[Halite]
NaCl
[Pl. 50]
Salt
Occurs in crystals, and in cleavable and granular masses; hardness, 2.5; specific gravity, 2.1; colorless to white; luster vitreous; transparent on thin edges.
Halite is common salt, occurring in cubic crystals, with perfect cubic cleavage. Its form, hardness, taste, and solubility in water make it easy to determine.
Halite is the most abundant salt in sea water, making about 2.5% out of the total of 3.5% of solids in solution. It is also a prominent, when not the leading, salt in solution in the waters of inland lakes, like Great Salt Lake, or the Dead Sea, there being 20% of halite in the former and 8% in the latter, though the total of solid in solution in the water of the Dead Sea is greater than that in Great Salt Lake.
The great salt deposits are mostly the result of the evaporation of the water of arms or isolated portions of former oceans; the salt, gypsum, etc., left by the drying sea, having been buried beneath later sediments. Other bodies of salt represent the disappearance of ancient lakes. There are also the curious “salt domes” of Louisiana and Texas, which are immense, roughly circular, subterranean masses of salt extending to as yet unknown depths which are thought to have been formed by masses of salt from some deep source bed pushing their way upward through the overlying formations by plastic flowage. As the upthrust took place the sediments were arched into domes. Some of these domes are today important sources of rock salt.
There are extensive beds of salt under parts of New York, Michigan, Ohio, Oklahoma, Kansas, etc., which are mostly worked by drilling wells into the salt layer, then introducing hot water to dissolve the salt. The brine thus formed is pumped to the surface, and the salt recovered by evaporation in pans. During the process, skeleton crystals of salt with concave faces may form, but in Nature the crystals are uniformly solid cubes.
[Boracite]
Mg₇Cl₂B₁₆O₃₀
Occurs in small crystals or granular masses; hardness of crystals, 7; of the masses, 4.5; specific gravity 3; colorless to white; luster vitreous; transparent to translucent on thin edges.
Small crystals, associated with salt and gypsum, occur in the beds and incrustations, which result from the drying up of alkaline lakes, especially in Nevada and southern California. The crystals are orthorhombic, but appear like perfect cubes, with the edges beveled and part of the corners cut. They are not easily dissolved in water, but quickly go into solution in hydrochloric acid.