An element with an atomic number of 19. The ⁴⁰K isotope is radioactive, decaying with the emission of a single gamma ray of 1. 46 MeV with a half-life of 1. 3 * 10⁹ years to give a stable isotope of argon. Potassium is the largest source of natural radioactivity. It occurs in illite, alkali feldspars, micas and some evaporite minerals. It also occurs in some drilling mud systems. The ⁴⁰K isotope is only a small fraction, about 0. 012%, of the total potassium, the main isotope being ³⁹K, which has an abundance of about 1. 7% in the Earth's crust. For the purposes of logging, the total potassium is calculated from the measured quantity of ⁴⁰K and scaled in percent by weight. It is a valuable aid in determining the mineral content of a formation.