Rock Salt Type, Composition, Formation, Occurrence, Mining & Uses

Rock Salt
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Rock salt is a sedimentary rock primarily composed of halite. Halite is a mineral of sodium chloride (common salt), NaCl.

Rock salt is usually white or colorless. However, impurities in the rock salt can make it appear light blue, pink, purple, yellow, orange, or gray. Colorless salt is rated the purest as it doesn’t contain any impurities. Rock salt is also known as halite. The word “halite” comes from the Greek word “háls” used to describe salt.

Rock Salt (Halite, NaCl)
Rock Salt (Halite, NaCl)

Rock Composition

Rock salt is mainly halite, a mineral made from sodium chloride, NaCl. Rock salt also contains trace amounts of clastic sedimentary particles like clay minerals. The clay minerals are usually crystals of secondary minerals like sylvite or hydrocarbons. Evaporative minerals like gypsum, anhydrite, or sylvite are also commonly found in rock salt.

Pink Rock Salt
Pink Rock Salt

Rock Salt Formation

Rock salt or halite usually occurs within sedimentary rocks. It was formed after seawater, or salty lake water, evaporated. Its reserves can also form after enclosed salty lakes evaporate.

The rock forms under a set of extraordinary geological conditions. Most of the deposits in the world were formed during the time of high sea levels when the shallow seas were spread over extensive areas.

A big deposit requires long periods of seawater evaporation in an arid climate, with fresh seawater regularly coming to that basin or a particular area. You can estimate how long the process is by understanding that one single foot of rock requires over 100 feet of water. Hence, thicker deposits may require up to miles of seawater evaporation.

Where is Rock Salt Found?

Rock salt reserves are located worldwide. The most noticeable North American reserves are in Kansas, Ohio, New Mexico, Nova Scotia, and Saskatchewan. Another big natural reserve is located at Khewera Mine, Islamabad, Pakistan. The salt produced from this mine is commonly known as pink salt or Himalayan salt and is exported worldwide.

Salt domes (vertical pipe-like salt reserves) are found in Germany, Spain, Netherlands, Romania, Denmark, and Iran. Iran has some salt glaciers as well.

France has reserves on unusual, purple, and fibrous vein-filling reserves. Australia has rare halite flowers found in caves of Nullarbor Plain.

Rock salt is being mined and processed for various uses. The largest underground salt mine is the Sifto salt mine, located in Canada. The United Kingdom has three salt mines, the largest of which is located at Winsford in Cheshire; it produces up to 1 million tons of salt annually.

A survey published by USGS states that almost every country has some salt deposits. The leading producers are China, the USA, India, Germany, Canada, Australia, Chile, and Mexico.

Khewera Salt Mine, Pakistan
Khewera Salt Mine, Pakistan

Rock Salt Production

Rock salt is produced in the following 4 ways.

  • Underground mining
  • Solution mining
  • Vacuum salt pan
  • Solar salt

Underground Mining: Underground mining is done in salt mines below the earth’s surface. There are a few factors that determine the success of the mining operation. The chemical and mineral composition, the salt’s purity, the mine’s depth, and transportation cost play an important role in underground mining. The depth of the mine is a critical factor in deterring mining costs like electricity, ventilation, salt extraction, human capital, and the cost of water in and out of the mine.

Solution Mining: Solution mining is done by injecting a hot water stream into a salt mine. The salt-infused water is then pulled out via a nearby recovery well. The water is then dried via solar energy or heating to produce salt crystals.

Vacuum Salt Pan: Vacuum salt pans produce pure salt in a series of heating tanks. The tanks are filled with brine and then heated to produce salt. The steam generated is moved to the next tank to reduce energy costs. The heating effect also boils many impurities and leaves pure salt behind.

Solar Salt: Solar salt refers to salt produced via sunlight and hot temperature. Seawater is captured in small ponds where solar energy and high temperatures increase evaporation rates.

The amount of salt produced depends on the climate, ponds’ surface area, and concentration of NaCl in the water. USA, Brazil, Mexico, and India are leading producers of solar salt.

Rock Salt Uses

Rock salt is an economically valuable rock. Its major uses include the following.

Highway De-icing: Almost 40% of the salt produced in the US is used for highway de-icing. The amount usually varies based on weather conditions. The rock salt is mined and crushed to the appropriate size and then spread over roads. It is applied wet to reduce scattering due to traffic. However, de-icing with rock salt has some environmental impacts.

Raw Material: Rock salt is used to produce chlorine, sodium hydroxide, and synthetic soda ash. These chemicals are used to manufacture plastic, paint, and solvents.

Cooking: Almost every processed or cooked meal contains salt in one form or another. Salt is added to increase flavor and color, as a preservative, and for various other uses.

Agriculture: Salt is essential for farm animals. Salt blocks are placed in the feeding areas for animals to lick them. These blocks are often processed to add iodine, sulfur, and trace elements to make up for animal nutrition.

Water Softening: Salt is frequently used to make hard water soft. The sodium ions in water replace calcium and magnesium in hard water and make it soft. Soft water is good for skin, appliances, and plumbing systems.

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