The structure-composition relationship of skeletal-dendritic ferrospheres (FSs) isolated from fly ash from the coal and lignite combustion has been studied systematically by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy. It is shown that illite is the aluminosilicate precursor determining the structure of globules in both cases. The formation of skeletal-dendritic globules occurs due to the "seed" of Al, Mg-ferrospinel that is formed in the thermochemical conversion of illite from initial coals. The dependence CaO = f(SiO2) that reflects the influence of glass-forming components reveals six groups of FSs, the composition of which is represented by linear regression equations. An analysis of SEM images of polished sections from six globule groups reveals that an increase in the concentration of glass-forming components in all groups is accompanied by gradual changes in the structure of globules, from the coarse-grained crystalline skeletal type to the fine crystalline dendritic type with a high content of the glass phase. The observed change in the structure is explained by expansion of the liquation region in the FeO-Fe2O3-SiO2 system, a rise in the oxidation potential, an increase in the proportion of ferrite complexes [Fe3+O2](-) and [Fe23+O5](4-) in high-calcium melts, and a decrease in the concentration of ferrospinel-forming Fe2+ and Fe3+ ions.