The solvus on the Mn-rich side in the system MnCO3-CaCO3 was studied experimentally at 10 kb and lower pressures. The critical point lies at 540 °C and Mn0.68Ca0.32CO3. Reaction rates of demising are extremely sluggish. X-ray calibration curves for (Mn, Ca)CO3 solid solutions are given. Depicting microprobe analyses of about 300 carbonates from manganese calc silicate marbles affected by different degrees of metamorphism confirm the experimentally derived solvus. Margules parameters were derived from the experimental solvus, giving for spinodal unmixing W a=20.1+0.032·T and W b=18.3-0.0033·T, and for binodal unmixing W a=−22.9+0.035 ·T and W b=3.36+0.0147·T (All W, in kJ/-mol) A physical interpretation of the Margules parameters based on deformations of the ionic radii in the solid solution is presented. The Margules parameters obtained by this model agree fairly well with the experimental data.