- a method for estimating the age of a mineral or rock, based on measurement of the rate of decay of radioactive potassium into argon.
- a technique for determining the age of minerals based on the occurrence in natural potassium of a small fixed amount of radioisotope 40 K that decays to the stable argon isotope 40 Ar with a half-life of 1.28 × 10 9 years. Measurement of the ratio of these isotopes thus gives the age of the mineral
