1. J. Griffiths, Rare-Earths-attracting increasing attention, Industrial Minerals, Apr. (1984).
2. F. Narin, M. P. Carpenter, P. Woolf, IEEE T. Eng. Manage., EM-314, 4. Nov. (1984).
3. C. Chao, Mining and Metallurgy (Taiwan), 3 (1974) 56.
4. K. L. Soong, Acta Oceanographica Taiwanica, 8 (1978) 43.
5. R. C. Vickery, Analytical Chemistry of The Rare Earths, Pergamon Press, New York, 1961.