Ore Processing Laboratory;
Institute of Geological and Mining Research;
P.O.Box 4110;
Yaounde;
Cameroon;
Department of Earth Sciences;
University of Yaounde 1.P.O.Box 812;
Yaounde;
Cameroon;
Niobium; Tantalum; Tapiolite-(Fe); Tantalite-(Mn); Ngoura placer; East Cameroon;