Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). Limestones often contain variable amounts of silica in the form of chert or flint, as well as amounts of clay, silt and sand as disseminations, nodules, or layers within the rock unit. The primary source of this calcite is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form is composed of oolites (oolitic limestone) and can be recognised by its granular appearance. Limestone makes up about 10 percent of the total volume of all sedimentary rocks.
Pure limestones are white or almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces. Limestone may be crystalline, clastic, granular, or massive, depending on the method of formation. Crystals of calcite, quartz, dolomite or barite may line small cavities in the rock.
Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution that is supersaturated with chemical constituents of calcite. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells.
During regional metamorphism limestone recrystallizes into marble.
Limestone is a parent material of Mollisol soil group.
www.wikipedia.com]== Limestone landscape ==
Limestone is partially soluble, especially in acid, and therefore forms many erosion landforms. These include limestone pavements, pot holes, caves and gorges. Such erosion landscapes are known as karsts. Limestone is less resistant than most igneous rocks, but more resistant than most other sedimentary rocks. Limestone is therefore usually associated with hills and downland and occurs in regions with other sedimentary rocks, typically clays.
Bands of limestone emerge from the Earth's surface in often spectacular rocky outcrops and islands. Examples include the Burren in Co. Clare, Ireland; the Verdon Gorge in France; Malham Cove in North Yorkshire, England; and the Ha Long Bay National Park in Vietnam.
Uses of limestone
Limestone is especially popular in architecture, and many landmarks around the world, especially in North America and Europe, are made primarily of the material. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, making it impractical for tall buildings; it is also quite expensive. Limestone was most popular in the early 20th and late 19th centuries. Train stations, banks and other structures from that era are normally limestone. Limestone is used as a facade on some skyscrapers, but only in thin plates for covering rather than solid blocks. In the United States, Indiana has long been noted as a source of high quality quarried building Indiana limestone, while many famous buildings in London are built from Portland limestone.
Though the limestone used for construction is good for humid climates, it is vulnerable to acids, making acid rain a problem when it occurs in places where limestone is used extensively. The acids in the water can wear away the details of statues and other art.
Limestone is also used to manufacture quicklime (calcium oxide).
Pulverized limestone is used as a soil conditioner to neutralize acid soil conditions.
See also
