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2. Niobium and tantalum — indispensable twins;Schulz;USGS Miner. Resour. Progr.,2014
3. Niobium and Tantalum Deposits;Abo Khashaba,2017
4. The Chemistry of Tantalum and Niobium Fluoride Compounds;Agulyonsky,2004
5. Final Conference Refractory Metals: Demand, Supply, Outlook and Open Issues;Christmann,2017