Found a mineral that can be a carrier of rock-forming and rare elements in nature

(ORDO NEWS) — An international team of scientists, which included researchers from the Kola Scientific Center, discovered a mineral that, according to scientists, can be a universal potential carrier of rock-forming and rare elements at pressures up to mantle pressures.

The Hatrurim complex (also known as the Spotted Zone) in the vicinity of the Dead Sea has already brought many discoveries to researchers. In particular, a mineral was discovered here, which was previously found only in meteorites, as well as phosphorus compounds , which with a high degree of probability could help the emergence of life on Earth.

Researchers representing St. Petersburg State University, the Kola Science Center of the Russian Academy of Sciences, the University of Bayreuth, the Bavarian Research Institute of Experimental Geochemistry and Geophysics and the University of Padua continue to study the composition of the rocks that make up this area.

Their new discovery is a mineral, which, according to scientists, can be a universal potential carrier of rock-forming and rare elements at pressures up to mantle pressures.

An article about the discovery was published in the journal American Mineralogist. The Spotted Zone is the world’s largest outcrop pyrometamorphic rock complex, scattered over a large area of ​​the Middle East from the southern sub-basin of the Dead Sea to the border between Northern Jordan and Syria.

Crystal structure of cubic perovskite and its hypothetical aristotype analogue

Its origin is still a matter of debate. It is assumed that it was formed during the calcination and melting of chalk marls and chalks at temperatures reaching 1400 ° C, however, the recent discovery of the mineral allabogdanite (Fe,Ni) 2 , which could be formed only at high pressure, raised many questions regarding the development of this rock complex. .

In samples collected in the Israeli Negev desert, the authors found the first natural perovskite (CaTiO 3 ) with a cubic crystal lattice (the structure of perovskite in nature is rhombic). Its clear, yellowish-brown crystals are up to 50 microns in size and have abundant micron inclusions of melilite glass.

The new mineral is stable under compression up to at least 50 GPa, and under atmospheric pressure it is stable up to 1250±50°C. At higher temperatures, its crystals fuse with embedded melilite glass, forming a mixture of titanite and anorthite.

Inclusions of melilite glass help elucidate the origin of cubic perovskite from the Hatrurim Formation. Apparently, the mineral was formed in a solid state and was never subjected to heating above 1250°C at atmospheric pressure.

The shape of the melilite glass inclusions indicates that they have become amorphous, remaining in the solid state. The melting point of melilite is 1400-1600 °C, so its melting would inevitably lead to fusion with perovskite.

Cubic perovskite and associated minerals. (a) Perovskite crystals in microcrystalline gelenite. Micrograph in scattered light. (b) The same area in reflected light. (c) Perovskite and α-(Fe,Ni). (d) Perovskite intergrown with α-(Fe,Ni). (e) poikilitic perovskite crystals densely filled with melilite glass. (f) Perovskite, α-(Fe, Ni) and silicon-rich fluorapatite in a gelenite matrix. Abbreviations: Prv – cubic perovskite, Gh – gehlenite, Si-Ap – silicon-containing fluorapatite

Solid state amorphization can be achieved in several ways, including alpha or electron beam irradiation, chemical processing, or mechanical grinding. The most probable is amorphization under a pressure not lower than 11 GPa, but most likely higher than 30 GPa.

If such an assumption is correct, the cubic perovskite was subjected to a pressure of several gigapascals, or could have been formed at such a pressure.

The mineral also has an unusually high silicon content. Perovskites with a high silicon content have previously been observed only as inclusions in diamonds of superdeep, mantle origin. However, there is no geological evidence of mantle rocks coming to the surface in the Dead Sea region or remnants of a large-scale impact structure.

Scientists speculate that evidence of high-pressure events or mantle rock outcrops may have been obliterated by later superimposed pyrometamorphic processes and extensive erosion in the Southern Levant, while geochemical anomalies in this area are still not fully explained.

Why is this discovery so important? First, it may provide a new point of view on the geochemical processes in the Spotted Zone, since cubic perovskite combines a number of geochemically significant elements in its composition.

Secondly, it must be remembered that the structure of perovskite is one of the “building blocks” from which the lattice of natural minerals is built, as well as the basis for the synthesis of new materials with interesting properties. Perovskite silicates are considered to be the main constituents of the Earth’s mantle.

The behavior of synthetic perovskite at high pressures has been studied in detail as a predictive model for the evolution of silicate perovskites under mantle conditions. At high pressures and temperatures, it can acquire a cubic shape.

This modification is reversible on cooling. In synthetic systems, the replacement of titanium for iron leads to the formation of hardened cubic perovskite, but natural perovskite is practically devoid of iron. Therefore, it was not previously assumed that cubic perovskite occurs in nature.

The stabilization of the cubic symmetry of the found mineral is achieved due to the previously unobserved displacement of the oxygen atom from its ideal structural position.

After analyzing the structure, the scientists suggested that it provides a mechanism for incorporating almost any element into the framework of natural perovskites. The stability of such disordered structures to very high pressures may indicate that such perovskites can be universal carriers of most elements of the Earth’s mantle.

The discovery of natural cubic perovskite allows us to consider it as a potential carrier of the majority of petrogenic and trace elements in the inner geospheres of our planet.

The stability of the discovered mineral at pressures at the mantle level makes it a candidate for the content of both rock-forming and less common elements in the interior of the planets.

That is, along with a new, disordered type of perovskite framework and structurally allowed oxygen deficiency, new ways are opened for modeling the behavior of such structures in various planetary environments.

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