Columbites may be radioactive because: 1) they contain isomorphous U4+(and Th4?) substituting for Fe2+; 2) they contain growth inclusions of other radioactive species (e.g., monazite); 3) they contain exsolved uraninite; 4) they are overgrown or partly replaced by other hypogene radioactive species(e.g., samarskite); 5) they are coated or veined by secondary uranyl minerals (e.g., parsonsite); 6) of various combinations of 1-5. Columbite from British Columbia (9-10% U3O8), although apparently homogeneous when examined by means of x-ray diffraction methods, is shown to be a mixture of non-radioactive columbite and radioactive, metamict pyrochlore by means of an electron microprobe analysis. The material toddite is shown to be a mixture of columbite and samarskite.
Radioactive columbite is related to a particular pegmatite district, its paragenetic position, and the presence of other accompanying radioactive species.