The phase behaviour of the carbonates in the system MgO sbnd CaO sbnd SiO 2sbnd CO 2 have been studied in a diamond-anvil press employing YAG laser heating from about 40 to 260 kbar at ∼ 1000°C. It is already known that at the CaCO 3sbnd MgCO 3 join calcite (CaCO 3) is stable only at relatively low pressures (< 30 kbar), that aragonite (CaCO 3) is stable between 30 and 400 kbar, and that magnesite (MgCO 3) is stable between 7 and 550 kbar at ∼ 1000°C. In the present study it has been found that the intermediate mineral dolomite (CaCO 3 · MgCO 3) decomposes into the mixture aragonite + magnesite in the pressure range 60-70 kbar at ∼ 1000°C. Another intermediate mineral, huntite (CaCO 3 · 3MgCO 3), transforms to a new phase at pressures below 40 kbar, and then decomposes to the mixture dolomite + magnesite at pressures greater than ∼ 60 kbar. The new phase with the huntite composition possesses an orthorhombic cell with ao = 9.933 ± 0.002, bo = 6.712 ± 0.001 and co = 24.06 ± 0.02 Å at ambient conditions. There are only two well-known silicate-carbonate minerals in the CaO sbnd SiO 2sbnd CO 2 system. These are spurrite and tilleyite. Both were found to decompose into their component silicates and carbonates at pressures below 40 kbar at ∼ 1000°C. On the basis of the present study it can be concluded that aragonite and magnesite are the only two carbonates in the entire CaO sbnd MgO sbnd SiO 2sbnd CO 2 system which are stable at pressures greater than about 70 kbar at ∼ 1000°C, if no interaction occurs between carbonates and silicates at still higher pressures.