Rare earths and other trace elements in cretaceous clays from central Portugal

Abstract

Eight rare-earth elements (REE), namely La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu, as well as other elements (Na, K, Sc, Cr, Fe, Co, Rb, Cs, Ba, Hf, Ta, and Th), have been determined in fifteen cretaceous clay samples of continental facies by instrumental neutron activation analysis. It was found that the REE contents are variable in absolute and relative values, but the means of these values are similar to those of European, shales. Analyses have also been made of the fractions <38 μm and <2 μm (clay-sized). The mineral contents of the clay-sized fraction were determined semi-quantitatively by X-ray diffraction. The results suggest the preferential presence of REE, Hf, and Th in fractions 2–38 μm, which can be explained by the presence of apatite, monazite, and zircon. A correlation study of chemical and mineralogical data of the clay-sized fraction showed that kaolinite is correlated with REE, specially the lighter ones; illite with K, Rb, and Cs; and smectite with Na.