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15 GPa before transforming into a high-pressure form, as described in the next section.
Iron represents an example of allotropy in a metal.
Fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust.
Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen and water.At least four allotropic forms of iron are known as α, γ, δ, and ε; at very high pressures and temperatures, some controversial experimental evidence exists for a stable β phase.As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a body-centered cubic (bcc) crystal structure.Further refinement with oxygen reduces the carbon content to the correct proportion to make steel.Steels and iron alloys formed with other metals (alloy steels) are by far the most common industrial metals because they have a great range of desirable properties and iron-bearing rock is abundant. Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores.