High pressure Raman study of Na4Ca(CO3)3 from the ambient pressure to 11 GPa

ABSTRACT Na-Ca double carbonates were found as the inclusions in ‘superdeep’ Juina diamonds and as the products of high pressure experiments and thus are regarded as the potential host of CO2 in the deep mantle. Recently, Na-rich carbonate (21.0% wt. of Na2O vs. 13.6% wt. of CaO) was reported as the product of experiment with carbonated eclogite treated at 21 GPa and 1200°C. Na/Ca ratio of this carbonate corresponds to Na4Ca(CO3)3 phase with cubic symmetry synthesized at 6 GPa. The stability of Na4Ca(CO3)3 at higher pressures has not been investigated yet. Herein, we report an in situ Raman study of Na4Ca(CO3)3 carbonate up to 11 GPa in order to reveal stability of its structure at mantle pressures. Obtained results strongly imply for reversible phase transition near 8 GPa associated with decrease of symmetry. The observed phase transition questions the stability of cubic Na4Ca(CO3)3 under conditions of mantle transition zone.