High-Pressure/Temperature Behavior of the Alkali/Calcium Carbonate Shortite (Na2Ca2(CO3)3): Implications for Carbon Sequestration in Earth's Transition Zone

Abstract

The behavior of shortite (Na2Ca2(CO3)3) has been probed using synchrotron-based single crystal X-ray diffraction and Raman spectroscopy at high pressures and following laser heating to illuminate carbon retention within the deep earth, and phase equilibria of alkali/calcium carbonate-rich systems. Above 15 GPa, a transition to the shortite-II structure occurs at 300 K. This phase is novel as it involves a large distortion of the carbonates, with an onset of 3 + 1 coordination and near-dimerization of carbonate groups. Above 22 GPa, shortite-II amorphizes. Samples laser heated at pressures between 12 and 30 GPa crystallize in a new structure, shortite-III. Below 12 GPa, this phase appears to decompose into a mixture of shortite, nyerereite (Na2Ca(CO3)2), and aragonite (CaCO3) in accord with prior phase equilibria results. The high-pressure behavior of nyerereite using Raman spectroscopy was also investigated to 25 GPa. The structural response of shortite to pressure is modulated by the sodium cations in the structure; hence, the behavior of alkali-rich carbonates within kimberlitic systems at depth is likely dependent on the bonding and local geometry of alkali cations. Our results show that complex, dense high-pressure structures are generated in the shortite system, and phase equilibria of the protoliths of carbonatites and kimberlites at deep upper mantle and transition zone pressures will involve intermediate alkali-calcium carbonate phases, including the high-pressure phases of shortite. Moreover, 3 + 1 coordination of carbon is observed at far lower pressures than other systems: this coordination could become important in complex carbonates and possibly liquids at substantially shallower depths than previously anticipated.