Limestone consists of calcium carbonate and is one of the most common sedimentary rocks. It sometimes includes corals and other fossils of organisms that have calcium carbonate in their composition. It has a light color and can be found all over the world. Karst environments, where caves are found, consist mainly of limestone.
Around ten percent of all the rocks formed by sedimentation found on Earth are a variety of limestone. Since it is easy to extract and widely available, it has been used for many purposes since ancient times. It is a popular construction material and an ingredient in the chemical industry. The deposits in Florida Keys and Niagara Falls are the best known in the USA.
Shallow waters in warm sea regions are the best areas where limestone is formed. It is a sedimentary rock of organic nature that results from the build-up of various materials like corals, shells, algae and other organic waste. Calcium carbonate in the water of lakes or oceans can also create limestone through a chemical reaction.
This sedimentary rock has been widely used by humans since deposits are commonly found all over the world. It is actually so widespread that approximately 10% of all sedimentary rocks on Earth are actually limestone, or mixtures that include it. Many people come in contact with this rock every day, without even knowing it. This is because it can look in many ways and the color or texture can vary a lot. As a result, identifying a particular mineral as limestone is not always easy.
The content of calcium carbonate or calcite must be at least 50% of the weight of a rock in order for it to be classified as limestone. It is rarely chemically pure and it includes many other materials. The most common such minerals are pyrite, siderite, quartz, feldspar or clay minerals, usually as small particles. Larger chunks of pyrite, siderite or chert can also be found.
Cold solutions with a concentration of 5% hydrochloric acid become effervescent when in contact with the calcium carbonate in limestone. This is often used to identify the composition of rocks.
Sedimentary rocks such as limestone are formed by the slow accumulation of sediments, which become rocks through a process of compression. Most of such deposits of sediments are formed in seas and oceans. Sediments accumulate on the ocean floor as debris after reefs, plants and animals die. Compression can happen due to the weight of other layers of sediments or accelerated by geological events. Oceans also generate forces that contribute to compression. Stalactites and other structures found in caves are examples of limestone formation through a slower and different process, the deposition of minerals from water.
Calcium carbonate is the main mineral found in limestone but these rocks are usually mixed with various other impurities. These can have a big influence on the color, texture and overall appearance of the rocks. For example, chalk and marble are both classified as limestone, even if they look different and don't have the same properties. Chalk is a soft rock with a white color that can be destroyed quickly. By contrast, marble is a hard rock with a crystalline structure. It can be white but many other colors are possible, including a mix of several nuances.
Limestones can have of a crystalline structure that becomes visible using a microscope, while others consist of small pieces of various materials that have been compressed together. It depends on how the rock was formed, which also determines the color. This depends on the content of other minerals and can be green, pink, cream, black, rust, brown, yellow, white or a mix. The color of limestone can be changed by the water that flows through deposits in time.
Marble and chalk are the most famous types of limestone but many other varieties exist, for example dolomite, oolite or marl. They are excellent building materials and many limestones have been used for this purpose for a very long time. Cement and animal fodder are other well-known uses of limestone, which is also needed for industrial purposes in printing plates for example. Paleontologists focus on the study of limestone because the rock often includes fossils that are very well preserved. These can reveal valuable information about ancient life forms, as well as the Earth's geological history.
Sedimentary rocks are classified in two major varieties: chemical and clastic rocks. Chemical sedimentary rocks, such as limestone, are formed when the minerals found in water solidify into a rock. However, these minerals remain soluble in water and react with acids, so they can be dissolved forming the typical karst landscapes.
The limestone that forms the bedrock of karst areas enters a chemical reaction with rainwater and other liquids. The result is strange formations like the stalactites and stalagmites found in caves. As the rocks dissolve, they create sinkholes on the ground. Groundwater can dissolve the calcium compounds in limestone, which are then carried away. This water is richer in minerals and has a higher pH than normal, being known as hard water.
The structure of limestone is very diverse and depends on the environment and conditions of formation. It can consist of distinctive individual crystals (crystalline structure), irregular particles (massive structure), grains of minerals (granular structure) or rock fragments bound together (clastic structure). The process known as metamorphism can transform any type of limestone into marble. Calcium carbonate is present in all types of limestone and releases bubbles when reacting with hydrochloric acid. This makes it very easy to identify calcite and limestone rocks and is considered one of the most reliable field tests.
Limestone comes in many varieties and has been given various names. These depend of the look of the rock, the chemical composition, the formation or other factors. Some of the most common types are described below:.
Chalk is a very soft rock with a fine texture. It is typically white but can also be grey in some cases. The shells of foraminifers and other tiny marine organisms with calcareous shells accumulate to form chalk, which can also be produced by the remains of some species of algae that have a content of calcium.
Coquina is a type of limestone made of shell debris that is loosely cemented together. It is usually formed by the action of waves, which accumulate together small shell fragments found on beaches.
The rocks that have a large number of visible fossils are known as fossiliferous limestones. These are the remains of the organisms that produced the limestone and consist of bones or shells.
Lithographic limestone is a type that consists of thin strips that separate easily and have a very smooth and plain surface. This is due to the very fine grain size of the rock, as well as its high density. It gave the name to lithography, an old printing technique developed in the 1700s. This relied on an oil ink to draw images on the limestone, which was then used to reproduce it several times by pressing.
Oolites are small spherical pieces of calcium carbonate. They are formed when the mineral is precipitated with a shell fragment or grain of sand as the core. The rock that consists of these spheres is known as oolitic limestone.
Travertine is usually found inside caves and is the material of stalactites, stalagmites, flowstone and other formations. It is formed through a process of evaporation. Water rich in calcium that precipitates on lake shores or in hot springs generates a type of limestone named tufa.
Limestone has many uses, since it is widely available. It is considered the most used of all rock varieties on Earth. It is a very popular construction material, usually after the limestone is crushed industrially. These crushed rocks are a quality railroad ballast or road base material. Limestone helps the aggregation of concrete and is used to produce cement, after it is mixed with crushed shale and fired up.
Limestones are very suitable for construction use due to their very dense structure, with very few pores and air pockets. As a result, they resist very well freezing cycles and are able to withstand abrasive damage. Silicate rocks are usually even better because of their superior strength. However, limestone is a lot easier to extract. Since it is softer, this calcium rock doesn't wear mining tools, crushers and the vehicles that transport it, so the overall cost is more competitive.
Shingles can resists better against heat when coated with a layer of roofing granules based on limestone. Rock blocks are a common material for construction items like facing stone, stair treads, floor tiles and window sills. Soil acidity can be reduced cheaply using limestone crushed finely to the size of sand particles.
Limestone is sometimes included as a filler ingredient in animal feed. It is especially needed by birds because they use calcium carbonate to produce strong shells for their eggs. The process of milking can cause cattle to lack enough calcium, which can be corrected by a diet that includes limestone. Limestone can also be sprayed in mines for safety purposes. When pulverized on coal, it reduces the amount of dust generated and boosts the overall light levels.
Many aggregates, chippings and grits based on limestone are used for decorative purposes in landscaping. They are found in car parks, footpaths or driveways, but also in gardens and rockeries as a cheap ground cover material.
The latex backing layer of carpets and rugs is commonly strengthened with a filler that includes limestone. This layer is the actual base of any carpet or rug and makes it stiff and durable by keeping the soft fibers in place. It is found on the back of most wowed items.