What is Limestone?

“Limestone” means any rock formed mostly of calcium carbonate (CaCO3), but to geologists, limestone is only one of several types of “carbonate rocks.” These rocks are composed of more than 50% carbonate minerals, generally the minerals calcite (pure CaCO3) or dolomite (calcium-magnesium carbonate, CaMg(CO3)2) or both.

How Does Limestone Form?

Most carbonate rocks were deposited from seawater. These sedimentary carbonate rocks are common on every continent and have formed through most of geologic history; they are still forming today in the tropics as coral reefs and at the bottoms of shallow seas.

Marine limestone forms because seawater has high concentrations of two key dissolved chemicals – calcium (Ca++) and bicarbonate (HCO3-) ions. In the near-surface layer of most oceans, corals, clams, and other sea-dwelling creatures use these two chemicals to make protective shells by combining them to form calcite or “aragonite,” which is the same chemical composition as calcite but has a different crystal form.

Formation of Dolomite from Limestone

Some limestones have been changed by the introduction of magnesium in ground water. Magnesium in ground water may CONVERT some or all of the calcite in the limestone to dolomite. Also, some rocks formed near the shores of ancient seas in arid climates were mostly dolomite at the time they were deposited.

Other Types of Carbonate Rock

Limestone comes in many different varieties. Chalk is a very fine grained, porous marine limestone composed almost entirely of microscopic fossils. Travertine is a freshwater sedimentary limestone that has very thin, crenulated layers and is commonly formed at springs. Marble is a carbonate rock, usually a marine limestone, that has been squeezed and deformed like plastic by great heat and pressure deep beneath the Earth’s surface. This process is called “metamorphism.” There are also rare “igneous” carbonate rocks that have crystallized from molten magma in the same way that lavas or granites have. These are called “carbonatites,” and this rock type is mined at a few places in the world as industrial limestone.

Mining Carbonate Rocks

Sedimentary limestone deposits can be extensive, covering hundreds of square miles, and can be relatively uniform in thickness and quality. Therefore, limestone quarries can be large and long lived, mining limestone layers that can be hundreds of feet thick over areas of several square miles. Many quarries produce multiple products, and crushed rocks that are not pure enough for certain uses may still be suitable as road aggregate. Marble quarries can also be very large. However, these rocks that were once regularly bedded have been metamorphosed into irregularly shaped bodies that are more difficult and costly to mine.

In large parts of the United States there are extensive deposits of marine limestone of various ages from a few thousand to more than 350 million years old. Some deposits have chemical grades as high as 95% CaCO3. However, some areas are COMPLETELY without any suitable limestone deposits. Most of the cost of limestone to the customer is determined by how far away it comes from and how it is shipped. Shipping by barge on water is cheaper than by train which, in turn, is cheaper than shipping by truck.

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 forms 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.

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