NATURE OF Diamond

What is Diamond, and why is it so hard? Briefly, the answer to both questions is this. Diamond is a crystalline form of carbon and in this crystal form the atoms are packed so closely and in so uniformly symmetrical a way that it is almost impossible (because of forces between them) to press the atoms any closer together than they are. This confers on the crystal great hardness and great resistance to compression.

It is a curiosity of nature that carbon (which exists on earth in a chemically combined form in millions upon millions of tons, in minerals, in petrol sources, in plants and vegetables, coal, even in carbon dioxide in the atmosphere) can occur in nature in two distinct crystal forms. One form is graphite (which is found as a mineral). This is the 'black-lead' of our pencils, a soft black crystal, with almost a greasy feel and which grows in the form of crystalline blocks which split down (cleave) easily to flat platelets.

It is a cheap material of considerable industrial use. It is a good conductor of electricity and it can withstand high temperatures, although when kept red hot in air it burns away to carbon dioxide. It is relatively light (of. density only 2∙2 times that of water), is very friable and smooth, indeed capable of acting as a lubricant. Graphite is a true crystal in which the pure carbon atoms from which it is constituted are organised in minute, thin, flat hexagonal platelets. These are loosely tied in such a way that the plate­lets can slide over each other when pushed sideways, just as if the crystal were constructed like a pack of cards. This is why graphite cleaves so easily and this is why it can act as a good lubricant. It is fairly resistant chemically.

There exists, however, another organisation of pure carbon atoms building up to a crystal form, and this is Diamond. Whereas in graphite the carbon atoms group into hexagonal platelets which build up in layers to form the crystal, in dia­mond the packing of the carbon atoms is very tight. Each atom is placed in a geometrical grouping such that it is linked equally to four other carbon atoms in the closest and tightest fashion possible. So close are the atoms that the Diamond material has a density 3∙5 times that of water. Actually, the graphite and Diamond crystals are closely related to one another; so much so, that under correct con­ditions there can be conversion from one to the other. Because graphite is less dense than Diamond it is the more stable form.

If a Diamond is heated to, say, 1200° C it slowly converts to graphite. If the temperature is raised to 1800° C the Diamond can transform so catastrophically fast that it shatters down into a heap of graphite. This graphitisation can be arrested by increasing the pressure around the Diamond. Indeed, accord­ing to theoretical prediction, in complementary fashion, if graphite is heated to, say, 2500° C and then very great pressure is applied (at least 100 000 atmospheres) it should be transformed to Diamond. In this sense there can be expected to be a two-way traffic in transformation, depending on temperature and pressure.

The pressure must be great enough to push together the carbon atoms of the graphite to achieve the necessary high density of Diamond. Possibly this is how dia­monds grew in the earth. At depths of many miles, the pres­sure in the earth's rocky covering is adequate for the purpose of converting graphite to Diamond. Furthermore, the Diamond crystal is what is called metastable: when it is formed at high temperature and pressure, provided the temperature cools down before the pressure is released, the Diamond, once made, stays put; even though it does want to revert to the stabler form (graphite) it is unable to do so until its temperature is raised. Raise the temperature of the metastable Diamond and it rapidly reverts to its parental graphite nature.

It is likely that far more Diamonds have formerly been formed in the earth than are now available, for if the temperature remained high when the pressure came off the Diamond would turn to graphite. The pressure could come off through Diamond-bear­ing rock being ejected up rapidly from below. It was this expected possibility of conversion of graphite into Diamond which inspired the researches which in due course successfully led to the large-scale manufacture of synthetic Diamond, such Diamond being now produced in quantities amounting to more than two tons a year, a development we shall later discuss in detail.