title: Tetrahedrally coordinated carbonates in Earth's lower mantle.
abstract: 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 the 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 infrared 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 threefold coordinated carbonates, as well as different chemical properties in the liquid state. Hence, this may have significant implications for carbon reservoirs and fluxes, and the global geodynamic carbon cycle.
authors.affiliation: Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA.
authors.forename: Eglantine
authors.initials: E
authors.lastname: Boulard
authors.affiliation: Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Ding
authors.initials: D
authors.lastname: Pan
authors.affiliation: Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA.
authors.forename: Giulia
authors.initials: G
authors.lastname: Galli
authors.affiliation: Geophysical Laboratory, Carnegie Institution of Washington, Washington, District Of Columbia 20015, USA.
authors.forename: Zhenxian
authors.initials: Z
authors.lastname: Liu
authors.affiliation: 1] Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA [2] Photon Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
authors.forename: Wendy L
authors.initials: WL
authors.lastname: Mao