Phase transitions and compressibility of alkali-bearing double carbonates at high pressures: a first-principles calculations study

Here, we investigated high-pressure behaviors of four end-members of K-Na-Ca-Mg alkali-bearing double carbonates (K₂Mg(CO₃)₂, K₂Ca(CO₃)₂, Na₂Mg(CO₃)₂, and Na₂Ca(CO₃)₂) using first-principles calculations up to ~ 25 GPa. For K₂Mg, K₂Ca, and Na₂Mg double carbonates, the transitions from rhombohedral structures (R \(\stackrel{\mathrm{-}}{3}\) m or R \(\stackrel{\mathrm{-}}{3}\)) to monoclinic (C2/m) or triclinic (P \(\stackrel{\mathrm{-}}{1}\)) structures are predicted. While for Na₂Ca(CO₃)₂, the P2₁ca structure remains stable across the calculated pressure range. But the high-pressure behavior of Na₂Ca double carbonate has changed over 8 GPa: the b-axis becomes more compressible than a-axis; [CO₃] –I groups tilt out of the a-b plane upon compression and reverse the direction of rotation at 8 GPa. The parameters for the equations of state of these minerals and their high-pressure phases were all theoretically determined. The predicted transformation is driven by the differences in the compressibility of structural units. The K⁺ and Na⁺ coordination polyhedra are more compressible in the structure, compared with the high axial rigidity of C–O bonds in the [CO₃] triangle along the a-b plane. Our results provide projections of the high-pressure behaviors of trigonal double carbonates, in part by helping to clarify the relation among the average metallic ionic radius (R_avg), the bulk modulus (K₀), and the transition pressure (P_T). The transition pressure (P_T) is anticorrelated to the average metallic ionic radius (R_avg), and a larger R_avg results in a lower bulk modulus (K₀) for the trigonal double carbonates. Furthermore, alkali-bearing double carbonates found as inclusions in the natural diamond may indicate a hydrous parental medium composition and a deeper genesis mechanism.