- © 2017 Mineralogical Society of America
Iron is a ubiquitous element with a rich (i.e., complex) chemical behavior. It possesses three oxidation states, metallic iron (Fe0), ferrous iron (Fe2+) and ferric iron (Fe3+). The distribution of these oxidation states is markedly stratified in the Earth.
Metallic iron is primarily present in the core, where it is alloyed with Ni, Co, and light elements such as S, Si or O. Some metallic iron may be present at depth in the mantle because Fe2+ in bridgmanite can disproportionate into Fe3+ and Fe0 (Frost et al. 2004). Natural metallic iron also exists at the surface of the Earth in rare occurrences in the form of meteorite falls, metallic iron produced by reduction of lavas through interaction with coal sediments as in Disko Island (Greenland), and Josephinite (awaruite) produced by serpentinization reaction in peridotites.
The main repository of ferrous iron is the mantle, where it is present in two spin states (the manner in which electrons fill the orbitals). At low pressure, mantle minerals contain iron in a high spin electronic state. Under the high-pressure conditions of the lower mantle, iron transitions into a low-spin electronic state (Badro et al. 2003; Lin et al. 2013). This spin transition influences the physical, chemical, and rheological properties of minerals.
The lower mantle presumably contains significant Fe3+ produced by Fe2+ disproportionation (Frost et al. 2004). At the pressures relevant to the upper mantle, iron is not disproportionated, yet Fe3+ represents ~3% of total iron (Canil et al. 1994). Ferric iron is much more common in surface oxygenated environments and in crustal rocks due to the presence of an oxygen-rich atmosphere. Iron is an essential micronutrient and the low solubility of Fe3+ in seawater has a significant influence on biomass productivity in the …