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Let's say I have a $\pu{1cm^3}$ diamond and I put it into an oven where the temperature is about $\pu{ 400°C}$. We all know that diamond oxidizes. Would it evaporate or turn into graphite? If so, what would be the rate of reaction? Is there some way to calculate it?

From Wikipedia:

Above $\pu{1700^\circ C}$ ($\pu{1973 K}$/$\pu{3583^\circ F}$) in vacuum or oxygen-free atmosphere, diamond converts into graphite; in air, transformation starts at $\pu{\approx 700^\circ C}$. Diamond's ignition point is $\pu{720–800^\circ C}$ in oxygen and $\pu{850–1000 ^\circ C}$ in air.
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But diamonds (sp3 carbon) are unstable against high temperature (above about $\pu{400 ^\circ C}$ ($\pu{752 ^\circ F}$)) under atmospheric pressure. The structure gradually changes into sp2 carbon above this temperature. Thus, diamonds should be reduced under this temperature.

So even if the diamond doesn't reach the temperature of $\pu{700^\circ C}$ it still oxidizes. That's why I'm asking.

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