Six samples of magnetospheres, recovered by magnetic separation from fly ashes, collected from power plants with high temperature boilers in the former USSR, were investigated by combination of instrumental techniques. The phase and chemical compositions, morphology and microstructure of magnetospheres have been studied by optical microscopy, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), Mossbauer spectroscopy. Iron in magnetospheres mainly occurs as Fe-spinels, hematite, and iron bearing glass. Structural formulas of spinels promising for catalysis applications were calculated on the base of obtained results. The magnetospheres have been characterized mainly as core-shell nanocomposites from spinel, hematite and quartz crystals in glass matrix divided in high-silica and high-ferrous glasses. The mechanism of magnetospheres formation was proposed. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.