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Limestone
Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3).
Description
Limestone often contains variable amounts of silica in the form of chert and/or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine biology|marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as Pelagic zone|pelagic ooze or alternatively are conglomerated in a coral reef (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by Supersaturation|supersaturated Meteorology|meteoric waters (groundwater that Precipitation (chemistry)|precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance.
Limestone makes up about 10% of the total volume of all sedimentary rocks.[ ][ ] Limestones may also form in both lacustrine and evaporite Sedimentary depositional environment|depositional environments[http://jgs.lyellcollection.org:/cgi/content/abstract/156/3/535|Trewin,N.H. & Davidson,R.G. 1999. Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed, Journal Geological Society, 156, 535-548][Oilfield Glossary: Term 'evaporite'].
Calcite can be either solvation|dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, acidity|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.
When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.
Karst topography and caves develop in carbonate rocks due to their solubility in dilute acidic groundwater. 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).
Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on Weathering|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. Folk classification|Folk and Dunham classification|Dunham classifications are used to describe limestones more precisely.
Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls and around Hot spring|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 Animal shell|shells.
During regional metamorphism that occurs during the mountain building process (orogeny) limestone recrystallizes into marble.
Limestone is a parent material of Mollisol soil group.
Limestone landscape
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 resistance (geology)|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 The Burren|Burren in Co. Clare, Ireland; the Verdon Gorge in France; Malham Cove in North Yorkshire and the Isle of Wight[Michigan Markers]
Uses of limestone
Limestone is very common in architecture, especially in North America and Europe. Many landmarks across the world, including the pyramids in Egypt, are made of limestone. So many buildings in Kingston, Ontario|Kingston, Ontario, Canada were 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. 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, 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, is 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 marble are very reactive to acid solutions, making acid rain a significant problem. 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:
- The manufacture of quicklime (calcium oxide) and slaked lime (calcium hydroxide);
- Cement and mortar (masonry)|mortar;
- Pulverized limestone is used as a soil conditioner to neutralize acidic soil conditions;
- Crushed for use as construction aggregate|aggregate—the solid base for many roads;
- Geological formations of limestone are among the best petroleum reservoirs;
- As a reagent in flue gas desulfurization|desulfurizations;
- Glass making, in some circumstances;
- Added to paper, plastics, paint, tiles, and other materials as both white pigment and a cheap filler.
- Toothpaste
- Suppression of methane explosions in underground coal mines
- Added to bread and cereals as a source of calcium
Notes
References
- Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1-103.http://www.wooster.edu/geology/Taylor%26Wilson2003.pdf
See also
- List of types of limestone
- Chalk
- Coral sand
- Calcium carbonate
- In Praise of Limestone
Category:Sedimentary rocks
Category:Limestone|
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Related Images- Iron impregnations in limestone
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