Potassium

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Isotopes

There are seventeen known isotopes of potassium. Three isotopes occur naturally: K-39 (93.3%), K-40 (0.01%) and K-41 (6.7%). Naturally occurring K-40 decays to stable Ar-40 (11.2%) by electron capture and by positron emission, and decays to stable Ca-40 (88.8%) by beta decay; K-40 has a half-life of 1.250 × 109 years.

Related Topics:
Isotope - Ar-40 - Electron capture - Positron emission - Ca-40 - Beta decay - Half-life

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The decay of K-40 to Ar-40 enables a commonly used method for dating rocks. The conventional K-Ar 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., Ar-40) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic Ar-40 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.

Related Topics:
Mineral - Biotite - Muscovite - Hornblende - Feldspar

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Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also be used for nutrient cycling studies because potassium is a macro-nutrient required for life.

Related Topics:
Tracer - Weathering - Nutrient cycling - Macro-nutrient - Life

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K-40 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.

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