Science

New Discovered Mineral Trapped in Deep Diamond

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It is the fourth most abundant mineral on earth, but we have never seen one… until today! Named davemaoite for the occasion, the mineral, with the chemical formula CaSiO3, was described Nov. 11 in the journal Science by a team of American researchers. Unusual fact: it was discovered in a diamond! Constituting about 5% of the lower mantle, the layer of rock that extends under our feet between 660 and 2900 km deep, geologists have long known, but had not yet been observed on the surface, because its existence requires pressures very high. .

First observed on the surface, Ca-perovskite changes its name and becomes… davemaoïte! Introduced as a new mineral in October 2020 under the name IMA2020-012a, the discovery had to wait for a scientific publication to be recognized. This has been done since November 11, when the journal Science released the study by Oliver Tschauner’s team that characterizes davemaoïte.

Found in the inclusion of a diamond mined at Orapa, the world’s largest diamond mine in central Botswana, the davemaoite takes its name from the Sino-American geologist Ho-Kwang (Dave) Mao, due to his significant contributions to the geophysics of the Earth. mantle.

The new mineral differs from wollastonite, a common rock on the Earth’s surface that shares the same CaSiO3 formula, due to its peculiar crystalline structure. It is a cubic perovskite, formed and normally only found at very high pressure. Perovskites are a family of minerals that share the same crystalline structure, among which are the main constituents of the Earth’s mantle.

The diamond from which the davemaoite was mined. © Aaron Celestian / Los Angeles County Museum of Natural History

In fact, the lower mantle (660 to 2900 km deep), solid, contains approximately 5% davemaoite, 20% magnesiumwüstite (Mg, Fe) O and 75% bridgmanite (Mg, Fe) SiO3. This last mineral is also a perovskite and, by the way, the most common terrestrial mineral: it only occupies more than 40% of the volume of the Earth! However, it wasn’t until 2009 to discover it in its natural state, in the form of a microscopic inclusion in a meteorite. How to explain that such common rocks are ultimately so rare?

Behind this apparent paradox there is a reality that we are not necessarily aware of, in our world with ultimately very homogeneous physicochemical conditions: a mineral is characterized by a specific atomic composition and by the particular arrangement of its atoms among them. . Therefore, a mineral exists only under certain conditions of pressure and temperature, beyond which its crystalline structure changes, and therefore its nature. Ascending from the Dantesque conditions that reign hundreds of kilometers deep, the vast majority of rocks gradually change to become banal surface rocks at the end of their journey.

Diamonds to the rescue

This is where diamonds come in. Due to their very limited elasticity, they allow maintaining the minerals they contained during their formation at high pressure, even once on the surface. That is why geologists are very interested in inclusions of “super-deep” diamonds, formed between 230 and 800 km deep, to study the lower mantle and the transition zone from the upper mantle to the lower mantle, still little known. , Which extends between 410 and 660 km deep. Thus, in 2018, Ca-perovskite had already been observed in impure form in a super deep South African diamond, formed in the lower mantle.

Botswana davemaoïte’s host diamond is type IaAB, classic in southern Africa. In addition to davemaoite, it contained inclusions of 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 have led to these inclusions, the researchers showed that the diamond had formed at around 30 GPa of pressure and 1500 ° C. Well beyond the 23 GPa that marks the beginning, a a depth of 660 km, from the lower mantle: we prefer to be about 900 km deep!

Trash can and lower mantle radiator

Unfortunately, the newly named mineral plays a thankless role in the lower mantle… Like the garnet in the upper mantle, it acts as a “garbage can”! In fact, its crystalline structure allows its calcium atom to be easily replaced by rare earths or even radioactive elements, which are mainly responsible for the production of heat in the mantle. These atoms cannot integrate with the crystal lattice of the other two lower mantle minerals and thus invariably end up in the lower mantle davemaoite, trash, and radiator.

Science

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