Feldspar

(feld's pahr,) or Felspar. [Ger. feldspath, literally, field-spar. ] (Min.) A mineral composed of silica, alumina, and potash, with traces of lime, and often of oxide of iron. It is one of the constituents of granite, is softer than quartz, and usually white, grayish, or reddish. The general figure of the numerous crystals of F. is an oblique prism, with unequally produced planes, whose number varies from 4 to 10. These prisms are terminated by summits, ordinarily composed of 2 large culminating faces, and several smaller faces, which seem to obey no constant law of arrangement; hence it results, that the forms of F. are among the most difficult of any to understand and describe. Common F., in which potash is the predominant alkali, is often called orthoclase. Where soda is the predominant alkali, it has received the name of albite In the variety called Labradorite, there is from 8 to 11 per cent. of lime present. A fourth variety is termed oligoclase. In its decomposed state, F. furnishes the petuntse, or Cornish stone, so much used in the porcelain and best pottery manufactures. [s. 358]