The proposal concerns the study of high-pressure phase transformations and elastic and structural behaviour of alkali-carbonates in the system Ca-Na-K-REE-C-O at Earth’s mantle pressures, by single crystal X-ray diffraction. We will focus on burbankite, (Na,Ca)3(Ca,La,Ce)3(CO3)5, and high pressure (Na,K)2Ca3(CO3)4 carbonate. Both structures are on the liquidus of carbonatitic melts at P > 4 GPa. The knowledge of their physical properties is fundamental to understand and quantitatively model the fractionation of trace elements and the processes related to the concentrations of strategic elements, such as those in REE-carbonatite. The possible identification of higher-pressure structures, at mantle transition zone and lower mantle will be fundamental to understand carbonatitic systems also at ultra-high-pressures and planetary conditions.