of pressure.  Remembering that pressure is  enormous 
in the core due to the planetesimal mass capable of differientiation,  you 
really can't quote meaningfully the melting temperatures you have and argue  
anything at all.  In a perfect world studied by scientists you would need a  
multidimensional phase-compositional diagram (Equation of State for the mix)  with 
enough interaction parameters functions to describe all the eutectic  points, 
alloys, even azeotropes, melting point depressions and all of the other  fun 
stuff from Physical Chemistry / Geology 201.  Only then could any such  
statements be made (or alternately, work out the quantum mechanics from first  
principles:) using some pretty miraculous Monte Carlo simulations or such.
 
Here are two papers that cover some beavior to give us an inkling of the  
sorts of things to suspect, one with a phase diagram of FeS and another where  
Caltech folks are still arguing over what the melting point of iron and its  
association formed are.  They also scratch the surface to start getting a  handle 
on the drivers of Steve's and your questions.
 
_http://www.aps.anl.gov/xfd/communicator/user2000/kavnera1.pdf_ 
(http://www.aps.anl.gov/xfd/communicator/user2000/kavnera1.pdf) 
 
_http://www.gps.caltech.edu/~sue/TJA_LindhurstLabWebsite/ListPublications/Pape
rs_pdf/Seismo_2069.pdf_ 
(http://www.gps.caltech.edu/~sue/TJA_LindhurstLabWebsite/ListPublications/Papers_pdf/Seismo_2069.pdf) 

Now throw in alloys to the mix (pun intended) and a host of other  
interacting fluids and solids and plasmas big and small and then we  can go to Princeton 
or Bern and work on a nobel prize, or watch the world go by  on the meteorite 
list and see that it is easy to dream up a scenario from the  two relevant 
phase diagrams I mention one from each paper in which at say 35 GPa  we are in 
the 1,927 degrees C range for FeS, and that iron might be there too  depending 
on which esteemed author you ask, or it might not.  I am sure all  would agree 
that with enough interacting components in the soup, that a sea  might not be 
a bad choice and miscibility arguments might actually be rather  reasonable, 
even if diffusion does make an important part of the story.   The core 
remember has all kinds of physical stresses happening, maybe a lot like  the ones in 
rivers that could even lead to blobby rocks, in addition.  Then  after we hit 
our head against the wall for a nobel prize in our own lifetime by  the next 
Einstein we realize we need to consider that alloy even compositions  are 
pressure and temperature dependent, and what you see then is not what you  get now 
until equilibrium is reached.
 
Better the fourth law of thermodynamics - a big mess tends to pick out its  
own melting temperature against the wishes of its components since no one is  
beyond interaction.  And whatever precipitates out first might just fall  down 
or even float up to make a golden crust.  My foolish humor aside,  combining 
all of the above with a miracle of the universe which would have it no  other 
way, some iron meteorites will certainly exhibit this FeS structure, and  
others where the sea was not optimum will not ... and those experimental  results 
were just cooked to agree with the hypothesis because the real  experiment is 
way too hard.
 
I wanted to say somewhere that while density may play a role sometimes in  
separation, not always.  For example at these temps and pressures, a simple  
test which in practice is difficult as heck, troilite or its precursor fluid may  
be quite miscible with Fe or Iron-Nickel right down to the freezing point 
when  separation happens.  That would be an easy thing to check on and would  
clear up some questions I have on this.  And about the filler argument you  
mention, at these pressures, it is not likely in my opinion that you could  squeeze 
much around in a core of formerly liquid metal under humongamountainous  
pressure, even as it freezes, the creater knows where any unfortunate soluble  gas 
would go as this thing froze, but I don't see many caverns forming,  either.
 
I hope that is a little food for thought for these deceptively easy  
questions that really have very difficult answered....I don't think anyone has  
actually watch this happen so I'm pretty satisfied with these ideas but  very open 
as well to new ones:)
Saludos, Doug
 
 
 

En un mensaje con fecha 01/03/2005 11:55:07 AM Mexico Standard Time,  
jk_unlimited at hotmail.com escribe:
Hi all,

A quick question regarding  rounded troilite inclusions in iron meteorites...

I believe FeS has a  significantly lower melting temperature (around 1000 
degrees C) than the  Fe-Ni alloy (around 1450 degrees C) that make most iron 
meteorites. In a  cooling planetismal, wouldn't one expect that troilite 
would be the last  dregs of molten liquid remaining in the cracks between 
crystallized Fe-Ni?  If that were the case, wouldn't troilite be expected to 
be a 'filler', with  an elongated morphology? So, why does troilite occur in 
rounded  inclusions?

Perhaps rounding from grain boundary diffusion occurs on a  long time-scale 
or the blebs are an indication of late stage impact melting  and rapid 
cooling... I'm not sure that I buy the surface tension idea where  troilite 
separates out from an ocean of liquid Fe-Ni  alloy.

Thanks,
John

From: MexicoDoug at aol.com
To:  steve_arnol60120 at yahoo.com, meteorite-list at meteoritecentral.com
Subject: Re:  [meteorite-list] triolite inclusions
Date: Mon, 3 Jan 2005 11:51:26  EST