New mineral trapped in deep diamond discovered

nouveau mineral piege diamant profondeurs

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It is the fourth most abundant mineral on earth, but to this day we have never seen it! The mineral, called davmavoid for this event – with the chemical formula CaSiO3 – was described in the November 11 issue. Science By a team of American researchers. Unusual fact: It was discovered in a diamond! About 5% of the mantle beneath the rock that stretches beneath our feet at depths of 660 to 2,900 km has been visible to geologists for a long time, but it has not yet been observed on the surface due to the high pressures required for its existence. .

Observed for the first time on the surface, Ca-perovskite changed its name … davemaoïte! Issued as a new mineral in October 2020 under the name IMA2020-012a, this discovery had to wait for a scientific publication to be approved. It has been under review since November 11th Science Dovemoid published the study of Oliver Schner’s team on characteristics.

Davmavoid, found in the addition of diamonds mined at Oraba, the world’s largest diamond mine in central Botswana, derives its name from the Sino-American geologist Ho-Guang (Dave) Mao for his contributions to Earth’s geophysics. Robe.

The new mineral differs from volastonite, a rock common on the Earth’s surface, which shares the same CaSiO3 formula with its specific crystalline structure. It is a heavy perovskite, which is usually found only at very high pressures. Ferroskites are a family of minerals that share a single crystal structure and are important components of the Earth’s crust.

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Dovemoid extracted diamond. © Aaron Celestian / Los Angeles County Natural History Museum

In fact, the lower mantle (660 to 2900 km depth) is solid, containing about 5% davmoite, 20% magnesiocyte (Mg, Fe) O and 75% bridzmonite (Mg, Fe) SiO3. The latter mineral is also a perovskite, and incidentally the most common terrestrial mineral: it is only abundant 40% The size of the earth! However, until 2009, in its natural state, it could not be found in the form of a microscopic compound in a meteorite. How to explain that such common rocks are ultimately very rare?

Behind this apparent contradiction hides a reality we do not know, ultimately in our world with identical physico-chemical conditions: a mineral is characterized by a specific atomic composition and by the specific arrangement of its atoms between them. Therefore a mineral exists only under certain conditions of pressure and temperature, beyond which its crystal structure and its nature change. Going beyond the Donetsk conditions at depths of hundreds of kilometers, most of the rocks will gradually change and become normal surface rocks at the end of their journey.

Diamonds for redemption

This is where diamonds come in. Due to their very low elasticity, they help to maintain the ores at high pressures, even once on the surface, during formation. That is why geologists are closely interested in adding “super-deep” diamonds formed at depths of 230 to 800 km, to study the transition zone from the lower mantle and the upper mantle, which is not yet properly understood. Ranges from 410 to 660 km deep. So, in 2018, Ca-perovskite was already there Observed Formed in impure form, bottom coat, in the deepest South African diamond.

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The host diamond of the Botswana davemaoïte belongs to the genus IaAB, endemic to South Africa. In addition to davemaoite, it contains ice VII – a crystalline form of ice formed at very high pressure, iron, wüstite and ilmenite.

By determining the pressure and temperature conditions that could lead to these additions, the researchers showed that the diamond was formed at a pressure of about 30 GPa and at 1500 C. Beyond the 23 GPa that marks the start, at a depth of 660 km, at the depth of the lower mantle: we will be at a depth of about 900 km!

Trash can and lower mantle radiator

Unfortunately, the newly named mineral plays an ungrateful role in the bottom coat … Like the cornet in the upper mantle, it acts as a “trash can”! In fact, its crystalline structure allows its calcium atom to be easily replaced by rare earths or radioactive elements, which are mainly responsible for heat production in the mantle. These atoms cannot coalesce with the crystal lattice of the other two lower mantle minerals, so the mantle of the lower mantle will always end up in the trash and radiator.

Source: Science

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