Carbonates are the main species that bring carbon deep into our planet
through subduction. They are an important rock-forming mineral group,
fundamentally distinct from silicates in Earth's crust in that carbon binds to
three oxygen atoms, while silicon is bonded to four oxygens. Here, we present
experimental evidence that under the sufficiently high pressures and high
temperatures existing in the lower mantle, ferromagnesian carbonates transform
to a phase with tetrahedrally coordinated carbons. Above 80 GPa, in situ
synchrotron infrared experiments show the unequivocal spectroscopic signature
of the high-pressure phase of (Mg,Fe)CO3​. Using ab-initio calculations, we
assign the new IR signature to C-O bands associated with tetrahedrally
coordinated carbon with asymmetric C-O bonds. Tetrahedrally coordinated
carbonates are expected to exhibit substantially different reactivity than low
pressure three-fold coordinated carbonates, as well as different chemical
properties in the liquid state. Hence this may have significant implications on
carbon reservoirs and fluxes and the global geodynamic carbon cycle