The phase diagrams of Na ${}_2$ CO ${}_3$ and K ${}_2$ CO ${}_3$ have been determined with multianvil (MA) and diamond anvil cell (DAC) techniques. In MA experiments with heating, $\gamma$ -Na ${}_2$ CO ${}_3$ is stable up to 12 GPa and above this pressure transforms to $P{6}_3$ /mcm-phase. At 26 GPa, Na ${}_2$ CO ${}_3$ - $P{6}_3$ /mcm transforms to the new phase with a diffraction pattern similar to that of the theoretically predicted Na ${}_2$ CO ${}_3$ - $P{2}_1$ /m. On cold compression in DAC experiments, $\gamma$ -Na ${}_2$ CO ${}_3$ is stable up to the maximum pressure reached of 25 GPa. K ${}_2$ CO ${}_3$ shows a more complex sequence of phase transitions. Unlike $\gamma$ -Na ${}_2$ CO ${}_3$ , $\gamma$ -K ${}_2$ CO ${}_3$ has a narrow stability field. At 3 GPa, K ${}_2$ CO ${}_3$ presents in the form of the new phase, called K ${}_2$ CO ${}_3$ -III, which transforms into another new phase, K ${}_2$ CO ${}_3$ -IV, above 9 GPa. In the pressure range of 9–15 GPa, another new phase or the mixture of phases III and IV is observed. The diffraction pattern of K ${}_2$ CO ${}_3$ -IV has similarities with that of the theoretically predicted K ${}_2$ CO ${}_3$ - $P{2}_1$ /m and most of the diffraction peaks can be indexed with this structure. Water has a dramatic effect on the phase transitions of K ${}_2$ CO ${}_3$ . Reconstruction of the diffraction pattern of $\gamma$ -K ${}_2$ CO ${}_3$ is observed at pressures of 0.5–3.1 GPa if the DAC is loaded on the air.

A rare mineral shortite, Na₂Ca₂(CO₃)₃, occurs among groundmass minerals in unaltered kimberlites, which suggests its participation in the evolution of kimberlite system. This work presents a high pressure Raman spectroscopic study of natural shortite (Udachnaya east kimberlites) compressed in KBr up to 8 GPa in a diamond anvil cell. At ambient pressure the spectrum contains two strong bands related to symmetric C-O stretching vibrations, four in-plane bending modes, and several low-frequency modes of lattice vibrations. Upon the pressure increase up to 8 GPa, almost all the bands exhibit positive shift with the rate of 1–4 cm⁻¹/GPa for the lattice modes and 3.6 and 3.9 cm⁻¹/GPa for the C-O stretching modes. The shifts of Raman modes are rather regular, which implies the absence of reconstructive phase transitions within the studied pressure range, similarly to the behavior of nyerereite, a related carbonate mineral. However, minor anomalies in the ν/P and FWHM/P dependences, observed at about 2 GPa, suggest some rearrangement and disordering of carbonate groups. The obtained data can be used for the estimation of residual pressure in shortite-bearing inclusions in deep-seated minerals.