For other uses, see Limestone (disambiguation).
In old USGS publications, dolomite was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolomites or magnesium-rich limestones.
About 10% of sedimentary rocks are limestones.
Most cave systems are found in 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, and as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains.
These organisms secrete shells made of aragonite or , and leave these shells behind when they die.
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 travertine (a precipitate of calcite and aragonite).
Another form taken by calcite is oolitic limestone, which can be recognized by its granular (oolite) appearance.
Other limestones are primarily composed of recrystalised lime mud, and are referred to as micrite.
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).
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.
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.
See also: List of types of limestone
Two major classification schemes, the Folk and Dunham, are used for identifying the types of carbonate rocks collectively known as limestone.
Main article: Folk's carbonate classification
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 to the cement.
For example, a limestone consisting mainly of ooids, with a crystalline matrix, would be termed an oosparite.
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.
Main article: Dunham classification
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, based on criteria such as whether 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.
A revised classification was proposed by Wright (1992).
It adds some diagenetic patterns to the classification scheme.
The solubility of calcium carbonate (CaCO3) is controlled largely by the partial pressure of carbon dioxide (CO2) in the water.
This is summarized in the reaction:
- CaCO3 + H2O + CO2 → Ca(aq) + 2 HCO3
Increases in temperature or decreases in pressure tend to reduce the partial pressure of CO2 and precipitate CaCO3.
Reduction in salinity also reduces the solubility of CaCO3, by several orders of magnitude for fresh water versus seawater.
Near-surface water of the earth's oceans are oversaturated with CaCO3 by a factor of more than six.
Although ooids may form through purely inorganic processes, the bulk of CaCO3 precipitation in the oceans is the result of biological activity.
Much of this takes place on carbonate platforms.
Main article: Karst topography
About 10% of all sedimentary rocks are limestones.
Limestone is partially soluble, especially in acid, and therefore forms many erosional landforms.
Such erosion landscapes are known as karsts.
It is therefore usually associated with hills and downland, and occurs in regions with other sedimentary rocks, typically clays.
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 Rock of Gibraltar, the Burren in County 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.
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.
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 (density 2.6), 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 were 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 alkali-based cleaner.
Other uses include:
- It is the raw material for the manufacture of quicklime (calcium oxide), slaked lime (calcium hydroxide), cement and mortar.
- Pulverized limestone is used as a soil conditioner to neutralize acidic soils (agricultural lime).
- Is crushed for use as aggregate—the solid base for many roads as well as in asphalt concrete.
- Geological formations of limestone are important petroleum reservoirs;
- As a reagent in flue-gas desulfurization, it reacts with sulfur dioxide for air pollution control.
- Glass making, in some circumstances, uses limestone.
- It is added to toothpaste, paper, plastics, paint, tiles, and other materials as both white pigment and a cheap filler.
- It can suppress methane explosions in underground coal mines.
- Purified, it is added to bread and cereals as a source of calcium.
- Calcium levels in livestock feed are supplemented with it, such as for poultry (when ground up).
- It can be used for remineralizing and increasing the alkalinity of purified water to prevent pipe corrosion and to restore essential nutrient levels.
- Used in blast furnaces, limestone binds with silica and other impurities to remove them from the iron.
Occupational safety and health
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/m total exposure and 5 mg/m 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/m total exposure and 5 mg/m respiratory exposure over an 8-hour workday.
Removing graffiti from weathered limestone is difficult because it is a porous and permeable material.
The surface is fragile so usual abrasion methods run the risk of "severe surface loss".
Because it is an acid-sensitive stone some cleaning agents cannot be used due to adverse effects.
Degradation by organisms
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Limestone.