Calcareous Spar, Or Calc Spar, crystallized carbonate of lime, a very common mineral. It is remarkable for the great variety of its crystalline forms derived from its primary obtuse rhomboid, no less than 600 modifications having been described and figured. It is seen in a pure state in the transparent rhomboidal crystals of Iceland spar, so called because the finest were originally brought from Iceland. These exhibit' the property of double refraction most perfectly. Calcareous spar is white or transparent, except when mixed with some foreign ingredients, which impart to it various shades. It is so soft as to be easily cut with a knife, its hardness being rated at 2.5 to 3.5. Its specific gravity is. 2.5 to 2.77. Acids dissolve it readily, causing a strong effervescence as the carbonic acid is expelled. This is also expelled by heat, the mineral being then converted into quicklime, or the protoxide of calcium. The proportion of this in calcareous spar is 56 per cent., and of carbonic acid 44 per cent. Some of the finest specimens of this mineral are from the Rossie lead mine of St. Lawrence co., N. Y., where a single crystal was found weighing 165 lbs. It is a common gangue in metallic veins, and often forms veins in rock formations of almost all ages, even when no ores are present.

It possesses no value different from that of ordinary limestones; and these are from their great abundance much more cheaply obtained for the manufacture of quicklime, or for fluxes of ores, than the crystallized mineral could be.