The transformation mechanism of Fe cations in natural olivine after thermal treatments in air has been studied using mainly57Fe Mössbauer spectroscopy. -Fe2O3 nanoparticles appear as the primary Fe3+ phase in Mössbauer spectra of olivine samples heated at 600-900 °C. These nanoparticles are thermally unstable and they are transformed to -Fe2O3 with the increase of heating time. Another transformation mechanism of iron related with the complete decomposition of olivine structure has been observed at temperatures of 1000 °C and higher. The mixed oxide MgFe2O4 with the spinel structure and enstatite MgSiO3 were identified as iron-bearing decomposition products.