The basis of this work is a detailed investigation of phase equilibria of the Mg-Sn-Ca system for wide ranges of composition and temperature by means of a combined approach of our own key experiments, first-principles calculations of finite-temperature properties of the compounds and Calphad-type modeling. In this report, the generated thermodynamic description is applied to predict the phase formation and discuss the solidification behavior of practically important Mg-rich alloys. Solidification calculations, based on computational thermochemistry and the present dataset, are successfully applied to the analysis of experimentally observed as-cast microstructures and thermal analysis data for the Mg-rich alloy samples.