An interesting piece on One Species at a Time featured NSF-funded Microbiologist David Emerson who studies microbes that feed on iron. Working at the Bigelow Laboratory for Ocean Sciences in Maine, Emerson studies Leptothrix lucretia and its potential for filtering water and other uses. However, the piece got us to thinking about iron and its many crystal forms, making it perfect for our Crystal of the Week in celebration of International Year of Crystallography.
Modern materials engineers carefully manipulate iron for a variety of applications, ranging from ship hulls to steel foundations in skyscrapers to magnets to cookware. But the history of iron engineering actually dates back thousands of years to the earliest civilizations, and before engineers used iron, some of the earliest life forms on the planet developed metabolic systems that relied on iron for energy.
Like other transition metals, iron can exist in its metallic, neutral-charge state, or iron can undergo reduction and oxidation (“redox”) reactions, where electrons are gained or lost, to attain new charge states. Changes in the electronic structure of iron dictate how it bonds with other elements, such as oxygen. For example, iron (II), where the iron atom has lost two electrons, readily forms black FeO. If oxidized further, iron (III) can combine with oxygen to create Fe2O3, otherwise known as hematite, which is not only the color of iron rust but also chemically related. Another familiar iron oxide is magnetite (Fe3O4), which is used in magnets (pictured above). The chemical structure of this mineral is complex, based on a combination of iron (II) and iron (III), which disrupts electronic pairing within the structure, giving rise to its magnetic properties.
These minerals have a lot more in common than just their chemistry. These iron oxides, and many other iron hydrides and oxyhydroxides, are regularly used as basic pigments in everything from plastic to paint. In fact, you’re probably already familiar with some iron oxide-based pigments, such as “iron black” and “burnt sienna.” And if you’ve ever wondered how mascara can make eyelashes appear longer – think magnetite!
The family of iron oxides, however, represents only a small fraction of the family of iron-based compounds. Most iron used by engineers is in the form of steel, where different elements are added in varying amounts to alter the mechanical properties of the steel, as well as its susceptibility to oxidation. And let us not forget that iron is the fourth most abundant element in Earth’s crust, with everything beneath being molten iron and nickel (another alloy of iron!).