Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history.Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.In adult human males are some 3.8 grams of iron, and 2.3 grams in females, for whom iron is distributed in hemoglobin and throughout the body.Its abundance in rocky planets like Earth is due to its abundant production by fusion in high-mass stars, where it is the last element to be produced with release of energy before the violent collapse of a supernova, which scatters the iron into space.
It is by mass the most common element on Earth, forming much of Earth's outer and inner core.
The inner core of the Earth is generally presumed to be an iron-nickel alloy with ε (or β) structure.
Somewhat confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.
At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.
Finally, at 770 °C (the Curie point, T) iron's magnetic ordering changes from paramagnetic to ferromagnetic.