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 (K 2 Mg(CO 3 ) 2 , K 2 Ca(CO 3 ) 2 , Na 2 Mg(CO 3 ) 2 , and Na 2 Ca(CO 3 ) 2 ) using first-principles calculations up to 25 GPa. For K 2 Mg, K 2 Ca, and Na 2 Mg double carbonates, the transitions from rhombohedral structures ( R -3 m or R -3 ) to monoclinic ( C 2/ m ) or triclinic ( P -1 ) structures are predicted. While for Na 2 Ca(CO 3 ) 2 , the P 2 1 ca structure remains stable across the calculated pressure range. But the high-pressure behavior of Na 2 Ca double carbonate has changed over 8 GPa: the b -axis becomes more compressible than a -axis; [CO 3 ] –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 [CO 3 ] 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.