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I know that coal (graphite) turns into diamond when a high enough pressure is applied, but I guessed it would stay as a diamond unless it was incinerated by a high enough temperature.

However, I recently took a closer look at the phase diagram of carbon, and it seems that diamond should turn back into graphite on low enough pressures even at room temperature:

enter image description here

I guess this is a relatively easy experiment to make, I just don't have the necessary funds to sacrifice a diamond, and I couldn't find such an experiment with a quick Google search.

What would really happen when a piece of diamond is put into a vacuum chamber and air is pumped out, as the pressure drops enough so that the subject leaves the "metastable diamond" phase? Does it remain the same? Does it just peacefully turn into a similarly shaped piece of graphite, which remains a normal piece of graphite after putting it back into atmospheric pressure? Or would something more spectacular happen?

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  • $\begingroup$ Standard atmospheric pressure is 0.000101325 GPa. Do you see diamonds turning into graphite everywhere? $\endgroup$ – Hot Licks Mar 15 '15 at 0:31
  • $\begingroup$ @HotLicks : No, and this is why I asked the question, because the phase diagram indicated it would change, and my assumptions were that it wouldn't. $\endgroup$ – vsz Mar 15 '15 at 10:48
  • $\begingroup$ There is the issue of the rate of any possible reaction. $\endgroup$ – Hot Licks Mar 15 '15 at 10:50
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Nothing happens. We have put many diamonds into vacuum chambers to do ion implantation. They remain diamond.

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    $\begingroup$ Does this mean that the phase diagram I linked, and the one seen on Wikipedia (upload.wikimedia.org/wikipedia/commons/4/46/…) are wrong? $\endgroup$ – vsz Mar 15 '15 at 0:00
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    $\begingroup$ Diamond is not thermodynamically stable at atmospheric pressure as indicated. However, the kinematics are such that it does not readily convert to graphite. $\endgroup$ – Jon Custer Mar 15 '15 at 4:15
  • $\begingroup$ In fact there are shock-produced micro-diamonds in space. They are not "stable", but the activation barrier is so high that they survive vacuums strong than any lab vacuum for billions of years. Cosmic rays and hard-gamma ray interaction will degrade them however. $\endgroup$ – Aabaakawad Oct 12 '15 at 6:54

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