It is frequently stated that although graphite is the more stable allotrope of carbon at STP, the activation energy of the diamond-to-graphite transformation is so high that our diamonds will never spontaneously turn into black dust.

Some sources add (correctly) that nothing is ever never, and reactions merely slow down with lower temperature, they never stop entirely. (The Arrhenius equation comes here). But nobody ever quantifies the non-neverness of the death of a diamond. So my question is:

Under normal conditions at room temperature, which of the following will happen first; when, and how fast?

  1. Diamond transforming to graphite.
  2. Diamond evaporating. Marshall and Norton ,J. Am. Chem. Soc., 1950, 72 (5), pp 2166–2171, seem to say that latent heat of carbon is 170kcal/mole, but I haven't got access to the whole paper to see what they say about the rate of evaporation.
  3. Diamond spontaneously combusting to CO2.
  4. Carbon decaying to iron via tunnelling. Barrow and Tipler, in The Anthropic Cosmological Principle (Oxford, 1986), p.654, quote 10^1500 years for this.
  • $\begingroup$ Possibly carbon decaying down. $\endgroup$
    – Horus
    Nov 1, 2015 at 15:16
  • 1
    $\begingroup$ At least the latter limit is purely on paper as one would expect other nuclear mechanisms to be much faster than this. Proton decay alone should empty out the universe much sooner and the interaction with cosmic rays will knock the diamond to pieces at vastly shorter time scales. Thermal conversion is trivial and quick, see e.g. youtube.com/watch?v=7L7BV3IBfFA. Diamond to graphite is easy. It't the reverse process that has kept people awake at night. $\endgroup$
    – CuriousOne
    Nov 1, 2015 at 15:23


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