A lustrous, iron-gray, malleable metallic element of the lanthanide series that occurs chiefly in the minerals monazite and bastnaesite, exists in four allotropic states, is a constituent of pyrophoric alloys, and is used in metallurgy and in the manufacture of glass, incandescent mantles, and hydrocarbon combustion catalysts. Atomic number 58; atomic weight 140.12; melting point 799°C; boiling point 3,443°C; specific gravity 6.77; valence 3, 4. See Periodic Table.