Magnesium, << mag NEE shee uhm or mag NEE zhee uhm, >> a silver-white metal, is the lightest metal that is strong enough to use in construction. It weighs only about two-thirds as much as aluminum, another widely used light metal. Magnesium was discovered in 1808 by the English chemist Sir Humphry Davy.
Magnesium is a fairly abundant metallic element. But pure magnesium does not occur in nature. Various minerals contain magnesium compounds. A few of these compounds, primarily magnesium chloride and magnesium sulfate, occur in dissolved form in seawater and in some pools of underground water. Seawater contains 0.13 percent magnesium and provides a practically unlimited source of the metal. Magnesium occurs in the minerals magnesite, brucite, and dolomite, as well as in amphibole asbestos, olivine, serpentine, talc, and in certain of the other silicate minerals (see Silicate ).
Magnesium plays a vital role in the life processes of plants and animals. Chlorophyll, which green plants use in photosynthesis, contains magnesium. Plants produce carbohydrates, a class of foods essential to living things, by means of photosynthesis. Magnesium takes part in the duplication of DNA (the genetic material of life) and in the production of proteins guided by the molecule RNA. Magnesium also activates many of the enzymes that speed up chemical reactions in the human body.
Uses.
Magnesium and its alloys are used in manufacturing many products. Their light weight makes them suitable for aircraft and automobile parts and for tools and equipment. Most magnesium alloys contain aluminum and zinc. These materials make magnesium alloys stronger and easier to shape. Some alloys may also contain small amounts of such elements as manganese, thorium, and zirconium that provide other properties.
Magnesium is used for a variety of nonstructural purposes because it is extremely active chemically. For example, pieces of magnesium are placed next to buried steel pipelines and water tanks. If magnesium were not present, oxygen and other chemicals in the earth would corrode the steel. Instead, the magnesium reacts with the chemicals. The pieces of magnesium can easily be replaced periodically at a cost much lower than that of replacing or repairing the steel. Protective strips of magnesium are also attached to the hulls of ships.
Steel manufacturers add magnesium to steel to remove sulfur and other impurities. In addition, magnesium is used in fireworks and flares because it burns with a brilliant white light. It also produces intense heat when it burns, making it useful for incendiary bombs.
Magnesium combines with other elements to form many useful compounds. These compounds include two commonly used medicines—milk of magnesia and Epsom salt (see Magnesia ). Magnesium oxide resists heat and is used to line special types of furnaces. Also, magnesium oxide forms on the surface of magnesium metal and prevents it from corroding readily at low temperatures. If it were not for this protective layer, magnesium would not be a suitable structural material. Magnesium chloride is an important catalyst (substance that speeds up a chemical reaction) in the preparation of organic compounds. Other magnesium compounds are used in tanning leather; in dyeing textiles; and in making cement, fertilizer, and insulating materials.
Properties.
Magnesium’s chemical symbol is Mg. It has a density of 1.738 grams per cubic centimeter at 20 °C. Its atomic number (number of protons in its nucleus) is 12. Its relative atomic mass is 24.3050. An element’s relative atomic mass equals its mass (amount of matter) divided by 1/12 of the mass of carbon 12, the most abundant form of carbon. Magnesium melts at 650 °C and boils at 1090 °C. Chemists classify magnesium as an alkaline earth metal . For information on the position of magnesium on the periodic table, see the article Periodic table .
Magnesium never occurs in nature as a pure metal because it is so active chemically. It readily combines with most acids and with many nonmetals, including nitrogen. When heated with the salts or oxides of many metals, magnesium replaces the other metal. In this process, called reduction, the magnesium purifies the other metal, preparing it for various uses.
How magnesium is obtained.
Two major methods are used to obtain magnesium. In the Dow process, magnesium is recovered from seawater. Lime obtained from the rock dolomite is mixed with seawater, which contains magnesium chloride. The lime and the magnesium chloride react to form magnesium hydroxide and calcium chloride. The magnesium hydroxide separates from the rest of the mixture. It is filtered out and mixed with hydrochloric acid, forming magnesium chloride and water. The water evaporates, leaving highly concentrated magnesium chloride, which is melted by being heated to a temperature above 708 °C. Then an electric current is passed through the melted compound. This kind of process is called electrolysis (see Electrolysis ). The current changes the magnesium chloride into magnesium and chlorine gas. The molten magnesium is poured off and cast into forms called ingots.
The other method of refining magnesium is known as the ferrosilicon process or Pidgeon process. This method, used primarily in Canada, involves heating dolomite in a vacuum with an alloy consisting of silicon and iron. The magnesium in the dolomite vaporizes, and then it condenses as crystals. The magnesium crystals are melted and cast into ingots.
