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Physical Chemistry

Carbonate mineral forms diamond on its own

New geological mechanism involving carbonate suggests Earth’s lower mantle may be rich in diamond

by Mitch Jacoby
March 5, 2018 | A version of this story appeared in Volume 96, Issue 10

When a meteorite slammed into Earth some 50,000 years ago, forming the bowl-shaped Xiuyan crater in northeast China, it left a treasure trove of geochemical research goodies. By applying microscopy and spectroscopy methods to analyze carbonate minerals found there, researchers have uncovered samples of diamond and a new mechanism for its formation (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1720619115).

Ming Chen of the Guangzhou Institute of Geochemistry, Ho-kwang Mao of the Center for High Pressure Science & Technology Advanced Research in Shanghai, and coworkers studied the effects of the high heat and pressures generated during impact on ankerite, a carbonate mineral containing calcium, iron, and magnesium.

They found that impact pressures of 25–45 GPa and temperatures of 800–900 °C were sufficient to decompose ankerite and form diamond. In that process, the carbonate component underwent self-reduction, turning into diamond, as iron changed oxidation states from Fe2+ to Fe3+ and formed a high-pressure polymorph of magnesioferrite (MgFe3+2O4). The transformation does not involve melting, and it represents a unique mechanism of diamond formation from carbonates in that it does not include a fluid and additional reductant.

The ability of carbonate to produce diamond by itself implies that diamond could be a very common mineral in Earth’s lower mantle, where carbonates are abundant and pressures and temperatures are high enough to drive this process, the researchers note.


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