Dense
β
‐K
2
Ca
3
(CO
3
)
Anna Yu. Likhacheva, Alexandr V. Romanenko, Sergey V. Goryainov, Sergey V. Rashchenko, Anton Shatsky
ABSTRACT
The high‐pressure β ‐K 2 Ca 3 (CO 3 ) 4 (space group Pnma ), synthesized at 6 GPa / 900°C–1300°C, is regarded as potential host for carbon and potassium involved in the alkali carbonatite magmatic processes in the deep Earth. The high‐pressure behavior and stability of synthetic β ‐K 2 Ca 3 (CO 3 ) 4 are characterized by in situ Raman spectroscopy up to 16 GPa and 400°C. Within this pressure range the Raman spectrum changes regularly at 25°C–200°C, the shift rates for the main C–O bands being the lowest among K‐Ca carbonates. The lower stability limit of β ‐K 2 Ca 3 (CO 3 ) 4 is roughly constrained within ~680°C / 4.6 GPa to ~450°C / 7 GPa according to the reaction “ β ‐K 2 Ca 3 (CO 3 ) 4 → bütschliite K 2 Ca(CO 3 ) 2 + aragonite CaCO 3 ”. In the low‐temperature part of K 2 Ca 3 (CO 3 ) 4 phase diagram (up to 680°C at 4.5 GPa) bütschliite is stable at 0–6.5 GPa. This implies that bütschliite, the only K‐Ca carbonate found in the inclusions in deep minerals, can be a product of decomposition of dense K‐Ca carbonates during their ascent from depth.