1. U.S. Dept. of Energy – Critical Materials Strategy 2011, 189 p.
2. British Geological Survey Risk List 2012.
3. European Commission, 2011 Inst. Energy & Transport of the Joint Research Centre (JRC) of the European Commission.
4. Grauch, R. and Mariano, A., 2008, Ion-Absorption Type Lanthanide Deposits, Abstract annual SME Conference, Salt Lake City.
5. Van Gosen, B.S., Verplanck, P.L., Long, K.R., Gambogi, J., Seal II, R.R., 2014, U.S. Dept. of the Interior, U.S. Geological Survey, Fact Sheet 2014-3078, http://dx.doi.org/10.3133/fs20143078.