Hahn and Fritz Strassmann, who later went on to discover nuclear fission in December 1938. Example For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar, K-feldspar, hornblende, biotite, and muscovite. Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed. Rubidium substitutes for potassium within the lattice of minerals at a rate proportional to its concentration within the melt. The ideal scenario according to Bowen's reaction series would see a granite melt begin crystallizing a cumulate assemblage of plagioclase and hornblende (i.e.; tonalite or diorite), which is low in K (and hence Rb) but high in Sr (as this substitutes for Ca), which proportionally enriches the melt in K and Rb. This then causes orthoclase and biotite, both K rich minerals into which Rb can substitute, to precipitate. The resulting Rb-Sr ratios and Rb and Sr abundances of both the whole rocks and their component minerals will be markedly different. This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses. Calculating the age The age of a sample is determined by analysing several minerals within multiple subsamples from different parts of the original sample. The 87Sr/86Sr ratio for each subsample is plotted against its 87Rb/86Sr ratio on a graph called an isochron. If these form a straight line then the subsamples are consistent, and the age probably reliable. The slope of the line dictates the age of the sample. Given the universal law of radioactive decay and the following rubidium beta decay: ^{87}_{37}Rb ->[{\beta^-}]~^{87}_{38}Sr ~+e^-\ + \bar{\nu}_e, we obtain the expression which describes the growth of Strontium-87 from the Rubidium-87 decay:\lambda being the decay constant of rubidium. Furthermore, we consider the number of ^{86}_{38}Sr as a constant, since it is stable and not radiogenic. Hence, is the isochron equation. After measurements of Rubidum and Strontium concentration in the mineral we can easily determine the age, the t value, of the sample. Sources of error Rb-Sr dating relies on correctly measuring the Rb-Sr ratio of a mineral or whole rock sample, plus deriving an accurate 87Sr/86Sr ratio for the mineral or whole rock sample. Several preconditions must be satisfied before a Rb-Sr date can