There are 25 known isotopes of potassium, three of which occur naturally: 39 K (93.3%), 40 K (0.0117%), and 41 K (6.7%) (by mole fraction). Naturally occurring 40 K has a half-life of 1.250×109 years. It decays to stable 40 Ar by electron capture or positron emission (11.2%) or to stable 40 Ca by beta decay (88.8%). The decay of 40 K to 40 Ar is the basis of a common 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 (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 best suited for dating include biotite, muscovite, metamorphichornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. Apart from dating, potassium isotopes have been used as tracers in studies of weathering and for nutrient cycling studies because potassium is a macronutrient required for life on Earth.