Geology

Formation in the earth

Diamond is a polymorph form of carbon. The other form is graphite. For carbon to turn into diamond, 3 factors are needed: time, very high temperature (around 1,000 degrees) and very high pressure above 30 kilobars (kb). The Pressure and Temperature conditions within the earth, are known as geotherms. Any changes of « P » (Pressure) and « T » (Temperature) within the environment where diamonds are formed, may reverse the cycle back to graphite or gas. The main bearing diamond rocks are kimberlite (the name proposed by Lewis in 1888 is for the Kimberley district in South Africa), eclogite, lamproite. Diamonds can be formed going down towards the mantle of the earth (subduction), going up towards the surface of the earth (obduction) and also on impact (meteorites).

The most common shape of kimberlite pipe is « the carrot shape » it also occur as dykes and rarely as sills. Within the kimberlite pipe 3 zones are recognized, the crater, diatreme and the root zones. The pipes do vary in size, up to 200 hectares. In contrast to kimberlites, which may show some vertical flaring over 1 to 2 kms, Lamproite bodies are shallower, around 0.5 kms in depth . Many lamproites have a champagne glass shape craters. The difference between kimberlites and lamproites pipe shapes has important implications for exploration (ore volume calculations).

Diamonds found in meteorites

Diamonds in meteorites were discovered in Arizona or South Pole and they contained minuscule crystals of diamonds. In 2004, The Harvard-Smithsonian Center for Astrophysics has announced the discovery of a mass of crystallized carbon formerly known as star BPM 37093, now known as the biggest diamond in the galaxy, fifty light years away from Earth in the constellation Centaurus. The diamond is estimated to be 2,500 miles across and weighs approximately 10 billion-trillion-trillion carats – a one, followed by 34 zeros = 10,000,000,000,000,000,000,000,000,000,000,000 carats.

Types of diamond deposits

Primary deposits

Secondary deposits

Diamantiferous kimberlite lamproite pipes are the « Primary deposits ». Diamonds are transported to the surface of the earth by the magma pushing upwards the kimberlite (diamond bearing rock ) at great velocity. When it reaches the surface, the volcanic eruption occurs releasing an enormous energy creating the birth of the volcano. The material blown out from the explosion probably landed in a pile around the vent. The material eventually washes away by rain or washes back into the central zone of the crater or is dispersed by other weathering processes. The kimberlite that remain in the cracks of the earth crust crystallize as dykes. The secondary deposits are formed by the weathering of the kimberlite and the lamproite. The diamonds are released from the rock and then, they are transported hundreds of kilometres away to be found in river beds, beach sands, old river beds (sometime found on top of hills deep jungle forest, deserts, etc… Diamonds may also have been transported by glaciers and if the journey has been hard, rough and long, they are not to be found as they have been broken and grinded into near dust.

World wide diamondiferous deposits

World wide diamondiferous deposits

Click to enlarge

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