CERUSSITE, a mineral consisting of lead carbonate, and forming an important ore of lead; it contains 77.5% of the metal. The name is derived from the Latin cerussa, "white lead." It crystallizes in the orthorhombic system, and often forms pseudo-hexagonal twins like those of aragonite, with which it is isomorphous. It also occurs in compact granular or fibrous forms. It is usually colourless or white, sometimes grey or greenish, and is usually more or less transparent : some well-formed crystals have a brilliant lustre. Its hardness is 3-3.5 and density 6.5. It effer vesces with dilute nitric acid. Cerussite occurs in large quantities in the upper oxidized portions of lead.