1. The 1985 “Red Book” (IUPAC Nomenclature of Inorganic Chemistry), p. 45.
2. In: Lanthanides, Kirk-Othmer encyclopedia of chemical technology;Bünzli,2013
3. Rare earth elements, in industrial minerals and rocks;Castor,2006
4. [Online] available at: http://www.statista.com/statistics/277268/rare-earth-reserves-by-country/;
5. The Statistics Portal; Statistics and Studies from more than 18,000 Sources.