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Carnallite

earth science Maturity 7-9 Next article →

Carnallite is a unique type of mineral known as an evaporite. An evaporite is a mineral sediment that forms when water evaporates from a sea or a salty basin.

Carnalit
Carnalit

Chemically, carnallite is a hydrated potassium magnesium chloride. Its specific chemical formula is KCl.MgCl2·6(H2O). This mineral is quite rare because it requires very specific environmental conditions to form. It is a double chloride mineral, meaning it combines two different types of salts. It is highly valued because it serves as an important source of both potassium and magnesium.

The formation of carnallite follows a precise sequence of chemical steps. It typically occurs in a saline marine environment or an endorheic basin.

Carnalit
Carnalit

In these settings, water flows in at a rate lower than the rate of evaporation. This creates a long period of evaporation that concentrates the salts. First, a mineral called sylvite precipitates from the mixed solutions of potassium, magnesium, and chloride ions. This process leaves behind a brine that is specifically enriched with magnesium. As the water continues to disappear, the carnallite then precipitates from this magnesium-rich liquid. In controlled experiments, these halides form when only 10% to 20% of the original water remains.

Carnallite has a complex internal structure involving shared octahedra. In this structure, there is a network of KCl6 octahedra. Two-thirds of these octahedra share faces with one another.

Carnalit
Carnalit

The magnesium ions are located within Mg(H2O)6 octahedra that occupy the open spaces. These water molecules are very important because they enclose the magnesium ions. This arrangement prevents the magnesium and chloride from interacting directly. Instead, the water molecules act as charge transmitters to keep the structure stable. This specific arrangement allows the mineral to follow Pauling's rules for crystal stability despite the face-sharing.

Physically, carnallite can appear in several different forms and colors. It is usually found in massive or fibrous shapes. While rare, it can also form pseudohexagonal orthorhombic crystals. The color of the mineral varies from white and yellow to reddish. It can also appear colorless or even blue.

Carnalit
Carnalit

Some specimens appear red because of inclusions of hematite, which is iron oxide. Carnallite is also deliquescent, which means it absorbs moisture from the air. If left in high humidity, it can absorb enough water to form an aqueous solution. Because of this, specimens must be stored in airtight containers. It also has a notably bitter taste.

The history of carnallite began in the mid-19th century. It was first described in 1856 from its type location in Germany.

Carnalit
Carnalit

This location is the Stassfurt Deposit in Saxony-Anhalt. The mineral was named in honor of Rudolf von Carnall. He was a Prussian mining engineer who lived from 1804 to 1874. Since that discovery, scientists have identified many other deposits around the world. These deposits often date back to the Devonian through the Permian Periods.

Carnallite is mined in several significant locations across the globe. In the United States, it is found in Carlsbad, New Mexico, and the Paradox Basin in Colorado and Utah. Large deposits also exist in the Perm Basin in Russia and the Williston Basin in Saskatchewan, Canada.

Carnalit
Carnalit

In China, it occurs in the Qaidam Basin. In Israel and Jordan, workers use evaporation pans to concentrate brine until carnallite precipitates. They then dredge the mineral from the pans. This process allows them to separate the potassium chloride from the magnesium chloride.

This mineral plays a vital role in global agriculture and industry. Its most common use is in the production of fertilizers. It is a major source of potash, which is a type of soluble potassium salt. Potash is essential because it is much easier to use than insoluble potassium feldspar. While sylvite is the most important source of potash, carnallite is also a key contributor. Additionally, carnallite is a minor source of magnesium worldwide. However, it serves as the main source of magnesium for Russia.

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