|"Calcium carbonate: inorganic crystalline "calcite or organic calcareous material|
Limestone is a "sedimentary rock, composed mainly of skeletal fragments of marine organisms such as "coral, "forams and "molluscs. Its major materials are the "minerals "calcite and "aragonite, which are different "crystal forms of "calcium carbonate (CaCO3).
About 10% of sedimentary rocks are limestones. The "solubility of limestone in water and weak acid solutions leads to "karst landscapes, in which water erodes the limestone over thousands to millions of years. Most "cave systems are through limestone bedrock.
Limestone has numerous uses: as a "building material, an essential component of "concrete ("Portland cement), as aggregate for the base of roads, as white pigment or filler in products such as "toothpaste or "paints, as a chemical "feedstock for the production of "lime, as a "soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as "coral or "foraminifera. These organisms secrete shells made of aragonite or calcite, and leave these shells behind when they die. Other "carbonate grains comprising limestones are "ooids, "peloids, "intraclasts, and extraclasts.
Limestone often contains variable amounts of "silica in the form of "chert ("chalcedony, "flint, "jasper, etc.) or siliceous skeletal fragment (sponge spicules, "diatoms, "radiolarians), and varying amounts of "clay, "silt and "sand ("terrestrial "detritus) carried in by rivers.
Some limestones do not consist of grains at all, and are formed completely by the chemical "precipitation of "calcite or "aragonite, i.e. "travertine. Secondary calcite may be deposited by "supersaturated "meteoric waters ("groundwater that "precipitates the material in caves). This produces "speleothems, such as "stalagmites and "stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular (oolite) appearance.
The primary source of the calcite in limestone is most commonly "marine organisms. Some of these organisms can construct mounds of rock known as reefs, building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone typically does not form in deeper waters (see "lysocline). Limestones may also form in "lacustrine and "evaporite "depositional environments.
Calcite can be "dissolved or "precipitated by groundwater, depending on several factors, including the water temperature, "pH, and dissolved "ion concentrations. Calcite exhibits an unusual characteristic called "retrograde solubility, in which it becomes less soluble in water as the temperature increases.
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. When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures.
"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 supersaturated with the 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.
Two major classification schemes, the Folk and the Dunham, are used for identifying limestone and carbonate rocks.
Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems (grains), matrix (mostly micrite), and cement (sparite). The Folk system uses two-part names; the first refers to the grains and the second is the root. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample.
The Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains that make up the limestone. Robert J. Dunham published his system for limestone in 1962; it focuses on the depositional fabric of carbonate rocks. Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are essentially for rock families. His efforts deal with the question of whether or not the grains were originally in mutual contact, and therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock. The Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample.
Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms. These include "limestone pavements, "pot holes, "cenotes, 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. It is therefore usually associated with hills and "downland, and occurs in regions with other sedimentary rocks, typically clays.
Karst "topography and caves develop in limestone rocks due to their "solubility in dilute "acidic "groundwater. The "solubility of limestone in water and weak acid solutions leads to "karst landscapes. Regions overlying limestone bedrock tend to have fewer visible above-ground sources (ponds and streams), as surface water easily drains downward through "joints in the limestone. While draining, water and organic acid from the soil slowly (over thousands or millions of years) enlarges these cracks, dissolving the calcium carbonate and carrying it away in "solution. Most "cave systems are through limestone bedrock. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.
Coastal limestones are often eroded by organisms which bore into the rock by various means. This process is known as "bioerosion. It is most common in the tropics, and it is known throughout the "fossil record (see Taylor and Wilson, 2003).
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 and the "Isle of Wight, England; the "Great Orme in Wales ; on "Fårö near the Swedish island of "Gotland, the "Niagara Escarpment in Canada/United States, "Notch Peak in Utah, the "Ha Long Bay National Park in Vietnam and the hills around the "Lijiang River and "Guilin city in China.
The "Florida Keys, islands off the south coast of "Florida, are composed mainly of "oolitic limestone (the Lower Keys) and the carbonate skeletons of "coral reefs (the Upper Keys), which thrived in the area during interglacial periods when sea level was higher than at present.
Unique habitats are found on "alvars, extremely level expanses of limestone with thin soil mantles. The largest such expanse in Europe is the "Stora Alvaret on the island of "Öland, Sweden. Another area with large quantities of limestone is the island of Gotland, Sweden. Huge quarries in northwestern Europe, such as those of Mount Saint Peter (Belgium/Netherlands), extend for more than a hundred kilometers.
Limestone is very common in architecture, especially in Europe and North America. Many landmarks across the world, including the "Great Pyramid and its associated "complex in "Giza, Egypt, were made of limestone. So many buildings in "Kingston, "Ontario, "Canada were, and continue to be, constructed from it that it is nicknamed the 'Limestone City'. On the island of "Malta, a variety of limestone called Globigerina limestone was, for a long time, the only building material available, and is still very frequently used on all types of buildings and sculptures. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. Ancient American sculptors valued limestone because it was easy to work and good for fine detail. Going back to the Late Preclassic period (by 200–100 BCE), the "Maya civilization (Ancient Mexico) created refined sculpture using limestone because of these excellent carving properties. The Maya would decorate the ceilings of their sacred buildings (known as "lintels) and cover the walls with carved limestone panels. Carved on these sculptures were political and social stories, and this helped communicate messages of the king to his people. Limestone is long-lasting and stands up well to exposure, which explains why many limestone ruins survive. However, it is very heavy, making it impractical for tall buildings, and relatively expensive as a building material.
Limestone was most popular in the late 19th and early 20th centuries. Train stations, banks and other structures from that era are normally made of limestone. It is used as a facade on some skyscrapers, but only in thin plates for covering, rather than solid blocks. In the United States, Indiana, most notably the Bloomington area, has long been a source of high quality quarried limestone, called "Indiana limestone. Many famous buildings in London are built from "Portland limestone.
Limestone was also a very popular building block in the Middle Ages in the areas where it occurred, since it is hard, durable, and commonly occurs in easily accessible surface exposures. Many medieval churches and castles in Europe are made of limestone. "Beer stone was a popular kind of limestone for medieval buildings in southern England.
Limestone and (to a lesser extent) marble are reactive to acid solutions, making "acid rain a significant problem to the preservation of artifacts made from this stone. Many limestone statues and building surfaces have suffered severe damage due to acid rain. Likewise limestone gravel has been used to protect lakes vulnerable to acid rain, acting as a pH buffering agent. Acid-based cleaning chemicals can also etch limestone, which should only be cleaned with a neutral or mild alkaline-based cleaner.
Other uses include:
People can be exposed to limestone in the workplace by inhalation of and eye contact with the dust.
The "Occupational Safety and Health Administration (OSHA) has set the legal limit ("permissible exposure limit) for limestone exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. The "National Institute for Occupational Safety and Health (NIOSH) has set a "recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday.