Magnesium chloride hexahydrate is a common mineral compound found globally, recognized for its high solubility and diverse applications. It is the most prevalent form of magnesium chloride that exists in nature and is widely utilized across various industries, from road maintenance to food production. Its chemical formula is \(\text{MgCl}_2 \cdot 6\text{H}_2\text{O}\), indicating that six water molecules are incorporated into its structure. Its widespread presence in marine environments and geological deposits makes it a readily accessible source of both magnesium and chloride ions.
Chemical Identity and Properties
The chemical structure of this compound is defined by its name, where the term “hexahydrate” signifies that six molecules of water are incorporated into its crystalline lattice. These six water molecules are coordinated to the central magnesium ion (\(\text{Mg}^{2+}\)), forming a stable structure. This coordination is what differentiates the hexahydrate from the anhydrous form of magnesium chloride, which lacks any water molecules.
In its pure state, magnesium chloride hexahydrate appears as a white or colorless crystalline solid, often presenting as flakes or granules. A defining physical characteristic is its extremely high solubility in water, dissolving quickly to form a clear solution. At standard temperatures, approximately 167 grams of the hexahydrate can dissolve in 100 milliliters of water.
This compound also exhibits deliquescence, meaning it readily absorbs moisture from the surrounding air until it dissolves entirely to form a liquid solution. This hygroscopic nature is due to the strong attractive force the magnesium and chloride ions have for water molecules. Because of this property, the material must be stored in tightly sealed containers to maintain its solid form.
Natural Sources and Extraction
Magnesium chloride hexahydrate occurs naturally in large quantities, primarily dissolved in bodies of water and concentrated in mineral deposits. Seawater is a vast source, though the highest concentrations are found in natural brines, such as those from the Dead Sea and the Great Salt Lake, which are major commercial sources.
The mineral form of the hexahydrate is known as bischofite, which is extracted through solution mining from ancient subterranean seabeds, like the Zechstein seabed in Europe. Another significant geological source is carnallite, a double salt mineral containing potassium chloride alongside magnesium chloride hexahydrate.
Commercial production typically involves evaporation and crystallization from these natural brines or from the leftover liquid, known as bittern, remaining after sodium chloride has been harvested from evaporated seawater. As the brine is concentrated, other salts precipitate out first, leaving a highly concentrated magnesium chloride solution. This final solution is then cooled to allow the hexahydrate crystals to form and be collected.
Industrial and Environmental Applications
The unique properties of magnesium chloride hexahydrate make it highly effective for large-scale industrial and environmental uses, particularly in road maintenance. Its strong hygroscopic nature is exploited in dust suppression, where it is applied to unpaved roads and construction sites. The compound draws moisture from the air, keeping the road surface damp and binding fine dust particles to the aggregate, which stabilizes the road and reduces airborne particulate matter.
As a de-icing agent, magnesium chloride is preferred over traditional rock salt (sodium chloride) for its effectiveness in colder conditions. It works by depressing the freezing point of water. Unlike rock salt, it is an exothermic compound, meaning it releases heat as it dissolves, which accelerates the melting process. This allows it to remain effective at temperatures as low as \(-13^{\circ}\text{F}\) (or \(-25^{\circ}\text{C}\)), whereas rock salt becomes largely ineffective below \(15^{\circ}\text{F}\) (or \(-9^{\circ}\text{C}\)).
In the construction industry, the hexahydrate is a primary component in the creation of magnesium oxychloride cement, often called Sorel cement. When mixed with magnesium oxide (\(\text{MgO}\)) and water, it undergoes a chemical reaction to form complex hydrates. This resulting cementitious material is known for its high strength, resistance to abrasion, and fire-retardant properties, making it suitable for industrial flooring and specialized paneling.
The compound is also utilized as an effective fire retardant, especially for wood and textiles. Its mechanism involves catalyzing the decomposition of the material at lower temperatures, promoting the formation of a carbon char instead of flammable gases. This char layer, along with the release of water vapor during heating, creates a protective barrier that shields the underlying material from the flame.
Role in Health and Nutrition
Magnesium chloride hexahydrate is a bioavailable source of magnesium, an element that acts as a cofactor in over 300 enzymatic reactions in the human body. As a dietary supplement, the compound is taken orally in capsules or solution to help correct magnesium deficiencies. Approximately one-third of the ingested magnesium is absorbed in the small intestine.
The compound is also the basis for “magnesium oil,” a topical application used for transdermal delivery of the mineral. Studies suggest that the magnesium ions are absorbed through the skin, with the hair follicles playing a significant role in this permeation. Topical application bypasses the digestive tract, which is advantageous for individuals who experience the common laxative effect associated with high oral doses of magnesium salts.
In food science, the hexahydrate is widely used as a coagulant in the production of tofu, where it is traditionally known by its Japanese name, Nigari. The \(\text{Mg}^{2+}\) ions cause the proteins in soy milk to denature and aggregate, forming the curds that are then pressed into tofu. Tofu made with this coagulant tends to have a fine, firm texture and a slightly higher mineral content.