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About Iron

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iron meteorite

Iron meteorite from Sikhote-Alin fall, Siberia. Half life size.

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Iron is serious stuff. It not only makes up the Earth's core, but it is the very base of civilization. Iron was long ago categorized as a base metal, as opposed to the noble metals like gold and silver, but in fact its fundamental role makes it a "base metal" in quite the other sense.

Only meteorites give us plain, solid iron, and they're very rare. Tools made from meteoritic iron are an expensive curiosity. To get useful amounts of metal requires ore and ways to smelt it. Earth is fortunate to have enormous bodies of iron ore, but they occur only in peculiar and extremely ancient settings. Whereas the human Iron Age began about 3000 years ago, a tiny fraction of our lifetime as a species, Earth's Iron Age lasted over two billion years.

The Precambrian Iron Age

Some 4 billion years ago, during the Eoarchean Era, the first blue-green algae or cyanobacteria went about their lives. This was a time when the atmosphere was a choking mix of nitrogen and carbon dioxide. In the shallow anoxic seas lapping upon the early continents, the cyanobacteria had the world to themselves, breathing carbon dioxide and making oxygen as a waste product. Ferrous iron in the water scavenged that poisonous waste, turned into solid minerals—magnetite and hematite—and settled in vast, thin layers.

For over 2 billion years this went on, until the iron ran out, and then the oxygen bubbled up into the atmosphere. The clear, oxygenated atmosphere we know dates from about 1700 million years ago, after two-thirds of Earth history. The vast layers of iron minerals stayed behind in rocks called banded iron formation, or BIF.

Iron Ores and Mining

BIFs are the algal legacy that we mine today. Nowadays, such iron deposits can form only in the airless bottoms of bogs in the form of hematite or goethite. "Bog ore" was the main source of iron in ancient and medieval times.

BIFs aren't very pretty, but in a few places this material has been altered into spectacular tiger iron. Old iron mines are rarely preserved, but there's a good example at the Soudan Mine in northern Minnesota, where the Mesabi and Vermilion Ranges have supported a great industry for well over a century. (That mine is being recycled as an underground physics laboratory, too.)

As the premium ores gave out and techniques arose to cope with lower grades, the iron industry changed into the dull, mammoth-scale undertakings in regions like Western Australia, the largest ore exporter in the eastern hemisphere. Now iron ore is a matter of geopolitics, as the former Soviet Union vies with the ore producers of the south—southern Africa, India, Australia and South America.

Today it may seem like there is no romance left in iron ore, just big industry and government agencies. But you can relive the days of iron mines and their iron men as you visit historical sites where old techniques and blast furnaces and ironworks from the 19th century are preserved. Visit the great ore ships that still ply the American Great Lakes, or ride the trains that carry the ore. And let us not forget the artisans for whom the industry began—the blacksmith in the village square.

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