a very heavy, soft, white, sweetish-tasting halide mineral, formed by the alteration of other mercury minerals, such as cinnabar or amalgams. Calomel is found together with native mercury, cinnabar, calcite, limonite, and clay, at Moschellandsberg, Ger.; Zimapán, Mexico; and Brewster county, Texas, U.S. For detailed physical properties, see halide mineral (table).

Once the most popular of cathartics, calomel has been used in medicine since the 16th century. The recognition of its potential toxicity (because of disassociation into mercury and mercuric chloride), together with the development of superior and safer cathartics, led to a decline in its use in internal medicine. It has found application in certain insecticides and fungicides, however.