The heat capacity of a natural monticellite and phase equilibria in the system CaO-MgO-SiO2-CO2

The heat capacity of a natural monticellite (Ca 1.00 Mg .09 Fe .91 Mn .01 Si 0.99 O 3.99 ) measured between 9.6 and 343 K using intermittent-heating, adiabatic calorimetry yields C p 0 (298) and S 298 0 of 123.64 ± 0.18 and 109.44 ± 0.16 J · mol −1 K −1 respectively. Extrapolation of this entropy value to end-member monticellite results in an S 0 298 = 108.1 ± 0.2 J · mol −1 K −1 . 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 (298–1700 K): S T 0 ( J · mol −1 K −1 ) = S 298 0 + 164.79 In T + 15.337 · 10 −3 T + 22.791 · 10 5 T −2 − 968.94. Phase equilibria in the CaO-MgO-SiO 2 system were calculated from 973 to 1673 K and 0 to 12 kbar with these new data combined with existing data for akermanite ( Ak ), diopside ( Di ), forsterite ( Fo ), 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 0 298 of Me is 250.2 J · mol −1 K −1 less than the measured value of 253.2 J · mol −1 K −1 . A best fit to the reversals for the solid-solid and decarbonation reactions in the CaO-MgO-SiO 2 -CO 2 system was obtained with the ΔG 0 298 ( kJ · mole −1 ) for the phases Ak (−3667), Di (−3025), Fo (−2051), Me (−4317) and Mo (−2133). The two invariant points − Wo and − Fo for the solid-solid reactions are located at 1008 ± 5 K and 6.3 ± 0.1 kbar, and 1361 ± 10 K and 10.2 ± 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.