High-pressure synthesis of U2[CO3]3 and U[CO3]2 as potential host phases for uranium in the Earth’s mantle

TL;DR

Scientists synthesized two uranium carbonates under high pressure, suggesting they could store uranium in Earth's mantle and contribute to heat generation.

Contribution

The discovery of two new uranium carbonate compounds and their potential role as uranium hosts in the Earth's mantle.

Findings

U2[CO3]3 and U[CO3]2 were synthesized under lower mantle conditions using a diamond anvil cell.

U2[CO3]3 contains trivalent uranium, while U[CO3]2 contains tetravalent uranium.

These carbonates could act as host phases for uranium in carbon-rich mantle lithologies.

Abstract

It is well established that a significant amount of heat produced in the Earth’s mantle is due to the decay of uranium. However, uranium cannot be incorporated in large amounts into the most common mantle minerals. Here, we suggest that carbonates could be host phases for uranium in carbon-rich mantle lithologies. Two anhydrous uranium carbonates, U2[CO3]3 and U[CO3]2, were simultaneously synthesized by a reaction of UO2 with CO2 in a laser-heated diamond anvil cell at 20(1) GPa and 1800(200) K. Their crystal structures were obtained from synchrotron-based single crystal diffraction data and reproduced by density functional theory-based calculations. In U2[CO3]3 trivalent uranium cations are present, while uranium is four-valent in U[CO3]2. The synthesis of U2[CO3]3 and U[CO3]2 is a significant extension of the chemistry of uranium compounds and we provide a straightforward synthesis route for a UIII-containing compound.

It is well established that a significant amount of heat produced in the Earth’s mantle is due to the decay of uranium, yet the incorporation of uranium in deep mantle phases remains poorly explored. Here, two chemically simple uranium carbonates (U2[CO3]3 and U[CO3]2) were synthesized by a reaction of UO2 with CO2 at lower mantle conditions, revealing that uranium carbonates could be host phases of uranium in carbon-rich lithologies in the Earth’s mantle.

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