Precious Stones Information

Monday, March 8, 2010

Thermal stability of Diamonds

Diamond and graphite are two different allotropes of carbon: pure forms of the same element that differ in structure.

Being a form of carbon, diamond oxidizes in air if it is heated over 700 °C. In the absence of oxygen, for e.g. in a flow of high-purity argon gas, diamond can be heated up to about 1700 °C. Its surface blackens, yet it can be recovered by re-polishing. At high pressure diamond can be heated up to 2500 °C, and a report published in 2009 suggests that diamond can withstand temperatures of 3000 °C and above. Diamonds are carbon crystals that usually form deep within the Earth under high temperatures and extreme pressures. At surface air pressure, diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (dH = -2 kJ / mol). So, contrary to De Beers' ad campaign extending from 1948 to at least 2006 under the slogan "A diamond is forever”, diamonds are certainly not forever. However, owing to a very large kinetic energy barrier, diamonds are generally metastable; they will not decay into graphite under normal conditions.

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Monday, March 8, 2010

Thermal stability of Diamonds