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EQUILIBRIUM REE FRACTIONATION BETWEEN MINERALS AND WATERS IN THE NORTH KAZAKHSTAN SALT LAKES
Abstract
REE determinations were conducted for two mineralized lakes of the Ishim steppe, situated between the Irtysh and Tobol rivers in the southern part of Western Siberia, by means of atomic absorption spectrometry (AAS). They are of Cl-Na-type but differ in salinity and pH. It was shown that REE distribution patterns for the Zhamantuz and Kishikaroy lake waters were identical. Nevertheless, coherent fractionation from LREE to HREE is disrupted after the equilibrium calculations. Thermodynamic modeling was performed with the пїЅHChпїЅ computer code and the UNITHERM database using a Gibbs free energy minimization algorithm at 25пїЅC and 1 bar total pressure. The 255 dissolved species and solid-phase thermodynamic data were incorporated into the UNITHERM database from the literature elsewhere. In addition to the 53 solid-phases of constant composition, the formation of ideal solid solutions (ss) is expected: of monazite (La,Ce,Pr,Nd,Sm,Eu,Gd)PO4, xenotime (Y,Dy,Er,Yb)PO4, REE-fluorite, REE-F-apatite and REE-calcite. Activity coefficients were calculated with the third approximation of the Debye-HпїЅckel equation. Both lake waters are supersaturated with respect to Goethite, K-Montmorilonite and REE-F-apatite. Microcline or AlOOH are presented in Zhamantuz and Kishikaroy lakes respectively. In addition, REE-bearing solid solution precipitated in Zhamantuz lake is REE-calcite (pH 8.5), but in Kishikaroy lake it is REE-fluorite. Important cause of the REE fractionation is differential solubility of REE-bearing Ca-minerals: apatite << fluorite < calcite. Different minerals tend to concentrate the light and heavy REE. According to model calculations, F-apatite contains more light REE, fluorite contains more heavy REE, calcite contain pronounced concentration of Y2(CO3)3. Fractionation may occur due to the variable stability of REE complexes in chloride lake waters. Our model presents recognition that rare earth fractionation could occur with time (when equilibrium will be reached) in different ways due to the various chemical composition of salt waters in spite of their coherent distribution in sampled solutions.
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