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2. 2) P. Enghag: Encyclopedia of the Elements, Trans. by T. Watanabe, et al., (Asakura Publishing Co. Ltd., Tokyo, 2007) pp. 295–304.
3. 3) USGS, 2006. Mineral Industry Surveys, US Dept. of the Interior, US Geological survey, Mar. 2008.
4. 4) F. Habashi: Handbook of extractive metallurgy III, (WILLEY-VCH, Weinheim, 1997) pp. 1471–1488.
5. 5) Japan Oil, Gas and Metals National Corporation, Mineral Resources Information Center: Metallic Resources Report, Mar. (2007).