Natural diamonds are mainly formed under high pressure and high temperature conditions, which can be produced in the Earth’s deep interior or by collisions between celestrial bodies.

Intensive research over the past half century showed that most diamonds from the Earth’s deep interior were formed via a reduction-oxidation reaction between C–O–H-bearing fluid and rocks, or between carbonate melt and reductant (for instance, metallic iron), in which the formation of diamonds requires the reduction of carbon to its bare elemental form by the presence of extra reductant.

In a study of shocked ferromagnesian carbonate at the Xiuyan impact crater, China, the researchers from Guangzhou Institute of Geochemistry of Chinese Academy of Sciences, Center for shanghai High Pressure Science and Technology Advanced Research, and Geophysical Laboratory, Carnegie Institution of Washington, found that diamond can be produced directly from ankerite Ca(Fe²⁺,Mg)(CO₃)₂, a ferromagnesian carbonate, via a subsolidus self-reduction- oxidation reaction without melting, fluid, and another reductant.

This study reported that ankerite in the shock-metamorphosed gneiss of Xiuyan impact crater had self-reduced to diamond by concurrent oxidation of Fe²⁺ to Fe³⁺ to form a high pressure phase of MgFe³⁺₂O₄. Pressure and temperature conditions for the formation of diamond in the crater were constrained to 25–45 GPa and 800–900 °C, respectively. In the reaction for the diamond formation, Fe²⁺ in ferromagnesian carbonate had actually behaved as a reductant.

Ferromagnesian carbonates are among the dominant carbonates in organic-rich oceanic sedimentary environments on the Earth, especially in the pre-1.8-Gy sedimentary sequences. The old oceanic crust with abundant ferromagnesian carbonates might subduct into the Earth’s deep mantle, and diamond could be produced from ferromagnesian carbonates via above self-reduction-oxidation reaction. Besides, they are chemical equivalent of ferromagnesian oxide plus CO₂.

Ferropericlase (Fe,Mg)O is a major component in the lower mantle. It appears that CO₂ produced from ferromagnesian carbonate or from decomposition of iron-free carbonates might react with (Fe,Mg)O to form diamonds and high pressure form of MgFe³⁺₂O₄.

The subsolidus self-reduction mechanism for the diamond formation indicated that another reductant, liquid and melting conditions are no longer required. Diamonds could be ubiquitously present as a dominant host for carbon in the Earth’s lower mantle where the carbonates are abundant and pressures and temperatures are sufficiently high.

This mechanism for natural diamond formation provided valuable insights to deep carbon in the Earth’s lower mantle. The study was published in PNAS.