Calcite

Calcite is a rock-forming mineral with a chemical formula of CaCO₃. It is extremely common and found throughout the world in sedimentary, metamorphic, and igneous rocks. Some geologists consider it to be a "ubiquitous mineral" - one that is found everywhere.
Calcite is the principal constituent of limestone and marble. These rocks are extremely common and make up a significant portion of Earth's crust. They serve as one of the largest carbon repositories on our planet.

The properties of calcite make it one of the most widely used minerals. It is used as a construction material, abrasive, agricultural soil treatment, construction aggregate, pigment, pharmaceutical and more. It has more uses than almost any other mineral.

Calcite as pink marble

Calcite as Limestone and Marble

Limestone is a sedimentary rock that is composed primarily of calcite. It forms from both the chemical precipitation of calcium carbonate and the transformation of shell, coral, fecal and algal debris into calcite during diagenesis. Limestone also forms as a deposit in caves from the precipitation of calcium carbonate.

Marble is a metamorphic rock that forms when limestone is subjected to heat and pressure. A close examination of a broken piece of marble will usually reveal obvious cleavage faces of calcite. The size of the calcite crystals is determined by the level of metamorphism. Marble that has been subjected to higher levels of metamorphism will generally have larger calcite crystals.

White Calcite as Marble

Uses of Calcite in Construction

The construction industry is the primary consumer of calcite in the form of limestone and marble. These rocks have been used as dimension stones and in mortar for thousands of years. Limestone blocks were the primary construction material used in many of the pyramids of Egypt and Latin America. Today, rough and polished limestone and marble are still an important material used in prestige architecture.

Modern construction uses calcite in the form of limestone and marble to produce cement and concrete. These materials are easily mixed, transported, and placed in the form of a slurry that will harden into a durable construction material. Concrete is used to make buildings, highways, bridges, walls, and many other structures.

Physical Properties of Calcite
Chemical Classification	Carbonate
Color	Usually white but also colorless, gray, red, green, blue, yellow, brown, orange
Streak	White
Luster	Vitreous
Diaphaneity	Transparent to translucent
Cleavage	Perfect, rhombohedral, three directions
Mohs Hardness	3
Specific Gravity	2.7
Diagnostic Properties	Rhombohedral cleavage, powdered form effervesces weakly in dilute HCl, curved crystal faces and frequent twinning
Chemical Composition	CaCO3
Crystal System	Hexagonal
Uses	Acid neutralization, a low-hardness abrasive, soil conditioner, heated for the production of lime

Calcium Carbonate Sorbents

Sorbents are substances that have the ability to "capture" another substance. Limestone is often treated and used as sorbent material during the burning of fossil fuels. Calcium carbonate reacts with sulfur dioxide and other gases in the combustion emissions, absorbs them, and prevents them from escaping to the atmosphere.

Monuments and Statuary

Marble is an attractive and easily worked rock that has long been used for monuments and sculptures. Its lack of significant porosity allows it to stand up well to freeze-thaw action outdoors, and its low hardness makes it an easy stone to work. It has been used in projects as large as the pyramids and as small as a figurine. It is widely used as cemetery markers, statues, mantles, benches, stairways, and much more.

Lincoln Memorial in Washington

Many Other Uses

In a powdered form, calcite often has an extremely white color. Powdered calcite is often used as a white pigment or "whiting." Some of the earliest paints were made with calcite. It is a primary ingredient in whitewash, and it is used as an inert coloring ingredient of paint.
Pulverized limestone and marble are often used as a dietary supplement in animal feed. Chickens that produce eggs and cattle that produce milk need to consume a calcium-rich diet. Small amounts of calcium carbonate are often added to their feeds to enhance their calcium intake.

Calcite has a hardness of three on the Mohs scale, and that makes it suitable as a low-hardness abrasive. It is softer than the stone, porcelain, and plastic surfaces found in kitchens and bathrooms but more durable than dried food and other debris that people want to remove. Its low hardness makes it an effective cleaning agent that does not damage the surface being cleaned.
Pulverized limestone is also used as a mine safety dust. This is a nonflammable dust that is sprayed onto the walls and roofs of underground coal mines to reduce the amount of coal dust in the air (which can be an explosion hazard). The mine safety dust adheres to the wall of the mine and immobilizes the coal dust. Its white color aids in illumination of the mine. It is the perfect material for this use.

• Calcite: A Carbon Dioxide Repository

Carbon dioxide is an important gas in Earth's environment. In the atmosphere it serves as a greenhouse gas that works to trap and hold heat near the surface of the planet. The process of limestone formation removes carbon dioxide from the atmosphere and stores it away for long periods of time. This process has been occurring for millions of years - producing enormous volumes of stored carbon dioxide. When these rocks are weathered, used to neutralize acids, heated to make cement or metamorphosed severely, some of their carbon dioxide is released and returned to the atmosphere. All of these processes of limestone formation and destruction have an impact on Earth's climate.

Sources: Geology.com