1. Smithells Metals Reference Book, 6, ed. E.A. Brandes (London: Butterworths, 1983).
2. Iron & Steelmaker, 24 (13) (1997), p. 2.
3. Light Metal Age, 55 (1,2) (1997), p. 8.
4. U.S. Geological Survey, Mineral Commodity Summaries, 2 (1997).
5. P.W. Harben and G.H. Edwards, JOM, 6 (1997), p. 21.