Indeed, given the universal law of radioactive decay and the following rubidium beta decay : <chem>^{87}_{37}Rb ->[{\beta -}][{}]~^{87}_{38}Sr ~+e^{-}~+~\bar{\nu_e}</chem> we obtain the expression whom describes the growth of Strontium 87 in the rubidium mineral : <chem>^{87}_{38}Sr(t)~=^{87}_{38}Sr(0)~+~^{87}_{37}Rb(e^{\lambda t}-1)</chem>, <chem>\lambda</chem> being the decay constant of rubidium. Furthermore, we could consider the number of <chem>^{86}_{38}Sr</chem> as a constant, for two reasons; firstly this isotope is stable, and secondly the half-time for <chem>^{86}_{37}Rb</chem> (whom produced the <chem>^{86}_{38}Sr</chem> radiogenic nucleide) is negligible compared to <chem>^{87}_{37}Rb</chem> half-time.