Ceramics-Silikáty 25, (3) 199 - 208 (1981)

Temperature dependences of the relative enthalpies of phases co-existing in equilibrium during incongruent decomposition of merwinite (merwinite, peritectic melt having the molar composition 1.193 CaO . 0.697 MgO . 1SiO₂, α - Ca₂SiO₄, MgO) and the material balance of this reaction were used for determining the heat of incongruent decomposition of merwinite at equilibrial temperature T_eq = 1848 K: Δ H_{in.dis, C₃MS₂} = (125 ± 15) kJ.mole^-1. The temperature dependences of relative enthalpies of merwinite and those of the peritectic melt were determined by the method of double calorimetry of the same sample. To determine the heat of incongruent decomposition of merwinite the same change in enthalpy during the cooling of MgO had to be measured and the heat of solution of γ - Ca₂SiO₄ and MgO had to be determined. The change in entropy due to incongruent decomposition of merwinite has the value ΔS_{in.dis, C₃MS₂} = (67.5 ± 8.1) J.mole^-1.K^-1.