Limestone
Limestone, a sedimentary rock that occurs in many areas of the world. Limestone consists chiefly of calcite (calcium carbonate), but it usually contains numerous other minerals, including dolomite, flint, alumina, bitumen, and pyrites.
Limestone containing a small proportion of magnesium carbonate is called magnesia limestone. If it contains a high percentage of the mineral dolomite it is called dolomite rock. Soft, powdery limestone is chalk. Limestone subjected to great heat and pressure beneath the earth's surface is converted into marble.
Limestone is used as a building stone, and as a flux in smelting iron and making steel. In agriculture, limestone is used to reduce soil acidity and to add calcium to the soil. Limestone is used to make lime and cement. Crushed limestone is used as gravel and in the making of concrete.
Most limestone was formed from calcium carbonate deposited at the bottom of ancient shallow seas. Most of these deposits consist of the fragmented remains of large numbers of organisms such as coral and foraminifera, protozoans that extract calcium carbonate from the water to form a shell. Limestone is often rich in fossils, and a type of limestone called coquina is especially noted for the fossils it contains. Some very fine-grained limestone comes from deposits created by the chemical precipitation of calcium carbonate dissolved in water.
Limestone is readily dissolved by water that has become slightly acidic, usually from carbon dioxide in the air. Caves are found where water has dissolved underground limestone and carried it away. In many caves, dripping water containing dissolved limestone slowly forms picturesque structures called stalactites and stalagmites. In addition to caves, depressions in the earth's surface called sinks, or sinkholes, are also found in limestone regions.