Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron oxides and oxyhydroxides are widespread in nature and play an important role in many geological and biological processes. They are used as iron ores, pigments, catalysts, and in thermite, and occur in hemoglobin. Iron oxides are inexpensive and durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy" end of the yellow/orange/red/brown/black range. When used as a food coloring, it has E number E172. Iron oxides feature as ferrous (Fe(II)) or ferric (Fe(III)) or both. They adopt octahedral or tetrahedral coordination geometry. Only a few oxides are significant at the earth's surface, particularly wüstite, magnetite, and hematite. Oxides of FeII FeO: iron(II) oxide, wüstite Mixed oxides of FeII and FeIII Fe3O4: Iron(II,III) oxide, magnetite Fe4O5 Fe5O6 Fe5O7 Fe25O32 Fe13O19 Oxide of FeIII Fe2O3: iron(III) oxide α-Fe2O3: alpha phase, hematite β-Fe2O3: beta phase γ-Fe2O3: gamma phase, maghemite ε-Fe2O3: epsilon phase iron(III) oxide-hydroxide goethite (α-FeOOH), akaganéite (β-FeOOH), lepidocrocite (γ-FeOOH), feroxyhyte (δ-FeOOH), ferrihydrite (Fe5HO8 · 4 H2O approx., or 5 Fe2O3 · 9 H2O, better recast as FeOOH · 0.4 H2O) high-pressure pyrite-structured FeOOH. Once dehydration is triggered, this phase may form FeO2Hx (0 < x < 1). green rust (Fe_IIIFe_IIOH3x + y − z (A−)z where A− is Cl− or 0.5 ) In blast furnaces and related factories, iron oxides are converted to the metal. Typical reducing agents are various forms of carbon. A representative reaction starts with ferric oxide: Iron is stored in many organisms in the form of ferritin, which is a ferrous oxide encased in a solubilizing protein sheath. Species of bacteria, including Shewanella oneidensis, Geobacter sulfurreducens and Geobacter metallireducens, use iron oxides as terminal electron acceptors.