Subject: Rock Salt

1.what is rock salt? is it formed? 3.where does it form? 4.what is it used for? 5.why is it important? and.... Is rock salt potassium sulfate K2SO4?

Here are the answers to your questions:

1. what is rock salt?
    This is the common name for the mineral "halite". Its chemical formula is NaCl. You might know this substance as table salt. and... Actually, rock salt is not K2SO4; it is NaCl. It can have impurities of gypsum (CaSO4) and sylvite (KCl) but it is very rare to find potassium sulfate as a mineral, although occasionally polyhalite (K2Ca2Mg(SO4)4.2H2O) is found associated with rock salt deposits.

2. how is it formed?
    It is typically formed by the evaporation of salty water (such as sea water) which contains dissolved Na+ and Cl- ions.

3. where does it form?
    One finds rock salt deposits ringing dry lake beds, inland marginal seas, and enclosed bays and estuaries in arid regions of the world. At various times in the geologic past, very large bodies of water (such as the Mediterranean Sea and an old ocean that sat where the Atlantic Ocean sits now) also evaporated and made enormous deposits of rock salt. These deposits were later burried by marine sediments, but since halite is less dense than the materials that make up the overlying sediments, the salt beds often "punched up" through the sediments to create dome-like structures. These are now mostly burried by additional sediments.

4. what is it used for?
    Table salt is essential for human life. A large amount of the comercially mined rock salt is prepared for human consumption. Rock salt is also applied to road beds in cold climates to help reduce the freezing point of water on the road, thereby allowing it to not freeze-over at 0 deg. C (although I have no first-hand knowledge of this since I live in Hawaii).

5. why is it important?
    As I mentioned above, Table salt is essential for human life.
    In addition, the formation of Rock Salt deposits from the world's oceans is the major mechanism through which the Ocean's can regulate their NaCl. content, thereby not becoming "overly" salty.
    Salt formations are also important in the rock record because they form one of the most important "traps" for petroleum in the crust. Geologists often use a variety of techniques to try and locate salt domes in the subsurface when they are exploring for new oil and gas resources.

Dr. Ken Rubin, Asst. Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822

Return to the Ask-An-Earth-Scientist © page