Using the methods of X-ray phase and X-ray Densitometric analysis, the phase equilibria between oxides of manganese, iron and antimony have been investigated in an air atmosphere at temperatures up to 1250? in an air mosphere at normal pressure. The phase diagram of the system at 1200? was built MnO-Mn₂O₃-FeO-Fe₂O-Sb₂O₃-Sb₂O₅. A new phase was found Mn_12-2x,sup>2+Fe_2x²⁺Sb³⁺Sb₅⁵⁺O₂₆(0?x?1), with edge compositions FeMn₅Sb₃O₁₃ and Mn₆Sb₃O₃ (a=8.5003?0.0025Å; b=8.0064?0.0025Å; c=11.5779?0.0025Å; Z = 3; ?obs. = 5.7 g/cm³; ?calc = 5.6991 g/cm³). The phase exists in the temperature range 1180-1230^oC and can be obtained by quenching, but always with a large admixture of Mn₂Sb₂O₇ and spinel Mn₁₁²⁺Mn₁₃³⁺Sb₉³⁺O₄₄.The reason for this behavior is that air molecules have different temperatures, as a result of which the phase composition of the reaction mixture cannot be strictly related to one temperature, and different phases can be stable at different temperatures.