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Isotopes of potassiumPotassium (K) has 24 known isotopes. Three isotopes occur naturally: ^{39}K (93.3%), ^{40}K (0.012%) and ^{41}K (6.7%). The standard atomic mass is 39.0983(1) u. Naturally occurring ^{40}K decays to stable ^{40}Ar (11.2%) by electron capture and by positron emission, and decays to stable ^{40}Ca (88.8%) by beta decay; ^{40}K has a halflife of 1.250×10^{9} years. The decay of ^{40}K to ^{40}Ar enables a commonly used method for dating rocks. The conventional KAr dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., ^{40}Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic ^{40}Ar that has accumulated. The minerals that are best suited for dating include biotite, muscovite, plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. Additional recommended knowledgeOutside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also been used for nutrient cycling studies because potassium is a macronutrient required for life. ^{40}K occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations. In healthy animals and people, ^{40}K represents the largest source of radioactivity, greater even than ^{14}C. In a human body of 70 kg mass, about 4,400 nuclei of ^{40}K decay per second.[1] Table
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Isotopes_of_potassium". A list of authors is available in Wikipedia. 