Calcite

Calcite is a common carbonate mineral. It is the most stable form of calcium carbonate (CaCO₃). You can find it in many places, especially in limestone.

Calcite has a hardness of 3 on the Mohs scale of mineral hardness. This means you can scratch it with a copper coin, but not with a fingernail.

Big calcite crystals are used to make optical equipment. Limestone with lots of calcite is used for building materials and farming.

There are other forms of calcium carbonate, such as aragonite and vaterite. Over time and with high heat, aragonite turns into calcite. Vaterite is even less stable than aragonite.

Etymology

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

Sometimes, people call a shiny, clear type of calcite alabaster. This is not what scientists mean when they use the word, because they usually use it for a 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 be confusing.

Calcite can also be described using two different kinds of unit cells — the small 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 the crystal.

Properties

Calcite is a common mineral. It has a hardness of 3 on the Mohs scale, which shows how easily a mineral can be scratched. It is usually white or clear, but can be gray, red, orange, yellow, green, blue, violet, brown, or black because of impurities.

Calcite comes in many shapes, such as rhombohedral and scalenohedral forms. It can look like prisms, flat shapes, or be fibrous and granular. It can be clear or cloudy and sometimes glows in the dark or under special light. When you look through clear calcite, things may look double because of a special optical property called birefringence.

Calcite dissolves in acids and makes bubbles of carbon dioxide gas. It can also dissolve or form from water, depending on temperature and acidity. This process helps make caves and change the landscape over time. Calcite can have small amounts of other elements like magnesium, iron, and manganese, which can change how it looks and works.

Distribution

Calcite is found all over the world. In the United States, a good example is the Calcite Quarry in Michigan. It 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 and linked it to their goddess Bast. Many other cultures also used calcite for carved objects.

A clear type of calcite called Iceland spar may have helped Vikings navigate on cloudy days. A 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. This helped sailors know where the sun was.

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

Calcite can also help clean soil and repair concrete. It helps manage mining waste and supports sustainable development. Calcite is important for making calcium carbonate used in paper production.

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

Calcite can help reduce water pollution caused by too much growth 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.

In traditional Chinese medicine, calcite is thought to have cooling properties.

Natural occurrence

Calcite is often found in sedimentary rocks, especially in limestone, which comes from the shells of tiny sea creatures. 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 some 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 in some shells, like those of oysters. Calcite is also an important part of bird eggshells.

Formation processes

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

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

In Earth history

Calcite seas were times long ago when the main kind of calcium carbonate in ocean water was low-magnesium calcite. Today, oceans usually form aragonite and high-magnesium calcite instead. These calcite seas changed with aragonite seas during the Phanerozoic time, especially in the Ordovician and Jurassic periods. Animals with shells made of calcium carbonate changed to use the type that was easier to make at the time.

Calcite might have helped important chemical reactions that could have started life. Its special surfaces may have helped certain molecules line up in a certain way.

Climate change

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