LANTHANIDE TETRAD EFFECT IN KIMURAITE [CAY2(CO3)4.6H2O] - IMPLICATION FOR A NEW GEOCHEMICAL INDEX

Close analysis of REEs in a REE carbonate mineral, kimuraite [CaY2(CO3)4 . 6H2O], has been conducted using isotope dilution-mass spectrometry (ID-MS) and inductively coupled plasma-mass spectrometry (ICP-MS). A standard solution whose REE composition matches kimuraite was used for the ICP-MS measurement. The resultant chondrite-normalized REE pattern revealed four distinct concave-upward segments, each segment covering four elements. The features of the patterns are characteristic of the lanthanide tetrad effect, which is theoretically understood in terms of the interelectron repulsion of electrons of 4f orbitals and has been observed prominently in the chemistry of lanthanide chelates. It is undeniable that the REE pattern of kimuraite very clearly reflects the tetrad effect. This conclusion will influence, in due course, the interpretation of the complicated REE patterns of a wide variety of other natural samples, such as seawater and marine products. The tetrad effect could be a unique geochemical index of chelating phenomena in nature.