Mossbauer and X-ray investigations of 3 MeV proton irradiation effects in biotite are reported. Mossbauer spectra of natural biotite samples in the temperature range from 77 K to 970 K were obtained and irreversible changes caused by heating of the biotite were investigated. Changes in the Mossbauer spectra of irradiated and heated samples were found to be similar and are interpreted as a result of oxidation of Fe2+ions to the Fe3+state. It is shown both by means of Mossbauer and X-ray spectroscopy that a heavy irradiation gives rise to a formation of Fe3O4particles of a size about 0.01 μm.