Structure and thermal equation of state of Ca3KNa(CO3)4 carbonate

The presence of alkali-carbonates in the Earth’s mantle is suggested by the occurrence of these minerals as diamond inclusion. We expect that alkali-carbonates may play a significant role in carbon-related processes (volcanism, mantle metasomatism, diamond formation…) in upper mantle and transition zone. High-pressures and temperatures experiments have recently revealed the possible occurrence of new class of Ca-alkali-carbonates above 4 GPa. We report the crystal structure of Ca3NaK(CO3)4 synthesized at 4 GPa. It appears in equilibrium with carbonatitic liquids. It presents a similar topology wih the reported Ca3Na2(CO3)4 high-pressure phase (Gavryushkin et al., 2014 Cryst Growth Des. 14, 4610) but a different unit cell and symmetry. Irregular coordination sites for Ca, K and Na cations and a partially disordered CO3 group arrangement reveal an anomalous elastic behaviour for the different cation sites, suggesting an interesting capability of trace element incorporation in the structure. Thermal equation of state of Ca3NaK(CO3)4 is determined by single crystal synchrotron X-ray diffraction with resistive heated DAC (ESRF, Grenoble, ID15a beamline) and the results are used to model the possibility for these phases to differentiate from liquids and stagnate in the upper mantle.