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Halite

Sodium Chloride

bristol dry lake
Salt Flat in Bristol Dry Lake - Mojave Desert

Halite (sodium chloride) comes from the Greek halos, meaning "salt" and lithos meaning "rock," and is in fact, better known as rock salt. Halite is called an evaporite because it is formed by the evaporation of saline water in partially enclosed basins. It is very common worldwide, deposited in solid underground masses, and as a dissolved solution in oceans and many arid-region inland lakes.

Huge halite deposits are mined for common table salt (NaCl), which is in great demand and has many uses, including:

food seasoning
road safety in the winter months to melt snow and ice
salt licks for cattle and horses to provide them with essential salt
water softeners
medicinal purposes
primary ore for both sodium and chlorine

Underground Halite

When halite occurs in non-arid regions, it is only possible for it to exist in underground deposits, as anything on the surface would dissolve from rainwater. Indeed, a great deal of halite is found in ancient bedrock all over the world where large extinct salt lakes and seas evaporated millions of years ago, leaving behind thick deposits of salt in beds that range from a few feet to more than 1,000 feet in thickness.

Deformation of some underground halite beds sometimes results in the extrusion of plugs of salt through overlying sediment, as in the salt domes and diapirs found in Texas and Louisiana. The cities of Cleveland and Detroit rest above huge halite deposits that are mined for road salt.

These deposits are mined by drilling wells into the salt layer, and pumping hot water to quickly dissolve the salt. When the water becomes saturated with dissolved salt (the solution is known as "brine"), it is pumped out. When the brine evaporates, the remainder is primarily sodium chloride, which is refined and utilized.

Aboveground Halite

Inland lakes that are rich in salt exist in many arid regions, especially in the desert Southwest. They usually have no outlet, as the Great Salt Lake of Utah, and are often below sea level, as with Badwater in Death Valley, which was once called Lake Manley. In such lakes there is more water being evaporated than the amount entering, causing the volume of water to decrease, while increasing its salinity.

In some lakes, concentrations of salt brine can become so extreme that a stick or stone placed in the water will become coated with a layer of halite. As the water evaporates, the shoreline recedes and chunks of salt are left crystallized on the beaches. Many of these inland lakes have already dried up completely, leaving enormous salt deposits that are commercially mined, as at Searles Lake, California in the Mohave Desert. Utah's Bonneville Salt Flats and California's Salton Sea are two other good locations to find halite.

Collection

Well crystallized specimens of halite cubes are very impressive and quite popular with rockhounds. Halite crystals form so fast and so well in some evaporative lakes that many such specimens that are sold worldwide in rock shops and at mineral shows were grown within the past year.

Some halite crystals are colored pinkish by bacterial debris trapped during crystallization while others are colored bright purple or blue, with a silky luster. Halite also forms some unusual variations that are prized by collectors. One variation, called a hopper crystal, forms what has been termed a skeleton of a crystal. Just the edges of a hopper crystal extend outward from the center, leaving hollow stairstep faces between these edges. Hopper crystals form due to the disparity of growth rates between the crystal edges and the crystal faces.

Since taste is an important property of salt, taste is one of the important means of identifying this mineral. The correct way to taste a mineral specimen is to first lick your index finger, rub it against the specimen and then taste the finger. Remember that there are poisonous minerals that resemble halite.

Hydrohalite is not strictly a variety of halite, but a very rare, similar mineral with the formula NaCl + 2H2O. This variation forms only under very unique conditions where the water does not dissolve the salt and becomes an integral part of the structure of the mineral.

 

Chemistry

Crystal System

Hardness
NaCl

Orthorhombic

2.5 (Mohs' scale)
Cleavage

 Fracture

Specific Gravity
perfect, all sides - cubic

Conchoidal

2.1 - 2.6

Color, Transparency,
Luster & Streak

VARIETIES

- Color: Colorless, white, red, yellow, orange, pink, green, blue, violet, gray
- Transparency: Transparent to Translucent
- Luster: Vitreous
- Streak: White

  • Rock Salt
  • Hydrohalite

 

-- Bob Katz

g
Environment & Geology Index Page
Things To Do: Rockhounding

 


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