Limestone

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The limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO₃), generally calcite, although it frequently presents traces of magnesite (MgCO₃) and other carbonates. It can also contain small amounts of minerals such as clay, hematite, siderite, quartz, etc., which modify (sometimes significantly) the color and degree of coherence of the rock. The practically monomineral character of the limestones makes it easy to recognize them thanks to two fundamental physical and chemical characteristics of calcite: it is less hard than copper (its hardness on the Mohs scale is 3) and it reacts with effervescence in the presence of acids such as hydrochloric acid.

In the field of industrial rocks or aggregates for construction, it is also called limestone. Along with dolomites and marls, limestones are part of what are known as carbonate or calcareous rocks.

If calcined (brought to a high temperature), limestone gives rise to lime (impure calcium oxide, CaO).

Training

Hydrothermal limestone training in Pamukkale, Turkey

They are very characteristic due to their light, whitish or gray color. Limestones form in the warm, shallow seas of tropical regions, in those areas where detrital contributions are insignificant. Two processes, which generally act together, contribute to the formation of limestone:

Water origin

Scheme of the formation process of caves and caves with stalactites and stalagmites in calcareous regions, due to the chemistry of the calcium carbonate

Calcium carbonate (CaCO₃) dissolves very easily in waters containing dissolved carbon dioxide (CO₂) gas, because it reacts with this and water to form calcium bicarbonate [Ca(HCO₃)₂], a highly soluble intermediate. However, in environments in which dissolved CO₂ is suddenly released into the atmosphere, the reverse reaction occurs, increasing the concentration of calcium carbonate (see law of mass action), whose excess over the saturation level precipitates. According to what has been described, the chemical equilibrium in solution follows the following equation:

{displaystyle CaCO_{3}+H_{2}O+CO_{2}rightleftharpoons Ca(HCO_{3})_{2}}

This release of CO₂ occurs, fundamentally, in two types of environments: on the coast when water laden with CO₂ reaches the surface and, on the continents, when groundwater reaches the surface. This is the fundamental process of formation of grottoes and caves with the presence of stalactites and stalagmites in many calcareous regions with limestone also called karsts, carsts or karsts. These last denominations of the calcareous regions come from the name of the Slovenian region of Carso, rich in these minerals and landscapes.

Biological origin

Current marine calcareous sedimentation
1: Carbonated Platforms; 2: Coral Reefs

Many organisms use calcium carbonate to build their mineral skeleton, due to the fact that it is an abundant compound and often almost saturated in the surface waters of oceans and lakes (being, therefore, relatively easy to induce its precipitation). After the death of these organisms, the accumulation of these mineral remains occurs in many environments in such quantities that they come to constitute sediments that are the origin of the vast majority of existing limestone.

Cacareous sedimentation was much more important in other times and is currently limited to a few tropical tidal regions. The limestone that can be seen on the continents was formed in times characterized by having a much warmer climate than today, when there was no ice at the poles and the sea level was much higher. Large parts of the continents were then covered by shallow epicontinental seas. At present, there are relatively few carbonate platforms [marked with (1) in the image above], with reefs (2) playing an important role.

Use of limestone

Dissolution of a limestone by water effect

The limestone, cut, carved or rough, is used as a construction or ornamental material, in the form of ashlars or covering plates. Examples of this use are numerous historical buildings, from the pyramids of Egypt to the Cathedral of Burgos. Crushed is used as construction aggregate.

It is an important component of the gray cement used in modern construction and can also be used as a main component, together with aggregates, to make the old lime mortar, fat paste for creating stucco or whitewashing grout (painting).) surfaces, as well as many other uses, for example in the pharmaceutical or fur industry.

Burgos Cathedral, built with limestone

It is an important rock as an oil reservoir, given its great porosity. It has a great resistance to weathering; This has allowed many ancient sculptures and buildings carved in limestone to reach the present day. However, the action of rainwater and rivers (especially when it is acidified by carbonic acid) causes its dissolution, creating a characteristic type of weathering called karst. However, it is used in the construction of riprap for maritime and port works such as breakwaters, breakwaters, breakwaters, among other stabilization and protection structures.

Limestone is within the classification of natural resources among non-renewable resources (minerals) and within this classification, in non-metallic ones, such as saltpeter, aljez and sulfur.

World production

World production in 2019, in millions of tons per year
1.	ChinaChina	310.0
2.	United States	16.9
3.	India	16,0
4.	Russia Russia	11,0
5.	BrazilBrazil	8.1
6.	JapanJapan	7.3
7.	GermanyGermany	7.1
8.	South KoreaSouth Korea	5.2
9.	TurkeyTurkey	4.6
10.	JapanJapan	4.3
11.	ItalyItaly	3.5
12.	IranIran	3.4
13.	PolandPoland	2.7
14.	France	2.6
15.	Ukraine Ukraine	2.2
16.	Australia	1.9
17.	Romania Romania	1.9
18.	SpainSpain	1,8

Source: USGS.

Confusion with marble

In the market, some limestones are offered as marbles, however, they are different rocks. Although marbles have the same major component as limestone (calcium carbonate, from shells and skeletal fragments of marine organisms such as corals, shells, etc.), they differ in their chemical structure. Therefore, both have different resistances and costs, marble being almost always more expensive. Basically, marble was a limestone that was recrystallized. This recrystallization occurred millions of years ago through a process called metamorphism, which means "change of type." Metamorphic rocks, in general, arise when the existing rocks in a certain place are physically and/or chemically transformed due to having been exposed to very high temperatures and very high pressure. In other words, limestone, subjected to these conditions, gives rise to marble (which is much more compact and more resistant to water, reaching a greater shine when polished). Since not all limestones suffered these conditions, some have continued to be limestones up to the present day. Aesthetically, many limestones show whole or broken fossil remains, like parts of shells, while high temperatures and pressures have meant that in no marble can the fossil remains be distinguished (but only its compound, calcium carbonate). The attractive and difficult to extract Italian Portoro limestone, which is sold worldwide as "Portoro marble", is one of the few exceptions in which the price of a limestone exceeds the price of the average marble, and even the price of Carrara marble.. However, that does not change the fact that it has lower resistance and is very sensitive to water. Another example of limestone generally polished and sold as marble is the "New Beige" from Egypt, which in some countries do not hesitate to offer as a "marble" that offers better value for money than other marbles (when compared to the costs of the real marbles).

Since calcium carbonate is white, it is the impurities that give limestone (and marble) its color differences.

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