Crystallographic mechanism of the elastic behaviour of synthetic bütschliite K2Ca(CO3)2 on compression to 20 GPa

Abstract

Bütschliite, K₂Ca(CO₃)₂, occurring as inclusions in mantle minerals, is regarded as one of the key phases to understand phase relationships of dense potassium carbonates and thus to evaluate their potential role within the Earth’s deep carbon cycle. Accordingly, the high-pressure behavior of synthetic bütschliite has been investigated by in-situ single-crystal X-ray diffraction under isothermal compression up to 20 GPa at T = 298 K. The compression mechanism before and after the trigonal-to-monoclinic (R-3m to C2/m) phase transition at ∼6 GPa, found previously, is characterized in terms of the evolution of the cation polyhedra and carbonate groups. On this basis, the modulation of the axial compression is interpreted, and the contribution of the cation polyhedra into the bulk compression is estimated. The refined compressibility of the monoclinic phase (K₀ = 44(2) GPa) fits to the trend of the carbonate bulk modulus versus average non-carbon cation radius. The analysis of the obtained and literature structural data suggests the distortion of a large cation polyhedron to be an effective tool to strengthen the carbonate structure at high pressure. On the other hand, the observed symmetrization of the cation polyhedra in trigonal bütschliite is apparently a crucial factor of its stabilization at high pressure upon the temperature rise observed previously. The structural crystallography provided in this study supports the enhanced stability of trigonal bütschliite at high P, T conditions and its significance of being considered as a constituent of the inclusions in deep minerals.