Zajacite or Zajacite-(Ce) is a rare radioactive fluoride mineral with formula: Na(REExCa1-x)(REEyCa1-y)F6. REE means rare earth elements mostly those belonging to the lanthanide series. It crystallizes in the trigonal - rhombohedral system It has a white vitreous appearance with a conchoidal fracture. It has a Mohs hardness of 3.5 and a specific gravity of 4.44 to 4.55. It is transparent with refractive indices of n?=1.483 and n?=1.503. It occurs as creamy to white grains and phenocrysts in pegmatite and aplite pods or lenses in a peralkaline igneous intrusion.
It was discovered in 1993 at Strange Lake, Quebec - Labrador (56°20'N , 64°10'W) and was named for Dr I. S. Zajac who lead the expedition responsible for its discovery.