Fluor Spar, fluoride of calcium, a mineral species consisting of fluorine 48.7 and calcium 51.3 per cent., named from the Latin fluere, in reference to its property of flowing when used as a flux. It is met with in cubical crystals, which easily cleave into octahedrons and tetrahedrons by removal of the solid angles. These crystals, collected in groups, their faces presenting a fine splendent lustre, and some brilliant shade of red, blue, green, or purple, constitute some of the most beautiful minera-logical specimens. They are sometimes transparent, but commonly translucent, and are brittle, breaking into splintery and conchoidal fragments. The hardness of the mineral is 4; its specific gravity 3.14 to 3.19. Coarsely pulverized and heated, it emits phosphorescent light of various colors. Before the blowpipe it decrepitates and fuses to an enamel. It is met with in veins in the metamorphic rocks, and in the limestones of formations as recent as the coal. In the north of England it is a common gangue of the lead veins which are found in the strata of the coal formation; and it is there most conveniently applied as a flux for the reduction of these ores, for which it is peculiarly adapted.

The most famous locality of fluor spar is at Castleton, in Derbyshire, England, whence the name of Derbyshire spar has been given to the mineral. It is there found in the fissures of the limestone of deep blue and purple colors, in specimens so large and beautiful that they are wrought into vases, inkstands, cups, tables, etc, which present fine colors and polish, but which from their softness are liable to be soon defaced. The blue color is often so intense that the articles cannot be worked thin enough to exhibit the shade; but by heating the stone nearly red-hot, the intensity is diminished and the blue changes to amethystine. If the heat is continued, the color disappears. The workmen call the stone blue John. They chip the block into a rude shape, and then heat it, so that on applying rosin over its surface this will fuse and penetrate slightly into the mass, the object of which is to check the tendency to cleave as the stone is afterward worked in the lathe; and as the particles are removed in this operation, the rosining is occasionally repeated. The manufacture is difficult, from the crystalline structure with its fourfold cleavage causing the lamina) to split up in unexpected places. The best workmen often fail in turning very thin hollow articles.

Fluor spar is found at many localities in the United States, and is now largely used for practical purposes. Fine crystals, commonly green and very large, are found in different places in Jefferson and St. Lawrence counties, N. Y., and at Rossie they have been used as a flux in smelting lead ores. In Illinois, below Shawneetown on the Ohio, it is found in large purple crystals, with the same associations of lead ores and coal that accompany it in the north of England. The lead veins of the meta-morphic rocks of New England often contain it as one of the gangues. From fluor spar is obtained fluorine, which, combined with hydrogen in the form of hydrofluoric acid, is used to etch glass. A variety of fluor spar has been discovered in Germany, which on the application of heat gives off an odor which Schonbein attributed to a modified oxygen, called anto-zone; the mineral is called antozonite.