zirconium , metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.22; m.p. about 1,852°C; b.p. 4,377°C; sp. gr. 6.5 at 20°C; valence +2, +3, or +4. Zirconium is a very strong, malleable, ductile, lustrous silver-gray metal. At ordinary temperatures it has a hexagonal close-packed crystalline structure. Its chemical and physical properties are similar to those of titanium, the element above it in Group 4 of the periodic table . Zirconium is extremely resistant to heat and corrosion. It forms a number of compounds, among them zirconate (ZrO 3-2 ) and zirconyl (ZrO +2 ) salts. The most important compound is the oxide zirconia (ZrO 2 ), used extensively as a refractory material in furnaces and crucibles, in ceramic glazes, and, formerly, in gas mantles. It occurs in nature as the silicate (ZrSiO 4 ) and is used as a gemstone; it may be clear or colored, and is usually called zircon or hyacinth. Zirconium compounds also have minor uses as catalysts, in the dye, textile, plastics, and paint industries, and in pharmaceuticals such as poison ivy lotions. The metal also has many other uses, among them in photographic flashbulbs and surgical instruments, in the removal of residual gases from electronic vacuum tubes, and as a hardening agent in alloys, especially steel. A major use of the metal is in nuclear reactors . It is employed in tubes for cladding uranium oxide fuel. It is well suited for this purpose because it is corrosion resistant and does not...