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.

High‐Pressure Na‐Ca Carbonates in the Deep Carbon Cycle

“…The Na-Ca carbonates recovered from experiments at 0.1, 3, and 6 GPa were characterized by Raman spectroscopy and their crystal structures were solved: Na 4 Ca(CO 3 ) 3 (Ia−3d) , Na 2 Ca(CO 3 ) 2 (P2 1 ca) nyerereite (Gavryushkin et al, 2016), Na 2 Ca 3 (CO 3 ) 4 (P1n1) (Gavryushkin et al, 2014;Shatskiy et al, 2015), Na 2 Ca 4 (CO 3 ) 5 (P6 3 mc) burbankite (Rashchenko, Bakakin, et al, 2017). Although in situ high-pressure studies of Na-Ca carbonates have just begun (Borodina et al, 2018;Vennari et al, 2018), they already suggest an existence of unquenchable high-pressure polymorphs of Na 2 Ca 2 (CO 3 ) 3 and probably Na 2 Ca(CO 3 ) 2 at pressures exceeding 15 GPa. The compounds with similar stoichiometries were also observed in quench experiments.…”

“…Room-temperature high-pressure behavior of nyerereite and shortite was recently studied by in situ Raman spectroscopy and X-ray diffraction (Borodina et al, 2018;Rashchenko, Goryainov, et al, 2017;Vennari et al, 2018). The equilibrium high-pressure investigation of the Na 2 CO 3 -CaCO 3 system was initiated by studies of its phase diagram at 3 and 6 GPa (Podborodnikov et al, 2018;Shatskiy et al, 2013Shatskiy et al, , 2015Litasov et al, this volume).…”

Structures and Crystal Chemistry of Carbonate at Earth's Mantle Conditions

“…But the more relevant result is the evidence of the progressive displacement of the C atom from a trigonal planar geometry to a 3+1 coordination, resulting in a dimerization of carbonate ions units. This effect is visible on half of the carbonate groups in the shortite II phase (Vennari et al, 2018).…”

“…However, a detailed study of different structures adopted by these carbonates at mantle conditions is still unknown. Recent published data on shortite (Na 2 Ca 2 [CO 3 ] 3 ) (Vennari et al, 2018) show how at pressures above 15-17 GPa the structure of shortite I transforms to shortite II, with a substantial density increase of about 2.5%. But the more relevant result is the evidence of the progressive displacement of the C atom from a trigonal planar geometry to a 3+1 coordination, resulting in a dimerization of carbonate ions units.…”

Ab initio исследования влияния давления на структуру, электронные и упругие свойства карбонатов щелочных--щелочно-земельных металлов

“…Исследования [14] показали, что эйтелит является стабильной полиморфной модификацией, по крайней мере, до 6.6 GPa. Поведение шортита под давлением изучалось в [15], где установлено, что выше 15 GPa появляется фаза шортит-II с группой симметрии Pm, а в интервале 12−30 GPa кристаллизуется фаза шортит-III, группа симметрии которой пока не определена. Теоретическими методами упругие, термодинамические и динамические свойства шортита, в том числе под давлением, изучались в [16].…”