THE ADSORPTION AND FRACTIONATION OF ND, GD, AND YB ON NATURAL AND SYNTHETIC CLAY MINERALS

Knowing the geochemical behavior of rare earth elements (REE) in clay mineral-dominated systems is essential to understanding observed REE fractionation in clay-hosted REE deposits, such as those in the Longnan district, south China. Previous work has focused on the behavior and distribution coefficients (Kd) of single REE on individual clay minerals and do not have the ability to show fractionation of REE. To test if fractionation is driven by processes involving clay minerals requires systematic study of adsorption behaviors for light and heavy REE as a function of varying clay mineral properties.

In this study, four Clay Mineral Society source clays, kaolinite (KGa-1), palygorskite (PFl-1), Na-montmorillonite (SWy-2), and synthetic mica-montmorillonite (Syn-1; Barasym SSM-100), selected on the basis of their surface areas and cation exchange capacities (CEC), were reacted with REE-enriched fluids to determine the Kd of each REE-clay mineral combination. The REE used in the study, Nd, Gd, and Yb, represent light, middle, and heavy portions of the lanthanide series. Each clay mineral was also equilibrated with a REE mix to evaluate REE fractionation. Adsorption isotherms were determined using purified and size-fractionated clay minerals at 1-atm pressure, constant T = 22^oC and pH that was buffered between 6.4-7.3. All adsorption isotherms followed the Freundlich equation.

Measured log Kd values, defined as the concentration ratio clay_REE/aqueous phase_REE, are the same for individual REE and for KGa-1, PFl-1, and SWy-2, ranging from 2.27 to 2.50. Log Kd values for Syn-1 are higher, ranging between 2.83-3.07, but do not vary between the REE. When REE are in competition, KGa-1 shows a slight preference for the heavier REE: Log Kd(Nd) is 2.05-2.27; log Kd values for Gd and Yb are 2.29-2.43. Other clay minerals show no fractionation of the REE mix.

The results of this study represent an internally consistent set of REE distribution coefficients obtained under identical conditions as well as data regarding the fractionation of REE on different clays. With the possible exception of Nd on kaolinite, the results of this study show that the observed fractionation of REE in clay mineral dominated environments is not attributable to clay mineral type, surface area, or CEC, but rather some other property of a weathering profile.