1. Research Institute of Mineral Dressing and Metallurgy (SENKEN), Tohoku University, Sendai 980, Japan
2. Waseda Research Institute of Mineral Dressing and Metallurgy (SENKEN), Tohoku University, Sendai 980, Japan
3. National Institute for Research in Inorganic Materials, 1 - 1 Namiki, Tsukuba 305, Japan
4. Faculty of Education, Iwate University, Morioka 020, Japan