calcita

Calcite is a widely distributed carbonate mineral with the chemical formula CaCO3. It crystallizes in the trigonal crystal system and commonly forms rhombohedral crystals. It is the most stable polymorph of calcium carbonate at ambient conditions. Calcite has a Mohs hardness of 3 and a specific gravity around 2.7. It exhibits perfect rhombohedral cleavage in three directions and strong birefringence, producing double images in transparent crystals. The refractive indices are approximately nω = 1.544–1.553 and nε = 1.654–1.670, giving a birefringence of about 0.109. It effervesces in dilute acids, releasing carbon dioxide.

Calcite commonly forms in sedimentary environments as cement and in limestone, and it constitutes marble when subjected to metamorphism through recrystallization. It also occurs in hydrothermal veins and as cave formations (speleothems) such as stalactites and stalagmites. Impurities impart color varieties ranging from colorless and white to yellow, brown, or blue. The transparent variety Iceland spar is notable for its strong double refraction and has historical use in optical experiments.

Etymology: the name derives from Latin calx, meaning lime. Economic importance: a principal source of lime and cement; a major component of limestone and marble; also used as a filler and pigment in various industrial applications. Calcite is one of the most common minerals in the Earth’s crust and occurs in many geological environments worldwide.