Fig. 460.—Spar crystal.
Calcium. This forms an important group of the minerals, which are very white in colour, and not very hard in substance. Calcium is the metallic basis of lime. Fluoride of calcium, known as fluor-spar, most frequently crystallizes in cubes in the first system. Anhydrite is the anhydrous sulphate of calcium. The hydrated sulphate is called gypsum. One variety of the hydrated sulphate is selenite, another is known as alabaster. Apatite, or asparagus stone, and pharmacolite are in this group.
Rhombohedron (r).
Primary rhombohedron (r).
Six-sided prism (g) regular.
