The distribution of lithium, rubidium and caesium in igneous and sedimentary rocks

Abstract

Lithium, rubidium and caesium have been determined on 186 samples and rubidium and caesium on an additional 53 samples, representing sedimentary and igneous rocks from geographically diverse areas and ranging in age from Pre-Cambrian to recent. Determinations were made by a flame photometric method. Lithium is closely related to magnesium and ferrous iron with a Mg/Li ratio varying from 2900 in ultramafic rocks to 120 in granites. Averages of 29 p.p.m. and 53 p.p.m. Li are reported for igneous and sedimentary rocks, respectively. Lithium is largely retained in the products of weathering because of concentration in clay minerals. This concentration is responsible for the high values obtained for shales. Impurities account for the greater part of the lithium content of carbonate rocks and sandstones. The ratio K/Rb is statistically constant at 270 in igneous rocks. Averages of 120 p.p.m. for igneous rocks and of 110 p.p.m. Rb for sedimentary rocks are reported. These values are approximately a third of those previously accepted. Rubidium is retained in weathering largely by those clays having a structural position for the ion, although adsorption plays an important part in both clays and shales. The bulk of the rubidium present in igneous rocks is, however, lost in solution during weathering. Diagenetic recrystallization of illite to orthoclase results in exclusion of rubidium. Like rubidium, caesium is related to potassium. Igneous rocks have an average ratio K/Cs of 7000. The average abundance of caesium in igneous rocks is less than 1 p.p.m. Cs. Sedimentary rocks average 4 p.p.m. Caesium reacts similarly to rubidium in the geochemical cycle.