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

Abstract

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 # 1000°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 # 1000°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 # 1000°C. Another intermediate mineral, huntite (CaCO3 . 3MgCO3), 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 a0 = 9.933 +/- 0.002, b0 = 6.712 +/- 0.001 and c0 = 24.06 +/- 0.02 a 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 # 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-MgO-SiO2-CO2 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.