In this article we show that heteroepitaxial CaF2 films can be induced on Si(lll) with a rapid thermal anneal. The change from preferentially oriented polycrystals to a single crystal with type-B epitaxy is visible by different structural techniques. The x-ray photoelectron spectroscopy results indicate the presence of a reacted layer at the CaF2/Si interface with a pronounced increase of fluorine atoms at this interface. Transmission electron microscopy results show that big structural changes occur due to the thermal pulse.