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Coal, Diamond, or Graphite
Name: Cheryl B.
Status: Other
Age: 50s
Location: N/A
Country: N/A
Date: July 2002
Question:
If you were to take a lump of coal and squeeze it for a
long time at very high temperatures, would you end up with a diamond, or
graphite?
Replies:
Cheryl,
I answered someone else's question about
this previously. You can find an answer to
this in the questions that have been answered on
the web site.
However, briefly, if the pressure were
high enough and last long enough, theoretically
you could get a diamond, but it would be quite
impure. Coal has a high level of impurities
(it is not all carbon), so any resulting
diamond would be of poor quality.
Squeezing a lump of coal would not produce
graphite. Diamonds were formed from graphite,
a pure form of carbon, under great pressure and heat.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
It really depends on the temperatures and pressures. Coal that is
metamorphosed may turn into graphite, but the pressures and temperatures
required for graphite are not the same as those required for diamond
formation. Diamond formation requires much higher temperatures and
pressures. The relationships between pressure and temperature and what form
of carbon exists can be plotted on a type of graph. There is a nice one at
http://invsee.asu.edu/Modules/carbon/condit.htm.
Interestingly, diamonds are
not permanently stable at the temperatures and pressures we experience at the
surface of the earth. Diamonds are very slowly changed into graphite.
Pat Rowe
Diamond is thermodynamically unstable with respect to graphite at ambient
conditions, 300 K and 1atm. At 300 K it requires about 14,500 atm to
transform graphite into diamond. That is how diamonds were formed
(presumably) at great depths below the earth's surface.
You can find the phase diagram for carbon's various forms at the web site:
http://www.nau.edu/~chem/courses/chm440/lectures/lec9/lec9.html
You may have to enter this link somewhere upstream of the complete URL here.
Vince Calder
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Update: June 2012
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