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

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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 (Ravg), the bulk modulus (K0), and the transition pressure (PT). The transition pressure (PT) is anticorrelated to the average metallic ionic radius (Ravg), and a larger Ravg 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.

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Acknowledgements

We would like to thank the Supercomputing Laboratory of IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences) for support.

Funding

This work was supported by the National Natural Science Foundation of China (Nos. 42072047, 41772034).

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Correspondence to Shan Qin.

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Hou, B., Huang, S. & Qin, S. Phase transitions and compressibility of alkali-bearing double carbonates at high pressures: a first-principles calculations study. Phys Chem Minerals 49, 34 (2022). https://doi.org/10.1007/s00269-022-01210-9

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  • DOI: https://doi.org/10.1007/s00269-022-01210-9

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