1. 1) Arumu Publishing Co. Ltd.: Industrial Rare Metals (Kogyo Rare Metal), No. 125, Annual Report 2009, (Arumu Publishing Co. Ltd., Tokyo, 2009) pp. 27-30, 90-97.
2. 2) Japan Oil, Gas and Metals National Corporation (JOGMEC): Rare Metal Handbook 2009, (Kinzoku Jihyo Co. Ltd., Tokyo, 2009) pp. 212-217.
3. 3) U.S. Geological Survey: Mineral Commodity Summaries 2008, (United States Government Printing Office, Washington, 2008) pp. 186-187.
4. 5) Bookers Inc.: Handbook of Recycling Technology of Precious and Rare Metals, (NTS Inc., Tokyo, 2007) pp. 388-396.