Periodic Table--Rubidium

Rubidium has two naturally occurring isotopes, ⁸⁵Rb (72.2%) and ⁸⁷Rb (27.8%). ⁸⁷Rb is radioactive, with a half-life of 48.8 x 10⁹ years. It readily substitutes for K in minerals, and is therefore fairly widespread. Rb has been used extensively in dating rocks; ⁸⁷Rb decays to stable ⁸⁷Sr by emission of a negative beta particle. During fractional crystallization, Sr tends to be come concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (10 or higher) occur in pegmatites. 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 dates indicate the true age of the minerals only if the rocks have not been subsequently altered. See Strontium for a more detailed discussion.

Rb has seen scant use in hydrology because of its ready substitution in minerals and because there is little chance of uptake by the biota. Furthermore, the half-life is far too long for any practical hydrologic application.

Source of text: This review was assembled by Dan Snyder from the references below.