Periodic Table--Potassium

Potassium has three naturally occurring isotopes: ³⁹K (93.3%), ⁴⁰K (0.01%) and ⁴¹K (6.7%). Naturally occurring ⁴⁰K decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and decays to stable ⁴⁰Ca (88.8%) by negatron emission; ⁴⁰K has a half-life of 1.250 x 10⁹ years. The decay of ⁴⁰K to ⁴⁰Ar is commonly used as a 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., ⁴⁰Ar) was quantitatively retained, i.e., closed system. Minerals are dated by measurement of the concentration of potassium, and the amount of radiogenic ⁴⁰Ar that has accumulated. The minerals that are best suited for dating include biotite, muscovite, and 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 (Faure, 1986). For a discussion of K-Ar dating see Dalrymple and Lanphere (1969), Faure (1986), and Dicken (1995).

Outside of dating, K isotopes have been used extensively in studies of weathering (Moldan and Cerny, 1993); K isotopes have also be used for nutrient cycling studies because K is a macro-nutrient required for life.

Source of text: This review was assembled by Dan Snyder, primarily from Faure (1986).