Calcite

Calcite is a common carbonate mineral and the most stable form of calcium carbonate (CaCO₃). It is found in many places, especially as part of limestone. Calcite is known for having a hardness of 3 on the Mohs scale of mineral hardness, which means it can be scratched by a copper coin but not by a fingernail.

Large crystals of calcite are important for making optical equipment. Limestone that contains a lot of calcite is used in many ways, such as in building materials and agriculture.

There are other forms of calcium carbonate, like aragonite and vaterite. Over time and at high temperatures, aragonite changes into calcite, and vaterite is even less stable than aragonite.

Etymology

The word "calcite" comes from a German word called Calcit. This German word was created in the 1800s and is based on the Latin word for lime, calx. Minerals often have names ending in ‑ite.

Sometimes, people use the word alabaster to describe a shiny, clear type of calcite. This is different from how scientists use the word, because they usually use it for a type of mineral called gypsum.

Unit cell and Miller indices

Scientists use special numbers called Miller indices to describe directions in crystals like calcite. There are two ways to write these numbers, which can make things a bit confusing.

Calcite can also be described using two different kinds of unit cells — the smaller space that repeats to make the whole crystal. One type was found by measuring crystal shapes, and the other was discovered using powerful X-ray tools. These two unit cells have different sizes and shapes, so scientists need to use different numbers when they talk about things like how the crystal breaks apart.

Properties

Calcite is a common mineral known for its hardness of 3 on the Mohs scale, which measures how easily a mineral can be scratched. It can be white or colorless, but impurities can make it appear in shades of gray, red, orange, yellow, green, blue, violet, brown, or even black.

Calcite comes in many shapes, including rhombohedral and scalenohedral forms. It can appear as prisms, tabular shapes, or in fibrous and granular forms. It is transparent to opaque and can sometimes glow in the dark or under special light. When viewed through clear calcite, objects may appear doubled due to a special optical property called birefringence.

Calcite dissolves in acids, producing bubbles of carbon dioxide gas. It can also dissolve or form from water depending on conditions like temperature and acidity. This process helps create caves and shapes the landscape over time. Calcite can contain small amounts of other elements like magnesium, iron, and manganese, which can affect its appearance and properties.

Distribution

Calcite is found all over the world. In the United States, a great example is the Calcite Quarry in Michigan, which is the largest carbonate mine in the world and has been used for over 85 years.

Calcite can also be found in Canada, such as in Thorold Quarry and Madawaska Mine in Ontario. In Mexico, lots of calcite is mined in the Santa Eulalia mining district in Chihuahua. In Iceland, large amounts of calcite are in the Helgustadir mine, which used to be the main place for "Iceland spar" but is now a nature reserve where mining is not allowed. Calcite is also found in parts of England, like Alston Moor, Egremont, and Frizington in Cumbria, as well as in St. Andreasberg in the Harz Mountains and Freiberg in Saxony, Germany.

Use and applications

Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster. Many other cultures have also used calcite for carved objects and similar purposes.

A clear type of calcite called Iceland spar may have helped Vikings navigate on cloudy days. A very pure calcite crystal can split sunlight into two images. By looking at the sky through the crystal and turning it until the two images look the same brightness, the rings of light around the sun can be seen even when the sky is covered. This helped sailors know where the sun was, guiding them during long sea trips.

During World War II, high-quality optical calcite was used in gun sights for bombs and anti-aircraft weapons. It acted as a polarizer before special plates were invented and is still used in some optical tools today. Scientists have also tested calcite for creating a cloak of invisibility.

Calcite can also be used to clean soil and repair concrete. It helps in managing mining waste and supports sustainable development in the mining industry. Calcite is important for making a type of calcium carbonate used in paper production and can help control the shape and size of particles.

In science, calcite from Carrara marble is used as a standard for measuring certain elements in mass spectrometry. It is also used in medical research to help build bone tissue because of its reliable properties.

Calcite can help reduce water pollution caused by too much growth of a type of tiny plants called cyanobacteria. This growth, called eutrophication, happens when there is too much phosphorus in the water. Calcite can help stop phosphorus from spreading into the water, which limits the growth of these tiny plants.

In traditional Chinese medicine, calcite is thought to have cooling properties and is used to balance what is called "shanghuo", or "heatiness".

Natural occurrence

Calcite is commonly found in sedimentary rocks, especially in limestone, which forms from the shells of tiny sea creatures. About 10% of rocks on Earth are made of limestone. It is the main mineral in metamorphic marble. You can also find calcite in deposits from hot springs, in caves as stalactites and stalagmites, and in special kinds of volcanic rocks.

Calcite is a key part of the shells of many sea creatures, like plankton and some sponges. It is also found in the hard parts of red algae and even in parts of some shells, like those of oysters. Amazingly, some tiny sea creatures that lived long ago used calcite to make the lenses in their eyes. Calcite is also an important part of bird eggshells.

Formation processes

Calcite can form in different ways. One way is through the crystallization of a substance called amorphous calcium carbonate, which first turns into vaterite and then changes into calcite. This process depends on the pH level and the amount of magnesium in the solution.

Calcite can also form deep underground due to the activity of tiny living things. These processes leave clues in the carbon isotope makeup of the calcite.

In Earth history

Calcite seas were times in Earth's past when the main type of calcium carbonate that formed in ocean water was low-magnesium calcite, instead of the aragonite and high-magnesium calcite that form today. These calcite seas changed with aragonite seas over the Phanerozoic time period, especially during the Ordovician and Jurassic periods. Animals that had shells made of calcium carbonate changed to use the type that was easier to make at the time, and kept using it for the rest of their history.

Calcite might have helped important chemical reactions that could have led to the start of life. Its special surfaces may have influenced the way certain molecules lined up, which could explain why life's building blocks have a specific arrangement.

Climate change

Climate change is making it harder for the ocean to create calcite. The ocean absorbs a lot of carbon dioxide from the air, which makes the water more acidic. This change in water can affect animals like shellfish and corals that help make calcite in nature. When the water becomes more acidic, it can reduce how much calcite these animals can produce.