Planetary occurrence
Iron meteorites, similar in composition to the Earth's inner- and outer core
Iron is the sixth most abundant element in the Universe, and the most common refractory element.[19] It is formed as the final exothermic stage of stellar nucleosynthesis, by silicon fusion in massive stars.
Metallic or native iron is rarely found on the surface of the Earth because it tends to oxidize, but its oxides are pervasive and represent the primary ores. While it makes up about 5% of the Earth's crust, both the Earth's inner and outer core are believed to consist largely of an iron-nickel alloy constituting 35% of the mass of the Earth as a whole. Iron is consequently the most abundant element on Earth, but only the fourth most abundant element in the Earth's crust.