Researchers Discover the ‘Diamond of Carbonate’

At the surface, pure carbon takes the form of graphite, a soft mineral composed of sheets in which every carbon atom is joined to a triangle of three others. Deep within Earth’s mantle, however, extreme temperatures and pressures convert graphite into diamond, a denser mineral where each carbon atom is joined to four other carbon atoms, forming a tetrahedral bonding geometry. Once thought unique, new research suggests that other carbon compounds may also make a similar phase shift under pressure. A new study provides strong evidence that carbon in calcium carbonate assumes a diamond-like bonding pattern in the lower mantle, as well.

Sergey Lobanov (Stony Brook University, USA, formerly at the Carnegie Institution of Washington, USA where he conducted this research), a member of the DCO Extreme Physics and Chemistry Community, and fellow DCO members Alex Goncharov (Carnegie Institution of Washington, USA), Naira Martirosyan (Sobolev Institute of Geology and Mineralogy, Russia), and Konstantin Litasov (Sobolev Institute of Geology and Mineralogy and Novosibirsk State University, Russia) explored the phase transition from triangular (CO3) to tetrahedral (CO4) calcium carbonate. They heated and squeezed calcium carbonate in the lab and used advanced analytic techniques to identify a chemical signature indicating that the bonding between carbon and oxygen atoms had changed and the phase shift had occurred. The researchers report their findings in a new paper in the journal Physical Review B [1].

“This is a new phase of calcium carbonate that we have discovered and now proven experimentally,” said Lobanov. “It is different from low-pressure calcium carbonate (calcite), which is one of the most abundant carbonate minerals on Earth, in the same way that diamond is different from graphite. We can think of this new phase as a ‘diamond of carbonate.’”

Lobanov adds, “This new calcium carbonate phase may have very different physical and chemical properties compared to its low-pressure­ forms, just as diamond differs from graphite.”

Previously, researchers had made theoretical predictions that tetrahedrally bonded calcium carbonate exists in the lower mantle, but no one had conclusively demonstrated its formation experimentally.

The researchers placed calcium carbonate into a diamond anvil cell, a device that generates great pressure, and heated the sample with a laser to simulate conditions in the lower mantle. They analyzed the resulting compounds using Raman spectroscopy, a technique where by shining a laser at a sample, researchers can use the light scattered by the chemical bonds to identify molecules. Additionally they performed X-ray diffraction, which uses the way that atoms in the crystal scatter a beam of X-rays to determine the atomic structure. Through Raman spectroscopy, the researchers identified a chemical signature that is evidence of tetrahedrally bonded carbon. They show that this transition occurs around 105 GPa, which is roughly 1 million times the pressure at sea level.

“We care about carbonates at high pressure and temperature conditions because they may be transported deep down to the lowermost mantle,” said Lobanov. “If calcium carbonates survive the trip that far below the surface, this would be the crystal structure that they form.“

In future work, the newly identified signature could help identify the same crystal structure in other carbonate minerals. Tetrahedrally bonded carbonates may react differently than carbonates with triangular bonds when interacting with silicate-containing minerals, which make up a large portion of the mantle. One of the next steps is to investigate the behavior of other carbonate minerals that may have these exotic new structures at high pressure.

Lobanov initially joined the Carnegie Institution in 2012, with partial funding from the DCO, to study chemical reactions in hydrocarbons at high temperature and pressure. He began working on carbonates and phase transitions “by accident,” but he is interested to see how this research field develops. This new work is in line with the Extreme Physics and Chemistry Community goal of discovering novel crystal structures occurring within deep Earth.

At surface temperatures and pressures, calcium carbonate most commonly exists in a white crystal form called calcite. This cleaved calcite single crystal is showing one of its unique physical properties, high birefringence, the ability to split a light beam into two rays travelling with different paths. Credit: APN MJM/Wikimedia Commons

At ambient temperature and pressure, the carbonate ion (CO32-) in calcium carbonate forms triangular structures (left), with carbon represented by pink and oxygen in white. At elevated temperature and pressure conditions, however, the carbonate shifts into a more compact tetrahedral formation, (right), a coordination that is similar to carbon coordination in the crystal structure of diamond. Credit: Public Domain/Wikimedia Commons

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