Editing Limestone (section)

== Description ==
[[File:Limestone Eocene deposit at Sinj Stari grad - Dalmatia - Croatia IMG 20210820 083857.jpg|thumb|This limestone deposit in the [[karst]] of [[Dinaric Alps]] near [[Sinj]], [[Croatia]], was formed in the [[Eocene]]. ]]
Limestone is composed mostly of the [[minerals]] [[calcite]] and [[aragonite]], which are different [[Polymorphism (materials science)|crystal forms]] of [[calcium carbonate]] ({{chem2|CaCO3}}). [[Dolomite (mineral)|Dolomite]], {{chem2|CaMg(CO3)2}}, is an uncommon mineral in limestone, and [[siderite]] or other [[carbonate mineral]]s are rare. However, the calcite in limestone often contains a few percent of [[magnesium]]. Calcite in limestone is divided into low-magnesium and high-magnesium calcite, with the dividing line placed at a composition of 4% magnesium. High-magnesium calcite retains the calcite mineral structure, which is distinct from dolomite. Aragonite does not usually contain significant magnesium.{{sfn|Blatt|Middleton|Murray|1980|p=448–449}} Most limestone is otherwise chemically fairly pure, with [[Detritus (geology)|clastic sediments]] (mainly fine-grained [[quartz]] and [[clay mineral]]s) making up less than 5%{{sfn|Blatt|Tracy|1996|p=295}} to 10%{{sfn|Boggs|2006|p=160}} of the composition. Organic matter typically makes up around 0.2% of a limestone and rarely exceeds 1%.{{sfn|Blatt|Middleton|Murray|1980|p=467}}

Limestone often contains variable amounts of [[silica]] in the form of [[chert]] or siliceous skeletal fragments (such as [[sponge]] spicules, [[diatoms]], or [[radiolarians]]).{{sfn|Blatt|Tracy|1996|pp=301-302}} [[Fossil]]s are also common in limestone.{{sfn|Boggs|2006|p=159}}

Limestone is commonly white to gray in color. Limestone that is unusually rich in organic matter can be almost black in color, while traces of [[iron]] or [[manganese]] can give limestone an off-white to yellow to red color. The density of limestone depends on its porosity, which varies from 0.1% for the densest limestone to 40% for chalk. The density correspondingly ranges from 1.5 to 2.7 g/cm<sup>3</sup>. Although relatively soft, with a [[Mohs hardness]] of 2 to 4, dense limestone can have a crushing strength of up to 180 [[MPa]].<ref>{{cite journal |last1=Oates |first1=Tony |title=Lime and Limestone |journal=Kirk-Othmer Encyclopedia of Chemical Technology |date=17 September 2010 |pages=1–53 |doi=10.1002/0471238961.1209130507212019.a01.pub3|isbn=978-0-471-23896-6 }}</ref> For comparison, [[concrete]] typically has a crushing strength of about 40 MPa.<ref>{{cite encyclopedia |chapter=Compressive strength test |title=Encyclopedia Britannica |chapter-url=https://www.britannica.com/technology/compressive-strength-test |access-date=4 February 2021}}</ref>

Although limestones show little variability in mineral composition, they show great diversity in texture.{{sfn|Blatt|Tracy|1996|pp=295-296}} However, most limestone consists of sand-sized grains in a carbonate mud matrix. Because limestones are often of biological origin and are usually composed of sediment that is deposited close to where it formed, classification of limestone is usually based on its grain type and mud content.{{sfn|Blatt|Tracy|1996|p=295}}

===Grains===
[[File:Ooids, Joulter Cays, Bahamas.jpg|thumb|[[Ooid]]s from a [[beach]] on Joulter's Cay, [[The Bahamas]] ]]
[[File:Ooids Carmel Formation Jurassic.jpg|thumb|Ooids in limestone of the [[Carmel Formation]] (Middle Jurassic) of southwestern Utah.]]
[[File:CarmelOoids.jpg|thumb|Thin-section view of a Middle [[Jurassic]] limestone in southern [[Utah]], U.S. The round grains are [[ooid]]s; the largest is {{cvt|1.2|mm|in|2}} in diameter. This limestone is an oosparite.]]
Most grains in limestone are skeletal fragments of marine organisms such as [[coral]] or [[foraminifera]].{{sfn|Blatt|Middleton|Murray|1980|p=452}} These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are [[ooids]], [[peloids]], and limeclasts ([[intraclasts]] and {{ill|extraclast|lt=extraclasts|ca}}).{{sfn|Blatt|Tracy|1996|pages=295–300}}

Skeletal grains have a composition reflecting the organisms that produced them and the environment in which they were produced.{{sfn|Blatt|Middleton|Murray|1980|p=449}} Low-magnesium calcite skeletal grains are typical of articulate [[brachiopod]]s, planktonic (free-floating) foraminifera, and [[coccolith]]s. High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, [[echinoderm]]s, and [[coralline algae]]. Aragonite skeletal grains are typical of [[mollusc]]s, calcareous [[green algae]], [[stromatoporoid]]s, [[coral]]s, and [[tube worm]]s. The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water).{{sfn|Boggs|2006|p=161–164}}

Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around a central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2&nbsp;mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an ''[[oolite]]'' or sometimes an ''oolitic limestone''. Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show [[Cross-bedding|crossbedding]] and other features associated with deposition in strong currents.{{sfn|Blatt|Tracy|1996|pp=297-299}}{{sfn|Boggs|2006|pp=164-165}}

''Oncoliths'' resemble ooids but show a radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment.{{sfn|Blatt|Tracy|1996|pp=297-299}}

Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes.<ref>{{cite journal |last1=Adachi |first1=Natsuko |last2=Ezaki |first2=Yoichi |last3=Liu |first3=Jianbo |title=The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China |journal=Sedimentary Geology |date=February 2004 |volume=164 |issue=1–2 |pages=161–178 |doi=10.1016/j.sedgeo.2003.10.007|bibcode=2004SedG..164..161A }}</ref> Many are thought to be fecal pellets produced by marine organisms. Others may be produced by [[endolithic]] (boring) algae{{sfn|Blatt|Tracy|1996|p=298}} or other microorganisms<ref>{{cite journal |last1=Chafetz |first1=Henry S. |title=Marine Peloids: A Product of Bacterially Induced Precipitation of Calcite |journal=SEPM Journal of Sedimentary Research |date=1986 |volume= 56 |issue=6 |pages=812–817 |doi=10.1306/212F8A58-2B24-11D7-8648000102C1865D}}</ref> or through breakdown of mollusc shells.<ref>{{cite journal |last1=Samankassou |first1=Elias |last2=Tresch |first2=Jonas |last3=Strasser |first3=André |title=Origin of peloids in Early Cretaceous deposits, Dorset, South England |journal=Facies |date=26 November 2005 |volume=51 |issue=1–4 |pages=264–274 |doi=10.1007/s10347-005-0002-8|bibcode=2005Faci...51..264S |s2cid=128851366 |url=http://doc.rero.ch/record/322424/files/10347_2005_Article_2.pdf }}</ref> They are difficult to see in a limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them.{{sfn|Blatt|Tracy|1996|p=298}}

Limeclasts are fragments of existing limestone or partially [[lithification|lithified]] carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include ''grapestone'', which is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a [[turbidity current]].{{sfn|Blatt|Tracy|1996|p=299–300, 304}}

===Mud===
The grains of most limestones are embedded in a matrix of carbonate mud. This is typically the largest fraction of an ancient carbonate rock.{{sfn|Blatt|Tracy|1996|p=298}} Mud consisting of individual crystals less than {{convert|5|µm|mil|abbr=in}} in length is described as ''micrite''.{{sfn|Blatt|Middleton|Murray|1980|p=460}} In fresh carbonate mud, micrite is mostly small aragonite needles, which may precipitate directly from seawater,{{sfn|Blatt|Tracy|1996|p=300}} be secreted by algae,{{sfn|Boggs|2006|p=166}} or be produced by abrasion of carbonate grains in a high-energy environment.<ref name="trower-etal-2019"/> This is converted to calcite within a few million years of deposition. Further recrystallization of micrite produces ''microspar'', with grains from {{convert|5|to|15|µm|mil|abbr=in}} in diameter.{{sfn|Blatt|Tracy|1996|p=300}}

Limestone often contains larger crystals of calcite, ranging in size from {{convert|0.02|to|0.1|mm|mil|abbr=in}}, that are described as ''sparry calcite'' or ''sparite''. Sparite is distinguished from micrite by a grain size of over {{convert|20|µm|mil|abbr=in}} and because sparite stands out under a hand lens or in thin section as white or transparent crystals. Sparite is distinguished from carbonate grains by its lack of internal structure and its characteristic crystal shapes.{{sfn|Boggs|2006|pp=166-167}}

Geologists are careful to distinguish between sparite deposited as cement and sparite formed by recrystallization of micrite or carbonate grains. Sparite cement was likely deposited in pore space between grains, suggesting a high-energy depositional environment that removed carbonate mud. Recrystallized sparite is not diagnostic of depositional environment.{{sfn|Boggs|2006|pp=166-167}}

===Other characteristics===
[[File:Seven Sisters 3.jpg|thumb|The [[Beachy Head]] cliffs are composed of chalk.]]
Limestone outcrops are recognized in the [[Field work|field]] by their softness (calcite and aragonite both have a Mohs hardness of less than 4, well below common silicate minerals) and because limestone bubbles vigorously when a drop of dilute [[hydrochloric acid]] is dropped on it. Dolomite is also soft but reacts only feebly with dilute hydrochloric acid, and it usually weathers to a characteristic dull yellow-brown color due to the presence of ferrous iron. This is released and oxidized as the dolomite weathers.{{sfn|Blatt|Tracy|1996|p=295}} Impurities (such as [[clay]], sand, organic remains, [[iron oxide]], and other materials) will cause limestones to exhibit different colors, especially with [[Weathering|weathered]] surfaces.

The makeup of a carbonate rock outcrop can be estimated in the field by etching the surface with dilute hydrochloric acid. This etches away the calcite and aragonite, leaving behind any silica or dolomite grains. The latter can be identified by their [[rhombohedral]] shape.{{sfn|Blatt|Tracy|1996|p=295}}

Crystals of calcite, [[quartz]], [[Dolomite (mineral)|dolomite]] or [[barite]] may line small cavities (''[[vugs]]'') in the rock. Vugs are a form of secondary porosity, formed in existing limestone by a change in environment that increases the solubility of calcite.{{sfn|Blatt|Tracy|1996|pp=315-317}}

Dense, massive limestone is sometimes described as "marble". For example, the famous [[Portoro marble|Portoro "marble"]] of Italy is actually a dense black limestone.<ref>{{cite journal |last1=Fratini |first1=Fabio |last2=Pecchioni |first2=Elena |last3=Cantisani |first3=Emma |last4=Antonelli |first4=Fabrizio |last5=Giamello |first5=Marco |last6=Lezzerini |first6=Marco |last7=Canova |first7=Roberta |title=Portoro, the black and gold Italian "marble" |journal=Rendiconti Lincei |date=December 2015 |volume=26 |issue=4 |pages=415–423 |doi=10.1007/s12210-015-0420-7|s2cid=129625906 }}</ref> True [[marble]] is produced by recrystallization of limestone during regional [[metamorphism]] that accompanies the mountain building process ([[orogeny]]). It is distinguished from dense limestone by its coarse crystalline texture and the formation of distinctive minerals from the silica and clay present in the original limestone.{{sfn|Blatt|Tracy|1996|pp=474}}

===Classification===
{{See also|List of types of limestone}}
[[File:Pamukkale 12.jpg|thumb|[[Travertine]] limestone terraces of [[Pamukkale]], [[Turkey]].]]
[[File:Luray Caverns, Dream Lake - mirror-lake of caverns (2015-05-09 14.03.28 by Stan Mouser).jpg|thumb|[[Cave formations|Cave limestone formations]] in the [[Luray Caverns]] of the northern [[Shenandoah Valley]]]]Two major classification schemes, the Folk and Dunham, are used for identifying the types of [[carbonate rocks]] collectively known as limestone.

====Folk classification====
{{main|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]].<ref>{{cite web| url = http://sepmstrata.org/page.aspx?pageid=89| title = Carbonate Classification: SEPM STRATA}}</ref> 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.<ref name="Folk">{{cite book |last=Folk |first=R. L. |year=1974 |title=Petrology of Sedimentary Rocks |publisher=Hemphill Publishing |location=Austin, Texas |isbn=0-914696-14-9}}</ref>

====Dunham classification====
{{main|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.<ref name="Dunham">{{cite book |last=Dunham |first=R. J. |year=1962 |chapter=Classification of carbonate rocks according to depositional textures |editor-last=Ham |editor-first=W. E. |title=Classification of Carbonate Rocks |series=American Association of Petroleum Geologists Memoirs |volume=1 |pages=108–121 }}</ref>

A revised classification was proposed by Wright (1992). It adds some diagenetic patterns to the classification scheme.<ref name="Wright1992">{{cite journal |last1=Wright |first1=V.P.|year=1992 |title=A revised Classification of Limestones |journal=Sedimentary Geology |volume=76 |issue=3–4 |pages=177–185 |doi=10.1016/0037-0738(92)90082-3 |bibcode=1992SedG...76..177W}}</ref>

====Other descriptive terms====
[[File:Chalk ("Upper Chalk" Formation, Upper Cretaceous; White Cliffs of Dover, England, southern Britain).jpg|thumb|Chalk from the White Cliffs of Dover ([[Chalk Group]]), England]]
''[[Travertine]]'' is a term applied to calcium carbonate deposits formed in freshwater environments, particularly [[waterfall]]s, cascades and [[hot springs]]. Such deposits are typically massive, dense, and banded. When the deposits are highly porous, so that they have a spongelike texture, they are typically described as ''[[tufa]]''. Secondary calcite deposited by [[Supersaturation|supersaturated]] [[meteoric water]]s ([[groundwater]]) in caves is also sometimes described as travertine. This produces [[speleothem]]s, such as [[stalagmite]]s and [[stalactite]]s.{{sfn|Blatt|Middleton|Murray|1980|p=479–480}}

''[[Coquina]]'' is a poorly consolidated limestone composed of abraded pieces of [[coral]], [[Exoskeleton|shells]], or other fossil debris. When better consolidated, it is described as ''coquinite''.{{sfn|Boggs|2006|p=172}}

''[[Chalk]]'' is a soft, earthy, fine-textured limestone composed of the tests of planktonic microorganisms such as foraminifera, while
''[[marl]]'' is an earthy mixture of carbonates and silicate sediments.{{sfn|Boggs|2006|p=172}}
{{clear}}