A radiometric dating method based on the radioactive decay of ⁸⁷Rb to ⁸⁷Sr. Rubidium has two isotopes (⁸⁵Rb 72.15%, ⁸⁷Rb 27.85%), but only ⁸⁷Rb is radioactive. ⁸⁷Rb disintegrates in a single step to ⁸⁷Sr by the emission of a low-energy beta particle (see beta decay). Unfortunately this low-energy disintegration makes it very difficult to assess the half-life (see decay constant) and two values (5.0×10¹⁰ years or 4.88×10¹⁰ years) have been in common use. When a mineral crystallizes, it will usually incorporate both rubidium and strontium ions and the ratio of Rb to Sr will vary depending on the mineral involved. This initial strontium in the mineral is known as ‘common strontium’ (see initial strontium ratio) and is normally in the proportion of ⁸⁸Sr 82.56%, ⁸⁷Sr 7.02%, ⁸⁶Sr 9.86%, and ⁸⁴Sr 0.56%. Using these proportions it is possible to identify the amount of radiogenic ⁸⁷Sr present. Originally the above proportions were assumed, but today it is more usual to plot ⁸⁷Sr:⁸⁶Sr against ⁸⁷Rb:⁸⁶Sr to produce a straight-line isochron from which the age of the mineral can be determined. When using the ⁸⁷Rb:⁸⁷Sr method it is customary to use whole-rock samples in the analysis, because although ⁸⁷Sr may leak from one mineral to adjacent minerals over time it usually remains in the system. Micas and potassium feldspars are the most suitable minerals for ⁸⁷Rb:⁸⁷Sr age determinations and the results can commonly be compared with ⁴⁰K:⁴⁰Ar age determinations from the same sample (see potassium–argon dating). The method has particularly been applied to ancient metamorphic rocks.