Iron—measured by mass—is the most abundant element in the earth. It forms much of our planet’s core. In the crust iron is the fourth-most abundant after oxygen, silicon, and aluminum.
Like most metals iron is not found in its metallic state, except for the occasional meteorite. Crustal iron is almost entirely oxides. All of us are familiar with rust. The chief ores of iron are the oxides hematite (Fe2O3) and magnetite (Fe3O4).
The symbol for iron is Fe from the Latin word ferrum. The pigment Prussian blue is created by the oxidation of ferrous ferrocyanide to the ferricyanide form. Ferro- means iron in the +2 oxidation state and ferri- means iron in the +3 oxidation state. Nowadays they use the old prefixes less and less and instead call Fe 2+ Iron(II) and Fe3+ Iron(III). You can see (if you remember your high school chemistry) that hematite is an iron(III) compound and is sometimes called ferric oxide. (Hint: oxygen is a -2 ion!) Magnetite is an iron(II)/iron(III) mix and is sometimes called ferrous-ferric oxide.
Iron is the most important commodity in the global economy except for perhaps oil. Iron is used to make steel and the modern world is built with steel. 98% of all the iron ore mined in the world goes into steel-making.
Iron ores are hard to smelt. You need a furnace or kiln that can get to 1500 degrees Celsius (2700 ºF). This is about 500 ºC (900 ºF) higher than copper. This is why the so-called Bronze Age happened first. Bronze is an alloy of copper and tin. Iron working didn’t emerge until about two thousand years before Christ.
In the modern world iron ores are heated in blast furnaces with coke (charcoal made from coal) to produce a high-carbon alloy called pig iron. This is further refined to reduce the carbon content to make cast iron. Further purification results in steel. There are many dozens of varieties of steel. Small amounts of elements like chromium are added to improve strength and corrosion resistance.
Steel-making is very energy intensive and is one of the biggest contributors to global greenhouse gas emissions. On the flip side, steel is one of the most recycled commodities in the global economy. Scrap steel is heated in an electric arc furnace to remove impurities so that alloying materials can then be added to the batch and new steel formed.
Iron is also biologically interesting. An adult human body has about four grams of iron in it. Hemoglobin and myoglobin both contain iron. Hemoglobin is necessary to transport oxygen in the blood. Myoglobin is found in the muscles and is also important in oxygen metabolism. Whales for example have a lot of myoglobin in their muscles. This allows them to function without breathing for long periods. Myoglobin contains iron compounds called “hemes” that give red meat its color.
Iron and its primary alloy, steel, are so ubiquitous that we probably don’t appreciate them much. We just go to work in our steel cars, cook in our steel pots, eat with our steel cutlery, garden with our steel tools, and watch the steel cranes unload the steel containers from the steel ships that carry everything across the world.