We have studied the composition of three types of individual spheres differing in microstructure (single-block, platelike, and skeletal–dendritic spheres) in the–40 + 32 μm size fraction of ferrospheres containing 92.72 wt % FeO. The results demonstrate that the single-block spheres have the highest Fe content (95–97% FeO), a basicity factor M b > 48, and an Fe/O atomic ratio of 0.68–0.71. They consist of block individuals of a partially martitized (oxidized to hematite) spinel ferrite. The platelike spheres feature high calcium content (11–12% CaO); lower basicity factor, M b ≃ 20–21; and a higher degree of oxidation of Fe, with an Fe/O atomic ratio of 0.63–0.66. They consist of spinel ferrite blocks with large percentages of FeO (up to 90%) and MgO (up to 6%), surrounded by regions with a platelike structure, reduced FeO content (down to 57%), and high CaO content (up to 34%). The skeletal–dendritic spheres feature increased percentages of SiO2 (≃ 4.7 %) and Al2O3 (3.6–4.6%) and low basicity factor: M b ≃ 10–11. They consist of unidirectional or branched crystalline spinel ferrite individuals with partial Mg2+ substitution for Fe2+, which prevents oxidation of the spinel to hematite, as evidenced by the large Fe/O atomic ratio: 0.72–0.73.