Only a small fraction of our planet’s total carbon budget is found at the surface. In fact, the Earth’s mantle is thought to be the largest carbon reservoir. Carbonates, and in particular ferromagnesite ((Mg,Fe)CO3), are likely candidates for deep-Earth carbon storage. The behavior of these carbonates at the high pressure and temperature conditions within the planet are therefore of great interest for understanding the global carbon cycle.
A paper from the Galli group in collaboration with the Mao group at Stanford University has been published in Nature Communications reporting unequivocal evidence of tetrahedrally coordinated carbon in high pressure carbonates.
Evidence of the new carbon-oxygen bond was obtained by combining in situ synchrotron infrared spectroscopic experiments in a diamond anvil cell with ab initio calculations. The paper, with first authors Eglantine Boulard and Ding Pan, identified a unique vibrational signature present only in the high pressure phase of ferromagnesite, which was assigned to a new carbon-oxygen bond formed under pressure. In particular, the new vibrational signature was associated to carbon bonded to four oxygen atoms with asymmetric C-O bonds.
Ferromagnesite represents an important rock-forming mineral group, fundamentally distinct from silicates in Earth’s crust because at low pressures, carbon bonds to three oxygen atoms, while silicon can bond to four. The tetrahedrally coordinated carbonates are expected to exhibit a dramatically different behavior compared to three-fold coordinated carbonates. For example, reactivity with other phases in the mantle will be altered and different chemical properties will arise when in a melt. Hence tetrahedrally coordinated carbonates may have significant implications on carbon reservoirs and fluxes and on the study of the global geodynamic carbon cycle.
Read the full press release on the Deep Carbon Observatory website.
Boulard E, Pan D, Galli G, Liu Z, Mao WL (2015). Tetrahedrally coordinated carbonates in Earth’s lower mantle. Nature Communications, 6, Article number: 6311 doi:10.1038/ncomms7311