Mineral Matters ZIRCON IN TECHNOLOGY AND EVERYDAY LIFE

The pages of this issue provide a glimpse into for melting high-temperature glasses and metals the scientific uses and advantages of zircon, such as platinum. Zircon itself has been slip- Synthetic zircon crystals grown from PbO–SiO2 melt which are both numerous and remarkable. cast as dense, refractory bricks for applications (scale bar is 200 microns) This extraordinary mineral is also valued for requiring strength, chemical durability, and more worldly reasons. Because of its high temperature resistance, and has even served as refractive index (~1.95) and dispersion (giving a container in experimental petrology (see it brilliance and “fire”), zircon was used as a photos). This versatile mineral has also been substitute for diamond before the manufacture proposed as a radioactive waste form for of cubic zirconia. Clear zircons are not uncom- transuranic elements (see Elements, volume 2, mon, and colored varieties include yellow issue 6). At atmospheric pressure, zircon breaks (hyacinth), orange, red, pink, green, brown, down to ZrO2 and SiO2 at ~1660°C. Together black, grey, and blue, the last of which is an these oxides melt in eutectic fashion at ~1690°C; alternative to topaz as the birthstone for the melting point of ZrO2 by itself is ~2700°C. December. In medieval times, zircon was Zirconium metal is used in a number of tech- believed to cure diseases, protect from poisons, nological applications, perhaps most impor- induce sleep, and aid digestion (in some mod- tantly in the cladding of uranium oxide fuel ern-day researchers it has caused insomnia and indigestion). Today, the main sources of gem- rods in nuclear reactors, where its corrosion quality zircon are Australia, Cambodia, Myanmar, resistance, mechanical strength and very low Sri Lanka, and Thailand. cross-section for thermal neutrons make it advantageous over all other metals. Added to Zircon is the primary source of zirconium niobium, Zr forms a superconducting alloy. Polycrystalline zircon brick oxide (ZrO2; zirconia), which was first isolated Zirconium (and hafnium) fluoride glasses are from zircon in 1789 (the reduced metal was not used in the core and cladding of infrared (IR) produced until 1824 because of the tenacity guide fibers because of their extremely high with which Zr forms bonds with both oxygen transmission in the IR region. and nitrogen). Zirconia is used as a commercial E. Bruce Watson refractory for furnace linings: porous, ZrO2- based ceramics make outstanding thermal insulators. Zirconia is also used as a container

Polycrystalline zircon capsules containing quenched basaltic melt

Faceted zircon crystals from Sri Lanka and Thailand, TOP INSERT: Zircon, Kandy, Sri Lanka. Brilliant-cut, heat- ranging in weight from 7.76 to 40.19 carats. treated, 208.65 cts. (largest fine gem zircon: PHOTO COURTESY AMERICAN MUSEUM OF NATURAL HISTORY / AMNH42724). PHOTO COURTESY AMERICAN MUSEUM OF Zircon structure (by EBW) VAN PELT PHOTOGRAPHERS. NATURAL HISTORY / VAN PELT PHOTOGRAPHERS. FEBRUARY 2007 E LEMENTS 52