Calcite Occurrence and Use

    Occurrence. Calcite is one of the most common and widely diffuse of minerals. It occurs as enormous and widespread sedimentary rock masses, in which it is the predominant, at times practically the only mineral present. Such rocks are the limestones, marbles (metamorphosed limestones), chalks, calcareous marks, calcareous sandstones, etc. The limestone rocks have, in great part, been formed by the deposition on a sea bottom of great thicknesses of calcareous material in the form of shells, skeletons of sea animals, etc. A smaller proportion of these rocks have been formed directly by precipitation of calcium carbonate. It occurs as a secondary mineral in lavas. It occurs in many sedimentary and metamorphic rocks in greater or less proportion. It is the cementing material in the light-colored sandstones; Calcite is also one of the most common of vein minerals, occurring as a gangue material, with all sorts of metallic ores.

    It would be quite impossible to specify all of the important districts for the occurrence of calcite in its various forms. Some of the more notable localities in which finely crystallized calcite is found are as follows: Andreasberg in the Harz Mountains; various places in Saxony; in Cumberlnd, Derbyshire, Durham, Cornwall, Lancashire, England; Iceland; Guanajuato, Mexico; Joplin, Missouri; Lake Superior copper district; Rossie, New York, etc.

    Use. The most important use for calcite is for the manufacture of lime for mortars and cements. Limestone when heated to about 1000° F. loses its carbonic acid, and is converted into quick-lime, CaO. This when mixes with water (slaked lime), swells, gives off much heat , and finally by absorption of carbon dioxide from the air hardens, or, as commonly termed “sets” Quicklime when mixed with sand forms the common mortar used in building. Certain limestones contain various clayey materials as impurities. Cements made from these limestones have he valuable property of hardening under water, and are known as hydraulic cements. Many hydraulic cements are made up artificially by combining their ingredients in experimentally determined proportions. The chemistry of the process of their hardening is not fully under- stood, but various silicates of calcium and aluminum are probable formed. Portland cement, used so largely in concrete construction, is a mixture of about 6 parts of lime, 2 parts of silica, and 1 part of alumina.

    Chalk is used as a fertilizer, for whiting and whitewash, for crayons, etc. It is found in many places in Europe, the chalk cliffs of Dover being famous.

    Limestone is largely used as a building material, and is obtained in the United States chiefly from Pennsylvania, Indiana, Ohio, Illinois, New York, Missouri, Wisconsin. Limestone is largely used as a flux for smelting various metallic ores. A fine-grained limestone is used in lithographing.

    Marbles are used very extensively as ornamental and building material. The most important marble quarries in the United States are found in Vermont, New York, Georgia, Tennessee, etc. Iceland spar is valuable for optical instruments, being used in the form of the Nicol prism to produce polarized light, Obtained at present only from Iceland.