Phase relations in the K2CO3-MgCO3 system have been studied in high-pressure high-temperature (HPHT) multi-anvil experiments using graphite capsules at 6.0 +/- 0.5 GPa pressures and 900-1450 degrees C temperatures. Subsolidus assemblies comprise the fields K2CO3+K2Mg(CO3)(2) and K2Mg(CO3)(2)+MgCO3 with the transition boundary near 50 mol% MgCO3 in the system. The K2CO3-K2Mg(CO3)(2) eutectic is established at 1200 degrees C and 25 mol% MgCO3. Melting of K2CO3 occurs between 1400 and 1450 degrees C. We propose that K2Mg(CO3)(2) disappears between 1200 and 1300 degrees C via congruent melting. Magnesite is observed as a subliquidus phase to temperatures in excess of 1300 degrees C. At 6 GPa, melting of the K2Mg(CO3)(2)+MgCO3 assemblage can be initiated either by heating to 1300 degrees C under "dry" conditions or by adding a certain amount of water at 900-1000 degrees C. Thus, the K2Mg(CO3)(2) could control the solidus temperature of the carbonated mantle under "dry" conditions and cause formation of the K- and Mg-rich carbonatite melts similar to those found as microinclusions in "fibrous" diamonds. The K2Mg(CO3)(2) compound was studied using in situ X-ray coupled with a DIA-type multi-anvil apparatus. At 6.5 GPa and 1000 degrees C, the structure of K2Mg(CO3)(2) was found to be orthorhombic with lattice parameters a = 8.8898(7), b = 7.8673(7), and c = 5.0528(5), V = 353.39(4). No structure change was observed during pressure decrease down to 1 GPa. However, recovered K2Mg(CO3)(2) exhibited a trigonal R (3) over barm structure previously established at ambient conditions.