Abstract: The invention is directed to single crystal alkaline earth metal fluoride optical elements having an adhesive, hermetic coating thereon, the coating being chemically bonded to the surface of the metal fluoride optical element with a bonding energy ?4 eV and not merely bonded by van der Walls forces. The materials that can be used for coating the optical elements are selected from the group consisting of SiO2, F—SiO2, Al2O3, F—Al2O3, SiON, HfO2, Si3N4, TiO2 and ZrO2, and mixtures (of any composition) of the foregoing, for example, SiO2; HfO2 and F—SiO2/ZrO2. The preferred alkali earth metal fluoride used for the optical elements is CaF2. Preferred coatings are SiO2, F—SiO2, SiO2/ZrO2 and F—SiO2/ZrO2.