The sedimentary rock made from aragonite and calcite minerals along with skeletal fragments composed of marine fossils is called as Limestone. The minerals aragonite and calcite are the different forms of crystallized calcium carbonate. The sedimentary rocks comprise of around 10% of limestone. The Limestone may also consist of dolomite or magnesium carbonate, iron carbonate, quartz, pyrite, clay and feldspar in a very low proportion.
The Limestone is often found with a granule texture that consists of silt, sand and clay, whereas some types of Limestone do not have grains, or the grain structure. Such Limestone is formed by the chemical reaction undergone by the aragonite and calcite mineral. The granules in the Limestone range from 0.001 mm to a few millimeters that can be viewed with the naked eye. In many other types, the grains are the small fragments of organic matter that can be viewed only through microscopes.
Composition of Limestone
The Limestone is basically a sedimentary rock, which consists of calcite mineral, 50% or more. Other than the calcium carbonate, limestone is also composed of other materials in a very low proportion. The organic material found in the limestone is the accumulation of the fossils under the sea, like the shells, algae, fecal debris and corals. The organisms, when alive, discharge the calcite and aragonite composed shells, which is left behind in the water after their extinction. These fossil shells accumulate in the limestone. Sometimes, the Limestone is formed by the chemical precipitation undergone by the calcium carbonate mineral (CaCO3) found in the ocean or lake water. The Limestone also consists of silica in the form of chert and different proportion of sand, clay and silt, carried into the seas by the rivers.
Types of Limestone
Limestone possesses many different names depending on the location, composition, appearance and many other factors. Some of the types of the Limestone are:
Chalk is a soft and fine textured limestone that is often found in gray or white color. This Limestone is mainly composed of the remains of the calcareous shells secreted by the microscopic organisms in the marine habitat. Basically these shell-remains come from the foraminifers. This rock is also formed from the calcareous organic remains of various types of algae found in the marine world.
The Limestone consisting of a huge amount of fossil remains is called as Fossilferous Limestone. These fossils consist of shells and skeletal remains of the dead creatures.
Coquina Limestone can be termed as a limestone with poor or nil cementing agents between the shell fragments. It is mainly composed of the broken shell remains. This type of Limestone is found on the beaches, where the continuous action of waves brings the shell fragments of the same size as the rock, and settles on the beach. Gradually, it turns into a rock.
The Travertine Limestone can be often seen in caves. It evolves as a result of evaporative precipitation, and forms stalactites, flowstone and stalagmites.
Lithographic Limestone is a fine textured and dense rock that has very small and uniform sized grains that develop on thin beds. This rock is so soft that it can be easily separated to form a very smooth surface.
This Limestone is formed when the calcium laden water undergoes the process of precipitation at lake shore or other locations.
Oolithic Limestone is formed by the concentric precipitation of its mainly composed constituent, calcium carbonate mineral. This mineral is also termed as “oolites”. The precipitation process results in the formation of small spheres of calcium carbonate on a shell fragment or a sand grain.
Formation of Limestone
Limestone is usually seen in marine environments. It is found in shallow, warm and calm waters, where the marine organisms are capable to produce calcium carbonate skeletons and shells. When the organisms are alive, they absorb the needed ingredients from the sea water to form shells and skeletons of calcium carbonate. After they die, these shells and skeletons turn into fossils, and as a sediment, they accumulate on hard surfaces, which later turn into a Limestone rock. The waste products in the water also contribute to the formation of the sediment mass. These types of Limestone are called biological sedimentary rocks. The study of the origin of this rock is possible by the study of the fossils present in it. The other way of formation is in the direct precipitation process. The calcium carbonate present in the marine water undergoes precipitation, which helps to form a chemical sedimentary rock. This type of rock is less found compared to the biological Limestone.
The other habitat of Limestone rock is in the caves. This type of Limestone is formed by the process of evaporation. Stalagmites, Stalactites and many other structures often termed as “speleothems” are some of the examples of Limestone formed by the evaporation process. The water flowing from the top of the caves enters in through the pores and fractures present in the cave ceiling. Before it drops down on the floor, this water gets evaporated. The calcium carbonate, if any, dissolved in the water, gets accumulated on the ceiling. As the time passes, the evaporation process results in the deposition of calcium carbonate in icicle-shape on the ceiling of the cave. These types of structures are referred as stalactites. Similarly, if these water drops fall to the floor, and get evaporated, then it will form a stalagmite structure that grows from the cave floor towards the ceiling.
The Limestone is very popular since ancient times. The landmarks across history, like Great Pyramid, Giza complex and many other structures were built with this unique rock. In the 19th and 20th century eras, all the structures like banks, train station and houses were built with Limestone. Many buildings in Ontario, Kingston and Canada were constructed using this rock, and these places are entitled to the nickname “Limestone City”. A type of Limestone, known as Globigerina Limestone, is the only material used in construction in Malta Island for a long time. Other than construction, it is also used as a raw material in the manufacturing of calcium oxide or quicklime, cement, slaked lime and mortar. Acidic soil is neutralized by using the pulverized Limestone. Most of the geological Limestone formations are the reservoirs of petroleum. It is also used in glass making as a raw material. The manufacturing sector makes use of limestone in making paper, paint, toothpaste, plastic, tiles and many other materials. Artists use it in carving sculptures due to its suitable texture. It is also often found in cosmetics and medicinal products.