Limestone is one of the most common types of sedimentary rock, found throughout the world in various forms. It is composed primarily of the mineral calcite (calcium carbonate), and is formed over long periods of time from the accumulation and compression of shell, coral, algal, and fecal debris. Limestone can be classified into several categories based on how it was formed, what it contains, and where it is located.
There are three main types of limestone classified by formation process: chemical, biochemical, and detrital. Chemical limestones form when calcium carbonate precipitates out of solution. Biochemical limestones are created by the accumulation of shell, coral, algal, and fecal debris, and are also known as organogenic limestones. Detrital limestones form from the compaction and cementation of calcareous sediments. In addition to differences in origin, limestones can be described based on composition and texture – whether made up of primarily calcite or aragonite, the size of the grains, and the amount of impurities present.
Some of the many varieties of limestone include chalk, travertine, coquina, fossiliferous limestone, oolitic limestone, and dolomitic limestone. Chalk is a fine-grained, white limestone originating from thecalcified skeletons of plankton. Travertine forms from chemically-precipitated calcium carbonate deposits around hot or cold springs. Coquina consists of poorly-cemented shell fragments. Oolitic limestone contains small spherical grains called oolites. Identifying key differences among the types of limestones provides insights into their physical properties, formation conditions, and potential uses.
How Limestone Forms
There are two main modes of formation for limestone:
Clastic Limestones
Clastic limestones form from the accumulation and cementation of pre-existing carbonate minerals and rocks. The fragments that compose these limestones may include pieces of older limestone formations, shells, coral fragments, and other carbonate debris. The source of the fragments is the erosion and weathering of uplifted carbonate platforms. As these carbonate particles are transported and deposited, they are lithified and cemented into clastic limestone.
Chemically-Precipitated Limestones
Chemically-precipitated limestones form when calcium carbonate precipitates out of solution. This can occur through both inorganic and biological processes.
Inorganic chemical precipitation happens when calcium carbonate saturates a solution and forms crystals. This may occur due to changes in temperature, pressure, or chemical composition of the solution.
Biologically-induced chemical precipitation occurs when organisms such as coral, shellfish, and algae remove calcium carbonate from seawater to construct their skeletal structures and shells. After their death, these skeletal elements accumulate on the sea floor and lithify into limestone.
Evaporative limestones also form through the evaporation of carbonate-rich water in restricted tidal flat or lagoon environments. The increase in alkalinity causes carbonate minerals to precipitate.
Limestone Composition and Properties
While the calcium carbonate mineral content gives limestone a chemical consistency across formations, there is a wide diversity in other properties like texture, porosity, and fossil contents.
Mineralogy
Most limestones consist of more than 50% calcite, with aragonite as the second most common carbonate mineral. Traces of dolomite, chert, silica, iron-rich carbonates like siderite, and fossils can also be found in many limestones. The relative proportions of these minerals and impurities influence the solubility and weathering patterns of limestone types.
Grain Size and Texture
Limestone can vary significantly in the size of the grains or particles it is composed of. Texture is classified based on grain size and sorting. Limestone can be coarse-grained with large fossil fragments visible or fine-grained with microcrystalline structures not discernable without magnification.
Porosity
Porosity measures the void spaces in a rock. In limestone these voids determine permeability, density, and resistance to weathering. Dense limestones are less porous and have low permeability while porous limestones allow fluid transport through interconnected pores.
Color
While most limestones are light-colored tones of grey to yellow, the presence of organic matter and impurities can create darker limestones in shades of brown, red, green, or black. Color can be a useful identification feature for limestone types.
Classification of Limestone Types
There are several classification systems used to categorize the diversity within limestone formations based on composition, depositional textures, and other attributes:
Folk Classification
Folk classification divides limestones into three broad categories based on the relative proportions of grains vs. cementing matrix:
- Grainstones contain over 2/3 grains cemented by sparry calcite
- Mudstones contain less than 1/3 grains in a micrite matrix
- Wackestones have a more balanced mixture of grains and matrix
Dunham Classification
The Dunham scheme focuses on depositional texture. Key groups include:
- Mudstones/micrites with fine-grained microcrystalline structures
- Wackestones with less than 10% grains over 2mm
- Packstones containing rounded fragments over 2mm
- Grainstones cemented by sparry calcite matrix
- Boundstones which exhibit rigid frameworks like stromatolites
Classification by Color and Grain Size
In addition to composition-based schemes, limestones can be categorized based on qualitative color and grain size attributes:
- Chalk is soft, fine-grained and white
- Travertine is banded and coarse-grained
- Coquina composed of shell fragments
- Oolitic limestone with spherical oolite grains
Where Limestone Forms
Limestone makes up 10% of all sedimentary rocks and occurs extensively across the world due to its alkaline chemistry. Here are some key limestone deposits:
- Chalk deposits in Northern Europe
- Extensive carbonate platforms surrounding the Bahamas
- The Mokattam formation in Egypt
- Limestone-marble bands of the Canadian Shield
- Travertine deposits in freshwater streams
- Coquina formations in Florida
Limestone also occurs in association with other geological features like reef complexes, lagoons, beaches, caves, and springs.
Limestone Structures and Textures
The sedimentary structures and textures preserved in limestone provide clues about the depositional environment and diagenetic history of the formation. Common limestone structures include:
- Cross bedding indicating shifting energy conditions
- Stromatolite domes created by trapped sediment behind microbial mats
- Styolites showing dissolution along pressure boundaries
- Fossil molds where shell material was weathered away
- Geopetal structures recording the original upright orientation
These features help geologists reconstruct the physical, chemical, and biological conditions at the time of deposition.
The Role of Limestone in Soil Science
In addition to its extensive use as a building material, limestone also plays an important role in soils and agriculture. Limestone raises pH and reduces acidity when spread on fields. The reaction of lime with clay minerals improves soil structure, aeration, drainage, and fertility. Calcium from limestone also activates plant growth processes. Understanding the composition of agricultural limestones allows scientists to select effective soil amendments.
Physical Properties of Limestone
The physical properties of limestone dictate its unique characteristics and variability. Some key attributes include:
- Hardness – Limestone has a hardness of 3-4 on the Mohs hardness scale. Chalk and travertine are on the softer end while dense limestones are harder.
- Color – Shades of gray, brown, yellow, red and white. Organic matter and iron create darker colorations.
- Luster – The surface light reflection. Ranges from earthy to vitreous.
- Streak – The powdered streak of limestone is white.
- Fracture – Limestone exhibits smooth, splintery and conchoidal fracture patterns.
- Porosity – Ranges from non-porous in mudstones/micrite to porous in grainstones. Influences weathering.
- Strength – Limestone has a compressive strength between 100-150 MPa. Chalk is the weakest while crystalline limestones are very sturdy.
The diversity of limestone properties equips it for uses ranging from construction to agriculture to industry. Careful examination of characteristics provides clues into the origins and best applications of different limestone types.
Major Limestone Varieties
With this foundation on limestone origins, composition and classifications, we can now survey some of the most prominent types of limestone found across different regions of the world:
Chalk
Chalk is a fine-grained, porous form of limestone composed of skeletal plankton debris such as the calcite plates of coccolithophores. It forms extensive deposits in deep marine basins where plankton production is high and detrital influx is low. The soft, white nature of chalk makes it useful for writing and marking. Major chalk deposits are found across northern Europe.
Travertine
Travertine is a banded, coarse-grained limestone that forms due to the rapid precipitation of calcium carbonate from hot or cold springs. Beautiful travertine terraces and formations occur near springs and rivers in Italy and Turkey. Travertine is less porous than other limestones and can be polished to a marble-like finish.
Coquina
Coquina is an organogenic limestone composed of loosely consolidated shell fragments. It forms in high-energy beach environments where shell debris is ground up and accumulates. The shell fragments visible in coquina distinguish it from other limestones. Major deposits occur along the southeastern US coastline.
Oolitic Limestone
Oolitic limestone consists of small spherical grains called oolites, which consist of concentric layers of calcium carbonate. It forms in shallow, turbulent, warm waters. Important oolitic limestone formations can be found in places like the Florida Keys and the Bahamas banks.
Fossiliferous Limestone
Fossiliferous limestones contain abundant marine organism fossils. The fossils provide information on prehistoric life and environments. The Tripoli formation in Libya and Solnhofen Limestone in Germany are renowned fossil-rich limestones.
Dolomitic Limestone
Dolomitic limestone contains a significant proportion of the mineral dolomite alongside calcite and/or aragonite. It commonly forms in post-depositional environments when magnesium replaces calcium in the crystal lattice. Dolomitic limestones are important reservoirs for petroleum and groundwater.
Comparison Table of Major Limestone Varieties
| Limestone Variety | Description | Composition | Texture | Location | Uses |
|---|---|---|---|---|---|
| Chalk | Fine-grained, soft, white. Composed of skeletal plankton debris. | 95% calcium carbonate. High porosity. | Very fine grain size, earthy luster. | Extensive deposits in Northern Europe, England. | Historic writing, marking. Building material. |
| Travertine | Banded limestone formed by mineral springs. Less porous than other limestones. | Composed of calcite. Concentric banding common. | Coarse crystalline structure. Polished travertine has a marble-like appearance. | Deposits near springs and rivers in Italy, Turkey. | Building and decorative stone. |
| Coquina | Limestone composed of loosely consolidated shell fragments. Very porous. | Mainly fragmented shells and fossil debris. Variable composition. | Grain supported. Shell fragments clearly visible. Poorly cemented. | Southeastern US coastline. | Building stone, source of lime. |
| Oolitic Limestone | Composed of small concentric oolite grains | Calcium carbonate oolites with concentric layered structure. | Fine to coarse grain size. Spherical oolite grains distinctive. | Florida Keys, Bahamas banks. | Ornamental stone, building material. |
| Fossiliferous Limestone | Contains abundant marine organism fossils | Variable composition. High fossil content. | Ranges from fine to coarse grains. | Libya’s Tripoli formation, Germany’s Solnhofen deposit. | Paleontological research, building material. |
| Dolomitic Limestone | Significant proportions of dolomite and calcite/aragonite. | Dolomite and calcite minerals. Variable porosity. | Wide ranging crystal sizes. | Widespread globally. | Reservoir for petroleum and groundwater. Industrial uses. |
FAQs about limestone
What is limestone made of?
Limestone is composed primarily of the mineral calcite (calcium carbonate, CaCO3). It also contains variable amounts of minerals like aragonite, dolomite, chert, silt, and clay.
How does limestone form?
Limestone forms through both biological and inorganic processes. It can form through the accumulation of shelled ocean organisms, precipitation of calcium carbonate, or lithification of carbonate sediments.
What are some different types of limestone?
Common types of limestone include chalk, travertine, coquina, fossiliferous limestone, oolitic limestone, and dolomitic limestone. They vary in composition, texture, fossils, and origin.
Where are the biggest limestone deposits located?
Major limestone deposits are found in Northern Europe (chalk), the Bahamas (oolitic), Florida (coquina), the Middle East (fossiliferous), and widespread marine shelves and platforms.
What are some uses of limestone?
Limestone has many uses including construction material, architectural stone, aggregate, agriculture lime, ingredient in cement, and source of lime. Different varieties have specialized uses.
What makes limestone change color?
Limestone color is influenced by mineral impurities like clays, sand, and organic matter. Iron oxide creates red/brown hues. Organics make dark grey to black limestone. Pure limestone is white.
How quickly does limestone form?
Limestone forms slowly over geological time. Most accumulations took millions of years. Rapid precipitation can occur in some hot spring travertine deposits.
Why is limestone important?
Limestone is an economically critical rock. It is a raw material for concrete, agriculture, architecture, and more. It also provides clues about prehistoric Earth environments and organisms.
Is limestone a renewable resource?
No, limestone is considered a non-renewable resource because it forms over extremely long geological timescales. Quarrying extracts limestone much more quickly than natural processes can replace it.
Conclusion
There are numerous other limestone varieties around the world that each have their own distinctive characteristics based on formation conditions. By understanding the different classification systems and properties of limestone, geologists can unravel the stories of these diverse sedimentary rocks. The next time you come across a limestone building or monument, consider what its origins and composition can reveal about the ancient environments preserved within.
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