An X-ray diffraction (XRD) and scanning electron microscopy (SEM) study of the phase composition and microstructure characteristics of the Mg(1-x)/3Nb(4-x)/6Fex/2O2 (x=0.5) chemical compound is presented. The samples were prepared by the conventional ceramic method and subjected to different heat treatments. Columbite (MgNb2O6) and iron niobium oxide (FeNbO4, Wolframite) were identified as intermediate compounds in the reaction. A new single phase precursor for the (1-x)Pb(Mg1/3Nb2/3)O-3-xPb(Fe1/2Nb1/2)O-3 (PMN-PFN) system identified as [Fe, Mg]NbO4, was obtained, isostructural with the FeNbO4 where Fe and Mg ions occupy the same crystal site (space group P1 21a 1). From the Rietveld refinement method the cell parameters of the monoclinic structure were determined. The microstructure analysis indicates that the particles are irregular in shape and the grain size tends to increase with the calcination temperature.