The transformation of monocrystalline and polycrystalline fullerite induced by high-power laser irradiation has been studied by Raman spectroscopy. The local heating of the samples promotes oxygen-driven cage opening reactions. Raman spectra show that fullerenes coalesce to form a new phase with an amorphous carbon nanostructure. The influence of the chemical environment and of the morphology of the sample on the onset of the thermal microreactions is presented.