The existence of alkali-bearing carbonates in deep mineral inclusions and the presence of K-rich carbonatitic melts in the upper mantle raised the attention to the role of potassium within the Earth's deep carbon cycle. A complex phase diagrams of binary K-Ca carbonates up to 3-6 GPa are experimentally derived only from quenched samples, but properties related to stability aspects, possible polymorphism and associated transitions remain unexplored so far. We propose in situ single crystal XRD experiments in diamond anvil cell to study the structure compression mechanisms, mechanical stability and compressibility of K-Ca-carbonates with different K:Ca stoichiometry, synthesized at HP,T-conditions. The obtained data are necessary to unravel their role within the deep carbon cycle associated to the existence of crystalline products of carbonatitic melts at mantle conditions.