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Doug,
Great explaination ! It's been a long time
since Chemistry class -- and we never got that deep. I guess I can turn
off the melting pot and put that 10 lb Campo back on the shelf.
One question, though. You stated "since
water (0.92 g/mL) in equilibrium at 0ÂC is denser than ice (1.00 g/mL)."
Did you mean that the other way? Water at
1.00 g/mL ?
Thanks
Fly Hill
----- Original Message -----
Sent: Thursday, December 11, 2003 10:43
PM
Subject: Re: [meteorite-list] Is there
room for a meteorite question ?
And for a more complete stab at answering your question
from a proud "newbee". Note you really asked two questions.
1. If upon cooling there is expansion and 2. If the density is the
same as non-crystalline materials formed on earth in the same ally
proportions.
To question 1: If I understand your question properly at
first reading you think that Fe-Ni mixtures upon cooling in the proper
proportions forming taenite / kamacite might expand? No way. Those
structures do not form until after the alloy is already solid, though in an
amorphous (non-crystalline) state with selective atom mobility. Those
migrations of atoms occur to release heat energy flowing out of the system
because of decreasing boundary temperatures and the law of heat flowing from
higher to colder temperatures. The migrations are happening as these new
crystal structures settle down from vibrations and fall into their little unit
cells, allowing for a more dense material. For an expansion they would
have to rip apart the already developed solid structure...causing perhaps a
very brittle substance even.
Now your other question, whether the
"terrestrial" amorphous material has the same density as a "crystalline"
meteorite material. That's something I'm sure you know if you think a
minute. Of course it does not have the same density. So save your
iron meteorites for better experiments, and don't melt them to see. The
amorphous material probably won't even have as an extremely precisely
definable density. It ought to vary because it is amorphous.
Whether statistics averages the density out or not is a separate question and
of course will depend on exactly what alloy %'s you have and how well they are
mixed on a macro scale...the history of how the sample was cooled, etc.
Does graphite have the same density as diamond? No.
You cite
water, when freezing, as expanding as your model, since water (0.92 g/mL) in
equilibrium at 0ÂC is denser than ice (1.00 g/mL). Water is weird, and
one of the anomalous things that explains life. The short answer on why
it expands upon decreasing temperature at atmospheric pressure from 4ÂC to 0ÂC
is a softening of proton (hydrogen) bonding and disruption of that nicely
organized structure as it is an ionic (solution) type attraction that is
destroyed as the molecules begin to fall down from their bounciness caused by
higher temperatures. So the destruction of the hydrogen bonds happens
upon cooling. Another way to say that is the melting of the hydrogen
bonds upon coolingâ In its amorphous, hydrogen-bondless state oxygen
atoms find themselves not tolerating such close density as before when all
were moderated by the hydrogen atoms. So they repel each other an
average of about 8% volume increase. Water would be less than 0.92 g/mL
at room temperature if not for the hydrogen bonding. Probably about
0.90, and then it would behave like most of the other molecules upon
freezing.
There are no comparably scaled ionic like attractions going
on in the metal alloys as they cool. So there is no comparison to the
special case of water. Additionally, you should know that water ice has
about 20 described crystalline structures, and they are, not coincidentally,
not all the same density! You need to know the history of it. Do
you think super cooled hail is the same as the cubes floating in your ice
tea? No. And the rockhounds can probably come up with a good list
of structures with different densities but the same molecular
proportions.
Every sample is individual: so back to the meteorite
experiment. Each meteorite has a different history as in temperatures
and pressures. Each has different impurities, so you will need to do
more that just reproduce the alloy proportions...actually by now it seems
pretty clear that the proposed experiment is not a good idea. All those
impurities disrupt perfect conditions. So oceans don't freeze at 0ÂC,
they are a few degrees colder, because of the impurities. All that
affects densities, not to mention any molecular flatulence causing micro
pores, uniformly or not. Ooops. Just checking to see if anyone
read this far...
PS Ice floats. Can you imagine solid pieces of
metal alloy floating as it is melted...that was the easier
explanation...
Saludos
Doug Dawn
Mexico
En un mensaje con
fecha 12/11/2003 2:58:13 PM Mexico Standard Time, [EMAIL PROTECTED]
escribe:
Question: When water turns to ice the
change to a crystal structure
expands the volume (decreases
density). Does this density/expansion change
also occur with the
development of taenite/kamacite
lattices? ie. at the same
temperature is the density of an iron meteorite
match the density of a
"terrestrial" matching mixture that is
"noncrystalline"? Guess you
could melt down one of your irons and compare?
Fly
Hill
>----- Original Message -----
>From: "mark ford"
<[EMAIL PROTECTED]>
>To:
<[EMAIL PROTECTED]>
>Sent: Tuesday, December
09, 2003 8:41 AM
>Subject: RE: [meteorite-list] Is there room for a
meteorite question
?
>
>
>>
>>
>>
>>
>>
>>On
the same note, thermal expansion...
>>
>>Presumably as the
core solidifies (cools down) it would contract, would
>>it not?,
what effect would that have on the crystalline
structure
>>(widmanstatten) would there be a gradual
stress/distortion in the
>>taenite/kamacite boundries
?
>>
>>
>>I guess as the core is such a big mass,
any significant thermal
>>contraction distortion would hardly be
noticeable in a small hand
>>specimen but I would imagine the
thermal contraction in something the
>>size of a large planetary
body would be massive.
>>
>>
>>
>>Mark
Ford
>>
>>
>>
>>
>
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