Most naturally-formed diamonds are from the Earth's mantle, generally understood to be formed as carbon is exposed to extreme temperature and pressure. A new study looks at the effects of a new parameter - electric fields - in the formation of these precious minerals.
Diamond is also a special solid form of carbon, much like graphite. Its crystalline structure and arrangement of carbon molecules create its signature hardness. Naturally, diamonds form at depths of at least 150 kilometers deep inside the Earth - to temperatures of more than 1,500 degrees Celsius and several gigapascals of pressure.
While varying theories explain how diamonds exactly form, most of them agree that the starting material for the process is carbonate-rich melts, such as compounds containing magnesium, silicon, and calcium that also contain oxygen and carbon.
A New Factor For Diamond Formation
Since the Earth's mantle is filled with molten metal and liquid materials - materials with generally high electrical conductivity - at high temperature, electrochemical processes also occur at this layer. Researchers from the V.S. Sobolev Institute of Geology and Mineralogy SB from the Russian Academy of Sciences Novosibirsk, led by Yuri Palyanov, developed a new model that describes the formation of diamonds that consider a new factor: localized electrical fields. Details of their report appear in the journal Science Advances.
In the proposed model, applying less than one volt is enough to provide electrons that could kickstart a chemical transformation process, making it possible for carbon-oxygen components of carbonate materials to become carbon dioxide through chemical reactions, ultimately leading to pure carbon, which can then form into a diamond.
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To test this theory, the Russian researchers developed their experimental setup that includes a miniature platinum capsule placed in a heating system, which in turn was encased in a high-pressure setup that could deliver up to 7.5 gigapascals of pressure. Small and precise electrodes lead into the capsule, which contains carbonate and carbonate-silicate powders. The setup was run at high temperatures - from 1300 to 1600 degrees Celsius - exposing the powders for as long as 40 hours at a time.
Small Electric Fields on Diamond Formation
As predicted, tiny diamonds started forming near the electrode after hours of exposure. However, this was only attained upon applying a small voltage, with close to 0.5 volts being enough to start the process. The resulting diamonds reached a diameter of 200 micrometers or about a fifth of a millimeter; these were generally smaller than a grain of sand, but diamonds nonetheless.
Additionally, other pure-carbon mineral graphite was also found to form under lower pressures. When researchers reversed the polarity of the voltage, diamonds formed on the other electrode, further lending credence to their theory. Running the setup without any voltage applied meant that diamonds, or even graphite, could form.
"The experimental facilities in Novosibirsk are absolutely impressive," says Michael Wiedenbeck, SIMS laboratory head at the GFZ, under Potsdam's Modular Earth Science Infrastructure (MESI) in Germany. His team has been working with the Russian researchers for more than ten years.
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