CALCITE

the most common form of natural calcium carbonate (CaCO3), a widely distributed mineral known for the beautiful development and great variety of its crystals. Calcite is the stable form of calcium carbonate at all temperatures and pressures encountered at or near the Earth's surface; because of this, all other forms probably invert to it with geologic time. It is polymorphous (same chemical formula but different crystal structure) with the minerals aragonite and vaterite and with several forms that apparently exist only under rather extreme experimental conditions. Calcite is the dominant mineral constituent of limestone and its metamorphic equivalent, marble, and is common in the shells of invertebrates. CaCO3 is very soluble in groundwater containing organic acids or dissolved carbon dioxide (CO2) and is deposited when the CO2 escapes; this mechanism is responsible for the frequent occurrence of limestone caves and the wide distribution of stalactites and stalagmites in caves, for massive banded deposits of travertine around hot springs, and for spongy calcareous tufa in ordinary streams and springs. Calcite is the most important mineral in limestones and marbles used in the building, steel, chemical, and glass industries. Iceland spar, first produced in the 17th century from the east coast of Iceland, is used in polarizing prisms such as the Nicol and Ahrens prisms and in polarizing microscopes, polariscopes, and other optical instruments. Although the name calcite has been used only since 1845, the mineral had long been known by the name calcareous spar, and the transparent variety called Iceland spar was greatly studied. The strong double refraction and perfect cleavage of Iceland spar were described in detail by Erasmus Bartholin in 1669; a study of the same variety led Christiaan Huygens to discover (1678) the laws of double refraction. From an investigation of the cleavage and crystal forms of calcite, R.J. Hay developed (17811801) a theory of crystal structure that played an important part in the evolution of modern structural crystallography. Sir Lawrence Bragg calculated (1924) the refractive indexes of calcite from its atomic arrangement, one of the first such calculations to be made. For detailed physical properties, see also carbonate mineral (table). the most common form of natural calcium carbonate (CaCo3), a widely distributed mineral known for the beautiful development and great variety of its crystals. It is polymorphous (same chemical formula but different crystal structure) with the minerals aragonite and vaterite and with several forms that apparently exist only under rather extreme experimental conditions. R.V. Dietrich Additional reading Terence P. Scoffin, An Introduction to Carbonate Sediments and Rocks (1987), is a well-illustrated, easy-to-read book about limestones and dolostones. Both Albert V. Carozzi, Carbonate Rock Depositional Models: A Microfacies Approach (1989), an extensively illustrated treatise of the author's ideas about the geneses of carbonate-rich sedimentary rocks; and Richard J. Reeder (ed.), Carbonates: Mineralogy and Chemistry (1983), a collection of summaries that deal with the carbonates, including calcite, are written at the professional level. R.V. Dietrich