DIAMONDS — LOGIE 



365 



Figure 3. 



at room temperature. The diamond is the preferred structure in a 

 region where the pressure is above 20,000 atmospheres, or about 

 300,000 p.s.i., while graphite is the preferred structure at normal 

 conditions of pressure and temperature. It is the enormous pressures 

 in the cooling magma of a volcano which is needed for the production 

 of a diamond. In more precise terms, one would say that the ther- 

 modynamic potential of gi'aphite is lower than that of diamond, 

 and it follows that if any transformation is to take place at all, at 

 room temperature it will be from diamond to graphite and not vice 

 versa. But thermodynamic stability is not the same as mechanical 

 stability, and although diamond has a thermodynamic permission to 

 transform itself into graphite, it has no mandate to do so. My wife 

 has worn her engagement ring these many years with no solicitude on 

 that score. 



The second strange fact is, that althougli diamond has a density 

 of 3.5 g./cc. and is the hardest known substance, its atoms are not 

 packed in the closest possible geometrical arrangement. It would be 

 much denser if each atom were surrounded, not by 4, but by 12 other 

 equidistant atoms as is the case for many metallic substances. In 

 fact, by packing spheres together in the diamond structure, only 

 46 percent of the available space is used. We would describe the 

 diamond lattice as having a very open structure. 



It was this fact that led us to attempt a few months ago to diffuse 

 impurity atoms into the diamond. The idea was that by heating the 

 diamond to several hundred degrees centigrade in a vacuum and 

 while it was embedded in a material like boron which has a small 



