Rock salt is the mineral halite, the same chemical used for ordinary table salt.
Rock salt is a common name for the mineral geologists call halite, the naturally occurring form of sodium chloride (NaCl). This is the same chemical compound you can find in the salt shaker on your kitchen table. Layers of halite occur in sedimentary basins around the world, in rocks of every age. Because halite is soluble in water, you are unlikely to see rock salt at the Earth's surface except in very dry areas. One well-known place where salt can be seen at the surface is the Bonneville Salt Flats, near Utah's Great Salt lake.
Formation
Rock salt is an evaporite, a sedimentary rock that forms when salty water evaporates. Evaporites form in place when the amount of water leaving a sedimentary basin by evaporation is greater than the amount entering from rainfall or streams and rivers. Salt crystals form and settle to the bottom when the water becomes saturated with dissolved sodium (Na+) and chlorine (Cl-) ions. Halite is not the only evaporite mineral; others include gypsum (calcium sulfate, CaSO4) and sylvite (potassium chloride, KCl).
Chemistry
Pure halite is entirely sodium chloride. Natural deposits of rock salt often contain impurities of other evaporite minerals, especially gypsum, and may contain thin beds contaminated with clay mud. These impurities are avoided or removed when mined rock salt is processed into salt for human consumption.
Mining
Rock salt dissolves in fresh water, a property that either aids those seeking salt or makes their job more difficult. Where the climate is too humid to allow halite to exist at the surface, salt must be mined from underground layers. Underground mines were once common around the Great Lakes and several are still active along the Gulf Coast in Louisiana. In dry climates, salt miners trap sea water or brine from groundwater wells in ponds at the surface and allow it to evaporate until halite crystals form.
Uses
Rock salt has several uses, though most of the salt produced worldwide ends up being consumed by humans and livestock. Consuming a small amount of salt is essential to human life. Rock salt has other uses as well, including uses in a variety of industries. Unprocessed crushed rock salt, impurities and all, has long been used to melt snow and ice off of roads in cold climates.
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