Silicates are by far the most abundant minerals in the Earth’s crust and mantle, while carbonates play an important role in carrying carbon from the surface into the interior as tectonic plates are subducted. To build a picture of the Earth’s deep carbon cycle, geoscientists need to understand how the two types of mineral interact at depth.

In ordinary conditions, carbon in carbonates is normally bonded to three oxygen atoms in flat, triangular units, whereas silicon in silicates is usually surrounded by four oxygen atoms in tetrahedra. These structures have been expected to change under extreme pressure, but quite how has been uncertain. Over a decade ago, theorists predicted the possibility of hybrid structures containing four-coordinated carbon and six-coordinated silicon, but experiments have struggled to achieve the necessary conditions.

Now an ESRF-led team has produced such a hybrid structure – calcium silicate-diorthocarbonate (CaSiC₂O₇), at pressures of around 122 GPa and temperatures of around 2,800 K – conditions approaching those at the core–mantle boundary. It was not what the researchers had originally been looking for. “It was a surprise,” says ESRF scientist Georgios Aprilis, the first author of the paper. “The synthesis of the silicate-carbonate was not the primary goal of our experiment.”

The material was produced in a laser-heated diamond anvil cell, using the double-sided laser-heating system installed at ID18. After heating, single-crystal X-ray diffraction at ID15B was used to identify and determine the structure of crystals of the new phase only a few micrometres across. For this, a small beam was essential. “Without the EBS we wouldn’t have been able to fully resolve the crystal structure and get data of good enough quality to build our model,” says Aprilis.

The resulting structure contains carbon surrounded by four oxygen atoms and silicon surrounded by six. According to Aprilis, it is similar to oranges stacked in pyramids at supermarkets, with the oranges – oxygen and calcium atoms – tightly packed together, leaving gaps that can accommodate the smaller carbon and silicon atoms. “These arrangements allow for both silicon and carbon to be present in the same dense crystal structure,” he says.

The discovery expands the range of minerals that could potentially host carbon inside the Earth. In principle, carbon carried down into the deep mantle within carbonates could react with the surrounding silicates and become incorporated into new silicate–carbonate phases. However, Aprilis stresses that this process would involve pressures that are too great for technological carbon sequestration in the fight against climate change. “The research is more relevant to the deep carbon cycle and our understanding of how carbon is stored in the Earth’s deep interior.”

Calculations also suggest that CaSiC₂O₇ has an unusually low shear-wave velocity. That could make it and similar compounds a contender for the makeup of “large low-shear-velocity provinces” (LLSVPs) – enormous, and so far unexplained, structures detected by seismic tomography just outside the core, beneath Africa and the Pacific Ocean. A definitive answer on that would require geoscientists to know whether silicate–carbonates occur naturally in those regions, however, as well as precisely which conditions are necessary for formation.

“We’re planning to study the formation conditions and chemical pathways that allow these phases to occur now, in more detail,” says Aprilis.

Reference:

Aprilis, Georgios et al. Calcium silicate-diorthocarbonate is a missing link in deep-mantle chemistry of silicates and carbonates. Nat Commun (2026) https://doi.org/10.1038/s41467-026-77204-w

Text by Jon Cartwright

Top image: “Stacked oranges” model of the CaSiC2O7 structure, obtained from single crystal X-Ray diffraction at the ID15B at a pressure of 122 GPa. The “oranges” or oxygen and calcium atoms (orange and grey, respectively) form a tightly packed pyramidal framework, within which silicon (blue) and carbon (yellow) atoms can reside. Yellow and blue volumes denote CO4 and SiO6 polyhedra, respectively.