Natural iron (26Fe) consists of four stable isotopes: 5.845% 54Fe (possibly radioactive with half-life >4.4×1020 years),[4] 91.754% 56Fe, 2.119% 57Fe and 0.286% 58Fe. There are 28 known radioisotopes and 8 nuclear isomers, the most stable of which are 60Fe (half-life 2.6 million years) and 55Fe (half-life 2.7 years).
Much of the past work on measuring the isotopic composition of iron has centered on determining 60Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, though applications to biological and industrial systems are beginning to emerge.[5]
^Bikit, I.; Krmar, M.; Slivka, J.; Vesković, M.; Čonkić, Lj.; Aničin, I. (1998). "New results on the double β decay of iron". Physical Review C. 58 (4): 2566–2567. Bibcode:1998PhRvC..58.2566B. doi:10.1103/PhysRevC.58.2566.