CALCITE, -sit, also known as calc-spar, a native carbonate of calcium, crystallizing in the rhombohedral system, and exhibiting over 300 distinct crystals of general forms or °hab its.° The mineral also occurs massive, fibrous, granular, lamellar, compact, earthy, stalactitic, nodular. In other forms it exhibits minute percentages of magnesium, iron, manganese, zinc and lead, replacing equivalents of cal cium. 'Its typical crystals exhibit a very per fect cleavage, commonly splitting up, from a blow, into many small rhombohedrons. Pure crystals have the composition carbon dioxide 44 per cent, lime 56 per cent. They show a vitreous or earthy lustre and have a specific gravity of about 2.72, and a hardness of about 3, though the latter varies somewhat with the face of the crystal. Calcite may be transparent, translucent or opaque, and in color may vary from white, or colorless, to black, also brown, violet, blue, green, yellow and red. It ex hibits the phenomenon of double refraction powerfully, and transparent crystals of it (called °Iceland spar° because first obtained from Iceland) are used in the manufacture of polarizing prisms. (See LIGHT). Limestone,
marble and chalk are commonly classed as massive or cryptocrystalline varieties of calcite. Oolite (q.v.) is a granular limestone composed of innumerable minute rounded concretions. Pisolite is a similar variety in which the spheres are as large as peas. The stalactites and stalagmites of many caves (q.v.) are cal cite: Mexican onyx, travertine and calc-tufa (q.v.) are a few of the many other varieties of calcite. Varieties containing other metallic carbonates are known as baricalcite, strontiano calcite, ferrocalcite, etc. Calcite effervesces briskly even in cold acid. It occurs abundantly all over the world; especially choice specimens of crystals come from Germany England, Guanajuato, Mexico; Rossie, N. Y.; Joplin, Mo., and Lake Superior. Examples of the stalactite forms are found in caves at Scho harie, N. Y., and at Wier's Cave in Virginia. The various rock forms of calcite are burned to make lime, and also in mixture with silica and alumina in the manufacture of Portland cement. See CALCIUM ; CEMENT.