Gagarinite-(Ce) previously zajacite-(Ce) is a rare radioactive fluoride (REE 6. REE refers to rare-earth elements , mostly those belonging to the lanthanide series. It crystallizes in the trigonal rhombohedral system and has a white vitreous appearance with a conchoidal fracture. It has a Mohs hardness of 4.44 to 4.55. Zajacite is transparent with refractive indices nω = 1.483 and nε = 1.503. Gagarinite-(Y) is a yttrium -rich analog.
It occurs as creamy to white anhedral to subhedral grains in pegmatite pods or lenses in a peralkaline.
It was discovered in 1993 at Strange Lake , Quebec, (56°20'N, 64°10'W) and was initially named for Ihor Stephan Zajac , who led the expedition responsible for its discovery, and who first recognized the presence of the new mineral. The mineral was renamed gagarinite-(Ce) in 2010 by the IMA The new name is for Russian cosmonaut (1934–1968).