[meteorite-list] triolite inclusions

From: MexicoDoug_at_aol.com <MexicoDoug_at_meteoritecentral.com>
Date: Mon Jan 3 14:58:29 2005
Message-ID: <da.1c5049c9.2f0afdd2_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@meteoritecentral.com
Subject: Re: [meteorite-list] triolite inclusions
Date: Mon, 3 Jan 2005 11:51:26 EST
Received on Mon 03 Jan 2005 02:58:10 PM PST