Calcite

Calcite is a carbonate mineral made of calcium carbonate (CaCO3).

It is the most stable polymorph of this chemical combination. A polymorph is a mineral that has the same chemistry as another but a different crystal structure. Calcite is very common in the Earth's crust. It is a primary component of limestone. Because it is so widespread, it plays a major role in geological processes. It is used in many ways, from ancient art to modern science.

One of the most famous traits of calcite is birefringence, or double refraction. This happens when light enters a clear crystal and splits into two different paths. This effect was first described by the Danish scientist Rasmus Bartholin in 1669. When you look through a clear piece of calcite, objects may appear doubled. This occurs because the mineral has two different refractive indices. At a wavelength of 590 nm, these indices are 1.658 and 1.486. A transparent variety of calcite called "Iceland spar" shows this effect very clearly.

Calcite crystals can take many different shapes, known as crystal habits. There are over 1,000 different crystallographic forms. Two common shapes are scalenohedra and rhombohedra. Scalenohedra often have sharp, pointed faces. Rhombohedra have faces that look like tilted cubes. The mineral also has perfect cleavage in three directions parallel to the rhombohedron form. This means it breaks easily along specific flat planes. The hardness of calcite is defined as 3 on the Mohs scale. This scale measures how easily one mineral can scratch another.

Humans have used calcite for thousands of years. Ancient Egyptians carved items from it, such as perfume jars found in the tomb of Tutankhamun. They associated the material with their goddess, Bast. In the stone trade, a fine-grained, translucent variety of calcite is often called alabaster. Historically, Viking navigators may have used Iceland spar to navigate. By rotating the crystal, they could find the sun's position even on cloudy days. This was possible because they could see the rings of polarized light around the sun.

Calcite is found in many large deposits worldwide. The Calcite Quarry in Michigan is the largest carbonate mine on Earth. It has been operating for more than 85 years. Other important locations include the Santa Eulalia district in Mexico and various mines in Canada. In Iceland, the Helgustadir mine was once a major source of Iceland spar. However, that mine is now a nature reserve where mining is forbidden. These locations show how important the mineral is to global industry.

The chemistry of calcite allows it to react with acids. When acid touches calcite, it produces carbon dioxide gas. This creates a bubbling effect called effervescence. This reaction can dissolve the mineral over long periods. If groundwater dissolves calcite, it can create large cave systems. This process leads to landscapes known as karst topography. Interestingly, calcite has retrograde solubility. This means it becomes less soluble in water as the temperature increases. It also becomes more soluble under higher pressures.

Calcite is also used in advanced modern technology. In World War II, high-grade calcite was used in bomb sights and anti-aircraft weaponry. Today, it is used in bone tissue engineering because scientists can control its properties. It is also used in environmental science to help clean up water. For example, it can reduce phosphorus levels in lakes to stop the growth of cyanobacteria. Even in medicine, some cultures use calcite, or "calcitum," for its perceived cooling properties. It remains a vital mineral for both nature and human progress.