Muséum national d'histoire naturelle

GENERAL DESCRIPTION

Calcite is a calcium carbonate crystallizing in the rhomboedric system. This mineral, very abundant in the sedimentary rocks such as limestones, is also common in hydrothermal deposits, where one can find it associated with quartz, sulphides, barite and fluorite.

It can be found in very rare volcanic rocks called carbonatites, but more often in the volcanic rocks weathered by rainwater. Calcite is the principal mineral of the stalactites/stalagmites and other concretions found in caves.

Calcite crystals show hundreds of different forms, many colours and its crystals can reach large-sized. The crystals are often rather well crystallized, forming a variety of aggregates. One can quote, among the current forms, scalenohedra and rhombohedra.

Pure calcite is colourless; it can be tinted in pink, yellow, green, blue, yellow or brown, depending on impurities and inclusions of other minerals (generally, iron and/or manganese) contained.

Calcite (and more generally limestone, which is composed of massive calcite) are used in the construction industry (cement, lime, bricks and ornament stones), as a flux in metallurgy and in optics. The calcite deposits are numerous and some known of long date, such as the deposit of Egremont, Cumberland, England that provided samples of superb quality. The deposit of Andreasberg in Germany started to be mined during the 18th century. It produced exceptional crystals with a typical hexagonal shape. The recently producing deposits include Dalnegorsk (Russia), Rio Grande do Sul in Brazil, many deposits in China, Missouri and Michigan (USA); this last site, producing calcites with copper inclusions.

In France, calcite crystals comparable with those of Andreasberg were discovered in Saint Julien les Rosiers (Gard). The deposits located close to Pau (Pyrénées-Atlantiques) and in Louverné (Mayenne) are also outstanding. This last gave gemmy crystals measuring up to 20 cm. At Bellecroix (near Fontainebleau, Paris suburb) superb aggregates of calcite crystals were replaced by sand.

The type is not definable because this species was already known before the conditions of deposit of the types were defined.

Did you know? In 1781, by examining a transparent calcite, the French mineralogist René-Just Haüy, first curator of the mineralogy collection of Muséum national d’Histoire naturelle in Paris, was surprised to observe a visual duplication of a text while looking through the calcite crystal placed over this text. Then, he has dropped the crystal, which has broken in a multitude of small fragments whose form is identical to that of the initial crystal ("cleavage"). The double refraction and the bases of crystallography had just been discovered.

Calcite

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Mineralogy