Most zirconium minerals contain 1 to 3% hafnium. Hafnium is a ductile metal with a brilliant silver lustre. Its properties are influenced considerably by the impurities of zirconium present. Of all the elements, zirconium and hafnium are two of the most difficult to separate. Hafnium is a Group 4 transition element.
Because hafnium has a good absorption cross section for thermal neutrons (almost 600 times that of zirconium), has excellent mechanical properties, and is extremely corrosion resistant, it is used for nuclear reactor control rods.