Carnallite

Let's talk about the Halide mineral before we get into Carnallite. Any of a group of naturally occurring inorganic compounds that are salts of the halogen acids is known as a halide mineral (e.g., hydrochloric acid). With the notable exceptions of halite (rock salt), sylvite, and fluorite, such compounds are uncommon and only found in small quantities. The simple halides, halide complexes, and oxyhydroxy-halides are known as three broad groups of halide minerals in terms of composition and structure. These categories are also distinguishable in terms of their modes of occurrence.

Alkali salts, alkaline earth, and transition metals are examples of basic halides. The transition-metal halides are unstable when exposed to air and are soluble in water. The most well-known evaporite mineral is halite, sodium chloride (NaCl); it is found in large beds with other evaporite minerals as a result of the deposition of brines and trapped oceanic water in impermeable basins and their evaporation. Sylvite, potassium chloride (KCl), is also present in small quantities in such beds. Among the simple halides, a few double salts such as carnallite and tachyhydrite formed under conditions close to the formation of halite. Carnallite is also a halide mineral.

Carnallite

Carnallite is a soft, white halide mineral that contains hydrated potassium and magnesium chloride and is used to make fertilisers. Carnallite is found in the upper layers of marine salt deposits, where it tends to be an alteration result of pre-existing salts, along with other chloride minerals. The mineral can be found mostly in salt deposits in northern Germany, as well as in Spain, Tunisia, and the southwestern United States.

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Carnallite Formula

Chemical Formula of Carnallite is KMgCl₃·6(H₂O). Slow crystallisation at 25°C will yield synthetic carnallite crystal specimens from 1.5 mole percent KCl and 98.5 mole percent MgCl₂·6H₂O. It has a density of 1.602 g/cm³. Grinding a mixture of hydrated magnesium chloride and potassium chloride can also yield carnallite.

Structure of Carnallite

We discussed the Chemical Formula of Carnallite. Now let’s look in detail about the Structure of Carnallite. 

• Carnallite basic structure has corner- and face-sharing. A network of KCl₆ octahedra exists, with two-thirds of them having the same faces.

• The open spaces within the KCl octahedra are occupied by Mg(H₂O)₆ octahedra.

• The interatomic distance between Mg and H₂O varies between 0.204 and 0.209 nanometers, with an average of 0.2045 nanometers. 

• K and Cl have an interatomic distance of 0.317 to 0.331 nm, with an average of 0.324 nm.

• The estimated density of the resulting structure is 1.587 g/cm³, which is very similar to the measured value of 1.602 g/cm³.

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Structure of Carnallite [Carnallite Formula is KMgCl3·6(H2O)]

According to the third of Pauling's law, sharing one's face increases the chances of instability. The magnesium ions are encased in water molecules in carnallite. The water molecules serve as charge transmitters, preventing the magnesium and chloride from interacting directly. Each of the five chloride anions is paired with two potassium cations and four water molecules. This means that each of the two potassium ions gives each chloride anion 1/6 of a +1 charge. Each of the four water molecules gives the chloride 1/6 of a +1 charge. The charges add up to six 1/6 positive charges, which offset the chloride's negative charge. Because of these two factors, the rare face sharing mentioned in Pauling's second and third rules is appropriate in the carnallite structure.

Physical Properties of Carnallite

The refractive index of carnallite varies from 1.467 to 1.494. Hematite (Fe₂O₃) inclusions in carnallite can cause it to be red. In the thin laminae of hematite, broken shards of iron oxide create red tints. In high humidity, carnallite also deliquesces. This indicates that it is highly soluble in water. Individual crystals are tabular and pseudo-hexagonal, but they are exceedingly uncommon. The forming environment, the lack of cleavage, and fracture are all field indicators of carnallite. Other factors to consider include density, taste, associations with local minerals, and luminescence. Carnallite has a sour flavour. Carnallite has the potential to be both fluorescent and phosphorescent. The potassium in carnallite readily fuses in a blaze, resulting in a violet hue. Carnallite can be distinguished from other evaporate minerals very easily. It has a bitter taste and, unlike halite, no cleavage. Carnallite has a specific gravity of just 1.6 and, unlike kieserite and other non-potassium salts, produces a violet flame when placed in a gas flame. Environment of formation, lack of cleavage, associations, density, deliquescence, fracture, and taste are the best Field Indicators.

Geologic Occurrence

Halite, anhydrite, dolomite, gypsum, kainite, kieserite, polyhalite, and sylvite are examples of mineral associations dependent on physical properties. Evaporites are mineral sediments that contain carnallite minerals. Evaporation of seawater concentrates evaporites. The water inflow must be less than the evaporation or usage levels. This results in a longer evaporation period. When 10%–20% of the original water sample remains after controlled environment experiments, the halides shape. Sylvite makes up about 10% of the total, followed by Carnallite. Carnallite is mainly present in marine deposits that are saline.

Uses

Carnallite is a mineral that is often used in fertilisers. It is a significant potash source. Only sylvite is more important in potash production than carnallite. They're both rare because they're among the last evaporites to form. The primary sources of fertiliser are soluble potassium salts. This is due to the difficulty of separating potassium from insoluble potassium feldspar. Carnallite is a minor magnesium source found all over the world. 

Conclusion

Carnallite is a valuable fertiliser since it contains a lot of potash. Potash is mostly obtained from sylvite, but carnallite also contributes significantly. The magnesium production of carnallite is much less important globally, but it is still Russia's most important source. Potassium is a common element, but it is unfortunately wrapped up in insoluble silicate minerals like potassium feldspars. Since potassium must be in a soluble state to be effective as a fertiliser, soluble potassium salts are the preferred source. Evaporite minerals like carnallite and sylvite, for example, are some of the last minerals to evaporate from sea water, making them difficult to form. In approximately that order, minerals including calcite, dolomite, gypsum, anhydrite, and halite crystallise first. The conditions required for potassium and magnesium salts to form include seawater contained in a cut-off, but not fully isolated basin, similar to the Black Sea. The Black Sea, on the other hand, does not form carnallite because it does not have a warm enough environment to allow for intensive evaporation (this is an evaporite mineral after all). The concentrated brine must not be allowed to leave the basin in order for the salinity to continue to rise. The brine will fall to the basin's bottom, allowing fresher water to join, bringing more magnesium into the basin. This has the effect of delaying the crystallisation of the salts and increasing the brine's salinity. If evaporation does not continue in this direction, the minerals mentioned above will fill the basin before the potassium salts crystallise.