What is 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.
The "Acid Test" for Carbonate Minerals
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.
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
||usually white but also colorless, gray, red, green, blue, yellow, brown, orange
||transparent to translucent
||perfect, rhombohedral, three directions
||rhombohedral cleavage, powdered form effervesces weakly in dilute HCl, curved crystal faces and frequent twinning
Uses in Acid Neutralization
Calcite has numerous uses as a neutralizer of acids. For hundreds of years, limestones
and marbles have been crushed and spread on fields as an acid-neutralizing soil treatment.
They are also heated to produce lime that has a much faster reaction rate in the soil.
Calcite is used as an acid neutralizer in the chemical industry. In areas were streams
are plagued with acid mine drainage, crushed limestone is dispensed into the streams to
neutralize their waters.
Calcium carbonate derived from high purity limestones or marbles is used in
medicine. Mixed with sugar and flavoring, calcium carbonate is made into chewable
tablets used in the neutralization of stomach acids. It is also an ingredient in
numerous medications used to treat digestive and other ailments.
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 out-of-doors 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.
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.
Contributor: Hobart King
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|Transparent calcite (known as "Iceland Spar") from Chihuahua, Mexico. This specimen shows excellent double refraction. Specimen is about four inches (ten centimeters) across .
|Calcite in the form of calcareous tufa from Mumford, New York. This specimen is approximately four inches (ten centimeters) across.|
|Calcite in the form of travertine from Tivoli, Italy. Specimen is about four inches (ten centimeters) across. |
|Calcite in the form of white, coarsely crystalline marble from Tate, Georgia. Specimen is about four inches (ten centimeters) across. |
|Calcite in the form of siliceous crystals from Bad Lands, South Dakota. The calcite grew as crystals in a sand, including the sand grains within its crystal structure. Specimen is about five inches (twelve centimeters) across. |
|A variety of marble with brown and black markings is known as "Picasso Stone". It is frequently cut at polished as cabochons or used to produce tumbled stones. It is popular for jewelry and ornamental crafts.||
|Calcite in the form of white marble was the primary stone used in the Supreme Court building. © iStockphoto and Gary Blakeley.|
|Calcite in the form of a pink marble from Tate, Georgia. This specimen is approximately four inches (ten centimeters) across. See below for more calcite photos.
|Calcite in the form of limestone is used to make cement and also used as the aggregate in most concrete. A concrete slurry can be pumped or hoisted from the ground and poured into forms to produce the structural elements of buildings. © iStockphoto / Frank Leung
|Acid neutralizing qualities of calcite make finely crushed limestone a preferred material for soil treatment. © iStockphoto / Krzch-34
|The acid neutralizing ability of calcite is used in medicine. High-purity calcite was used to make these antacid tablets. © iStockphoto / Rudi Tapper
|White marble blocks for monuments or statuary waiting transport from a quarry in Portugal. © iStockphoto / Manuel Ribeiro|
|Calcite cave formations of Luray Caverns, Virginia, USA. © iStockphoto / Daniel Yost|
|Transparent calcite from Baxter Springs, Kansas, showing characteristic cleavage. Specimen is approximately four inches (10 centimeters) across.|
|Calcite in the form of oolitic limestone from Bedford, Indiana. Specimen is about four inches (ten centimeters) across.|
|Calcite in the form of chalk from Dover, England. Specimen is about 4 inches (10 centimeters) across.|
|Calcite in the form of lithographic limestone from Solenhofn, Bavaria. Note the fine, uniform texture that is characteristic of lithographic limestone. Specimen is about 4 inches (ten centimeters) across.|
|Calcite in the form of oolitic limestone from Tyrone, Pennsylvania. This specimen is approximately four inches (ten centimeters) across.|
|Calcite in the form of translucent onyx from Tecali, Mexico. Specimen is about four inches (ten centimeters) across.|