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What Is Limestone and How Is It Used?
Sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris.
Also a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.
What is Limestone?
Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.
Limestone-Forming Environment – Marine
Most limestones form in shallow, calm, warm marine waters. That type of environment is where organisms capable of forming calcium carbonate shells and skeletons can easily extract the needed ingredients from ocean water. When these animals die their shell and skeletal debris accumulate as a sediment that might be lithified into limestone. Their waste products can also contribute to the sediment mass. Limestones formed from this type of sediment are biological sedimentary rocks. Their biological origin is often revealed in the rock by the presence of fossils.
Some limestones can form by direct precipitation of calcium carbonate from marine or fresh water. Limestones formed this way are chemical sedimentary rocks. They are thought to be less abundant than biological limestones.
Today Earth has many limestone-forming environments. Most of them are found in shallow water areas between 30 degrees north latitude and 30 degrees south latitude. Limestone is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean islands and within the Indonesian archipelago.
One of these areas is the Bahamas Platform, located in the Atlantic Ocean about 100 miles southeast of southern Florida (see satellite image at right). There abundant corals, shellfish, algae and other organisms produce vast amounts of calcium carbonate skeletal debris that completely blankets the platform. This is producing an extensive limestone deposit.
Limestone-Forming Environment – Evaporative
Limestone can also form through evaporation. Stalactites, stalagmites and other cave formations (often called “speleothems”) are examples of limestone that formed through evaporation. In a cave, droplets of water seeping down from above enter the cave through fractures or other pore spaces in the cave ceiling. There they might evaporate before falling to the cave floor. When the water evaporates, any calcium carbonate that was dissolved in the water will be deposited on the cave ceiling. Over time this evaporative process can result in an accumulation of icicle-shaped calcium carbonate on the cave ceiling. These deposits are known as stalactites. If the droplet falls to the floor and evaporates there a stalagmite could grow upwards from the cave floor.
The limestone that makes up these cave formations is known as “travertine” and is a chemical sedimentary rock. A rock known as “tufa” is a limestone formed by evaporation at a hot spring, lake shore, or other area.
Uses of Limestone
Limestone has many industrial uses and can be used as mined or processed into a wide variety of products. It is the raw material for a large variety of construction, agricultural, environmental, and industrial materials.
Limestone is used in construction almost everywhere. In 2007, crushed limestone was 68% of all crushed rock produced in the United States. Also, limestone is the key ingredient in making Portland cement. Despite our Nation’s abundance of limestone, there have been cement shortages in recent years.
Some of the purest of natural limestones are marbles. For centuries, marble has been the decorative stone of choice in government buildings and public statues. Travertine is also used as a dimension stone in tiles and tabletops. Some white limestone is simply crushed and sieved for use in landscaping and roofing.
Powdered limestone is used to remove impurities from molten metals like steel. It can also remove toxic compounds from the exhaust of coal-burning power plants. Limestone is used as a filler in a variety of products, including paper, plastic, and paint. The purest limestone is even used in foods and medicines such as breakfast cereals and calcium pills.
Limestone is also the raw material for making lime (CaO) that is used to treat soils, purify water, and smelt copper. Lime has many additional uses in the chemical industries.
Dolomites are commonly less suitable than other industrial limestones for most applications. Most dolomite that is mined is simply crushed and sieved for use as aggregate in concrete or asphalt.