An infrared study of carbon-oxygen bonding in magnesite to 60 GPa

Infrared spectra of magnesite, MgCO3, have been measured in the diamond anvil cell from 0 to 60 GPa at 300 K. Mode shifts were determined for the in-plane bend, the asymmetric-stretch, and the out-of-plane bend of the carbonate group, as well as a magnesium ion translational mode. The in-plane bend shows a small positive shift and ultimately merges with the out-of-plane bend, exhibiting an accidental degeneracy by 60 GPa. The asymmetric stretch exhibits highly nonlinear behavior, shifting monotonically to pressures of similar to 30 GPa, at which point its shift markedly decreases to similar to 50 GPa. At 50 GPa, it again shifts strongly with pressure. We calculate force constants for the carbonate group under pressure from our data, and correlate these with the C-O bond length. Our data are consistent with a slight lengthening (similar to 0.01 angstrom) of the C-O bond between 30 and 50 GPa to a maximum bond distance of similar to 1.27 angstrom. Although our finding of bond lengthening is in general agreement with previously published Rietveld refinements of X-ray diffraction data for magnesite under pressure, our data indicate that the lengthening is likely substantially smaller than previously proposed. The expansion of the C-O bond in magnesite can be produced by rotations of the MgO6 octahedra, and such non-ionic crystal-chemical effects may be a key contributor to the remarkable stability of R (3) over barc-structured carbonates to very high pressures.