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The diamond more valuable and rare in the world call the attention by their size, quality, beauty or proportions. But not just for that. Researchers from the Gemological Institute of America (GIA) has confirmed that these jewels are also remarkable because of its origins. Formed in the bowels of the Earth, about 700 km beneath our feet, to three times deeper than the vast majority of the diamonds are more common.
The scientists, who presented their results at the conference of geochemistry Goldschmidt, believe that this is the origin of parts of heart attack as the Hope of the National Museum of Natural History, Smithsonian Institution (45,52 carats), known as the “stone cursed” for having the reputation of giving bad luck to their owners, or of a heavy weight, the Cullinan, (3.100 carats), the largest known, and the centerpiece of the British Crown Jewels.
to reach this conclusion, the team of Evan Smith, GIA, studied two diamonds: a large blue diamond of 20 carats of type IIb, as the Hope, of a mine in south Africa, and another large diamond 124 carat of the mine Letseng in Lesotho, similar to the Cullinan.
diamonds are formed under high pressure in the mantle of the Earth, the intermediate layer between the superficial cortex and the central core. While most occur at the base of the tectonic plates continental, to depths of 150-200 km, some more rare are formed deeper in the mantle. These diamonds are “super deep” originate beneath the continental plates, rigid and stable, towards the bottom, where the mantle moves very slowly.
The Hope is classified as a diamond is type IIb, which contains the element boron, which can cause a blue tint. They are very rare; less than one in every one thousand diamonds are classified as this type. Until now there has been uncertainty about whether they are in an environment shallow or deep. In particular, the uncertainty revolved around the source of the large diamonds of type IIb, larger than 3 carats (approximately the size of a pea).
The team of Smith detected the remains of the mineral bridgmanita in the large blue diamond of 20 carats from a mine in south Africa, which was crucial to solve the problem. “To find these remains of the elusive mineral bridgmanita is significant. It is very common in the deep Earth, in the conditions of extreme pressure of the lower mantle, below 660 km, even more profound than the majority of diamonds super deep”, explains the researcher. The bridgmanita does not exist in the upper mantle, or on the surface. What we actually see in the diamonds when they reach the surface is not bridgmanita, but the minerals that are left behind when it breaks down as you decrease the pressure. “To find these minerals trapped in a diamond means that the diamond must have crystallized to a depth where there is bridgmanita, very deep within the Earth”, he concludes.
When you point a laser to the small inclusions trapped within the piece, the researchers discovered that the way in which extended the light (using a Raman spectrometer) was characteristic of the decomposition products of the bridgmanita.
The “nut” of Lesotho
The team also examined the great diamond of Lesotho. The size of a walnut, it is very pure, contains no nitrogen in its crystal structure and is known as a CLIPPIR, the same kind of diamond-the Cullinan. This large diamond showed the same characteristic products of rupture of bridgmanita, which means that they had been formed as a diamond super deep. This is the first CLIPPIR that researchers can be assigned firmly to a source of mantle lower, i.e., below 660 km, “Earlier, it was speculated that the depth of origin was from 360 to 750 km depth, but in reality we had not seen any that was definitely from the extreme depths of that window,” says Smith. This means that there is “some overlap in the birth place of diamonds CLIPPIR, such as the Cullinan, and the diamonds are type IIb, like the Hope diamond. This is the first time that it has been found”.
“to Discover the origin of deep mantle means that the material in these diamonds experienced a remarkable journey. We believe that boron, which gives the Hope diamond its characteristic blue color, originates in the bottom of the oceans. From there, the tectonic plates dragging it hundreds of miles down in the mantle, where it can be incorporated into the diamond. Shows that there is a route of recycling giant, which takes elements from the surface to the interior of the Earth, and then occasionally returns beautiful diamonds to the surface, like a passenger in volcanic eruptions”, explains the researcher.
“This fascinating work confirms that the Hope diamond is extraordinary and special, and truly one of the most rare of the Earth,” says Jeff Post, curator in charge of gems and minerals at the National Museum of Natural History of the Smithsonian institution.