Other than 87Rb, the longest-lived radioisotopes are 83Rb with a half-life of 86.2 days, 84Rb with a half-life of 33.1 days, and 86Rb with a half-life of 18.642 days. See rubidium–strontium dating for a more detailed discussion. The dates indicate the true age of the minerals only if the rocks have not been subsequently altered. If the initial amount of Sr is known or can be extrapolated, the age can be determined by measurement of the Rb and Sr concentrations and the 87Sr/ 86Sr ratio.
The highest ratios (10 or higher) occur in pegmatites. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. 87Rb has been used extensively in dating rocks 87Rb decays to stable strontium-87 by emission of a negative beta particle, i.e. It readily substitutes for potassium in minerals, and is therefore fairly widespread.
Normal mixes of rubidium are radioactive enough to fog photographic film in approximately 30 to 60 days.Ĩ7Rb has a half-life of 4.92 ×10 10 years. Rubidium ( 37Rb) has 36 isotopes, with naturally occurring rubidium being composed of just two isotopes 85Rb (72.2%) and the radioactive 87Rb (27.8%).
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