High-pressure behavior and stability of synthetic buetschliite K2Ca(CO3)2 up to 19 GPa and 300°C

The occurrence of buetschliite, K2Ca(CO3)(2), as inclusions in mantle minerals, is considered as one of the keys to understanding phase relationships of dense carbonates and outlines the potential role of potassium carbonates in the Earth's deep carbon cycle. Within this scope, the high-pressure behavior of synthetic buetschliite is characterized by in situ Raman spectroscopy up to 19 GPa and 300 degrees C. Up to 6 GPa, the compression is regular, then the splitting of some of the lattice and internal modes defines the transition to a low-symmetry phase, in analogy to that observed previously in K2Mg(CO3)(2). The temperature rise to 300 degrees C shifts the transition pressure from similar to 6 to similar to 8 GPa, but on the whole, it does not change the high-pressure behavior of K2Ca(CO3)(2). The observed pressure-induced spectral changes are fully reversible at room and elevated temperature. The findings show the expansion of buetschliite baric stability with temperature, which confirms its importance as a constituent of carbonate inclusions in deep minerals.