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Is it possible to melt diamond? And if possible while let it cool will it became diamond again?

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While I agree in principle with David Lynch's answer, I think it's good to take a closer look at the phase diagram (adapted from http://upload.wikimedia.org/wikipedia/commons/4/46/Carbon_basic_phase_diagram.png):

enter image description here

I added the arrows to show possible paths you might follow.

Red path: diamond would become graphite before melting; the molten carbon becomes diamond just above 10 GPa, and you cool it down while maintaining the pressure. Once the diamond is cool enough it can be depressurised slowly without changing phase (the hashed region has to be traversed carefully).

Blue path: if you just heat your diamond, it will turn to graphite and then vaporize (sublimate) around 4000 K.

The green path shows the only "sure" way to melt diamond - starting at a very high pressure, then raising the temperature; above 5000 K one could either continue raising the temperature, or lower the pressure.

Note that there is a real problem with doing this - there are no containers that I know of that will allow that combination of temperature and pressure to be maintained. Synthetic diamonds have been made, but typically not by growing from the melt...

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  • $\begingroup$ If you look at the phase diagram, if you start by pressurizing the diamond and then heat it, it should be possible to "melt diamond" - follow the green arrows. $\endgroup$ – Floris Nov 21 '14 at 2:18
  • $\begingroup$ Note that at standard temperature and pressure ("ordinary" conditions), diamond is unstable. Diamonds are somewhat rare because they form slowly in the Earth's mantle, then they must be carried up to the surface relatively quickly so that they don't have time to "relax" into graphite. $\endgroup$ – Beta Nov 21 '14 at 3:48
  • $\begingroup$ @Beta very good point. That "gray" area in the phase diagram is there for a reason... $\endgroup$ – Floris Nov 21 '14 at 4:08
  • $\begingroup$ Do I read correctly that if you put a diamond into near vacuum while maintaining room temperature, it will turn into graphite? If we project the edge of the region where diamond is stable, it crosses T=0 at just about 1 MPa. $\endgroup$ – John Dvorak Nov 21 '14 at 13:31
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    $\begingroup$ @JanDvorak - the diagram is theoretical and approximate - it is not trying to tell you exactly where diamond stops existing. There are issues like impurities and catalysts (surface modifications, seeds) that can affect the stability - the point being that really the whole bottom left quadrant wants to be graphite. $\endgroup$ – Floris Nov 21 '14 at 15:44
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Carbon, at atmospheric pressure, has no melting point as its triple point is at 10.8 ± 0.2 MPa and 4,600 ± 300 K (~4,330°C or 7,820°F), so it sublimes at about 3,900 K. You can make it liquid at larger pressures, in an inert gas atmosphere. When you cool it, what it becomes depends on the pressure. Below about 10 GPa it will become graphite, above that it will become diamond again. You can see a carbon phase diagram here.

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