Calcite

Calcite, from the Latin calx, lime, the chief mineral form of calcium carbonate, which substance being dimorphous also crystallises in the prismatic system, and is then known as aragonite (q.v.). Calcite occurs in several hundred distinct crystalline forms belonging to the rhombohedral or hexagonal system, of which the chief are the scalenohedron or dog-tooth spar and the obtuse rhombohedron, the primary form which can be obtained from all the others by cleavage. It also occurs in stalactites, stalagmites, and other massive forms. When pure it is transparent, colourless, and vitreous, with a specific gravity of 2.7, and a hardness which is 3 in the scale. This form is known, from the source of the finest crystals, as Iceland spar; or, from the exceptionally wide divergence of the ordinary and extraordinary rays of transmitted light, as doubly-refracting spar. It is used as a polariser in the Nicol's prism (q.v.). Calcite is frequently tinted red, yellow, brown, or grey from the presence of impurities, Fontainebleau limestone being a variety crystallising in rhombohedra, but opaque from the inclosure of 65 per cent. of sand. Almost all the forms yield a white streak. Before the blowpipe calcite is reduced to quicklime, and glows intensely, the carbon dioxide being driven off. Even with dilute acids, such as ordinary vinegar, it effervesces freely from the escape of the same gas. Limestones, many of which are made up of animal remains, are merely impure massive forms of calcite. When earthy they are known as chalk (q.v.); when in small rounded concentric granules, as oolite (q.v.); when capable of taking a polish, as marble (q.v.). Many of these latter forms are entirely made up of small crystals, and are then termed saccharoid marble. Limestones are largely burnt into quick-lime, and impure varieties that contain clay furnish what is termed "hydraulic cement" which sets under water, and are therefore known as cement-stone.