Novosibirsk scientists have reproduced the growth conditions of the Star of Africa diamond in the laboratory.

The temperature, pressure and fluids required for this were experimentally selected

The growth of the diamond is triggered by the garnet. Geologists from the V. S. Sobolev Institute of Geology and Mineralogy SB RAS modeled the conditions in the Earth’s mantle necessary for the growth of diamonds. For this purpose, a high-pressure apparatus was used – a “cut sphere”, simulating the bowels of the earth. It created a pressure corresponding to that created at a depth of about 200 meters, and a temperature of 950–1550 degrees Celsius.

It is known that the main companion of diamond is the garnet stone; geologists used these minerals to find deposits of the valuable crystal at the beginning of the last century. Studies have shown that in diamond formation, in addition to garnet, melts and fluids—liquid components of the Earth’s mantle—also play an important role. They contain carbon dioxide, and this is the source of the main component of diamond – carbon.

Novosibirsk scientists decided to test theoretical data in practice for the first time: they studied in detail the transformations that occur with garnet when interacting with carbon dioxide and hydrocarbon dioxide fluids in the presence of carbon in the Earth’s mantle.

As MK was told by the Russian Science Foundation, the experiment took place in the institute’s laboratory. Garnet samples, “seed” diamond crystals 0.5 millimeters in size, graphite, and silver oxalate, a salt of silver and oxalic acid, which served as a source of carbon dioxide and hydrocarbon dioxide fluids, were placed in an installation simulating the interior of the planet. These components were kept under high pressure and temperature for four days, after which the experimenters analyzed the structure and chemical composition of the minerals.

It turned out that the fluid changed the chemical composition of the surface layers of garnet: the content of magnesium and calcium oxides decreased in them, and inclusions of carbon dioxide fluid, carbonates, carbonate-silicate melts, and graphite appeared. Changes were also found on diamond crystals: a layer of carbon atoms had grown on them.

Scientists calculated that diamonds in the presence of garnet grew at a rate of 0.013 to 0.8 micrometers per hour, depending on temperature. Thus, they came to the conclusion that in nature, the growth of a diamond crystal weighing one carat (0.2 grams) would take from 4.5 months at a temperature of 1550 degrees Celsius to 17.5 years at 1150 degrees, and the crystallization of the largest world, the Cullinan (or Star of Africa) diamond, weighing 3106.75 carats or 621 grams, would have taken from 56 to 3000 years.