Calcite is a mineral that occurs in small amounts in most types of rocks and makes up the largest part of limestone and marble. Pure calcite is transparent or white, but a variety of impurities can give this mineral certain colors. Nickel impurities produce green calcite; manganese, pink calcite; and iron, brown calcite. Calcite in the form of limestone is the chief source of lime. Limestone also serves as a raw material in cement and mortar and as a building stone.
Pure calcite is a form of calcium carbonate. Calcite crystals can split into perfect rhombohedrons, six-sided objects in which the opposite sides are parallel. The crystal structure of calcite causes it to polarize light (see Polarized light). Calcite is fairly soft; it can be scratched by a copper coin. Because of its softness, calcite serves as a scratchless scouring ingredient in some cleaning powders. Calcite fizzes in a weak acid solution.
Many invertebrates (animals without backbones) have skeletons of calcite. Some beaches in Florida along the Gulf of Mexico consist primarily of calcite shells and coral from marine invertebrates.
See also Calcium carbonate; Chalk; Lime; Limestone; Marble.
