THE NONMETALLIC MINERALS. 



195 



HI. HALIDES. 

 1. HALITE; SODIUM CHLOKIDE; OR COMMON SALT. 



Composition Na Cl,= sodium 60.6 per cent; chlorine 39.4 per cent. 

 The natural substance is nearly always more or less impure, as noted 

 later. Hardness, 2.5; specific gravity, 2.1 to 2.6 per cent. Colorless 

 or white when pure, but often yellowish or red or purplish by the 

 presence of metallic oxides and organic matter. Readily soluble in 

 cold water, and has a saline taste. Crystallizes in the isometric system, 



Fig. 2. 



CLUSTER OP HALITE CRYSTALS. 



Stassfurt, Germany. 

 Specimen No. 40222, U.8.N.M. 



usually in cubes (fig. 2, Specimen No. 40222, U.S.N.M.), but some- 

 times in octahedrons, the faces of the crystals (particularly when pre- 

 pared artificially) being often cavernous or hopper shaped. Sometimes 

 occurs in fibrous forms, which it has been suggested are pseudomor- 

 phous after fibrous gypsum (Specimen No. 64733, U.S.N.M.). Often 

 found in the form of massive, crystalline granular aggregates com- 

 monly known as rock salt (Specimens Nos. 67558, 64736, 62946, 

 U.S.N.M.). 



Sylvite, the chloride of potassium, sometimes occurs associated with 

 halite, where.it has formed under similar conditions. From halite 



