Calcite

Calcite, mineral consisting largely of calcium carbonate (CaCO₃). Next to quartz, it is the most abundant of the Earth's minerals. Crystallizing in the hexagonal system, (see Crystal), calcite is noted for its wide variety of crystalline forms. It also occurs in massive or cryptocrystalline formations. Examples of the crystalline varieties are nailhead spar, dogtooth spar, satin spar, and Iceland spar. The last named is the only pure form of calcite found in nature. Limestone, chalk, travertine, Oriental alabaster, and marble are among the most common of the massive forms of the mineral. Calcite is also found as stalactites and calcareous tufas, forms deposited by mineral waters. It is also the chief component of many molluscs' shells.

Colourless, with a hardness of 3 on the Mohs scale and a relative density of 2.72, pure calcite is readily identified by the ease with which it is cut or cleaved and by the rapidity with which it reacts with dilute acids. Such contaminants as magnesium, ferrous iron, manganese, and zinc alter the properties of the mineral in varying degrees, depending on the amounts present.