COMPOUNDS OF CALCIUM. 



215 



Calcite. — Calc Spar. Calcium Carbonate. 



Rhombohedral. JiAE (fig. 1) = 105°5'. Cleavage easy, 

 parallel with the faces of the fundamental rhombohedron. 



Calcite with the form in fig. 7 is often called dog-tooth 

 spar. Occurs fibrous with a silky lustre ; sometimes lamel- 

 lar ; often coarse or fine granular, and compact. 



The purest crystals are transparent with a vitreous lustre ; 

 the impure massive varieties are often opaque, and without 

 lustre, and even earthy. The colors of the crystals are 

 either white or some light grayish, reddish or yellowish 

 tint, rarely deep red ; occasionally topaz-yellow, rose or 

 violet. The massive varieties are of various shades from 

 white to black, generally dull unless polished. H. = 3. 

 G. = 2-5-2-8. 



Composition. Ca0 3 C = Carbon dioxide 44, lime 56 = 100. 

 Sometimes impure from mixture with iron, silica, clay, 

 bitumen, and other substances. B.B. infusible; colors the 

 flame reddish, gives up its carbon dioxide, is thereby made 

 caustic, in which state it gives an alkaline reaction. Effer- 

 vesces in dilute cold hydrochloric acid. Many varieties 

 phosphoresce when heated. 



The folloAving are the principal varieties. 



Iceland spar. Transparent crystalline calcite, first 

 brought from Iceland. Shows well double refraction. 



Satin spar. A finely fibrous variety with a satin lustre. 

 Receives a handsome polish. Occurs usually in veins tra- 

 versing rocks of different kinds. 



Chalk. White and earthy, without lustre, and so soft as 



