lutetium, formerly lutecium (both: lōtēˈshēəm) [key], metallic chemical element; symbol Lu; atomic number 71; at. wt. 174.9668; m.p. about 1,663°C; b.p. about 3,395°C; sp. gr. 9.835 at 25°C; valence +3. Lutetium is a silver-white metal that is relatively stable in air. One of the rare-earth metals, it is the last member of the lanthanide series in Group 3 of the periodic table. The metal may be prepared by reduction of the chloride or fluoride with an alkali or alkaline earth metal. Rare and expensive, it has few commercial uses. The chief commercial source of lutetium is the mineral monazite, which contains lutetium in a concentration of about three parts per hundred thousand. A process for separating lutecia (lutetium oxide, a rare earth) from ytterbia was described in 1907 by Georges Urbain, a French chemist, who is credited with the discovery of the element. It was discovered independently in 1908 by Carl Auer von Welsbach, an Austrian chemist, who called the element cassiopeium.