A group of scientists in California report the possibility of icebergs of diamond afloat in oceans of carbon on Neptune and Uranus. Natural diamonds are believed to have formed when carbon solidified under great pressure, at some point in the violent geological past of the earth. Attempts to reproduce these conditions in the laboratory have not completely failed and there is a thriving industry of ‘synthetic’ diamonds, with a market both for jewellery and industry.

Dr Jon Eggert of the Lawrence Livermore National Laboratory in California and colleagues placed a small diamond, a tenth of a carat, under intense lasers and subjected it to pressures of millions of atmospheres and temperatures above 50,0000C. The result was a drop of melted diamond, kept liquid because of the pressure. When the pressure dropped to 11 atmospheres, diamond crystals formed and floated on the surface of the melt. As the pressure dropped, the temperature remained the same, but the diamond condensate became larger, and not sinking, but afloat. Diamond hence forms under these pressures and shows anomalous expansion when it freezes.

These kinds of pressures and temperatures are considered to exist within the core of the giant planets, Neptune and Uranus. These planets also consist of carbon to about 10 per cent. In the core, the carbon would be in liquid state and at a pressure of millions of earth atmospheres. The carbon that condenses in these conditions would crystalise as diamond and float like icebergs on the ocean of molten carbon, under pressure.

Allotropes of carbon

Diamond of course is pure carbon, the same as a piece of coal, or the graphite in our ‘lead’ pencils. Carbon has an atomic structure that makes it versatile both in combination with other elements, to form compounds, as well as with other carbon atoms, to form crystals.

The atoms of all elements consist of a massive, positively charged core, surrounded by tiny, negatively charged electrons, equal in number to the charge of the core. These electrons, which keep from falling into the core by whirling round, like planets around a sun, are arranged in ‘shells’, which become the most stable when they consist of eight electrons.

All elements then have a few extra electrons in the last shell, which then tries, through combination with other atoms, to achieve the numbers of two or eight. Carbon is thus able to mix and match, and can form versatile chemical forms, including the ‘ring’ and the ‘chain’ forms of organic chemistry, when it combines with other elements, or the different crystal forms, in the pure state. 

These crystalline forms are called allotropes and in carbon, we have eight different forms – the structure-less, amorphous carbon like coal, the 2-D structure in sheets, which is graphite and is easily deformed, the very hard 3-D structure of diamond, a hexagonal structure that we find in meteorites and then the structures of spheres and cylinders, the buckyballs and buckytubes.

The diamond structure, the rarest of natural forms is the iridescent, very hard form, used as a gem stone, as an industrial abrasive and in electronics for its special heat-conducting and electric properties.

Synthetic diamond

Which allotropic form carbon takes when it solidifies depends on the conditions. When soot forms, it is usually the amorphous form. For graphite, there is need for high temperatures, when the carbon atoms form into sheets. Depositing from the vapour can result in balls and tubes, the Bucky Fullerenes. But for the 3-D, ‘cubic’ form of diamond, we need tremendous pressures, which may have existed when the rocks condensed and formed on the earth or may have existed in meteorites, which also contain diamonds.

Normal crystals are produced by melting the substance and allowing the melt to cool slowly, usually in the form of a cone, where the vertex provides a pole, from which the crystal can begin to form. This cannot work with diamond, because when diamond is heated, we don’t get melted diamond but melted graphite, and when it cools, what forms is just graphite.

High temperatures, great pressure

For diamonds, we need the melt under tremendous pressure, which also implies very high temperatures. Attempts to create these conditions have been made since 1797, when it was discovered that diamond was nothing but carbon. Several well-known attempts are on record, of dissolving carbon in molten metals and then rapidly freezing the solvent. The pressure created by the solidification was expected to squeeze the carbon into diamond. Similar innovative methods were tried, with success reported, but not replicated right till the 1940s and 50s.

They were able to reach temperatures of 3,5000C and pressures of about 35,000 atmospheres. But they were able to actually create diamond grains only in 1954, with an arrangement to reach more than 1,00,000 atmospheres and a temperature over 2,0000C.

The first gem-quality diamonds were created in 1971, and these were found to be ‘nitrogen doped’, which gave them the yellow tint of natural yellow diamonds. It was possible to limit the nitrogen and achieve colourless diamonds, but the effort was too costly to be worth it. The methods have been refined into a series of HPHT (high pressure, high temperature) methods, with steadily better results.