HIGH-PRESSURE PHASE-TRANSFORMATIONS OF CARBONATES IN THE SYSTEM CAO-MGO-SIO2-CO2

The phase behaviour of the carbonates in the system MgO-CaO-SiO2-CO2 have been studied in a diamond-anvil press employing YAG laser heating from about 40 to 260 kbar at similar to 1000 degrees C. It is already known that at the CaCO3-MgCO3 join calcite (CaCO3) is stable only at relatively low pressures (< 30 kbar), that aragonite (CaCO3) is stable between 30 and 400 kbar, and that magnesite (MgCO3) is stable between 7 and 550 kbar at similar to 1000 degrees C. In the present study it has been found that the intermediate mineral dolomite (CaCO3 MgCO3) decomposes into the mixture aragonite + magnesite in the pressure range 60-70 kbar at similar to 1000 degrees C. Another intermediate mineral, huntite (CaCO3 3MgCO(3)), transforms to a new phase at pressures below 40 kbar, and then decomposes to the mixture dolomite + magnesite at pressures greater than similar to 60 kbar. The new phase with the huntite composition possesses an orthorhombic cell with a(o) = 9.933 +/- 0.002, b(o) = 6.712 +/- 0.001 and c(o) = 24.06 +/- 0.02 Angstrom at ambient conditions. There are only two well-known silicate-carbonate minerals in the CaO-SiO2-CO2 system. These are spurrite and tilleyite. Both were found to decompose into their component silicates and carbonates at pressures below 40 kbar at similar to 1000 degrees 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-MgO-SiO2-CO2 system which are stable at pressures greater than about 70 kbar at similar to 1000 degrees C, if no interaction occurs between carbonates and silicates at still higher pressures.