CHAPTER VII 



THE HALOID COMPOUNDS 



HALITE 



Halite. Common salt ; NaCI, Sodium chloride ; Na = 

 39.4, Cl = 60.6 ; Isometric ; Type, Ditesseral Central ; Common 

 forms, a (100), o(lll); Cleavage, cubic perfect; Twins rare; 

 Brittle, fracture conchoidal; H. = 2.5; G. = 2.1-2.6; Color, 

 white, gray, blue, yellowish, or red ; Streak, white ; Luster, vitre- 

 ous; Transparent to translucent; n = 1.542. 



B.B. Fuses at two (815), often with decrepitation, and colors 

 the flame intense yellow (Na). With S. Ph. bead saturated with 

 copper oxide yields an azure blue flame (Cl). Dissolves in water 

 easily and has a salty taste. 



General Description. Crystals are cubes, while other forms 

 in combination are rare. When crystallized from solution con- 

 taining caustic soda it separates in octahedrons, or in combinations 

 of the cube and octahedron, according to the alkalinity of the solu- 

 tion. Natural crystals showing the cube and octahedron are rare. 

 Twins are not known, except in some cases thin lamellae have been 

 noted which have probably been formed by pressure. Pure salt 

 is white ; the yellow and reddish colors are caused by such impuri- 

 ties as oxide of iron or clay. Calcium and magnesium sulphates 

 and chlorides are often present, and these chlorides cause the deli- 

 quescence of many specimens of rock salt. The beautiful blue of 

 some specimens from Stassfurt has been attributed to the presence 

 of the subchloride of sodium or to small amounts of metallic sodium, 

 as the blue color may be produced by exposing crystals to the vapors 

 of sodium, also to the cathode ray, or to the radiations of radium. 



Both halite and sylvite are diathermous and allow the non- 

 luminous heat rays to pass with very little absorption. 



Salt deposits are found in the sediments of all ages; those of 

 New York are Upper Silurian, while those of Michigan are Carbon- 

 iferous. They have been formed and deposited from solution, the 



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