Tantalum occurs in great variety of oxide minerals. The most common are columbite-tantalite, (Fe,Mn)(Nb,Ta) 2 O 6, and microlite, (REE, U, Y, Ca,Na)2(Ta,Nb) 2 O 6 (O,OH,F)), which belong to the pyrochlore group minerals. Also tantalum can be in the structure of cassiterite, SnO 2, in substitution of