Magnesium is a chemical element with symbol Mg and atomic number 12. Its common oxidation number is +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole.
The name magnesium originates from the Greek word for a district in Thessaly called Magnesia. It is related to magnetite and manganese, which also originated from this area, and required differentiation as separate substances.
Magnesium is the fourth most common element in the Earth as a whole (behind iron, oxygen and silicon), making up 13% of the planet's mass and a large fraction of the planet's mantle. The relative abundance of magnesium is related to the fact that it is easily built up in supernova stars from a sequential addition of three helium nuclei to carbon (which in turn is made from three helium nuclei). Due to magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater.
The free element (metal) is not found naturally on Earth, as it is highly reactive (though once produced, it is coated in a thin layer of oxide, which partly masks this reactivity). The free metal burns with a characteristic brilliant white light, making it a useful ingredient in flares. The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. Commercially, the chief use for the metal is as an alloying agent to make aluminium-magnesium alloys, sometimes called "magnalium" or "magnelium". Since magnesium is less dense than aluminium, these alloys are prized for their relative lightness and strength.
The metal itself was first produced by Sir Humphry Davy in England in 1808 using electrolysis of a mixture of magnesia and mercuric oxide. Antoine Bussy prepared it in coherent form in 1831. Davy's first suggestion for a name was magnium, but the name magnesium is now used.
In human biology, magnesium is the eleventh most abundant element by mass in the human body; its ions are essential to all living cells, where they play a major role in manipulating important biological polyphosphate compounds like ATP, DNA, and RNA. Hundreds of enzymes thus require magnesium ions to function. Magnesium compounds are used medicinally as common laxatives, antacids (e.g., milk of magnesia), and in a number of situations where stabilization of abnormal nerve excitation and blood vessel spasm is required (e.g., to treat eclampsia). Magnesium ions are sour to the taste, and in low concentrations they help to impart a natural tartness to fresh mineral waters.
In vegetation magnesium is the metallic ion at the center of chlorophyll, and is thus a common additive to fertilizers.
Magnesium has a specific gravity of approximately 1.7; its alloys are the lightest of all engineering metals employed. The melting point of magnesium is 650° C; its boiling point is 1007° C. Magnesium is very inflammable and burns with a dazzlingflame, developing a great deal of heat.
The mechanical properties of magnesium, especially the tensile strength, are very low and therefore pure magnesium is not employed in engineering.
The alloys of magnesium possess much better mechanical properties which ensure their wide application. The principal alloying elements in magnesium alloys are aluminium, zinc and manganese. Aluminium, added in amounts up to 11 per cent, increases the hardness, tensile strength and fluidity of the alloy. Up to 2 per cent zinc is added to improve the ductility (relative elongation) and castability. The addition of 0.1-0.5 per cent manganese raises the corrosion resistance of magnesium alloys. Small additions of cerium, zirconium and beryllium enable a fine-grained structure to be obtained; they also increase the ductility and oxidation resistance of the alloys at elevated temperatures.
Magnesium alloys are classified into two groups: wrought alloys, grades MA1, MA2, casting alloys, grades MЛ4, MЛ5.
Wrought magnesium alloys MA1 and MA2 are chiefly used for hot smith and closed-die forged machine pants. They are less frequently used as sheets, tubing or bar stock.
Magnesium casting alloys MЛ4 and MЛ5 are widely used as foundry material though their castability is inferior to that of aluminium-base alloys.
The United States has traditionally been the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Today, the US market share is at 7%, with a single domestic producer left, US Magnesium, a Renco Group company in Utah born from now-defunct Magcorp.
As of 2005, China has taken over as the dominant supplier, pegged at 60% world market share, which increased from 4% in 1995. Unlike the above-described electrolytic process, China is almost completely reliant on a different method of obtaining the metal from its ores, the silicothermic Pidgeon process (the reduction of the oxide at high temperatures with silicon).
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