Limestone

Limestone is a sedimentary rock composed chiefly of calcium carbonate (CaCO3) in the form of the mineral calcite, with some varieties containing aragonite or dolomite. It is one of the most abundant and economically important sedimentary rocks.

Most limestone is biological in origin, built from the accumulated shells, skeletons, and fragments of marine organisms such as corals, mollusks, foraminifera, and algae. As a result, limestone is often rich in fossils.

Because calcite dissolves in mildly acidic water, limestone landscapes develop distinctive karst features such as caves, sinkholes, and underground rivers. The rock is soft enough to scratch with a knife and reacts vigorously with dilute acid.

Most limestone forms in warm, shallow, clear marine settings where carbonate-secreting organisms thrive. Their shells and skeletons accumulate on the seafloor and, along with chemically precipitated carbonate mud, are compacted and cemented into rock.

Some limestone forms by direct chemical precipitation of calcium carbonate, for example as oolites (tiny rounded grains) or as travertine and tufa around springs. Reef limestones preserve ancient coral and shell frameworks in growth position.

Great limestone deposits include the White Cliffs of Dover (chalk, a soft limestone), the Indiana Limestone of the USA, the Jura Mountains of Europe, the Guilin karst of China, and the Yucatan Peninsula in Mexico.

Look for a fine- to medium-grained rock, usually pale grey, cream, white, or tan, that is fairly soft (hardness about 3) and can be scratched by a steel knife or even a coin.

The definitive test is its reaction to dilute hydrochloric acid: limestone fizzes briskly as CO2 is released. Many specimens contain visible fossils, shell fragments, or ooids. The streak is white.

Look-alikes include dolostone (fizzes only weakly or when powdered), marble (the harder, crystalline, metamorphosed equivalent with interlocking calcite grains), and chalk (a very soft, porous, fine limestone). The combination of softness, pale color, fossils, and vigorous acid fizz identifies limestone.

Limestone is one of the most useful rocks in industry. It is the principal raw material for cement and lime, crushed for road base and concrete aggregate, and used as a flux in steelmaking. Agricultural lime ('aglime') neutralizes acidic soils.

As a dimension stone it is widely used in building and sculpture; the Pyramids of Giza and countless cathedrals and public buildings are clad or built in limestone. It is also used in glassmaking, water treatment, and as a filler in products from paper to toothpaste.

Geologically, limestone is a vast reservoir of Earth's carbon, an important aquifer and petroleum reservoir rock, and a rich archive of fossils and ancient marine environments.