Iron is the fourth most common metal in Earth's crust and forms the basis for much of modern industry. Iron is less common in other planets and in stars. It is the tenth most common element in the universe. Its atomic symbol is Fe, from the Latin word ferrum.
Iron in nature
Iron occurs naturally in combination with oxygen in the metal ores hematite, limonite, and magnetite. These minerals have an iron content of about 50 percent. Since high-grade ores such as these have largely been exhausted, the use of lower-grade ores such as taconite (with 20 to 40 percent iron content) has increased. In addition, recycled iron and steel have become essential raw materials in steel production.
Iron in the blood
Iron is also an essential constituent of nearly all organisms. It occurs most commonly in compounds known as metalloproteins, complex structures in which an iron atom is buried at the center of the molecule. The most common metalloprotein is heme, which occurs in a number of enzymes and in oxygen-carrying substances such as hemoglobin and myoglobin. The red color of blood is dues to the iron in hemoglobin. When blood circulates through the lungs, it picks up oxygen, with combines with iron in heme to form oxyhemoglobin. When blood travels to cells, oxyhemoglobin breaks down, releasing oxygen to cells.
Primary sources of iron in the diet are meats, especially liver and nuts and legumes (beans and peas). Discovery of this dietary element was made by American physician and medical researcher George Whipple (1878-1976) in 1925 during his research on the human liver.
Iron in industry
As a metal, iron has been in use since before recorded history. Historians believe that humans first made iron tools in about 4000 BC, using iron extracted from meteorites that had landed on Earth. The Iron Age eventually spread across Europe and into China. Early people learned about iron either by heating it or while fashioning tools from stone that contained iron. Later, forges were created to smelt or separate the metal from the ore and to eliminate the carbon and other impurities before working it into the desired shapes.
Over time, more sophisticated iron production methods were invented. Iron production began in earnest with English inventor Abraham Darby's (1678-1717) coke-burning furnaces, which yielded iron in commercial quantities and made it affordable for industry. Because of this invention, Darby is considered by some historians to be the father of the Industrial Revolution (England, 1760-1870).
Pig iron is extracted from iron ore in blast furnaces. It contains about five percent impurities, mostly carbon. Most pig iron is used for steelmaking. It was formerly used to make cast iron, but now cast iron accounts for only about eight percent of pig iron used.
Cast iron is noted for its strength and its ability to be easily formed, or cast, into shapes. Cast iron was one of the most popular building materials during the 1800s. It is used today for engine blocks, sewer lids, cookware, and fire hydrants.
Wrought iron contains slag, the remains left from the reduction process. It is stronger than cast iron, but is less easily formed. In the past, it has been used for fences, railings, and farm implements, but it is rarely used today.
When iron is alloyed (mixed) with varying amounts of carbon and other metals, a wide range of grades and types of steels are the result. The conversion process invented by English inventor Henry Bessemer (1813-1898) in 1856 made steel affordable.