Limestone

What is limestone?

Limestone is a sedimentary rock that is primarily composed of Calcium carbonate in the form of calcite mineral. It forms mostly in the clear, warm and shallow marine water accumulates in the form of organic debris as coral, shells, algal or fecal deposits. It can also be formed by the precipitation of calcium carbonate dissolved in the water running in the ground water. 

Composition of limestone

Limetone as the name indicates is mainly composed of calcium carbonate, greater than 50%. A few percentage of other minerals are also present as all limestone contains them. The other minerals can be quartz, siderite, pyrite, clay minerals feldspar and other minerals. It also includes large nodules of chert, pyrite or siderite. The calcium carbonate can easily be identified when treated with acid, it reacts. 

A Limestone-Forming Environment: An underwater view of a coral reef system from the Kerama Islands in the East China Sea southwest of Okinawa. Here the entire seafloor is covered by a wide variety of corals which produce calcium carbonate skeletons.

Environment of deposition

Marine

The limestone is mainly deposited in calm, warm marine water at shallow depths where organism are capable of generating calcium carbonate shells and skeleton. These organism when die accumulate in the area to form limestone deposits. The waste product of these organism also contribute to sediment mass which in turn when lithifies produces limestone. The limestone produce from this type are biological sedimentary rocks. 

Other limestone is formed by direct precipitation from the marine or fresh water and this type is called chemical sedimentary rock. 

Evaporite

The limestone formed by evaporation are often formed in the caves where water droplet enters the fracture and when reaching cave ceiling evaporates. When water evaporates the calcium carbonate present precipitates to form limestone.

The Bahamas Platform: A NASA satellite image of the Bahamas Platform where active limestone formation occurs today. The main platform is over 100 miles wide, and a great thickness of calcium carbonate sediments have accumulated there. In this image the dark blue areas are deep ocean waters. The shallow Bahamas Platform appears as light blue

Description

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. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts. These organisms secrete shells made of aragonite or calcite, 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 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 recognised 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. Limestone 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.

Impurities (such as clay, sand, organic remains, iron oxide, and other materials) will cause limestone to exhibit different colours, especially with 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. 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.

During regional metamorphism that occurs during the mountain building process (orogeny), limestone recrystallises into marble. Limestone is a parent material of Mollisol soil group.

Classification

Two major classification schemes, the Folk and the Dunham, are used for identifying limestone and carbonate rocks.

Folk 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 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.

Dunham classification

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 characterised 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 landscape

About 10% of all sedimentary rocks are limestone. 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 groundwater will also create conditions suitable for cave formation.

Coastal limestone 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,[9] 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 kilometres.

The world's largest limestone quarry is at Michigan Limestone and Chemical Company in Rogers City, Michigan.

Varieties of limestone

There are many varieties of limestone based on the composition of limestone.

• Chalk A soft limestone that is white or grey in colour which is mainly formed from calcareous shell remains of microscopic marine organisms.
• Coquina A poorly cemented limestone that is composed mainly of broken shell debris.
• Fossiliferous limestone A limestone with prominent and abundant fossils.
• Oolitic limestone Limestone composed mainly of calcium carbonate oolites, spheres concentric precipitates of calcium carbonate.
• Travertine Limestone formed by precipitation method mainly in caves.
• Tufa Precipitation of limestone at hot springs or lake shore.

Uses of limestone

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, are 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. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, 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 artefacts made from this stone. Many limestone statues and building surfaces have suffered severe damage due to acid rain. Acid-based cleaning chemicals can also etch limestone, which should only be cleaned with a neutral or mild alkaline-based cleaner.

Other uses include:

• It is the raw material for the manufacture of quicklime (calcium oxide), slaked lime (calcium hydroxide), cement and mortar.
• Pulverised limestone is used as a soil conditioner to neutralise 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 among the best petroleum reservoirs;
• As a reagent in flue-gas desulphurization, it reacts with sulphur 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.
• It is often found in medicines and cosmetics.
• It is used in sculptures because of its suitability for carving.

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