The methods of Mossbauer spectroscopy, magnetic measurements, chemical analysis, and thermodynamic calculation of the phase composition were used in systematic studies of oxide iron-aluminosilicate (SiO2-Al2O3-Fe2O3) system in a wide range of its macrocomponent composition: 0.6-65.6wt.% SiO2; 0.05-20.5 wt.% Al2O3; 4.6-92.5 wt.% Fe2O3. These correspond to the composition of magnetic microspheres and cenospheres from fly ashes originated from powdered-coal combustion of five varieties of energy coals. It was established that the main phase governing the magnetic properties is magnetite with a defective structure. The microspheres saturation magnetization increases with the iron content range of 4.6-80.0 wt.% Fe2O3 is caused by the increase of the spinet phase content. The magnetization reduction in ferruginous microspheres with the high iron content (83.0-92.5 wt.% Fe2O3) is the result of the hematite-magnetite displacement. (C) 2008 Elsevier B.V. All rights reserved.