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 similar to 25 GPa. For K2Mg, K2Ca, and Na2Mg double carbonates, the transitions from rhombohedral structures (R (3) m or R (3)) to monoclinic (C2/m) or triclinic (P (1)) structures are predicted. While for Na2Ca(CO3)(2), the P2(1)ca 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 (K-0), 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 (K-0) 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.