In mineralogy, diamond is the allotrope of carbon where the carbon atoms are arranged in an isometric-hexoctahedral crystal lattice. Its hardness and high dispersion of light make it useful for industrial applications and jewelry. It is the hardest known naturally-occuring mineral. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds (known as Type-II diamonds) that are harder than the diamonds used in hardness gauges.
Roughly 49% of diamonds originate from central and southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia. They are mined from kimberlite and lamproite volcanic pipes, which can bring diamond crystals, originating from deep within the Earth where high pressures and temperatures enable them to form, to the surface.
|FORMATION OF DIAMOND|
The formation of natural diamond requires very specific conditions. Diamond formation requires exposure of carbon-bearing materials to high pressure, ranging approximately between 45 and 60 kilobars, but at a comparatively low temperature range between approximately 1652-2372 °F (900-1300 °C). These conditions are known to be met in two places on Earth; in the lithospheric mantle below relatively stable continental plates, and at the site of a meteorite strike.
The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the aforementioned requirements of temperature and pressure. These depths are estimated to be in between 140-190 kilometers (90-120 miles) though occasionally diamonds have crystallized at depths of 300-400 km (180-250 miles) as well. The rate at which temperature changes with increasing depth into the Earth varies greatly in different parts of the Earth. In particular, under oceanic plates the temperature rises more quickly with depth, beyond the range required for diamond formation at the depth required. The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist.Long residence in the cratonic lithosphere allows diamond crystals to grow larger.
Diamonds can also form in other natural high-pressure events. Very small diamonds, known as microdiamonds or nanodiamonds, have been found in meteorite impact craters. Such impact events create shock zones of high pressure and temperature suitable for diamond formation. Impact-type microdiamonds can be used as one indicator of ancient impact craters.
Diamond-bearing rock is brought close to the surface through deep-origin volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed, 150 km (90 miles) deep or more (three times or more the depth of source magma for most volcanoes); this is a relatively rare occurrence. These typically small surface volcanic craters extend downward in formations known as volcanic pipes. The pipes contain material that was transported toward the surface by volcanic action, but was not ejected before the volcanic activity ceased. During eruption these pipes are open to the surface, resulting in open circulation; many xenoliths of surface rock and even wood and/or fossils are found in volcanic pipes. Diamond-bearing volcanic pipes are closely related to the oldest, coolest regions of continental crust (cratons). This is because cratons are very thick, and their lithospheric mantle extends to great enough depth that diamonds are stable. Not all pipes contain diamonds, and even fewer contain enough diamonds to make mining economically viable.
The magma in volcanic pipes is usually one of two characteristic types, which cool into igneous rock known as either kimberlite or lamproite. The magma itself does not contain diamond; instead, it acts as an elevator that carries deep-formed rocks (xenoliths), minerals (xenocrysts), and fluids upward. These rocks are characteristically rich in magnesium-bearing olivine, pyroxene, and amphibole minerals which are often altered to serpentine by heat and fluids during and after eruption. Certain indicator minerals typically occur within diamondiferous kimberlites and are used as mineralogic tracers by prospectors, who follow the indicator trail back to the volcanic pipe which may contain diamonds. These minerals are rich in chromium (Cr) or titanium (Ti), elements which impart bright colors to the minerals. The most common indicator minerals are chromian garnets (usually bright red Cr-pyrope, and occasionally green ugrandite-series garnets), eclogitic garnets, orange Ti-pyrope, red high-Cr spinels, dark chromite, bright green Cr-diopside, glassy green olivine, black picroilmenite, and magnetite. Kimberlite deposits are known as blue ground for the deeper serpentinized part of the deposits, or as yellow ground for the near surface smectite clay and carbonate weathered and oxidized portion.
Schematic diagram of a volcanic pipe
|Once diamonds have been transported to the surface by magma in a volcanic pipe, they may erode out and be distributed over a large area. A volcanic pipe containing diamonds is known as a primary source of diamonds. Secondary sources of diamonds include all areas where a significant number of diamonds, eroded out of their kimberlite or lamproite matrix, accumulate because of water or wind action. These include alluvial deposits and deposits along existing and ancient shorelines, where loose diamonds tend to accumulate because of their approximate size and density. Diamonds have also rarely been found in deposits left behind by glaciers (notably in Wisconsin and Indiana); however, in contrast to alluvial deposits, glacial deposits are not known to be of significant concentration and are therefore not viable commercial sources of diamond.|
|DIAMOND IN INDIA|
In India, diamond deposits occur in three types of geological settings viz. Kimberlite, conglomerates and alluvial gravels. At present only Majhgawan Diamond pipe in Panna district of Madhya Pradesh is under exploitation by NMDC.
In India diamonds are found in kimberlite-lamproites of Chattishgarh, Madhya Pradesh and Andhra Pradesh and as placers in Proterozoic conglomerates within Kurnool and Vindhyan rocks and also in recent and sub-recent gravel horizons along the river basins. Apart from the only diamond mine in Panna, the geological domain potential for search of diamond are:-
Granite-greenstone terrain of Andhra Pradesh. Cuddapah basin of Andhra Pradesh.
Granite-greenstone terrain of Karnataka in Gulbarga-Raichur-Bellary-Bijapur,Tumkur and Chitradurga districts.
Bastar, Raigarh and Raipur districts in Bastar Craton. Upper reaches of Ong and Jira rivers in parts of Bargarh and Sonepur districts,Orissa.
Terrain exposing Vindhyan Supergroup, Bundelkhand Granite, Bijawars and Gwalior Group of rocks, Madhya Pradesh. Amgaon Gneiss, Bhandara district, Maharashtra.
In potential geological domain search for kimberlite/lamproite, the source rock for diamond would continue. Besides, primary rock search for diamond will be taken up from secondary sources in favourable geological milieu in Andhra Pradesh. Apart from diamond, India has diverse source of semi-precious stones and precious stones from different geological set up. The most potential gem tract of India which deserves attention are:-
Gem tract of southern granulite terrain of Kerala and Tamil Nadu.
Gem tract of Eastern Ghat granulite belt of Andhra Pradesh and Orissa.
Granulite terrain in Chhattisgarh.
Gem tract of Rajasthan and Karnataka.
The Panna belt in Madhya Pradesh is the main diamond producing area. Recoverable reserves of about 1 Mcts have been estimated. An additional 265 000 carats have been classified as”submarginal reserves”. Here the National Mineral Development Corporation (NMDC) of India operates the country’s only diamond mine, Majhgawan in Panna. The Dharwar craton has been the target for India’s diamondfierous kimberlite potential.
De Beers, Rio Tinto, BHP Billiton (in joint venture with Dwyka diamonds), Oropa and Indian Resources are exploring for diamonds in several states of India. India is the largest processor of diamonds in the world. By volume, India accounts for over 90% of the global market share in diamond cutting and polishing but for nearer 60% by value.