Limestone

Limestone is a sedimentary rock composed predominantly of calcium carbonate (CaCO3), most often in the form of the mineral calcite. The rock is the building stone of much of the world's sedimentary landscape — the carbonate platforms that became the Florida peninsula, much of the Bahamas, the Cretaceous chalk cliffs of southern England and Normandy, and the karst landscapes of central Indiana, central Tennessee, Kentucky's caves, and the Texas Hill Country are all limestone or limestone-derived.

Most limestone forms in shallow marine environments where dissolved calcium and bicarbonate ions in seawater are removed (by living organisms and chemical precipitation) and accumulated as carbonate sediment on the seafloor. Compaction and cementation over millions of years convert the loose sediment into solid rock. Many limestones are essentially compressed remains of coral reefs, shellfish, and microscopic plankton (foraminifera and coccolithophores) — the rock preserves billions of years of marine biological history.

Limestone is unusually soluble compared to most rocks. Slightly acidic rainwater (carbonic acid, formed when atmospheric CO2 dissolves in rain) gradually dissolves limestone over time, creating karst topography — sinkholes, caves, underground rivers, spring-fed streams, and the spectacular cave systems including Mammoth Cave (Kentucky), Carlsbad Caverns (New Mexico), Wind Cave (South Dakota), and Cave of the Mounds (Wisconsin).

Limestone has been the most widely used building stone in human history. The Egyptian pyramids of Giza are made primarily of limestone blocks; the Roman Colosseum is limestone (specifically travertine, a variety formed at hot springs); the Parthenon is limestone-derived marble; many medieval European cathedrals are limestone; the Empire State Building is clad in Indiana limestone; the Pentagon is constructed primarily of limestone.

Crushed limestone is the most important non-metallic mineral commodity in the United States by volume, used as aggregate for concrete and road construction, as the calcium source for cement manufacture, as agricultural lime to neutralize acidic soil, and in steelmaking as a flux to remove impurities from iron ore.

The species "limestone" is geologically diverse — ranges from pure white chalk to gray fossiliferous rock to nearly black bituminous limestones. Indiana's "Salem Limestone" is the U.S. standard for high-quality dimension stone.