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Naturally occurring manganese is composed of one stable isotope, Mn. 25 radioisotopes have been characterized, with the most stable being Mn with a half-life of 3.7 million years, Mn with a half-life of 312.3 days, and Mn with a half-life of 5.591 days. All of the remaining radioactive isotopes have half-lives that are less than 3 hours and the majority of these have half-lives that are less than a minute, but only Mn has an unknown half-life. The least stable is Mn with a half-life shorter than 105 nanoseconds. This element also has 3 meta states.
Manganese is part of the iron group of elements, which are thought to be synthesized in large stars shortly before supernova explosions. Mn decays to Cr with a half-life of 3.7 million years. Because of its relatively short half-life, Mn occurs only in tiny amounts due to the action of cosmic rays on iron in rocks. Manganese isotopic contents are typically combined with chromium isotopic contents and have found application in isotope geology and radiometric dating. Mn−Cr isotopic ratios reinforce the evidence from Al and Pd for the early history of the Solar System. Variations in Cr/Cr and Mn/Cr ratios from several meteorites indicate an initial Mn/Mn ratio that suggests Mn−Cr isotopic systematics must result from in-situ decay of Mn in differentiated planetary bodies. Hence Mn provides additional evidence for nucleosynthetic processes immediately before coalescence of the Solar System.
The isotopes of manganese range in atomic weight from 44 u to 69 u. The primary decay mode before the most abundant stable isotope, Mn, is electron capture and the primary mode after is beta decay.