Sparklers in the Ice
(appeared on 18th Dec 2013)

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The Antarctic may turn out to be rich with diamonds, says S.Ananthanarayanan.

Diamonds, valued for their lustre in jewelry, and for their hardness and as a conductor of heat, in industry, are found mostly in Africa, India, Australia and South America. Now, rocks that could contain diamonds have been found in Antarctica. This discovery suggests that Antarctica too formed part of the ancient Gondwana, to which the other landmasses once belonged. But finding diamond-rich veins in Antarctica may not lead to a miners’ rush, both because of an International accord and also because of the difficult working conditions.

Gregory M. Yaxley, Vadim S. Kamenetsky, Geoffrey T. Nichols, Roland Maas, Elena Belousova, Anja Rosenthal and Marc Norman from institutes in Australia report in their article in the journal, Nature Communications, the discovery of kimberlites, the classic rock from which diamond is extracted, in the Prince Charles Mountains in eastern Antarctica. “The samples are texturally, mineralogically and geochemically typical of Group I kimberlites from more classical localities,” say the authors, which indicates that the Antarctic was part of the supercontinent which broke up to form the main land masses of the southern hemisphere.

Diamond

Diamond is a crystalline variety of ordinary carbon, or graphite, but which is quite unlike the common forms of carbon. The diamond crystal consists of tetrahedrons of carbon atoms, securely bound to each other by shared outer shell electrons. As the carbon atom has four outer shell electrons, bonding with four other atoms gives each atom a saturated shell of eight electrons and the structure has unequaled stability, strength and hardness. The structure also results in properties of transparence and very strong bending of light. This, along with many reflecting planes and surfaces, gives diamonds their legendary sparkle and iridescence. The structure and the hardness also permit easy transfer of mechanical disturbances, which makes diamond the best known natural conductor of heat. These properties, of hardness and heat conduction, make diamond valuable in industry, apart from its traditional leadership in jewelry.

The way diamonds are formed is when carbon is subjected to very high pressures of 45,000 to 60,000 atmospheres, at a temperature of 900 to 1300°C. This temperature and pressure are found naturally only at 140 km to 190 km underground, in the earth’s mantle, below the continental plates, or when a meteorite strikes the earth. While diamonds can thus form in quantity deep underground, the conditions required are best met under the thick, ancient and stable parts of the continental plates. And the diamonds that have been transported to the earth’s surface are generally a billion to 3.3 billion years old.

Diamond bearing rock rises from the earth’s mantle to the surface through deep volcanic eruptions. The origin of the eruptions needs to be from three times or more than the depth of the source of most volcanoes.

These are quite rare and the typically small surface craters of volcanoes of this group extend downwards in formations known as volcanic pipes. And not all volcanic pipes contain diamonds and of the few that do, only a proportion contain enough for mining to be economical. The picture shows the section of a volcanic pipe, with the depths where diamond bearing rock in mined in the mines at Kimberley, Jagersfontein and Opara, in South Africa. The town of Kimberley, incidentally, lends its name to the form of rock in which diamonds are found.

Kimberlites

Kimberlites are igneous rocks, or rocks formed by the cooling of magma, the molten mineral matter that rises from volcanoes, and are best known because they sometimes contain diamonds. Although they do not form a large part of volcanic rocks, they are studied because they arise from greater depths than other forms of igneous rocks, a conclusion drawn from their lesser content of silica. Kimberlites are thus the most deep-under-the-earth derived materials accessible on the earth. Their study may hence help understand the composition of the deep mantle and the melting processes between the earth’s rocky outer surface and the relatively fluid underlayer.

Kimberlites have been found in almost all stable and rocky formations, below younger sedimentary rock, in all continents, with the exception of Antarctica. It is now accepted that the continents formed by the separation of parts of the ancient Pangean supercontinent, the first land mass, which was surrounded by Panthalassa, the single ocean. The first part to break away was the North American continent, with the formation of the Atlantic Ocean, some 175 million years ago, leaving Pangea as Gondwana. Later, in the early Cretaceous, or some 150 million years ago, Gondwana broke up into South America, Africa, Australia, India and Antarctica, or the major part of the southern hemisphere - India, Madagascar and Antarctica having first formed a combined landmass.

The current theory of continental drift, for the formation of the continents, was proposed in the early 1900s and is supported by biological evidence. Many of the same life forms, both plant and animal, are found in parts of Australia, India, Africa and South America, which proves that they were once interconnected. The fact that they all contain kimberlite, which has origin deep within below the surface and in great antiquity, is also evidence of the continents having once formed the same land mass.

Antarctica

While there is evidence of Antarctica having been part of portion which broke away from Gondwana, before separating from India, the presence of kimberlites had not been proved in Antarctica. The article in Nature Communications now describes detailed analysis of the composition of the mineral discovered in Antarctica, which shows that it is the same variety of kimberlite as found in other continents. The location in Antarctica also tallies with the mechanism proposed for the rifting that brought about the separation from the Indian sub-continent.

While discovery of kimberlite and hence diamonds in Antarctica may suggest that leaders in the diamonds industry would rush to the South Pole for mining, mining in the Antarctic is forbidden till 2041 under the Madrid protocols, where the nations of the world designated the Antarctic as a “‘natural reserve, devoted to peace and science” and specifically prohibited mining. Considering the difficulties in attempting to mine the frozen desert that is Antarctica, it is likely that the same prohibition would be extended beyond 2041.

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