Isotopes of aluminium
Aluminium or "aluminum" (Al) has 22 known isotopes from Al to Al and 4 known isomers. Only Al (stable isotope) and Al (radioactive isotope, t = ) occur naturally, however Al comprises nearly all natural aluminium. Other than Al, all radioisotopes have half-lives under 7 minutes, most under a second. The standard atomic weight is . Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of Al to Be has  been used to study the role of sediment transport, deposition, and storage, as well as burial times, and erosion, on 10 to 10 year time scales. Al has also played a significant role in the study of meteorites. Aluminium-26. Cosmogenic aluminium-26 was first applied in studies of the Moon and meteorites. Meteorite fragments, after departure from their parent bodies, are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial Al production. After falling to Earth, atmospheric shielding protects the meteorite fragments from further Al production, and its decay can then be used to determine the meteorite 's terrestrial age. Meteorite research has also shown that Al was relatively abundant at the time of formation of our planetary system. Most meteoriticists believe that the energy released by the decay of Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.