A metallic element resembling lead in physical properties; the metal is silvery-white, but turns bluish-gray in air. Symbol, Tl. Occurs in crooksite, lorandite, and hutchinsonite. It is also present in pyrites and is recovered from the roasting of this ore in the manufacture of sulfuric acid and from the smelting of lead and zinc ores. Used in low-melting glasses, photocells, and infrared detectors.