© 2018 ISIJ. The thermodynamic model of a silico-ferrites of calcium and aluminum solution, SFCA phase (Ca2 (Fe,Ca)6Oct (Fe,Al,Si)T6etO20 ) was newly developed in the framework of the Compound Energy Formalism (CEF). Preferred substitution of Al atoms to tetrahedral sites in the SFCA solution was verified by X-ray absorption near edge structure (XANES) analysis. On considering crystallographic information in particular the shortrange-ordering nature in the SFCA solution, the Ca8 (Fe3+)2Oct(CaSi6+,FeFe6+,FeAI6+)3Paired(CaSi6+)1Paired(Fe3+,AI3+)Tet20O80 structure was considered for modeling the SFCA solution. The optimized Gibbs energies of all endmembers can successfully reproduce the experimental single phase region of the SFCA solution.