Limestone Rock Type, Composition, Formation, & Uses

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Limestone is a non-clastic sedimentary rock consisting of more than 50% calcite (a calcium carbonate mineral, CaCO3). Limestone is mostly a biological rock that forms in clear, warm, and shallow marine waters.

Limestone exists in various types and colors. Its composition varies slightly from one reserve to another, but all types contain calcium carbonate as the primary ingredient. Although the mineral composition varies slightly, limestones show great texture diversity.

Limestone is usually classified depending on grain type and mud content. This is because most limestones consist of sand-sized grains and are composed of biological sediment that is deposited close to where it is formed.

Limestone Sample
Limestone Sample

Rock Composition

Limestone is primarily made of the mineral calcite and aragonite. Both these minerals are different crystallized forms of calcium carbonate. The calcite is limestone and often contains some magnesium in it. Calcite in limestone is divided into two types based on magnesium content. The dividing point is 4% magnesium content. Calcites with more than 4% magnesium are considered high-magnesium calcites, and those with less than 4% magnesium are considered low-magnesium calcites. High magnesium calcites retain the calcite mineral structure. Aragonite does not have a significant magnesium concentration.

Other trace minerals found in limestone include dolomite and siderite. Some specimens also contain different amounts of silica  (in the form of chert). Fossils are also commonly found in most limestone reserves.

The calcium carbonate in limestone makes it easy to identify the rock. Limestones effervesce when it comes in contact with 5% hydrochloric acid.

Physical Properties

Limestone is usually white to gray, but its concentration often imparts different colors. Limestones rich in organic matter have darker colors, like black. Iron or manganese presence makes limestone off-white, yellow or red.

It is a relatively soft stone with a rating of 3 to 4 on the Mohs Hardness scale. Its density depends on porosity. The porosity varies from 0.1% to 40% from one limestone type to another. The chalk is the most porous limestone with a porosity greater than 30%.

Although limestone is a soft rock, dense limestone shows a crushing strength of up to 180 MPa. Concrete has a crushing strength of about 40 MPa. Most limestones consist of sand-sized grains in a carbonate mud matrix.

Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain
Limestone outcrop in the Torcal de Antequera nature reserve of Málaga, Spain

Limestone Formation

The rock forms from accumulated organic debris from shells, corals, algae, and fecal matter. Some reserves are also formed by chemical sedimentary processes such as calcium carbonate precipitation from lake or ocean water.

Biological limestone usually forms in clear, warm, and peaceful shallow marine waters. This environment facilitates the growth of organisms with calcium carbonate shells and skeletons. Their shells and skeleton accumulate when these organisms die. The debris is then lithified into limestone. Most biological limestones often have fossils in varying percentages.

Chemical limestone forms after the direct precipitation of calcium carbonate from fresh or marine water. Chemical limestones are less abundant than biological limestones.

El Capitan, an ancient limestone reef
El Capitan, an ancient limestone reef

Limestone Types

Limestone has different types with unique names based on the rock formation process, appearance, composition, and physical properties. The most common types of limestones are defined below.

Chalk: It is one of the most common types of limestone and forms from calcareous shell remains of microscopic marine organisms like foraminifera and algae.

It is a soft limestone with high porosity and fine texture. Chalk is used to write on blackboards and can be crushed with bare hands.

Chalk Limestone: The White Cliffs of Dover are composed of chalk.
Chalk Limestone: The White Cliffs of Dover are composed of chalk.

Coquina: It is a poorly cemented limestone consisting of sand-size fragments of calcium carbonate shells or coral debris. The grains in the rock are bound by calcium carbonate cement.

Coquina is usually found on beaches where steady wave currents deliver the shells. Coquina can be composed of different invertebrate remains like gastropods, brachiopods, trilobites, coral, or ostracods.

Coquina outcrop on the beach at Washington Oaks State Gardens, Florida
Coquina outcrop on the beach at Washington Oaks State Gardens, Florida

Crystalline Limestone: Limestones exposed to metamorphic heat and pressure are known as crystalline limestones. This alters the rock, and crystals form and recrystallize. The recrystallization continues depending on the intensity of the metamorphic process. Once the crystals have fully grown and can be seen with the naked eye, the rock is called a marble or crystalline limestone.

Marble is a commonly used rock that is made up of metamorphosed limestone. It contains abundant calcium carbonate and other minerals like mica, quartz, pyrite, and iron oxide.

Dolomitic Limestone: Dolomitic limestone mostly contains calcite, but some of the calcite has been converted to dolomite in a chemical process known as dolomitization. Dolomitization happens when magnesium-rich groundwater converts calcite to dolomite.

The extent of dolomitization varies from one place to another. Sometimes a rock is completely converted to dolomite, and sometimes only partial conversion occurs. The rock is called dolomitic limestone as a result of partial conversion.

Fossiliferous Limestone: It consists of abundant fossils from marine invertebrates like crinoids, mollusks, gastropods, and brachiopods. These limestones are used to study and investigate the geological age of the rock.

Lithographic Limestone: It is a dense rock with extremely fine and uniform grain size. It was used in printing machines in the 1700s.

Oolitic Limestone: These rocks are sand-sized clasts consisting mainly of calcium carbonate with an ovate or spherical shape. The rocks form when concentrated layers of calcium carbonate are formed around a sand grain or a shell fragmen0074 nucleus.

Travertine: It forms when geothermally heated alkaline water, powered by dissolved gases and minerals emerge at the surface. The water slowly cools down and evaporates, resulting in the rock formation. Travertine’s largest reserves are located in Egypt and Italy. The rock is used as floor tiles, wall panels, and stair steps.

Tufa: It is a porous limestone produced from calcium carbonate precipitation from hot springs or water bodies that contain calcium carbonate in abundance.

Where is Limestone Found?

Limestone is an economically valuable rock. Many countries mine millions of tons and export it worldwide to increase revenues. Many limestone-forming environments are active on the earth today. The most notable sites are the Caribbean sea, the Indian Ocean, the gulf of Mexico, and the Persian Gulf.

Limestone is also found in Michigan, Indiana, and Illinois. The area around the great lake has plenty of limestone reserves.

Limestone Uses

Limestone is one of the few rocks that have so many uses. Its major uses are outlined below.

  • It is crushed to use as railroad ballast, road base, foundation stone, and aggregate in various construction projects. Limestone is suited for these applications because it is easier to mine, transport and crush. However, some harder rocks may be more suited for these applications.
  • Limestone is cut into slabs and pieces to be used as dimension stone. It is commonly used for facing stones, floor tiles, window sills, and stair steps.
  • Limestone is used as roofing granules because it is weather-resistant.
  • Crushed limestone is used for metal refining processes.
  • Limestone is often used in cement manufacturing.
  • It is used as a neutralizing agent due to the high calcium carbonate content in the rock.
  • Limestone is often used as a substitute in the chicken diet to fulfill calcium needs for stronger bones and egg shells.
  • Crushed limestone is used as filler in the paint, paper, and rubber industry.
  • It is also used as filler in on-site sewage disposal systems and an absorbent at coal-burning facilities.

Limestone Problems

  • Some varieties of limestone can deteriorate due to weathering events such as wind, rain, and thermal change.
  • Limestone can erode due to natural weathering events. Erosion is generally a localized phenomenon based on handling or exposure of the rock.
  • Limestone is sensitive to oil/grease stains, dyes, and inks, organic and metallic stains.
  • Limestone can crack or crumble due to inherent weakness or external factors affecting the rock.
  • Limestone rocks can start peeling due to inherent weakness or improper finishing of the rock.

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