The heat capacity of a natural monticellite and phase equilibria in the system CaO-MgO-Si02-C02*

The heat capacity of a natural monticellite (Ca,.,Mg.,,Fe.~Mn.o,Sio.9903.99) measured between 9.6 and 343 K using intermittent-heating, adiabatic calorimetry yields C$298) and S&s of 123.64 f 0.18 and 109.44 +0.16 J mol-’ K-’ respectively. Extrapolation ofthis entropy value to end-member monticellite results in an S$, = 108.1 + 0.2 J. mol-’ K-‘. High-temperature heat-capacity data were measured between 340-1000 K with a differential scanning calorimeter. The high-temperature data were combined with the 290-350 K adiabatic values, extrapolated to 1700 K, and integrated to yield the following entropy equation for end-member monticellite (2981700 K): S$(J.mol-’ K-‘) = .S’&, + 164.79 In T + 15.337. 10m3 T + 22.791 . lo5 T-I 968.94 Phase equilibria in the CaO-MgO-Si02 system were calculated from 973 to 1673 K and 0 to 12 kbar with these new data combined with existing data for akennanite (Ak), diopside (Di), forsterite (I%), merwinite (Me) and wollastonite (Wo). The location of the calculated reactions involving the phases MO and Fo is affected by their mutual solid solution. A best fit of the thermodynamically generated curves to all experiments is made when the S&, of Me is 250.2 J mol-’ K-‘, less than the measured value of 253.2 J * mol-’ K-l. A best fit to the reversals for the solid-solid and decarbonation reactions in the CaO-MgO-Si02-CO2 system was obtained with the AGqg8 (kJ mole-‘) for the phases Ak (-3667), Di (-3025), Fo (-205 1). Me (-43 17) and MO (-2 133). The two invariant points Wo and -Fo for the solid-solid reactions are located at 1008 ? 5 K and 6.3 f 0.1 kbar, and 1361 ? 10 K and 10.2 I? 0.2 kbar respectively. The location of the thermodynamically generated curves is in excellent agreement with most experimental data on decarbonation equilibria involving these phases.