Magnesium

Magnesium (Mg) is a silvery white metal that is similar in appearance to aluminum but weighs one-third less. With a density of only 1. 738 grams per cubic centimetre, it is the lightest structural metal known. It has a hexagonal close-packed (hcp) crystalline structure, so that, like most metals of this structure, it lacks ductility when worked at lower temperatures. In addition, in its pure form, it lacks sufficient strength for most structural applications.

However, the addition of alloying elements improves its properties to such an extent that both cast and wrought magnesium alloys are widely used, particularly where light weight and high strength are important. Magnesium is strongly reactive with oxygen at high temperatures; above 645 °C (1,190 °F) in dry air, it burns with a bright white light and intense heat. For this reason, magnesium powders are used in pyrotechnics. At room temperature, a stable film of water-insoluble magnesium hydroxide forms on the metal’s surface, protecting it from corrosion in most atmospheres. Being a strong reactant that forms stable compounds with chlorine, oxygen, and sulfur, magnesium has several metallurgical applications, such as in the production of titanium from titanium tetrachloride and in the desulfurization of blast-furnace iron.

Its chemical reactivity is also evident in the magnesium compounds that have wide application in industry, medicine, and agriculture. Magnesium derives its name from magnesite, a magnesium carbonate mineral, and this mineral in turn is said to owe its name to magnesite deposits found in Magnesia, a district in the ancient Greek region of Thessaly. The eighth most abundant element in nature, magnesium constitutes 2. 4 percent of Earth’s crust. Because of its strong reactivity, it does not occur in the native state, but rather it is found in a wide variety of compounds in seawater, brines, and rocks.

Among the ore minerals, the most common are the carbonates dolomite (a compound of magnesium and calcium carbonates, MgCO3·CaCO3) and magnesite (magnesium carbonate, MgCO3). Less common is the hydroxide mineral brucite, Mg(OH)2, and the halide mineral carnallite (a compound of magnesium and potassium chlorides and water, MgCl2·KCl·6H2O). Magnesium chloride is recoverable from naturally occurring brines such as the Great Salt Lake (typically containing 1. 1 percent by weight magnesium) and the Dead Sea (3. 4 percent), but by far the largest source is the oceans of the world.

Although seawater is only approximately 0. 13 percent magnesium, it represents an almost inexhaustible source. Both dolomite and magnesite are mined and concentrated by conventional methods. Carnallite is dug as ore or separated from other salt compounds that are brought to the surface by solution mining. Naturally occurring magnesium-containing brines are concentrated in large ponds by solar evaporation.