Magnesium chloride

Lore & Background

Magnesium chloride can be extracted from brine or sea water. In North America and South America, it is obtained primarily from Great Salt Lake brine. In the Jordan Valley, it is obtained from the Dead Sea. The mineral bischofite (MgCl2·6H2O) is extracted by solution mining out of ancient seabeds, for example, the Zechstein seabed in northwest Europe. Some deposits result from high content of magnesium chloride in the primordial ocean. Some magnesium chloride is made from evaporation of seawater. In the Dow process, magnesium chloride is regenerated from magnesium hydroxide using hydrochloric acid. It can also be prepared from magnesium carbonate by a similar reaction. At ambient pressure, anhydrous MgCl2 adopts a layered structure often described as a distorted variant of the cadmium chloride (CdCl2) type, with a different stacking sequence. In the hexahydrate, the Mg2+ is octahedral, being coordinated to six water ligands. The octahydrate and the dodecahydrate can be crystallized from water only below about −3 °C and −16 °C, respectively. As verified by X-ray crystallography, these 'higher' hydrates also feature [Mg(H2O)6]2+ ions. A decahydrate has been reported but is not a well-established, widely recognized hydrate; the known stable hydrates are typically n=1, 2, 4, 6, 8, and 12. Anhydrous MgCl2 is produced industrially by heating the complex salt named hexamminemagnesium dichloride [Mg(NH3)6]2+(Cl−)2. The thermal dehydration of the hydrates MgCl2·nH2O (n = 6, 12) does not occur straightforwardly. As suggested by the existence of hydrates, anhydrous MgCl2 is a Lewis acid, although a weak one.

Reader's Guide

Magnesium chloride is significant primarily as the main precursor to metallic magnesium, which is produced on a large scale via electrolysis of molten anhydrous MgCl2. This process is essential for producing magnesium metal, used in alloys and lightweight materials. The compound also has widespread practical applications: it is used for low-temperature de-icing of highways, sidewalks, and parking lots, and for dust control, soil stabilization, and wind erosion mitigation. In industry, MgCl2 serves as a catalyst support in Ziegler-Natta catalysts for polyolefin production and as a Lewis acid catalyst in aldol reactions. In cuisine, it is an important coagulant for making tofu, sold as nigari in Japan and lushui in China. It is also used in nutraceutical and pharmaceutical preparations, as an electrolyte, and in gardening to correct magnesium deficiency in plants via foliar feeding. In wastewater treatment, it supplies magnesium to precipitate phosphorus as struvite. Its occurrence in nature includes being a major component of Dead Sea minerals and the mineral bischofite. The compound's deliquescence reflects its Lewis acidity, and its toxicity is generally low in healthy individuals, though overapplication in gardening can lead to chloride toxicity in plants.

Did You Know?

• Anhydrous magnesium chloride is the principal precursor to magnesium metal, produced on a large scale via electrolysis of molten salt.
• Magnesium chloride is used for low-temperature de-icing of highways, sidewalks, and parking lots.
• In Japan, magnesium chloride is sold as nigari, a white powder used as a coagulant for tofu.
• The mineral bischofite (MgCl2·6H2O) is extracted from ancient seabeds such as the Zechstein seabed in northwest Europe.

Sourcing from the Earth and Sea

Magnesium chloride is found in abundance across the planet's water bodies and geological formations, making its extraction a globally distributed endeavor. In the Americas, the primary source is the brine of the Great Salt Lake, while in the Middle East, the Dead Sea in the Jordan Valley provides another rich reservoir. In northwest Europe, the ancient Zechstein seabed yields a mineral called bischofite, which is essentially the hexahydrate form of magnesium chloride, recovered through solution mining techniques. Some geological deposits trace their origin to magnesium-rich compositions of the primordial ocean. Beyond direct extraction, the compound can be produced by evaporating seawater or through the Dow process, in which magnesium hydroxide is treated with hydrochloric acid to regenerate the chloride salt. A similar acid-based route starting from magnesium carbonate is also viable. The hydrated form is the most readily available variant in commerce, while the anhydrous version requires more specialized preparation, typically involving the thermal decomposition of a complex ammine salt.

Crystal Architecture and the Chemistry of Hydration

The solid-state behavior of magnesium chloride is dominated by its strong affinity for water molecules. The anhydrous salt crystallizes in the same structural motif as cadmium chloride, and when heated it sheds water in a stepwise fashion: the dodecahydrate releases molecules at minus 16.4 degrees Celsius, the octahydrate at minus 3.4, the hexahydrate at 116.7, the tetrahydrate at 181, and the dihydrate near 300 degrees. X-ray crystallographic studies confirm that even in the higher hydrates the magnesium ion retains an octahedral coordination sphere of six water ligands, forming the hexaaquamagnesium cation. Both octahydrate and dodecahydrate crystals can form from aqueous solution below 298 kelvin, and a decahydrate has also been isolated. The anhydrous compound is a weak Lewis acid, a property vividly demonstrated by its deliquescence: it pulls moisture from ambient air until the solid dissolves into a liquid. This Lewis acidity also allows it to form adducts such as the TMEDA complex and a coordination polymer with dioxane, as well as the tetraethylammonium tetrachloromagnesate salt.

Forging Magnesium Metal and Enabling Polymer Chemistry

The single most consequential industrial role of anhydrous magnesium chloride is serving as the feedstock for metallic magnesium production. Because the reduction potential required to liberate magnesium from its ion falls outside the stability window of liquid water, aqueous electrolysis is impossible; any magnesium produced would instantly react with the solvent, and hydrogen gas would be evolved instead. The solution is molten-salt electrolysis, in which the salt is melted and current is passed through it. At the cathode, magnesium ions gain electrons and deposit as the silvery metal, while at the anode chloride ions are oxidized, releasing chlorine gas. This operation runs at massive industrial scale. In a different but equally important domain, magnesium chloride acts as a support material in Ziegler-Natta catalysts. Its introduction dramatically boosted the activity of earlier catalyst systems and made possible the highly stereospecific catalysts now used to manufacture polypropylene. The compound also functions as a Lewis acid catalyst in aldol reactions, bridging inorganic and organic synthetic chemistry.

From Tofu to Roadways: Everyday Encounters

Few inorganic compounds touch daily life as broadly as magnesium chloride. In kitchens across East Asia, it plays an irreplaceable role as a coagulant for tofu, listed under the food-additive code E511. In Japan it is known as nigari, a term rooted in the word for bitter, and in China as lushui. The commercial product is not fully refined: it retains up to five percent magnesium sulfate along with trace minerals, and the raw crystals are harvested from lakes in Qinghai province before being reprocessed in Japan. On winter roadways, magnesium chloride serves as a de-icing agent effective at low temperatures. It can be applied as an anti-icing spread to stop snow from bonding to pavement, sprayed as a prewetting liquid onto salt to help it adhere, or blended with salt before loading onto spreaders. Compared with calcium chloride, it damages concrete at roughly half the rate, though environmental pollution from runoff still demands careful dose management. In gardens, it substitutes for Epsom salt as a foliar magnesium source, though its chlorine content requires a lower application threshold to avoid phytotoxicity.

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