Calcite is one of the most abundant and versatile minerals in the Earth’s crust. Belonging to the carbonate group, it is composed of calcium carbonate (CaCO₃) and forms in a wide variety of geological environments, including sedimentary rocks such as limestone and marble, hydrothermal veins, and igneous deposits.
The name 'calcite' is derived from the German 'calcit', a term from the 19th century that came from the Latin word for lime, 'calx' (genitive 'calcis'), with the suffix '-ite' used to name minerals.
The mineral crystallises in the trigonal crystal system, and its crystal habits are probably the most diverse of any other mineral. To date, over 1,000 crystallographic forms have been reported, ranging from sharp rhombohedra ('nailhead-calcite') and scalenohedra ('dogtooth-calcite') to blocky prisms. Furthermore, calcite exhibits several twinning types that add to the observed habits. It may also occur as fibrous, granular, lamellar, or compact. Combined, therefore, calcite provides collectors with an almost endless wealth of forms and shapes to collect and admire.
One of calcite’s most distinctive characteristics is its reaction with dilute hydrochloric acid, releasing bubbles of carbon dioxide. This is often used as a test to differentiate calcite from other similar minerals.
Calcite shows perfect rhombohedral cleavage and has a hardness of 3 on the Mohs scale, making it relatively soft and easily scratched. A remarkable property of some clear crystals, particularly the transparent “Iceland spar” variety, is double refraction: when an image is viewed through the crystal, it appears doubled. This optical feature, called birefringence, was used in early scientific instruments and probably in navigation by the Vikings, who may have used Iceland spar to locate the sun’s position on cloudy days.
Industrially, calcite is extremely important. It is the principal component of limestone, widely quarried for cement and lime production, and finely ground calcite is used as a filler in paper, paint, plastics, and even toothpaste. Beyond this, calcite plays a role in controlling the pH of natural waters and in carbon cycling, making it vital to Earth’s environmental systems.
In World War II, high-grade optical calcite was used for gun sights, specifically in bomb sights and anti-aircraft weaponry. It was also used as a polariser (in Nicol prisms) before the invention of Polaroid plates, and it still finds use in optical instruments.
Collectors seek calcite because of its colour and variety of crystals. The Elmwood Mine in Tennessee is world-famous for large golden to amber crystals, often associated with fluorite and barite. Equally striking specimens come from Tsumeb in Namibia, where brilliant transparent crystals occur, and from Pribram in the Czech Republic, known for complex twinning. Chinese localities produce spectacular stalactitic and cave-grown calcites, many with unusual textures. The Cumbrian iron mines in England are famous for highly transparent, gemmy, blocky crystals.
An interesting fact about calcite is that many specimens fluoresce under ultraviolet light, glowing in shades of red, blue, green or pink depending on impurities. Another is that calcite often forms in cave systems as stalactites and stalagmites, creating some of the world’s most beautiful subterranean landscapes. These traits, combined with its importance to science and industry, secure calcite’s place as one of the most studied and admired minerals worldwide.
The Assay House—Mineral Dealers of quality Mineral Specimens
Image Credits - CC Ed Utman, Géry PARENT, Thomas Spann & James St. John