The transformation mechanism of Fe2+ cations in two samples of natural olivine (Fe,Mg)2SiO4 after heat treatment in air in the temperature range 400–1180 °C has been studied using 57Fe Mössbauer spectroscopy and XRD measurements. The influence of heating duration (from one to 96 hr) on the behavior of Fe2+ ions in the olivine structure was investigated. Fe3+ ions dominate as a primary product of the thermal treatment in air above 1000 °C. Heating at 1180 °C led to decomposition of the olivine structure. The formation of ferriolivine as a result of low-temperature oxidation was also observed.